Interaction of polyurethane with wood-based sandwich panels ; Wechselwirkung von Polyurethan mit holzbasierten Sandwichplatten

The continuous one-step production process for light wood-based panels patented by Lüdtke et al. (2007) allows for the expansion of a foam core in a hot press after the compaction of surface layers. Thermoplastic materials have been successfully applied as the core material but require a final cooling phase in a production unit in order to stabilize the expanded polymer foam after pressing. Polyurethane (PUR), as a thermoset, does not require this process step. The self-adhesive property, low thermal conductivity and tailor- made preparation based on selected components and formulations, make PUR an attractive core layer. However, isocyanate's high reactivity challenges the control of PUR foam reaction in the one-step production of wood-based panels. Besides, PUR may be fluid when compressed due to its liquid nature, so it does not remain static during the pressing process. The potential PUR components need to be scattered, able to mix well with wood chips and inactive under normal conditions, yet have the ability to be triggered by external parameters in the process. In order to develop PUR components into an efficient precursor to fulfill the core material requirements of the one-step production process, this thesis focuses on chemical and physical solutions: blocked isocyanates to trigger the foaming reaction thermally and wood filler addition to improve the distribution within the core layer. Blocked isocyanates are an adduct formed by the reaction of isocyanate with an active hydrogen compound, stable at normal temperature. The isocyanate group could be re- released at elevated temperatures. Regarding the mechanism, blocked isocyanates from pMDI and 4,4’-MDI were prepared using NaHSO3 and ɛ-caprolactam as blocking agents. Blocked isocyanates were mixed with sucrose-based polyether polyol to prepare the one-component (1-C) PUR mixture, which must be cured at high temperatures in the hot press to polymerize and create a foam core. Blocked isocyanates were successfully prepared according to FTIR. The ...