Influence of Single/Collective Use of Curing Agents on the Curing Behavior and Bond Strength of Soy Protein-Melamine-Urea-Formaldehyde (SMUF) Resin for Plywood Assembly

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.

Synthesis of Lignin-Based Polyacid Catalyst and Its Utilization to Improve Water Resistance of Urea–formaldehyde Resins

“…Urea-formaldehyde resins, generated by the polycondensation of formaldehyde, urea, and other modifiers, are one of the most important members of thermosetting resins [1][2][3]. Due to their simple synthesis process, excellent thermal properties, low curing temperature, resistance to microorganism, low cost, and excellent mechanical properties, UF resins have been widely used as wood adhesives.…”

“…Currently, UF resins can be effectively catalyzed in practical production by inorganic acids or acidic salts such as ammonium chloride, ammonium sulfate, phosphoric acid, etc. [1,4]. For example, Li et al have investigated the effects of curing agents, including ammonium chloride and hexamethylenetetramine, on the properties and performance of the UF resin [4].…”

Improved performance of soy protein adhesive with melamine–urea–formaldehyde prepolymer

“…36 The peaks at 1655 and 814 cm À1 were assigned to C]O stretching and the triazine ring out-of-plane vibrations, respectively. 29 Further, the weak peak at 2960 cm À1 and the strong peak at 1003 cm À1 were assigned to stretching vibrations of -CH 2and bending vibrations of the C-O from hydroxymethyl groups (-CH 2 OH), respectively. 29,32 The results indicated that urea and melamine were successfully hydroxymethylated.…”

“…29 Further, the weak peak at 2960 cm À1 and the strong peak at 1003 cm À1 were assigned to stretching vibrations of -CH 2and bending vibrations of the C-O from hydroxymethyl groups (-CH 2 OH), respectively. 29,32 The results indicated that urea and melamine were successfully hydroxymethylated. Moreover, according to the GPC spectrum (Fig.…”

“…Moreover, some studies have successfully copolymerized soy protein hydrolysate with amino formaldehyde resins to prepare biodegradable resins, which further conrmed the reaction between hydroxymethyl and soy protein. 28,29 However, the modication efficiency of these amino formaldehyde resins is low due to their large molecular weight and poor reactive groups. High amount (usually greater than 10% of the total mass of the adhesive) of synthetic resin additive can effectively improve the performance of the soy-based adhesive, but it also causes a waste of non-renewable petroleum resources.…”