Investigation of combination catalyst system for UF resin in particleboard and MDF manufacturing. Part II. DSC analysis of UF resin curing behavior in the presence of seven catalysts
Ammonium chloride is one of the catalysts that have been commonly used to catalyze urea-formaldehyde (UF) resin curing in particleboard and medium density fiberboard (MDF) manufacturing in North America. There are some limitations with this catalyst. It is known that ammonium chloride reacts with the free formaldehyde in the resin to generate strong acid for accelerating the resin curing rate. Thus, the catalytic impact of this catalyst on resin curing depends on the amounts of free formaldehyde available in the resin. It has been reported that recycling particleboard made with UF resins catalyzed by ammonium chloride may create polychlorinated dioxine compounds, which are classified as toxic materials.
To explore a suitable catalyst system as a substitute for ammonium chloride, four one-component catalyst systems and three combination-catalyst systems were characterized by differential scanning calorimetry (DSC) analysis, in terms of the resin curing rate and activation energy as influenced by the resin solid content and catalyst content. These catalyst systems included (1) ammonium chloride, (2) ammonium persulfate, (3) aluminum chloride, (4) oxalic acid, (5) combination catalyst I (ammonium chloride + ammonium persulfate), (6) combination catalyst II (ammonium chloride + triethanolamine + phosphoric acid), and (7) combination catalyst III (ammonium chloride + triethanolamine + phosphoric acid + hexamethylenetetramine). The test results showed that combination catalysts II and III had great potential as substitutes for ammonium chloride for catalyzing UF resin (particularly with a low F/U molar ratio), in terms of stability (higher pH values of catalyst solutions than ammonium chloride solution at room temperature) and catalytic impact (comparable to ammonium chloride). Ammonium persulfate appeared to be a more powerful catalyst, while combination catalyst I (ammonium chloride/ammonium persulfate) seemed to be a slightly stronger catalyst as compared with ammonium chloride. However, the catalyst solutions of ammonium persulfate and combination catalyst I showed lower pH values than that of ammonium chloride, which may influence the stability of resin system in terms of pot life. Aluminum chloride and oxalic acid had much greater catalytic impacts on the resin curing than the other five catalysts. The resin started curing after mixing with either aluminum chloride or oxalic acid, which would lead to a significantly shortened resin pot life. Therefore, these two catalysts cannot be used directly as catalyst for UF resin curing.