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The radiation curing industry is searching for new polymerisable materials as alternatives to
acrylates. Whilst acrylates possess many admirable features such as speed of cure, and there is a plethora of commercially available materials which give coatings having a wide range of properties, there is still concern about their toxicity and this is fuelling the search for new materials. Currently there is great interest in a totally different radical-mediated reaction,
namely the initiated copolymensation of vinyl ethers with maleate and other unsaturated
esters 1]. The closely related copolymerisation reaction of vinyl ethers with maleimides is
particularly interesting since irradiation of mixtures of the reactants, in the absence of a
photoinitiator, leads to polymerisation 2]. A further advantageous feature of these systems is
that they are somewhat less sensitive to oxygen inhibition than acrylate based systems.
Apart from the curing characteristics of these systems it is important to consider the toxicity of
the components. Maleate esters and related species and maleimides undergo the Michael
addition reaction and therefore may be expected to be irritants. Vinyl ethers appear to be of
low toxicity but they are very susceptible to acid-catalysed hydrolysis which releases pungent
smelling acetaldehyde. The closely related species, propenyl ethers are also susceptible to
acid-catalysed hydrolysis which generates the odorous propionaldehyde. This propensity to
undergo hydrolysis may be reduced, but in many cases not eliminated, by either introducing
an amine into the formulation or appending hydrophobic groups to the unsaturated ethers.
Vinyl and propenyl ethers also undergo cationic curing. These reactions are particularly rapid
and do not suffer from oxygen inhibition. The rate of cure of vinyl ethers appears to be more
rapid than that of oxiranes (epoxides) and the cure appears to be less sensitive to high
humidity. However two negative features of the reaction is the presence of hydrolysis
products in the coatings and as with every other cationic system, the poor selection of
commercially available initiators. To obtain photoinitiating systems that work at> 350nm. one
has to resort to the use of sensitisers.