# Studies of the Cracking Behaviour of High-Pressure Laminates
## 1 Introduction  
High-pressure laminate (HPL) and continuous-pressure laminate (CPL) consist of several layers of papers impregnated with melamine resin or phenolic resin with a ratio of about 50-70% (w) paper and 30-50% thermosetting resin [1; 2]. Commonly, the laminates are glued to wood-based panels, often particle boards. In situations where increased fire safety is required, inorganic panels based on gypsum, vermiculite, or foamed glass are used as carrier boards. PVAc, spray adhesives, and Urea Formaldehyde (UF) resins are common adhesives for laminates. Due to their high chemical and mechanical resistance, laminates are widely used for kitchen tops, furniture, and doors [3; 4; 5; 6]. The production of laminates is still managed empirically, resorting to rules of thumb and workshop experience. Published research on the properties of laminates focuses mainly on the reduction in warping [7; 8; 9; 10; 11] and the improvement of surface properties [3; 12; 13; 14]. In rare cases, laminates develop cracks during use. The NEMA Standards Publication LD 3-2005 (2005) [15] addresses several factors that may lead to such cracking:  
* The dimensional movement of the carrier board does not match the dimensional movement of the laminate (also mentioned by [16]). * Cutouts with a too-low radius and too-small drill diameter for fittings/screws. * Overly wide spans without a sturdy framework. * Non-uniform glue lines and foreign bodies in the glue line. * Lack of acclimatization of laminate and substrate before assembly. In addition to the factors that fall into the responsibility of the manufacturing of the boards, the quality of the laminate itself and extreme dry use conditions have been observed to cause cracking. Cracks in laminates are more frequent in public buildings, namely, hospitals, hotels, and ships, due to the combination of the high exchange of air, well-heated conditions, and fire requirements [17]. The high air exchange may lead to extremely dry conditions in cold winters and fireproof minerals or aluminium-based panels do not follow the shrinkage of the laminate. Additionally, laminates with exceptionally large moisture-induced movement and production faults the laminate, e.g., the use of inadequately resin-impregnated papers have been observed to cause cracks in laminates. With few exceptions, cracks in laminates tend to be rather straight and often at a small angle to the direction of production. Single cracks often originate from cutouts, however, in the absence of cutouts, the common failure pattern consists of multiple parallel cracks, distanced 50 mm to 200 mm from each other [17]. The European standard EN 438-2 [18] describes a method for the determination of the resistance to cracking under stress (paragraph 23 of the standard). The laminates are rigidly clamped in a preheated steel fixture and stored for 6 h at 50 \({}^{\circ}\)C (Figure 1). The resistance to cracking is assessed by visual examination. In the EN 438-2 setup (chapter 23 of the standard), the samples are loaded with tensile stress resulting from the laminate's drying shrinkage counteracted by the laminate's thermal expansion. Reported values for the swelling and shrinking of laminates are scarce. According to [19], the swelling of a melamine paper is in the range of 0.006% per percent change in relative humidity (RH) between 30% and 80% RH. A technical paper on the HPL industry mentions 0.005%/% in the range of 10% and 90% RH [20]. The thermal expansion of the laminate counteracts the shrinkage in the EN 438-2 (23) testing. According to the literature, mixtures of melamine resins and paper show a coefficient of thermal expansion in the range of 40 to 60 \(\times\) 10\({}^{-6}\)/K [2], 22 \(\times\) 10\({}^{-6}\)/K [21], and 40 to 45 \(\times\) 10\({}^{-6}\)/K [22]. Regarding HPL, values of 14 to 17 \(\times\) 10\({}^{-6}\)/K have been reported by [19]. The temperature  
Figure 1: Schematic illustration of the EN 438-2 (23) procedure.