Melamine-bridged alkyl resorcinol modified urea - formaldehyde resin for bonding hardwood plywood
A powdery product was obtained by the reaction of methylolated melamine with alkyl resorcinols to form melamine-bridged alkyl resorcinols (MARs). The effects of the addition of this powder on the bonding strength and formaldehyde emission of urea–formaldehyde (UF) resins were investigated. Three types of UF resins with a formaldehyde/urea molar ratio of 1.3 synthesized by condensation at pH 1.0 (UF-1.0), pH 4.5 (UF-4.5), and pH 5.0 (UF-5.0) were fabricated. The addition of MAR to UF-4.5 and UF-5.0 for bonding hardwood plywood enhanced the bonding strength and reduced formaldehyde emission. For UF-1.0, the addition of MAR adversely affected the bonding strength. However, the UF-1.0 resin yielded the lowest formaldehyde emission of all of the UF resins in the study. The effects of the MAR addition were related to the molecular structures of the UF resins. UF-1.0 contained a large amount of free urea, a considerable number of urons, and a highly methylenelinked, ring-structured higher molecular weight fraction and had a smaller number of methylol groups. Therefore, the addition of MAR was considered to cause a shortage of the methylol groups, which in turn, led to incomplete resin curing. In contrast to UF-1.0, UF-5.0 contained a smaller amount of free urea and a linearly structured higher molecular weight fraction and had a larger number of methylol groups. In this case, MAR was considered to effectively react with the methylol groups to develop a three-dimensional crosslinked polymer network to enhance the bonding strength and suppress the generation of free formaldehyde to reduce formaldehyde emission.