Abstract
This study aimed to improve the bonding strength and water resistance of soy protein-based adhesives (SPAs) by modifying with larch tannin-based resins (TRs). This is especially important because of their eco-beneficial effects. The TR was characterized by Fourier Transform Infrared (FTIR) and Thermogravimetric/Derivative Thermogravimetric (TG/DTG) in order to demonstrate the formation of the self-crosslinking structure. Rheological properties, fracture morphology, solubility, and crosslinking density were characterized in detail. Three-ply poplar plywood was fabricated and the wet shear strength was measured. The experimental data showed that the addition of TR improved the moisture uptake, residual rate, and shear strength of SPA. This improvement was attributed to the crosslink reactions of TR with the relevant active functional groups of the side chains of soy protein molecules. The crosslinking structure joined with the TR self-crosslinking structure to form an interpenetrating network, which promoted a uniform and compact cured structure. The 5 wt % TR additions in the SPA was found to yield optimum results by improving the wet shear strength of the plywood by 105.4% to 1.13 MPa, which meets the interior-use plywood requirement. Therefore, the larch tannin could be applied in the modification of soy protein adhesive.
Highlights
Studies on eco-friendly materials that are based on agricultural biomass resources used for coatings, packaging, and furnishings have gained popularity
The addition of 5 wt % tannin-based resins (TRs) in soy protein-based adhesives (SPAs) increased the residual rate of the resultant adhesive by 38.4%
The wet shear strength of the resultant plywood by 105.4%; The water resistance and the crosslinking density of the adhesive were improved because of the following mechanisms: (a) a crosslinking structure formed through the reaction between the TR
Summary
Studies on eco-friendly materials that are based on agricultural biomass resources (e.g., proteins, polysaccharides, gelatin, lignin, and tannin) used for coatings, packaging, and furnishings have gained popularity. Many manufacturers and researchers are eager to study novel environmentally-friendly materials in the wood industry [1,2]. Soy proteins have significant potential in the wood industry as a type of biodegradable, abundant, inexpensive, and environmentally-friendly biomass. Soy protein-based adhesives (SPAs) are eco-friendly products and the resulting plywood, having very low emissions of formaldehyde, is able to meet the current requirements in terms of formaldehyde emissions [3,4]. These adhesives have a low water resistance. Previous researchers have attempted to improve the performance of SPAs by denaturation [5] and crosslinking agents [6,7]
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