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Why Fish Eyes develop in an Epoxy or Polyurethane Coating?
Why Fish Eyes develop in an Epoxy or Polyurethane Coating?

When a coating is applied to a substrate that is contaminated with low surface energy particles such as oil, wax, grease, or silicone, fish eyes may develop in the coating as it is applied. These fish eyes are produced because the coating is unable to wet out the contaminated area.

 

All substrates have a surface energy and as a general rule, adhesion is achieved when the surface energy of the substrate is 8-10 dynes/cm greater than the surface tension of the liquid being applied to the substrate. Epoxy and polyurethane coatings may have a surface tension in the range of 38 dynes/cm.

 

So, applying this coating to a substrate that has a surface energy equal to or greater than 46 dynes/cm should wet out and provide a satisfactory bond. Many waxes and polishes contain silicone which has a surface energy of 24 dynes/cm. If the surface cleaning of the substrate leaves behind these polishing contaminates, the coating with a higher surface tension will not wet out these low surface energy particles leaving behind fish eyes as a result.

 

This is why low surface energy polymers such as polypropylene (30 dynes/cm), natural rubber (24 dynes/cm) and Teflon (20 dynes/cm) are difficult to bond to as coatings typically do not have surface tensions low enough to wet out these substrates.

 

Many automotive manufacturing plants will not allow silicone based products in their plants, because silicone contamination is difficult and almost impossible to remove and control. A very small quantity of silicone in sanding dust or in a cleaning rag can contaminate a surface being prepared for coating/bonding.

 

For more information on preparing a surface for bonding or coating, please refer to our “Surface Preparation” section on our website under “Tips & Tricks”.

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