Auto-cross-linking reactions

Flexible foam is made from long-chain diols with a small amount of triols for cross-linking to give strength and resilience. Foaming is produced by reaction between measured amounts of isocyanate and water to liberate carbon dioxide. Molded products are made by pouring the reactive liquid mixture into a mold cavity at 50°C. They foam and cure in 2-10 min, after which they are temporarily crushed or vacuum-shocked to open the cell walls and insure softness. Then they are allowed to condition for several hours to finish the cure. The major products are auto seating and headrests, and furniture cushions. 

Other reactions must be mentioned beside the major reactions described above. These reactions may be responsible for the transformation of a toxic metabolite into the ultimate toxicant. Rearrangements and cyclizations are examples of reactions involved in these processes. In the case of the solvent hexane (Figure 33.19), the toxic metabolite, 2, 5-hexanedione, is formed by four successive oxidations of the molecule. The condensation of the -dicetone with the lysyl amino group of a neurofilament protein is followed by a Paal-Knorr cyclization reaction. This is the initial process that explains the hexane-induced neurotoxicity." A further auto-oxidation of the A-pyrrolyl derivatives leads to the cross-linking of the axonal intermediate filament proteins and the subsequent occurrence of peripheral neurotoxicity." ... 

As it was noted above, cross-linked polymers formal curing kinetics could be described by the Eq. (16) of Chapter 2. Besides, there are two modified variants of the indicated equation accounting for curing reaction proceeding with autoacceleration or auto-deceleration [29] ... 

Auto acceleration at relatively high conversion can also be caused by formation of fi"ee radicals from the polymer that has been produced (Woods and Pikaev 1994). The yield of radicals from the polymer, which is usually a saturated chain, maybe higher than the yield from the unsaturated monomer leading to an increase in the rate of polymerization due to the increased radical population. Termination reactions of these radicals with chain-end radicals give rise to branched chains (graft homopolymer), while self-termination of mid-chain radicals brings about cross-links. 

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