Abstract
The purpose of this study was to improve the performance of soy protein isolate (SPI) adhesives using a polyurethane elastomer. Triglycidylamine (TGA), SPI, thermoplastic polyurethane elastomer (TPU), and γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH-560) were used to develop a novel SPI-based adhesive. The residual rate, functional groups, thermal stability, and fracture surface micrographs of the cured adhesives were characterized. Three-ply plywood was fabricated, and the dry/wet shear strength was determined. The experimental results suggested that introducing 2% TGA improved the residual rate of the SPI/TGA adhesive by 4.1% because of the chemical cross-linking reaction between epoxy groups and protein molecules. Incorporating 7% TPU into the SPI/TGA adhesive, the residual rate of the adhesive increased by 5.2% and the dry/wet shear strength of plywood bonded by SPI/TGA/TPU adhesive increased by 10.7%/67.7%, respectively, compared with that of SPI/TGA adhesive. When using KH-560 and TPU together, the residual rate of the adhesive improved by 0.9% compared with that of SPI/TGA/TPU adhesive. The dry and wet shear strength of the plywood bonded by the SPI/TGA/TPU/KG-560 adhesive further increased by 23.2% and 23.6% respectively when compared with that of SPI/TGA/TPU adhesive. TPU physically combined with the SPI/TGA adhesive to form a interpenetration network and KH-560 acted as a bridge to connect TPU and SPI/TGA to form a joined crosslinking network, which improved the thermo stability/toughness of the adhesive and created a uniform ductile fracture section of the adhesive.
Highlights
In recent years, soy protein-based adhesives (SPIAs) have garnered great research attention as a substitute for formaldehyde-based adhesives, to eliminate formaldehyde emission from wood-based panels [1]
Introducing TGA and KH-560 as a cross-linker reacted with the functions of the soy protein and effectively increased the water resistance of the soy protein isolate adhesive
The wet shear strength of plywood bonded by soy protein isolate (SPI)/TGA and SPI/TGA/KH-560 adhesive was 0.96 and 1.39 MPa, improving by 26% and 83%
Summary
Soy protein-based adhesives (SPIAs) have garnered great research attention as a substitute for formaldehyde-based adhesives, to eliminate formaldehyde emission from wood-based panels [1]. Chemical and physical methods were applied to enhance water resistance of the SPIAs, including denaturation [5], cross-linking agents [6], synthetic resin [7], nano-material [8] and biomimetic modification [9]. Among those modifications, the most effective way is using cross-linking agents and synthetic resins, such as phenol formaldehyde resin [10], polyisocynates [11], polyamidoamine—epichlorohydrin resin [12] and epoxide [13]. The active functions of the cross-linker or synthetic resin react with the –NH2 , –NH– and –COOH groups
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