Chain 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. 

Once cured, PDMS networks are essentially made of dimethylsiloxane polymeric chains crosslinked with organic linkages. The general and inherent molecular properties of the PDMS polymers are therefore conferred to the silicone network. Low surface energy and flexibility of siloxane segments are two inherent properties very useful in adhesion technology. 

In 1983, radiation curable 100 % solvent-free silicone acrylates were introduced into the market [4], This system provide the opportunity to be cured by either ultraviolet light (UV) or electron beam (EB). Similar in concept to peroxide initiation, silicone acrylate systems employ photoinitiators to generate free radicals and initiate cure, which is based on the polymerization of the acrylic C=C double bond via a radical chain reaction. 

There are numerous applications where cure reaction by-products are not acceptable as they may contaminate other sensitive areas of the devices, and where fast deep section cure is required. These constraints have directed the choice to the platinum-catalyzed hydrosilylation addition cure system. The reaction involves the addition of a hydrido silane (SiH) to an alkenyl organic group (-CH=CH2), typically the vinyl or hexenyl groups. The product of the reaction is the ethylenic bridge (-CH2-CH2-). The alkenyl groups are usually placed at the end of the polymer base chains in structures such as The SiH groups are usually placed in a combed structure within the polymeric or copolymeric chains called cross-linker M-D -M or M-D -Dy-M. (The M, D, etc. nomenclature is explained in Silicones structures.) Typical values of n cover a wide range from 100 to 1000, while the values of x and y vary from 3 to 100. 

Pb(C2H5)4 is used as a catalyst for side-chain alkylation of toluene with ethene [731], for the reaction of alkenes with SO2 [635], for the halogenation of alkanes such as ethane, propane, or pentane in the gas and liquid phase [123, 650], for vulcanizing a mixture of acetaldehyde copolymers with acrolein or other unsaturated compounds, and a natural rubber [858], for curing caoutchouc [629], and for the polymerization of rosin or rosin acid derivatives [493, 704]. Pb(C2H5)4 is part of room-temperature, light-vulcanizable silicone rubber compositions [875]. 

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