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Thermoplastics

For our purposes, we need to divide thermoplastics into two types: condensation polymers (such as polyamides and polyesters) and addition polymers (such as polyethylene and polypropylene).

The condensation polymers are polar and have groups that are able to react with certain silane functionalities, such as amino, during melt processing. It is thus easy to obtain beneficial coupling effects and silanes are most widely used with this type of thermoplastic.

The polyolefins, on the other hand, are fairly inert and much harder to react with, due to their chemical inertness. A number of approaches have been used in the past including, for a short time, a very reactive azido silane. The use of silane coupling agents as additives for thermoplastics has also been reviewed by Godlewski and Heggs (1989). Today, the favored approach for coupling fillers into thermoplastics is to use acid functional derivatives of the polymer matrix (see earlier). Where these cannot react with the filler directly (e.g., with silicas and silicates), then the filler can be pretreated with an amino-silane. Coupling is then achieved by amide formation between the amine and acid functionalities.

Both “in situ” and precoating methods are utilized, but unlike elastomers and thermosets, precoating is more common. This is due to the difficulties in adding liquids during melt processing and also to problems that can arise from alcohol release in compounding machinery.

The reader is referred to Borup and Weissenbach (2010) for a recent review on silane coupling agents.

 
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