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...

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Lignin has received wide attention due to the enormous renewable supply and potential use of this inexpensive organic raw material. We used straw lignin to partly replace bisphenol A for preparing epoxy resins. This process not only greatly reduces the environmental pollution caused by lignin, but also decreases the production cost of epoxy resins. The lignin coming from agricultural waste was modified by Mannich reaction. At the beginning, we would get the mixture after diethanolamine was reacted with formaldehyde for 2 hr. Then the modified lignin was prepared by adding dropwise the mixture into lignin solution. The contents of active groups, including phenolic hydroxyl and hydroxymethyl, in modified lignin by Mannich reaction had been improved effectively. In addition, lignin-based epoxy resins which we prepared by modified lignin have good thermal stability. POLYM. ENG. SCI., 54:2777-2784

Alkali lignin was successfully depolymerized into polyols with high hydroxyl number via direct hydrolysis using sodium hydroxide (NaOH) as a catalyst, without any organic solvent agent. Hydrolysis of lignin can produce a multitude of high-value products via alkali-catalyzed cleavage. This process usually gives good results with respect to the yield of phenols. Through this method, the numbers of the hydroxymethyl and phenolic hydroxyl groups of lignin had been dramatically increased, reaching 2.11%, nearly four times higher than that in the original one. Meanwhile, we added the same amounts (20 wt %) of different depolymerization of lignin (DL) into epoxy resin (EP), and the results showed that DL could not only increase the decomposition temperature of EP, but also remarkably improve its mechanical properties. The optimum reaction time was 1.5 h, the reaction temperature was 250 C, and the optimum sodium hydroxide concentration was 15 wt % for depolymerizing lignin. The mechanical and thermal properties of cured lignin-based epoxy resin (LEP) were compared with cured neat EP. The cured DL-based epoxy resin (DLEP) showed the highest adhesive shear strength up to 2.66 MPa, which displayed 122% of the adhesive shear strength of EP.

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