Low Mole Ratio Urea–Melamine–Formaldehyde Resins Entailing Increased Methylene-Ether Group Contents and Their Formaldehyde Emission Potentials of Wood Composite Boards

A previous study of synthesizing low mole ratio urea–formaldehyde (UF) and urea–melamine–formaldehyde (UMF) resins, which included an acidic reaction step at the beginning of the typical resin synthesis procedure to obtain higher uron-type methylene–ether group contents, was repeated with the acidic reaction step extended to a higher viscosity. Compared to previous resins, the synthesized resins showed additional increases in the uron-type and linear-type methylene–ether groups, resulting in longer storage times, longer pot lives, longer gel times, and comparable internal bond strengths and water absorption values of particleboards; however, the formaldehyde contents (FC) of boards increased. It was concluded that the extended acidic reaction resulted in increased formaldehyde emission potential of boards because of additional methylene–ether groups formed. The results led to the hypothesis that the FC values of current UF resin-bonded boards are mainly due to the methylene–ether-type groups present in significant levels in UF and UMF resins.