Effects of resin open time and melamine addition on cold pre-pressing performance of a urea–formaldehyde resin
The pre-pressing performance of urea formaldehyde resins has a significant impact on plywood production efficiency. This paper reports on the effects of the time elapsed after the addition of the hardener (“resin open time”) and of a melamine addition on the cold pre-pressing performance of UF resins. The changes in pH and viscosity, the thermal behaviour, and the structural composition as well as the performance during cold pre-pressing of veneers were investigated. Cold pre-pressing performance was measured by testing pre-pressed three-ply plywood stacks. The results showed that the cold pre-pressing performance is enhanced with longer time after preparation of the resin mix before application onto the veneers and also by longer pre-pressing times. All plywood stacks with at least 2 h pre-pressing time reached the so-called G1 grade (where the re-opened area of the cold pre-pressed plywood stack was 0), which meets the industrially accepted requirements for plywood fabrication. Further, the addition of melamine improved the cold pre-pressing behaviour compared to the investigated UF resin without melamine, showing an earlier start of the development of the pre-pressing shear strength and higher G grades at the various cold pressing times. There are two main reasons for the enhanced cold pre-pressing performance of the resin under the investigated conditions: (1) influence of the “resin open time”: a certain further condensation of the resin after preparing the glue mix (addition of hardener) leading to increased molar mass and viscosity ultimately transforming the resin from liquid-state to gel-state; (2) a higher methylol content in the resin after incorporation of melamine into the resin, which enhanced the formation of hydrogen bonds between the resin and the wood surface. These conclusions represent a feasible approach for the improvement of the cold pre-pressing properties and thus the practical applicability of UF resins with low molar ratios.
This research was financially supported by National Natural Science Foundation of China (Project 31400501).
- Chen LO, Gao Q, Zhang JZ, Chen H, Zhang SF, Li JZ (2014) Effects of assembly time on wet shear strength and formaldehyde emission of plywood bonded by urea formaldehyde resin. BioResources 9:2975–2986Google Scholar
- Gao Q, Qin ZY, Li CC, Zhang SF, Li JZ (2013) Preparation of wood adhesives based on soybean meal modified with PEGDA as a crosslinker and viscosity reducer. BioResources 8:5380–5391Google Scholar
- Luo JL, Zhang JZ, Luo J, Li JZ, Gao Q (2015) Effect of melamine allocation proportion on chemical structures and properties of melamine–urea–formaldehyde resins. Bioresources 10:3265–3276Google Scholar
- Mao A, Kim MG (2013) The effects of adding melamine at different resin synthesis points of low mole ratio urea–melamine–formaldehyde (UMF) resins. BioResources 8:5733–5748Google Scholar
- Wang SM, Shi JY (2008) Performance of plywood made with low-toxicity urea–formaldehyde resin. China Wood Ind (in Chinese) 22:32–34Google Scholar
- Zhang JZ, Wang XM, Zhang SF, Gao Q, Li JZ (2013) Effects of melamine addition stage on the performance and curing behavior of melamine–urea–formaldehyde (MUF) resin. BioResources 8:5500–5514Google Scholar