Thermoset polymers are chemically cross-linked and are ideal for use with reactive modifiers such as silane coupling agents. Indeed, much of the early development of the silanes was in the glass fiber-filled thermosets. The silane types most commonly used are vinyl, methacryl, epoxy, and amino. Among the fillers, commonly treated are various silicas and silicates and the flame retardant aluminum hydroxide. As with elastomers, the “in situ” treatment method is favored over precoating, and there is the possibility of some alcohol remaining in the product.
In thermosets, the main effects of coupling agents are seen in property retention after exposure to moisture, rather than in initial properties. This property loss has long been a weak point of reinforced thermosets and so the effect is most important. In addition, improvements in initial impact strength are also often seen.
More details will be found in the chapter on thermosets (? Chap. 6, “Particulate Fillers in Elastomers”).