Dark cure

Epoxy resins and other polymers (tetrahydrofuran, vinyl ethers, styrene, etc.) can be cured when exposed to an acid or cation intermediate species. The photoactive catalyst system commonly used to cure epoxy resins and multifunctional vinyl ether materials is composed of salts of aryldiazonium, triarylsulfonium, and diaryliodonium. These systems are commonly employed in coatings and adhesives for electronic products. The acid initiator generated from the photoinitiator continues to be active even after uv curing, and so conversion of reactants and crosslinking continue even in the absence of uv light. This phenomenon is typically referred to as dark cure. 

Sims, J. L., and Wells, K. L. (1990). Response of burley and dark-cured... 

In the presence of triarylsulfonium and diaryliodonium salts, polymerization continues even if UV irradiation is terminated. This phenomenon is called dark cure and is due to the living nature of the superacid generated cation. The cure regime can be thought of as UV-initiated but thermally cured. Thermally initiated cationic catalysts are also available (129). 

Cationically cured UV laminating adhesives based on cycloaliphatic epoxies are emerging as an alternative to solvent-hased adhesives. The dark cure of cationics allows UV exposure and post lamination in hne. This process does not require UV exposure through the plastic barrier material. 

The postpolymerisafion, which is known to occur just after the UV-exposure in cationic photocuring, was easily monitored by RTIR spectroscopy, as shown in Fig. 7 for DVE-3 (dashed curve). It can be seen that, after a 0.7 s exposure to UV-radiation, the vinyl ether continues to polymerise in the dark, the conversion rising from 10% to over 90% within a few seconds. One can take advantage of such an important dark curing process to assemble non-transparent foils by means of... 

UV-curable adhesives. Fig. 8 shows a schematic representation of the experimental setup. By running the UV-line at a speed of 1 m/s, the second foil will be laminated on top of the irradiated foil only 0.5 s (50 cm) after the UV-exposure, so that most of the VE polymerisation will occur in the dark. UV-curable epoxy resins were also successfully used as adhesives to laminate opaque films by such an ultrafast dark-cure mechanism [56,62]. 

Carter, J.W., Lamb, K.T., Cationic UV-adhesives laminate opaque fihn via dark cure mechanism (pp. 9 9099), Adhesives and Sealants Industry August 1999. Cationic UV-cure of Cyracure cycloaliphatic epoxides (pp. 1-40), Technical brochure from Union Carbide Corporation (now Dow Chemicals), 1999. 

Pappas, S.P. and Woods, J UV-curable adhesives with auxiliary dark cure processes (pp. 195-200), Proc. RadTechAsia, 1991. 

Carter JW, Lamb KT. Cationic UV-adhesives laminate opaque film via dark cure mechanism. Adhesives and Sealants Industry, August 1999 90-99. 

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