Methacrylation of kraft lignin for UV-curable coatings: Process optimization using response surface methodology

Highlights: • Methacrylated lignin (ML) proved to have improved hydrophobic properties. • The methacrylation process was optimized by response surface methodology (RSM). • Lignin methacrylation at the optimal conditions led to 146.5% weight gain. • ML demonstrated to be a promising bio-based UV-curable material for coatings. -- Abstract: Lignin, as the world's second most abundant renewable polymer, is an under-utilized sustainable resource. Most technical lignin, in particular kraft lignin (KL) – a byproduct from pulping process, is used as a fuel for recovery boilers. Thus, developing approaches to transform this under-utilized sustainable resource into a high-value bio-product, such as methacrylated lignin (ML) for UV-curable coatings, is of utmost importance to sustainability. In this work, methacrylated lignin (ML) was produced by methacrylation of KL, and the process was optimized using response surface methodology employing a central composite design (CCD). Three variables were examined and optimized via the CCD: reaction time, reaction temperature, and catalyst/lignin molar ratio, and their corresponding effects on the product yield were investigated. The mathematical model that was derived from the employed CCD was accurate in predicting the optimal reaction conditions. At the optimal reaction conditions (54 min, 54 °C and catalyst/lignin molar ratio of 0.225), the ML product yield of 146.5% was achieved. The ML materials were characterized using ¹H NMR and FTIR spectroscopies, as well as thermogravimetric analysis. A UV-curable coating system containing 30 wt% ML with a hexafunctional siloxane-based crosslinker and photoinitiator was shown to be promising.