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Polymers 2016, 8(6), 209;

Demethylation of Wheat Straw Alkali Lignin for Application in Phenol Formaldehyde Adhesives

Faculty of Materials Science & Engineering, Changzhou University, Changzhou 213164, China
Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, China
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON M5S 3B3, Canada
Author to whom correspondence should be addressed.
Academic Editor: Antonio Pizzi
Received: 18 April 2016 / Revised: 13 May 2016 / Accepted: 24 May 2016 / Published: 30 May 2016
(This article belongs to the Special Issue Renewable Polymeric Adhesives)
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Lignin is a natural biopolymer with a complex three-dimensional network. It is the second most abundant natural polymer on earth. Commercially, lignin is largely obtained from the waste liquors of pulping and bioethanol productions. In this study, wheat straw alkali lignin (WSAL) was demethylated by using an in-situ generated Lewis acid under an optimized demethylation process. The demethylation process was monitored by a semi-quantitative Fourier Transform Infrared Spectroscopy (FTIR) method. The demethylated wheat straw alkali lignin (D-WSAL) was further characterized by Proton Nuclear Magnetic Resonance (1H NMR), Gel Permeation Chromatography (GPC), and titration methods. After the demethylation process, it was found that the relative value of the methoxy group decreased significantly from 0.82 to 0.17 and the phenolic hydroxyl group increased from 5.2% to 16.0%. Meanwhile, the hydroxyl content increased from 6.6% to 10.3%. GPC results suggested that the weighted averaged molecular weight of D-WSAL was lower than that of WSAL with a smaller polydispersity index. The D-WSAL was then used to replace 60 wt % of phenol to prepare lignin-based phenol formaldehyde adhesives (D-LPF). It was found that both the free formaldehyde content and the free phenol content in D-LPF were less than those of the lignin-based phenol formaldehyde adhesives without lignin demethylation (LPF). Gel time of D-LPF was shortened. Furthermore, the wet and dry bonding strengths of lap shear wood samples bonded using D-LPF were higher than those of the samples bonded using LPF. Therefore, D-WSAL has shown good potential for application in phenol formaldehyde adhesives. View Full-Text
Keywords: wheat straw alkali lignin; Lewis acid; demethylation; phenol formaldehyde adhesive wheat straw alkali lignin; Lewis acid; demethylation; phenol formaldehyde adhesive

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Song, Y.; Wang, Z.; Yan, N.; Zhang, R.; Li, J. Demethylation of Wheat Straw Alkali Lignin for Application in Phenol Formaldehyde Adhesives. Polymers 2016, 8, 209.

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