Radiation chemistry aspects of polymerization and crosslinking. A review and future environmental trends in 'non-acrylate' chemistry

Radiation induced polymerization of acrylate based materials has been in industrial use for almost twenty years. This field is still growing rapidly and the advantages of the technology, e.g. being solvent free and rapid 'cure', are very attractive properties from an industrial and environmental point of view. Mixtures of mono- and multifunctional acrylates are today the 'heart' of radiation curable systems. However, serious concerns about the health hazards connected with the handling of the liquid coating is an important issue. Different approaches to overcome this problem have been addressed and the obvious thing to start with was the investigation of a great number of acrylates regarding their potential health effects. Acrylates do exhibit some specific unattractive properties, regarding their toxicology profile. Sensitization and skin irritation are considered to be limiting factors for a continuing rapid expansion of this successful technology. A further development and improvement of the photoinitiator chemistry together with a more efficient use of high powered irradiators with a narrow bandwidth distribution, will improve still the acrylate technology. The importance of degree of conversion of carbon-carbon double bonds as a function of dose rates, i.e. residual unpolymerized acrylic monomers, will be discussed. An additional approach, since the acrylate technology is well established, is to further optimize the 'reactivity' and conversion on 'new' acrylates in order to obtain maximum conversion, thus minimizing the residual amount of monomers that can migrate out of the coating. The increasing functionality of the acrylate/oligomer will of course result in an increasing probability of monomer/oligomer being attached to the crosslinked network. Furthermore, this paper emphasizes potential acrylate replacements, and 'new' or alternative chemistry for acrylates will be introduced. The state of the art and the associated problems for cationically induced polymerization, free radical alternating copolymerization, hybrid systems and direct photolysis of donor/acceptor pairs are discussed.