# Studies of the Cracking Behaviour of High-Pressure Laminates
## 3 Results
### Properties of Laminates on Carrier Boards  
After 14 days at 25% RH and the subsequent 14 days at 10% RH, no mentionable cracks were observed on the 560 mm \(\times\) 560 mm laminate-covered boards. Only the following storage at 50 \({}^{\circ}\)C for 24 h led to the formation of cracks on 26 of the 126 faces. By far, the most numerous and longest cracks appeared in the combination of laminate L6, preconditioned at 65% RH, and glued with PVAc on a gypsum board. With this combination, an 80 cm crack length per face originated from the slit and 164 cm of the crack length per face originated independently from the slit (Figure 6); the cracks of all other combinations started at the corners of the slit only. Figure 7 shows the statistical evaluation as the sum of squares of the combined parameters. All displayed factors and combinations are highly significant with \(p<0.0001\). Figure 6: Cracking pattern of laminate L6 (**left**) and laminate L 35 (**right**). The cracks are highlighted with a red marker (the length of the slits is 5 cm). The sum of the crack length and the low crack resistance of the laminate L6 corresponded well with the time to crack as determined by EN 438-2 (23) (Table 2). The two laminates that did not crack after 240 h of the prolonged EN 438-2 (23) procedure showed a max of 12 cm cracks per face of the large samples. The laminate failing during the 240 h in the prolonged EN 438-2 (23) procedure had 45 cm and the laminate that failed within 6 h (regular test time of EN 438-2 (23)) showed 244 cm of crack length per face. According to the infrequently reported shrinking values of laminates [11; 19; 20], particle boards shrink very similarly to laminates (0.006%/% to 0.0103%/% for HPL and 0.0024%/% to 0.01%/% for particle boards). Consequently, the resulting stress in laminates on particle boards may be rather low. The shrinkage of gypsum boards is reported to be 0.0000065%/%) [23]. This almost negligible reaction to moisture may cause significant stresses on laminates in dry conditions. The strong effect of higher drying stresses caused by gluing laminates stored at increased RH demonstrates the strong influence of shrinkage on the occurrence of cracking and confirms the practical observation of a much-increased risk for the cracking of laminates on fireproof mineral-based boards, e.g., in public buildings and passenger ships [17]. For particle boards, which are the typical carrier board for laminates, the coefficient of thermal expansion is reported to be 9 \(\times\) 10\({}^{-6}\)/K [19] and 10 to 15 \(\times\) 10\({}^{-6}\)/K [24]. With a typical coefficient of 16.7 \(\times\) 10\({}^{-6}\)/K, gypsum boards expand in a similar range [23]. The increased cracking on the gypsum board caused by different thermal expansions is seemingly unlikely. The advantage of using UF as an adhesive for laminates can be traced back to the balanced transfer of stresses into the carrier board. Contrary to what is often assumed in the industry, the higher flexibility of thermoplastic adhesives, e.g., PVAc, may not necessarily contribute to an even stress distribution. As a hypothesis, the higher flexibility may indeed favour stress accumulation by creep in certain areas. This finding is supported by the incidences of assumed higher crack susceptibility of laminates glued with very flexible and rather inhomogeneously applied spray adhesives [17]. Laminates are known to be sensitive to sharp cutaways. The trial demonstrated that at least small internal grooves from plate joiners are not necessarily very critical to all \begin{table}
\begin{tabular}{c c c c c} \hline  & **Laminate L6** & **Laminate L15** & **Laminate L35** & **Laminate L20** \\ \hline Sum of crack length per face & 244 cm & 45 cm & 12 cm & 4 cm \\ \hline Time to crack EN 438-2 (23) & \textless{}6 h & \textless{}240 h & \textgreater{}240 h & \textgreater{}240 h \\ \hline \end{tabular}
\end{table} 
Table 2: Sum of the crack length of the four laminates glued to a carrier board and the corresponding time to crack in the modified EN 438-2 (23) testing.