Effect of the initial F/U molar ratio in urea-formaldehyde resins synthesis and its influence on the performance of medium density fiberboard bonded with them

The formaldehyde to urea molar ratio (F/U) plays an important role on the properties of urea formaldehyde (UF) resins and of the medium density fiber board (MDFs) bonded with them. This work presents a hypothesis that besides the final F/U molar ratio in UF resin preparation, the initial F/U affects both the formaldehyde emission levels and the physico-mechanical properties of the boards. Three initial molar ratios F/U of 1.9, 2.1 and 2.3 in resin synthesis have been examined in this work. The structural changes and thermal curing behavior of UF resins were tracked by C-13 NMR and DSC, respectively. At parity of final F/U molar ratio, the resin with initial F/U = 2.1 yielded the highest proportion of linear methylol groups, resulted in the best internal bond strength and lowest thickness swelling of the board. A resin synthesized with the initial F/U = 1.9 provided the highest proportion of total methylene linkages, most methylene ether linkage and lowest free formaldehyde, consistently yielding the lowest formaldehyde emission. These results partially explained why UF resins with lower F/U molar ratios showed relatively poor adhesion when used to manufacture wood-based composites. MDFs bonded with a UF resin having the initial F/U = 2.3 had a relatively good performance with the exception of free formaldehyde, but also showed the best storage stability compared to the other resins having lower initial F/U molar ratios.