The effect of pressing parameters and hardener content on the properties of plywood bonded with propylamine-UF adhesive

Research on reducing the formaldehyde emission from wood-based materials bonded with urea-formaldehyde (UF) resin has attracted the attention of scientists for years. Many studies investigated the possibility of using amines for this purpose; however, the outcomes of the vast majority of them have shown that the limiting factor is reduced resin reactivity and consequently, deterioration of the strength of the resultant materials as well. Therefore, the aim of this research work was to investigate and evaluate the effect of the pressing protocol applied (pressing temperature and time), and hardener content (20% ammonium nitrate solution) on the properties of plywood bonded with UF adhesive system modified with 1% of propylamine. Manufactured plywood was tested in terms of shear strength (in dry and wet conditions) and formaldehyde emission (measured initially and after 4 weeks). Based on the results, it was found that it is possible to produce plywood with equally good characteristics as the reference, non-modified variant by increasing hardener content from 3% to 7%, increasing pressing temperature from 120°C to 150°C, and extending the pressing time from 240 to 300 s. Markedly, the implementation of these adjustments also contributed to a further decrease in formaldehyde emission.