Abstract
Chemical modification of soy protein, via crosslinking, is the preferred method for creating non-toxic, renewable, environmentally friendly wood adhesives. The denaturing process of protein is important for the adhesive performance improvement. In order to investigate the effect of different denaturing agents on the performance of soy protein-based adhesives before and after crosslinking modification. In this study, three different denaturing agents—urea (U), sodium dodecyl sulfate (SDS), and sodium hydrogen sulfite (SHS) and an epoxide crosslinking agent—Triglycidylamine (CA) were used to prepare soy protein-based adhesives. The results showed: (1) The denaturing agent unfolded protein molecules and exposed more hydrophobic groups to prevent water intrusion, which was mainly a contribution for the water resistance and performance improvement of soy protein-based adhesives. The wet shear strength was improved up to 91.3% (denaturing by urea). (2) After modifying by the crosslinking agent, the properties and performance improvement was due to the fact that the active groups on soybean protein molecules reacted with the crosslinking agent to form a crosslinking structure, and there is no obvious correlation with the hydrophobic groups of the protein. (3) The unfolded soybean protein molecules also expose hydrophilic groups, which facilitates the reaction between the crosslinking agent and protein to form a denser crosslinking structure to improve the performance of the adhesive. Particularly, after denaturing with SHS, the wet shear strength of the plywood bonded by the SPI-SHS-CA adhesive increased by 217.24%.
Highlights
The wood-based panel industry in China primarily uses formaldehyde synthetic resin adhesives and their modified products [1].As such, throughout the wood-panels’ production and use life-cycle, formaldehyde and other harmful substances are released into the environment, thereby generating a pollution problem and endangering human health
(2) After modifying by the crosslinking agent, the properties and performance improvement was due to the fact that the active groups on soybean protein molecules reacted with the crosslinking agent to form a crosslinking structure, and there is no obvious correlation with the hydrophobic groups of the protein
After the plywood specimens were cooled for 10 min to room temperature, their wet shear strengths were tested using a tensile machine at an operating speed of 10.0 mm/min
Summary
The wood-based panel industry in China primarily uses formaldehyde synthetic resin adhesives and their modified products (e.g., urea-formaldehyde, phenolic, and melamine-formaldehyde resins) [1]. The effects of different denaturing agents and crosslinking agents on the properties and performance of soybean protein-based adhesives were investigated. Such knowledge is useful and important for appropriate modification methods in the soy protein-based adhesive preparation and to develop more effective modification methods. Urea, SDS and SHS are common denaturants used to enhance the performance of soybean protein-based adhesives. Three denaturants (urea, SDS and SHS) were individually incorporated into an SPI adhesive solution in this study; which was subsequently mixed with the epoxy crosslinker (CA) to prepare three modified soy protein-based adhesives. The different denaturants’ effects on the soy protein’s surface hydrophobicity, residual rate, fracture surface micrograph and resultant adhesive performance were quantitatively assessed
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