Surface Technology

Research project

Avoidance of cracks in melamine resin-impregnated coating papers for wood-based materials

Melamine resin surfaces are hard, durable and, with the exception of strong acids, very insensitive to chemicals. With a market share of over 70 percent, melamine resin-impregnated papers are the dominant coating material for wood-based materials for indoor applications such as laminate floorings, kitchen and laboratory furniture, household furniture and interior doors. In rare cases, cracks can occur on such surfaces - often weeks or months after delivery. At present, it is difficult for the industry to take appropriate measures for the avoidance of cracking, as the causes of the crack formation are only known to a certain degree. In this research project, we are identifying the significant parameters in order to achieve adequate crack resistance in melamine resin-impregnated papers and are thereby developing appropriate testing methods. 

© Fraunhofer WKI | Dirk Lukowsky

Detail of a branched crack in a melamine resin surface. For better contrast, blue marking paste was used. The cracks initiate from larger craters in the melamine resin surface or branch out from there.

© Fraunhofer WKI | Dirk Lukowsky

Cracks in an HPL surface. The parallel crack path at a shallow angle to the direction of production is typical.

© Fraunhofer WKI | Dirk Lukowsky

Typical cracks in a direct coating. The slight bulge at the crack flanks is recognizable through the shadow on the cracks. The bulge occurs because the cracks continue in the carrier plate.

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.

 

Our research approach is based on the following working hypotheses:

  • Through the systematic examination of different material and process parameters, the most important influencing factors for the crack formation in melamine resin-impregnated papers can be identified and their influence can be quantified, thereby opening up the possibility of reducing this type of damage.
  • One of the known influences on the crack formation is over-curing of the utilized melamine resins. Through a quantification of the degree of cure of melamine resins by means of conventional UV spectrometers, the quality of melamine resin-impregnated papers can be monitored and improved. Crack formation can be prevented through this quality assurance measure. It is furthermore possible to carry out the measurement semi-automatically. A demonstrator for the semi-automatic quantification of the hydrolyzed melamine can be produced during the project.
  • Through systematic investigations, the causes of cracks in melamine resin-impregnated papers can also be determined retrospectively. In the future, it will be possible to assign such cracks to the root cause, thereby enabling the possibility of a targeted modification of process and material parameters.

Funding

Arbeitsgemeinschaften industrieller Forschungsvereinigungen (German Federation of Industrial Research Associations, AiF)

Internationaler Verein für Technische Holzfragen (International Association for Technical Issues Related to Wood e.V., iVTH)