Curing conditions effects on the characteristics of thermosetting adhesives-bonded wood joints : Part 2: Hot postcuring improvement of UF particleboards and its temperature forecasting model | Einflu\ der Aushärtebedingungen auf das Verhalten von Holzverbindungen mit hitzehärtenden Klebern : Teil 2 : Verbesserung von UF-Spanplatten durch Nachhärten und Vorhersage des Temperaturverlaufs

Thermomechanical analysis (TMA) results indicate that simple post-treatment (such as some postcuring) conditions capable of improving the mechanical performance of aminoplastic resins-bonded particleboard without any degradation do indeed exist. A model to describe the decrease in temperature under different conditions of a particleboard after hot pressing has been developed and shown to correlate well with experimental results of board temperature variation after pressing, both on cooling and during postcuring under different conditions. From this, conditions of temperature and time favourable to improve panel performance by postcuring treatments were also determined. The validity of the improvements forecasted under such conditions was then confirmed at molecular level for urea-formaldehyde (UF) adhesive/wood joints by TMA testing, and finally confirmed by testing the mechanical performance of laboratory UF-bonded boards prepared under the postcuring treatment conditions identified. Noticeable improvements in several UF-bonded board properties such as internal bond (IB) strength, thickness swelling, MOE and MOR were observed by implementing short postcuring periods at temperatures much lower than pressing temperature. The panels performance improvements observed were explained on the basis of the shifts in relative importance of already described and well-known molecular level rearrangements of the cured adhesive network in modern, lower formaldehyde content UF adhesives. The conclusion was that modern, lower formaldehyde content UF adhesives can considerably benefit from short, hot postcuring periods as regards board performance, a trend in clear contrast with the degradation and loss of performance this practice was known to induce in the older, very much higher formaldehyde content aminoplastic resins of the past. The coupling of the simple and very rapid TMA technique with the developed model allows the rapid scan of many other post-treatment schedules and thus to forecast other still possible improvements in the performance of UF-bonded and other-adhesives-bonded particleboard.