Know the most common culprits of your insulated wall panel quality problems

Insulated wall panel manufacturers usually face some quality problems with their laminated composite products that lead to a reduction of their productivity and an increase in their production costs.

Two of those most frequent quality problems are a lack of flatness on the face sheets and debonding of the foam material or core material (from face sheets). The lack of flatness of the surface appears as a consequence of mechanical grinding in order to increase the roughness for better bonding. This way liquid adhesive can be applied on the surface filling into the micro cavities and micro pores creating a solid film that connects the core material with the face sheet.

On the other hand, debonding occurs as a result of face sheets debonding from the foam due to the lack of the adhesive in some regions of the sheets’ surface. This flaw commonly happens by the cause of manual gluing of faces sheets during insulated wall panel production.

At ARCEO Engineering, we take advantage of the coil coating technology to produce prepainted galvanized steel, cold rolled steel, stainless steel and aluminium coils capable of being used as raw materials for high quality insulated wall panels.

An epoxy-based adhesive is applied on the face sheet to replace either the manual gluing or the harsh mechanical grinding. As a result, you will have a continuous solid adhesive on your face sheet with no lack of material in some regions leading to a possible foam debonding.  

Epoxy adhesives: knowing them better

An epoxy is a compound having an oxirane group, which is a ring system having two carbon atoms and one oxygen (figure 1). Oxygen atoms have, in general, more affinity for electrons than carbon atoms. For this reason, the oxygen of the oxirane group will pull toward itself an electron from each carbon atom. 

Figure 1. Oxirane group. The oxygen atom is “lifting” the two carbon atoms by pulling toward itself their electrons making the molecule be strained. 

This electron pulling action of the oxygen is “lifting” the two carbon atoms from the plane where they are and this is making the molecule be strained. This strain energy is the main reason for the oxirane group to be highly reactive as it is not stable, and therefore, it is prone to be opened.  

Epoxy coatings are produced by reaction of epoxy resins (molecules having oxirane groups) with harderners to produce an epoxy molecule of low molecular weight (figure 2). This latter, will react with other molecules of the harderner through their oxirane groups still available to continue the polymerization reaction until an epoxy-based polymer network is obtained.

Figure 2. Chemical reaction between an epoxy resin and a hardener producing an epoxy-based polymer. These types of polymers are the basis of coatings applied on galvanized steel, cold rolled steel, stainless steel or aluminium at ARCEO Engineering in order to make sandwich panel face sheets.    

These polymeric networks are materials of a high strength (no flexibility and no permanent change under high temperatures) as well as materials very well known for their adhesion properties.  

Epoxy, Polyurethane and Polyester adhesives: any difference?

Epoxy coatings are normally used as primers due to their adhesion ability to almost all types of substrates. Besides epoxies, we can also find other kinds of polymers that are usually used as primers. These are Polyurethanes and Polyesters. 

The essential differences between these coatings are summarized in the following chart:

Epoxies
Polyurethanes
Polyesters
Excellent adhesion to almost all materials
Excellent adhesion mostly to porous materials (wood and textiles)
Usually used in thermoset plastics bonding, metal bonding and repair works in the automobile industry
Good thermal resistance and excellent chemical resistance
Good thermal and chemical resistance
Low thermal resistance and resistance to a variety of chemicals
High resistance to humidity
No resistance to humidity
Good resistance to humidity
Low flexibility
High flexibility even at low temperatures
Not much impact resistance as it is a porous material
High degree of crosslinking that confers them high strength and almost no shrinking
Variable crosslinking degree depending on the raw materials, so variable shrinking
Usually low shrinking

Polyester-based coatings are usually applied as universal primers for a basic corrosion protection, but as adhesives for insulated wall panels, depending on the operation conditions, polyester primers could not be an ideal solution due to their low impact resistance.

Epoxies have the advantage over polyurethanes of being water resistance, which means that primed metallic face sheets can be stored without damage. When talking about steel substrates, epoxy-based coatings are more resistance against humidity when they are applied on cold rolled steel than when they are applied on galvanized steel.  

On the other hand, epoxy-based coatings have a greater degree of functionality than polyurethanes, which means that, depending on the raw materials for producing the epoxy resin, epoxy-based coatings will have many more reactive groups like the isopropyl group and oxirane group (figure 2) to be used as a source of adhesion to materials.

As an experienced coil coater with a R&D background, we are more than capable of working in collaboration with our customers to come with an innovative solution tailored to their technical and process requirements. Contact us! 

References

  • F. Avilés, L.A. Carlsson. Experimental studies of compression failure of sandwich specimens with face/core debond. Delamination Behaviour of composites. 2008: pages 344-363.
  •  A. Katunin, M. Chuda-Kowalska. Assessment of internal damages in sandwich panels via Active Thermography. Engineering Transactions. Vol. 63. Issue 2. 2015: pages 171-180.
  • A. Pizzi, K. L. Mittal. Handbook of adhesive technology. 2nd ed. 2003. New York: Marcel Dekker.     
  • D. Swern. Reactions of the oxirane group. Journal of the American Oil Chemists Society. Vol. 47. Issue 11. 1970: pages 424-429.  
  • M. Nakazawa. Mechanism of adhesion of epoxy resin to steel surface. Nippon Steel Technical Report. Vol. 63. 1994: 16-22.
  • 2K Polymer Systems Limited. (2015). Polyester. Retrieved from www.2kps.net/p-polyester

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