NSBR) and Styrene Butadiene Rubber (SBR

PRÜFEN UND MESSEN TESTING AND MEASURING

Rubber blend · styrene butadiene rubber · nitrile rubber · acrylonitrile-styre- Comparison on Properties of ne-butadiene rubber · oil resistance · mechanical properties Acrylonitrile Styrene Butadiene Compounds and vulcanisates of acrylo- Rubber (NSBR) and Styrene nitrile-styrene-butadiene rubber (NSBR) were prepared, and their properties were compared with those of conventi- Butadiene Rubber (SBR) / Nitrile onal blends between styrene butadiene rubber (SBR) and acrylonitrile-butadie- Rubber (NBR) Blends ne rubber (NBR). Results reveal discrepancy in cure behaviour of NSBR from SBR/NBR blends to some extent, i.e., The main purpose of blending two or mer with those of SBR/NBR blends. By the NSBR yields inferior crosslink densi- more rubbers together is typically to this means, a general guideline for selec- ty to the SBR/NBR blends, and thus the combine inherent properties of the indi- ting the appropriate materials having a hardness. By contrast, the cured NSBR vidual components to a single material. balance of processability, resistances to specimens offer superior tensile Such enhanced properties include me- abrasion, crack growth, and hydrocarbon strength and processability. Payne chanical properties and processability, oil could be drawn. effect as an indication of filler network and in some circumstances the product formation magnitude is slightly greater cost could be diminished [1-4]. In practi- Experimental in the NSBR system. In summary, it is ce, a physical blending is the most widely Materials possible to substitute the SBR/NBR used technique for preparing rubber All materials used in this work were used blends with single NSBR eliminating blends because of its ease of operation as-received. Styrene butadiene rubber the complexity in blending process with cost effectiveness [1]. However, the (SBR 1502; styrene content of 23.5%) and while giving advantageous process- blending technique is still restricted by a acrylonitrile–butadiene rubber (NBR: ability and tensile strength. difficulty in mixing process particularly N230; acrylonitrile content of 35%) as in the blends of polarity and viscosity raw rubbers were manufactured by mismatching systems. Bangkok Synthetics Co., Ltd. (Rayong, Styrene butadiene rubber (SBR) is one Thailand) and Japan Synthetic Rubber Eigenschaftsvergleich von Nit- of the most useful synthetic rubbers, par- Co., Ltd. (Tokyo, Japan), respectively. Acry- ril-Styrol-Butadienkautschuk ticularly in tyre industry, because of its lonitrile-styrene-butadiene rubber (NS- good resistances to abrasion and crack (or BR: Sabor™ DT100; acrylonitrile content mit Verschnitten aus Styrol- cut) growth [5]. However, its oil resistance and styrene content of 30% and 5%, res- Butadien Kautschuk und Nit- is considerably poor. Conversely, acrylonit- pectively) was produced by Lion Copoly- rilkautschuk rile-butadiene or nitrile rubber (NBR) is a mer, LLC. (Baton Rouge, USA). Carbon polar elastomer possessing high resis- Black (N550) as reinforcing filler was Kautschukverschnitte · Styrol-Butadien tance to non-polar substances such as supplied by Thai Carbon Product Co., Ltd. Kautschuk · Nitrilkautschuk · SBR/NBR hydrocarbons oils and solvents, but its (Bangkok, Thailand). Stearic acid as cure Verschnitte · Ölbeständigkeit · mecha- mechanical properties are considerably activator was purchased from Petch Thai nische Eigenschaften poor compared with those of SBR. There- Chemical Co., Ltd. (Bangkok, Thailand). fore, blending of these two polymers is Zinc oxide (ZnO) as cure activator and Mischungen und Vulkanisate aus Nitril- possible to combine the individual pro- sulphur (S8) as vulcanising agent were Styrol-Butadienkautschuk (NSBR) wur- perties to a single material. Notably, be- den hergestellt und die rheologischen Ei- cause of the difference in polarity bet- genschaften, Vernetzungscharakteristik ween SBR and NBR, the thermodynamic sowie die mechanischen Eigenschaften incompatibility with poor batch-to-batch (Härte, Zugfestigkeit, Rollwiderstand) Authors consistency is resulted, leading to poor und Ölbeständigkeit im Vergleich zu kon- mechanical properties [6]. Typically, the Yotwadee Chokanandsombat, ventionellen SBR/NBR Verschnitten er- control of state-of-mix and/or the incor- Supanut Owjinda, Chakrit mittelt. Die Ergebnisse zeigen Diskrepan- poration of compatibiliser are used to Sirisinha, Bangkok zen bezüglich des Vernetzungsverhaltens overcome such problem. The production und der Härte auf. Vulkanisate aus NSBR of a copolymer, namely, acrylonitrile-sty- Corresponding author: zeigen eine bessere Verarbeitbarkeit und rene-butadiene rubber (NSBR) is therefore Chakrit Sirisinha eine höhere Zugfestigkeit. Der Payne Ef- an alternative to the SBR/NBR blends be- Department of Chemistry and fekt als Ausdruck des Füllstoffnetzwerks, cause the NSBR integrates advantages of Center of Excellence for Innovati- ist bei NSBR geringfügig stärker ausge- SBR in terms of processability and abrasion in Chemistry Faculty of Science prägt. Daher können SBR/NBR Verschnit- on resistance, and of NBR in views of oil Mahidol University te durch NSBR ersetzt werden. resistance. The present work aims to in- Rama 6 Rd. Figures and Tables: vestigate the potential use of NSBR to Bangkok 10400, Thailand By a kind approval of the authors substitute conventional SBR/NBR blends FAX: +662-441-0511 by comparing properties of NSBR copoly- E-Mail: [email protected] www.kgk-rubberpoint.de KGK · 11/12 2012 41 PRÜFEN UND MESSEN TESTING AND MEASURING

supplied by Chemmin Co. Ltd., (Samuth- 1 Table 1. Rubber formulation (in phr*) prakarn, Thailand). N-t-butyl-2-benzo- Chemicals Sample codename thiazolesulfenamide (TBBS) as cure acce- S0N100 S25N75 S50N50 S75N25 S100N0 NSBR lerator was purchased from Kitpiboon SBR1502 0 25 50 75 100 0 chemical Ltd., Part. (Bangkok, Thailand). NBR 100 75 50 25 0 0 Rubber preparation NSBR 0 0 0 0 0 100 The rubber blends were prepared at 5 dif- ZnO 3 3 3 3 3 3 ferent SBR/NBR composition ratios of Stearic acid 1 1 1 1 1 1 100/0, 75/25, 50/50, 25/75 and 0/100. Sulphur 1.75 1.75 1.75 1.75 1.75 1.75 The compounding ingredients, as shown TBBS 1 1 1 1 1 1 in Table 1, were prepared with a laborato- CB; N550 50 50 50 50 50 50 ry-size internal mixer (Haake Rheomix90, *parts per hundred part of rubber Germany) equipped with cam-type rotors. The rubber compounds were prepared 2 based on a fill factor of 0.75 with a two- Table 2. Cure characteristics of rubber compounds stage mixing technique at set tempera- Rubber systems ts2; Scorch tc90; Cure Minimum Maximum Torque ture and rotor speed of 50 °C and 50 rpm, time (min) time Torque Torque (dNm) difference (dNm) (dNm) respectively. In the first mixing stage, the ZnO, stearic acid and carbon black were S0N100 2.41 9.14 1.497 22.72 21.22 incorporated sequentially into raw rubber S25N75 3.49 10.13 2.126 22.57 20.44 with total mixing time of 12 minutes. In S50N50 4.15 10.25 2.603 21.75 19.15 the second stage, the rubber mix prepa- S75N25 4.57 10.41 2.653 21.76 19.11 red from the first mixing stage was masti- S100N0 6.09 17.08 2.329 19.47 17.14 cated for 2 minutes at 50 °C, and then the sulphur and TBBS were incorporated into NSBR 3.52 13.50 2.031 20.89 18.85 the rubber mix, and allowed 5 minutes to achieve good distribution and dispersion 1 of all ingredients. 200 ain 180 Characterisation 160 Cure behaviour was monitored with the 140 use of rubber process analyser (RPA; Al- 120 pha Technology, USA). Cure characteristics of rubber compounds, including 100

scorch time (ts2), cure time (tc90), mini- ain - G‘ @ 100%str 80 mum torque (ML) and maximum torque 60 (MH), were investigated at 155 °C. Torque

56%str 40 difference (MH – ML) was taken as an 20 indication of the crosslink densities of 0 the vulcanisates [7]. In order to prepare G‘ @ 0. vulcanised specimens (aka. vulcanisates), SON100 S25N75 S50N50 S75N25 S100NO NSBR the uncured compounds were compres- Sample codename sion moulded using a hydraulic hot-press Payne effect of the SBR/NBR blends and NSBR compound filled with 50 phr carbon black under pressure of 16 MPa at 155 °C for the optimum cure time (tc90) as pre-determined from the RPA. Apart from cure was scanned from -60 to 80 °C at 2 °C/ of oil resistance [9]. characteristics, the rheological behavi- min under static strain, dynamic strain σ 0–2 our of rubber compounds was determi- and frequency of 1%, 0.1% and 10 Hz, Relative tensile strength = σ 0–1 ned by RPA with strain sweep tests at respectively. The tanδ at 60 °C was used Equation (1) temperature and frequency of 100 °C to represent the magnitude of rolling re- and 0.99 rad/s, respectively. sistance of vulcanised rubber [5, 7]. Glass where σ 0–1 and σ 0–2 are the tensi- Hardness of vulcanisates was measu- transition temperature (Tg) was determi- le strength before and after oil immersi- red using a durometer with Shore A scale ned from the temperature at the dam- on, respectively. (Cogenix Wallace, UK) as per ASTM ping peaks [8]. Bound rubber content is known to be D2240. Tensile properties were determi- Measurement of oil resistance in this a measure of the filler-rubber interaction ned using a universal testing machine work was based on changes in tensile [10]. As the filled compound is extracted (Instron 5566, USA) according to ASTM D properties, according to ASTM D471. The with good solvent, the gel-like rubber 412, Die C. Dynamic mechanical test of specimens were immersed in hydraulic bound to the filler surfaces will not be vulcanisates was carried out by dynamic oil at room temperature for 7 days. Rela- soluble while the rest will go into soluti- mechanical analyser (Gabo, Eplexor 25N, tive tensile properties as calculated from on. In this work, the determination of Germany) in tension mode. Temperature Equation (1) were used as an indication bound, impenetrable (and therefore in-

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2 are shown in Table 2. Evidently, in SBR/ 45 NBR blends, scorch time and cure time are Immersed in toluene shortened as NBR content increases. Simi- 40 lar findings were reported elsewhere [1, 35 Immersed in toluene and acetone sequentially 2]. The results might be explained by the polarity of the bulk, depending strongly 30 on NBR content. Such polarity might af- 25 fect the decomposition of TBBS to form ammonium mercaptide, which functions 20 as actual cure accelerator [13]. Compared 15 with the blend systems, the NSBR offers % Bound rubber similar tendency of scorch safety and 10 short cure time to the 25/75 SBR/NBR 5 blend, which is due probably to the corresponding acrylonitrile content in NSBR. 0 Also, the minimum torque (ML) of NSBR is SON100 S25N75 S50N50 S75N25 S100NO NSBR slightly lower than SBR/NBR rubber blend Sample codename systems, indicating the superior processability of the NSBR to the blend systems. Bound rubber of SBR/NBR blend and NSBR compounds filled with 50 phr carbon black The maximum torque (MH) and torque difference (MH-ML) of SBR/NBR blends 3 somewhat decrease with increasing SBR content. However, the NSBR system shows slightly smaller magnitude of crosslink density, compared with the blend systems, as evidenced by its somewhat lower values of MH-ML. It has been reported that the storage modulus (G’) of rubber compounds is cont- rolled mainly by following effects: (i) hyd- rodynamic effect of rigid filler in rubber matrix; (ii) crosslink network connecting the polymer molecules; (iii) chemical and physical interactions of rubber-filler and filler-filler [14]. Since the filler-filler-interaction is strain-dependent, the filler transient network could be destroyed at suffici- ently high strain. Such strain is considerably smaller than the strain required for dis- rupting the polymer-polymer network, i.e., well below 100% strain [14]. The decrease in G’ with increased strain is sometimes known as “Payne effect” which is used as a Schematic representation of bound rubber and occluded rubber in rubber compound measure of filler transient network magni- filled with carbon black [modified on Leblanc’s work as reported in ref 19] tude in rubber compounds [14-17]. Referred to Figure 1, the magnitude of soluble) rubber content based on the the rubber specimens were dried at 80 °C discrepancy in G’ at low (i.e., 0.56% gravimetric method was conducted with for 24 hours. The percentages of bound strain) and high strains (i.e., 100% strain), toluene as good solvent for SBR, and ace- rubber contents were calculated from is considered as the Payne effect magni- tone as good solvent for NBR and NSBR Equation (2) [8, 12]. tude. Evidently, the magnitude of Payne W – W[m /(m + m )] [11]. Small pieces of approximately 1.5 g fg f f p effect is highest in the NSBR system. In % Bound rubber content (% BRC) = x 100 W[m /(m + m )] of uncured specimens were immersed in p f p SBR/NBR blends, the Payne effect is more 80 mL of solvent for a total of 7 days at Equation (2) pronounced with increasing SBR content, room temperature, and then filtered out. implying the high magnitude of transi-

The residues after extraction were dried where Wfg is the weight of filler–rub- ent filler network formation. Additio- in hot-air oven for 24 hours at 80°C. In ber gel, W is the weight of the test speci- nally, the magnitude of Payne effect of the case of SBR/NBR blends, the uncured mens. The mf and mp are the weights of the blends is found to be greater than rubbers were immersed in 80 mL of to- filler and polymer in the rubber com- that of filled SBR and filled NBR, demons- luene during 7 days. Thereafter the ma- pound, respectively. trating the “synergistic effect” in SBR/ terials were immersed in acetone for 7 NBR blends. Not only the Payne effect, days, to remove non-bound NBR portion Results and Discussion but also the mechanical properties exhi- being insoluble in toluene. Subsequently, Cure characteristics of rubber compounds bit the synergistic effect, which is advan- www.kgk-rubberpoint.de KGK · 11/12 2012 43 PRÜFEN UND MESSEN TESTING AND MEASURING

tageous for the blend systems over the 4 single SBR and NBR systems. Detail of 72 mechanical properties will be given later. 70 Figure 2 shows the results of bound rubber content (BRC) in SBR/NBR blends 68 and NSBR systems. The error bars indica- te the standard deviations of the measu- 66 rements for 3 specimens in each rubber system. In SBR/NBR blends, with the use 64 of toluene as solvent, the unexpected sy- 62 nergistic effect is observed, and the grea- test BRC is noticeable at the SBR/NBR 60 blend ratio of 25/75. The high BRC values Hardness (Shore A) found in SBR/NBR blends might partly be 58 caused by the inclusion of unbound NBR 56 portion in the blends which is insoluble SON100 S25N75 S50N50 S75N25 S100NO NSBR in toluene. To support the proposed ex- planation, further experiment was per- Sample codename formed by immersing the rubber com- Hardness of SBR/NBR blends and NSBR vulcanisates pounds in toluene and acetone sequentially. By this means, the unbound NBR would dissolve in acetone as its good 5 solvent. Results obtained are shown in 6 Figure 2 where the synergistic effect dis- appears. The BRC values of SBR/NBR 5 blends are in between the single SBR and NBR systems. In NSBR copolymer, the re- 4 latively high BRC is observed, compared with SBR/NBR blends. This suggests the 3 greater magnitude of rubber-carbon black interaction in NSBR copolymer. M100 Referred to results of Payne effect and 2 BRC, the NSBR compound exhibits relatively high magnitude of the Payne effect, 1 implying the high amount of tri-dimensi- onal filler transient network of carbon 0 black in rubber compounds. Such network SON100 S25N75 S50N50 S75N25 S100NO NSBR might be composed of plenty of voids Sample codename between black agglomerates in which some portion of rubber could reside in the Modulus at 100% strain (M100) of vulcanised rubber voids as illustrated in Figure 3 [18, 19]. The occluded rubber in such voids leads to the high BRC found in NSBR system. of the SBR/NBR blends. The hardness re- vulcanisates of SBR/NBR blends and Figure 4 reveals hardness results of sult appears to be in line with the result NSBR copolymer. Since the NSBR vulca- rubber vulcanisates in which the rela- of crosslink density where the NSBR pos- nisate possesses relatively poor filler tively high hardness is found in the blend sesses lower crosslink density (as deter- dispersion degree (as implied by the systems in conjunction with the synergi- mined from torque difference illustrated high magnitude of Payne effect) in as- stic effect. Among the blend systems, the in Table 2). In other words, the hardness sociation with low crosslink density NBR-rich system demonstrates relatively of NSBR is dominated by crosslink densi- magnitude, the relatively low ultimate low hardness despite its relatively high ty over the filler network formation. Ad- tensile strength was initially anticipa- crosslink density as determined from cu- ditionally, it is known that the hardness ted. Evidently, the highest tensile re torque difference (MH-ML) as illustra- could be used to predict modulus espe- strength is observed in the NSBR sys- ted in Table 2. It is proposed that the ap- cially at small deformation of vulcanisa- tem followed by filled NBR and filled parent results of hardness in SBR/NBR tes in many circumstances, i.e., the grea- SBR systems, respectively. In addition, blends might be governed dominantly by ter the hardness, the higher the low- the strength of SBR/NBR blends is infe- the magnitude of filler transient network strain modulus [13]. Figure 5 reveals the rior to the neat and NSBR systems, im- formation as evidenced from the results results of modulus at 100% strain (M100) plying “antagonistic effect” which is of Payne effect (see Figure 1). Despite the of vulcanisates in which the synergistic believed to be the result of poor inter- high magnitude of Payne effect in NSBR effect is also noticed in a similar fashion facial adhesion (via polarity mismat- system, the hardness of NSBR is in bet- to those of hardness. Thus similar expla- ching) between NBR and SBR phases in ween that of filled SBR and filled NBR nation could be applied. Figure 6 shows blends [20]. Similar result trend is also systems, and evidently lower than that a comparison of tensile strength in found in elongation at break, as illust-

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6 rated in Figure 7. Comparison of oil re- 30 sistance of NSBR and SBR/NBR blends is exhibited in Figure 8. According to previ- 25 ous work, the relative tensile properties could be used an indication of for oil re- 20 sistance in rubber vulcanisates [9]. Ex- pectedly, the NBR single system offers 15 highest oil resistance while the NSBR and NBR-rich blends give comparable oil 10 resistance. This means the NSBR is practically advantageous due to its

ensile strength (Mpa) compromise in its mechanical strength,

T 5 flexibility and oil resistance. Furthermo- re, the NSBR system requires no com- 0 plex blending procedure for controlling SON100 S25N75 S50N50 S75N25 S100NO NSBR consistent phase morphology in rubber Sample codename matrix, leading to a significant reduc- tion in production cost and time. Among Tensile strength of vulcanised rubber SBR/NBR blends, the oil resistance increases with increasing NBR content in 7 blends, which is not unusual. This is be- 600 cause the NBR contains acrylonitrile group as a polar group which is known to yield high resistance to hydrocarbon 500 oil. Figure 9 exhibits the plots of loss factor (tan delta) against temperature 400 of SBR/NBR and NSBR vulcanisates. The glass transition temperature (Tg) as de- 300 termined from the temperature at the damping peak of NSBR is close to that 200 of the filled NBR and the NBR-rich blend (S25N75). The results suggest compara-

Elongation at break (%) 100 ble service temperature range of NSBR, NBR and S25N75. However, the inferio- 0 rity of S25N75 to NSBR and NBR is its SON100 S25N75 S50N50 S75N25 S100NO NSBR complexity in preparation as well as its Sample codename presence of discrete phase (as evidenced by its 2 tan delta peaks) leading to its poor mechanical properties. Com- Elongation at break of vulcanised rubber pared with NBR, the NSBR is more practical in its balance of mechanical 8 strength, flexibility and oil resistance. In some applications such as industrial 1.2 rollers where magnitude of rolling resistance is concerned, the value of tan 1 delta at 60 °C is capable of predicting the rolling resistance of rubber [5, 7]. It 0.8 is apparent from Figure 10 that the tan delta values at the temperature range 0.6 of 40-80 °C of NBR and NSBR are comparable, and significantly higher than that of SBR single system. As one can 0.4 e tensile strength expect, in SBR/NBR blends, the tan delta appears to increase with increasing 0.2

elativ NBR content, due mainly to the dilution R effect. The fraction of SBR having low 0 tan delta is substituted by that of NBR SON100 S25N75 S50N50 S75N25 S100NO NSBR possessing high tan delta. This might Sample codename be considered as a main disadvantage of NBR-rich blends and NSBR which could practically be alleviated by the Relative tensile strength (oil resistance) of rubber vulcanisates adjustment of crosslink density www.kgk-rubberpoint.de KGK · 11/12 2012 45 PRÜFEN UND MESSEN TESTING AND MEASURING

Conclusions 9 The property comparison of filled NSBR copolymer and SBR/NBR blends was con- 1.2 ducted, and the following conclusions S25N75 could be drawn: 1 S50N50 ■■ The NSBR system and SBR/NBR S75N25 blends offer comparable or slight 0.8 discrepancies in cure characteristics, SON100 depending on the NBR content in 0.6 S100NO

blends. an δ T ■■ Main advantages of NSBR are its su- NSBR periority in processability, tensile 0.4 strength, flexibility and oil resistance with the expenses of inferiority in 0.2 hardness, modulus and hysteresis. The crosslink density of NSBR is 0 slightly lower than that in NBR-rich -80 -60 -40 -20 0 20 40 60 80 100 blends. Temperature (°C) Acknowledgement The authors thank the Center of Excel- Loss factor (tan) of rubber vulcanisates lence for Innovation in Chemistry (PERCH-CIC) for financial support throughout this work, and MDR Inter- 10 national Co., Ltd. for kindly supplying the acrylonitrile-styrene-butadiene 0.16 rubber (NSBR: Sabor™ DT100) used in this work. 0.14

References 0.12 [1] N. Z. Noriman, H. Ismail and A. Rashid, Po- lym-Plast Technol 47 (2008) 1016. 0.1 [2] K. Habeeb Rahiman, G. Unnikrishnan, A. Su- jith and C. K. Radhakrishnan, Mater Lett 59 0.08 (2005) 633. an δ T [3] N. A. Darwish, A. B. Shehata, A. A. Abd El-Me- geed, S. F. Halim and A. Mounir, Polym-Plast 0.06 S25N75 S50N50 S75N25 Technol 44 (2005) 1297. [4] N. A. Darwish, N. Abd El-Aal and A. A. Abd El- 0.04 Megeed, Polym-Plast 46 (2007) 345. SON100 S100NO NSBR [5] J. White, S. K. De and K. Naskar, “Rubber 0.02 Technologist’s Handbook Volume 1”, Repra Technology Ltd., United Kingdom, (2009). 0 35 45 55 65 75 85 [6] L. D. Pérez, L. Sierra and B. L. López, Polym Eng Sci 48 (2008) 1986. Temperature (°C) [7] A. Neau and M. Rangstedt, Rubber World (2009) 21.  [8] C. Sirisinha, P. Sae-oui and J. Guaysomboon, Loss factor (tan ) of rubber vulcanisates at the temperature range of 40 °C to 80 °C (en- larged from Figure 9) Polymer 45 (2004) 4909. [9] C. Sirisinha and W. Sittichokchuchai, J Appl Polym Sci 76 (2000) 1542. K. U. Kelting and C. Van de Pol, KGK 56 [10] V. M. Litvinov, R. A. Orza, M. Klüppel, M. V. (2003) 338. Duin and P. C. M. M. Magusin, Macromole- [15] S. Dasgupta, S. L. Agrawal, S. Bandyopadhy- Corresponding author cules 44 (2011) 4887. ay, R. Mukhopadhyay R, R. K. Malkani and S. Dr. C. Sirisinha is the Associate Prof. in [11] S. Wolff, M. J. Wang and E. H. Tan, Rubber C. Ameta, Polym Test 28 (2009) 251. the Department of Chemistry and Cen- Chem Technol 66 (1993) 163. [16] P. Cassagnau, Polymer 44 (2003) 2455. ter of Excellence for Innovation in Che- [12] K. Sahakaro and S. Beraheng, J Appl Polym [17] N. Phewthongin, P. Saeoui and C. Sirisinha, J mistry (PERCH-CIC), Faculty of Science, Sci 109 (2008) 3839. Appl Polym Sci 100 (2006) 2565. Mahidol University, Thailand, and also [13] W. Hofmann, Vulcanisation and vulcanising [18] M. T. Ramesan, J Polym Res 11 (2004) 333. the researcher based in the Research agents; 1st ed. United Kingdom: Maclaren [19] J. L. Leblanc, Prog Polym Sci 27 (2002) 627. and Development Centre for Thai Rub- and son Ltd; 1967. [20] http://shodhganga.inflibnet.ac.in/bitstream/ ber Industry (RDCTRI) at Mahidol Uni- [14] W. Dierkes, J. W. M. Noordermeer, M. Rinker, 10603/3562/13/13_chapter%207.pdf. versity, Thailand.

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