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... Adhesive Bonding Strength 90 5.4 Conclusions 92 5.5 References 92 Chapter The Effect of Aqueous Polyurethane Dispersion on Non- yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear. .. solvent borne polyurethane Solution and aqueous polyurethane dispersion Figure 6.6 101 Comparison the color appearance of both solvent borne polyurethane solution and aqueous polyurethane dispersion. .. 3.2 Preparation of aqueous polyurethane dispersion 50 Figure 3.3 Form and dimensions of test pieces for shear tests 53 Figure 3.4 Form and dimensions of test pieces for peel strength test 54 Figure
AQUEOUS POLYURETHANE DISPERSION WITH NON-YELLOWING AND GOOD BONDING STRENGTH FOR WATER BORNE POLYURETHANE FOOTWEAR ADHESIVES APPLICATIONS KWEE KOK YEE (BSc.(Hons), Acadia University, Canada) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2005 Acknowledgment It is my great pleasure to express my sincere thanks to my supervisor, Professor Goh Suat Hong for his invaluable guidance, support and enthusiastic encouragement throughout the course of my research work. Special thanks is also extended to Rhodia Asia Pacific Pte Ltd for the financial support of my M.Sc. Course. The assistance of the staff in NUS analytical laboratory and Rhodia PC&S-INCO laboratory staff is also gratefully acknowledged. Last but not least, I wish to express my greatest gratitude to my husband, Kenny Lim , my children Andrew Lim and Alfonsine Lim for their love, support, encouragement and sacrifices which enable the completion of these studies. i Table of Contents Acknowledgment i Table of Contents ii Summary vii Glossary ix List of the Tables xi List of the Figures xiii List of Publications xviii Chapter 1 1 1.1 Introduction References Chapter 2 Theoretical Background 3 5 2.1 Introduction To Polyurethane 5 2.2 Types Of Polyurethane 6 2.2.1 Foamed Type 6 2.2.2 Solid Type 6 2.3 Polyurethane Adhesives 7 2.3.1 Types of Adhesives Technology 8 2.4 Application of Polyurethane 9 2.5 Market Trends – Rising Significance Of Aqueous Polyurethanes 10 2.6 Aqueous Polyurethane Dispersion 11 2.6.1 Various Methods Of Making Polyurethane Dispersions 14 ii 2.6.1.1 Emulsifier-Containing Dispersions 14 2.6.1.2 Ionomer Dispersions 14 2.6.1.3 Non-Ionic Dispersion 19 2.7 Ingredients For Aqueous Polyurethane Dispersions 21 2.7.1 Isocyanates crosslinkers 21 2.7.1.1 Aromatic isocyanates 21 2.7.1.2 Aliphatic isocyanates 21 2.7.1.3 Chemistry Of Isocyanates 25 Polyols Resins 30 2.7.2.1 Polyether Polyols 32 2.7.2.2 Polyester Polyols 32 Other Additives 35 2.7.3.1 Catalysts 35 2.7.3.2 Neutralizing Agents 37 2.7.3.3 Dimethylolpropionic Acid 38 2.7.3.4 Chain Extenders 39 2.8 Application Test 40 2.8.1 Strength And Adhesion 40 2.9 Introduction Of Shoe Making 42 2.7.2 2.7.3 2.9.1 Methods Of Shoe Construction 43 2.9.1.1 Method 1 : Moccasin Construction 44 2.9.1.2 Method 2: Cement Construction 44 2.9.1.3 Method 3 : Stitchdown Construction 45 iii 2.10 2.9.1.4 Method 4 : Moulded Method 45 2.9.1.5 Method 5 : Force Lasting Construction 46 References 47 Chapter 3 Experimental 48 3.1 Material 48 3.2 Preparation of Aqueous Polyurethane Dispersion 48 3.3 Preparation of Two Component (2K) Water Borne Polyurethane Footwear Adhesives 51 3.4 Gel Permeation Chromatography (GPC) Measurement 51 3.5 Isocyanate Functionality Determination 51 3.6 Particle Size Analysis 52 3.7 FT-IR Analysis 53 3.8 Shear and Peel Strength Measurement 53 3.8.1 Shear Strength Measurement 53 3.8.2 Peel Strength Measurement 54 3.9 References 56 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 57 4.1 Introduction 57 4.2 Experiment 58 4.3 Results and Discussion 62 iv 4.3.1 The Effect of NCO/OH Ratio 62 4.3.2 The Effect of DMPA Content 65 4.3.3 The Effect of TEA/DMPA Molar Ratio 67 4.4 Conclusions 71 4.5 References 72 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 74 5.1 Introduction 74 5.2 Experiment 75 5.3 Results and Discussion 81 5.3.1 Effect of Different Types of Chain Extenders 81 5.3.2 FT-IR Analysis of Aqueous Polyurethane Dispersion (PUD) 81 5.3.2.1 Formation of PUD 81 5.3.2.2 FT-IR Analysis of Residual NCO Functionality in PUD 82 5.3.3 Growth of Average Molecular Weight during the Chain Extension 86 5.3.4 Effect of the Degree of Chain Extension on the Adhesive Bonding Strength 90 5.4 Conclusions 92 5.5 References 92 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives 94 v 6.1 Introduction 94 6.2 Experiment 99 6.3 Results and Discussion 101 6.3.1 Color Appearance and Durability Comparison 101 6.3.2 Adhesive Bonding Strength Comparison 103 6.4 Conclusions 108 6.5 References 109 Chapter 7 Conclusions 110 vi Summary It is imperative to develop aqueous polyurethane dispersions mainly due to the evolution of legislation towards reducing the VOC (volatile organic concentration) and the creation of environmentally friendly products. In view of the footwear industry, the big multi-national footwear producers like Nike, Reebok and Adidas have already embarked on the campaign to demand their suppliers to supply water borne footwear adhesives. Aqueous polyurethane dispersions are binary colloidal systems having polyurethane particles dispersed in aqueous phase, which can be classified into anionic, cationic and nonionic systems. In this research, the polyurethane ionomers were prepared by anionic dispersion process using polyester diol derived from caprolactone monomer terminated by primary hydroxyl groups (CAPA®2205), isophorone diisocyanate (IPDI) and dimethylol propionic acid (DMPA) as the potential ionic center with 1-methyl-2pyrrolidone (NMP) as the co-solvent. 1,6-Hexanediamine (HDA) was selected as a chain extender for the chain extension process. The reaction parameters were NCO/OH ratio, DMPA content, neutralization degree and chain extension. The influence of the molecular weight and particle size of the PUD on the adhesive bonding strength i.e. peel and shear strength have been determined. The weight-average molecular weight (Mw) and adhesive bonding strength were significantly affected by the NCO/OH ratio, the DMPA content, the degree of neturalization and the level of chain extension. As the NCO/OH ratio increases, the Mw increases and the adhesive bonding strength also increases. The lower the DMPA content , the higher the Mw and particle size of PUD, but lower the formation of hard segments vii in the polyurethane main chain. Perhaps, the increase in the adhesive bonding strength would be more influenced by the Mw than the ratio of soft and hard segments. When the chain extension increases, the Mw increases, leading to the increase in adhesive bonding strength. The higher the degree of neutralization, the lower the particle size, and consequently the molecular weight and the adhesive bonding strength increased. To obtain an aqueous polyurethane dispersion with the optimum performance, the reaction parameters are as follows: NCO/OH ratio is 3, the DMPA content is 5% and the degree of neutralization is 100%. The non-yellowing property of the footwear adhesive was achieved by using aliphatic isocyanate (IPDI). The merit of this research is that, we are able to develop an aqueous polyurethane dispersion (PUD) with the properties fulfilling the industrial requirements such as good compatibility, durability (non-yellowing), low VOC and excellent adhesive bonding strength. Moreover, the performance of PUD developed in this research has also been assessed and compared with the commercial PUD in both 1K and 2K footwear adhesives formulations. Our PUD showed superior performance in adhesive bonding strength than the commercial product. Therefore, the PUD developed in this research may attract attention from footwear adhesive producers in the market. viii Glossary BDMAEE Bis(N,N-Dimethylaminoethyl)ether CAGR Compound annual growth rate DABCO 1,4-Diazabicyclo[2,2,2]octane DBBT Di-n-butylamine back titration DLATGS L-alanine-dooped duterated triglycine sulfate DMAEE 2-(2-Dimethylaminoethoxy)-ethanol DMCHA N,N-Dimethylcyclohexylamine DMEA N,N-Dimethylethanolamine DMPA Dimethylol propionic acid DMF Dimethylformamide DMT Dimethyl terephthalate DETDA Diethyl toluene diamine DI Deionized water DBTL Di-n-butyltin-di-laurate EDA Ethylenediamine FT-IR Fourier-tranform infrared spectroscopy GPC Gel permeation chromatography H12MDI 4,4’-Diisocyanatodicyclohexylmethane HDI Hexamethylene diisocyanate HDA 1,6-Hexanediamine IPDI Isophorone diisocyanate Mw Weight-average molecular weight ix Mn Number-average molecular weight MDI Methylene diphenyl diisocyanate NDI 1,5-Naphthalenediisocyanate NMP 1-Methyl-2-pyrrolidone PUD Polyurethane dispersion PIR Polyisocyanurate rigid foam PET Poly(ethylene terephthalate) PMMA Polymethylmethacrylate PMDETA N,N,N’,N’,N”-Pentamethyldiethylenetriamine PS Polystryene TDI Toluene diisocyanate TDA Toluene diamine TEA Triethylamine THF Tetrahydrofurane VOC Volatile organic concentration W/O Water/oil 2K Two-component 1K One-component x List of Tables Table 2.1. Types of polyurethane adhesives 9 Table 2.2. Characteristic features of polyurethane dispersions 20 Table 2.3. Tertiary amine catalysts and their application 36 Table 2.4. Organometallic catalysts and their application 37 Table 4.1. Formulation of aqueous polyurethane dispersion (Sequence 1) – DMPA content is constant and NCO/OH ratio varies from 2.8 to 3.4 Table 4.2. Formulation of aqueous polyurethane dispersion (Sequence 2) – NCO/OH ratio is constant and DMPA varies from 4 to 7 Table 4.3. 61 61 Formulation of aqueous polyurethane dispersion (Sequence 3) – NCO/OH ratio and DMPA content are constant but the degree of neutralization varies from 85% to 115% Table 5.1. Characteristics and formulations of aqueous polyurethane dispersion using HDA as chin extender with different degree of neutralization Table 5.2. 78 Characteristics and formulations of aqueous polyurethane dispersion using EDA as chin extender with different degree of neutralization Table 5.3. 62 79 Characteristics and formulations of aqueous polyurethane dispersion using Dytek® A Amine as chain extender with different degree of neutralization Table 5.4. Appearance of the finishing PUD product with different types of chain extenders Table 5.5. 80 81 The residual NCO content of the polyurethane prepolymer by the di-nButylamine back titration method 84 xi Table 6.1. One-component (1K) water borne PU footwear adhesive formulation Table 6.2. 100 Two-component (2K) water borne PU footwear adhesive formulation 100 xii List of Figures Figure 1.1. Formation of aqueous dispersion 3 Figure 2.1. Polyurethane adhesives market segment 8 Figure 2.2. Reaction of polyol and isocyanate 12 Figure 2.3. Amine catalyst reaction mechanism 12 Figure 2.4. Anionic polyurethane dispersion with carboxylate groups 14 Figure 2.5. Non-ionic polyurethane dispersion 15 Figure 2.6. Preparation of aqueous polyurethane dispersion by acetone process 16 Figure 2.7. Preparation of aqueous polyurethane dispersion byprepolymer mixing process 17 Figure 2.8. Preparation of aqueous polyurethane dispersion by Ketimine and ketazine Process 18 Figure 2.9. Global split of isocyanate market in year 2000 21 Figure 2.10. The reaction rate for different types isocyanates 24 Figure 2.11. Water Reaction 26 Figure 2.12. Urea formation 27 Figure 2.13. Biuret formation and equilibria 28 Figure 2.14. Allophonate formation 29 Figure 2.15. Other isocyanates reactions 30 Figure 2.16. Polyol types used in polyurethane 31 Figure 2.17. Worldwide demand for polyester polyols by application 33 Figure 2.18. Adhesion/cohesive strength testing formats 41 Figure 2.19. Single lap joint testing 42 xiii Figure 2.20. Parts of a shoe 43 Figure 2.21. Moccasin method 44 Figure 2.22. Stitchdown / Veldschoen method 45 Figure 2.23. Moulded method for various types of footwear 46 Figure 2.24. Slip lasting / strobel stitched method 47 Figure 3.1. Set up of apparatus for the synthesis of polyurethane prepolymer 49 Figure 3.2. Preparation of aqueous polyurethane dispersion 50 Figure 3.3. Form and dimensions of test pieces for shear tests 53 Figure 3.4. Form and dimensions of test pieces for peel strength test 54 Figure 3.5. Process for applying the adhesive 55 Figure 4.1. Preparation of aqueous polyurethane dispersion 60 Figure 4.2. Mw and Mn of PUD as a function of the NCO/OH ratio 63 Figure 4.3. Change of Mw with different NCO/OH ratio during the chain extension reaction 64 Figure 4.4. Effect of NCO/OH ratio on the adhesive bonding strength 65 Figure 4.5. Mw and particle size of PUD as function DMPA content 66 Figure 4.6. Effect of Mw on bonding strength of the aqueous polyurethane dispersion with different DMPA content Figure 4.7. 67 Effect of neutralization degree on the particle size of aqueous polyurethane dispersion (PUD) 68 Figure 4.8. Particle size controlled by TEA/DMPA ratio 68 Figure 4.9. Evolution of Mw of PUD with varying the degree of neutralization from 85% to 115% during chain extension reaction 69 xiv Figure 4.10. Mw and Mn of PUD as a function of degree of neutralization 70 Figure 4.11. Effect of neutralization on adhesive bonding strength 71 Figure 5.1. FT-IR spectra of polyol, IPDI and PUD 82 Figure 5.2. FT-IR spectra of PUD before (a) after (b) chain extension 83 Figure 5.3. Absorption FT-IR spectra of PUD of varying particle size before chain extension : (a) 1.65µm, (b) 2.60µm, (c) 3.65µm, (d) 4.05µm (NCO/OH ratio = 2.8) Figure 5.4. 84 Change of FT-IR spectra during preparation of PUD: (a) polyester polyol + IPDI + DMPA, (b) after neutralization/ before dispersion, (c) before chain extension, (d) adding 20% of chain extender (theortically), (e) 40%, (f) 60%, (g) 80% and (h) 100% (NCO/OH = 3.0; particle size =2.65µm) Figure 5.5. 85 Amount of residual NCO groups versus the sizes of PUD particles in chain extension stage at average particle size 2.65µm, 2.00µm and 1.50µm at NCO/OH =3.0 Figure 5.6. 86 The change of average molecular weight in chain extension stage with different particle sizes and different NCO/OH ratio (particle sizes varied from 1.50µm to 2.56µm; NCO/OH ratio varied from 2.8 to 3.0) Figure 5.7. 87 Effect of particle size on the maximum value of chain extension (CEmax) 88 Figure 5.8. The reaction of residual NCO groups 89 Figure 5.9. Two competitive reactions of residual –NCO groups on PU particle 89 xv Figure 5.10. Effect of the degree of chain extension on adhesive bonding strength (shear and peel strength) at NCO/OH ratio 2.8 Figure 5.11. Effect of the degree of chain extension on the shear strength with different particle sizes at NCO/OH ratio 3.0 Figure 5.12. 90 91 Effect of the degree of chain extension on the peel strength with different Particle sizes at NCO/OH ratio 3.0 91 Figure 6.1. Total output of adhesives + sealant and PU adhesive in China 94 Figure 6.2. Segmentation of PU adhesives markets (by consumption volume, total 1666 thousands tonnes) in year 2002 96 Figure 6.3. The largest footwear producer in the world, China 97 Figure 6.4. Total footwear production in China from year 1985 to 2002 97 Figure 6.5. Comparison the color appearance of both solvent borne polyurethane Solution and aqueous polyurethane dispersion Figure 6.6. 101 Comparison the color appearance of both solvent borne polyurethane solution and aqueous polyurethane dispersion after storage for 6 months at ambient temperature and humidity at 55% environment Figure 6.7. 102 Comparison of the color appearance of both 2K solvent borne and water borne PU footwear adhesives after exposure to sunlight for 3 months Figure 6.8. 102 Comparison of initial peel strength of our 1K water borne PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD) 104 xvi Figure 6.9. Comparison of final peel strength of our 1K water borne PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD) Figure 6.10. Shear strength comparison of our 1K water borne PU footwear adhesive versus commercial product Figure 6.11. 104 105 Comparison of initial peel strength of 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD) Figure 6.12. 106 Comparison of final peel strength of 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD) Figure 6.13. 106 Shear strength comparison of our 2K water borne PU footwear adhesive versus commercial product 107 xvii List of Publications 1. Two Pack Water Borne Polyurethane for Furniture Coatings K.Y. Kwee, V. Granier and C. Varron, J. Eur. Coat. 2002, 27. 2. Overview of Aliphatic Polyisocyanates Used in Polyurethane Coatings: Chemistry and Market Trends K.Y Kwee, E. Charriere, J. Asi. Coat. 2003, 37. 3. Fast Drying Aliphatic Polyisocyanate for 2K PU Automotive Coatings K.Y. Kwee, Coating Manufacturing Technology for China’s Automobile Industry 2004, 18. xviii Chapter 1 Introduction Chapter 1 Introduction The term “aqueous polyurethane dispersion” refers to aqueous dispersions of polymers containing urethane groups and optionally urea groups. Aqueous polyurethane disperisons are well known and used in the production of a variety of useful polyurethane products, for example, adhesives, coatings and sealants etc. Such dispersions are produced by dispersing a water-dispersible, isocyanate-terminated polyurethane prepolymer in an aqueous medium together with an active hydrogen-containing chain extender, such as diamine. It is vital to develop aqueous polyurethane dispersions mainly due to the evolution of legislation towards reducing the VOC (volatile organic concentration) and the creation of environmentally friendly products. Continuous increase in solvent prices, low raw material cost and easy to clean up the reactor system made aqueous polyurethane system more popular in the industry. Aqueous polyurethane dispersions can be classified into anionic, cationic and nonionic systems.1,2 They can be obtained by different processes, however, the earliest process to prepare the aqueous polyurethane dispersion is known as acetone process. This process has remained technically important so far.3,5 Within the last three decades several new processes have been developed such as prepolymer mixing process, hot melt process and ketamine/ketazine process. The basic principle involved in producing NCO-terminated polyurethane prepolymer with appropriate molecular weights.6 Distinctly different step among several processes lies in the chain extension step that is generally performed using diamines (-NH2 ) and /or diols (-OH).7 In the chain extension step, it is most important to control of extremely fast reaction between NCO 1 Chapter 1 Introduction groups and NH2 groups accompanied by the viscosity rise.2 The prepolymer mixing process that we have used in this study has the advantage of avoiding the use of a large amount of organic solvent. In this process, NCO-terminated polyurethane prepolymer containing pendant acid group i.e. dimethylol or 2,2-bis(hydroxymethyl) propionic acid (DMPA) is neutralized with base to form internal ionic emulsifier and dispersed in the aqueous phase i.e. water to form an aqueous dispersion (see figure 1.1). The chain extension step is accomplished by the addition of diamine to the aqueous dispersion. Molecular weight of polyurethane dispersion increases by the formation of urea linkage with NCO-terminate prepolymers and diamines through the chain extension step. Hence, the most important step to determine molecular weight of polyurethane dispersion is the chain extension step, which is the reaction between residual NCO groups and amine groups. The chain extension is influenced by the amount of residual NCO groups and particle diameter. The amount of residual NCO groups is determined by the molar ratio of NCO:OH. In additional, both hydrophilic acid group contents and their degree of neutralization can affect particle diameter.8 Consequently, the molecular weights (Mw) can be controlled with varying these process variables. In general, the molecular weights (Mw) of polymer materials have a significant effect on their mechanical properties. Therefore, the control of the mean Mw can be used as a indicator to obtain the optimum mechanical properties i.e. the adhesive bonding strength of aqueous polyurethane dispersion. In this research, the aqueous polyurethane dispersions had been synthesized with different formulations by varying their NCO/OH molar ratios, DMPA contents, degrees of neutralization, types of chain extenders and different degree of chain extension to 2 Chapter 1 Introduction obtain the best finishing aqueous polyurethane dispersion. The footwear (PU) adhesive was formulated using this newly developed aqueous polyurethane dispersion and then its performances were being compared against the commercialized polyurethane dispersion. 1st Step : Preparation of polyurethane prepolymer Hard Segment (-NHCOO -) or CH2 (-NHCOOCH2CCH2COONH-) COOH or COO- NH+ (Et)3 Soft Segment Hard Segment Soft Segment (-CH2-CH2-) 2nd Step : Dispersion process COOH or COO- NH+ (Et)3 PU particle Repulsion Electrical double layer Figure 1.1. Formation of aqueous dispersion 1.1 References 1. J.W. Rothause and K. Nachtkam, Advances in Urethane Science and Techology 1987,10, p.121. 2. B.K. Kim, Coll. Polym. Sci., 1996, 274, p.559. 3 Chapter 1 Introduction 3. C. Hepburn, Polyurethane Elastomers, Second ed., Elsevier, New York, 1992, p.281. 4. G. Woods, The ICI Polyurethane Book, ICI Polyurethanes, 1987, p.197. 5. G. Oertel, Polyurethane Handbook, Carl Hanser, Munich, 1985, p.31. 6. P. Thomas, Waterborne and Solvent Based Surface Coating Resins and their Applicaions – Polyurethanes, Vol. III, SITA Technology, London, 1999, p.59. 7. H.T. Lee, Y.T. Hwang, N.S. Chang, C.C.T. Huang, H.C. Li, Waterborne, High-Solids and Powder Coatings Symposium, New Orleans, 22-24 February, 1995, p.224. 8. H. Xiao, H.X. Xiao, K.C. Frisch, N. Malwitz, J. Appl. Polym. Sci., 1994, 54, p.1643. 9. Y.K. John, I.W. Cheong, J.H. Kim, Colloids Surfaces A Physicochem. Eng. Aspects 2001, 179 (1), p.71-78. 4 Chapter 2 Theoretical Background Chapter 2 Theoretical Background 2.1 Introduction to Polyurethanes The reaction between isocyanate and hydroxyl compounds was originally identified in the 19th century; the foundations of the polyurethanes industry were laid in the late 1930s with the discovery, by Otto Bayer, of the chemistry of the polyaddition reaction between diisocyanate and diols to form polyurethane.1 Polyurethanes are now all around us, playing a vital role in many industries – from furniture to footwear, construction to cars. Polyurethane can appear in many different forms, making them the most versatile of any family of plastic materials. Commercially, polyurethanes are produced by the exothermic reaction of molecules containing 2 or more isocyanate groups with polyol molecules containing 2 or more hydroxyl groups. Relatively few basic isocynates and a far broader range of polyols of different molecules weights and functionalities are used to produce the whole spectrum of polyurethane materials. Additionally, several other chemical reactions of isocyanates are used to modify or extend the range of isocyanate-based polymeric materials. The unique advantage of polyurethanes lies in the wide variety of highperformance materials that can be produced. They also differ from most other plastic materials because the processor is able to change and control the nature and the properties of the final product, even during the production process. It is possible because most polyurethanes are made using reactive processing machines, which mix together the polyurethane chemicals that then react to make the polymer required.1,2 5 Chapter 2 Theoretical Background 2.2 Types of Polyurethane 2.2.1 Foamed Types By itself the polymerization reaction produces solid polyurethane and it is by foaming gas bubbles in the polymerizing mixture, often refer to as ‘blowing’. Three foam types are, in quantity terms, particularly significant: low density flexible foams, low density rigid foams and high-density flexible foams, commonly referred to as microcellular elastomers and integral skin foams. Low density flexible foams have densities in the range 10 to 80 kg/m3. Low density flexible foams have densities in the range 10 to 80 kg/m3 , made from a lightly crossed-linked polymer with an open cell macro structure. There are no barriers between adjacent cells, which result in a continuous path in the foam, allowing air to flow. Low density rigid foams are highly cross-linked polymers with an essentially closed cell structure and a density range of 28 to 50 kg/m3. The individual cells in the foam are isolated from each other by thin polymer walls, which effectively stop the flow of gas through the foam. High density flexible foams are defined as those having densities above 100 kg/m3. 2.2.2 Solid Types Solid polyurethanes are used in many diverse applications. Cast polyurethane elastomers are simply made by mixing and pouring a degassed reactive liquid mixture 6 Chapter 2 Theoretical Background into a mould. These materials have good abrasion resistance, many common non-polar solvent resistance etc. They are used in the production of printing rollers tyres and so.1,7 Polyurethane elastomeric fibres are produced by spinning from a solvent, usually dimethylformamide (DMF), or by extrusion from an elastomer melt. The major applications are in clothing where these fibres have effectively replaced natural rubber. Thermoplastic polyurethane is supplied as granules or pellets for processing by well established thermoplastic processing techniques such as injection moulding and extrusion. By these means elastomeric mouldings having an excellent combination of high strength with high abrasion and environmental resistance, can be mass produced to precise dimensions. Applications include hose and cable sheathing and so on. Polyurethanes are also used in flexible coatings or textiles and adhesives for film, fabric laminates and footwear. Paints and coatings give the highest wear resistance to floors and aircraft surfaces. Binders are used increasingly in the composite wood products market for oriented strand board and laminated beams for high performance applications. 2.3 Polyurethane Adhesives Polyurethane adhesives, which vary widely in composition, are used in many application areas due to their outstanding properties, their simple and economical processing and their high strength. They account for about eight percent of the global adhesives market, at around 530,000 tonnes, excluding their use as binders for wood and other materials. Polyurethanes are a major element in the high value reactive adhesives category because of their versatility and moderate pricing. The market segments in which polyurethanes find most use are construction 31 percent, flexible packaging 27 percent, 7 Chapter 2 Theoretical Background footwear 17 percent, woodworking 17 percent and transportation including assembly 8 percent.1,4 This can be illustrated in the figure 2.1. PU Adhesives ~ 530,000 tonnes (about 8% of global adhesives market) 8% 31% 17% 17% 27% Construction Footwear Transportation (including assembly) Flexible packaging Woodworking Figure 2.1. Polyurethane adhesives market segment Polyurethane adhesives are normally defined as those adhesives that contain a number of urethane groups in the molecular backbone or where such groups are formed during use, regardless of the chemical composition of the rest of the chain. Thus, a typical urethane adhesive may contain, in addition to urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea and allophanate groups. A common factor for all polyurethane adhesives is that they cure to produce essentially thin films used to bond two similar or dissimilar surfaces together, if the correct type of polymer structure for the end application. 2.3.1 Types Of Adhesives Technology Polyurethane adhesives can be divided into two main classes: non-reactive and reactive. In both cases, the aim is to put a thin continuous layer of high molecular weight 8 Chapter 2 Theoretical Background polyurethane between the two surfaces to be joined. Non-reactive adhesives are based on high molecular weight, which are applied as solvent-borne, water-borne or as hot-melts. Film forming occurs through evaporation of the solvent or water for the first two whilst hot-melts are applied at high temperature and films form upon cooling. Reactive adhesives are supplied as one- or two-component systems or as hot melts. The one-component reactive systems, based on low isocyanate prepolymers are usually moisture-cured, whilst the prepolymer for two-component systems are reacted with a mix of polyol and chain extenders.1,3 The types of technology are summarized in table 2.1. Table 2.1. Types of polyurethane adhesives Type Non-reactive: Solvent-borne Water –borne Hot-melt Form at room temperature Curing at film forming mechanism Liquid Liquid (dispersion) Solid Physical evaporation of solvent Physical evaporation of water Physical cooling Reactive: One-component Two-component Reactive hot-melt Liquid Liquid Solid Cross-linker Liquid Chemical cure, NCO + Moisture Chemical cure, NCO + Polyol Physical cooling + Chemical cure, NCO + moisture Chemical cure, NCO + active H 2.4 Applications of Polyurethane Footwear – some of the soles are made from synthetic material like polyurethane to give high performances. Polyurethane adhesives are widely used in the shoe industry and coatings are used to improve appearance and wear resistance of shoe uppers. Automotives – applications include seating, interior padding such as steering 9 Chapter 2 Theoretical Background wheels and dashboards, complete soft front-ends, components for instrument assemblies and accessories such as mirror. Furniture – the market for cushioning materials is mainly supplied by polyurethane flexible foam, which competes with cotton, polyester fibre etc. It is ideal where strong, tough, but decorative integral-skinned flexible or rigid foam structures are needed. Thermal insulation – rigid polyurethane foam offers unrivalled technical advantages in the thermal insulation of buildings, refrigerators and other domestic appliances. 2.5 Market Trends - Rising Significance Of Aqueous Polyurethanes The fact that aqueous/water borne polyurethanes have become increasingly important commercially in recent years is due to three reasons: 1. The increasingly stringent environment legislation requires the development of ecologically and physiologically tolerated products for which the emissions of solvents and other volatile organic compounds (VOC) have been reduced to a minimum. 2. The use of expensive organic solvents in conventional and aqueous polyurethanes is undesirable for economic reasons. 3. The performance of aqueous polyurethanes reaches or surpasses that of conventional isocyanate- and/or solvent-containing polyurethanes. Due to these reasons, companies like Nike, Rebok, New Balance etc. global based multi-national footwear manufacturers want to be the environmental oriented companies. 10 Chapter 2 Theoretical Background They are making a lot of efforts in demanding their shoes adhesives producers to produce aqueous polyurethane adhesives for their shoes. As the environmental problems grow bigger, it is expected that other shoe manufactures become more interested in the environmental policy, thus adopting more water borne products in the future. Industries other than shoe industry, such as automobile, furniture and electronic industries are expected to adopt the water borne adhesives only when the products are supplied in a stable and consistent manner. The overall market size for the aqueous polyurethanes will grow tremendously in the very near future. 2.6 Aqueous Polyurethane Dispersion Aqueous polyurethane dispersions (PUD) are fully-reacted polyurethane systems produced as small discrete particles, 0.1 to 3.0 micron, dispersed in water to provide a product that is both chemically and colloidally stable, which only contains minor amounts of solvents and thus emit very little volatile organic compounds. Aqueous PUDs are based on aliphatic – IPDI or H12MDI – or aromatic – MDI or TDI – isocyanates, modified polyether and/or polyester polyols, chain extenders, catalysts plus additives to modify the coalescence, flow, thickness, coagulation and defoaming properties. Aqueous PUD is used in many application areas to coat a wide range of substrates – for example footwear adhesives, wood lacquers for flooring and furniture, leather finishing, vinyl upholstery topcoats, plastic coatings, printing inks and automotive base coats.2,8-9 Aqueous PUD is produced in conventional stirred reactor fitted with distillation equipment. The first step in the manufacture of an anionically-stabilised PUD is to 11 Chapter 2 Theoretical Background prepare a prepolymer from isocyanate, polyol (containing either carboxylate or sulpfonate side chains) and chain extenders in a water-miscible solvent such as acetone.3,5 H O R' N C O R R OH + R' N C O Polyol Isocyanate urethane (carbamate) Figure 2.2. Reaction of polyol and isocyanate. The reaction product is an isocyanate-terminated polyurethane or polyurea with pendent carboxylate or sulpfonate groups. These groups can be converted to salts by adding a tertiary amine compound, which, as water is added to the prepolymer/solvent solution, disperses the prepolymer in the water. R-N=C=O + R"3N R-N C O R'OH O R-HN-C + R"3N O-R' N R" R" R" Figure 2.3. Amine catalyst reaction mechanism The carboxylate groups are generally neutralized before or during dispersion of the polyurethane prepolymer into water with a tertiary amine compound (see figure 2.3). An organic bases are less convenient, since the polyurethane will generally coagulate when they are applied or it will provide highly water sensitive films or coatings. To prevent coagulation it is suitable to incorporate a great number of hydrophilic polyethoxy chains 12 Chapter 2 Theoretical Background into the polymer system, but the coatings prepared from these dispersions will be sensitive to water as well. The next step in the synthesis is the reaction of the remaining isocyanate groups with more chain extender or a cross-linker or a mixture of both. The solvent is then stripped, leaving the water-borne polyurethane dispersion with only a low solvent content. The critical factor is achieving a fine enough particle size of the fully reacted polyurethane so that it maintains a stable dispersion once the solvent is removed. The final dispersions contain 35 to 50 wt% of dispersed particles.3,4 Alternatively, a low molecular weight hydrophilic prepolymer can be chain extended at the same time as the aqueous dispersion is formed, providing that the isocyanate reacts preferentially with the amine rather than water. In each case, the final polymer contains a mixture of urethane and urea groups. The majority of polyurethane dispersions are made from the slower-reacting aliphatic isocyanates. Hybrid systems, especially urethanes-acrylates, are also increasingly used. Simple blending of two individual dispersions can be used, but the film properties are better if a mixed synthesis is used, giving a continuous phase on drying with final film properties typical of polyurethane on its own. PUD can be applied by a range of techniques- such as brush, spray, dip, curtain and the aqueous dispersions form films by a coalescence process in which the individual particles are forced together, as water is lost during drying, so that the particles deform and eventually fuse together. The process is dependent on a number of parameters with a faster rate obtained from a small particle size, low polymer glass transition temperature, increasing temperature and the addition of a coalescing agent to achieve sufficient flow and fusion of the particles. Cross-linkers, such as isocyanates, aziridine, carbodiimide, 13 Chapter 2 Theoretical Background and melamine, can be added just prior to application to improve the performance of the coating or adhesives, but then the blends have a pot life and usually need temperature activation in order to achieve full cure. 2.6.1 Various Methods Of Making Polyurethane Dispersions 2.6.1.1 Emulsifier-Containing Dispersions Depending on the emulsifier used, the resulting dispersions are mainly anionic or non-ionic, but seldom cationic. Often they contain small amounts of solvents, for example toluene. 2.6.1.2 Ionomer Dispersions The most important dispersions are emulsifier-free ionomer dispersions. The resulting dispersions are mainly anionic or non-ionic (see Figure 2.4 and 2.5), which are characterized by high mechanical and chemical stability, excellent film forming properties, good adhesion and the potential for wide variations in composition and property level.4,5 Figure 2.4. Anionic polyurethane dispersion with carboxylate groups 14 Chapter 2 Theoretical Background Figure 2.5. Non-ionic polyurethane dispersion Due to the fact that ionomers are self-dispersing, the preparation procedure does not require emulsifiers or high shear forces. The following preparation processes have gained technical importance: 1. The acetone process : First a solution of high molecular weight polyurethane – especially a polyurethane urea – ionomer is prepared in a hydrophilic organic solvent, for example acetone. The solution is subsequently mixed with water and then the organic solvent is removed by distillation (see Figure 2.6). An aqueous solution or dispersion of the polyurethane ionomer is obtained. Depending on ionic group content and concentration, the dispersion will be formed either by precipitation of the hydrophobic segments or by invasion of the phases of a primary formed W/O emulsion. Advantages of this process are the wide 15 Chapter 2 Theoretical Background variety of possibilities in the molecular weight build up of the polymer and the control of the average particle size, as well as the high quality of the final products, and the good reproducibility from batch to batch.2,3 Figure 2.6. Preparation of aqueous polyurethane dispersion by acetone process 2. The prepolymer ionomer mix process : Prepolymers with terminal NCO groups can be mixed with water to yield reactive O/W emulsions; particularly when the molecular weight of the prepolymers does not exceed approximately 8000. Prepolymers containing ionic centers or hydrophilic polyether segments are self-emulsifiable. This means that upon mixing with water they spontaneously form emulsions with particle sizes which decrease as hydrophilicity increases. The reactivity of NCO groups towards water increases in the same order. Hydrophobic NCO prepolymers necessitate the use of emulsifiers and high shear forces 16 Chapter 2 Theoretical Background to disperse them in water. Emulsifiers which are chemically similar to the substrate to be dispersed are most efficient.3,5 Highly viscous prepolymers must be diluted with organic solvents, which do not necessarily have to be miscible with water. The resulting aqueous emulsions can be further chain-extended by the addition of di- or polyamines. When high molecular weight polyurethanes containing hydrophilic centers or external emulsifiers are to be dispersed with water, preferably a solution of these polymers in hydrophilic solvents is prepared, mixed with water, and the solvent is subsequently removed. The prepolymer ionomer mix process can be demonstrated in Figure 2.7. Figure 2.7. Preparation of aqueous polyurethane dispersion by prepolymer mixing process 3. The ‘melt dispersion’ process with formaldehyde polycondensation : Reaction of an NCO-terminated ionic modified prepolymer with, for example 17 Chapter 2 Theoretical Background ammonia or urea results in a prepolymer with terminal urea or biuret groups, respectively. These are methylolated with formaldehyde. Before, during, or after the reaction with formaldehyde, the hot melt is mixed with water, forming dispersion spontaneously. Afterwards, chain-extension or cross-linking takes place through polycondensation (lowering the pH, increasing the temperature). 4. Ketimine and ketazine process : Diamines and especially hydrazine are reacted with ketones to yield ketimines and ketazines, respectively. These can be mixed with NCO prepolymers containing ionic groups without premature chain extension. These mixtures can be emulsified with water even in the absence of co-solvents.2,3 Reactions with water liberate the diamine or hydrazine, which then reacts with the prepolymer (see Figure 2.8). Figure 2.8. Preparation of aqueous polyurethane dispersion by Ketimine and ketazine process 18 Chapter 2 Theoretical Background 5. Spontaneous dispersion process for solids : Ionic and/or non-ionic hydrophilic modified oligomers, which have an average molecular weight of less than 8000 and which are glassy solids at room temperature, can be dispersed in water without the need of high shear forces, emulsifiers or thermal treatment. Due to this feature, these products can be shipped as 100% solid resin precursors for aqueous dispersion. Once dispersed in water, cross-linkers can be added and high molecular weight polyurethane coatings can be obtained after cure on a substrate. 2.6.1.3 Non-Ionic Dispersion Non-ionic dispersions can be prepared similarly to ionomer dispersions if the ionic center is replaced by lateral or terminal hydrophilic ether chain, having a molecular weight of approximately 600 to 1500. The preparation is the same as described in ionomer dispersions, except that the dispersing process temperature has to be kept below 60oC. This is because polyethylene glycol ether units lose their hydrophilicity with increasing temperature and result in unstable dispersions. Non-ionic dispersions are stable towards freezing, pH-changes and addition of electrolytes. Also, they can be thermally coagulated.3,5 19 Chapter 2 Theoretical Background Table 2.2. Characteristic features of polyurethane dispersions Acetone process Prepolymer mixing process Polyhydroxy Compound Dispersant shear force process Polyether (liquid) Meltdispersion process Variable Ketimine/ ketazine process Variable Solids selfdispersing process Variable Linear, variable Polyethers, some polyesters Diisocyanate TDI Variable TDI,IPDI, H12MDI TDI,HDI, IDPI Variable Mainly ionic Variable - Dimethylol propionic acid - - - Softening point of prepolymer >40oC MW < 8,0000 - Glycols Only small amounts + Variable Solvent 5 to 10% toluene 40to70% acetone often 10 to 30% Nmethyl pyrrolidone - possibly 5 to 30% acetone - Shear force mixer + - - - - - Temperature of dispersion Product before dispersion ~ 20oC 20 to 80oC 50 to 130oC 50 to 80oC 15 to 30oC Dispersant ~ 50oC Nonionic NCO prepolymers Polyurethane NCO prepolymerionomer Ionic-biuretprepolymers NCO prepolymer + ketimine/ ketazine Prepolymer Procedure after dispersion Amine extension Acetone distill. Amine extension Polycondensation possibly acetone distillation Curing agent added End product Polyurethane urea Polyurethane Polyurethaneurea Polyurethane urea ionomer Polyurethane Biuret Polyurethane urea Polyurethane Solvent contents of the final dispersion 2 to 8% toluene < 0.5% Often 5 to 15% Nmethyl-pyrrolidone - possibly < 2% acetone - Particle size (nm) 700 to 3,000 30 to 100,000 100 to 500 30 to 10,000 30 to 10,000 30 to 500 Post curing temperature 100oC - - 50 to 150oC 50 to 150oC >120oC 20 Chapter 2 Theoretical Background 2.7 Ingredients For Aqueous Polyurethane Dispersions 2.7.1 Isocyanates Crosslinkers Isocyanates can be classified into the following two main types: 2.7.1.1 Aromatic isocyanates: Methylene diphenyl diisocyanate (MDI), Toluene diisocyanate (TDI) and 1,5-Naphthalenediisocyanate (NDI) 2.7.1.2 Aliphatic isocyanates: Hexamethylene diisocyanate (HDI), Isophorone diisocyanate (IPDI) and 4,4’-Diisocyanatodicyclohexylmethane (H12MDI) Presently, the isocyanates dominating the market are the aromatic isocyantes. The major ones are MDI and TDI. However, the second major isoyanates are from the aliphatic group, HDI and IPDI. Below is a pie chart showing the percentage production of various isocyanates in 2000 market.3,4 Total Market Size for Isocyanate ~ 4.4 million tonees ( in year 2000) 3.40%1.20% MDI 34.10% TDI HDI & IPDI 61.30% Others Figure 2.9. Global split of isocyanate market in year 2000 21 Chapter 2 Theoretical Background I. Methylene diphenyl diisocyanate (MDI) Pure 4,4’-MDI is a symmetrical molecule with two aromatic isocyanate groups of equal reactivity. Commercial products normally contain one to two percent of the 2,4’ isomer and have hydrolysable chlorine levels below five ppm. 2,4’-MDI is an asymmetrical molecule with two aromatic isocyanates of different reactivity. The 4-position is approximately four times more reactive than the 2-position and is of similar reactivity to the two groups in 4,4’-MDI. It is normally commercially available as a mixture with the 4,4’ isomer (2,4’/4’4, 55/45).typical hydrolysable chlorine levels are less than 50 ppm. It is an aromatic isocyanate thus not light-stable and causes yellowing appearance after exposure of sun light for a period of time.2,3 II. Toluene diisocyanate (TDI) The isocyanate groups on 2,4-TDI have different reactivities with the 4- position approximately four times the reactivity of the 2-position and about 50 percent more reactive than the 4-position group in MDI, whilst for the 2,6 isomer the groups have equal reactivity that is approximately the same as that of the 2-position in 2,4TDI. Due to its aromatic structure, therefore it is not light-stable and gives rise to yellowing appearance particularly after exposure to sun light for a period of time.1,3 III. 1,5-Naphthalenediisocyanate (NDI) NDI (1,5-naphthalenediisocyanate) is a very bulky and symmetrical molecule with two aromatic isocyanate groups of equal reactivity, and similar to that in the 4position in MDI. It is normally supplied in flake form and requires melting at 130oC or dissolving in the solvent for processing. It is not light-stable due to its aromatic feature thus gives rise to poor durability or weatherability (yellowing appearance) after exposure to sun light.1,3 22 Chapter 2 Theoretical Background IV. Hexamethylene diisocyanate (HDI), HDI is a flexible, linear, symmetrical molecule with two primary aliphatic isocyanate groups of equal reactivity. Their reactivity is at least two orders of magnitude lower than that in the 4-position of MDI. Of all the commercially available polyisocyanates, it has the highest isocyanate content. It is because it is totally aliphatic; it gives rise to light-stable (non-yellowing) polyurethanes.3,10-12 V. Isophorone diisocyanate (IPDI) IPDI is a bulky and a very asymmetric molecule. In fact, of all the commercially available polyisocyanates, it is the only one with no degree of symmetry. It is totally aliphatic, therefore giving rise to light-stable (non-yellowing) polyurethanes. It is commercially available as a mixture of two isomeric forms (25/75 cis/trans). Because of this, it has effectively four different isocyanate groups. Two are secondary aliphatic groups with similar reactivity, about half that in HDI. The other two are primary groups, but both are sterically hindered, rendering them even slower, by a factor of about five than MDI. Thus, IPDI has the slowest reactivity of all the commercially available polyisocyanates.1,3 VI. 4,4’-Diisocyanatodicyclohexylmethane (H12MDI) H12MDI is commercially available as a 90/10 blend of the 4,4’/2,4’isomers. The predominant 4,4’-diisocyanatodicyclohexylmethane consists of three conformational isomers, cis-cis, cis-trans and trans-trans. The two different isocyanate groups, either of which can be axial or equatorial, are secondary and are of similar reactivity to the secondary isocyanate groups in IPDI. The 10 percent of 2,4’diisocyanatodicyclohexylmethane, derived from the 2,4 isomer in the MDA, is made up of four conformational isomers, cis-cis, cis-trans, trans-cis and trans-trans. Because it is totally aliphatic it gives rise to light-stable polyurethanes.2,3 23 Chapter 2 Theoretical Background VII. Other Diisocyanates All other diisocyanates are only commercially available in limited or developmental quantities, so only have niche and specialized applications. Figure 2.10 shows the characteristics of reaction rate K1 and K2 for different types of isocyanates used in the aqueous polyurethane dispersion. Figure 2.10. The reaction rate for different types isocyanates 24 Chapter 2 Theoretical Background 2.7.1.3 Chemistry of Isocyanates In polyurethane chemistry the major focus is on the reactions of isocyanates with compounds that contain active hydrogen groups such as hydroxyl, water, amines, urea and urethane, but also other reactions of isocyanates needs to be considered. I. Isocyanate Reactions With Hydroxyl Groups The most important reaction in the manufacture of polyurethanes is between isocyanate and hydroxyl groups, Figure 2.2. The reaction product is a carbamate, which is called a urethane in the case of high molecular weight polymers. The reaction is exothermic and reversible going back to the isocyanate and alcohol.1,2 Aliphatic primary alcohols are the most reactive and react much faster than secondary and tertiary alcohols due to steric reasons, but urethanes made from tertiary alcohols do not regenerate free isocyanate instead dissociating to yield the corresponding amine, alkene and carbon dioxide. The urethane back reaction starts at 250oC for aliphatic isocyanates, but is closer to 200oC for aromatic isocyanates. The reaction between isocyanates and alcohols is accelerated by the addition of catalysts such as acids, bases (most aliphatic tertiary amines) and metal complexes (organo tin compounds). Catalysts also promote the dissociation of urethanes and so the deblocking of blocked isocyanates can occur at lower temperature. II. Isocyanate Reaction With Water The reaction of isocyanates with water to produce an amine and carbon dioxide is highly exothermic. The initial reaction product is a carbamaic acid, which breaks down into carbon dioxide and a primary amine (Figure 2.11). The amine will then react immediately with another isocyanate to form symmetric urea. Due to the formation of carbon dioxide the water reaction is often 25 Chapter 2 Theoretical Background used as a blowing agent as the level of blow can be tailored, simply by adjusting the amount of water in the formation. Figure 2.11. Water Reaction Diisocyanates having isocyanate groups of similar reactivity such as MDI, tend to chain extend to give crystalline polymeric urea. On the other hand, 2,4-TDI has an isocyanate group in the 2-position far less reactive than the one in the 4position. Consequently, urea will be formed rapidly between TDI molecules in the 4position leaving the 2-position unaffected, below 50oC. Despite the highly exothermic nature, the reaction with water is generally slow in the absence of catalyst, and one of the main reasons is that isocyanates such as MDI and TDI are not very soluble in water. III. Isocaynate Reaction With Amines Isocyanates react with primary and secondary amines to produce di- and tri- substituted urea respectively whilst tertiary amines form labile 1:1 adducts, but generally do not react with isocyanates (Figure 2.12). 26 Chapter 2 Theoretical Background Figure 2.12. Urea formation These conversions are exothermic and diamines are used as chain extending and curing agents in polyurethane manufacture. The resulting polyureá segments increase the potential for cross-linking. The reaction of unhindered isocyanates with primary amines at room temperature and in the absence of catalyst is 100 to 1000 times faster than the reaction with primary alcohols. The reactivity of an amine increases with its basicity and consequently, aliphatic amines are much more reactive than aromatic amines. The reactivity of amines can be slowed down by the presence of electron withdrawing groups. Another way is to increase the steric hindrance by branching on the carbon next to the nitrogen or introducing substituents in the ortho position of an aromatic amine.2,3 The kinetics of the reaction of amines with isocyanates is complicated by strong product catalysis. Since the product urea is a much weaker base and more hindered than the amine, its catalysis is bi-functional and based on hydrogen bonds between urea and both amine and isocyanate. 27 Chapter 2 Theoretical Background IV. Isocyanate Reaction With Urea Biurets are formed from the exothermic reaction of an isocyanate with a urea. With di-substituted urea, a biuret is formed through the active hydrogen (Figure 2.13). Figure 2.13. Biuret formation and equilibria This reaction is significantly faster than the allophonate reaction and occurs at lower temperature, about 100 oC compared to 120 oC to 140 oC. In polyurethane systems this reaction, that is reversible upon heating, is often a source for additional cross-linking. Another important feature of this urea-biuret equilibrium is the potential for redistribution of the biuret across the spectrum of isocyanate species. For instance, if polymeric MDI and a diisocyanate prepolymer are mixed together, the molecules of the di, tri, tetra and higher species are not initially smoothly distributed across the spectrum of derivatives – biuret, allophonate, uretonimine. However, they slowly redistribute through the various reversible reactions. This redistribution will be faster at higher temperatures, resulting ultimately in a product stable in composition and viscosity.3,10-11 V. Isocyanate Reaction With Urethanes An allophonate group is the result of an exothermic reaction of isocyanate with the active hydrogen on a urethane group (Figure 2.14). 28 Chapter 2 Theoretical Background Figure 2.14. Allophonate formation This reaction is slow compared to biuret formation and usually takes place uncatalysed at about 120 oC to 140 oC. The reaction is reversible at temperatures above 150 oC and so, as with biurets, the reaction increases cross-linking in polyurethane systems. This reverse reaction takes place at lower temperature than with biurets so that the interchange of isocyanate homologues is faster. If the allophanates are heated with a third equivalent of isocyanate, the cyclic triisocyanurate or trimers can be obtained.3,6 VI. Other Reactions Of Isocyanates There are many other reactions of isocyanates that can influence the polyurethane process and a few special cases are illustrated in Figure 2.15. 29 Chapter 2 Theoretical Background Figure 2.15. Other isocyanates reactions 2.7.2 Polyols Resins The term ‘polyol’ describes compounds with hydroxyl groups that react with isocyanates to produce polyurethane polymers. Typically ‘polyols’ contain two to eight reactive hydroxyl groups and have average molecular weights from 200 to 8000. The two key classes of product are polyethers and polyesters. 30 Chapter 2 Theoretical Background The initial polyols used were predominantly polyesters until, in the late 1950s, it was realized that polyether polyols were particularly well suited for the manufacture of flexible slabstock foam. They quickly became the dominant class of polyol, and now account for 80 percent of total consumption. Total polyol use had grown from 1.75 million tonnes in 1985 to 4.5 million tonnes in 2000. A major factor in the choice of polyol for a polyurethane application, apart from its technical effect, is cost. A selected polyol must be competitive with other polyols and also enables the final polyurethane product to be cost competitive with other materials in the end application. Figure 2.16. Polyol types used in polyurethane 31 Chapter 2 Theoretical Background 2.7.2.1 Polyether Polyols About 90% of the polyols used in polyurethane manufacture are hydroxylterminated polyethers. These are made by the addition of alkylene oxides, usually propylene oxide, onto alcohols or amines which are usually called starters or ‘initiators’. The addition polymerization of propylene oxide occurs with either anionic (basic) and cationic (acidic) catalysis. Polyether based on propylene oxide thus contains predominantly secondary hydroxyl end-groups. Secondary hydroxyl endgroups are several time less reactive with isocyanates than primary hydroxyl groups and for some applications polyether based only on propylene oxide may have inconveniently low reactivity. The primary hydroxyl content may be increased by a separate reaction of the polyoxypropylene polyols with ethylene oxide to form a block copolymer with an oxyethylene ‘tip’. 2.7.2.2 Polyester Polyols There are four main classes of polyester polyols: • Linear or lightly branched aliphatic polyester polyols (mainly adipates) with terminal hydroxyl groups. • Low molecular weight aromatic polyesters for rigid foam applications. • Polycaprolactones. • Polycarbonate polyols. (Aliphatic and aromatic polyester polyols will be discussed) The worldwide demand for polyester polyols in the polyurethane industry is estimated at around 850000 tonnes, growing at four to five percent a year and is broken down in Figure 2.17. Applications include the manufacture of flexible foam for textile lining, where superior resistance to dry cleaning solvents, flame bonding performance, elongation 32 Chapter 2 Theoretical Background and tensile properties make polyester polyols the product of choice. The outstanding abrasion resistance of polyester polyol-based polyurethanes has led to their extensive use in surface coating and footwear applications, and the superior thermal and oxidative stability of the aromatic polyesters is exploited in the manufacture of rigid isocyanurate foams.3,4 Global consumption (2000): 850000 tonnes Flexible slabstock foam (textile laminates for apparel and automotive applications), 12% Rigid foams (aromatic polyester polyols), 29% Synthetic leather, 9% Adhesives & sealants (flexible packaging, footwear, automotive), 6% Paints & coatings (specialist applications requiring high performance), 13% Elastomers (footwear, cast elastomers, TPU, fibres and shock absorbers), 31% Figure 2.17. Worldwide demand for polyester polyols by application I. Linear Or Lightly Branched Aliphatic Polyester Polyols Aliphatic polyester polyols are produced by direct esterification in a condensation reaction. This is a reversible equilibrium reaction, with water being removed during reaction to drive the process. As the reaction precedes transesterification reactions also occur on the forming polymer backbone, giving rise to a relatively wide molecular weight distribution in the final polyester polyol 33 Chapter 2 Theoretical Background (especially when compared to polyether polyols and polycaprolactones). Further, when polyesters are made from two or more glycols, they will be incorporated into the polymer chain in a statistical distribution irrespective of their sequence of addition. Careful control of the ratio of the ingredients is needed to ensure the product has the required hydroxyl, and not acid, end groups. II. Aromatic Polyester Polyols The use of polyesters in rigid foams was traditionally very limited, with polyether polyols being preferred. Following their introduction in the early 1980s, it was discovered that aromatic polyester polyols offered significant advantages in polyisocyanurate rigid foam (PIR) systems, where the highly cross-linked trimer structure can compensate for low functionality of the polyester polyol. Based on recycled or by-product streams, the aromatic polyester polyols are lower cost than polyether polyols and give superior performance in fire tests. They quickly became the polyols of choice in North America for the production of boardstock for building insulation; in combination with polyether polyols in spray systems; and in other applications, such as appliances and pour-in-place foam, as a diluent to cheapen the formulation. There are three types of aromatic polyester polyol used today: 1. Products derived from the process residues of dimethyl terephthalate (DMT) production, commonly referred to as DMT. They are typically transesterified at 180oC to 230oC with at least one mole of diethylene or dipropylene glycol per equivalent of acid to produce a simple hydroxyl-ended, glycol-capped aromatic polyester polyol. 2. Products derived from the glycolysis of recycled poly(ethylene terephthalate) (PET) bottles or magnetic tape with subsequent re-esterification with do-acids or reaction with alkylene oxides. 34 Chapter 2 Theoretical Background 3. Products derived by direct esterification of phthalic anhydride. The polyesters have functionalities between two and three, typically closer to two, and hydroxyl values in the range 200 to 330 mg KOH/g. Compatibilisers and surfactants are often added during manufacture to reduce viscosity and to improve miscibility with blowing agents, other polyols and isocyanates. 2.7.3 Other Additives In addition to the basic materials needed to make polyurethanes, isocyanates and polyols, a wide range of other chemicals can be added to modify and control both the polyurethane chemical reaction as well as the properties of the final polymer. 2.7.3.1 Catalysts Catalysis plays a vital role in the preparation of urethane and ureathane-urea polymers, because it not only affects the rates of the chemical reactions responsible for chain propagation, extension, and cross-linking but also affects the ultimate properties of the resulting polymers. Catalysts are employed whose functions are not only to bring about faster rate of reaction but also to establish a proper balance between the chain-propagation reaction (primarily the hydroxyl-isocyanate reaction) and the foaming reaction. Another important function of catalysts is to bring about completion of the reactions resulting in an adequate ‘cure’ of the polymers. The catalysts most commonly employed are tertiary amines and metal catalysts, especially tin catalysts. Tertiary amines are catalysts for the isocyanatehydroxyl and the isocyanate-water reactions. The efficiency of tertiary amine catalysts depend on upon their chemical structure. It generally increases as the basicity of the amine increases and the steric shielding of the amino nitrogen decreases. Some of the most commonly used tertiary amine catalysts are triethylenediamine, N-alkyl 35 Chapter 2 Theoretical Background morpholines, N,N,N’,N’-Tetramethylethylenediamine, N,N,N’,N’-Tetramethyl-1,3butanediamine, N,N’-substituted piperazines, and dialkylanolamines. Organometallic catalysts are mainly seen as gelation catalysts although they do affect the isocyanate-water blowing reaction. Organotins are the most widely used, but organomercury and organolead catalysts are also used. The mercury catalysts are very good for elastomers because they give a long working time with a rapid cure and very good selectivity towards the gelation. The lead catalysts are often used in rigid spray foams. However, both mercury and lead catalysts have unfavorable hazard properties so alternatives are always being sought.3,4 Table 2.3. Tertiary amine catalysts and their application Catalyst Formulae N,N-Dimethylethanolamine (DMEA) (CH3)2NCH2CH2OH N,N-Dimethylcyclohexylamine (DMCHA) C6H11N(CH3)2 Bis(N,N-Dimethylaminoethyl) ether (BDMAEE) (CH3)2NCH2CH2O(CH3)CH2CH2N(CH3)2 N,N,N’,N’,N”Pentamethyldiethylenetriamine (PMDETA) (CH3)2NCH2CH2N(CH3)CH2CH2N(CH3)2 1,4-Diazabicyclo[2,2,2]octane (DABCO) (Also referred to as triethylenediamine (TEDA) 2-(2-Dimethylaminoethoxy)ethanol (DMAEE) N(CH2CH2)3N 2-(2-Dimethylaminoethoxy)ethyl methyl-amino)ethanol (CH3)2NCH2CH2OCH2CH2N(CH3)CH2 CH2OH 1-(Bis(3-dimethylamino)propyl)amino-2-propanol (Also referred to as N”hydroxypropyl-N,N,N’,N’tetramethyliminobispropylamine (CH3)2N(CH2)3N(CH2CHOHCH3)(CH2)3N(CH3)2 (CH3)2NCH2CH2OCH2CH2OH Characteristic and use Inexpensive, used in flexible foams and in rigid foams. Acid scavenger for rigid-ester foams and fire retarded foams. Inexpensive, has a high odour, is used mainly in rigid foams. Excellent blowing catalyst used in flexible, high resilience and cold moulded foams. Good blowing catalyst used in isocyanurate board stock and moulded rigid foams. Very good amine gelation catalyst. Used in all types of foams. Reactive catalyst used in low density packaging foams. Excellent reactive low odour blowing catalyst used in high resilience and flexible foams. Low vinyl staining. Low odour reactive catalyst used in rigid and high resilience foams. Replaces DMCHA in spray and is low vinyl staining. 36 Chapter 2 Theoretical Background N,N’,N’- Tris(3dimethylaminopropyl)hexahydrotriazine (NRCH2)3 Where R= (CH2)3N(CH3)2 Dimorpholinodiethylether (DMDEE) (O((CH2)2)2N)(CH2)2O(CH2)2(N(CH2)2)2O) N,N-Dimethylbenzylamine C6H5CH2N(CH3)2 N,N,N’,N”,N”Pentamethyldipropylenetriamine (CH3)2N(CH2)3N(CH3)(CH2)3N(CH3)2 N,N’-Diethylpiperazine CH3CH2N(CH2CH2)2NCH3CH2 Isocyanurate catalyst that provides back end cure. Decreases demould time of appliance foams. Low odour catalyst used in one-component foams and sealants. Characteristic smell used in polyester-based flexible foams, integral skin foams and for making prepolymers. Strong ammoniacal odour used for polyether-based slabstock foams and in semi-rigid foam moulding. Low odour balanced blow cure catalyst for flexible and semi-flexible systems. Table 2.4. Organometallic catalysts and their application Catalyst Stannous octoate Dibutyltin dilaurate (DBTDL) Dibutyltin mercaptide Phenylmercuric propionate Lead octoate Potassium acetate/octoate (KA/KO) Quaternary ammonium formates (QAF) Ferric acetylacetonate Characteristic and use Slabstock polyether-based flexible foams, moulded flexible foams. Microcellular foams, elastomers, moulding system, RIM. Hydrolysis resistant catalyst for storage stable twocomponent systems. Delayed action catalyst for elastomers. Rigid spray foams. Isocyanurate foams. Isocyanurate foams. Cast elastomers system especially those based on TDI. 2.7.3.2 Neutralizing Agents The neutralizing component consists of one or more bases which serve for neutralizing some or all of the carboxyl and/or sulfo groups. For example, tertiary amines, such triethanolamine, as N,N-Dimethylethanolamine, N,N-Dimethylisopropanolamine, N-Methyldiethanolamine, N-Methyldiisopropanolamine, triisopropanolamine, N-Methylmorpholine, N-Ethylmorpholine, triethylamine or ammonia, or alkali metal hydroxides, such as lithium hydroxide, sodium hydroxide, potassium hydroxide or mixtures thereof, can be used as suitable bases. Tertiary amines and in particular triethylamine are preferably used. 37 Chapter 2 Theoretical Background The neutralizing component is added in an amount such that the degree of neutralization, based on the free carboxyl and /or sulfo groups of the polyurethane prepolymer, is preferably 70 to 100 equivalent %, particularly preferably 80 to 90 equivalent %. During the neutralization, carboxylate and/or sulfonate groups are formed from the carboxyl and/or sulfo groups and serve for anionic modification or stabilization of the polyurethane dispersion. 2.7.3.3 Dimethylolpropionic Acid Dimethyopropinonic acid is a main raw material for manufacturing watersoluble polyurethane; presently, DMPA has been widely applied to the production of emulsified coating agent for leather. Besides, it can be applied to the manufacturing of polyester dope, photosynthetic substance, liquid crystal of new type, adhesive and magnetic recording materials etc. Adding DMPA can improve the stability, hydrophilic property, homogeneous property, and endurance property. In a typical anionic polyurethane dispersion process, anionic groups (carboxylic and sulfonic) are introduced along the length of the polymer chain by using hydrophilic monomers or internal emulsifiers. DMPA improves the hydrophilic property by serving as the potential ionic center with NMP as the co-solvent. In polyurethane dispersions, particle size is governed by the hydrophilicity of the polymer which in turn depends on the number of ionic centers present in it. Study has shown that the particle size of dispersion decreases with increasing DMPA. Therefore, increased amount of DMPA leads to more ionic centers in the PUD backbone and thereby increasing hydrophilicity of the polymer and hence reductions in particle size.4,12-14 38 Chapter 2 Theoretical Background 2.7.3.4 Chain Extenders This is a low molecular weight polyfunctional compounds, reactive with isocyanates and are also known as curing agent. Chain extenders are difunctional glycols, diamines or hydroxyl amines and are use in adhesives, flexible foams, elastomers and RIM systems. The chain-extender reacts with an isocyanate to form a polyurethane or polyurea segment in the polyurethane polymer. Through reactons with excess isocyanate, allophonates and biuret can be formed, transforming the chain-extender effectively into thermo-reversible cross-linker. Simple diamines are, in general, too reactive for a high level of addition and special amines have been developed such as aromatic amines with bulky substituents ortho to the amino group. A widely use chain extender in RIM applications is DETDA (diethyl toluene diamine). The steric factors are responsible for lowering the reactivity of the amino groups as compared to TDA (toluene diamine). The reaction of one amino group with the isocyanate introduces a urea substituent on the aromatic ring, which lowers the reactivity of the second amino group. A more recent development is themoreversible chain extension by hydrogen bonding through polyols containing special end groups. Typical chain-extending agents are as follows: 1. water 2. diethylene glycol 3. hydroquinone dihydroxyethyl ether 4. ethanolamine 5. bisphenol A bis(hydroxyethylether) 6. DETDA (diethyl toluene diamine) 39 Chapter 2 Theoretical Background In the chain extension step, it is most important to control the extremely fast reaction between NCO groups and NH2 group accompanied by the viscosity rise. Molecular weight of PUD increases by the formation of urea linkage with NCOterminated prepolymer and diamines through the chain extension step. Therefore, the most important step to determine molecular weight of polyurethane dispersion is the chain extension step, which is the reaction between residual NCO groups and amine groups. Incidentally, chain extension is influenced by the amount of residual NCO groups, particle size diameter and so on. Generally molecular weight affects the mechanical properties of PUD.4,5 The efficiency of chain extension increased as total surface area of particles increases. Increasing the NCO/OH ratio enriched residual NCO groups that react with HDA. In chain extension urea linkage contribute to hard segments of polyurethane, therefore the mechanical properties increased with the increase of the hard segment and molecular weight. 2.8 Application Test 2.8.1 Strength And Adhesion Mechanical properties are often the most significant in determining whether a particular product can be used in a given application and there are a number of methods for assessing the strength of adhesion to a substrate, depending on whether the substrate is rigid or flexible. Common tests are illustrated in Figure 23-4. Lap shear tests are used both to assess the adhesion to a substrate and the cohesive strength of an adhesive, whilst peel and blister tests are used to measure the adhesion to the substrate. For simple cases, the peel force is a direct measure of the test energy. Test designed to apply a well-defined stress to the bond, and to minimize the amount of energy that is absorbed by deformation either of the substrate or the 40 Chapter 2 Theoretical Background coating/adhesive, so that the strength measured is the strength of the bond formed. The size and shapes of the bonds and joints are precisely controlled, so that stresses are applied accurately and the results can be compared. For single lap joints, care must be taken to ensure that the joint is correctly aligned and gripped with spacers in the testing machines, so that the stress applied when the bond is pulled is pure shear and does not twist the sample, Figure 2.15. Typical measurements of the strength of the bond are normalized stress at failure, modulus and elongation. More empirical methods are also used to test for adhesion, such as the crosshatch and mandrel bend test, which are, respectively, used for films on rigid and flexible substrates. In the cross-hatch test, pressure-sensitive tape is applied and removed over a series of cuts that have been made in film, simulating scratches or damage to the coating. The cut may be linear, or cross- or straight-hatched. The tape used is appropriate for the level of adhesion the coating will need in practice, and the level of damage from repeated applications of the tape is assessed. In the mandrel bend test, the coating on a flexible substrate is rolled around either a cylindrical or conical mandrel of smaller and smaller size to produce tighter bends and the cracking of the coating measured. As well as giving a measure of coating adhesion, this test simulates what might happen to a coating when in use on a substrate that is bent.4,6 Figure 2.18. Adhesion/cohesive strength testing formats 41 Chapter 2 Theoretical Background Figure 2.19. Single lap joint testing 2.9 Introduction Of Shoe Making Shoes both protect feet as well as, when incompatible in size and shape, present exciting factors in inflammatory conditions e.g. bunion. Despite the presence of pain, people are reluctant to change their footwear styles. The main function of modern footwear is to provide feet with protection from hard and rough surfaces, as well as climate and environmental exposure. To the wearer the appearance of their footgear is often more important than its function. Consumer resistance to change style is common. Informed decisions of shoe styles are thought to occur when the benefits of alternative shoe styles are carefully explained and footwear habits discussed in a culturally sensitive manner. Figure 2.22 shows the different parts of a shoe.6,15 42 Chapter 2 Theoretical Background Figure 2.20. Parts of a shoe 2.9.1 Methods Of Shoe Construction There are many ways to attach the sole to the upper but commercially only a few methods are preferred. Shoes were traditionally made by moulding leather to a wooden last. Modern technology has introduced many new materials and mechanised much of the manufacture. Remarkable as it may seem the manufacture of shoes remains fairly labour intensive. No matter the type of construction the first stage in construction is to attach the insole to the undersurface of the last. Two main operations follow: Lasting describes the upper sections are shaped to the last and insole. Followed by bottoming, where the sole is attached to the upper. The process of bottoming will determine price, quality and performance of the shoe. 43 Chapter 2 Theoretical Background 2.9.1.1 Method 1: Moccasin Construction3 Thought to be the oldest shoe construction, this consists of a single layer section, which forms the insole, vamp and quarters. The piece is moulded upwards from the under surface of the last. An apron is then stitched to the gathered edges of the vamp and the sole is stitched to the base of the shoe. This method is used for flexible fashion footwear. The imitation moccasin has a visual appearance of a moccasin but does not have the wrap around construction of the genuine moccasin. I. High oil content leathers This direct stitching method allows us to use leathers that have a much higher oil content than can be used normally. The oil keeps the leather nourished’ and supple, and is much softer and more comfortable to wear. II. Wax, rot proof thread Water won’t weaken the stitching. III. Upper fully Blake-Stitched on to Sole (or mid-sole depending on style) Much more secure bond to the sole Figure 2.21. Moccasin method 2.9.1.2 Method 2: Cement Construction3,4-11 Under this method the upper is stretched over the last and attached to the inner sole. The leather is then 'roughed up' to allow the adhesive to grip, and cement 44 Chapter 2 Theoretical Background bonded to the sole using the best quality polyurethane cement. Only leathers with a maximum of 12% oil/fat content can be used under this construction method.6 2.9.1.3 Method 3: Stitchdown Construction3,6-13 Here the upper is stretched over the last, folded or flanged out and glued to the midsole. They are then stitched with a "lockstitch" machine and cement bonded to the soles using a neoprene adhesive. A lockstitch has a top and bottom stitch which is interstitched. This stitching will not unravel even if a stitch is removed. Stitchdown construction shoes can use leathers of higher oil, fat content than cement construction and therefore have a more suppler feel. When 12 cord rot-proof stitching thread is used, the shoes will not rot like cheaper imitations. Meanwhile, Rivers shoes are stitched using the lock-stitch method for greater security.1,6 Figure 2.22. Stitchdown / Veldschoen method 2.9.1.4 Method 4 : Moulded Method3,4-14 The lasted upper is placed in a mould and the sole formed around it by injecting liquid synthetic soling material (PVC, urethane). Alternatively, the sole may 45 Chapter 2 Theoretical Background be vulcanized by converting uncured rubber into a stable compound by heat and pressure. When the materials in the moulds cool the sole-upper bonding is complete. These methods combine the upper permanently into the sole and such shoes cannot therefore be repaired easily. Moulded methods can be used to make most types of footwear.6-10 Figure 2.23. Moulded method for various types of footwear 2.9.1.5 Method 5 : Force Lasting Construction 3,6-16 Force lasting has evolved from sport shoes but is increasingly used in other footwear. The Strobel-stitched method (or sew in sock) describes one of many force lasting techniques. The upper is sewn directly to a sock by means of an overlooking machine (Strobel stitcher). The upper is then pulled (force lasted) onto a last or moulding foot. Unit soles with raised walls or moulded soles are attached to completely cover the seam. This technique is sometimes known as the Californian process or slip lasting.3,6 46 Chapter 2 Theoretical Background Figure 2.24. Slip lasting / strobel stitched method 2.9 References 1. George Woods, The ICI Polyurethanes Book, 2nd Edition, 1987, p.197. 2. J.W. Rothause, Advances in Urethane Sci. and Techology, 1987, 10, p.121. 3. Günter Oertel, Polyurethane Handbook, Hanser Publishers, 1985, p.31. 4. David Randell and Steve Lee, Polyurethane Book, 2nd Editors, John Wiley & Sons, 2000, p.10-20. 5. B.K. Kim, Colloid Polymer Science, 1996, 274, p.599. 6. Paul F. Bruins, Polyurethane Technology, 1969, p. 1990. 7. Y. Chen and Y.L. Chen, J. Appl. Polym. Sci., 1992, 46, 435. 8. B.K. Kim and L.Y. Min, J. Appl. Polym. Sci., 1994, 54, 1809. 9. S. Ramesh and G. Radhakrishna, Polym. Sci., 1994, 1, 418. 10. K. Matsuda, H. Ohmura and T. Sakai, J. Appl. Polym. Sci., 1979, 23, 141. 11. H.A. Al-Salah and C.K. Frisch, J. Appl. Polym. Sci., 1987, 25, 2127. 12. D. Dieterich, Progr. Organic Coatings, 1981, 9, 281. 13. P.B. Jacobs and P.C. Yu, J. Coat. Tech., 1993, 65, 222. 14. J.W. Rosthauser and K.J. Nachtkamp, J. Coat. Fabrics, 1986, 16, 39. 15. C.K. Kim and H.M. Jeong, Colloid Polym. Sci., 1994, 53, 371. 16. H. Xiao and K.C. Frisch, Pure Appl. Chem., 1995, 32, 169. 47 Chapter 3 Experimental Chapter 3 Experimental 3.1 Material The linear polyester diol derived from caprolactone monomer, terminated by primary hydroxyl groups (CAPA® 2205, white waxy solid, mean molecular weight 2000, hyroxyl value 56mg KOH/g, Solvay Caprolactones) was required to melt at 50°C before use. Other ingredients used for polymerisation including isophorone diisocyanate (IPDI, Rhodia, France), 2,2-bis(hydroxymethyl) propionic acid (DMPA, Aldrich), 1,6-hexanediamine (HDA, Aldrich), ethylenediamine (EDA, Aldrich), 2methylpentamethylenediamine (Dytec® A Amine), triethylamine (TEA, Merck), 1methyl-2-pyrrolidone (NMP, Merck) were used as received. Deionized (DI) water was used throughout the experiment. Reagents including hydrochloric Acid (1 mol/L), Di-n-butylamine (solution in pure toluene about 1.25 mol/L) and Bromophenol Blue (1N), pH 3.0 – 4.6 (BPB) were used for back titration to determine the residual NCO content. The catalyst di-nbutyltin-di-laurate (DBTL, Air Product) was used in the experiment. 3.2 Preparation of Aqueous Polyurethane Dispersion An aqueous polyurethane dispersion (PUD) was prepared by forming a NCO prepolymer initially. Subsequently chain extension was performed in the aqueous phase in the presence of a polyamine chain extenders. The prepolymer was formed by reacting an active hydrogen containing compound such as a linear polyester diol (CAPA® 2205) with isophorone diisocyanate (IPDI) and 2,2-bis(hydroxymethyl) propionic acid (DMPA). 48 Chapter 3 Experimental Three sequences of PUD were synthesized : in sequence 1, the DMPA content was held constant, while the ratio of NCO:OH was varied from 1.2 to 3.4. In sequence 2, the ratio of NCO:OH was fixed constant and the DMPA content was varied from 4 to 7. This resulted as an increase of ionic group thus lead to the increment of hard segment content. For sequence 3, the degree of neutralization was varied from 85 to 115 while the ratio of NCO:OH and the DMPA content were kept constant. The details of formulations and evaluation are described in Chapter 4. The parameters studied in this experiment also involved of the reaction of the chain extension as the variation of residual NCO group, molecular weight and particle size of the polyurethane during the chain extension step. Change of molecular weight and timedependent variation of residual NCO group were investigated by using GPC and FTIR with different degree of chain extension and particle size of the aqueous polyurethane dispersion (details of studies are described in Chapter 5). Polyurethane prepolymer was synthesized in a 1-L four-neck round-bottom glass reactor equipped with a mechanical stirrer, an electronic temperature controller, a temperature probe, a reflux condenser and a nitrogen inlet (Figure 3.1). Reflux condenser Mechanical stirrer Temperature probe Electronic temperature controller Nitrogen gas Four neck roundbottom glass reactor Figure 3.1. Set up of apparatus for the synthesis of polyurethane prepolymer 49 Chapter 3 Experimental Reaction was carried out in nitrogen atmosphere. Polyester diol (CAPA® 2205), IPDI and DMPA (pre-dissolved in NMP solvent) were charged into the reactor and the mixture was stirred and heated to 80°C. The reaction proceed at 80°C until the amount of residual NCO content reached at 20% above the theoretical value. The amount of residual NCO (%) was checked at every hour interval using di-nbutylamine back titration method.1,2 After the residual NCO (%) reached to end-point, the temperature was lowered to approximately 60oC, and TEA was added whilst stirring. The prepolymer was stirred continuously for another 10 minutes. The required amount of HDA to be added was determined based on the formulae given below : [% NCO/ MWt NCO X (MWt HDA/2] = required amount HDA(g)/100g prepolymer where MWt is the molecular weight of NCO = 42 and HDA = 116.2 The required amount of DI water was poured into a metal container. The required amount of the prepolymer was introduced slowly into the water with high speed strring using Dispermat machine. The HDA and DI water were premixed and then added slowly into the aqueous dispersion under stirring for about 20 minutes. The finishing polyurethane dispersion (PUD) was then formed (Figure 3.2). Figure 3.2. Preparation of aqueous polyurethane dispersion 50 Chapter 3 Experimental 3.3 Preparation of Two Component (2K) Water Borne Polyurethane Footwear Adhesives The water borne 2K footwear adhesives were formulated using the aqueous polyurethane dispersion (PUD) which is successfully developed in this experiment, combined with the water borne polyisocyanate crosslinker, Rhodocoat WAT-1 from Rhodia, France. The key properties of the adhesive bonding strength (i.e. shear and peel strength) had been evaluated using the Zwick universal shear/peel strength test equipment.3-5 The performances of this newly developed PUD were also compared against one commercially available PUD in the market currently. Details of the studies are described in Chapter 6. 3.4 Gel Permeation Chromatography (GPC) Measurement The molecular weight of the PUD was determined using a Water gel permeation chromatograph. Polystyrene standards of known molecular weights were used for the calibration curve for this instrument. Tetrahydrofuran (THF) was used as an eluent. The elution was monitored using a Waters 410 differential refraction detector which is connected to a microprocessor. 3.5 Isocyanate Functionality Determination NCO functionality is determined by titration method. 20 ml of di-n- butylamine solution (1.25 mol/L in pure toluene) and 2.5 g of prepolymer were added into a 250-ml conical flask. The flask was stopped and agitated until complete homogenization was obtained. The test solution was kept at ambient temperature for at least 15 minutes prior titration. After that, 150 ml of acetone was added to the test solution. Hydrochloric acid solution (1 mol/L) was used as a titratant. A few drops of 51 Chapter 3 Experimental bromophenol blue solution (1 g/L in the acetone) was used as an indicator. Titration was continued until the violet or yellow colouration was stable for 15 second. In a ponderal percentage of the isocyanate functions (N=C=O) by means of the following formula: N=C=O % = [42.02 (Vo-V1) / 1000m] x 100 In isocyanate equivalent corresponding with the number of functions for 100g by means of the following formula: [(Vo-V1) x 100] / 1000m Where Vo : volume (ml) of hydrochloric acid solution used for the blank test. V1 : volume (ml) of hydrochloric acid solution used for the sample and m : mass of the test sample, expressed in g. The NCO/OH ratio is determined by the following formula : (Weight of isocyanate x NCO % x 17) / (42 x weight of polyol x OH % x solid of polyol). 3.6 Particle Size Analysis The particle size of the aqueous polyurethane dispersion (PUD) was determined by the Mastersizer, Malvern MAF 5001 Mastersizer Micro Plus, based on the principle of laser ensemble light scattering. It falls into the category of non imaging optical systems due to the fact that sizing is accomplished without forming an image of the particle onto a detector. Laser light scattering is an exceptionally flexible sizing technique able, in principle to measure the size structure of the test material. The resolution goes up to 100 size bands displayed covering a range up to 18000:1 in size capability on any single range. The test PUD samples were introduced into the sample dispersion unit which contained of DI water. The test samples were 52 Chapter 3 Experimental dispersed under ultrasonic condition. The particle size of the tested PUD was then analysed based on laser light scattering. 3.7 FT-IR Analysis FTIR spectra were recorded on a Shimadzu FTIR-8400S spectrophotometer with resolution of 2 cm-1. FTIR-8400S uses a high sensitivity pyroelectric detector with a DLATGS (L-alanine-dooped deuterated triglycine sulfate) element. The detector relies upon the temperature dependent “pyroelectric effect” created on the crystal surface by spontaneous ferroelectric polarization. As the DLATGS Curie temperature is as low as 61°C, temperature control is required. The prepolymers samples for FTIR analysis were prepared by casting onto KBr disk. 3.8 Shear and Peel Strength Measurement 3.8.1 Shear Strength Measurement The test pieces were cut by a sharp cutting knife to 80± 2 mm long and (20± 0.2) mm wide with an overlap of (10± 0.2) mm, as shown in Figure 3.3. Figure 3.3. Form and dimensions of test pieces for shear tests The test pieces were sanded by sandpaper. After sanding, the surface of the test pieces were cleaned with a cloth, cotton wool and a suitable solvent like acetone 53 Chapter 3 Experimental or ethyl acetate, 1,1,1-trichlorethane or light petroleum with boiling range 80 oC to 110 oC. The test pieces were dried for about (30± 5) minutes at (23 ± 5 oC) at a relative humidity of less than 70% in order to allow the solvent to evaporate off. The two-component water borne polyurethance adhesive was then prepared and applied onto the test pieces by brush. The adhesive was then dried for 10 to 15 minutes at a controlled temperature of (23 ± 5 oC) before in contact bonding between the application of the adhesive and the assembling of the bond. The test pieces were pressed evenly by a weight load roller for about 15 seconds. The test pieces were further dried at the ambient temperature (23 ± 5oC, relative humidity 55%) for 4 days. For shear strength test, the test piece was clamped in the jaws of the Zwick universal shear/peel strength test equipment to obtain a free test length of (110 ± 2 ) mm. The test piece was then loaded at a constant rate of traverse of (25 ± 2 ) mm/min until breakage. The maximum force in Newton (N) during this process was recorded. The shear strength was calculated based on the following formula. Shear strength = Maximum value of the force (N) during separation Area of overlap in mm squared (mm2) 3.8.2 Peel Strength Measurement The test pieces were cut by a sharp cutting knife to (100± 2) mm long and (25.4± 0.5) mm wide with an overlap of (60± 2) mm (Figure 3.4). Figure 3.4. Form and dimensions of test pieces for peel strength test 54 Chapter 3 Experimental The test pieces were sanded by sandpaper. After sanding, the surfaces of the test pieces were cleaned with a cloth, cotton wool and a suitable solvent (e.g. acetone) or cleaning agent. The test pieces were dried for about (10 ± 5) minutes at (60 ± 5 oC) in the oven. The adhesive was then applied onto the test pieces (first coat) by brush to obtain a uniform coating of the adhesive under test. This form and dimensions of the test piece for peel test is illustrated in Figure 3.4. The adhesive was dried for 4 minutes at 55 oC. The adhesive was then applied onto the test pieces again (second coat) and dried for 5 minutes at 55 oC. After that, the test pieces were cooled for 4-6 minutes before the two test pieces were pressed together with pressure. The test pieces were left in contact for 5 minutes prior subjected to peel strength test. The experiment was repeated with different contact time of 15, 30 and 60 minutes respectively (Figure 3.5). Surface Treatment First Coat OVEN Second Coat OVEN 55oC x 10 mins OVEN 55oC x 4 mins Apply Adhesive Pressure Bond 55oC x 5 mins Apply Adhesive Cool for 4 mins Figure 3.5. Process for applying the adhesive The peel strength was determined by using a Zwick universal shear/peel strength test equipment. The test speed of pulling is (500 ±10) m/min. Peel strength is measured in term of N/mm. 55 Chapter 3 Experimental The initial bonding strength was determined by taking the average peel strength after the test pieces was in contact for 5, 15, 30 and 60 minutes respectively. The 24 hours bonding strength was determined by taking the peel strength after the test pieces was in contact for 24 hours. Peel strength is the mean peel force per unit width, in N/mm, calculated from the trace over the course of separation as follows: Peel Strength = Mean peel force in newton (N) during separation / Width of test piece in mm. 3.9 References : 1. C. Hepburn, Polyurethane Elastomers, 2nd Edition, Elsevier, New York, 1992, p.281. 2. George Woods, The ICI Polyurethanes Book, 2nd Edition, ICI Polyurethanes, 1987, p.197. 3. Paul F. Bruins, Polyurethane Technology, 1969, p1990. 4. David Randell and Steve Lee, Polyurethane Book, 2nd Editors, John Wiley & Sons, 2000, p.10-20. 5. J. W. Rothause, Advances in Urethane Sci. and Technology, 1987, 10, p.121. 56 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 4.1 Introduction An aqueous polyurethane dispersion (PUD) is a binary colloidal system where polyurethane particles are dispersed in a continuous aqueous medium. Conventional polyurethane is insoluble in water and phase separates in large domains. To be dispersible in water, polyurethane should contain ionic and/or non-ionic hydrophilic segments in its structure. Particle size is governed by internal and external factors. Among them, the most important factor is the hydrophilicity of polyurethane. In the application of adhesives market, aqueous polyurethane has been developed and studied1,2 in view of its unique properties and the environmental regulations prohibiting VOC.3,4 The earliest process to prepare the polyurethane dispersion was the acetone process, which has remained technically important so far.5,6 Within the last three decades several new processes have been developed. However, these processes have a common feature that is the preparation of NCOterminated polyurethane prepolymer with appropriate molecular weight.7,14 Distinctly different step among several processes lies in the chain extension step that is generally performed using diamines (-NH2) and/or diols (-OH).8-10 In the chain extension step, it is most important to control the extremely fast reaction between NCO groups and NH2 groups accompanied by the viscosity rise.9,14 The prepolymer mixing process that we have used in this study has the advantage of avoiding the use of a large amount of organic solvent. In this process, NCO-terminated polyurethane prepolymer containing pendant acid group, such as dimethylol propionic acid (DMPA) is neutralized with 57 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion base to form internal ionic emulsifier and dispersed in the aqueous phase to form an aqueous dispersion. Afterwards, the chain extension step is accomplished by the addition of diamine to the aqueous dispersion. Molecular weight of polyurethane dispersion increases by the formation of urea linkage with NCO-terminate prepolymer and diamines through the chain extension step. Therefore, the most important step to determine the molecular weight of polyurethane dispersion is the chain extension step, which is the reaction between residual NCO groups and amine groups. Incidentally, the chain extension is influenced by the amount of residual NCO groups, particle diameter, and so on.10,15-17 The amount of residual NCO groups is determined by the molar ratio of NCO to OH (NCO/OH). In addition, both hydrophilic acid group contents and their degree of neutralization can affect particle diameter.10,11 Accordingly, the molecular weight can be controlled with varying these process variables. Generally, the molecular weights of polymeric materials have a remarkable effect on their mechanical properties. Therefore, the control of molecular weight can be expected to obtain the optimum mechanical properties of polyurethane dispersion. In this experiment, several aqueous polyurethane dispersions were prepared by varying the NCO/OH ratio, the DMPA content and the degree of neutralization. Then their molecular weights and mechanical properties such as adhesive bonding strength (shear and peel strength) were evaluated to study the effect of process variables and the relationship between molecular weight and mechanical properties. 4.2 Experiment The characteristics of all the raw materials used in this experiment such as the linear polyester diol derived from caprolactone monomer terminated by primary hydroxyl groups (CAPA® 2205, Solvay Caprolactones), IPDI isophorone diisocyanate 58 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion (2,2-bis(hydroxymethyl), Rhodia France) for polymerisation, dimethylol propionic acid (DMPA, Aldrich), 1,6-hexanediamine (HDA, Aldrich), triethylamine (TEA, Merck), 1-methyl-2-pyrrolidone (NMP, Merck) are shown in Chapter 3, section 3.1. Three sequences of PUD were prepared. In sequence 1, the DMPA content was fixed, while the NCO/OH ratio was varied from 2.8 to 3.4. In sequence 2, the NCO/OH ratio was held constant, and the DMPA content was varied. In sequence 3, the NCO/OH ratio, the DMPA content and the total solid were held constant, and the degree of neutralization was controlled by varing the TEA from 85% to 115% based on DMPA content. These formulations are shown in Tables 4.1, 4.2 and 4.3. Polyurethane prepolymer was polymerised in a 1-L round-bottom glass reactor equipped with a mechnical stirrer, a thermometer, a reflux condenser, a temperature controller and a nitrogen inlet. Reaction was conducted under the nitrogen atmosphere. The linear polyester diol (CAPA® 2205) and DMPA were pre-dissolved in NMP in the reactor flask. The mixture was heated and stirred at 80°C; IPDI were then added to the mixture. The amount of residual NCO(%) was checked at one-hour interval using di-n-butylamine back titration method. The reaction was allowed to proceed until the residual NCO(%) became 20-30% above the theoretical residual NCO(%). After the required residual NCO(%) was reached, the temperature was lowered to approximately 60°C and TEA was then added whilst stirring to neutralize the carboxylic acid in the DMPA. The reaction mixture was stirred continuously for another 10 minutes. The aqueous PUD was then formed by phase inversion process. The required reaction mixture or prepolymer was poured slowly into a metal container which contained the required amount of DI water. The dispersion was then obtained under high speed stirring using a Dispermat stirrer. For the chain extension, the required amount HDA (calculated by the formulae [%NCO/MW NCO X MW 59 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion HDA/2] = Required amount of HDA (g)/100g prepolymer) was premixed with DI water and then added slowly into the prepared dispersion phase. The preparation of aqueous polyurethane dispersion is illustrated in Figure 4.1. + NCO HO--------------------OH OCN POLYOL IPDI O H O O OCN O----O N N NCO N N O ------- O H H O H CH3 OH HO DMPA COOH O O H O O OCN N O ---O H O N N C OOH O H O--O N O N NC O H N H O H NCO - terminated prepolymer N(Et)3 Triethyl amine O O OCN N H H O O O ---O O N H N N H H O O -- O N O N NCO H O C O O -N H + ( E t ) 3 Water + H N H N H 1,6-Hexanediamine H Dispersion and Chain Extension Step Figure 4.1. Preparation of an aqueous polyurethane dispersion 60 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 4.1. Formulation of aqueous polyurethane dispersion (Sequence 1) – DMPA content is constant and NCO/OH ratio varies from 2.8 to 3.4. Raw Materials Weight (grams) Polyester diol (CAPA 2205) 157.58 Variablea IPDI DMPA 7.88 NMP 15.76 DBTL Catalyst (0.08% of prepolymer) 0.19 TEA 5.94 HDA (Theoretical) per 100g prepolymer Variablee DI Water Variableb Variablea = NCO/OH ratio is from 2.8, 3.0, 3.2 or 3.4 Variableb = Total solid content maintained at 50% Variablee = Based on the residual NCO content DMPA content fixed at 5% wt of polyester diol Neutralization fixed at 100% Table 4.2. Formulation of aqueous polyurethane dispersion (Sequence 2) – NCO/OH ratio is constant and DMPA varies from 4 to 7 Raw Materials Weight (grams) Polyester diol (CAPA 2205) 157.58 IPDI 52.95 DMPA Variablec NMP 15.76 DBTL Catalyst (0.08% of prepolymer) 0.19 TEA 5.94 HDA (Theoretical) per 100g prepolymer Variablee DI Water Variableb Variablec = DMPA content is from 4, 5, 6 or 7 Variableb = Total solid content maintained at 50% Variablee = Based on the residual NCO content NCO/OH ratio fixed at 3.0 Neutralization fixed at 100% 61 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 4.3. Formulation of aqueous polyurethane dispersion (Sequence 3) – NCO/OH ratio and DMPA content are constant but the degree of neutralization varies from 85% to 115%. Raw Materials Weight (grams) Polyester diol (CAPA 2205) 157.58 IPDI 52.95 DMPA 7.88 NMP 15.76 DBTL Catalyst (0.08% of prepolymer) 0.19 TEA Variabled HDA (Theoretical) per 100g prepolymer Variablee DI Water Variableb Variabled = Degree of neutralization is from 85%, 95%, 100%, 105% or 115%. Variableb = Total solid content maintained at 50% Variablee = Based on the residual NCO content DMPA content fixed at 5% wt of polyester diol 4.3 Results and Discussion 4.3.1 The Effect of NCO/OH Ratio The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the PUD with different NCO/OH ratio are given in Figure 4.2. The Mw value of PUD varied from 256,350 to 475,400 with increasing the NCO/OH from 2.8 to 3.4. However, the Mn value increased gradually (with small degree of changes). The polydisperisty index could be influenced by both Mw and Mn, as the NCO/OH ratio increases, the polydisperisty index (Mw/Mn) becomes larger. The results showed that the higher the NCO/OH ratio, the higher the molecular weight of PUD. In this case, increasing the NCO/OH enriched residual NCO groups that react with HDA. In addition, the chain extension produced urea linkages that contribute to hard segments 62 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion of polyurethane. Therefore, the adhesive bonding strength increased with increasing Molecularg/mol Weight (g/mol) molecular weight of the hard segment.10-14 500000 450000 400000 350000 300000 250000 200000 150000 100000 50000 0 Mw Mn 2.8 3 3.2 3.4 NCO/OH ratio Figure 4.2. Mw and Mn of PUD as a function of the NCO/OH ratio The change of Mw during the chain extension step is shown in Figure 4.3. The final Mw was determined by the amount of the residual NCO groups that could react with the HDA chain extender. These residual NCO groups increased with higher NCO/OH ratio. However, insignificant change of the molecular weight was observed at lower NCO/OH ratio i.e. at 2.8. This could be due to the side reaction where the residual NCO group reacted with the water molecules instead of with the hydroxyl functionality from the polyol. At higher NCO/OH ratio such as 3.0 and 3.2, the molecular weight reached the optimum when about 40-50% of chain extender has been added. This implies that the efficiency of chain extension was about 40-50% in these formulations. This result indicates that the residual NCO groups on the polyurethane particles did not completely react with the chain extender. The prepolymer chains were extended to the particle surface due to the high viscosity of 63 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion the particle at low temperature during the chain extension step. So the chain extender required a longer time to diffuse to the particles. Hence, the efficiency of chain extension increased as the total surface area of the particle increased.15-18 400000 350000 Mw (g/mol) 300000 250000 200000 150000 NCO/OH 2.8 NCO/OH 3.0 NCO/OH 3.2 100000 50000 0 0 20 40 60 80 100 Degree of Chain Extension (%) Figure 4.3. Change of Mw with different NCO/OH ratio during the chain extension reaction Figure 4.4 shows the effect of NCO/OH ratio on the adhesive bonding strength. The shear and peel strength increased as the NCO/OH ratio increased. 64 40 8 35 7 30 6 25 5 20 4 15 3 Shear Strength 10 2 Peel Strength, N/mm Shear Strength, N/mm2 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Peel Strength 5 1 0 0 2.8 3 3.2 3.4 NCO/OH ratio Figure 4.4. Effect of NCO/OH ratio on the adhesive bonding strength 4.3.2 The Effect of DMPA Content The variations of Mw with DMPA content are shown in Figure 4.5. The Mw decreased from 276,000 to 80,000 g/mol with increasing DMPA content. The concentration of DMPA increased from 5 to 8 weight % based on total polyester polyol used. The molecular weight of the linear polyester polyol (terminated by primary hydroxyl group) CAPA 2205 is 2000 but that of DMPA is 134.15. If there is no significant difference in the reactivity between the polyester polyol and DMPA, the prepolymer chain should be shorter as the DMPA content increases at a constant NCO/OH ratio. The average particle size as a function of DMPA content is also shown in the Figure 4.5. In PUD, the average particle size could be controlled to some extent by emulsification conditions such as stirring speed or dispersing temperature which have an effect on the viscosity of prepolymer but it is mostly governed by the 65 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion concentration of hydrophilic groups, i.e. carboxylic acids.11,21-22 The average particle size decreased as the DMPA content increased. The decreases of the particle size with increasing of DMPA content could be due to the stabilizing mechanism of the ionomer. Dispersion polyurethane ionomer is stabilized as electrical double layers formed by the ionic groups.3,19-21 300000 3 Mw Particle Size 2.5 200000 2 150000 1.5 100000 1 50000 Particle Size (µm) Mw (g/mol) 250000 0.5 0 0 5 6 7 8 DMPA Content Figure 4.5. Mw and particle size of PUD as function DMPA content Figure 4.6 shows the impact of the molecular weight to adhesive bonding strength. The hard segments increases as DMPA content increases at a fixed NCO/OH ratio because DMPA molecules formed hard segments on polyurethane main chain. Consequently, it was anticipated that the increase of hard segment content contributed to the improvement of adhesive bonding strength such as shear and peel strength. Contrary, the bonding strength at 5% DMPA content showed the best bonding strength as compared to the other DMPA contents. This is due to the increase in 66 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion molecular weight as DMPA content decreases at a constant NCO/OH ratio. This result indicates that the bonding strength would be more influenced by the Mw than the ratio of soft segment to hard segment.4,17-22 6 20 18 Shear Strength 16 5 Peel Strength 14 4 3 10 N/mm N/mm 2 12 8 2 6 4 1 2 0 0 80020 100200 177340 276330 Mw (g/mol) Figure 4.6. Effect of Mw on bonding strength of the aqueous polyurethane dispersion with different DMPA content 4.3.3 The Effect of TEA/DMPA Molar Ratio The effect of neutralization degree of carboxylic acid in DMPA on the average particle sizes and molecular weight of the aqueous PUD is shown in Figure 4.7. The average particle size decreased as the mole ratio of TEA to DMPA (TEA/DMPA) increased from 0.85 to 1.00. The decrease of the particle size was due to the variation of the number of carboxylic acid group that could be neutralized by TEA. In the aqueous PUD, usually the greater the hydrophilicity the smaller the particle size because the degree of dissociation depends on the degree of neutralization. When the 67 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion degree of neutralization was over 100%, the particle size increased as the degree of neturalization increased. Excess amount of the TEA increased the ionic strength of a continuous phase. The electrostatic replusion can be decreased due to the contraction of electrical double layers among the polyurethane particles as the ionic strength increased.6,18-22 This is demonstrated in Figure 4.8. Particle Size (µm) 6 5 4 3 2 1 0 85% 95% 100% 105% 115% Degree of Neutralization Figure 4.7. Effect of neutralization degree on the particle size of aqueous polyurethane dispersion (PUD). Figure 4.8. Particle size controlled by TEA/DMPA ratio 68 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Figure 4.9 indicates the change of Mw of PUD as a function of degree of chain extension, with varying degree of neutralization i.e. TEA/DMPA molar percentage varied from 85% to 115% when the NCO/OH ratio fixed at 3.0. Highest Mw was obtained when the neutralization reached 100%. This result demonstrated that the efficiency of chain extension was closely related to the average particle size of PUD and the diffusion-dominant reaction. 350000 300000 Mw (g/mol) 250000 200000 150000 85% 100000 100% 115% 50000 0 0 20 40 80 100 Degree of Neutralization (% ) Figure 4.9. Evoluation of Mw of PUD with varying the degree of neutralization from 85% to 115% during chain extension reaction As shown in Figure 4.10, the Mw of PUD reached a maximum value when the degree of the neutralization was 100%. 69 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 300000 Mw Mn g/mol 250000 200000 150000 100000 50000 0 85% 95% 100% 105% 115% Degree of Neutralization Figure 4.10. Mw and Mn of PUD as a function of degree of neutralization The effect of degree of neutralization on the adhesive bonding strength is illustrated in Figure 4.11. The excess TEA remained in the polyurethane prepolymer could react with the water which exist in the polyester polyol (even very small quantity < 0.05% water) to form urera linkages in the prepolymer main chain. In addition, TEA could cause NCO-terminated prepolymer to form polyfunctional branched prepolymer chains. These branched chains may form cross-linking during the chain extension by the reaction with HDA. As shown in Figure 4.9, the adhesive bonding strength such as shear and peel strength increased as the degree of neutralization increased. However, a maximum value was obtained when the neutralization reached 100%. Above 100% neutralization, the adhesive bonding strength decreased.17-22 70 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 30 7 Shear Strength 5 2 Shear Strength, N/mm 6 20 4 15 3 10 2 5 Peel Strength, N/mm Peel Strength 25 1 0 0 85 95 100 105 Degree of Neutralization (% ) 115 Figure 4.11. Effect of neutralization on adhesive bonding strength 4.4 Conclusions The effect of NCO/OH ratio, DMPA content and degree of neutralization on molecular weight and adhesive bonding strength for the prepared aqueous polyurethane dispersion (PUD) have been studied. As the NCO/OH ratio increased, the Mw increased. It was due to the chain extension reaction which depended on the amount of residual NCO groups. The adhesive bonding strength as shown by shear and peel strength increased significantly, mainly due to the increases of molecule weight and hard segment content. The molecular weight and adhesive bonding strength properties are not seriously affected by the DMPA content, especially when the NCO/OH ratio is 2.8. When the DMPA content decreased, polyol content increased, and consequently, the 71 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion molecular weight of polyurethane prepolymer increased slightly. Hence, the adhesive bonding strength, which was mainly affected by the molecular weight, was improved slightly. The Mw increased as the neutralization degree reached 100% due to the increase of chain extension efficiency. In addition, the mechanical properties such as shear and peel strength reached a maximum value where the degree of neutralization approached 100%. 4.5 References : 1. D. Dieterich, Prog. Org. Coat, 1981, 9, 281. 2. P.B. Jacobs, P.C. Yu, J. Coat. Tech. 1993, 65, 222. 3. S.H. Son, H.J. Lee, J.H. Kim, Colloids Surfaces A: Physicochem. Eng. Aspects, 1998, 133, 295. 4. D.S . Chen, M. Hsien, US Patent 5, 1994, 306,764. 5. C. Hepburn, Polyurethane Elastomers, second ed., Elsevier, New York, 1992, p.281. 6. G. Oertel, Polyurethane Handbook, Carl Hanser, Munich, 1985, p31. 7. P. Thomas, Water Based and Solvent Based Surface Coating Resins and their Applications – Polyurethanes, vol. III, SITA Technology, London, 1999, p59. 8. H. Xiao, H.X. Xiao, K.C. Frisch, N. Malwitz, J. Appl. Polym. Sci., 1994, 54, 1643. 9. B.K. Kim, Colloid, Polymer Sci., 1996, 274, 599. 10. Y.K. Jhon, I.W. Cheong, J.H. Kim, Colloids Surfaces A Physicochem. Eng. Aspects, 2001, 179 (1), 71-78. 11. S.Y. Lee, J.S. Lee, B.K. Kim, Polym. Int., 1997, 42, 67. 72 Chapter 4 Effect of Process Variables on Molecular Weight and Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 12. B.K. Kim, T.K. Kim, H.M. Jeong, J. Appl. Polym. Sci., 1994, 53, 371. 13. H.T. Lee, Y.T. Hwang, N.S. Chang, C.C.T. Huang, H.C. Li, Waterborne High Solids and Powder Coatings Symposium, New Orleans, 22-24 February, 1995, 224. 14. H.R. Allcock and F.W. Lampe, Contemporary Polymer Chemistry, New York, 1990, p.43. 15. S. Kaizerman and R.R. Aloia, US Patent 4, 1985, 198, 330. 16. R. S. Buckanin, US Patent 4, 1985, 705, 840. 17. P.H. Markusch, J.W. Posthauser and M.C. Beatty, US Patent 4, 1985, 501,852. 18. J. Weikard and E. Luhmann, US Patent 6, 2003, 541, 536. 19. C. Irle and W. Kremer, US Patent 6, 2003, 559, 225. 20. T.D. Salatin and A.M. Budde, US Patent 5, 1993, 236, 995. 21. G.A. Anderle and S.L. Lenhard, US Patent 6, 2003, 576, 702. 22. H.P. Muller and H. Gruttmann, US Patent 6, 2001, 172, 126. 73 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 5.1 Introduction During the past several decades, aqueous polyurethane dispersion has been investigated by many researchers. However, very little systematic work has been conducted and reported in details on the chain extension process. The chain extension step is important and critical to the molecular weight and particle size of the aqueous polyurethane dispersion. As a result, the chain extension step has important impact of the physical and mechanical i.e. bonding strength properties of polyurethane. In the chain extension step, it is most important to control the fast reaction between the residual NCO group and amine group (NH2) as it will easily increase the viscosity. Generally, it has been reported that residual NCO groups are measured by din-butylamine back titration method (DBBT method).1,2 This method, however, can be used to determine the NCO content in a diisocyanate intermediate or the free reactive isocyanate available in the prepolymer. In other words, this method is not applicable after neutralizating agent is introduced because it is impossible to determine the residual NCO groups due to the presence of various side reaction and other base materials such as chain extender, neutralizing agent etc. Some earlier research work has been tried to avoid the reaction between residual NCO with the water in the preparation of aqueous polyurethane dispersion by using blocking agent and controlling the process temperature. However, both approaches could not stop the reaction completely. In view of this, one must determine the concentration of NCO group and use appropriate amount of chain extender for stoichiometric reaction 74 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion between the two components. Excess chain extenders may cause poor chain extension efficiency and subsequently the deterioration of mechanical properties of the polyurethane dispersion. In addition, it is important to know where the locus of the chain extension reaction, particle surface or inner particle for understanding the reaction mechanism and controlling particle morphology.3,6-9 In this experiment, the aqueous polyurethane dispersion was prepared by prepolymer mixing and neutralization emulsification method. The parameters studied involved of the reaction of the chain extension as the variation of residual NCO group, molecular weight and particle size of the polyurethane during the chain extension step. Change of molecular weight and time-dependent variation of residual NCO group were investigated by using GPC and FTIR with different degree of chain extension, and particle size of the aqueous polyurethane dispersion was measured with a Mastersizer analyzer. 5.2 Experiment The characteristics of all the raw materials used in this experiment such as the linear polyester diol derived from caprolactone monomer terminated by primary hydroxyl groups (CAPA® 2205, Solvay Caprolactones), isophorone diisocyanate ( 2,2-Bis(hydroxymethyl), IPDI, Rhodia France), propionic acid (DMPA, Aldrich), 1,6-hexanediamine (HDA, Aldrich), ethylenediamine (EDA, Aldrich), 2- methylpentamethylenediamine (Dytek® A Amine), triethylamine (TEA, Merck), 1methyl-2-pyrrolidone (NMP, Merck) are shown in Chapter 3, section 3.1. Polyurethane prepolymer was synthesized in a 1L four-neck round-bottom glass reactor equipped with a mechanical stirrer, an electronic temperature controller, a temperature probe, a reflux condenser and a nitrogen inlet (see Figure 3.1). 75 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Polyester diol (CAPA® 2205) and DMPA (pre-dissolved in NMP solvent) were charged into the reactor and the mixture was stirred and heated to 80°C under nitrogen atmosphere. The crosslinker IPDI was then added. The reaction was allowed to proceed at 80°C until the amount of residual NCO content reached 20-30% above the theoretical residual NCO content. The end point was hence reached and subsequently NCO-terminated prepolymer was obtained. The amount of residual NCO (%) was checked at every hour interval using di-n-butylamine back titration method.3,7-11 TEA was added to neutralize the COOH groups at 60°C and polyurethane anionomers were obtained consequently. The polyurethane anionomer was then dispersed in DI water and chain extension reaction proceeded with the addition of 1,6 hexanediamine. The particle sizes of polyurethane dispersion were analyzed by the Mastersizer which is based on the principal of laser ensemble light scattering (Malvern MAF 5001 Mastersizer Micro Plus). The relative amount of NCO groups in the polyurethane was measured by FT-IR spectroscopy (Shimadzu FTIR-8400S). The average molecular weight was measured by a GPC (Waters 501) equipped with refractive index detector (Water 410). Tetrahydrofurane (THF) was used as an eluent at 1.0 mL/min flow rate and 1 X 103 Pa pressure. One column (Polymer Laboratories gel, 1000 °A) was used for the analysis of low molecular weight products and two columns of Millipore microstyreagel HR 3 and HR 4 were connected for the analysis of high molecular weight polymers. Number- and weight-average molar weights were calibrated with PMMA (polymethyl-methacrylate, mean Mw =3000, 11800, 95100 and 1456000) and PS (polystryene, mean Mw = 35000, 490000 and 2780000) standards. The polyurethane prepolymer was synthesized in a 1-L round-bottom glass reactor equipped with a mechnical stirrer, a thermometer, a reflux condenser, a 76 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion temperature controller and a nitrogen inlet. Reaction was conducted under a nitrogen atmosphere. The polyester diol (CAPA® 2205) and DMPA were pre-dissolved in NMP in the reactor flask. The aliphatic isocyanate crosslinker, IPDI, was then added to the mixture. The mixture was heated and stirred at 80 °C. The amount of residual NCO(%) was then checked at every one hour interval using di-n-butylamine back titration method. The reaction was allowed to proceed until the residual NCO(%) reached the end point (20-30% above the theoretical residual NCO content). Then the temperature was lowered to approximately 60 °C and TEA was then added whilst stirring to neutralize the carboxylic acid in the DMPA. The reaction mixture was stirred continuously for another 10 minutes. The aqueous PUD was then formed by phase inversion process. The required reaction mixture or prepolymer was poured into a metal container and cooled to ambient temperature. The dispersion was obtained by introducing DI water slowly into the prepolymer under high speed stirring using a Dispermat stirrer. For the chain extension, the required amount HDA (calculated by the formulae [%NCO/MW NCO X MW HDA/2] = Required amount of HDA(g)/100g prepolymer) was premixed with DI water and then added slowly into the prepared dispersion phase. The adhesive bonding strength (i.e. shear and peel strength) was evaluated by using the Zwick universal shear/peel strength test equipment. The details of test method was described in Chapter 3, section 3.8. 77 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 5.1. Characteristics and formulations of aqueous polyurethane dispersion using HDA as chain extender with different degree of neutralization. Characteristics F1 F2 F3 F4 NCO/OH 3 3 3 3 DMPA 5 5 5 5 Total solid % 50 50 50 50 Neutralization % 85 95 100 115 2.80 2.79 2.78 2.78 Weight (grams) Weight (grams) Weight (grams) Weight (grams) CAPA 2205 (Polyester Diol) 157.58 157.58 157.58 157.58 IPDI 52.95 52.95 52.95 52.95 DMPA 7.88 7.88 7.88 7.88 NMP 15.76 15.76 15.76 15.76 DBTL Catalyst - 0.08% of prepolymer 0.19 0.19 0.19 0.19 TEA 5.05 5.64 5.94 6.83 HDA Chain Extender (Theoretical) per 100g prepolymer 3.87 3.86 3.85 3.85 243.28 243.86 244.15 245.04 Theoretical Residual NCO % Raw Materials Total 78 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 5.2. Characteristics and formulations of aqueous polyurethane dispersion using EDA as chain extender with different degree of neutralization. Characteristics F5 F6 F7 F8 NCO/OH 3 3 3 3 DMPA 5 5 5 5 Total solid % 50 50 50 50 Neutralization % 85 95 100 115 2.80 2.79 2.78 2.78 Weight (grams) Weight (grams) Weight (grams) Weight (grams) CAPA 2205 (Polyester Diol) 157.58 157.58 157.58 157.58 IPDI 52.95 52.95 52.95 52.95 DMPA 7.88 7.88 7.88 7.88 NMP 15.76 15.76 15.76 15.76 DBTL Catalyst - 0.08% of prepolymer 0.19 0.19 0.19 0.19 TEA 5.05 5.64 5.94 6.83 EDA Chain Extender (Theoretical) per 100g prepolymer 2.00 2.00 1.99 1.99 241.41 242.00 242.29 243.18 Theoretical Residual NCO % Raw Materials Total 79 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Table 5.3. Characteristics and formulations of aqueous polyurethane dispersion using Dytek® A Amine as chain extender with different degree of neutralization. Characteristics F9 F10 F11 F12 NCO/OH 3 3 3 3 DMPA 5 5 5 5 Total solid % 50 50 50 50 Neutralization % 85 95 100 115 2.80 2.79 2.78 2.78 Weight (grams) Weight (grams) Weight (grams) Weight (grams) CAPA 2205 (Polyester Diol) 157.58 157.58 157.58 157.58 IPDI 52.95 52.95 52.95 52.95 DMPA 7.88 7.88 7.88 7.88 NMP 15.76 15.76 15.76 15.76 DBTL Catalyst - 0.08% of prepolymer 0.19 0.19 0.19 0.19 TEA Dytek chain extender (Theoretical) per 100g prepolymer Total 5.05 5.64 5.94 6.83 3.87 3.86 3.85 3.85 243.28 243.86 244.15 245.04 Theoretical Residual NCO % Raw Materisla 80 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 5.3 Results and Discussion 5.3.1 Effect of Different Types of Chain Extenders Three types of chain extenders i.e. 1,6-hexanediamine (HDA), ethylenediamine (EDA) and 2-methylpentamethylenediamine (Dytek® A Amine) were chosen in this research. Table 5.1 shows that using HDA as the chain extender for preparing the PUD, the final appearance of the product is superior than using either EDA or Dytek® A Amine chain extender. Table 5.4. extenders Appearance of the finishing PUD product with different types of chain Types of extenders chain Parameters : NCO/OH ratio Neutralization Particle size (µm) Appearance 1,6hexanediamine (HDA) ethylenediamine (EDA) 2-Methylpentamethylenediamine (Dytek® A Amine) 3.0 100% 6.93 3.0 100% 4.42 3.0 100% 36.94 Undesirable Milky white with semi-gel product Undesirable Hazy with lots of air bubbles. Very viscous. Desirable Milky white liquid PUD using HDA as chain extender had a satisfactory finishing appearance (i.e. milky white and liquid form). On the other hand, the use of the other two chain extenders, EDA and Dytek® A Amine, gave undersirable appearance for the end product of PUD. Hence, HDA was selected as a chain extender in further studies in this research. 5.3.2 5.3.2.1 FT-IR Analysis of Aqueous Polyurethane Dispersion (PUD) Formation of PUD Figure 5.1 demonstrates the FT-IR spectra of the main component of polyester polyol (CAPA2205), isophorone diisocyanate (IPDI) and the PUD. The 1750 -1740 cm-1 81 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion band indicatse the C=O stretching in polyol; the 2280 – 2260 cm-1 band is due to N=C=O antisymmetric stretching in isocyanate. Bands at 1560 – 1530 and 1610 1560 cm-1 are due to N-H bending and COO- antisymmetric stretching in the PUD4,8-10 Absorbance . These bands confirm the PUD structure. PUD IPDI Polyester Polyol (CAPA 2205) 4400 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 1/cm Figure 5.1. FT-IR spectra of polyol, IPDI and PUD 5.3.2.2 FT-IR Analysis of Residual NCO Functionality in PUD The absorption spectra of PUD is shown in Figure 5.2. The presence of characteristic peaks, C=O [1733, 1703 cm-1 ] and N-H [1550 cm-1 ], confirmed the formation of urethane group [-NHCOO-]. The absence of the N=C=O [2270 cm-1 ] stretching band showed that all the –NCO functionalities were consumed after chain extension. 82 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion B e fo re c h a in e x te n s io n %T -N = C = O A fte r c h a in e x te n s io n 4400 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 1 /c m Figure 5.2. FT-IR spectra of PUD before (a) and after (b) chain extension Figure 5.3 shows the FT-IR spectra of the PUD with different average particle sizes at a constant NCO/OH ratio of 2.8 before chain extension. The absence of N=C=O stretch bands in the spectra indicated that all the residual NCO groups were consumed completely due to the water molecules at the surface of the polyurethane particle since only a small amount of free NCO group was present initially. 83 Absorbance Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion (a) (b) (c) (d) 4400 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 1/cm Figure 5.3. Absorption FT-IR spectra of PUD of varying particle size before chain extension : (a) 1.65µm, (b) 2.60µm, (c) 3.65µm, (d) 4.05µm (NCO/OH ratio =2.8) Table 5.5 shows that the NCO content of the polyurethane prepolymer determined by the di-n-butylamine back titration method. The result showed that the NCO groups of IPDI were sufficiently reacted with hydroxyl groups of polyol at the first step and with the DMPA at the next step. Table 5.5. The residual NCO content of the polyurethane prepolymer by the din-butylamine back titration method NCO : OH ratio 2.8 3.0 3.2 3.4 Polyol + IPDI +DMP 3.95 4.45 4.54 5.37 % residual NCO Polyol + IPDI + DMP + Cataylst + 1hr 3.36 3.61 4.11 4.68 Polyol + IPDI + DMP + Cataylst After 2 hrs 3.10 3.20 4.08 4.61 84 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Figure 5.4 shows the time-dependent change of NCO bands during the chain extension process. No residual NCO peak was observed after the completion of chain extension. 0.3 0.25 Absorbance [ h] 0.2 0.15 [ g] [f] [ e] 0.1 [ d] 0.05 [c] [ b] 0 [ a] 4500 4200 3900 3600 3300 3000 2700 2400 1950 1800 1650 1500 1350 1200 1050 900 750 1/cm Figure 5.4. Change of FT-IR spectra during preparation of PUD: (a) polyester polyol + IPDI + DMPA, (b) after neutralization / before dispersion, (c) before chain extension, (d) adding 20% of chain extender (theortically), (e) 40%, (f) 60%, (g) 80% & (h) 100% (NCO/OH = 3.0; particle size = 2.65µm) 85 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Figure 5.5 shows the effect of particle size on the needed amount of chain extender for 100% extension. Relative amount of residual NCO group was calculated by taking an alkane (-CH2 -) stretching vibration at about 2855 cm-1 as a reference because CH2 groups were not changed during the entire reaction period.2,9-12 120 Ave. Particle Size : 2.65 Ave. Particle Size : 2.00 Ave. Particle Size : 1.50 Amount of residual NCO (%) 100 80 60 40 20 0 0 20 40 50 60 80 100 Amount of chain extender (% theoretical) Figure 5.5. Amount of residual NCO groups versus the sizes of PUD particles in chain extension stage at average particle size 2.65µ, 2.00µ and 1.50µat NCO/OH =3.0 5.3.3 Growth of Average Molecular Weight during the Chain Extension Figure 5.6 demonstrates the change of average molecular weight during the chain extension process with different NCO/OH ratio and particle sizes. When the NCO/OH ratio is low such as 2.8, the molecular weight of PUD did not change significantly during the chain extension stage. This result indicated that the side reaction occurred between the residual NCO groups and water during and after the dispersion stage. This phenomenon can be further verified by the absence of the N=C=O stretch band at 2270-2280cm-1 in the FT-IR spectra as shown in Figure 5.3. 86 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 350000 Molecular weight (g/mol) 300000 250000 200000 150000 Particle size : 2.56µ (NCO/OH =2.8) Particle size : 2.00µ (NCO/OH =2.8) Particle size : 1.50µ (NCO/OH =2.8) Particle size : 2.56µ (NCO/OH =3.0) Particle size : 2.00µ (NCO/OH =3.0) Particle size : 1.50µ (NCO/OH =3.0) 100000 50000 0 0 20 40 60 80 100 Amount of chain extender (% theoretical) Figure 5.6. The change of average molecular weight in chain extension stage with different particle sizes and different NCO/OH ratio (particle sizes varied from 1.50µm to 2.56µm; NCO/OH ratio varied from 2.8 to 3.0) Figure 5.6 also showed the increase in molecular weight of PUD with increasing amount of chain extender. About half of the residual NCO groups reacted with water molecules in the dispersion process and the rest could react with the chain extender. In the case of 4,4’-methylenebis-phenyl isocyanate (MDI), it was reported that almost all the residual NCO groups reacted with water during the dispersion process.7-13 Therefore, it was difficult to prepare chain extended MDI-based polyurethane dispersion. In the IPDI-based polyurethane dispersion, the reactivity of NCO group in IPDI is much lower than that of MDI. The relative reactivity of NCO groups in IPDI with various functionalities can be illustrated as below : Aliphatic NH2 > Aromatic NH2 > Primary OH > Water > Secondary OH > Tertiary OH > Phenolic OH > COOH.12-15 87 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion Value of chain extension (% of chain extender) 70 60 50 40 30 20 10 0 0 0.5 1 1.5 2 2.5 3 3.5 Number-average particle size of PUD (µm) Figure 5.7. Effect of particle size on the maximum value of chain extension (CEmax) Figure 5.7 shows the relationship between number-average particle size and maximum value of chain extension, CEmax (50% at 2.6µm). With the decrease of the average particle size, the value of chain extension increased. This result does not indicate that the number of residual NCO groups locate at the surface of the particle is proportional to the total surface area. However, possible reaction locus of the chain extension is considered as particle surface since water-soluble chain extender was used and CEmax was influenced by the total surface area. Moreover, available free NCO group should be incorporated with carboxyl group near the end of prepolymer molecule since DMPA was present between excess amount of IPDI and polyester polyol during the preparation of prepolymer.3,14-17 This indicated that the residual NCO-groups on the particle surface have more favourable condition than inner particle spaces have to react with chain extender (Figure 5.8). 88 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion (A) H2O (A) H2N NCO - NCO NH2 - - H2N H2O (B) NH2 - NCO - H2O NCO H2O - Chain Extension - (B) - - - NH2 - H2N - - H2N NH2 - - PU particle Figure 5.8. The reaction of residual NCO groups. Figure 5.9 below demonstrates the reaction scheme of residual –NCO groups, and the two competitive reactions may occur simultaneously. 2(R-N=C=O) + H 2O ----å R-NH-CO-NC-R + CO 2 ---------------(1) Urea Linkage 2(R-N=C=O) + H 2N (C6 H 12)NH 2---å R-NH-CO-NH (C6H12)N-CO-NH-R ---(2) Urea Linkage Figure 5.9. Two competitive reactions of residual –NCO groups on PU particle Urea linkage is developed in both ways. The first reaction may occur in inner particle and also on particle surface. However, the second reaction is supposed to occur only on particle surface. Probably, polyurethane ionomer particle is swelled with water even though swelling ratio is not so high. However, the possibility of side reaction between residual NCO groups and water molecules inside of the particle is related to the particle size or volume.15-17 89 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 5.3.4 Effect of the Degree of Chain Extension on the Adhesive Bonding Strength Figures 5.10, 5.11 and 5.12 show that the adhesive bonding strength (shear and peel strength) versus the degree of chain extension with different NCO/OH ratios. From Figure 5.3, it was found that there were no free NCO groups available due to the small amount of NCO group and reaction with water molecules at dispersion stage. Therefore the adhesive bonding strength decreased even though the amount of chain extender increased. The excess chain extender became impurities and consequently caused undesirable side effect on the bonding strength (see Figure 5.10). In the case of higher mole ratio such as 3.0, even though the free NCO groups reacted with water molecule, there were still some residual NCO groups remained in water phase. Therefore, the bonding strength increased to the point of CEmax . The adhesive bonding strength decreased again after this point, as shown in Figure 5.11 and 5.12. 30 9 8 Shear Strength, N/mm2 7 6 20 5 15 4 Peel Strength, N/mm 25 3 10 Shear Strength 2 Peel Strength 5 1 0 0 0 20 40 60 80 100 Amount of chain extender (% theoretical) Figure 5.10. Effect of the degree of chain extension on adhesive bonding strength (shear and peel strength) at NCO/OH ratio 2.8 90 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 40 35 Shear strength, N/mm2 30 25 20 15 Shear Strength (particle size=1.50) 10 Shear Strength (particle size=2.00) Shear Strength (particle size=2.56) 5 0 0 20 40 50 60 80 100 Amount of chain extender (% theoretical) Figure 5.11. Effect of the degree of chain extension on the shear strength with different particle sizes at NCO/OH ratio 3.0 30 Peel Strength, N/mm 25 20 15 Peel Strength (particle size=1.50) 10 Peel Strength (particle size=2.00) Peel Strength (particle size=2.56) 5 0 0 20 40 50 60 80 100 Amount of chain extender (% theoretical) Figure 5.12. Effect of the degree of chain extension on the peel strength with different particle sizes at NCO/OH ratio 3.0 91 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 5.4 Conclusions This study focused on the effect of chain extenders. The aqueous polyurethane dispersion was prepared and the effect of the chain extension was investigated. At a low NCO/OH ratio, no free NCO group was found due to the reaction with water molecules from the beginning of chain extension reaction. At a NCO/OH ratio of 3.0, about half of residual NCO groups were remained and reacted with the chain extender. The amount of residual NCO group varied with the total surface areas or the particle sizes at the same NCO/OH ratio. The required amounts of chain extender for the optimal chain extension do not correspond with the theoretical residual NCO group. With decreasing polyurethane particle size, the amounts of optimal chain extender logarithmically increased. For larger particle, residual NCO group could not be founded. Therefore, most chain extenders reacted with NCO groups in particle surface more than in inner particle. This is also further verified with the results of adhesive bonding strength. The excess amines had an unfavourable influence on the bonding strength. 5.5 References 1. D.S. Chen, M. Hsien, US Patent 5, 1994, 306, 764. 2. Lee, H.T, Hwang, Y.T, Chang, N.S, Huang, C.C. T, Li, H.C, Water Borne, High-Solid and Power Coatings Symposium, New Orleans, 22-24 February, 1995, p.224. 3. C. Hepburn, Polyurethane Elastomers, Second ed., Elsevier, New York, 1992. 92 Chapter 5 Effect of Chain Extension on Adhesive Bonding Strength of Aqueous Polyurethane Dispersion 4. J.B. Lambert. D.A. Lightner, H.F Shurvell, R.G. Cooks, Introduction to Organic Spectroscopy, Macmillan, New York, 1987, p.169 5. P. Thomas, Waterborne and Solvent Based Surface Coating Resins and their Applications in Polyurethanes, Vol III, SITA Technology Ltd London, 1999, p.59. 6. T.D. Salatin and A.M. Budde, US Patent 5, 1993, 236, 995. 7. J. Weikard and E. Luhmann, US Patent 6, 2003, 541, 536. 8. S. Kaizerman and R.R. Aloia, US Patent 4, 1985, 198, 330. 9. R. S. Buckanin, US Patent 4, 1985, 705, 840. 10. H.R. Allcock and F.W. Lampe, Contemporary Polymer Chemistry, New York, 1990, p.43. 11. W. Koonce and F. Parks, US Patent 6, 2002, 451, 908. 12. P.H. Markusch, J.W. Posthauser and M.C. Beatty, US Patent 4, 1985, 501,852. 13. C. Irle and W. Kremer, US Patent 6, 2003, 559, 225. 14. G.A. Anderle and S.L. Lenhard, US Patent 6, 2003, 576, 702. 15. H.P. Muller and H. Gruttmann, US Patent 6, 2001, 172, 126. 16. L.C. Hesselmans, US Patent 6, 2003, 599, 977. 17. M.A. Schafheutle and A. Artz, US Patent 6, 2002, 429, 254. 93 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives 6.1 Introduction Polyurethane (PU) adhesives consumption has been estimated at 216 million lb in 1991, with a value of approximately USD 301 million. As compared to others adhesives, PU adhesives for footwear have a great demand, especially in China, Taiwan, Korea, Thailand and Malaysia. Overall the PU adhesives market grew at approximately 3% per year from 1986 to 1991. Currently, the output of adhesives (including PU adhesive) and sealants in China is roughly at 3 million tonnes in year 2002, or 7% of the global production,1,4 as shown in Figure 6.1. 4 Adhesive & Sealant PU Adhesive 3.54 3.5 CAGR for adhesive & Sealant is ~ 10% per year Million Tonnes 3 2.5 2.27 3 2.61 2.44 2.07 2 1.5 1 CAGR for PU adhesive is ~ 14% per year 0.5 0.10 0.11 0.12 0.14 0.17 0.20 0 1998 1999 2000 2001 2002 2005 Figure 6.1. Total output of adhesives + sealant and PU adhesive in China 94 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives It was estimated that in year 2005, the output will reach 3.54 million tonnes. The total adhesives + sealant grew in average about 10% per year. However, for the PU adhesive, the average growth was estimated to be 14% per year from 1998 to 2005. Recently, the adhesive market in China has grown more aggressively than other countries in the world, as the average growth rate is about 10% as compared to 1~1.5% in US and 2.5% globally.2,4-7 PU adhesives used to attach soles to footwear make a sizeable niche. PU adhesives compete primarily with neoprene-based adhesives and have replaced much of the neoprene due to improved performance. For more than 30 years, solvent-based PU adhesives have been used in application for attaching soles in the shoe industry. They have high initial and final bond strengths, excellent heat resistance and the ability to be used in wet bonding or heat reactivation application as compared to the traditional type of shoe adhesive i.e. neoprene. Figure 6.2 shows the segmentation of the PU adhesives in various application fields. The major market segment for PU adhesives was in footwear industry, about 60% of the total market with consumption volume at 100 thousands tonnes in year 2002 while the total output of PU adhesives (all applications) was 166 thousands tonnes.3,6-10 95 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives Footwear 60% Sealant and Others 10% General 18% Compound Packaging Film 12% Figure 6.2. Segmentation of PU adhesives markets (by consumption volume, total 166 thousands tonnes) in year 2002 Transportation in China depends on buses, bicycles and feet. Bicycles do not required a lot of adhesives. The story is different for footwear. There are at least 1 billion domestic customers. Per capita, there is an annual average consumption of 1.5 pairs of footwear for Chinese and approximately five pairs of westerners depending on gender and out door activities. Finally, it becomes clear that in China, footwear industries play an important role among light industries because 70-80% of today's footwear is glued together. Thus, it is natural for China to become the number one exporter for footwear with the 1992 production of approximately 2 billion pairs. For the period of 1991 through May 1994, Chinese footwear occupied 13.9% of the total US import. For this, a lot of shoe manufacturer and adhesives makers have moved to China. Figure 6.3 shows the largest footwear producer in the world, which is China, with about 54% of global market share.2,7-12 96 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives Asia (excl China) 21.5% Western Europe 7.3% South America 6.8% China Middle East 3.0% 54 % Eastern Europe 2.8% North and Central America 2.6% Oceania 0.1% Africa 1.9% Figure 6.3. The largest footwear producer in the world, China Figure 6.4 illustrates the total production of footwear in China from year 1985 to 2002, from 1.6 billion pairs increased to 6.6 billion pairs of footwear. China Footwear Production (billion pairs) 7 6.3 6.5 6.6 1997 2001 2002 5.7 6 5 4 3 2 1.6 1 0 1985 1995 Figure 6.4. Total footwear production in China from year 1985 to 2002. 97 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives In year 2002, the total production capacity for footwear in the whole Asia was estimated to be 76% of global production, however, for China alone, it was already about 54% of global production with 6.6 billion pairs of footwear produced. The adhesives for footwear are mainly PU and neoprene (or named chloroprene). China imports chloroprene and MDI, methylene diisocyanate (for PU). The first large-scale production line, amounting to 1000 MT of PU, was installed at the Da-Cang Factory in Jiangsu Province. Currently, there are at least 20 factories with a total capacity of 6000 MT of PU, and there are about 2000 MT of PU solutions available for footwear manufacturing. However, the problems of some PU solutions have been poor stability, low initial viscosity and the yellowing of adhesives.3,13-15 In recent years, aliphatic polyisocyanate has been chosen to replace aromatic polyisocyanate like MDI. Aliphatic polyisocyanate has several advantages over aromatic polyisocyanates: 1) Aliphatic polyisocyanate has better durability than aromatic polyisocyanate. This means that aliphatic polyisocyanate has non-yellowing properties after exposure to sunlight for long time. 2) Aliphatic polyisocyanate is a more environmentally friendly product (in term of less toxicity) than aromatic polyisocyanate. 3) Aliphatic polyisocyanate is more stable and having longer pot-life than aromatic polyisocyanate. As the global trends shifting footwear production from U.S.A and Europe to Asia, footwear industry has expanded enormously in China in the recent years. PU adhesives are replacing the conventional type of footwear adhesives in the market due to their high performance in adhesive bonding strength, non-yellowing and less toxicity (using aliphatic isocyanate as a crosslinker in the footwear adhesives) 98 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives properties. As environmental demands on the adhesives industry have increased and the need for adhesives with low VOC (volatile organic concentration) or no-solvent content has developed, water borne polyurethane footwear adhesives have been created to address these needs.7,15-17 The main component in the water borne PU adhesives is the aqueous polyurethane dispersion (PUD). In this research, the PUDs based on aliphatic isocyanates which were developed in the previous research work (Chapter 4 and 5) were used in formulating the water borne footwear adhesives. The key properties such as the appearance (colour) and adhesive bonding strength have been assessed. In addition, the adhesion bonding strength i.e shear and peel strength of the footwear adhesives on different shoe substrates have also been evaluated and compared with one of the commercialized PUD, Dispercoll U54 (from Bayer, Germany). 6.2 Experiment The water borne polyurethane footwear adhesives were prepared using the aqueous polyurethane dispersions (PUDs) which were obtained from the previous research described in Chapters 4 and 5. Two types of formulations known as onecomponent (1K) and two-component (2K) water borne PU footwear adhesives were prepared (see Tables 6.1 and 6.2), and the performances were then evaluated by comparing our research developed PUD versus the commercialized PUD, Disperoll U54. 99 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives Table 6.1. One-component (1K) water born PU footwear adhesive formulation Ingredients Weight (grams) Aqueous polyurethane dispersion i.e. research developed PUDs or Dispercoll U54 Tafigel PUR 40 thickener 100.00 Total 100.47 0.47 Table 6.2. Two-component (2K) water borne PU footwear adhesive formulation Ingredients Weight (grams) Component A Aqueous polyurethane dispersion i.e. reserach developed PUDs or Dispercoll U54 Tafigel PUR 40 thickener Component B Rhodocoat WAT-1 water polyisocyanate crosslinker Total borne aliphatic 100.00 0.47 3.00 103.47 The appearance (color) of our research developed aqueous polyurethane dispersion and the adhesives were compared against the commercialized solvent borne PU footwear adhesive. The shear and peel strength of our research developed footwear adhesives were evaluated on different substrates i.e. PVC, PU and NBR and compared with the commercialized water borne PU footwear adhesives. Both shear strength (based on EN1392 standard) and peel strength were measured using the Zwick universal shear/peel strength tester. The details of both test methods were described in Chapter 3, section 3.8. 100 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives 6.3 Results and Discussion 6.3.1 Color appearance and durability comparison The aqueous polyurethane dispersion which developed earlier was used to formulate the water borne polyurethane footwear adhesive and then compared with the commercial solvent borne polyurethane footwear adhesive. Figure 6.5 shows the color appearance of our newly prepared product and the commercial solvent borne polyurethane solution. The aqueous polyurethane dispersion is white and milky. However, the commercial solvent borne polyurethane solution is yellowish. Newly made commercial solvent borne polyurethane solution Newly made aqueous polyurethane dispersion Figure 6.5. Comparison the color appearance of both solvent borne polyurethane solution and aqueous polyurethane dispersion After storage both solvent borne polyurethane solution and aqueous polyurethane dispersion for a period of times i.e. 6 months at ambient temperature and humidity at 55% environment, the solvent borne polyurethane solution changed to dark yellowish. However, the milky white color of aqueous polyurethane dispersion remained unchanged (see Figure 6.6). This indicates that the durability and stability of our aqueous polyurethane dispersion is better than the commercial solvent borne polyurethane solution. 101 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives The commercial solvent borne polyurethane solution after storage for 6 months The aqueous polyurethane dispersion after storage for 6 months Figure 6.6. Comparison the color appearance of both solvent borne polyurethane solution and aqueous polyurethane dispersion after storage for 6 months at ambient temperature and humidity at 55% environment Figure 6.7 shows the color appearance of both two component (2K) solvent borne and water borne polyurethane (PU) footwear adhesives. Both adhesives were prepared by using the commercial PU solution (for solvent borne) and our PUD (for water borne). Both prepared footwear adhesives were then applied onto a white shoe sole base, dried and exposed to sunlight for 3 months. Apply/brush evening on the white shoe sole base side by side and then exposure to sunlight for 3 months A drop of 2K solvent borne PU adhesive White shoe sole base A drop of 2K water borne PU adhesive (using PUD) 2K solvent borne PU adhesive 2K water borne PU adhesive (using PUD) Figure 6.7. Comparison of the color appearance of both 2K solvent borne and water borne PU footwear adhesives after exposure to sunlight for 3 months 102 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives After exposure the prepared footwear adhesives to sunlight for 3 months, the commercial 2K solvent borne PU adhesive was yellowish in color whereas our 2K water borne PU adhesive had satisfactory appearance (transparent after drying) and no color changed after exposure to sunlight for a 3-month period. This indicates that our 2K water borne PU adhesive has a better durability property as compared to the commercial 2K solvent borne PU adhesive. This property is particularly important for footwear industry especially for those shoes bases which are in white or light color. 6.3.2 Adhesive bonding strength comparison Based on the previous research studies on the effect of different process parameters (Chapters 4 and 5), the optimal parameters for preparing the PUD have been identified i.e. the NCO/OH ratio to be 3.0, DMPA content to be 5% and degree of neutralization to be 100%. Our polyurethane dispersion was prepared according to these optimal parameters and then formulated into the one-component (1K) and twocomponent (2K) water borne polyurethane (PU) footwear adhesives. The adhesive bonding strengths such as peel and shear strength of these research prepared water borne polyurethane footwear adhesives were then compared with the commercial products. Figures 6.8 and 6.9 demonstrate the initial (5 minutes after bonding) and final (24 hours after bonding) peel strengths of both our 1K water borne polyurethane footwear adhesive (based on formulation in Table 6.1) versus the commercial 1K water borne polyurethane footwear adhesive on different shoe substrates i.e. PVC, PU and NBR. 103 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives 40 Commercial product 35 Our product Peel Strength (N/mm) 30 25 20 15 10 5 0 PVC PU NBR Shoe substrates Figure 6.8. Comparison of initial peel strength of our 1K water born PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD) Commercial product 100 Our product 90 Peel Strength (N/mm) 80 70 60 50 40 30 20 10 0 PVC PU Shoe substrates NBR Figure 6.9. Comparison of final peel strength of our 1K water born PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD) 104 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives The initial and final peel strengths of our 1K water borne footwear adhesive were found to be superior than the commercial product when applied on different shoe substrates i.e. PVC, PU and NBR. Figure 6.10 shows the shear strengths of our 1K water borne PU footwear adhesive and the commercial product. 140 Commercial product Shear strength (N/mm2) 120 Our product 100 80 60 40 20 0 PVC PU NBR Shoe substrates Figure 6.10. Shear strength comparison of our 1K water borne PU footwear adhesive versus commercial product Once again, our 1K water borne PU footwear adhesive showed better shear strength than the commercial product when applied onto the 3 different shoe substarates i.e. PVC, PU and NBR. Figures 6.11 and 6.12 show the initial and final peel strengths of our 2K water borne PU footwear adhesives and the commercial product (based on Disperoll U54 PUD). 105 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives 60 Peel strength (N/mm) Commercial product Our product 50 40 30 20 10 0 PVC PU Shoe substrates NBR Figure 6.11. Comparison of initial peel strength of our 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD) 120 Commercial product Our product Peel strength (N/mm) 100 80 60 40 20 0 PVC PU Shoe substrates NBR Figure 6.12. Comparison of final peel strength of our 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD) 106 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives In the comparison of initial and final peel strength for our 2K water borne PU footwear adhesive and the commercial product, it was found that our 2K water borne PU footwear adhesive has a superior peel strength than the commercial product. Similarly, the shear strength of our 2K water borne PU footwear adhesive is higher than that of the commercial product (Figure 6.9). 160 Commercial product Our product Shear strength (N/mm2) 140 120 100 80 60 40 20 0 PVC PU Shoe substrates NBR Figure 6.13. Shear strength comparison of our 2K water borne PU footwear adhesive versus commercial product The 2K water borne PU footwear adhesives provide higher adhesive bonding strength (in term of peel and shear strength) than the 1K water borne PU footwear adhesive. This is mainly due to the incorporation of external water borne aliphatic isocyanate crosslinker (Rhodocoat WAT-1 from Rhodia Co.) in the 2nd part of the formulation. With this additional crosslinker, the crosslinking network in the polyurethane chain was increased and so enhanced the adhesive bonding strength of the footwear adhesives between the shoe substrates. High adhesive bonding strength is particularly needed for shoes subjected to higher degree of strain or bending. 107 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives In the initial peel strength test, both 1K and 2K water borne PU footwear adhesives with PVC substrate give the best result than the other two substrates. However, in the final peel strength test, adhesive with PU substrate gives the best result. This is because the surface energy (wetness) of the PU substrate is the highest among all the substrates, due to its active molecular structure. With the higher surface energy, the PUD is therefore able to create a strong covalent bond . 6.4 Conclusions The aqueous polyurethane dispersion developed in this research provides desirable color appearance i.e. a white milky liquid. When formulated into 1K and 2K water borne PU footwear adhesives, the dispersion retained the same white milky color in wet form (liquid form). However, when the dispersion was coated as the adhesive on shoe substrates and dried, it was transparent. In addition, it offered better durability (non-yellowing appearance after exposure to sunlight for a long period of times) as compared to the commercial solvent borne type of footwear adhesives. In terms of adhesive bonding strength, both 1K and 2K water borne PU footwear adhesives developed in this research showed superior peel and shear strengths than the commercial product regardless of shoe substrates materials used i.e. PVC, PU and NBR. 6.5 References 1. B.S, Jackson, Industrial Adhesives and Sealants, 1995, p.10. 2. C. Hepburn, Polyurethane Elastomers, Second ed., Elsevier, New York, 1992, p.281. 108 Chapter 6 The Effect of Aqueous Polyurethane Dispersion on Non-yellowing and Adhesive Bonding Strength Properties in Formulating the Footwear Adhesives 3. P. Thomas, Waterborne and Solvent Based Surface Coating Resins and their Applications in Polyurethanes, Vol III, SITA Technology Ltd London, 1999, p.59. 4. G. Oertel, Polyurethane Handbook, Carl Hanser, Munich, 1985, p.31. 5. George Woods, The ICI Polyurethanes Book, 2nd Edition, ICI Polyurethanes, 1987, p.197. 6. J.W. Rothause and K. Nachtkam, Advances in Urethane Sci. and Techology, 1987, 10, p.121. 7. G. Schneberger and M. Dekker, Adhesives in Manufacturing, 1983, p2. 8. J. Weikard and E. Luhmann, US Patent 6, 2003, 541, 536. 9. H.P. Muller and H. Gruttmann, US Patent 6, 2001, 172, 126. 10. S. Kaizerman and R.R. Aloia, US Patent 4, 1985, 198, 330. 11. W. Koonce and F. Parks, US Patent 6, 2002, 451, 908. 12. R. S. Buckanin, US Patent 4, 1985, 705, 840. 13. H.R. Allcock and F.W. Lampe, Contemporary Polymer Chemistry, New York, 1990, p.43. 14. T.D. Salatin and A.M. Budde, US Patent 5, 1993, 236, 995. 15. G.A. Anderle and S.L. Lenhard, US Patent 6, 2003, 576, 702. 16. B.K. Kim and Y.M. Lee, J. Appl. Polym. Sci., 1994, 54, 1809. 17. P.H. Markusch, J.W. Posthauser and M.C. Beatty, US Patent 4, 1985, 501,852. . 109 Chapter 7 Conclusions Chapter 7 Conclusions With the evolution of legislation towards reducing the VOC and the creation of environmental friendly products, there is a great demand for the development of water borne products for the industries. Solvent borne adhesives are used extensively in the footwear industry for a long time. Currently, there are several solvent borne footwear adhesives available in the market, namely neoprene, grafted choroprene and the polyurethane based systems. The polyurethane adhesives are generally accepted for their good bonding strength and good resistance to water, fat, oils, chemical and solvents. Owing to the large varieties of polyurethane adhesives systems, they are classified into one-component (1K) and two-components (2K) systems. Two-component polyurethane adhesives are characterized essentially by using polyisocyanates as crosslinkers and oligomeric diols or polyols as the back-bone resin. They have the advantages of presenting no great problems in terms of shelf life. By a skillful choice and targeted reactivity of the monomers, it is possible to formulate systems having different pot lives, bonding strength and chemical resistance to meet different requirements. Due to the polyaddition reactions, these adhesives do not release any elimination products during the crosslinking. Therefore, the two-component system is generally well accepted in the industry. As the regulators are implementing the policy to protect the environment across these regions, big multi-national organizations like Nike, Reebok and Adidas have already embarked on the campaign to demand their suppliers to supply environmental friendly adhesives (water borne type) for their applications. This has generated a big demand for the water borne adhesives and all suppliers are gearing 110 Chapter 7 Conclusions their R&D in this direction. The adhesives industry is therefore gearing to produce water borne adhesives with non-yellowing property for the shoes markets especially those with white or light based sport shoes. In view of the fact that the trend is towards a high demand for environment friendly products, the research on the development of an aqueous polyurethane dispersion to form the water borne polyurethane footwear adhesives (1K and 2K systems) with non-yellowing and good adhesive bonding strength properties were therefore designed. To achieve this objective, the research has focused on the following areas : 1) Formulation of an aqueous polyurethane dispersion (PUD) by forming a NCO prepolymer initially. The chain was subsequently extended in the aqueous phase in the presence of a polyamine chain extender. 2) The prepolymer is formed by reacting an active hydrogen containing compound such as linear polyester diol (CAPA® 2205, white waxy solid, mean molecular weight 2000 and hyroxyl value 56mg KOH/g) with aliphatic polyisocyanate such as isophorone diisocyanate (IPDI), 2,2-bis(hydroxymethyl) propionic acid (DMPA) and the chain extender 1,6-hexanediamine (HDA). 3) Various formulations were designed to study the effects of process parameters such as NCO/OH ratio, DMPA content, degree of neutralization and the degree of chain extension. Based on these studies, it was found that the molecular weight and adhesive bonding strength of the PUD were significantly affect by the DMPA content and the degree of neutralization. The molecular weight of the PUD was found to increase when the NCO/OH ratio was increased. As the particle size decreased, the amount of chain extender needed to optimize the chain extension decreased. The nonyellowing property could be achieved by using an aliphatic isocyanate (IPDI). From 111 Chapter 7 Conclusions the obtained results, the optimal process parameters for formulating the aqueous polyurethane dispersion with optimum performance were therefore identified to be : NCO/OH ratio is 3, DMPA content is 5% and degree of neutralization is 100%. 4) The aqueous polyurethane dispersion (PUD) which developed in this research was used to formulate the 1K and 2K water borne PU footwear adhesives. Their adhesive bonding strengths i.e. shear and peel strenghts were then assessed and compared versus the commercial product. Our PUD gave good compatibility and outstanding peel and shear strength than the commercial product. In conclusion, we are able to generate a series of good and valuable data for use to develop the aqueous polyurethane dispersion (PUD) with good compatibility, durability (non-yellowing), low VOC (environmental friendly) and outstanding adhesive bonding strength properties. These properties are actually the key requirements for the footwear adhesives market now. Therefore, the present research could bring attractivenss and added value to the adhesive producers as well as the footwear industry. 112 [...]... both solvent borne polyurethane solution and aqueous polyurethane dispersion after storage for 6 months at ambient temperature and humidity at 55% environment Figure 6.7 102 Comparison of the color appearance of both 2K solvent borne and water borne PU footwear adhesives after exposure to sunlight for 3 months Figure 6.8 102 Comparison of initial peel strength of our 1K water borne PU footwear adhesive... One-component (1K) water borne PU footwear adhesive formulation Table 6.2 100 Two-component (2K) water borne PU footwear adhesive formulation 100 xii List of Figures Figure 1.1 Formation of aqueous dispersion 3 Figure 2.1 Polyurethane adhesives market segment 8 Figure 2.2 Reaction of polyol and isocyanate 12 Figure 2.3 Amine catalyst reaction mechanism 12 Figure 2.4 Anionic polyurethane dispersion with carboxylate... initial peel strength of 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD) Figure 6.12 106 Comparison of final peel strength of 2K water borne PU footwear adhesive versus commercial 2K water borne footwear adhesive (based on Disperoll U54 PUD) Figure 6.13 106 Shear strength comparison of our 2K water borne PU footwear adhesive versus commercial... Table 4.3 61 61 Formulation of aqueous polyurethane dispersion (Sequence 3) – NCO/OH ratio and DMPA content are constant but the degree of neutralization varies from 85% to 115% Table 5.1 Characteristics and formulations of aqueous polyurethane dispersion using HDA as chin extender with different degree of neutralization Table 5.2 78 Characteristics and formulations of aqueous polyurethane dispersion using... The term aqueous polyurethane dispersion refers to aqueous dispersions of polymers containing urethane groups and optionally urea groups Aqueous polyurethane disperisons are well known and used in the production of a variety of useful polyurethane products, for example, adhesives, coatings and sealants etc Such dispersions are produced by dispersing a water- dispersible, isocyanate-terminated polyurethane. .. composite wood products market for oriented strand board and laminated beams for high performance applications 2.3 Polyurethane Adhesives Polyurethane adhesives, which vary widely in composition, are used in many application areas due to their outstanding properties, their simple and economical processing and their high strength They account for about eight percent of the global adhesives market, at around... commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD) 104 xvi Figure 6.9 Comparison of final peel strength of our 1K water borne PU footwear adhesive versus commercial 1K water borne footwear adhesive (based on Disperoll U54 PUD) Figure 6.10 Shear strength comparison of our 1K water borne PU footwear adhesive versus commercial product Figure 6.11 104 105 Comparison of initial peel strength. .. Moulded method for various types of footwear 46 Figure 2.24 Slip lasting / strobel stitched method 47 Figure 3.1 Set up of apparatus for the synthesis of polyurethane prepolymer 49 Figure 3.2 Preparation of aqueous polyurethane dispersion 50 Figure 3.3 Form and dimensions of test pieces for shear tests 53 Figure 3.4 Form and dimensions of test pieces for peel strength test 54 Figure 3.5 Process for applying... more water borne products in the future Industries other than shoe industry, such as automobile, furniture and electronic industries are expected to adopt the water borne adhesives only when the products are supplied in a stable and consistent manner The overall market size for the aqueous polyurethanes will grow tremendously in the very near future 2.6 Aqueous Polyurethane Dispersion Aqueous polyurethane. .. an excellent combination of high strength with high abrasion and environmental resistance, can be mass produced to precise dimensions Applications include hose and cable sheathing and so on Polyurethanes are also used in flexible coatings or textiles and adhesives for film, fabric laminates and footwear Paints and coatings give the highest wear resistance to floors and aircraft surfaces Binders are