A photo-curing study of a pigmented UV-curable alkyd

The photo-curing of a pigmented UV-curable tung oil alkyd system (UVTA) was investigated. The UV-curable alkyd was a Diels–Alder Adduct of trimethylolpropane trimethacrylate with a tung oil-based alkyd. A mixture of yellow iron oxide, red iron oxide, titanium dioxide and lamp black pigments were chosen to obtain an observed yellow color. The pigment mixture, UV-curable alkyd and a reactive diluent were formulated together and photo-cured via a free radical mechanism. The through and surface cure was assessed by thumb twist, blocking resistance and MEK single rub. The through cure and surface cure assessment showed that optimum cure was achieved using UV-A as a source with two photoinitiators: phenylbis(2,4,6-trimethylbenzoyl-phosphine oxide) at 2 wt% and 1-hydroxy-cyclohexyl-phenyl-ketone at 4 wt%. The effect of reactive diluent was also investigated. Ten different reactive diluents were used. After curing, pencil hardness, conical mandrel, impact resistance, cross cut adhesion and gloss were measured. In addition, dynamic mechanical thermal analysis (DMTA) was used to evaluate selected systems. The coatings tests showed that the use of long chain linear structure diluents, such as isodecyl methacrylate, provided UV-curable paint films with better impact resistance, flexibility, and adhesion over other reactive diluents.