Dual-cure adhesive systems

The disadvantages of UV adhesives are that one transparent substrate is normally required they suffer from oxygen inhibition and only a limited depth of cure can be achieved. The latter problem has been tackled by the development of dual-cure adhesives. In these systems, two independent curing mechanisms are incorporated into a single system. Thus the adhesive is cured first to a chemically stable form by UV irradiation and subsequently led to fiiU cure by a second means, for example, thermal cure. 

Dual-cure resins for wood composites. These resins (heat and EB irradiation systems) are based on urethane-acrylate thermoplastic adhesive demonstrate efficacy to bond wood. Eow amounfs of EB radiation were needed to produce a wood bond that survived shipping and that improved the composite modulus. The system demonstrated promise and a new direction for wood adhesives. 

When UV-curable resins are used as laminated adhesives to protect three-dimensional objects having complex shapes, it may happen that some remote areas wiU not be accessible to UV-radiation and therefore remain uncured. To address the issue of such shadowed areas, dual-cure systems combining UV-irradiation and thermal treatment have been developed [71]. They contain two types of reactive functions a UV-curable functional group (usually an acrylate double bond)... 

The cross-linked nature of UV/EB-cured adhesives results in good chemical, heat, and abrasion resistance toughness dimensional stability and adhesion to many substrates. Unlike thermal curing, EB or UV curing requires pure raw material grades. The use of dual-curing systems allows opaque substrates to be cured. 

However, an adhesive s ability to adhere to specific substrates must be considered. Further progress has been achieved with dual-cure systems the outstanding adhesive performance of polyurethane systems has been combined with the quick curing of free-radical formulations. If including isocyanate components one needs to consider the residual isocyanate monomer content in isocyanate-terminated compounds... 

Recently, Endo and coworkers have developed novel sulfonium type initiators that can initiate polymerization either upon irradiation or upon thermal treatment. In addition, these photoinitiators are shown to be functional for both cationic and radical polymerizations. This dual activity is particularly important in hybrid curing systems for coatings and adhesions [28]. Scheme 11.6 contains the general steps for the synthesis of these photoinitiators from sulfides. 

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