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

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.

Chemical Structures of Adhesive and Interphase Parts in Sucrose/Citric Acid Type Adhesive Wood-Based Molding Derived from Japanese Cedar (Cryptomeria japonica)

“…Some of the wood components were esterified with citric acid although the content of the citric acid in the sucrose/citric acid type adhesive wood-based molding was lower than that in the citric acid type adhesive wood-based molding [ 32 ]. Namely, it was elucidated that the interface between the furan polymer and wood powder was bonded via citric acid ( Figure 4 ).…”

“…Prior to the FT, the data matrices were zero-filled up to 2048 and 1024 points in the 1 H and 13 C dimensions, respectively. The spectra of wood powder, shown in Figure 1 a, Figure 2 a and Figure 3 a), were referred from those in the previous report [ 32 ] for comparing with the molding, because the same wood powder is the starting material in this study.…”

“…In a previous report [ 32 ], the structure of the citric acid-based adhesive in the molding was investigated with HSQC-NMR analysis to clarify the esterification of wood components with citric acid. It is elucidated that the citric acid type adhesive is a chemical covalent-bonding-type adhesive and citric acid functions as a clamp between wood powder particles.…”

Branched/Hyperbranched Copolyesters from Poly(vinyl alcohol) and Citric Acid as Delivery Agents and Tissue Regeneration Scaffolds

“…More CA content in the feed led to more branching which resulted into higher steric repulsion and eventually reduced both the rate of reaction and the yield. [23] However, more branching consequently developed lesser amount of chain entanglement and generated more void or free space in between the molecular segments of copolyesters. That eventually allowed more water molecules (solvent) to penetrate the polymeric network and thereby increased the solubility.…”

Influence of Citric Acid on the Bond Strength of Beech Wood

“…Katović et al [ 25 ] esterified fir and beech with citric acid, and the results showed a good improvement in the dimensional stability of wood. Ando and Umemura [ 26 ] concluded that citric acid bonds with wood components and that the bonds improve the wood molding properties, although polysaccharides are damaged slightly in this process. Šefc [ 11 ] impregnated fir and beech wood samples with citric acid and enhanced dimensional stability; compressive strength parallel to the grain was not affected but reduced its micro tensile strength.…”