Surface preparation, application environment, application technique and improper formulation to a product’s manufacturing process and end use are among the most common causes for coatings failures and defects. Coating failure can also present itself when foreign materials are added to the coating that are not recommended by the experienced coating formulator.
Improper surface preparation is a common cause for coatings failures. A surface must be properly prepared to enable sufficient adhesion between the coating and substrate. Whether you are relying on a chemical or mechanical bond, the presence of even trace amounts of surface or chemical contaminants can severely hinder your coating’s longevity. It is essential that the surface be clear of contaminants so that they do not wind up under the polymer film, which can lead to a host of problems discussed below.
Some common defects and failures of improper surface preparation include cratering, detaching, improper wetting, blistering, uneven gloss, orange peel and crawling.
Cratering, commonly referred to as “fish eyes” in the coatings industry, can be caused by inadequate surface preparation or surface contamination, leaving small craters as the coating dries. You can prevent cratering by applying a mist coat to the coated surface or more effective, make sure that the substrate is free of surface contaminations and imperfections.
Blistering occurs when a coating goes over moisture, soluble salts or other contaminants, preventing proper adhesion between the coating and the surface. It is characterized by dome-shaped projections in dry paint film that allow a coating to dry without any bond to the initial substrate.
The temperature, humidity, air purity, and cleanliness of a coating’s application environment can either set it up for success or failure. For liquid coatings, it’s equally important to ensure your paint operation, especially air lines in spray guns, are impeccably clean.
When ambient temperature is too high or low, coatings can experience wetting failures. When this occurs, your coating will not be spread evenly over the substrate because it does not flow out in a continuous wet film.
Contamination in paint spray air lines can lead to a multitude of issues, including blistering, solvent popping, cratering and many other visible surface defects.
Another important factor in determining the success or failure of a coating is the application technique employed. Generally, coatings are designed around a specified application method, which must be followed to maximize results. The most common application techniques for liquid industrial coatings are spray, dip, brush, and roll. Spraying can be broken down into four subgroups: airless, conventional air, electrostatic and high-volume, low-pressure (HVLP). Just as selecting the proper application technique is critical to a coating line’s success, proper execution of that application technique is also crucial to avoid common coatings failures and costly rework.
Two of the more common defects associated with incorrect application technique are orange peel and sagging or running. Orange peel usually occurs due to insufficient coating atomization resulting from poor application technique. This defect is characterized by uniform pock-marking along the coated surface, causing the coating to resemble (you guessed it) the skin of an orange. Sags and runs occur where paint is applied at an excessive thickness, causing it to flow downward after application before the paint sets, resulting in an uneven area with a thick lower edge.
An industrial or OEM coating should be tailored to fit seamlessly into a manufacturing process while achieving the performance required in a product’s end use environment. If a coating is formulated inadequately, it is likely to fail regardless of any efforts to optimize any of the factors discussed above. Some common formulation-related failures include chalking, erosion, checking, alligatoring, cracking, and mud-cracking.
Your process should not need to be altered to overcome the obstacles that an improperly formulated coating. Allow your coating supplier to better understand your process and conditions before formulating a coating that accommodates your manufacturing needs.
These costly formulation-related failures can be avoided by working with an experienced coatings formulator that understands your product’s end-use environment. Ideally, your industrial coatings formulator will test the prescribed formulation with accelerated environmental exposure tests, such as ultraviolet light (QUV) exposure tests, salt spray (fog) tests for corrosion resistance and humidity tests for water resistance to ensure the coating will stand up to its end use environment.
By considering these factors when conducting a coatings failure analysis, or more importantly, during the coatings formulation stage, you will uncover, and hopefully avoid, most root failure causes. Working with an experienced industrial coatings formulator is a strong first step in addressing these issues.