# Studies of the Cracking Behaviour of High-Pressure Laminates
## 2 Materials and Methods  
Forty-three laminates (CPL and HPL) with a thickness range of 0.15 mm to 1.42 mm were provided by five companies. The laminates were selected to account for a wide range of materials including pure melamine laminates and laminates with a paper backing. The first set of experiments was performed in order to identify the influences of the standard test EN 438-2 (23). The sample's dimensions were in accordance with the EN 438-2 (23) (150 mm in length, 50 mm in width, and a borehole 10 mm in diameter in the centre of the sample). The length of all samples was oriented perpendicular to the direction of production, thereby focusing the load on the orientation most commonly encountering cracks.  
The following properties were determined on samples that were conditioned at 23 \({}^{\circ}\)C/50% RH for at least 14 days before further exposure:  
* Resistance to cracking under stress (EN 438-2 (23)): Laminates were clamped in a metal device that had been heated to 50 \({}^{\circ}\)C. After 6 h storage at 50 \({}^{\circ}\)C, the samples were removed from the oven and the clamps and rated for cracking. Diverging from the standard, the number of replicas for the 6 h testing time was increased from one to three. Additionally, the test was repeated for a prolonged time in the oven (24 and 240 h). These prolonged tests were made with a single sample per time interval.
* Tensile strength at 50 \({}^{\circ}\)C: This test was created to simulate the conditions of resistance to the cracking test (EN 438-2(23)). Five samples in accordance with the standard (including the drill hole) were stored at 50 \({}^{\circ}\)C for 6 h. Subsequently, the tensile strength was determined with a speed of 0.1 mm/min within a climate chamber at 50 \({}^{\circ}\)C.
* Change in length and mass at 50 \({}^{\circ}\)C: Three samples per laminate were stored at 50 \({}^{\circ}\)C. The length and the mass of the three replicas were determined in accordance with EN 318 after 6-, 24, and 240-h storage. In order to exclude the possible effects of the temperature during the measurement, two sets of samples were measured independently: one set was measured within a larger climate chamber at 50 \({}^{\circ}\)C and the other set was measured 10 min after moving the samples from the oven to a room at 23 \({}^{\circ}\)C.
* With the intention to determine the time needed to reach the equilibrium moisture content, the mass loss of five of the laminates was also monitored in further time intervals.
* Infrared Spectroscopy (IR-ATR): The powder of the surface was scratched from the backing layers using a doctor's blade. The powder was analysed in a Tensor 27 (Bruker) with 32 scans. The baseline-corrected spectra were normalized to the peak at 1023 cm\({}^{-1}\).  
Based on the results of the first set of experiments, four laminates were chosen for a full factorial experiment to distinguish the influences of the carrier board, adhesive, and the initial moisture of the laminate on the development of cracks (Figure 2):Half of these four laminates were conditioned at either 20 \({}^{\circ}\)C/65% RH or 20 \({}^{\circ}\)C/30% RH. The laminates were hot pressed on both sides of a standard particle board or a gypsum board with 40 g/m\({}^{2}\) polyvinyl acetate (PVAc) or urea-formaldehyde adhesive (UF). Pressing parameters: 80 \({}^{\circ}\)C, 30 s for PVAc, and 130 \({}^{\circ}\)C, 18 s for UF. After cutting the edges straight, a 3 mm thick edge band was glued to the resulting 560 mm \(\times\) 560 mm boards. In order to provoke cracks and to replicate regular occurring cutouts a 55 mm \(\times\) 5 mm slit for a plate joiner was machined in each side of the boards. The boards were stored for 14 days in a climate at 20 \({}^{\circ}\)C/25% RH, followed by 14 days at 20 \({}^{\circ}\)C/10% RH, and final storage for 24 h at 50 \({}^{\circ}\)C. The development of cracks was recorded after each climate.