Effect of Melamine Allocation Proportion on Chemical Structures and Properties of Melamine-Urea-Formaldehyde Resins

Melamine-urea-formaldehyde (MUF) resin was synthesized by adding melamine with a first and second portion of urea, named M1 and M2, respectively. Different allocation proportions of M1:M2 (4:0, 3:1, 2:2, 1:3, and 0:4) were used to develop five MUF (MUF-1, 2, 3, 4, and 5) resins with an F/(U+M) molar ratio of 1.05. The chemical structures, curing behaviors, and cross-section morphology of the resins were characterized. Three-ply plywood was fabricated to evaluate wet shear strength and formaldehyde emission. Results showed that when the melamine allocation proportion was increased from 0:4 to 4:0, the total methylene ether group content increased, the curing rate was elevated, and a heterogeneous cross-section of the cured resin formed, which led to a wet shear strength increase of 42.11%. When the melamine allocation proportion decreased, the free melamine content increased, the pot life was prolonged, and a highly homogeneous morphology was formed, which resulted in a decrease of 42.86% in formaldehyde emission of the resulting plywood. These results suggested that a high melamine allocation proportion, meaning that more of the MUF resin was added initially, improved the water resistance of the resulting resin, whereas a low melamine allocation proportion decreased the formaldehyde emission reduction of the resulting plywood.