Redox and photoinitiated crosslinking polymerization

The redox polymerization of acrylic resins initiated by the benzoyl peroxide (BPO)/amine system has been used, in addition to photopolymerization, to harden the shadow areas of UV-cured clearcoats as well as pigmented coatings. Both reactions (redox- and photopolymerization) were followed quantitatively by infrared spectroscopy through the decay of the characteristic infrared band of acrylate double bonds and by hardness measurements. The spontaneous redox initiation was more specifically followed by pyrometry and near-IR spectroscopy. The BPO/amine redox system was shown to greatly accelerate the hardening of the coating in the shadow areas, a hard coating being obtained after a few minutes at room temperature. Because the formulation has a short potlife, it must be used as a two-pack system. Moreover, this type of redox initiation proved to be highly sensitive to oxygen. The viscoelastic properties of the thermally cured material were characterized by dynamic mechanical analysis, and were shown to be similar to those obtained by UV-curing alone. This exothermal initiation process is also highly efficient to cure thick pigmented specimens, and it has no detrimental effect on the weathering resistance of the coating.