Binders and Coatings

Research project

Avoidance of cracks in melamine resin surfaces

Whether laminate flooring, kitchen and laboratory furniture, household furniture or interior doors: the majority of these items consist of wood-based materials which are coated with melamine resin-impregnated papers. The melamine resin surfaces are very resistant. In rare cases, however, cracks and deformations may occur, which can result in costly complaints and dismantling measures. In a previous project, we were successful in narrowing down the causes of crack formation and in developing promising methods for the problem solution. For a reliable quality assurance and damage analysis, we will optimize the methods during this project. Furthermore, we will extend the investigations to include mixed resins.

Microscope image shows a cell-like, brownish structure which is covered by a cell-like, white structure. In the white layer, there is a narrow, blue area which extends from the very top down to the brownish layer. At the upper end of the blue discoloration, the white layer bulges upwards like a crater.
© Fraunhofer WKI | Dirk Lukowsky
Blue contrasted crack due to direct coating in cross-section.
Microscope image shows a white area interspersed with blue lines. The lines originate from larger craters and branch out in different directions.
© Fraunhofer WKI | Dirk Lukowsky
Detail of a branched crack in a melamine resin surface. For better contrast, blue marking paste was used.

With a market share of over 70 percent, melamine resin-impregnated papers are the dominant coating product for wood-based materials for interior applications. For the wood-based materials industry as well as the processing companies, crack-resistant melamine resin papers are therefore of considerable economic importance.

In our previous project, we determined important process-engineering factors which influence the formation of cracks in coating papers impregnated with melamine resin. Furthermore, we have successfully developed methods with which, for the first time, the hygric and mechanical properties of direct pressure laminates (DPL) “welded” to the carrier plate can be quantified in an isolated manner.

We are now in the process of optimizing the methods in order to precisely determine the mechanical and hygric properties. With these characteristic values, we are creating the prerequisites for the development of less-sensitive direct coatings in the wood-based materials industry and for targeted quality assurance in the processing companies.

The new methods should furthermore serve the reliable identification of the cause of damage in the case of complaints. Until now, it was often not clear as to whether the damage was the result of production, processing or improper use. With our investigations, we are facilitating the legal clarification as to who is responsible for the damage.

Our solution approach:

  1. Optimized sample geometry, avoidance of inhomogeneities during pressing
  2. Comparison of the properties of papers cured without a carrier plate (free films)
  3. Improved consideration of the stresses induced during pressing via mathematical modelling (FEM)
  4. Validation of the test in a wide range of properties
  5. Integration of the increasingly utilized mixed resins
View from above of a piece of OSB board partially covered by a white layer which becomes thinner from right to left. Dark, irregular lines can be seen in the white layer.
© Fraunhofer WKI | Dirk Lukowsky
By means of the tapered-grinding technique, the crack patterns can be displayed in the depth of the coating
Image montage of four square photographs which have been assembled to form a large square. Pale, irregular lines and isolated point-like fragments can be seen on the brownish wood-based material surfaces. In addition, a dark structure can be seen, once in the form of a line (groove) and once in the form of a circle (borehole).
© Fraunhofer WKI | Mark Meder
Montage composed of four crack patterns on different wood-based materials (upper left: coarse particle board; upper right and lower left: particle board with groove and borehole; lower right: birch plywood). The cracks in the melamine coating were photographed and, after the coating had been sanded off, were projected onto the original location on the raw material surfaces by means of image processing.
Microscope image shows a light grey area which is intersected by a large number of dark grey, irregular lines and point-like fragments.
© Fraunhofer WKI | Dirk Lukowsky
Micro-cracks documented through fluorescence microscopy.

Funding

Project title (original): Vermeidung von Rissen in Melaminharzimprägnierten Beschichtungspapieren für Holzwerkstoffe

Funding body: German Federal Ministry for Economic Affairs and Energy (BMWi)

Funding reference: IGF 20908 N

Project management: Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” e.V. (German Federation of Industrial Research Associations, AiF) via the Internationale Verein für Technische Holzfragen e. V. (International Association for Technical Issues Related to Wood e.V., iVTH)

Duration: 1.11.2019 to 30.6.2021