Preparation of polyurethane foam from lignin and crude glycerol

Abstract

The development and implementation of lignocellulose-based biorefineries which produce renewable fuels, chemicals and pharmaceuticals are expected to grow, but still face technological challenges. Economic feasibility could be improved through the optimization of feedstock utilisation and in this work a method is proposed and demonstrated which utilise two by-product streams from an integrated biorefinery concept to produce a higher value product. More specifically the study focused on the preparation of biobased products rich in hydroxyl groups which could be used as polyols for polyurethane preparation. The process entails lignin liquefaction in crude glycerol. Lignin is viewed as a potentially high volume by-product of future biorefineries and is an existing by-product of the pulp and paper industry. Crude glycerol is a by-product of conventional biodiesel preparation through transesterification of fats and oils with alcohols. In this work crude glycerol was prepared from ethanol and sunflower oil and used in the liquefaction of three technical lignins, i.e. softwood kraft lignin, hardwood lignosulphonate and organosolv lignin from sugarcane bagasse. The lignin structure differs, depending on the source and isolation method and was expected to influence product properties. 1H and 31P NMR spectroscopy was used to study the lignins, with 31P NMR specifically capable of quantifying hydroxyl groups. The solid phase liquefaction products differed in degree of functionality while the liquid phase composition varied. The glycerol and fatty acid ethyl ester (FAEE) contents were reduced, while the monoacylglycerol (MAG) and diacylglycerol contents were also altered in the liquid phase. From size-exclusion chromatography (SEC) and NMR results it was concluded that the modification of lignin was to some extent correlated with the ratio of aliphatic and phenolic hydroxyl group contents in lignin, as well as MAG and glycerol reduction during liquefaction. Higher molar mass lignin derivatives were detected with SEC in the organosolv lignin liquid phase product, supported by NMR and FTIR observations. The solid phase products showed an increase of ether and ester bonds, as well as aliphatic content relative to that in lignin, both with FTIR and 1H NMR spectroscopy. It was concluded that the solid phase products consisted of lignin derivatives functionalised with glycerol, MAG and FAEE. The liquefaction products had hydroxyl contents similar to commercial polyurethane polyols and urethane bond formation through reaction with diphenylmethane-4,4'-diisocyanate was confirmed with FTIR. The product of each respective lignin was used as the sole polyol component to prepare rigid polyurethane foam (PUF) and the variation in material and thermal properties revealed the influence of the lignin type. The biobased contents of the prepared foams were as high as 55 wt%. The thermal conductivity and compressive strength of the kraft lignin-based foams were superior with values of 0.039 W m-1 K-1 and 345 kPa. An evaluation of the biodegradability in soil of the prepared PUF and commercial petroleum-derived PUF, over a 30 month period, did not reveal significant differences in degradation. Based on the properties of the foams it is concluded that the proposed strategy can be a viable valorisation route for these by-products in biorefineries and feasibility should be further evaluated