Novel processing agents for silica-filled rubber.

Great interest has been devoted to the use of silica as a reinforcing filler for tire treads, because of improved wet traction, snow traction and reduced rolling resistance (ref. 1). The quantity of silica used in the tire tread compound has increased every year and silica-filled tires may become dominant. Silane coupling agents have played an important role in the silica-filled compounds to attain improved reinforcement, and the detailed mechanism of the reactions between silane coupling agents and silanol groups on the silica surface has been reported (ref. 2). Especially, a curable silane coupling agent, TEPST [bis(3-triethoxsilylpropyl)tetrasulfide], being available as Si69 from Degussa AG, has made an important contribution in developing the silica-filled tire treads.

On the other hand, the problems of silica-filled compounds, such as the viscosity increase of the unvulcanized compounds, the delayed cure rate and the cost of the coupling agent, still remain. It is significant for the tire industry to facilitate the use of silica without such drawbacks. Some processing additives for improving extrusion rate and silane coupling efficiency were reported (ref. 3). In our study, polysiloxanes having bifunctional groups, silica-reactive and rubber-affinitive, in a sense, surface-active agents for mixing rubber and silica, have been developed (figure 1).

[Figure 1 ILLUSTRATION OMITTED]

This presentation describes investigations relating to the alkoxy polysiloxanes, i.e., preparation of the polysiloxanes, their effects on processing characteristics, property of the vulcanized compounds and the reaction mechanism of the polysiloxane with silica in the compounds.

Experimental

Materials

Poly(methylhydrogensiloxane) being available as KF99 from Shin-etsu Chemical Industry, TEPST (Si69 from Degussa AG), methanol, ethanol and isopropanol were synthesized and used as received. Alcoxy polysiloxanes were used in this study (figure 2). The synthesis was carded out by alcoholysis of poly(methylhydrogensiloxane) in the presence of a catalyst.

[Figure 2 ILLUSTRATION OMITTED]

Procedure

Preparation of alcoxy polysiloxane

A representative procedure is described below. To a 1,000 ml three-necked flask, equipped with a reflux condenser, a dropping funnel and magnetic stirrer, 75 g ethanol and 50 gl 2.5% isopropanol solution of hexachloroplatinic acid ([H.SUB.2]Pt-[Cl.sub.6] [H.SUB.2]0) were added. Poly(methylhydrogensiloxane), 100 g, was added dropwise to the stirred reaction mixture at 60 [degrees] C adjusting the rate of dropping to control the evolution of hydrogen. The reaction temperature was raised to 80[degrees]C and 50 [micro]l platinum catalyst was added again after the evolution of hydrogen ceased. Then the reaction was continued for eight hours. Subsequently, the residual ethanol was removed under reduced pressure. Poly(methylethoxysiloxane) is a transparent, colorless and viscious liquid.

Reaction of polysiloxanes with silica

In order to determine the reactivities of polysiloxanes and silane coupling agent, silica .was reacted with polysiloxane and/or silane coupling agent for one hour at a temperature of 100[degrees]C and unreacted silicone compounds were analyzed.

Formulation of silica-filled compounds

A series of silica-filled compounds was prepared with formulations shown in tables 1 and 2.

Table 1 - formulation used in the effect of alkoxyl
groups, effect of amounts and curing behavior

Ingredient                            Parts

NR                                     50
SBR (Zeon NS116)                       50
Silica (Nipsil AQ)                     50
Diethylene glycol                       2
Zinc oxide                              3
Stearic acid                            1
Antioxidant (6C)                        1
Sulfur                                1.70
DPG accelerator                         1
CBS accelerator                       0.50
Silane coupling agent (Si69)       2.5 or 5.0
Alkoxy polysiloxane                 2.5 - 5.0
Table 2 - formulated used in the effect of conversion

Ingredient                            Parts

SBR (SBR 1500)                         100
Silica (Nipsil AQ)                     40
Diethylene glycol                       2
Zinc oxide                              3
Stearic acid                            1
Antioxidant (6C)                        1
Sulfur                                 1.7
DPG accelerator                         1
CBS accelerator                        0.5
Silane coupling agent (Si69)            2
Alkoxy polysiloxane                     2

Results and discussion

Preparation of alcoxy polysiloxane

In general, alcoxy polysiloxane with 2 to 5 Si atoms in a chain is prepared by partial hydrolysis of alkyltrialcoxysilane as scheme 1 (figure 3) (ref. 4). It is difficult to control the degree of polymerization by this method. In this study, alkoxy polysiloxanes having methoxy, ethoxy and isopropoxy moieties, respectively, were prepared by the reaction of poly(methylhydrogensiloxane) and corresponding alcohol in the presence of platinum catalysts as scheme 2. The reaction of Si-H group and alcohol proceeded with the vigorous evolution of hydrogen. Obtained alkoxy polysiloxanes were identified by [sup.1]H NMR spectra.

[Figures 3-4 ILLUSTRATION OMITTED]

In the NMR spectra of the polysiloxane in CDC13, the peak at 4.70 ppm owing to Si-H groups of the polysiloxane is significant information regarding the determination of the conversion of alcoholysis. Figure 4 shows a [sup.1]H NMR spectrum of ethoxy polysiloxane with 92% conversion.

Effect of alkoxy groups

Effects of alkoxy groups in the polysiloxanes on the processing characteristics were evaluated. Although every polysiloxane improves the processing characteristics, some differences are recognized among the kinds of alkoxyl group. It is particularly apparent that the methoxy and ethoxy polysiloxanes decrease compound viscosity and cure time, and that the ethoxy polysiloxane also increases scorch time. The processing characteristics of silica-filled compounds such as viscosity, cure time and scorch time could depend largely on the amounts of silanol groups on the silica surface. In this sense, reduced amounts of the silanol groups would give high processing performances. Here, the reactivity of the polysiloxane to silanol group should be affected by the kind of alkoxy groups. The order of the reactivity is known as: [CH.SUB.3]0 [right arrow] [CH.SUB.3]C[H.SUB.2]0 [right arrow] ([CH.SUB.3])2CHO-. Although the methoxy group is considered to be superior to diminish merely the silanol groups of the silica during the mixing in the silica-filled compounds, these results indicate that the ethoxy group is the best alkoxy group to improve the processing characteristics.

Effect of conversion for ethanolysis

Ethoxy polysiloxane was prepared by the reaction of poly(methylhydrogensiloxane) and ethanol as mentioned above. It was easy to convert Si-H groups into ethoxy groups at low conversions, while very difficult to complete the reaction. A high conversion of the ethanolysis is technically important for the production of ethoxy polysiloxane. The effect of conversion of ethanolysis on the processing characteristics is evaluated.

There is no significant difference in the compound viscosity and scorch time when the conversion is increased. On the other hand, cure time is a little shorter at the low conversion of ethanolysis. It is uncertain whether Si-H groups accelerate the cure or prevent the adsorption of accelerator on the surface of silica, which is considered to be one of the reasons for the delayed cure rate in the silica-filled compound. At any rate, the conversion of ethanolysis is not considered a significant factor to influence the processing characteristics.

Reduced compound viscosity, rapid cure and elongated scorch time are found to be achieved for the silica-filled compounds by using various ethoxy polysiloxanes.

Effect of amounts of ethoxy polysiloxane

As mentioned above, ethoxy polysiloxane among the several polysiloxanes shows an apparent contribution to the improvement of processing characteristics in the silica-filled compounds. These results are considered to be due to depressed adsorption of cure accelerator and increasing affinity of silica to rubber by protecting acidic silanol groups by using ethoxy polysiloxane.

The effect of amounts of ethoxy polysiloxane on abrasion resistance is investigated. Figure 5 shows the change of abrasion resistance using 5 wt% of TEPST [bis(3-triethoxysilylproply)tetrasulfide] corresponding to a half amount of general practice (10 wt% of silica) and variable ethoxy polysiloxane. Abrasion resistance of the compound with 2 wt% ethoxy polysiloxane and 5 wt% TEPST correspond to that with 10 wt% TEPST. This result suggests that ethoxy polysiloxane probably improves the dispersity of silica in the compounds and/or the efficiency of TEPST. Actually, according to the photomicrographs of silica-filled compounds, the dispersity of silica was improved by adopting ethoxy polysiloxane. The results indicate that costly silane coupling agent can be reduced by adopting ethoxy polysiloxane.

[Figure 5 ILLUSTRATION OMITTED]

Curing behavior

Figure 6 shows the curing behavior of silica-filled compounds with and without ethoxy polysiloxane. Time-torque curves of the silica-filled compound after adding ethoxy polysiloxane and a half amount of TEPST (5 wt% for silica) at 160oC show a fast cure rate after a long induction period. On the other hand, the compound with only TEPST added shows a much slower cure rate after a short induction period, a typical curing pattern of silica-filled compounds. These torque curves clearly indicate the effects of the addition of ethoxy polysiloxane, rapid cure and scorch safety.

[Figure 6 ILLUSTRATION OMITTED]

Dynamic property

Figure 7 shows the effect of ethoxy polysiloxane on the dynamic properties in the silica-filled compounds. It is well known that silica-filled compounds have a low tan 8 value at 60 [degrees] and a high tan [Delta] value at -10[degrees]C compared with blackfilled compounds. The trend indicates wet performance and rolling resistance.

[Figure 7 ILLUSTRATION OMITTED]

By adopting ethoxy polysiloxane, further improvements both at 60 [degrees] C and -10 [degrees] C are shown to be obtained in the silica-filled compound. The reason for the further improvement is considered to be the improved dispersion of silica particles. Because double the quantity of ethoxy polysiloxane gave no further improvement of tan [Delta] values.

Mechanism of improving processing characteristics

The silanol groups on the silica surface react with the alkoxysilyl groups to form siloxane bonds. A detailed study of the reaction mechanism of TEPST with silica has been reported (ref. 2). The reaction of silica with TEPST and/or ethoxy polysiloxane were evaluated to clarify the function of ethoxy polysiloxane in the silica-filled compound. Table 3 shows the results of the reaction carried out at 100[degrees]C for one hour.

Table 3 - reactions of silica with TEPST and/or
ethoxy polusiloxane

Silica                    100   100   100      100    100     100
TEPST                20          10    20              10
Ethoxy
polysiloxane                     20    10              20      10
Ti[(OiPr).sub.4]                               0.5    0.5     0.5
Conversion (%)(1)          51    65    67(2)    80     99   99(2)

1) Determined by weighing unreacted TEPST or ethoxy
polysiloxane

2) Conversion of TEPST

Conversion of the reaction for TEPST and silica was only 51%, which is lower than that of ethoxy polysiloxane. The reaction of TEPST in the presence of ethoxy polysiloxane was accelerated. Furthermore an almost quantitative conversion of the reaction for TEPST and silica was obtained in the presence of ethoxy polysiloxane and an accelerating catalyst such as tetraisopropoxytitane. This is one of the reasons why the efficiency of TEPST was increased by using ethoxy polysiloxane. Further, silica treated with ethoxy polysiloxane becomes hydrophobic and can float on the surface of water.

The roles of ethoxy polysiloxane in the reaction of TESPT are postulated as follows:

* Ethoxy polysiloxane prevents TEPST from moving into the inside of silica. TEPST reacted with silanol in the inside of silica is considered to be poorly reactive to the polymers.

* Ethoxy polysiloxanes support the reaction of TEPST with silica. TEPST reacts with silica not only directly but also with the ethoxy polysiloxane which has reacted with silica.

Conclusions

* Novel processing aids, polysiloxanes having alkoxysilyl moieties are developed in order to improve the processing characteristics of silica-filled compounds. They can react easily with silanol groups on the silica surface.

* Ethoxy group is the best as the alkoxy group of the polysiloxane for improving the processing characteristics of the compounds.

* Improved compound properties, both unvulcanized and vulcanized, were obtained by adopting the ethoxy polysiloxane with a reduced amount of the silane coupling agent.

* Reaction of silane coupling agent to silica was accelerated by using ethoxy polysiloxane as a processing agent.

* The role of ethoxy polysiloxane concludes that the efficiency of silane coupling agent and dispersity of silica were improved.

References

[1.] S. Wolff, 129th Meeting of the Rubber Division, American Chemical Society, New York, April 8-11, 1986.

[2.] U. Gorl and A. Hunsche, 150th Meeting of the Rubber Division, American Chemical Society, October 8-11, 1996.

[3.] J.P.V. Kooi, R. Carmany, J. Pawlikowski and J. Sherritt, Int. Rubber Conference, Kobe, 26C-7, October 23-27, 1995.

[4.] H.J. Fletcher and M.J. Hunter, J. Am. Chem. Soc., 71, 2918(1949).