Abstract
Soybean protein hydrolysate, melamine, urea, and concentrated formaldehyde were used to synthesize an environmentally friendly soybean protein-melamine-urea-formaldehyde (SMUF) co-condensation resin. (NH4)2SO4, (NH4)2HPO4, (NH4)2HPO4 + (NH4)2SO4, (NH4)2HPO4 + (NH4)2S2O8, and (NH4)2HPO4 + (NH4)2SO4 + (NH4)2S2O8 were employed as curing agents for SMUF resin. The curing and thermal behaviors of the SMUF resin were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results revealed the following: (1) (NH4)2SO4 alone could not cure the SMUF resin completely; thus, the final shear strength accomplished plywood with the resin was low, and its water resistance was poor, while the adhesive section was loose and porous/brittle after curing. (2) (NH4)2HPO4 could be hydrolyzed to generate H+ and promote SMUF curing, but it could also form polyphosphoric acids, resulting in crosslinking reactions with SMUF in parallel; thereby, the curing properties were improved. (3) When (NH4)2HPO4 + (NH4)2SO4 + (NH4)2S2O8 were engaged collectively as curing agent, the shear strength, water resistance, and heat resistance of SMUF attained were the best possible whereas the curing temperature was decreased and the heat released by curing was elevated substantially, which signifies maximized extent of crosslinking was achieved. Further, the adhesive section exhibited mostly a crosslinking intertexture as demonstrated by means of SEM. Accordingly, this study may serve as a guide for the curing of amino resins, with low-molar ratio of formaldehyde to amine in adhesives, which are applied to plywood production.
Highlights
Great interest is being dedicated to the modification and applications of melamine-ureaformaldehyde (MUF) resin adhesive in wood panel industry, such as particleboard (PB), plywood and medium density fiberboards (MDF), due to the excellent application prospects of this co-condensation resin [1,2,3]
The properties of soybean protein-melamine-urea-formaldehyde (SMUF) treated different curing agents are shown in Table 3. (NH4)2SO4 can react with formaldehyde to generate H2SO4 and promote SMUF curing (CA-1)
The curing characteristics, thermal response, and interfacial adhesion of SMUF resin were found be dependent on the type of employed curing agent
Summary
Great interest is being dedicated to the modification and applications of melamine-ureaformaldehyde (MUF) resin adhesive in wood panel industry, such as particleboard (PB), plywood and medium density fiberboards (MDF), due to the excellent application prospects of this co-condensation resin [1,2,3]. Various modification methods exist currently, among which biomass-based co-condensation technology has drawn much attention This technology made considerable use of renewable raw materials bearing in mind the resin properties are guaranteed. Biomass raw materials, such as soybean protein, tannin and starch, are either blended with MUF resin directly before the preparation of plywood [9,10,11] or used to form biomass-based polymerization resin systems together with MUF [12,13,14,15,16] These two methods brought about ideal modification effects as they control the cost of resin synthesis and reduce free formaldehyde emissions and improve the properties of the resulting MUF resin adhesives in such a way that the derived wood panels meet the standard requirements. The current study investigates a prospect for managing the required balance with the use of collective curing systems for soybean protein-melamine-urea-formaldehyde (SMUF) resin and the impact of this on the curing behavior and end-product properties
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