Formulation and optimization of radiation-curable nonisocyanate polyurethane wood coatings by mixture experimental design

Abstract

Radiation-curable urethane acrylates have been extensively used and successfully implemented in industrial wood coatings thanks to their capability to provide a balance of mechanical and chemical properties. However, isocyanates, one of the main building blocks in the conventional urethane acrylates, pose toxicity and health hazards both in the manufacturing and application, and therefore, are targeted for restricted use and limited occupational exposure in the impending environmental regulations. In this regard, this study presents the development of urethane acrylate oligomers using nonisocyanate approaches and their application in industrial wood coatings. Two acrylic-functional nonisocyanate polyurethane oligomers (NIPU-ACs), NIPU-AC-2 with longer urethane chains and higher flexibility and NIPU-AC-1 with shorter urethane chains, were synthesized as main building blocks of radiation-curable wood coatings. Next, a series of 20 wood coating systems were formulated using a D-optimal mixture experimental design methodology to find the optimized quaternary mixtures composed of the two synthetic NIPU-ACs, and trimethylolpropane triacrylate (TMPTA) and dipropylene glycol diacrylate (DPGDA) as reactive diluents. The results of the study revealed that at a constant 5 wt% of TMPTA, formulations rich in NIPU-AC-2 showed higher impact resistance, scratch resistance, and pull-off adhesion, which was attributed to the flexible structure of this oligomer. On the other hand, coatings including higher contents of NIPU-AC-1, which induces higher crosslink density (XLD) to the system, demonstrated higher Tg and pendulum hardness. Evaluation of the optimal coatings not only validated the predictability of models, but also determined that the coatings have promising stain and abrasion resistance, and good thermal stability. The results show that NIPU-ACs developed in this study have excellent potential as an alternative to conventional urethane acrylates in the development of low-VOC and sustainable industrial wood coatings.