Silane Coupling Agent

One of the problems when dealing with silica and clay products is the poor bonding between rubber polymer and silica or clay. As an effective way of overcoming this issue, chemical suppliers came up with the Silane Coupling Agents. The silane coupling agent when used would produce results of a reduced viscosity, and a significant increased of modulus and tensile strength.

The issue of poor bonding of silica with rubber is due to the fact that silica is hydrophilic and incompatible. Silica to silica attraction is high and large aggregates are formed which then impedes the flow. The mix of silica in rubber thus becomes very stiff and difficult. For example a 55 PHR of common silica will provide a Mooney viscosity (at 1+4′ ML 100°CO) of about 90, which is very stiff making the rubber very difficult for further processing. Although silica would provide reinforcing effect the compounding issues and dispersion makes it difficult for processing especially for higher loading of silica.

One of the ways to overcome the compounding of silica and clay is with the use of silane coupling agents and it can be used with great effect though the use of silane coupling agent may be costly, it would still provides enhance properties which would otherwise be difficult to achieve.

Two coupling agents are most frequently used, mercaptopropyltrimethosysilane and bis (3- (triethosxisily) propyl) tetrasulfane. A typical commercial example of the first type of silane coupling agent made was by Union Carbide with the grade A189 and the second is the Degussa’s reinforcing agent called the Si 69. According to Degussa, the reinforcing agent Si69 reacts chemically via the filler reactive ethoxisilyl groups with the silanol groups of the silica surface forming a stable siloxane bonds. In addition, the chemical reactions between the tetrasulfane group and the elastomer during the vulcanizing process links the silica filler with the elastomer matrix.

The usage of silane coupling agents, and depending on the polymers used and the compounding techniques employed, a wide variations in properties can be obtained. Tensiles can go up as much as 4200psi, elongations up to 850%, and hardness can be varied from 40 to 80 Shore A. Compression set (ASTM method B) of around 70 after 3 days at 100°C may be high but can be considerably reduced using an efficient curing system.

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