Investigation of combination catalyst system for UF resin in particleboard and MDF manufacturing. Part V. Evaluation of combination catalyst system as curing agent for UF resin in PB panel manufacturing
To evaluate the catalytic influence of combination catalyst system on urea-formaldehyde (UF) resin curing rate and bonding quality, a number of catalyst systems were selected based on the previous test results of this project. These systems included D-1 (10.0 wt% ammonium chloride + 90.0 wt% water), F-10 (7.5 wt% ammonium sulfate + 2.5 wt% ammonium persulfate + 2.0 wt% urea + 88.0 wt% water), G-2 (7.5 wt% ammonium sulfate + 1.0 wt% aluminum sulfate + 1.5 wt% triethanolamine + 0.5 wt% phosphoric acid + 2.0 wt% urea + 87.5 wt% water), G-4 (7.5 wt% ammonium sulfate + 1.5 wt% triethanolamine + 1.0 wt% phosphoric acid + 2.0 wt% urea + 88.0 wt% water), B-1 (6.0 wt% ammonium sulfate + 2.0 wt% ammonium persulfate + 1.2 wt% triethanolamine + 0.8 wt% phosphoric acid + 2.0 wt% urea + 88.0 wt% water) and B-4 (7.5 wt% ammonium sulfate + 1.5 wt% triethanolamine + 1.0 wt% aminosulfonic acid + 2.0 wt% urea + 88.0 wt% water). These catalyst systems were first characterized for the curing behaviors of UF resin in the presence of the catalysts with differential scanning calorimetry (DSC) analysis and the chemical structures of the cured resin with CP/MAS 13C nuclear magnetic resonance (NMR) spectroscopy. The catalyst systems were then evaluated by manufacturing a series of particleboards. The resulting boards were tested for internal bond (IB) strength, modulus of rupture (MOR) and modulus of elasticity (MOE), 24-h thickness swelling (TS) and water absorption (WA), and free formaldehyde (FF) emission.
This study was conducted in two phases. In Phase I, six catalyst systems (D-1, F-10, G-2, G-4, B-1 and B-4) were selected to manufacture particleboard with 1.2% catalyst content and a 170-second press cycle. The test results indicated that combination catalyst G-4 resulted in stronger particleboard than the other catalysts in terms of IB, MOR and MOE properties; catalyst type seemed to have a little influence on board water resistance and free formaldehyde emission. In general, G-4 seemed to be more powerful in catalyzing UF resin curing and producing stronger particleboard than the conventional catalyst D-1 (ammonium chloride) and the other four combination catalysts.
In Phase II of this study, the combination catalyst G-4 was further evaluated by manufacturing particleboard against control catalyst D-1 with three press cycles (140, 155 and 170 seconds) and three levels of catalyst content (0.9%, 1.2%, and 1.5%). The analysis of variance (ANOVA) indicated that board manufacturing conditions had a significant influence on IB strength and free formaldehyde (FF) emission rather than on other board properties. G-4 resulted in better quality boards than D-1 in terms of consistently higher IB strength and lower FF emission under each board manufacturing condition. This study also showed that G-4 was more powerful in catalyzing UF resin curing than D-1; thus, using G-4 has great potential for cutting down press time and reducing catalyst content. The improved bond quality and durability of the UF resin catalyzed by G-4 is probably due to the improved chemical structure of the resin in terms of the greater degree of resin crosslinking.