aTo improve the economic viability of polymer from renewable resources, a value-added lignin polymer is increasingly important. The lignin was successfully modified by three chemical methods: hydroxymethylation, epoxidation, and phenolation. Through these methods, the percentage of impurity had decreased, and the number of phenolic hydroxyl groups of lignin had dramatically increased, particularly by phenolation, reaching 9.41%, nearly three times higher than that in the unmodified one. Meanwhile, we added different amounts of modified lignin into polyurethane foams, and the results showed that 1 wt% modified lignin could not only increase the decomposition temperature of the foam material but also remarkably improve its mechanical properties. The optimum reaction time was 4 hr, and the reaction temperature was 80°C for blends. Copyright © 2014 John Wiley & Sons, Ltd.Keywords: lignin; polyurethane foam; modification; renewable resources
INTRODUCTIONLignin, a natural irregular phenolic polymer that is synthesized via unique biosynthesis pathways in lignocellulosic plants, has been found wild in plant cell tissue, and it is the second most abundant fiber polymer after cellulose on earth. [1,2] Because of its complicated structure, lignin has limited use in industries and always been obtained as by-products or residues from papermaking and emerging cellulosic ethanol production. [3][4][5][6][7] In recent years, only about 1% of it has been fully utilized in manufacture process.[8] Therefore, it is a great challenge to urgently realize the large-scale application of lignin in various ways with the aim of resources recycling and environmental sustainability. [9,10] Lignin contains various functional groups that could hardly participate in the process of making polymer materials with the low activity. Therefore, modifying lignin through chemical method, which can not only increase the molecular activity of lignin but also represent a fine dispersion in the organics, is good for the preparation of lignin-based polymer materials. [11][12][13][14][15][16][17] In the past decades, the researchers have carried out a great deal of work in the development of lignin polymer, [18][19][20][21][22] and what is more, the chemical modification of lignin for its use in the preparation of polyurethane foams (PUFs) has received crucial attention. [23,24] Therefore, on the basis of the properties and structural features, three different ways are employed to handle the selected lignin in this study: hydroxymethylation, epoxidation, and phenolation, which had been used by some previous researchers. [25][26][27] But we have utilized modified lignin for the synthesis of PUFs and compared their properties for the first time. Through these three methods, the number of reactive functional groups of the selected lignin has increased, especially the number of the phenolic hydroxyl group. It also lays the foundation for the follow-up production of polyurethane (PU) material.Polyurethanes, synthesized from polyether glycol (PEG) and 4,4-methylene...