Preparation and properties of a chitosan-lignin wood adhesive

Abstract

A highly efficient method was developed for preparing chitosan-lignin wood adhesives with high performance using chitosan and ammonium lignosulfonate as main raw material and performance of the resulting adhesives was evaluated by manufacturing medium density fiberboard (MDF). The effects of chitosan-lignin adhesive content and lignin/chitosan weight ratio on the physical and mechanical properties of MDF and the optimal method to prepare and utilize chitosan-lignin wood adhesives were investigated. The results indicated that chitosan had important effects on bonding strength and water resistance and the performance of chitosan-lignin adhesive improved significantly with the optimal method: chitosan-lignin adhesive content of 6% and lignin/chitosan weight ratio of 1:2. The fractures in the bonded joints and the chitosan-lignin adhesive were analyzed via scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The SEM image analysis verified the high performance of the chitosan-lignin adhesive. The FTIR analysis indicated that interactions due to hydrogen bonding between chitosan and lignin occurred. The results of thermal stability analysis validated this and showed that the thermal stability of the interactions was not high.

Introduction

In recent years the growing problem of timber shortage in China has drawn the attention of specialists, who are trying to develop multiple methods for efficient utilization of timber resources. One of the methods generally used for this purpose is to use waste materials and small diameter timbers or some remains to make medium density fiberboard (MDF) as a substrate of panel-type furniture and hence it requires a lot of wood adhesives [1]. In the MDF industry, urea-formaldehyde (UF) resin, phenol formaldehyde (PF) resin and melamine formaldehyde (MF) resin are the most widely used wood adhesives but they produce environmental and human health hazards because of the release of free formaldehyde [2], [3], [4], [5]. To overcome the shortcomings of adhesives containing formaldehyde, specialists are trying to find low toxicity or non-toxic wood adhesives through reducing the molar ratio and adding a formaldehyde catcher in the preparation process [6], [7]. However, it cannot address the problem at its roots. Besides, oil resources, which are being used to produce UF, PF and MF resins, are running out rapidly. Therefore, it is of great importance to develop formaldehyde free adhesives from non-oil resources.

Lignin is one of the most widespread renewable natural raw materials on earth and is an amorphous polymeric material that is based on a phenyl propane derivate. At present most lignin is discarded as waste from the pulp and paper making industry, and is mainly used as fuel. However, as a non-toxic natural polymer, lignin is bio-renewable and inexpensive, which has thus resulted in a considerable amount of research to determine the extent to which lignin could be used in the manufacture of materials with a significant added value. Its macromolecular structure is chemically complex, and the main monomer units constituting lignin molecules are 2-methoxy-4-propyl phenol (guaiacol) in soft wood and a mixture of guaiacol and 1,5-dimethoxy-4-propylphenol (syringol) in hardwood [8]. The chemical structures of the three principal phenolic molecules of lignin are shown in Fig. 1. As a cheap, renewable, phenolic hydroxyl containing biopolymer, lignin offers attractive potential as the basis of adhesives, but isolated lignin is generally a poor adhesive for wood composites when compared to conventional resin systems such as PF resins [9]. Most of the research for modifying lignin-based adhesives has focused on the incorporation of lignin with phenolic wood adhesives for panel products [10], which cannot resolve the problem of formaldehyde release fundamentally.

Blending with other biopolymers is a promising way to modify lignin and to extend its application potential. Chitosan, the second most abundant polysaccharide on Earth [11], is an ideal biopolymer to blend with lignin for formulating wood adhesives. This polysaccharide, obtained by alkaline deacetylation of chitin, has received considerable attention because of its potential application as a bioadhesive. The main developments of bioadhesives were carried out in the biomedical field and more recently in the wood construction industry [12]. Parameters influencing the attractive characteristics of chitosan in the field of bioadhesives are its molecular weight as well as its degree of deacetylation and several studies have clearly shown that adhesive properties were altered when the degree of deacetylation and molecular weight decreased [13]. Besides, industrially chitin is obtained mainly from the exoskeletons of shrimps and crabs, which are a seafood processing waste. All of these properties permit efficient chitinous waste management through the utilization of the biopolymer in wood adhesive applications.

The potential of chitosan–lignin composites for dye and metal ion adsorption has been extensively studied in recent years [14], [15]. In addition to their adsorption capabilities, such composites have also been shown to be inexpensive and biodegradable, thus demonstrating potential applications as relatively low cost -friendly wood adhesives. In this work, novel adhesives for wood were prepared, and mixed with wood fiber to prepare environmentally friendly medium density fiber board (MDF). Then, the mechanical properties of the MDF were determined to select the best method to prepare and use chitosan–lignin adhesives (CLA). Finally, the novel adhesives were characterized using various techniques.