Wood-based composites such as plywood and particleboard are produced with a thermoset adhesive via a hot-press process. At present, UF is the most commonly used thermoset adhesive for production of decorative plywood. The emission of the carcinogenic formaldehyde from UF-bonded panels during the use of panels affects the indoor air quality and causes many health problems. In the first part of my PhD research, two formaldehyde-free wood adhesives that consist of soybean flour (SF) with poly (glycidyl methacrylate-co-styrene) (PGS) emulsions were investigated. PGS emulsions were synthesized through a free radical initiated emulsion polymerization of glycidyl methacrylate (GMA) and styrene. In the first SF-PGS adhesive, NaOH was used as a catalyst; and polyethylenimine (PEI) replaced NaOH in the second SF-PGS adhesive. Seven-ply plywood panels were prepared with the two SF-PGS adhesives via a hot press process. The resulting plywood panels were evaluated for their water resistance according to the American National Standard For Hardwood And Decorative Plywood/Hardwood Plywood and Veneer Association (ANSI/HPVA HP-1-2000). Effects of the weight ratio of the adhesive ingredients, hot press temperature, and hot press time on the water resistance of the resulting plywood panels were investigated. The curing mechanisms of the adhesives were investigated and discussed. The plywood panels made with both SF-PGS adhesives met the industrial property requirements for interior applications.
Thick wood-based composites such as cross-laminated timber (CLT), glulam, and laminated veneer lumber are produced with a cold-set adhesive via a cold-press process. Resorcinol-formaldehyde (RF), phenol-resorcinol-formaldehyde (PRF), melamine-urea-formaldehyde (MUF), and polyurethanes (PUR) are the currently used cold-set wood adhesives. They are all based on carcinogenic formaldehyde-based or very toxic isocyanates and release hazardous air pollutants during the production of the wood-based composites. In the second part of my PhD research, two formaldehyde-free and isocyanates-free cold-set wood adhesives that were based epoxy-resins were investigated. The first adhesive was comprised of bisphenol A diglycidyl ether (BPADGE), polyethylenimine (PEI), and a polyamidoamine (PAA), and the second one was comprised of bisphenol A diglycidyl ether (BPADGE), polyethylenimine (PEI), and soy protein isolated (SPI). In the second adhesive, the petrochemical-based PAA in the first adhesive was successfully replaced with renewable SPI. The adhesives were investigated for the production of exterior plywood panels. Three-ply plywood panels were prepared via a cold-press process, and evaluated for their shear strengths and water resistance in accordance with the American National Standard For Hardwood And Decorative Plywood/Hardwood Plywood and Veneer Association (ANSI/HPVA HP-1-2000) for exterior plywood. The effects of the weight ratios of the adhesive ingredients and the conditions for the preparations of the adhesives on the water resistance and the shear strengths of the plywood panels were investigated. The potlife of the adhesives were measured. The effects of the pressing time at room temperature on the water resistance and the shear strengths of the resulting panels were investigated. The curing mechanisms of the adhesives were investigated and discussed. The plywood panels made with the BPADGE–PAA–PEI and BPADGE–SPI–PEI adhesives met the industrial property requirements for exterior applications.