Adhesive bonding of wood and wood based products Part 4: Different types of adhesives: thermosetting adhesives, PU, hot melts, etc…

These adhesives or glues have been developed a long time ago: urea-formaldehyde ( UF ) and phenolics (PF) started in the years 1920, melamine-formaldehyde ( MF ) and resorcinol-formaldehyde ( RF )a little bit later. However all are still in use, in very large amounts for the UF which are mainly used as binders for wood based panels. The performances of these adhesives are very interesting, and, besides this, UF are quite cheap. The glue manufacturer reacts the urea and the formaldehyde in acidic conditions at varying molar ratio, pH and temperature in order to reach the required molecular weight and viscosity. The molecules are ended by methylol groups by using an excess of formaldehyde. The reaction is stopped by cooling and adjusting the pH. Many different chemical reactions occur during the manufacturing of UF glues because the reaction between urea and formaldehyde is very complex and leads to linear, branched or tridimension networks. And HCl provides acidic conditions so that the condensation may proceed further.