Heretofore, carbon black has been used as a reinforcing filler for rubber because carbon black provides higher reinforcement and more excellent abrasion resistance than other fillers. Recently, because of social requirements to save energy and to save resources, particularly to cut down fuel consumption of automobiles, a decrease in the heat buildup of rubber compositions is also required.

For decreasing the heat buildup of rubber compositions by using carbon black, use of a small amount of carbon black or carbon black having a large particle size is considered. It is, however, well known that, in both methods, decreasing heat buildup is in a contradictory relation with improving reinforcement and abrasion resistance of a rubber composition.

On the other hand, silica is known as a filler which provides decreased heat buildup of a rubber composition. However, silica particles tend to cohere together due to hydrogen bonding of silanol groups which are functional groups on the surfaces of the silica particles. For improving the dispersion of silica particles into rubber, the mixing time must be increased. When dispersion of silica particles into rubber is insufficient, a problem arises in that processability in processes such as extrusion and the like deteriorates due to the increase in the Mooney viscosity.

Moreover, the surfaces of the silica particles are acidic. Therefore, there are problems in that basic substances used as vulcanization accelerators are absorbed such that vulcanization is not carried out sufficiently, and a sufficient modulus of elasticity is not obtained.

In order to solve these problems, we developed various types of silane coupling agents. For example, SiSiB® PC2000 and SiSiB® PC2200. In the following article we would like to inform you about the silane coupling agent SiSiB® PC2000, SiSiB® PC2200 and its other commercial forms SiSiB® PC2000B and SiSiB® PC2000S.

SiSiB® PC2000 is a bifunctional polysulfidic organosilane for the rubber industry defined chemically as Bis(3-triethoxysilylpropyl)tetrasulfide. It is used to improve the reinforcing capability of fillers with silanol group on their surface (e.g., silicas, silicates, clay, etc.), and also as an integral part of curing systems to improve crosslinking network properties.

However, when the mixing temperature of rubber is low, the sufficient reinforcing effect is not obtained by this type of silane coupling agent. Dispersion of the silica into the rubber is also inferior, and this causes deterioration of the low heat buildup property that is the strong point of a rubber composition containing silica. Moreover, the silane coupling agent is hydrolyzed, and the generated ethanol does not vaporize sufficiently and vaporizes during extrusion. Thus, there is a drawback in that blisters are formed.

On the other hand, when mixing is conducted at high temperatures of 150.degree. C. or more, the reinforcing property is improved. However, as a drawback, the polysulfide is to decarbolize to form free sulfur during mixing at high temperatures of 150°C or higher. The free sulfur will result in the vulcanization of rubber at the temperatures. So that, in this temperature range, gelation of the polymer reduces the rheological property of system. Thus, processing in later stages becomes impossible in actuality.

To prevent gelation of the polymer, it is necessary to reduce the content of high polysulfide silane such as pentasulfide silane, heptasulfide silane, hexasulfide silane, and the like, in the polysulfide Silane. Therefore, as a result we developed a novel silane coupling agent SiSiB® PC2200 which is suitable for mixing at a high temperature.

The novel silane coupling agent SiSiB® PC2200 solves the above problems of the conventional art. The polysulfide structure in which the distribution of sulfur is specified can ensure its stability at high temperature. Thus, it can be avoided to decarbolize to form free sulfur. The silanol group on the surface of silica can react sufficiently with the silane coupling agent. And at the same time, the dispersion of the silica into a rubber is remarkably improved, furthermore there are no effect on the property processing. Thus it can improve the productivity of rubber processing and the low heat buildup property and abrasion resistance is improved.