Лако-красочные материалы — производство

The free radicals first react with the chemical inhibitor which has previously been added to the resin, since the inhibitor material must be chemically dissipated before any reaction between free radicals and the ^=C double bonds can proceed [5]. Apparently, the free radicals serve to open the double bonds in the polyester linear chain to set in motion that portion of the polymerization process designated as initiation. Either the opened double bonds react with the vinyl groups of the monomer, or the free radicals serve to also open (add to) these latter unsaturated C=C bonds, permitting them to perform their cross­linking function, uniting the polyester chains into a three-dimensional network. There is further evidence that free radicals may also, to some degree, react with the unsaturated monomer to form various products of decomposition [5].

Theoretically, the reaction of polyesters should go to completion with all the double bonds reacted upon by free radicals and complete cross-linking established under the most favorable conditions. However, in actual practice, as determined by iodometric analysis, the true amount of residual unsaturation (indicating how far the polymerization has not gone) has been traced in the actual curing of polyesters, and can be summarized as follows:

1. What may be considered as an optimum cure with full-properties potential realized occurs when 92 to 95% of the unsaturation has been converted. Neither extra catalyst nor postcuring will convert this slight amount of remain­ing unreacted material.

2. The failure of all unsaturated sites to become reacted during final cure accounts for the discoloration of polyesters upon weathering and long-term aging. The unreacted double bonds eventually take up oxygen due to the action of sunlight and other factors, and peroxides are formed, creating a yellowish or amber color.