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

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.