Bond Structures between Wood Components and Citric Acid in Wood-Based Molding.

Daisuke Ando,Kenji Umemura

doi:10.3390/polym13010058

Abstract

Citric acid-based wood adhesive is considered a chemical-bonding wood adhesive. However, the detailed structures of the bonds between wood components and citric acid remain unknown. Here, we examine the chemical bonding structures between citric acid and wood by heteronuclear single quantum coherence-nuclear magnetic resonance (HSQC-NMR) analysis of wood-based molding using Japanese cedar (Cryptomeria japonica) and citric acid. In the HSQC-NMR spectrum of the wood molding, some esterified C/H correlation peaks appeared. The primary hydroxyl groups of polysaccharides, such as cellulose and galactoglucomannan, and the primary hydroxyl groups of the β-O-4 and β-5 substructures in lignin were found to be esterified with citric acid. In contrast, the secondary hydroxyl groups, except for xylan, barely reacted because of the steric hindrance. Additionally, the C/H correlation peak volumes of the reducing ends of mannan and xylan in the anomeric region increased after molding. It was clarified that the glycosidic bonds in the hemicelluloses were cleaved under the acidic molding condition with citric acid. The HSQC-NMR analysis revealed that the esterification of hemicellulose and lignin, and degradation of hemicellulose, proceeded under the molding condition. These results will promote understanding of the adhesive mechanism of citric acid-based wood adhesive and of the properties of the molding.

Highlights

Wood adhesives have long been used for the efficient utilization of lignocellulosic resources, and many of these adhesives, are derived from petrochemicals [1,2]
To clarify the chemical bonds formed between wood and a citric acid-based wood adhesive, we analyzed a wood-based molding, derived from wood powder and citric acid using Fourier-transform infrared spectroscopy (FT-IR) and HSQC-NMR spectroscopy
The carbonyl peak at 1740 cm−1 of the esters formed between citric acid and the hydroxyl groups in wood or carboxylic acids from citric acid appears in the FT-IR spectrum (Figure 1b) [22,34,35]

Summary

Introduction

Wood adhesives have long been used for the efficient utilization of lignocellulosic resources, and many of these adhesives, are derived from petrochemicals [1,2]. Bio-based wood adhesives have recently attracted much attention. Various types of bio-based wood adhesives have been reported, including proteinbased [7], oil-based [8], carbohydrate-based [9,10,11], tannin-based [12,13,14], and lignin-based adhesives [15,16,17,18]. Citric acid-based wood adhesive has been investigated as one of the few known chemical bonding-type wood adhesives [19,20]. Wood adhesive mechanisms can be categorized into three types: those involving mechanical interactions, physical interactions, and chemical interactions. Chemical interactions are the strongest because of the covalent bond between adhesive and wood

Methods

Results

Conclusion