Hot postcuring improvement of MUF-bonded particleboards and its temperature forecasting model C. Zhao, A. Pizzi &) ENSTIB, University of Nancy 1, P.B. 1041, 88051 Epinal, France Materials and methods The two MUF resins used were com- mercial particleboard adhesive type of M + U):F molar ra- tio  1:1.2 and 1:1.5, both of 65% resin solids content, and both of M:U weight ratio of 47:53. MUF-bonded particleboards were then produced at a density  0.680 g/cm 3 , resin solids con- tent  9%, non-resinated MC 6.9%, 2% NH 4 Cl hardener, board thickness  14 mm, target press platens temperatures  150°, 180°, 190°, pressing cycle  60/50/60 seconds at pressures of 3.0 MPa/1.1 MPa/0.5 MPa. These particleboard panels were then post-treated as indicated in Table 1 in an oven set at 100 °C real temp  96 °C), 90 °C real temp  88 °C), for 30 minutes before being allowed to cool at ambient temperature. One set of panels used as control was allowed to cool without any post- treatment. The panels were tested after 2 days conditioning at ambient temperature for dry internal bond IB) strength according to standard speci®cations. The results are shown in Table 1. The MUF resins above were also tested dynamically by thermomechanical analysis TMA) on a Mettler 40 TMA apparatus. Samples of beech wood alone, and of two beech wood plys bonded with each system of liquid polycondensate resins in a layer of 350 lm, for a total samples dimensions of 21 ´ 6 ´ 1.1 mm were each tested with a schedule of a non- isothermal mode followed by an isothermal mode to reproduce the increase and holding of the temperature characteristic of the core of a wood particleboard during its pressing schedule. Each sample was placed at ambient temperature in the TMA sample holder and the system then heated at 60 °C/minute to imitate the rise in temperature of the core of a particleboard panel during hot pressing, until a maximum temperature of 150 °C, 180 °C, and 190 °C, for the different cases at which the schedule was maintained isothermal at 150 °C, 180 °C, and 190 °C, for total heating times leading to constant modulus for all cases tested to ensure that the joint had cured completely and that no adhesive degradation had intervened. The samples were then cooled down to ambient temperature and then re-heated in the TMA apparatus, but to a lower temperature than that used for the original curing of the joint. The re-heating temperatures used were of 100 °C for the 150 °C and 180 °C curing), and of 90 °C for the 190 °C curing). All tests were done in three points bending on a span of 18 mm exercising a force cycle of 0.1 N/0.5 N on the specimens with each force cycle of 12 seconds 6 s/6 s) and the resulting modulus curves as a function of both temperature and time obtained. The classical mechanics relation between force and de¯ection E  [L 3 /4bh 3 )][DF/Df] allows the calculation of the Young's modulus E for each of the cases tested Lu and Pizzi 1998). Results and discussion The single model equation which was used for UF bonded particleboard Lu and Pizzi 1998), can be written for the process of hot-stacking at a given temperature MUF-bonded particleboard, both adhesives being aminoplastic acid-setting wood adhesives. Thermomechanical analysis was undertaken to ascertain if experimental results obtained for curing of bonded wood joints by TMA could be correlated with what occurs during and after pressing in a particleboard. This TMA schedule, entailing an increase in temperature and sub- sequent stabilization of the temperature of the joint, was used to imitate the characteristic temperature increase and stabili- zation pro®le of a particleboard core during pressing. The cured, hardened joint was then retested, after curing, again by TMA, to ascertain what the effect of post-curing by hot-stacking could be on the modulus of the joint itself. Examples of TMA experiments done in this manner are shown in Fig. 1a, b, c, where the modulus MOE) values for isothermal TMA lower temperature `postcuring' of the joint are compared with the ®rst step MOE curve during joint curing characteristic of the initial TMA higher temperature `pressing' schedule. From Fig. 1 it is evident that: 1) postcuring can be used under well-de®ned conditions to improve the performance of MUF-bonded panels without any further joint and hardened adhesive degradation, as Table 1. Post-treatment conditions and results for laboratory MUF-bonded particleboards Tabelle 1. Bedingungen fu Èr die Nachbehandlung und Ergebnisse an MUF-gebundenen Laborplatten MUF M + U):F molar ratio Press temper ature °C) Cooled down naturally after being pressed min.) Heated in oven for 30 minutes °C) TMA max MOE before postcure MPa) TMA max MOE after postcure MPa) Board I.B. Strength MPa) Cooled down surrounding temperature °C) 1.5 190 till being tested No 3575 ± 0.87 16.7 1.5 190 5 90 3575 6425 1.12 17 1.5 180 Till being tested No 3200 ± 0.76 17 1.5 180 5 100 3200 4000 0.91 17 1.5 150 5 100 3423 3558 ± 17 1.2 190 Till being tested No 4000 ± 0.81 17 1.2 190 5 90 4000 4956 0.93 19 1.2 180 5 100 3461 4807 0.92 19 1.2 150 5 100 2500 4135 ± 19 Kurz-Originalia á Brief Originals Holz als Roh- und Werkstoff 58 2000) 307±308 Ó Springer-Verlag 2000 307