These are specialist coupling agents, specifically designed for use in sulfur-curing elastomers. The two principle forms are y-mercaptopropyl trialkoxysilane and various polysulfidic silanes (especially tetrasulphide).
Another recent development is the use of oligomerized silanes. These are partly selfcondensed products and have the advantage of reduced volatility and decreased alcohol release (Mack 2002).
Examples of Silane Coupling Agent Effects in Filled Polymers
A brief account of the use of silanes in various polymer systems follows.
Elastomer applications are a major consumer of silane coupling agents, largely due to the size of the tire market. The polysulfidic types have the bulk of the market, but mercapto, vinyl, and amino also have significant use. They are mainly used with precipitated silicas and clays, with the “in situ” method being far more common than precoating. One limitation of this method is the retention of some of the eliminated alcohol in the final product. This is important where high addition levels are used (e.g., in some tire applications) and can cause problems with VOC (volatile organic constituent) regulations.
There is also a specialized market for silane-modified fillers in silicone elastomers. Here the main filler is fumed silica and precoating is more common.
The silane coupling agents markedly improve the adhesion between the filler and polymer, especially at the high strains often encountered in elastomer applications. The main benefits to be observed from the improved adhesion are higher modulus, higher tear strength, better fatigue resistance, and higher abrasion resistance. The polysulfides used with precipitated silica have played a major role in the development of fuelsaving tire tread formulations (green or energy tires); details of this can be found in the chapter on elastomer (? Chap. 6, “Particulate Fillers in Elastomers”).