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

Yan Song,Zhixin Wang,Rong Zhang,Jinchun Li,Ning Yan

doi:10.3390/polym8060209

Yan Song, Zhixin Wang + Show 3 more

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https://doi.org/10.3390/polym8060209

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Journal: Polymers	Publication Date: May 30, 2016
Citations: 93	License type: CC BY 4.0

Affiliation: Changzhou University, University of Toronto

Abstract

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.

Highlights

Adhesives are key components in the manufacturing of particleboards, wood panels, fiberboards, and plywood
The hydroxyl group in softwood kraft lignin (SKL) increased by 28% and the demethylated SKL was more reactive than the unmodified SKL
The Fourier Transform Infrared Spectroscopy (FTIR) semi-quantitative method has been used to characterize the relative content of specific groups, in which the phenyl group’s skeleton vibration peak at 1503 cm1 was selected as the reference for calculating the relative values of other groups by area ratios of the peaks [32]

Summary

Introduction

Adhesives are key components in the manufacturing of particleboards, wood panels, fiberboards, and plywood. Commercial PF adhesives are synthesized mostly using petroleum-derived feedstock. With the dwindling petroleum resources [3,4], there is a growing interest to explore alternative raw materials from renewable resources to make bio-based adhesives [5,6,7,8]. Lignin is a natural polymer with a three-dimensional network structure that consists of p-hydroxyphenyl propane, guaiacyl propane, and syringyl propane units (Figure 1). The three units are connected through C–C and C–O–C [9,10] bonds. Lignin is one of the most abundant organic polymeric materials on earth, only second to cellulose. Commercial lignin is mainly obtained from waste liquor in the pulping industry or as a by-product of bio-ethanol production [11,12]

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