Optimisation of lignin liquefaction with polyethylene glycol/ glycerol through response surface methodology modelling

Abstract

In order to diminish the dependence on oil lignin, which is the most abundant biopolymer of phenolic origin on Earth and can be utilised in different industrial endeavours, such as, the production of polyurethanes. In this study, hardwood (Eucalyptus globulus) and softwood (Pinus radiata) organosolv lignins were employed to produce bio-polyols trough microwave-assisted liquefaction. The resulting bio-polyols possessed specific properties to be employed in the synthesis of rigid and elastic polyurethanes. The values of the reaction parameter were optimized using response surface methodology to determine the most effective conditions for producing bio-polyols from both types of lignins for the purpose of rigid and elastic polyurethane formulation. The effect of catalyst concentration (%wt.), temperature (ºC) and Polyethylene glycol/Glycerol weight ratio on the molecular weight (Mw) and hydroxyl number (IOH) of bio-polyols was evaluated. The optimum reaction conditions of bio-polyols production for rigid polyurethanes were virtually equal for the two lignins, 159–161 ºC, Polyethylene glycol/Glycerol ratio of 3 without catalyst. On the contrary, the bio-polyols for elastic polyurethanes required different reaction parameters depending on the lignin used. For hardwood lignin the optimised conditions were 180 ºC, 7.57 (Polyethylene glycol/Glycerol ratio) and 5.00% of catalyst while for softwood lignin were 160 ºC, 7.34 (Polyethylene glycol/Glycerol) and 3.85% of catalyst. Additionally, the bio-polyols obtained at optimised conditions were fully characterised and acid number, polydispersity index, functionality and the rheological behaviour was studied.