Interactions Between Surface Treated Ultrafine Mineral Filler and Silicone Rubber Matrix

Interactions Between Surface Treated Ultrafine Filler and Silicone Rubber Matrix Interactions Between Surface Treated Ultrafine Filler and Silicone Rubber Matrix Jihuai Wu*, Zhen Shen, Congrong Wei, Yike Chen and Donghong Hu Institute for Materials Physical Chemistry College of Materials Science and Engineering, Huaqiao University, Quanzhou, Fujian 362011, P. R. China E-mail: jhwu@hqu.edu.cn Received: 8th November 2000; Accepted: 3rd January 2001 SUMMARY Silicone rubber filled with surface treated ultrafine mineral powder was prepared. The bound rubber in uncured silicone rubber was investigated using different mineral powders as fillers. The cross-link density in the vulcanizate was researched, and the mechanical properties were measured. It was found that there are strong relationships between T (bound rubber in toluene solvent), D T (total cross-link density) and tensile strength for vulcanizate.there are also strong links between E (bound rubber in basic solvent), D C (chemical cross-link density) and 100% modulus for vulcanizate. Meanwhile, the bound rubber, cross-link density and mechanical properties of the silicone rubber products are affected by the surface chemical treatment of the mineral filler. INTRODUCTION powders have not been used to reinforce silicone rubbers so far, though they have been used in other rubbers 1. If a natural mineral powder, after being processed by physical and/or chemical methods, can be used as a reinforcing filler for silicone rubber to replace expensive fumed silica, it will not only promote the development of the silicone rubber industry, but also enhance the usefulness of the minerals 2. It is important to understand the interactions between a silicone rubber matrix and an ultrafine mineral filler 3. Unfortunately, it is rare to see research on the interactions between a mineral filler and a silicone rubber matrix. In this paper, a silicone rubber, filled with a surface treated ultrafine mineral filler, was prepared. The bound rubber content in the uncured mix was investigated; the cross-link density of the vulcanizate was researched, and the mechanical properties of the vulcanizate were measured. As a result, the connection between the surface treatment of the mineral filler, the amount of bound rubber in the uncured mix, the cross-link density and the mechanical properties were elucidated. *Correspondence author Materials EXPERIMENTAL On the basis of research into the influence of minerals on the reinforcement of silicone rubber 4,5, the minerals talc, quartz and wollastonite were chosen as starting materials to prepare fillers. A silane coupling agent A- 151 [(CH 3 CH 2 O) 3 -Si-CH=CH 2 ] vinyl triethoxy silane (made by the Shuguang Chemical Company, Nanjing, China) and a borate coupling agent B-5 [RO-B-(OR') 2 ] (made by the Fuzhou Chemical Company, Fuzhou, China) were used to modify the mineral powder. Methyl vinyl silicone resin, diphenyl dihydroxy silicone and benzoyl peroxide were used to prepare the silicone rubber mixes and the vulcanizate. Acetone, toluene and ethylenediamine (NH 2 -CH 2 -CH 2 -NH 2 ) were used to measure the bound rubber content. Instruments QS-350 Supersonic Speed Gas Steam Grinder (Qudong, Shanghai, China) GH-10 Electric-heating Constant Thermal Mixer (Beijing, China) SK-160B Two Roll Mill (Shanghai, China) QLB-D Vulcanizator (Qindao, China) Tensile Testing Machine (Guangzhou, China) Polymers & Polymer Composites, Vol. 9, No. 3, 2001 169

Jihuai Wu, Zhen Shen, Congrong Wei, Yike Chen and Donghong Hu Preparation of the surface treated ultrafine mineral powder After the minerals had been purified, graded and subjected to preliminary grinding, mineral micro powders were obtained. The mineral powders were then crushed into ultrafine powders (average particle size < 2 µm and specific surface area > 100 m 2 /g) with a Supersonic Speed Gas Steam Grinder 4. Using the liquid coupling agent A-151 as surface treatment agent, the ultrafine mineral powders were chemically modified with an electrically-heated Constant Thermal Mixer. Using the solid coupling agent B-5 as modifying agent, the ultrafine mineral powder was mechanochemically modified with the Supersonic Speed Gas Steam Grinder 4. As a result, surface treated ultrafine mineral powders T A,, Q A,, W A and were prepared, where T, Q and W were talc, quartz and wollastonite ultrafine powders; subscripts A and B refer to ultrafine mineral powders treated with coupling agents A-151 and B-5, respectively. Preparation of silicone rubber sample An uncured silicone rubber sample was prepared, according to the formulation shown in Table 1, using surface treated ultrafine mineral powder as filler, after mixing for 50 min on a two-roll mill. The uncured mix became a cured vulcanizate after the mix was vulcanized at 140 C, under 14 MPa for 15 min on the Vulcanizator. In order to compare the test results conveniently, all the uncured mixes and vulcanizates in this paper were prepared by using this condition and the formulation shown in Table 1, and the only variation was the kind of mineral filler. The mechanical properties of the vulcanizates were measured using standard methods. Table 1 Formulation Compound Parts by weight Methyl vinyl silicone resin 120 Benzyl peroxide 2 Diphenyl dihydroxyl silicone 2. 5 Fumed silica 40 filler 40 Measurement of the bound rubber content The extraction method 6 was adopted to measure the bound rubber content in the uncured mix. About 0.5 g of uncured mix was precisely weighed and wrapped with filter paper. The wrap was immersed in 100 cm 3 organic solvent (pure toluene, or the mixed liquid with 90 cm 3 toluene and 10 cm 3 ethylenediamine) at room temperature for 9 days; the solvent was renewed after every 3 days. After the 9 days, the wrap was immersed in 100 cm 3 acetone solvent for 3 days to remove any remaining toluene. Finally, the uncured mix was dried and weighed. The bound rubber content () in the uncured mix was calculated according to the following equation: ( M M M B = F D) 100% M where M B is the mass of the uncured mix before immersing; M F is the mass of the filler in the uncured mix, which can be calculated from the mix formulation shown in Table 1; M D is the mass of rubber dissolved in the organic solvent, which is equal to the difference between the mass of the uncured mix before and after immersion. Measurement of the cross-link density B The cross-link density of the vulcanizate was measured by a mechanical method 7,8. According to the Mooney-Rivlin equation, σ C = C + P 2 C 2( λ λ ) λ where σ is the stress exerted on the vulcanizate sample, λ is the strain, C C is a constant related to chemical cross-linking of the vulcanizate sample, and C P is a constant related to physical cross-linking of the vulcanizate sample. According to classical rubber elasticity theory, there is a relationship as follows 7,8 : C = ρrtd (1) (2) (3) where C (C C or C P ) is obtained from equation (2), ρ is the density of the vulcanizate sample which is measured experimentally, T is the temperature; R is the gas constant, and D is the cross-link density of the sample. The measurement of σ for different λ values allows equation (2) to be applied for each strain λ and a straight line to be drawn whose slope is C P and whose intercept is C C. The chemical cross-link density (D C ) and physical cross-link density (D P ) in the vulcanizate sample are calculated according to equation (3). Total cross-link density (D T ) in the vulcanizate sample can be obtained from the relation D T = D C + D P. In these experiments, a dumbbellshaped vulcanizate sample was stamped out with a thickness of 2.0 ± 0.3 mm, and the sample was 170 Polymers & Polymer Composites, Vol. 9, No. 3, 2001

Interactions Between Surface Treated Ultrafine Filler and Silicone Rubber Matrix stretched to 25%, 50%, 75% and 100%, respectively, at a speed of 50 mm/min on a tensile testing machine. The stress (σ) for each strain (λ) was recorded. D T and D C of the vulcanizate samples were determined by the above method. RESULTS AND DISCUSSION Influence of surface modification of mineral filler on bound rubber Bound rubber is the percentage of rubber that can no longer be separated from the filler when the rubber mix is extracted in a good rubber solvent (such as toluene) over a specific period of time, usually at room temperature 3. In general, there are two kinds of bound rubber, one ( T ) is the rubber that remains undissolved in pure toluene solvent, and the other ( E ) is the rubber remaining undissolved in basic toluene solvent [here, 90 toluene: 10 ethylenediamine (v/v)]. The former reflects mainly interactions between rubber and filler; the latter reflects chemical interactions between rubber and filler in the uncured mix 9. In order to understand the influence of the surface modification of mineral fillers on bound rubber in uncured silicone rubber, mineral powders with different concentrations of coupling agent were made, and the corresponding uncured mixes were prepared. The bound rubber contents were measured and the results are shown in Table 2. From Table 2, using the wollastonite powder treated with coupling agent A-151 as filler, the uncured mix s T and E all increase with an increase in the concentration of coupling agent, from 1 wt.% to 4 wt.%. On the other hand, using talc powder treated with coupling agent B-5 as filler, the uncured mix s T increases, and E decreases, with increasing concentration of coupling agent from 1 wt.% to 4 wt.%. The above phenomenon is attributed to surface modification of the mineral filler. After the mineral powder is surface treated, on the one hand, the surface energy of the mineral powder decreases and the mineral filler disperses in the silicone rubber organic phase more even and easily. The contact area between mineral filler and silicone rubber increases, and the interaction between mineral filler and silicone rubber matrix increases 10, which results in an increase in T, since T reflects mainly the interaction between filler and rubber matrix. On the other hand, after the mineral powder is surface modified, the organic group of the coupling agent blocks off active hydroxyl groups on the surface of the mineral powder, and chemical interaction between silicone rubber and mineral filler decreases. This causes a decrease in E, since E reflects the chemical interaction between rubber matrix and filler. Obviously, with an increase in the concentration of coupling agent, T increases and E decreases further. The above explanation is applied to fillers surface-treated with many coupling agents, such as B-5. But for the filler surface treated with coupling agent A-151 [(CH 3 CH 2 O) 3 -Si-CH=CH 2 ], the situation is different, because coupling agent A-151 contains an active vinyl group (-CH=CH 2 ). During mixing, the vinyl group on the surface of the mineral filler will react with rubber molecules or another filler particle. The chemical interactions between filler and rubber, filler and filler are strengthened, which results in an increase in E, besides an increase in T. The bound rubber contents for different kinds of mineral powders are shown in Table 3. From Table 3, T and E are larger with T A as filler than with T as filler; on the other hand, T is larger with as filler than with T as Filler, and E is smaller with as filler than with T as filler. The effect is the same for the other two kinds of mineral powder and is different from the findings of reference 11. In other words, using coupling agent A-151, T and E all increase; using coupling agent B-5, T increases but E decreases. These results further confirm the phenomenon in Table 2 and the explanation by us. Table 2 Surface modification of mineral filler versus bound rubber filler ( 1% ) ( 2% ) ( 4% ) W A ( 1% ) W A ( 2% ) W A (4%) * E (%) 7.16 7.04 6.76 14. 6 15. 2 15. 9 **T (%) 42. 9 43. 5 48. 0 46. 2 47. 7 47. 8 * T is the bound rubber content using pure toluene as solvent **E is the bound rubber content using toluene (90 cm3) and ethylenediamine (10 cm3) as solvent Polymers & Polymer Composites, Vol. 9, No. 3, 2001 171

Jihuai Wu, Zhen Shen, Congrong Wei, Yike Chen and Donghong Hu Table 3 Kinds of mineral filler versus bound rubber filler T T A Q Q A W W A T (%) 41. 2 47. 5 43. 5 36. 4 45. 9 41. 6 43. 1 47. 7 46. 4 E (%) 8.79 9.20 7.04 7.21 8.36 6.33 13. 5 15. 2 9.50 Influence of surface modification of filler on cross-link density The cross-link density of vulcanizate is the density of cross-linking between rubber molecules and filler. The cross-linking is caused by chemical bonds (D C ), physical entanglements and embedding (D P ) between rubber matrix and filler. D T, D C and D P reflect different interactions between rubber matrix and filler. The influence of the surface modification of mineral fillers with different concentrations of coupling agent on the cross-link density in silicone rubber vulcanizate is shown in Table 4. From Table 4, with an increase in the amount of coupling agent A-151 on the mineral filler, the corresponding D T and D C increase. With an increase in the amount of coupling agent B-5 on mineral filler, the corresponding D T increases but D C decreases. The influence of surface modification of the mineral filler on the cross-link density in the vulcanizate is similar to that of the bound rubber content in the uncured mix. The surface modification of mineral filler reduces the surface energy of the mineral powder, leads to an increase in total interaction between filler and rubber in vulcanizate, and an increase in D T, since D T reflects total interaction in the vulcanizate. This rule is the same whether using coupling agent A-151 or B-5. On the other hand, coupling agent A-151 contains active groups (-CH=CH 2 ) which enhance chemical interaction and lead to an increase in D C. Coupling agent B-5 does not contain active groups and blocks off any active groups (OH) on the mineral filler, which reduces chemical interaction and results in a decrease in D C. When different mineral powders were used as filler, the influence of surface treatment of the filler on the cross-link density of the vulcanizate also has to be taken into account. From Table 5, D T and D C are larger with T A as filler than with T as filler; on the other hand, D T is larger with as filler than with T as filler, and D C is less with as filler than with T as filler. The result is the same for the other two kinds of mineral filler. These results indicate that the surface treatment of the filler affects the interaction between the filler and the rubber matrix, and bring about a change in the cross-link density of the vulcanizate. Influence of surface treatment of the mineral filler on mechanical property The surface treatment of the mineral filler not only affects the bound rubber content in the uncured mix and the cross-link density in the vulcanizate, but also affects the mechanical properties of the vulcanizate. Table 6 shows the tensile strength and the 100% modulus of the silicone rubber vulcanizate filled with different mineral fillers. In Table 6, the tensile strength of the vulcanizate using treated powder, such as and T A, as filler is larger than that using untreated powder, such as T, as filler. But the 100% modulus increases with T A as filler and declines with as filler, compared to T. This rule is the same for the other two kinds of filler, i.e. coupling agent A-151 increases the tensile strength and the 100% modulus of the vulcanizate, whereas coupling agent B-5 enhances the tensile strength and reduces the 100 % modulus. This rule becomes clear in Table 7. It is known that the tensile strength is Table 4 Surface modification of mineral filler versus crosslink density filler ( 1% ) ( 2% ) ( 4% ) W A ( 1% ) W A ( 2% ) W A (4%) DT (104 mol/g) 5.31 5.62 4.12 4.98 5.26 DC (104 mol/g) 1.35 1.20 1.14 3.33 3.68 3.97 Table 5 Kinds of mineral filler versus crosslink density filler T T A Q Q A W W A D T ( 104 mol/g) 4.36 5.48 3.11 3.30 3.23 4.79 5.77 4.98 D C ( 104 mol.g) 1.52 1.58 1.37 1.94 2.08 1.89 2.29 3.68 1.45 172 Polymers & Polymer Composites, Vol. 9, No. 3, 2001

Interactions Between Surface Treated Ultrafine Filler and Silicone Rubber Matrix Table 6 Kinds of mineral filler versus mechanical property of vulcanizate filler T T A Q Q A W W A Tensile strength (MPa) 5.30 5.64 5.67 6.36 5.84 4.74 6.22 5.13 100% modulus (MPa) 4.36 4.49 3.54 3.24 3.63 3.18 4.64 4.84 4.17 Table 7 Surface modification of mineral filler versus mechanical properties of vulcanizate filler ( 1% ) ( 2% ) ( 4% ) W A ( 1% ) W A ( 2% ) W A (4%) Tensile strength (MPa) 5.38 5.87 6.12 6.22 6.26 100% modulus (MPa) 3.83 3.54 3.30 4.77 4.84 5.24 related to the force applied at break, and the 100% modulus is related to the force when the length of the vulcanizate sample is stretched to 100%. The former implies the total interaction between filler and rubber, the latter relates to chemical interaction. Based on discussion above, the total interaction and chemical interaction are both enhanced by using coupling agent A-151, but the total interaction is enhanced and the chemical interaction is reduced by using coupling agent B-5. Therefore, different reinforcing effects appear with different coupling agent. Relation between surface treatment of mineral filler, bound rubber in uncured mix, cross-link density and mechanical properties of vulcanizate In order to establish the relationship between the surface treatment of the mineral filler, the bound rubber in the uncured mix, the cross-link density in the vulcanizate and the mechanical properties of the vulcanizate, Tables 3, 5 and 6 were combined to give Table 8. According to Table 8, using mineral powder treated with A-151 as filler, T and E of the uncured mix, D T and DC of the vulcanizate, tensile strength and 100 % modulus of the vulcanizate all increase. Using mineral powder treated with B-5 as filler, T, D T and tensile strength increase; but E, D C and 100% modulus all decrease. This applies to all three kinds of minerals chosen. In other words, there are strong relationships between connections T, D T and tensile strength, and between E, D C and 100 % modulus. T, D T and tensile strength reflect, from different angles, the total interaction between the mineral filler and the silicone rubber matrix, which leads to a strong relationship between T, D T and tensile strength. E, D C and 100% modulus also reflect, from different angles, chemical interactions between silicone rubber and mineral filler. CONCLUSIONS 1. Using the mineral powder treated with coupling agent A-151 as filler, T and E all increase; using mineral powder surface treated with coupling agent B-5, T increases and E Table 8 The relation between surface modification of mineral filler, bound rubber in uncured mix, crosslink density and mechanical properties of vulcanizate filler T) E) D T (% (% ( 104 mol/g) D C ( 104 mol/g) Tensile strength (MPa) 100% modulus (MPa) T 41. 2 8.79 4.36 1.52 5.30 4.36 T A 47. 5 9.20 5.48 1.58 5.64 4.49 43. 5 7.04 1.37 3.54 Q 36. 4 7.21 3.11 1.94 5.67 3.24 Q A 45. 9 8.36 3.30 2.08 6.36 3.63 41. 6 6.33 3.23 1.89 5.84 3.18 W 43. 1 13. 5 4.79 2.29 4.74 4.64 W A 47. 7 15. 3 5.77 3.68 6.22 4.84 46. 4 9.50 4.98 1.45 5.13 4.17 Polymers & Polymer Composites, Vol. 9, No. 3, 2001 173

Jihuai Wu, Zhen Shen, Congrong Wei, Yike Chen and Donghong Hu decreases. It is probably due to the fact that A- 151 contains an active group (-CH=CH 2 ) and B- 5 does not. 2. Total cross-link density and chemical crosslink density in the vulcanizates all increase when using a mineral filler treated with A-151. Total cross-link density increases but chemical cross-link density declines when using a mineral filler treated with B-5. The surface modification of the filler plays an important role in improving the interaction between the mineral filler and the silicone rubber matrix. 3. Coupling agent A-151 increases the tensile strength and the 100% modulus of the vulcanizate. Coupling agent B-5 increases the tensile strength of the vulcanizate and decreases the 100% modulus. 4. There are more compact relations among T, DT and tensile strength, and connections among E, D C and 100 % modulus. Because T, D T and tensile strength reflect, from different angles, the total interaction between mineral filler and silicone rubber matrix, E, D C and 100% modulus reflect chemical interaction. On the basis of the researches above, natural minerals were prepared in ultrafine form treated with coupling agent. Silicone rubber fillers with high reinforcing power were prepared 2. ACKNOWLEDGEMENTS This program was supported by the National Natural Science Foundation of China (No. 59572033). REFERENCES 1. Dannenenberg E.M., Filler Choices in the Rubber Industry, Rubb. Chem. & Technol. 55, (1982) 860 2. Wu J-H., Huang J-L. and Chen N-S., Chemical Modification of s and Its Application as Silicone Rubber Reinforcing Filler, Chemistry Letters (6), (1998) 509 3. Wolff S., Chemical Aspects of Rubber Reinforcement by Fillers, Rubb. Chem. & Technol. 69, (1969) 325 4. Wu J-H., Huang J-L. and Chen N-S., Deep Processing of s and Preparation of Silicone Rubber Reinforcement Filler, Chinese Acta Petrologica et Minetalogica 16 (4), (1997) 207 5. Wu J-H., Wei C-R. and Wu W-D., Rubber Reinforcing Filler Made by Surface Modified Clay Micropowder, Chinese J. of Materials Research 11 (5), (1997) 535 6. Wu J-H., Huang J-L. and Chen N-S., Study on Bound Rubber in Silicone Rubber Filled with Modified Ultrafine Powder, Rubb. Chem. & Technol. 73 (1), (2000) 19 7. Sombatsompop N., Practical Concerns Regarding the Use of the Mooney-Rivlin Equation to Assess Degree of Crosslinking of Swollen Rubber Vulcanisates, Polymer & Polymer Composites 7 (1), (1999) 41 8. Sombatsompop N. and Chrustodoulou K., J., Penetration of Aromatic Hydrocarbons into Natural Rubber, Polymer & Polymer Composites 5 (5), (1997) 377 9. Ou Y-C. and Yu Z-Z., Rubb. Chem. & Technol. 67, (1994) 834 10. Wu J-H., Huang J-L. and Chen N-S., Surface Energy of Powder and Interaction between Silicone Rubber Matrix and Filler, J. of Material Science Letters 18 (6), (1999) 461 11. Dannenenberg E. M., Bound Rubber and Carbon Black Reinforcement, Rubb. Chem. & Technol. 59, (1986) 512 174 Polymers & Polymer Composites, Vol. 9, No. 3, 2001