Compression and tensile strength of low-density straw-protein particleboard

Wheat straw can be used as a raw material for particleboard, and soybean protein can be used as an adhesive in particleboard fabrication. The objective of this research was to characterize the mechanical properties of low-density wheat straw-soy protein particleboard as affected by protein modification/denaturation, initial straw moisture content, and chemical treatment of the straw. Methylene diphenyl diisocyanate (MDI), considered an environmental-friendly resin, was also used as a binder for comparison. The major factors that determined the mechanical properties of wheat-straw particleboard were the types of adhesives, straw surface properties, and moisture content of straw. Surface treatment of straw with bleach gave better results than treatments with hydroperoxide alone or combined with sodium hydroxide treatment. Sodium hydroxide, urea, and dodecylbenzene sulfonic acid were used in the modification of soy protein. Particleboard with sodium hydroxide-modified soy protein had the best mechanical properties. The bondability of modified soy protein was highly dependent on straw moisture content. About 30-40% initial straw moisture content was needed to obtain good bonding. Methylene diphenyl diisocyanate showed better bondability than modified soy protein adhesives. Equilibrium moisture content of straw particleboard increased as the relative humidity increased. The equilibrium moisture contents were similar for straw particleboard with sodium hydroxide-modified soy protein and methylene diphenyl diisocyanate adhesives at 30 or 60% relative humidity. Although the soy protein-based adhesive resulted in low-density particleboard with poorer mechanical properties than the methylene diphenyl diisocyanate adhesive, it is environmental-friendly and could be used in filters or light-weight core material where the requirement for mechanical strength is not stringent.