New UV-PUD for Outdoor Application on Wood

摘要

Radiation curable polyurethane dispersions (called UV-PUD) have benefited over the last years from a continuous growth in a market that recognizes in them a high-end performance for innovative markets. These products combine the advantage of (ⅰ) the polyurethane chemistry (ⅱ) the radiation curing technology and (ⅲ) the physico-chemistry of aqueous colloidal dispersions. The chemical structure of UV-PUDs is presented in the framework of new coating developments for improved outdoor resistance. These coatings offer a competitive advantage for certain industrial applications on wood, like joinery. The mechanisms of polymer degradation are shortly summarized. In particular, the model chemical structure of an unsaturated polyurethane is outlined and the essential chemical modifications of the backbone are described for promoting an increased resistance to hydrolysis and ultraviolet light. The polymer modifications involve the stoechiometric replacement of diisocyanates and several linear telechelic diols based on polyesters, polyethers, polyacrylates, polybutadienes, polycarbonates and polysiloxanes. The comparative effect of dimethylol propionic acid against dimethylol butanoic acid is also reported. Finally, the influence of critical additives like UV-absorbers, light stabilizers (HALS) and photo-initiators are outlined using electron beam curing as a comparison against UV-irradiation. The formulation of the aqueous dispersion and its application on wood by spraying are described in detail. The mechanical and chemical resistance of the coatings are assessed using a standard protocol. The test method for artificial weathering uses repeated cycles of ultraviolet light with defined temperature & relative humidity patterns (Atlas C4000 Weather-O-Meter). It is inspired by the indications of the European directive EN927. The comparative aging results of the coatings are statistically analyzed by determining a critical time (tc) linked either to the significant apparition of defects on the wood surface or to the complete destruction of the sample. The results are accompanied by pictures that visually show the evolution of the samples during the test. Some aging correlations have been made with the same coatings on a neutral, treated metallic substrate and with free films submitted to mechanical analysis. In the case of a polyester-based unsaturated polyurethane model, the use of photo-initiators reduces considerably the life time of the coated sample while, surprisingly, no significant improvement has been recorded following the use of Uvabsorber and light stabilizers (HALS) in the coating. The best performances were obtained with a particular polycarbonatebased variant. In that case, it was possible to significantly increase the longevity of the coated wood in our test conditions, while presenting a similar level of mechanical and chemical performance.