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

Thermomechanical analysis (TMA) results indicate that simple post-treatment (such as some postcuring) conditions capable of improving the mechanical p erformance of aminoplastic resins-bonded particleboard without any degradat ion do indeed exist. A model to describe the decrease in temperature under different conditions of a particleboard after hot pressing has been develop ed and shown to correlate well with experimental results of board temperatu re variation after pressing, both on cooling and during postcuring under di fferent conditions. From this, conditions of temperature and time favourabl e to improve panel performance by postcuring treatments were also determine d. The validity of the improvements forecasted under such conditions was th en confirmed at molecular level for urea-formaldehyde (UF) adhesive/wood jo ints by TMA testing, and finally confirmed by testing the mechanical perfor mance of laboratory UF-bonded boards prepared under the postcuring treatmen t conditions identified. Noticeable improvements in several UF-bonded board properties such as internal bond (IB) strength, thickness swelling, MOE an d MOR were observed by implementing short postcuring periods at temperature s 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 c ured adhesive network in modern, lower formaldehyde content UF adhesives. T he conclusion was that modern, lower formaldehyde content UF adhesives can considerably benefit from short, hot postcuring periods as regards board pe rformance, a trend in clear contrast with the degradation and loss of perfo rmance this practice was known to induce in the older, very much higher for maldehyde content aminoplastic resins of the past. The coupling of the simp le and very rapid TMA technique with the developed model allows the rapid s can of many other posttreatment schedules and thus to forecast other still possible improvements in the performance of UF-bonded and other-adhesives-b onded particleboard.