Studies of the Cracking Behaviour of High-Pressure Laminates

The European standard EN 438-2 (2019) describes a method to assess the resistance to cracking of high-pressure laminate (HPL). In the EN 438-2 setup, the samples’ shrinkage at a temperature of 50 °C for 6 h is restrained by a metal clamp. The resulting tensile stress may lead to the cracking of the laminate. With the intention to gain additional insight into the parameters and influencing factors of the standard test, the resistance to cracking of 43 laminates was tested in accordance with the standard and with a prolonged testing time. Additionally, the tensile strength and shrinkage of the laminates were determined under the conditions of the standard test. The standard test uses free laminates (laminates not glued to a carrier board), so in order to correlate the results to applications that are close to real-life situations, four of the laminates were also tested when glued to carrier boards the size of furniture doors. In this full factorial test, particle board and gypsum board were used as carrier boards, UF and PVAc as adhesives, and the possible shrinkage of the laminates was adjusted by two moisture conditions of the laminates. The resistance to cracking in the standard test turned out to be mainly influenced by the composition of the laminate. Thin laminates with a pure paper backing were especially prone to cracking in the setup. Within the 6 h storage at 50 °C of the standard test, the laminates did not reach moisture equilibrium. Prolonged storage at 50 °C until equilibrium was reached (which took up to 10 days) allowed an improved differentiation of the cracking resistance, especially of thicker laminates. Four of the laminates were also tested on wooden and mineral-based carrier boards with either PVAc or UF as adhesive and two moisture contents of the laminates. The following parameters were identified as a critical combination for cracking: laminate with a low cracking resistance according to the EN 438-2 test in combination with a non-hygroscopic mineral-based carrier board, increased moisture of the laminate before being glued on the board, and the use of PVAc adhesive.