Studies of the cracking of direct pressed laminates

Direct pressed laminate (DPL) is a common surface finish for decorative wood-based panels. It typically consists of one or two papers impregnated with thermosetting resins which are hot pressed directly onto a wood-based carrier board. The cured resin forms an inseparable bond to the carrier board, and the resulting surface is mechanical and chemical resistant. In rare cases DPL develops cracks during use. Failure analysis of cracked DPL is restricted by the inseparability of cured resin, paper and carrier board. To gain some fundamental insights in the factors that may lead to cracking, this study systematically evaluated the influence of pressing time and pressing temperature on one type of DPL. To distinguish the influence of board and DPL, we also prepared free films of cured DPL without carrier board and included uncoated particle board and paper without resin impregnation. MOE and MOR of the DPL-coated particle board and free films of DPL correlated well, except for overcured material. Microscopic evaluation demonstrated the development of microcracks in the DPL and a good correlation between the occurrence of microcracking and the susceptibility to visible cracks. No microcracking occurred in the free films. This is most likely due to stresses being relaxed in the free films after the press was opened. In DPL on particleboard, the carrier board restricts the relaxation of these stresses, which can result in the development of microcracks. Therefore, documenting these microcracks could be a useful way of analyzing future DPL failures. Comparing the tensile strength of the free films with the zero-span tensile strength of the raw paper shows that the strength of the individual paper fibers predominantly influences the tensile strength of the DPL. In this regard, the resin solely acts as a binder for the paper fibers, so we propose improving the cracking resistance of DPL by selecting papers with a high zero-span strength. However, it remains unanswered whether the zero-span strength of the paper can be determined retrospectively in the process of failure analysis.