Influence of Coupling Agent on Properties of Carbon Black-Reinforced SBR and NR/SBR Vulcanizates

Transcription

1 J. Ind. Eng. Chem., Vol. 13, No. 6, (2007) Influence of Coupling Agent on Properties of Carbon Black-Reinforced SBR and NR/SBR Vulcanizates Sung-Seen Choi, Jong-Chul Kim, Ji-Eun Ko, Yi Seok Cho*, and Wae Gi Shin* Department of Chemistry, Sejong University, Seoul , Korea R & D Center, Pyunghwa Industrial Co., Bonri-ri, Nongong-eup, Daegu , Korea Received July 3, 2007; Accepted August 9, 2007 Abstract: Influence of the coupling agent on properties of carbon black-reinforced rubber compounds and vulcanizates was studied. Four styrene-butadiene rubber (SBR) compounds with different SBR types and five NR/SBR blend compounds with different rubber compositions were compared. Bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) was used as the coupling agent. For the SBR specimens, the modulus and tensile strength were enhanced by adding the coupling agent although the elongation at break was reduced. For the NR/SBR specimens, the crosslink density and modulus were enhanced by adding the coupling agent while the bound rubber content was decreased. The elongation at break and tensile strength of the NR/SBR specimens with higher SBR content were enhanced by adding the coupling agent. Change of the properties of the rubber specimens by adding the coupling agent was explained with modification of the carbon black surface and donation of sulfur from TESPT. Keywords: coupling agent, SBR vulcanizates, NR/SBR vulcanizates, physical properties, crosslink density, bound rubber content Introduction 1) Carbon black and silica have been used as the main reinforcing agents in rubber compounds but their surface chemistries are very different [1-7]. Silica has a number of hydroxyl groups (silanol, Si-OH), which results in strong filler-filler interactions and adsorption of polar materials by hydrogen bonds [6,8]. Since intermolecular hydrogen bonds between silanol groups on the silica surface are very strong, it can aggregate tightly [7,9]. Its property can cause a poor dispersion of silica in a less polar rubber compound. In general, a silane coupling agent such as bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) is used to improve the filler dispersion and to prevent adsorption of curatives on the silica surface by modification of the silica surface [10-13]. Carbon black has lots of functional groups such as hydroxyl, carboxyl, ketone, and aldehyde on the surface although the quantity is very small [14-16]. Functional To whom all correspondence should be addressed. ( sschoi@sejong.ac.kr) groups on the carbon black can also react with TESPT as shown in Scheme 1. The ethoxy groups of TESPT, (C 2 H 5 O) 3 Si-(CH 2 ) 3 -S x -(CH 2 ) 3 -Si-(OC 2 H 5 ) 3 can react with the functional groups on the carbon black surface to form C-O-Si~ bond. The silane molecule is bound to the carbon black surface and can react with a rubber molecule to form crosslink between carbon black and rubber. A rubber vulcanizate filled with silane(vinyl triethoxysilane)-treated carbon black showed a tremendous improvement in the fatigue behavior compared to a vulcanizate filled with untreated furnace carbon black [17] and addition of the silane coupling agent, N-3-(-N-vinyl benzyl amino) ethyl-g-amino propyl trimethoxy silane monohydrochloride, enhanced the polymer-filler interactions [18]. Crosslink density of a rubber vulcanizate determines the physical properties such as modulus, hardness, resilience, elongation at break, heat build-up, and so forth [19]. By increasing the crosslink density, the modulus, hardness, resilience, and abrasion resistance increase, whereas the elongation at break, heat build-up, and stress relaxation decrease. Natural rubber (NR) is the most commonly

2 1018 Sung-Seen Choi, Jong-Chul Kim, Ji-Eun Ko, Yi Seok Cho, and Wae Gi Shin Table 1. Formulations of NR/SBR Blend Compounds (phr) Compound No SMR CV SBR N Si HPPD Wax ZnO Stearic acid Paraffinic oil TBBS Sulfur SMR CV60: Malaysian standard rubber (natural rubber) with Mooney viscosity of 60 N330: carbon black Si69: silane coupling agent, bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) HPPD: N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine TBBS: N-tert-butyl-2-benzothiazole sulfenamide physical properties were compared. Experimental Scheme 1. Mechanism for the reactions of TESPT with carbon black and rubber used rubber and it is blended with a synthetic rubber to improve physical and chemical properties. Styrene-butadiene rubber (SBR) is a copolymer of styrene and butadiene. NR/SBR blend is compatible and is one of useful NR blends [20]. In the present work, influence of the silane coupling agent on the properties of carbon black-reinforced rubber compounds and vulcanizates was studied. Bound rubber contents of the rubber compounds were measured to investigate level of reinforcement, the apparent crosslink densities were also measured, and the Ten carbon black-reinforced NR/SBR blend compounds and eight carbon black-reinforced SBR compounds were made of rubbers (NR and SBR), carbon black, silane coupling agent, processing oil, cure activators (stearic acid and ZnO), antidegradants (HPPD and wax), and curatives (TBBS and sulfur). For the NR/SBR blend compounds, SBR 1502 and SMR CV60 were used as SBR and NR. For SBR compounds, SBR 1502, SBR 1507, SBR 1712, and SBR 1778 of Kumho Petroleum Chemical Co. of Korea were employed as SBR. Si69, bis-(3- (triethoxysilyl)-propyl)-tetrasulfide (TESPT), of Degussa Co. was used as coupling agent. Content of the coupling was set 3 % of the filler content. Formulations for the NR/SBR blend compounds and the SBR compounds were given in Tables 1 and 2, respectively. The SBR 1502 and SBR 1507 compounds have oil of 37.5 phr since the SBR 1712 and SBR 1778 have 37.5 wt% of oil as raw materials but the SBR 1502 and SBR 1507 do not contain oil as raw materials. Mixing was performed in a Banbury type mixer at a rotor speed of 40 and 30 rpm for master batch (MB) and final mixing (FM) stages, respectively. The initial temperatures of the mixer were 110 and 80 o C for the MB and FM stages, respectively. The vulcanizates were prepared by curing at 160 o C for 13.3 and 15.0 min for the NR/SBR compounds and SBR compounds, respectively, in a press mold. Physical properties of the vulcanizates were measured with the universal testing machine (Instron 6021). The crosslink densities of the samples were measured

3 Influence of Coupling Agent on Properties of Carbon Black-Reinforced SBR and NR/SBR Vulcanizates 1019 Table 2. Formulations of SBR Compounds (phr) Compound No SBR SBR SBR SBR N Si HPPD Wax ZnO Stearic acid Aromatic oil TBBS Sulfur SBR 1502: ML1 + 4 at 100 o C = 52 SBR 1507: ML1 + 4 at 100 o C = 35 SBR 1712: ML1 + 4 at 100 o C = 49, aromatic oil 37.5 wt% SBR 1778: ML1 + 4 at 100 o C = 46, naphthenic oil 37.5 wt% Table 3. Bound Rubber Contents (wt%) of the SBR Compounds and Apparent Crosslink Densities (1/Q) of the SBR Vulcanizates SBR type SBR 1502 SBR 1507 SBR 1712 SBR 1778 Without the coupling agent Bound rubber content Apparent crosslink density Containing the coupling agent Bound rubber content Apparent crosslink density by the swelling method. The procedure to measure the crosslink density was as follows: initially organic additives in the samples were removed by extraction with THF and n-hexane for 2 days each and were dried for 2 days at room temperature; the weights of the organic materials-extracted samples were measured; they were soaked in toluene for 2 days and the weights of the swollen samples were measured. The swelling ratio (Q) was calculated by the equation (1). Q = (W s - W u )/W u, (1) where W s and W u are weights of the swollen and unswollen samples, respectively. The reciprocal swelling ratio (1/Q) was used as the apparent crosslink density. Contents of bound rubber were determined by successive extracting the unbound materials such as ingredients and free rubbers with toluene for 7 days and n-hexane for 1 day and then drying for 2 days at room temperature. Weights of the samples before and after the extraction were measured and the bound rubber contents were calculated as equation (2). R b (%) = 100 [W fg W t [m f /(m f + m r )]]/W t [m r /(m f + m r )] (2) where R b is the bound rubber content, W fg is the weight of filler and gel, W t is the weight of the sample, m f is the fraction of the filler in the compound, m r is the fraction of the rubber in the compound. Results and Discussion The SBR 1712 and SBR 1778 as raw materials contain oil of 37.5 phr since their molecular weights are very high whereas the SBR 1502 and SBR 1507 as raw materials do not contain oil. Bound rubber contents of the SBR compounds and apparent crosslink densities of the SBR vulcanizates were summarized in Table 3. Bound rubber contents of the SBR 1502 and SBR 1507 compounds containing TESPT are lower than those without the coupling agent while bound rubber contents of the SBR 1712 and SBR 1778 compounds are enhanced by adding the coupling agent. Bound rubber contents of the SBR 1712 and SBR 1778 compounds are much higher than those of the SBR 1502 and SBR 1507 ones. It is nat-

4 1020 Sung-Seen Choi, Jong-Chul Kim, Ji-Eun Ko, Yi Seok Cho, and Wae Gi Shin Table 4. Physical Properties of the SBR Vulcanizates SBR type SBR 1502 SBR 1507 SBR 1712 SBR 1778 Without the coupling agent Modulus at 100 % (kg/cm 2 ) Elongation at break (%) Tensile strength (kg/cm 2 ) Containing the coupling agent Modulus at 100 % (kg/cm 2 ) Elongation at break (%) Tensile strength (kg/cm 2 ) Figure 1. Variation of the bound rubber content of the carbon black-filled NR/SBR vulcanizate as a function of the SBR content. The open and solid symbols indicate the specimens without and containing the silane coupling agent, respectively. ural because the bound rubber content is a proportional property to the molecular weight [21]. Apparent crosslink densities of the SBR vulcanizates are enhanced by adding the coupling agent as shown in Table 3. This is due to the sulfur of TESPT. The silane coupling agent has about four sulfur atoms every one molecule of TESPT. Elemental sulfur can be isolated from the silane coupling agent by heating during vulcanization [10]. Thus, the total sulfur content is increased by adding TESPT and amount of the sulfur crosslinks is also enhanced. Modulus is a proportional property to the crosslink density [19]. Physical properties of the SBR vulcanizates were summarized in Table 4. Moduli of the SBR specimens are enhanced by adding the coupling agent. This may be due to the increased crosslink density and the formation of crosslinks between carbon black and rubber by the coupling agent. Increment of the moduli for the SBR 1712 and SBR 1778 specimens is larger than for the SBR 1502 and SBR 1507 ones. This can be explained with the increased bound rubber contents. Bound rubber contents of the SBR 1712 and SBR 1778 compounds are increased by adding TESPT whereas those of the SBR 1502 and SBR 1507 ones are reduced as shown in Table 3. Increase of the bound rubber content of a rubber compound leads to enhancement of the modulus [22]. Elongation at break is an inversely proportional property to the crosslink density [19]. Elongations at break of the SBR specimens containing TESPT are lower than those without the coupling agent except the SBR 1507 specimens. This is also due to the increased crosslink density by adding the coupling agent. In general, tensile strength of a rubber composite is enhanced by increasing the elongation at break but tensile strengths of the SBR specimens containing TESPT are on the whole higher than those without the coupling agent though the elongations at break are reduced by adding the coupling agent as shown in Table 4. This may be due to the formation of crosslinks between carbon black and rubber by the coupling agent. Tensile strengths of the SBR specimens with high molecular weight (SBR 1712 and SBR 1778) are much larger than those with low ones (SBR 1502 and SBR 1507) although elongations at break of the former specimens are shorter than those of the latter ones. This may be due to the higher bound rubber content to enhance reinforcement. For the SBR 1507 specimens, the tensile strength is notably enhanced by adding the coupling agent due to the increased elongation at break, while the tensile strength of only the SBR 1712 specimen is slightly decreased by adding the coupling agent due to the strikingly reduced elongation at break. Experimental results of the NR/SBR blends were summarized in Figures 1 5 plotted as a function of the SBR content to investigate the blend ratio effect. Figure 1 shows variation of the bound rubber contents with the SBR ratio and there is specific trend with the SBR ratio. The bound rubber content is reduced by increasing the SBR content. The bound rubber contents are decreased about 0.8 and 0.7 % every 10.0 phr SBR increase for the NR/SBR compounds without and containing the cou-

5 Influence of Coupling Agent on Properties of Carbon Black-Reinforced SBR and NR/SBR Vulcanizates 1021 Figure 2. Variation of the increased crosslink density of the carbon black-filled NR/SBR vulcanizate by adding Si69 as a function of the SBR content. The increased ratio (%) of the apparent crosslink density was calculated by dividing the difference of the apparent crosslink densities of the samples containing and without Si69 by the apparent crosslink densities of the samples without Si69. pling agent, respectively. This is because molecular weight of NR is higher than that of SBR 1502 since the bound rubber content is a proportional property to the molecular weight [21]. But, the coupling agent effect on the bound rubber content was not observed. This means that modification of the carbon black surface by TESPT did not strikingly improve the filler-polymer interactions because carbon black is compatible with the polymers of NR and SBR. The apparent crosslink densities are increased by adding the coupling agent as shown in Figure 2. The experimental results were plotted with the crosslink density changes not the values of the apparent crosslink density because interaction parameters of the swelling solvent (toluene) with NR and SBR are different. The enhanced crosslink densities may be due to the increased total sulfur content by adding TESPT. Elemental sulfur can be isolated from TESPT by heating [10]. Thus, the total sulfur content is increased by adding TESPT and amount of the sulfur crosslinks is also enhanced. Degree of the increased crosslink density is reduced by increasing the SBR content. Increments of the crosslink density are decreased about 1.5 % every 10.0 phr SBR increase. The coupling agent played an important role to increase the modulus of the NR/SBR specimen. The moduli of the NR/SBR vulcanizates containing TESPT are larger than those without the coupling agent by about 13 % as shown in Figure 3. The modulus is reduced by increasing the SBR content. The moduli are decreased about 0.3 kg/cm 2 every 10.0 phr SBR increase. Elongation at break is an inversely proportional property to the crosslink density Figure 3. Variation of the 100 % modulus of the carbon blackfilled NR/SBR vulcanizate as a function of the SBR content. The open and solid symbols indicate the specimens without and containing the silane coupling agent, respectively. Figure 4. Variation of the elongation at break of the carbon black-filled NR/SBR vulcanizate as a function of the SBR content. The open and solid symbols indicate the specimens without and containing the silane coupling agent, respectively. but the NR/SBR specimens containing TESPT with higher SBR content have longer elongations at break than those without the coupling agent though the formers have higher crosslink densities than the latters as shown in Figure 4. This indicates that SBR is more favorable to improve physical properties by adding the coupling agent than NR for the NR/SBR blends. The elongation at break is reduced by increasing the SBR content. The elongations at break are decreased about 14 and 5 % every 10.0 phr SBR increase for the NR/SBR specimens without and containing the coupling agent, respectively. This is because molecular weight of NR is higher than that of SBR Variation of the tensile strength shows a similar trend with the elongation at break as shown in Figure

6 1022 Sung-Seen Choi, Jong-Chul Kim, Ji-Eun Ko, Yi Seok Cho, and Wae Gi Shin References Figure 5. Variation of the tensile strength of the carbon black-filled NR/SBR vulcanizate as a function of the SBR content. The open and solid symbols indicate the specimens without and containing the silane coupling agent, respectively. 5. The NR/SBR specimens containing TESPT with higher SBR content ratio have also larger tensile strength than those without the coupling agent. The experimental results lead to a conclusion that improvement of the physical properties of the NR/SBR blend composites by adding the coupling agent is mainly due to SBR not NR. Conclusion For the SBR specimens, the bound rubber contents were not enhanced by adding the silane coupling agent but the apparent crosslink densities were on the whole increased by adding TESPT. Modulus and tensile strength of the SBR specimen were on the whole enhanced by adding the coupling agent although the elongation at break was reduced. For the NR/SBR blend specimens, the crosslink density was enhanced by adding the coupling agent but it was reduced by increasing the SBR content. The modulus of the NR/SBR blend vulcanizate was increased by adding the coupling agent and the elongation at break and tensile strength were also enhanced for the specimens with higher SBR content. The coupling agent was found to improve tensile properties of the SBR specimens and the NR/SBR blends, especially the NR/SBR specimens with higher SBR content ratios. 1. G. R. Cotton, Rubber Chem. Technol., 57, 118 (1984). 2. S. Wolff and U. Grl, Kautsch. Gummi Kunstst., 44, 941 (1991). 3. T. C. Gruber and C. R. Herd, Rubber Chem. Technol., 70, 727 (1997). 4. A. K. Ghosh and B. Adhikari, Kautsch. Gummi Kunstst., 52, 681 (1999). 5. H. Raab, J. Frhlich, and D. Göritz, Kautsch. Gummi Kunstst., 53, 137 (2000). 6. J. T. Byers, Rubber World, 218, 38 (1998). 7. S. Wolff and M.-J. Wang, Rubber Chem. Technol., 65, 329 (1992). 8. Y.-C. Ou, Z.-Z. Yu, A. Vidal, and J. B. Donnet, Rubber Chem. Technol., 67, 834 (1994). 9. Y. Li, M. J. Wang, T. Zhang, F. Zhang, and X. Fu, Rubber Chem. Technol., 67, 693 (1994). 10. U. Görl and A. Hunsche, in Proceedings of the Rubber Division 151st Meeting, American Chemical Society, Paper No. 38 (1997). 11. A. S. Hashim, B. Azahari, Y. Ikeda, and S. Kohjiya, Rubber Chem. Technol., 71, 289 (1998). 12. C. R. Herd, in Proceedings of the Rubber Division 149th Meeting, American Chemical Society, Education Symposium Paper No. 38 (1996). 13. A. K. Manna, P. P. De, D. K. Tripathy, and S. K. De, Rubber Chem. Technol., 70, 624 (1997). 14. M.-J. Wang, K. Mahmud, L. Murphy, and W. J. Patterson, in Proceedings of the Rubber Division 151st Meeting, American Chemical Society, Paper No. 24 (1997). 15. S.-S. Choi, Elastomer, 36, 37 (2001). 16. S.-S. Choi and S.-J. Choi, Bull. Kor. Chem. Soc., 27, 1473 (2006). 17. S. K. Rana, J. Appl. Polym. Sci., 78, 2235 (2000). 18. A. K. Manna, A. K. Bhattacharyya, P. P. De, D. K. Tripathy, S. K. De, and D. G. Peiffer, J. Appl. Polym. Sci., 71, 557 (1999). 19. N. J. Morrison and M. Porter, Rubber Chem. Technol., 57, 63 (1984). 20. A. L. G. Saad and S. El-Sabbagh, J. Appl. Polym. Sci., 79, 60 (2001). 21. G. Kraus and J. T. Gruver, Rubber Chem. Technol., 41, 1256 (1968). 22. S.-S. Choi, J. Appl. Polym. Sci., 93, 1001 (2004).