The depletion of natural resources and increasing environmental apprehension regarding the reduction of harmful isocyanates employed in manufacturing polyurethanes (PUs) have generated significant attention from both industrial and academic sectors. This attention is focused on advancing bio-based non-isocyanate polyurethane (NIPU) resins as viable and sustainable substitutes, possessing satisfactory properties. This review presents a comprehensive analysis of the progress made in developing bio-based NIPU polymers for wood adhesive applications. The main aim of this paper is to conduct a comprehensive analysis of the latest advancements in the production of high-performance bio-based NIPU resins derived from lignin and tannin for wood composites. A comprehensive evaluation was conducted on scholarly publications retrieved from the Scopus database, encompassing the period from January 2010 to April 2023. In NIPU adhesive manufacturing, the exploration of substitute materials for isocyanates is imperative, due to their inherent toxicity, high cost, and limited availability. The process of demethylation and carbonation of lignin and tannin has the potential to produce polyphenolic compounds that possess hydroxyl and carbonyl functional groups. Bio-based NIPUs can be synthesized through the reaction involving diamine molecules. Previous studies have provided evidence indicating that NIPUs derived from lignin and tannin exhibit enhanced mechanical properties, decreased curing temperatures and shortened pressing durations, and are devoid of isocyanates. The characterization of NIPU adhesives based on lignin and tannin was conducted using various analytical techniques, including Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), matrix-assisted laser desorption/ionization with time-of-flight (MALDI-TOF) mass spectrometry, and gel permeation chromatography (GPC). The adhesive performance of tannin-based NIPU resins was shown to be superior to that of lignin-based NIPUs. This paper elucidates the potential of lignin and tannin as alternate sources for polyols in the manufacturing of NIPUs, specifically for their application as wood adhesives.

The citric acid (CA)-based adhesive, as a new environment-friendly adhesive, has been developed and used for the manufacturing of plywood to resolve the problem of formaldehyde-based adhesive for human health. The effects of pressing time and temperature on physical and mechanical properties of plywood were examined. Three-layer plywood panels were bonded using CA adhesive with a 134 g/m² of single glue spread on White jabon wood veneer. The plywood panels were hot-pressed at 5 MPa with three different pressing temperatures, 180°C, 190°C, 200°C, for 10 min, and also different pressing times, 4, 7, 10 min at 190°C. The physical and mechanical properties, such as moisture content, delamination, and shear strength, were evaluated according to JAS 233:2003. The results show that the plywood with pressing temperature of 190°C for 10 min had lower delamination and greater tensile shear strength than those of other plywood panels. Moreover, some properties of the plywood fulfilled the requirement of JAS 233:2003. Based on the FTIR analysis, citric acid plays an important role in the formation of ester linkages (R-COO-R) via esterification between carboxyl groups of citric acid and hydroxyl groups of White jabon wood veneer, resulting excellent properties of plywood.