|MadSci Network: Chemistry|
Not MEK (methyl ethyl ketone), MEK peroxide (methyl ethyl ketone peroxide, which is trimeric).
The weak peroxide linkage thermally undergoes symmetric scission to act as an energetic free radical source. The initiating free radicals then attack unsaturated polyester moieties as with acrylic polymerizations. The trick is to use an initiator that gives you the right rate of decomposition at the operating temperature. Too slow and you can wait forever as trace oxygen and other impurites grab the radicals before much happens. Too fast and the exotherm goes wild, as well as suffering radical recombination (second order kinetics prior to gelation - Trammesdorf effect?) which leads to low molecular weights and incomplete polymerizations.
Compare with Bondo, a typical filled styrene/unsaturated polyester resin where the resin contains a trace of dimethylaniline and the hardener is a phthalate ester paste of benzoyl peroxide. Benzoyl peroxide requires rather hight temps to set off (safety), but not in the presence of tertiary amines of the right one-electron redox potential.
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