The design, synthesis, and characterization of environmentally friendly wood adhesives
Increasing environmental awareness and the recognized hazards of formaldehyde-based wood glues has created a strong demand for more environmentally-friendly wood composites. In response to the threat of formaldehyde toxicity and their emissions from OF resins, we have designed, synthesized, and characterized a novel, formaldehyde-free wood adhesive using renewable, agricultural-based raw materials. The objective of this research has been to build on soybean's previous success as a wood adhesive in the early 1900s by improving its poor water resistance that led to its replacement with petroleum-based resins. To that end, soy protein isolate (SPI) has been successfully blended with vegetable oil derivatives to create non-toxic, non-aqueous binders that possess enhanced water resistance. Furthermore, wood furnish combined with the soy-based adhesive and cured using commercial guidelines for time, temperature, and pressure exhibit the binder's high water resistance, while maintaining strength. The key to increasing the water resistance of soy protein-based adhesive was found to be the in situ reaction between a maleinized methyl ester of tung oil (MMETO) and polyol plasticizer, namely glycerol. The glycerol is used to improve SPI flow while the MMETO counteracts the polarity of the plasticizer. During soy adhesive characterization, it was recognized that traditional rheometry and melt flow index methods were unsuitable for evaluating flow and screening soy protein adhesives. Hence, a new method was developed involving dynamic mechanical analysis that not only isolated the effects of formulations, but also evaluated adhesive flow over large temperature, pressure, and ingredient concentration ranges. The ability to synthesize 100% formaldehyde-free wood adhesive using soybeans represents the development of a new generation of wood composites; one that focuses on better utilization of agricultural resources to replace petroleum-based raw materials. The production of the adhesives from agricultural raw materials is important because it not only introduces environmental benefits via elimination of formaldehyde, but also adds commercial value to the agricultural resources since the wood composite arena is a large-volume business.