Adhesion, Part I.

Adhesion is not a defect, but lack of adhesion or loss of it certainly is, With the exception of paint films that are designed to be temporary, adhesion is critical to the performance of coatings. Without it, a coating will be easily scraped off or otherwise removed, leaving ugly damaged areas and inadequate protection for the substrate (which may be a coating as well as wood, metal, or plastic). What are the requirements for good adhesion? The first is that the paint must come into intimate contact with the substrate via the process of wetting (see the October 2004 issue of JCT COATINGSTECH for more on wetting). Whether this occurs depends on surface tension factors, but wetting usually is adequate over wood, clean metal, and pretreated metal surfaces. Existing coatings may be difficult to topcoat or recoat, and plastics may need cleaning and special treatments for wetting to occur.

Although wetting is necessary for adhesion, it does not guarantee good adhesion. Drying or baking may destroy the initial bonding by changing the characteristics of the coating so that it is no longer compatible with the substrate. This compatibility is the next requirement due to the need for one material or layer to adsorb onto the other. The old adage that "like dissolves like" also holds for wetting and adhesion. This enables us to say that "like adheres to like." The third requirement is for polar groups on both the coating and the substrate to aid in the formation of adhesive bonds. There is evidence in the literature that matching the polarities of the cured film and the substrate contributes to good adhesion. This explains why polar coatings do not adhere well to nonpolar or low polarity materials (such as many plastics).

Adhesion failure may happen soon after application or may occur over a period of time, particularly with weathering. Immediate failure may be due to a contaminated surface, such as oil on steel, dirt or chalking on wood, or mold release residues on plastic. Another possibility is an overbaked undercoat that offers no opportunity for the topcoat solvents to bite into it to allow component mixing at the interface. Painting over decayed wood often leads to failure, but it is cohesive failure in the decayed wood rather than true adhesive failure.

One possible root cause for delayed failure is the migration of a substrate component to the interface between the substrate and a coating. A weak boundary layer is formed in the interface between the coating and substrate and adhesion is lost. This layer may be composed of plasticizer, unreacted crosslinker, or an internal mold release agent. Outdoor exposure can cause further crosslinking in a coating, leading to embrittlement and the build up of internal stress. Temperature and humidity cycling can cause spikes in internal stress to the point of cracking and lifting. If topcoats do not absorb enough UV light, then the primer under them will be at risk. A number of years ago, there was an epidemic of severe delamination of automotive topcoats because of UV degradation and chalking of the underlying electrodeposition primer. It only takes a small amount of degradation at an interface for adhesion to be compromised. Thin coatings, low pigment loadings, and loss of UV absorbers can allow UV transmission. Pigments provide UV protection by blocking out the light. They may also absorb UV. Additives are available that absorb UV light and change the energy to heat energy or act as free radical scavengers.

Many adhesion problems can be prevented by better surface preparation practices. Waiting to repaint the house until the existing surface has been cleaned, mildew and chalk has been removed, chipped or brittle areas have been sanded or wire brushed, etc. would seem to be self-evident, but often this does not occur. Metals must be cleaned of all oils and then pretreated to provide a wettable, uniform surface that promotes adhesion. Plastics must have mold release agents and other contaminants removed and may need power washing and solvent wiping to accomplish satisfactory preparation. They also may require special treatments or adhesion promoters to enable coatings to stick.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].