8 General Compounding Harry G. Moneypenny Moneypenny Tire & Rubber Consultants, Den Haag, The Netherlands Karl-Hans Menting Schill + Seilacher ‘‘Struktol’’ Aktiengesellschaft, Hamburg, Germany F. Michael Gragg ExxonMobil Lubricants & Petroleum Specialties Company, Fairfax, Virginia, U.S.A. I. INTRODUCTION In conjunction with the chemicals used in a rubber formulation to ensure acceptable product characteristics, a number of ingredients may be incorpo- rated to allow or improve processing with the manufacturing equipment available in the plant. The stages of rubber processing may be broken down into raw materials handling, mixing, forming, and vulcanization. Some of the factors that may influence the process economics and product acceptability in these stages are listed in Table 1. The function of the processing additives is to minimize or overcome any problems associated with product fabrication while maintaining, or even improving, product performance. Before going into detail, some examples of acceptable performance criteria for processing additives at the four stages in product manufacture are presented briefly. Note: Throughout this chapter the authors make reference to suppliers of particular materials and their trade names. Mention of any company does not imply that it is the sole supplier of this material. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 365 Copyright © 2004 by Taylor & Francis Table 1 Rubber Processing—Performance Factors Stage Operation Performance factors Raw materials Storage, handling, 1. Temperature control weighing, blending, 2. Humidity control delivery 3. Handling of dusty and hazardous materials 4. Automatic handling and weighing 5. Weighing accuracy with small quantities 6. Uniformity of blending Mixing Internal mixer mill 1. Viscosity reduction in 2. Viscosity control 3. Heat generation 4. Filler incorporation 5. Filler dispersion 6. Hydrophobation reaction with silica 7. Homogenization 8. Sticking and release 9. Mix time Forming Extrusion, hot/cold feed 1. Flow 2. Sticking and releasecalendering, sheet/fabric 3. Shrinkage and stretchingcalendering, profile 4. Die swellcutting/joining fabric, 5. Dimensional stabilitybuilding 6. Tack 7. Green strength 8. Scorch 9. Surface appearance 10. Bloom 11. Fabric cord penetration Vulcanization Compression molding, 1. Scorch 2. Flowtransfer molding, 3. Component state of cureinjection molding, 4. Curative migration and dispersion continuous vulcanization 5. Mold release, fouling, cleaning 6. Surface appearance Source: Schill+Seilacher, Hamburg, Germany. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 366 Copyright © 2004 by Taylor & Francis A. Raw Materials Handling Chemicals are frequently dusty powders that are difficult to handle and to disperse. They can become electrostatically charged, and as a result incorpo- ration into a product is made more difficult. Also, dusty powders are undesirable for environmental reasons, and this has led to the use of binders and dispersing agents to improve materials handling and weighing. Generally preparations are coated, nondusting powders, granules, and masterbatches. B. Mixing During mixing in the internal mixer or open mill the additives should facilitate homogeneous blending of different polymers and enable faster incorporation of fillers and other compounding materials. Mixing should be optimized with respect to time, temperature, and energy. Compound viscosity should be reduced only to that level which allows acceptable processing in the ongoing manufacturing stages. Uniform distribution and optimum dispersion of all compounding materials should be achieved, and the influence on scorch time has to be minimal and/or controllable. If possible, the tackiness of the com- pound should be controlled. Both excessive sticking to the machines and bagging on the mill due to a lack of stickiness must be avoided. C. Forming Down-line processing, i.e., shaping of semiproducts, requires compounds with good flow properties. Profile compounds should calender and extrude easily, fast, and uniformly. The profiles should exhibit dimensional stability, smooth surface appearance, and exact edge definition. Temperature and die swell or shrinkage should be controllable and acceptable. For sheet calen- dering, a smooth surface, uniform shrinkage, and freedom from blisters are required. For metal wire or textile calendering, cutting, and joining, good flow properties and acceptable tack are required. Last but not least, bloom should be avoided. D. Vulcanization In the vulcanization process good flow properties are needed in order to 1. Obtain adequate compound–compound adhesion 2. Obtain compound–metal and/or compound–textile adhesion 3. Fill the mold quickly, uniformly, and free of blisters or trapped air, particularly with transfer and injection molding equipment. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 367 Copyright © 2004 by Taylor & Francis Finally, the vulcanizates should demold easily without tear and not produce mold-fouling residues. Processing additives may be subdivided according to their chemical structures (Table 2), or according to their application (Table 3). Several classes of substances can have more than one application. For example, fatty acid esters act as lubricants and dispersing agents. Mineral oils act as physical lubricants in rubber compounds, reducing viscosity, and also help in the filler dispersion process. In this chapter we discuss the following compounding ingredients with respect to their influence on processing behavior and their relevant compound vulcanizate properties: Physical and chemical peptizers Lubricants Homogenizing agents Dispersing agents Tackifiers Plasticizers Masterbatches—i.e. sulfur, accelerator, etc. Mineral oils II. PHYSICAL AND CHEMICAL PEPTIZERS A. Mastication Mastication is the process whereby the average molecular weight of a polymer is reduced by mechanical work. The resulting lower viscosity of the polymer Table 2 Processing Additives—Chemical Structure Group Examples Mainly hydrocarbons Mineral oils Paraffin waxes Petroleum resins Fatty acid derivatives Fatty acids Fatty acid esters Fatty alcohols Metal soaps Fatty acid amides Synthetic resins Phenolic resins Low M.W. polymers Polyethylenes Polybutenes Organothio compounds Peptizers Source: Schill+Seilacher, Hamburg, Germany. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 368 Copyright © 2004 by Taylor & Francis facilitates the incorporation of fillers and other compounding ingredients and can improve their dispersion. Because it is often difficult to homogeneously blend rubbers with very different viscosities, mastication of the higher vis- cosity rubber will enable improved blending with other, lower viscosity elas- tomers. Improved compound flow leads to easier down-line processing such as calendering and extrusion. Shorter processing time and lower power con- sumption are generally obtained. Table 3 Processing Additives—Applications Processing aid Application Examples Chemical peptizer Reduces polymer viscosity by chain scission 2,2V -Dibenzamidodiphenyl- disulfide Pentachlorothiophenol Physical peptizer Reduces polymer viscosity by internal lubrication Zinc soaps Dispersing agent Improves filler dispersion Mineral oils Reduces mixing time Fatty acid esters Reduces mixing energy Metal soaps Fatty alcohols Lubrication agent Improves compound flow and release Mineral oils Metal soaps Fatty acid esters Fatty acid amides Fatty acids Homogenizing agent Improves polymer blend compatibility Improves compound uniformity Resin blends Tackifier Improves green tack Hydrocarbon resins Phenolic resins Plasticizer Improves product performance at low and high temperatures Aromatic di- and triesters Aliphatic diesters Alkyl and alkylether monoesters Stiffening agent Increases hardness High styrene resin rubber Masterbatches Phenolic resins Trans-Polyoctenamer Softening agent Lowers hardness Mineral oils Mold release agent Eases product release from mold Decreases mold contamination Organosilicones Fatty acid esters Metal soaps Fatty acid amides Source: Schill+Seilacher, Hamburg, Germany. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 369 Copyright © 2004 by Taylor & Francis Because most of today’s synthetic rubbers are supplied with easy-to- process viscosity levels, the mastication process is mainly restricted to natural rubber. Although the natural rubber mastication process may be accomplished on an open mill, it is generally carried out in an internal mixer. During mechanical breakdown the long-chain rubber molecules are broken under the influence of high shear from the mixing equipment. Chain fragments with terminal free radicals are formed, which recombine to form long-chain molecules if they are not stabilized (Fig. 1). Through atmospheric oxygen the radicals are saturated and stabilized. The chains are shorter, the molecular weight is reduced, and the viscosity drops. The course of the chain breakdown of natural rubber is shown in Figures 2 and 3. Temperature is an important factor in the mastication of natural rubber. When the breakdown of natural rubber is plotted versus temperature (Fig. 4), it can be seen that the effect is lowest in the range of 100–130jC. Chain cleavage by the mechanical process is more efficient at low temper- Figure 1 Physical peptization of rubber. (Courtesy of Schill+Seilacher.) Figure 2 Physical peptization of rubber—reaction sequence. (Courtesy of Schill+Seilacher.) 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 370 Copyright © 2004 by Taylor & Francis atures (below 90jC) because, owing to the viscoelastic nature of elastomers, the shear is higher the lower the temperature. With increasing temperature the mobility of the polymer chains increases; they slide over one another, and the energy input and generated shear force drop. However, although the mechanical breakdown process is minimal around 120jC, above this temper- ature another breakdown process with a different mechanism, thermo- oxidative scission of the polymer chains, takes over and becomes more severe as temperature increases. An envelope curve is formed by the curves of the thermomechanical mastication and thermo-oxidative breakdown at elevated temperatures. In practice, the two reaction modes superimpose. Whereas the mechanical breakdown at low temperatures largely depends on the mixing parameters, the thermo-oxidative breakdown is accelerated by temperature and catalysts, i.e., peptizing agents. Free radicals are generated when the molecular chains of the rubber are broken by mechanical or thermo-oxidative means. These radicals may re- Figure 3 Physical peptization of polyisoprene. (Courtesy of Schill+Seilacher.) Figure 4 Peptization of NR. Viscosity reduction vs. temperature. (Courtesy of Schill+Seilacher.) 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 371 Copyright © 2004 by Taylor & Francis combine, and consequently no reduction in molecular weight and viscosity will be observed. Moreover, branching is likely to occur. The peptizing agents can act as radical acceptors, thus preventing recombination of the generated chain-end free radicals. All peptizing agents shift the start of thermo-oxidative breakdown to lower temperatures. Of the peptizing agents used in former times (Fig. 5), only combinations of specific activators with thiophenols, aromatic disulfides, and mixtures of the activators with fatty acid salts are now available. Note that for environmental reasons the chlorine-containing or polychlorinated thiophe- nols have largely been removed from use. The activators used in combination with a peptizing agent permit breakdown to start at lower temperatures and accelerate the thermo-oxida- tive process. They are chelates—complexes of ketoxime, phthalocyanine, or acetylacetone with metals such as iron, cobalt, nickel, or copper, but now- adays almost exclusively iron complexes. These chelates facilitate the oxygen transfer by formation of unstable coordination complexes between the metal atom and the oxygen molecule. This loosens the OUO bond, and the oxygen becomes more reactive. Because of the high effectiveness of the activators or boosters they are used only in small proportions in the peptizing agents. During recent times physical peptizers have gained major importance. They act as internal lubricants and reduce viscosity without breaking the Figure 5 Common peptizing agents. (Courtesy of Schill+Seilacher.) 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 372 Copyright © 2004 by Taylor & Francis polymer chains. Generally, zinc soaps have proved to be very effective in this role. Mechanical and chemical breakdown of the elastomer results in chain scission, lower molecular weight, broader molecular weight distribution, and an increased number of free chain ends. Normally this leads to an increase in compound heat buildup and a decrease in abrasion resistance. Lubricants do not change the molecular chains, i.e., the chains are not broken. As mentioned previously, synthetic rubbers are normally supplied with easy-to-process viscosity levels. If viscosity reduction is needed, mechanical mastication in an internal mixer has virtually no effect. In comparison to natural rubber, viscosity reduction of synthetic rubbers is more difficult owing to the 1) lower number of double bonds (SBR, NBR); 2) electron-attracting groups in the chain, which stabilize the double bonds; 3) vinyl side groups, which foster cyclization at high temperatures (NBR, SBR, CR); and 4) lower green strength due to the absence of strain-induced crystallization (NBR, SBR). Synthetic rubbers can be broken down by means of peptizing agents. However, they require higher dosage levels and temperatures than natural rubber. For this reason they are nowadays mostly physically peptized with salts of unsaturated fatty acids. B. Processing with Peptizing Agents At one time it was common practice to have a separate mastication stage whereby the peptizer was added to the NR and the mixing cycle was con- trolled to obtain an acceptable viscosity reduction. Nowadays normally only one stage is used, with the filler addition being delayed in order to allow the peptizing agent to be incorporated in the rubber. The early addition of the filler, while enhancing shearing and breakdown, also has a positive effect on dispersion. However, as the activators used in combination with the pep- tizing agent may be adsorbed by the filler, it is normal to increase loading slightly. When natural rubber is blended with synthetic rubber that has a lower viscosity, it is useful to peptize the natural rubber before the synthetic rubber is added. Because antioxidants inhibit the oxidative breakdown of rubber, they should be added late in the mixing cycle during the processing of natural rubber. With synthetic rubbers an early antioxidant addition can avoid cyclization. Figure 6 shows the influence of a number of chemical and physical peptizing agents on the breakdown, as measured by Mooney viscosity, of natural rubber (RSS1) in a 1 L laboratory internal mixer at 65 and 49 rpm and a start temperature of 90jC. Samples for Mooney viscosity testing were taken after 6, 9, 12, and 15 min. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 373 Copyright © 2004 by Taylor & Francis Comparable results are obtained when physical peptizers are used at higher dosage levels than the chemical peptizers. The raw RSS1 had a Mooney viscosity of 104. C. Influence of Peptizing Agents on Vulcanizate Properties The effects of a chemical peptizer (STRUKTOLR* A 86, an aromatic di- sulfide in combination with a metal organic activator), a physical peptizer (STRUKTOLR A 60, based on unsaturated fatty acid salts of zinc), and mechanical mastication on viscosity reduction and the tensile properties of NR (SIR 5 L) have been investigated. Apart from the usual evaluation of viscosity at low shear rates (i.e., Mooney viscosity, ML 1 + 4V, 100jC), vis- cosity at higher shear rates, using a rubber processing analyzer (RPA), was measured. The data are shown in Table 4. Under low shear conditions the chemical peptizer is by far the more effective method for viscosity reduction. However, under higher shear, which Figure 6 Chemical vs. physical peptizers in NR. STRUKTOLR A 82 is a chemical peptizer containing an organic metal complex booster. STRUKTOLR A 86 combines a chemical peptizer and a booster. Its composition is similar to that of STRUKTOLR A 82 but with a higher concentration of active substance. STRUKTOLR A50P contains zinc soaps of high molecular weight fatty acids. STRUKTOLR A 60 is similar to STRUKTOLR A 50 P but has a lower melting range, allowing open mill mixing. (Courtesy of Schill+Seilacher.) * STRUKTOL is a registered trademark of Schill+Seilacher ‘‘ Struktol’’ AG, Hamburg, Germany. 4871-9_Rodgers_Ch08_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 374 Copyright © 2004 by Taylor & Francis [...]... in Figure 14 R is typically a tertiary alkyl group Wolney and Lamb (21) studied, in a blend of oil-extended SBR and NR, the effect on tack of novolaks prepared using o-sec-butylphenol, p-sec-butylphenol, p-tert-butylphenol, p-tert-amylphenol, and p-tert-octylphenol All polymers had approximately the same molecular weight, free monomer level, and melting point It was found that novolaks based upon the... tack retention decreased with increasing free monomer level The optimum number-average molecular weights for tack with resins based on p-tert-butylphenol and p-tert-octylphenol were 850 and 1350, respectively Rhee and Andries (17) studied the effect of molecular weight and loading of tackifying resin on autohesion of NR and SBR rubbers Figure 14 General structure of phenol–formaldehyde novolak resin Copyright... calendering and demolding and reduce mold fouling in critical polymers such as ethylene oxide epichlorohydrin copolymer (ECO) or fluoropolymers such as FKM Polyethylene and polypropylene waxes of low molecular weight are easily dispersed in natural rubber and synthetic rubbers They act as lubricants and release agents They improve the extrusion and calendering of dry compounds in particular and reduce... STRUKTOLR A 50 P 10 13.6 19. 1 430 60 21 9 10.7 18.3 480 60 24 12 10.3 18.4 480 60 24 Source: Schill+Seilacher, Hamburg, Germany Copyright © 2004 by Taylor & Francis IV HOMOGENIZING AGENTS A Examples and Function Homogenizing agents are used to improve the homogeneity of difficult-toblend elastomers They assist in the incorporation of other compounding materials, and intrabatch and batch-to-batch viscosity variation... these parts is between 1 and 5 phr Because of their high effectiveness, however, low dosages are often sufficient Very high filler loadings may require higher dosages A typical product is STRUKTOLR W 33, a mixture of fatty acid esters and metal soaps that allows fillers to be rapidly incorporated and dispersed, particularly when high loadings have to be processed Agglomerations are avoided and batch-to-batch...Table 4 Influence of Chemical and Physical Peptizers on Viscosity Reduction Mechanical mastication Chemical peptizer Physical peptizer ML (1+4V), 100jC 94 65 83 Shear stress (secÀ1) 55.3 99 .7 299 .1 5.017 3.125 1.344 Viscosity (Pa-sec) 5.536 3.125 1.3 29 4.325 2.626 1.113 Source: Schill+Seilacher, Hamburg, Germany is a better simulation of factory... distribution and in their degree of unsaturation The most important fatty acids are listed in Table 7 Separation and purification processes lead to specified technical grade fatty acids that are the basis for tailor-made lubricants in rubber processing The fatty acids tend to be incompatible and therefore insoluble in the rubber hydrocarbon, and consequently they can migrate to the surface of the uncured rubber. .. reasons, and this led to the relatively early use of binders and dispersing agents by the chemical industry Generally preparations are coated, nondusting powders, granules, and masterbatches; a few are pastes Powders that are easy to process are mostly mixtures of fine particle size chemicals with oil and/ or dispersing agents The very homogeneous mixtures are nondusting, are easy to handle and weigh, and. .. straight polystyrene can hardly be processed in rubber compounds, copolymers of styrene and butadiene with higher styrene contents have proven their worth Terpene resins are very compatible with rubber and give high tackiness However, they are used mainly for adhesives The polymers are based on aand h-pinene The cyclobutane ring is opened during polymerization and polyalkylated compounds are formed (Fig... mineral rubber and is a good processing additive, for example, in difficult-to-process compounds that have a high percentage of polybutadiene Mineral rubber is also successfully used to improve the collapse resistance of extrusions Rosins are natural products obtained from pine trees They are mixtures of organic substances, for the most part doubly unsaturated acids, such as abietic acid, pimaric acid, and . difficult-to- blend elastomers. They assist in the incorporation of other compounding materials, and intrabatch and batch-to-batch viscosity variation are reduced by their use. They are resin-based. FKM. Polyethylene and polypropylene waxes of low molecular weight are easily dispersed in natural rubber and synthetic rubbers. They act as lubri- cants and release agents. They improve the extrusion and calendering. Reduction Mechanical mastication Chemical peptizer Physical peptizer ML (1+4V), 100jC94 65 83 Shear stress (sec À1 ) Viscosity (Pa-sec) 55.3 5.017 5.536 4.325 99 .7 3.125 3.125 2.626 299 .1 1.344 1.3 29 1.113 Source: Schill+Seilacher, Hamburg,