Melamine Paper Grading Rules: Part 1 Quality Control of the Surface Defects

Melamine Paper Grading Rules: Part 1 Quality Control of the Surface Defects

December 27th,2018

 

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.

The laminating process of wood-based panels with surface materials such as decor foils, HPL (high pressure decorative laminate) and melamine films is a well established method. Nevertheless, insufficient bonding or invisible defects below the surface are regularly recurring problems in quality assurance.

The classification of surface defects is one of the most important tasks in the quality control stage of

panels with melamine surfaces;  .In particular, the classification of  defects is of great importance, since on the one hand, it allows generating statistics that help to correct the processes that cause the defects and, on the other hand, they allow classifying the quality of the panels depending on the types of detected defects. Currently, there are numerous works that point to the automated classification of defects. However, aspects such as reflections on shiny surfaces, surfaces

with low contrast, complex color relationships, small defects, complex designs, among others, make these automated solutions have poor performance when inspecting this type of surfaces.  Such as relief areas or dents, cracks, moisture variances or different materials .
In essence, a melamine board is a particle board coated with a decorative sheet impregnated with
melamine resins. The particle board consists of small fragments of pine, usually called pine shavings,
crushed and selected to later mix with a special adhesive, generally made from water, resin, and chemical
hardeners. After the mix is ready, the particle-board and the sheet are fused using a heating system.
As stated in the previous section, at each step of the fabrication process a defect could appear that
will alter the final quality of the product. In this work, we focus on classifying five different types of
defects, paper scrap, stains, white stops, paper displacement, and bubbles (see Figure 1). Each one of
those defects appears due to one or more of the following reasons, the presence of dust or suspension
particles, dirt or zones without adhesives.
Figure 1.
Example of different defects that can appear in melamine boards ((a)Paper scrap, (b) Stains,(c) White spots, (d) Paper displacement, (e) Bubbles).

 

Special Thanks to the following sources :

1.https://www.wki.fraunhofer.de/en/departments/ot/profile/research-projects/cracks-melamine-wood-based-materials.html

2.https://www.researchgate.net/publication/328590729_Melamine_Faced_Panels_Defect_Classification_beyond_the_Visible_Spectrum

3.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6263934/

4.http://www.mactac.eu/datas/files/Technical%20Bulletin%207%208%20Laminating%20problems%20%20solutions.pdf

5.https://www.mdpi.com/1424-8220/18/11/3644/htm

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