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a study on epoxidised natural rubber
1 A STUDY ON EPOXIDISED NATURAL RUBBER FOR POSSIBLE APPLICATION IN TUBELESS TYRE INNER LINER Roland Ngeow # , Khaw Pei Chin and Che Su Mt Saad Technology & Engineering Division, Rubber Research Institute of Malaysia, Malaysian Rubber Board, 47000 Sungai Buloh, Selangor, Malaysia. # Corresponding author. Tel.: +6 (0)3 6156 1121; fax: +6 (0)3 6156 4418; E-mail address: ywngeow@lgm.gov.my ABSTRACT Epoxidised Natural Rubber containing 50 mole % epoxidation (ENR 50) has been shown to have unique set of properties. One of these properties is reflected in vulcanisates with excellent air retention characteristics which is comparable to that of butyl rubber. A low air permeability characteristic is important for tubeless tyre inner liner to ensure the performance and durability of the tyre during service. Other requirements for tyre inner liner compound are good processibility and good physical properties. In this paper, ENR 50 is studied and compared with blends of ENR 50 and SMR 20 at different ratios. Natural Rubber (NR) possesses good processing properties including high green strength, as well as excellent cured physical properties. Keywords: Epoxidised natural rubber (ENR), ENR 50, tyre, inner liner 1.1 Introduction Epoxidised Natural Rubber (ENR) is produced by modification of natural rubber (NR) latex. In general, the desire % mole epoxidation can be achieved at any level, however only two grades are available commercially. These grades are marketed as Ekoprena 25 and Ekoprena 50 by Malaysian Rubber Board for ENR 25 and ENR 50 with epoxide contents of 25 % mole and 50 % mole respectively. The epoxidation of NR has progressively reduced the air permeability ability of NR as the % mole epoxidation increased. Studies has found that the air retention characteristic of ENR 50 is as good as halogenated butyl rubber which is an important requirement of tubeless tyre inner liner application 1,2 . Tyre inner liner is used for air barrier in the inner surface of a tyre to retain the internal tyre pressure. Tyre with inflation pressure at optimum level ensure the durability and performance of a tyre, resulting in better performance in term of better fuel economy, handling and ride compared to tyre that is not inflated at optimum pressure. Besides that, tyre with optimum pressure can prolongs the tyre carcass and tread lives and contributes to even tread wear. According to French tyres manufacturer - Michelin, the rolling resistance of a tyre is accounts for 20% - 30% energy consumption in an average vehicle 3,4 and 25% of world wide carbon dioxide emissions are generated by road traffic 4 . This showed that maintaining the optimised inflation pressure of a tyre plays an important role in our environment. 2 Other requirements for tyre inner liner compound are good processibility and physical properties. The compound must be easily calendered to give smooth and even thickness during tyre manufacturing process. NR is known of possessing good processing characteristics and high green strength. Blending NR with ENR 50 provides compound that exhibits the unique qualities of these respective polymers for tyre inner liner application and promote the usage of sustainable materials for tyre compound development. According to International Rubber Study Group, 44% of 22.7 million tonnes of world rubber consumption is source from natural rubber in year 2009 5 and majority of this rubber is used for the tyre industry. Tyre inner liner compound occupied approximately 5% of the total tyre weight. In general synthetic rubber such as halogenated butyl rubber (HIIR) is conventionally used for tyre inner liner. Replacing part of the tyre components with sustainable materials will greatly reduce the usage of our depleting natural resources and meeting the stringent carbon dioxide gas emission regulations made by policy makers. In this study, ENR 50 is blend with NR to obtain the optimum properties for tubeless tyre inner liner application. Further study is carried out to compare ENR 50/ NR blend at 70 : 30 % by weight with Bromobutyl (BIIR) compounds. 2.1 Material and Method 2.2 Preparation of ENR/ NR Compounds Raw rubbers ENR 50 and SMR 20 are obtained from Malaysian Rubber Board. All the vulcanisates are prepared according to standard techniques. Laboratory Banbury mixer size 1.6 L (BRL600) is used to blend the polymers and chemicals. The mixer is set at 110 r.p.m. rotor speed and the initial temperature is set at 70 o C with 0.7 filled factor. The mixing cycle is as follows for ENR 50/ NR blend: 0 min : Add polymers + calcium stearate ½ min : Add powders (zinc oxide, stearic acid, antioxidant, etc.) 1 min : Add Carbon Black (CB) and processing oil 2 ½ min : Sweep down feed chute 3 ½ min : Dump In all the ENR 50/ NR compounds preparation, insoluble sulphur and accelerators are added later on a Carter 9” x 18” two-roll mill and the compounds are cured to optimum (t 95 ) at 150 0 C using electrical press. The semi-EV ENR 50/ NR tyre inner liner formulation employed in this study is shown in Table 1: 3 Table 1: ENR 50/ NR Tyre Inner Liner Formulations Ingredient p.p.h.r. Polymers 100 Calcium Stearate 2 Zinc Oxide 4 Stearic Acid 2 Carbon Black N660 55 Escorez Resin 5 Tudalen 65 10 Santoflex 13 1 Permanax TMQ 1 Insoluble Sulphur 0.7 TBBS 1.8 The ratios of ENR 50 and NR blends are shown in Table 2. ENR 50 are taken as control. Six compounds are prepared and shown in Table 2. Table 2: ENR 50/ NR Blend Ratios Used for Tyre Inner Liner Study The physical testing procedures are done in accordance to the International Standard Organisation (ISO) or British Standard. 2.3 Scanning Electron Microscope (SEM) Analysis Joel FESEM JSM 6701F model is used in this study for morphology analysis. The instrument is operated at 2kV with 15mm working distance. The analysis is based on semi-quantitative measurement of element extracted from a specific window size. Samples are placed onto the specimen stub and examined by evaporative coating with ultra-thin layer of platinum under high vacumm which provided a conducting layer that permits SEM examinations. 2.4 Filler Dispersion Analysis Reflective Light Microscope disperGrader+ TM of Dynisco is used to measure the filler dispersion. Samples are cut to generate a ‘fresh face’ for analysis. By utilising the reflected light method, the disperGrader scanned the samples surface, measured and quantify filler dispersion and agglomerate size. The ‘flat areas’ are seen as black (no reflected light) and agglomerates reflect light to the sensor and are seen as grey to white. This device is complies with ISO 11345 with 30x magnification. Polymer Polymer Ratio 1 2 3 4 5 6 ENR 50 100 80 70 65 60 50 NR 0 20 30 35 40 50 4 3.1 Results and Discussions 3.2 Processing Properties Understanding the processing properties of compound are important in the tyre industry to ensure the consistency and quality of the final product. Table 3 showed the processing properties of ENR 50/ NR compounds. Table 3: Processing Properties of ENR 50/ NR Compounds From Table 3, the cure time (t 95 ) decreased with increasing NR contents in general except for compound 4 and compound 6. This may due to the homogeneity of the blends. It is observed that NR has an effect on the cure time (t 95 ) and the Mooney Scorch (t 5 ) of the compounds. It is noteworthy that in general, the cure time (t 95 ) and the Mooney Scorch (t 5 ) are reduced when NR contents increased. The rheographs of compounds are shown in Figure 1. Compound 1 consists of ENR 50 without any blend with NR showed the lowest minimum torque. From the figure, it is clear that compound 1 is undergoing cure reversion which is not good for tyre application. However, it is no longer observed when NR is incorporated into ENR 50. It is postulated that NR increases the crosslink density in the rubber matrix due to higher double bond density in NR and improves the cure reversion resistance of ENR 50/ NR compounds. Figure 1: Rheographs of Compounds Processing Properties Compound 1 2 3 4 5 6 Compound Viscosity (ML 1+4 at 100 o C) 37.7 35.1 36.2 37.1 36.5 37.2 Mooney Scorch, t 5 (Min: Sec t 120 o C) 22:15 21:29 20:51 20:41 20:23 19:52 Rheo Cure, t 95 (Min: Sec) 4:48 4:37 4:32 4:40 4:15 4:18 0 1 2 3 4 5 6 7 0 500 1000 1500 2000 Time (Second) Torque (dNm) Compound 1 Compound 2 Compound 3 Compound 4 Compound 5 Compound 6 Torque (dNm) Time (second) 5 3.3 Scanning Electron Microscope (SEM) Analysis The physical properties of the blends are largely determined by phase morphology and compatibility. These effects are particularly marked with regard to air permeability efficiency, tear strength and elongation at break. SEM micrographs of ENR 50/ NR compounds at different blend ratios are presented in Figure 2. Figure 2: SEM Micrographs of Compounds ENR 50/ NR at 100:0 % by weight showed finer morphology with relatively smooth surface compared with the rest of the compounds due to the presence of single polymer. In the 65:35 %, 60:40 % and 50:50 % by weight of ENR 50/ NR compounds, the rough surfaces became apparent and higher number appearance of agglomeration are observed in the rubber matrix when ENR 50 contents decreased. The morphology of these blends indicates that the continuous phase of ENR 50 components are reducing and moving towards the co-continuous phase between ENR 50 and NR components. The SEM morphology study indicates lower chemicals dispersion rates when ENR 50 contents decrease. 6 3.4 Filler Dispersion Analysis Reflective light microscope is used to measure the filler dispersion. Many mechanical properties of vulcanisates are directly affected by filler dispersion, i.e. tensile strength and tear strength. Figure 3 and Figure 4 showed the average filler agglomeration sizes and percentage of fillers dispersion in the compounds. Figure 3: Average Agglomeration Sizes of Compounds Figure 4: Percentage of Fillers Dispersion in The Compounds It is observed that the percentage of CB dispersion decreased when the ENR 50 contents decreased. It is also found that the average CB agglomerate size increased when ENR 50 is decreased. The observation is consistence with SEM micrographs analysis where higher number of agglomerates are observed when ENR 50 contents decreased. The percentage of CB dispersion is above 97% for blending of ENR 50 above 65% by weight in the compound. The study postulated that ENR 50 improved the CB dispersion in the blending of ENR 50 and NR. 6 8 10 12 14 123456 Average Agglomeration Size (μm) Compound 0 20 40 60 80 100 123456 Dispersion (%) Compound 7 3.5 Physical Properties of Compounds The physical properties of the ENR 50/ NR compounds are shown in Table 4. It is observed that hardness values are approximately the same for ENR 50/ NR compounds. It is also observed that the tear strength increased with the increased of NR contents. Table 4: Physical Properties of Compounds A small volume of discrete phase (NR) may determine the path of tear strength tensile strength and elongation at break. Some of the physical properties are inconsistent when NR contents increased. Is it postulated that Payne effect 6 may have occurred due to stronger filler-filler interaction hence poorer CB dispersion as observed in filler dispersion analysis and lead to inconsistency of these properties. The air permeation coefficient of the compounds are measured according to ISO 2782 and the results are showed in Figure 6. The lower the air permeation coefficient, the better the tyre inner liner in retaining the tyre pressure. As expected, the air permeation coefficient of compounds are dependent on the level of ENR 50 contents in the compounds. Figure 6: Air Permeation Coefficient of Compounds at 23 o C Physical Properties Compound 12345 6 Hardness (IRHD) 56 60 63 63 62 62 Tear Strength (N/mm) 22.7 23.9 26.5 30.8 31.4 41.4 M100 (MPa) 1.46 1.5 1.52 1.56 1.49 1.49 M300 (MPa) 5.76 6.0 5.85 5.88 5.66 5.73 Tensile Strength (MPa) 12.6 13.0 15.0 14.0 14.2 14.0 Elongation at Break (%) 520 513 576 565 574 562 0.0 1.0 2.0 3.0 4.0 5.0 6.0 123456 Air Permeation Coefficient (m 2 /Pa.s x 10 -17 ) Compound 8 The results showed that ENR 50/ NR compound at 70 : 30 % by weight will be generally be good for tyre inner liner with high tensile strength and elongation at break as well as relatively moderate modulus physical properties and air permeability. 3.6 COMPARISON OF BROMOBUTYL AND ENR 50/ NR BLEND In this study, comparison between ENR 50/ NR blend at 70:30 % by weight and bromobutyl rubber (BIIR) are carried out. ENR 50 and NR compounds are prepared as control. Table 5 showed the physical properties of the compounds. Table 5: Physical Properties of Compounds Table 5 showed that BIIR tear strength and elongation at break are higher than ENR 50/ NR compound. However, the hardness, tensile strength, M100 and M300 of ENR 50/ NR compound are higher compared with BIIR compound due to higher filler- rubber interaction and crosslink density. The results for air permeation coefficient of BIIR and ENR 50 compounds are comparable. The air permeation coefficient of ENR 50/ NR is higher than BIIR compound due to the reduction of ENR 50 contents in ENR 50/ NR blend. However, the air permeation coefficient of ENR 50/ NR at 70 : 30 % by weight is acceptable for tyre inner liner 7 . Besides, tyre inner liner air permeability does not necessary causes the whole tyre loss its pressure. The tyre construction parameters do have effect on the whole tyre inflation pressure loss rate. 4.1 CONCLUSIONS The study showed that the physical properties of ENR 50/ NR blends are largely determined by the phase morphology. The average filler agglomerate size increased is in consistence with the observation through SEM micrographs where relatively higher number of agglomerates are observed. The study postulated that ENR 50 improved the dispersion of CB in ENR 50/ NR compound. Blending NR with ENR 50 has improved the processing property of ENR 50/ NR blend. In general, the cure time (t 95 ) and the Mooney Scorch (t 5 ) are reduced when NR Physical Properties Compound BIIR ENR 50/ NR 70/30 ENR 50 NR Hardness (IRHD) 53.0 60.0 59.0 44.0 Tear Strength (N/mm) 34.7 24.7 19.2 55.3 M100 (MPa) 0.71 1.58 1.47 0.83 M300 (MPa) 2.37 6.72 6.58 4.05 Tensile Strength (MPa) 7.65 19.2 18.4 18.3 Elongation at Break (%) 686 619 582 638 Air Permeation Coefficient at 23 o C (m 2 / Pa.s x 10 -17 ) 0.534 2.43 0.646 8.64 9 contents increased. Incorporation of ENR 50 in NR gives improvements in vulcanisation rates and cure reversion resistance. The tensile strength of ENR 50/ NR compound are higher than BIIR compound. However, the tear strength, modulus and air impermeation of ENR 50/ NR is lower than BIIR compound. ENR 50/ NR at 70 : 30 % by weight compound is preferred for tubeless tyre inner liner application in corresponding with the rest of the ENR 50/ NR blend ratios. References 1. The Rubber Research Institute of Malaysia, Epoxidised Natural Rubber, (1984). 2. P.C. Loh and M.S. See Toh, Epoxidised Natural Rubber in Tubeless Tyre Inner Liners, Int. Rubb. Conf., (1985). 3. Alexander H. Tullo, Stretching Tires ‘Magic Triangle’. Chemical & Engineering News, Vol. 87 (46),10-14. 4. Joachim Neubauer, Get a grip, Tire technology International, 24 – 26 5. International Rubber Study Group Vol. 64 (1-3), (July – September 2009). 6. Payne AR. A, Note on Conductivity and Molulus of Carbon Black Loaded Rubber, J. Appl. Polym. Sci., (1965), Vol.9, 1073-1082. 7. Uday Karmarkar, Ana Barbur, Edward R. Terrill, Mark Centea, Larry R. Evans, James D. MacIsaac Jr., Tire Aging Test – Tire Inner Liner Analysis, National Highway Traffic Safety Administration, (2010). 8. C.S.L. Baker, I.R. Gelling and I.R. Wallace, Recent Development in Natural Rubber for Tyres, Proc. Int. Rubber Technology. Conf., (1988), 467-491. 9. C.S.L. Baker, I.R. Gelling and R. Newell, Epoxidised Natural Rubber, Rubber Chem. Technology, Vol.58(1), (1985), 67-85. 10. Haidzir Abdul Rahman, I.R. Gelling and P.K. Freakley, Influence of Phase Morphology on the Properties of Natural Rubber and Epoxidised Natural Rubber Blends., J. Nat. Rubb. Res., Vol.9(4), (1994), 213-225. 11. I.R. Gelling, Epoxidised Natural Rubber, J. Natural Rubber Research, Vol. 6(3), (1991), 184-205. 12. LANXESS International SA, LANXESS Butyl Rubber, (2008), LANXESS International SA, Switzerland. 13. R.P. Brown, S. Cook, J. Patel, and A.J. Thinker, Enhance Passenger Tire Performance from Sustainable Resources. Proc. of Tire Technology Expo Hamburg, (2009).