Ionic curing, reaction mechanism

FIGURE 5.2 Reaction mechanism for ionic curing. (FromArcella, V. and Ferro, R., in Modem Fluoropolymers, Scheirs, J., Ed., John Wiley Sons, New York, 1997. With permission.)... 

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11... 

The disadvantage of curing fluoroelastomers by ionic mechanism is that dehy-drofluorination required for this reaction produces considerably more double bonds than required for the cross-linking itself. This excess of unsaturation represents weak points in the polymeric chain, which can be attacked by basic substances contained in a contact fluid. This has been actually found when parts cured by this method were exposed to new oil and fuels containing basic additives.19 20... 

The general disadvantage of curing fluoroelastomers by ionic mechanism is that dehydrofluorination required for this reaction produces considerably more double... 

Those mechanisms in which the TPP directly catalyzes the reaction can be called into question by the diffusion issue as well. Direct catalysis would require the bulky TPP to diffuse through the mixture as it polymerizes. The ionic conductivily decreases by many orders of magnitude as cure progresses, and the mobility of TPP decreases correspondingly. These mechanisms require that TPP orient with both epoxy and with phenol. Such favorable orientations between three reactants would become increasingly unlikely as the crosslink density increases and the resulting mobility decreases. 

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