Investigation of combination catalyst system for UF resin in particleboard and MDF manufacturing. Part IV. Evaluation of combination catalyst system as curing agent for UF resin in MDF panel manufacturing
This study was conducted in three phases to characterize a number of selected and optimized catalyst systems for pH of catalyst solution, gel time and curing behavior of resin by a differential scanning calorimetry (DSC) analysis. These catalyst systems were also evaluated for bond quality by manufacturing medium density fiberboard (MDF) panels and testing the resulting panels 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.
In Phase I, four catalyst systems were selected to manufacture MDF panels with urea-formaldehyde (UF) resin having F/U molar ratio of 1.08 based on the previous study on the characterization of 24 catalyst systems (Wang and Wan 2008), including D-1 (ammonium chloride), F-10 (7.5 wt% ammonium sulfate + 2.5 wt% ammonium persulfate + 2.0 wt% urea + 88 wt% water), G-2 (7.5 wt% ammonium sulfate + 1.0 wt% aluminum sulfate + 1.5 wt% triethanolamine + 2.0 wt% urea + 0.5 wt% phosphoric acid + 87.5 wt% water) and G-4 (7.5 wt% ammonium sulfate + 1.0 wt% phosphoric acid + 1.5 wt% triethanolamine + 2.0 wt% urea + 88 wt% water). The test results of the panels indicated that combination catalyst systems G-4 and F-10 showed potential as substitutes for control catalyst D-1 (ammonium chloride).
In Phase II, seven catalyst systems were selected and evaluated by manufacturing MDF panel with UF resin having F/U molar ratio of 1.08, including three control catalysts, D-1, F-10 and G-4. The three new catalysts were derived from formulations of F-10 and G-4 by introducing triethanolamine and phosphoric acid from G-4 into F-10 and adjusting their contents in the catalyst formulations, including 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 wt% water), B-2 (6.0 wt% ammonium sulfate + 1.0 wt% ammonium persulfate + 1.8 wt% triethanolamine + 1.2 wt% phosphoric acid + 2.0 wt% urea + 88 wt% water) and B-3 (6.0 wt% ammonium sulfate + 1.0 wt% ammonium persulfate + 1.5 wt% triethanolamine + 1.8 wt% phosphoric acid + 2.0 wt% urea + 87.7 wt% water). The new catalyst B-4 (7.5 wt% ammonium sulfate + 1.5 wt% triethanolamine + 1.0 wt% aminosulfonic acid + 2.0 wt% urea + 88 wt% water) was derived from the formulation of G-4 by substituting aminosulfonic acid for phosphoric acid. Based on the overall panel performance, the combination catalysts B-4 and B-1 seemed to be the most promising as substitutes for control catalyst D-1 (ammonium chloride).
In Phase III, the combination catalyst B-1 was selected to further evaluate its potential against the control catalyst D-1 by manufacturing MDF panels with five different formulations of UF and MUF resins in terms of F/U molar ratio (1.10, 10.8, 1.05) and melamine content (0.1%, 2.5%, 5%). These five resins included Resin A (F/U 1.10/0.1% melamine), Resin B (F/U 1.08/0.1% melamine), Resin C (F/U 1.05/0.1% melamine), Resin D (F/U 1.08/2.5% melamine), and Resin E (F/U 1.08/5% melamine). The test results indicated that all five resins cured faster with B-1 than with D-1 in terms of shortened gel time and lower activation energy. Resin B and Resin C catalyzed by B-1 produced the overall best quality panels than other resins catalyzed by B-1 and D-1, which implies that B-1 would be more powerful for catalyzing UF resin curing and producing better bond quality compared with D-1. The study also implies that catalyst type and catalyst content should be properly adjusted according to resin formulation (such as F/U molar ratio and melamine content) in combination with pressing conditions (such as press time and temperature) in order to achieve the maximum bond quality in panels.