The influences on the crack formation of melamine resins are only partially understood. At present, four influencing groups can be primarily identified: the properties of the resin, the structure of the paper, the drying and curing conditions and the shrinkage of the impregnated papers during manufacture and in use.
Unmodified melamine resins are very brittle and cannot generally be applied for the impregnation of papers. Through suitable additives, however, the flexibility of the resins can be significantly increased. In order to improve the flexibility - and thereby also the crack resistance - methanol partially-etherified resins, for example, are deployed or the resins are modified using glycols, ε-caprolactam, sugar or similar compounds.
The cellulose fibers of the utilized paper can withstand relatively high tensile stresses and therefore prevent cracking. The paper quality, the degree of resin impregnation, the layer structure and the bonding of the resin to the paper fibers all influence the occurrence of micro and macro cracks.
The degree of cure is a particularly important parameter for the properties of melamine resin-impregnated papers. Over-curing of the melamine resins can lead to cracks, under-curing to insufficient resistance to chemicals. At present, however, there is no recognized method for the quantification of the exceedance of the curing conditions.
As with all polycondensation resins, melamine resin shrinks during curing through elimination of water (condensation). Furthermore, the impregnated papers shrink due to the thermal linear deformation during cooling following pressing. Melamine resin papers therefore demonstrate shrinkage stress compared to the carrier plate following curing. Papers which are already cured when they are bonded with an additional adhesive (HPL, CPL, finish foils) demonstrate these tensions to a much lesser extent. Additional tensions occur when the papers are exposed to variations in the relative humidity during use.
Fraunhofer Institute for Wood Research 
