Step polymerization silicone cure

The mechanism of cure is quite different for cyanoacrylates on one hand and silicones and isocyanates on the other. In the former case, cure is by Chain polymerization initiated by water on the surface of the substrate, while in the latter, water from the atmosphere diffuses into the adhesive or sealant and then participates in a Step polymerization. One consequence of this is that only catalytic qnantities of water are reqnired to cure cyanoacrylates, while much larger, stoichiometric quantities are needed for silicones and isocyanates. 

Silicones are widely used in industry, for example, as adhesives, sealants and release agents see Silicone properties and Silicone adhesion. Silicone adhesives may cure by condensation or addition reactions. The former is considered here and the latter in Silicone adhesives addition cure (see also Step polymerization). 

Curing due to moisture also takes place with some silicone adhesives with hydrolysable end groups (acetates in the example shown below). Hydrolysis produces silanol end groups, which then react in a step polymerization. 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

Rigid composites are obtained by mixing graphite powder with a non-conducting polymeric matrix into a soft paste that becomes rigid after a curing step [52,53]. They could be classified according to the nature of the binder or the polymeric matrix into epoxy composites, methacrylate composites or silicone composites. 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

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