Abstract
In this study, corn stalk lignin was used to react with non-volatile and non-toxic glyoxal under the catalysis of a sodium hydroxide solution, and a wood adhesive based on glyoxalated corn stalk lignin was prepared. The effect of the hydroxylation reaction on the structure and properties of lignin were studied using Fourier transform infrared spectroscopy (FTIR), ultraviolet spectrophotometry (UV), thermogravimetric analysis (TGA), titration tests, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Compared with unmodified lignin, the glyoxalated corn stalk lignin had a significant improvement in hydroxyl content, activation, and thermal stability. At the same time, results from the GPC showed that the molecular weight increased compared with original corn stalk, possibly due to the secondary polycondensation reaction between lignin and glyoxal. Lignin-based environmental wood adhesives were prepared by mixing modified lignin and epichlorohydrin (ECH), and the dry strength of plywood reached 1.58 MPa. The mechanical strength and water resistance of plywood was improved significantly by mixing some aqueous emulsion into lignin-based adhesives, e.g., polyacrylic ester (AE) emulsion and aqueous polyurethane (PU) emulsion.
Highlights
Traditional fossil-fuel-based wood adhesives, e.g., urea-formaldehyde resin and phenolformaldehyde resin, have been widely used in wood-based panel industry since the 20th century, due to their own high performance–price ratio [1]
The increase of the hydroxyl groups indirectly indicated the occurrence of a glyoxalation reaction
From the comparison between enzymatic hydrolysis lignin (EHL)/ECH and GEHL/ECH, it is clear that dry strength and wet strength of adhesive from GEHL was significantly higher than from EHL, which reached 1.58 MPa and 1.08 MPa (63 ◦ C wet strength) for GEHL/ECH. These results indicate that the content of active groups and reactivity for GEHL increase significantly with the introduction of glyoxal
Summary
Traditional fossil-fuel-based wood adhesives, e.g., urea-formaldehyde resin and phenolformaldehyde resin, have been widely used in wood-based panel industry since the 20th century, due to their own high performance–price ratio [1] Some of these adhesives have already won the recognition of the market, there were some shortcomings and hidden dangers that have appeared gradually to this day, such as the decrease of raw materials, as well as environmental and health concerns. All of these problems intensified with the growing interest in environment-friendly biomass-based wood adhesives. Most industrialization attempts in this field were not really achieved, due to some availability reasons from the technique, both process and economic
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