Q: We have been developing new polymers for powder coatings, and one of the most critical properties is impact resistance. Our test results have been inconsistent and vary significantly depending on substrate used.
In some cases, the coating passes 160 inch-lbs, and with other conditions it only gets 80 inch-lbs. What could be the cause of this variation, and how can we eliminate these variations?
A: First of all, kudos to you and your colleagues for developing new polymers. Most advancements in powder coating technology are based on new and improved polymer chemistry. It’s tough for formulators to create new coatings without innovative polymers.
The most common impact resistance test method is described in ASTM D-2794 Standard Test Method for the Rapid Deformation of Organic Coatings (Impact) and ISO 6272 (1 and 2) Rapid-deformation (impact resistance) tests. The values typically quoted in impact resistance are inch-pounds (the metric centric folks use Newton-meters) and refer to the weight times the distance (height) of the impact.
Another parameter is the radius of the “ball” or hemisphere that impacts the surface of the coating. North America typically uses a 5/8-in. diameter ball, although some specifications call for a 1/2-in. one. The ISO standard uses a 20-mm ball. The smaller the diameter, the higher the deformation and hence lower impact resistance.
In addition to the coating’s inherent flexibility, substrate has the most significant influence on film performance. Adhesion is affected by the type of metal, the surface profile, cleanliness, and pretreatment. Impact resistance is affected by adhesion, so poorly pretreated substrate can cause unexpected impact failure. On the other hand, well-pretreated metal will enhance impact resistance by providing a strong bond of the powder coating to the substrate.
In addition to metal preparation, the gauge and type of substrate significantly influences impact resistance due to the degree of deformation of the substrate. Hence, 80 inch-lbs on a ductile grade of aluminum will have more deformation than carbon steel. And certainly, the gauge or thickness of the metal will also influence deformation and therefore impact resistance. Thicker gauge metal will deform less, and the coating therefore experiences less stress.
Coating parameters to control are mainly film thickness and degree of cure. Thinner films tend to exhibit higher impact resistance, whereas thicker ones are more brittle. Complete cure is highly recommended, as under-cured powders will usually have lower impact resistance. Make sure your coating receives the time at temperature specified by the coating supplier.
When characterizing a coating for impact resistance, the challenge is to use substrate(s) that are relevant to the target market(s). For general industrial end uses, cold-rolled steel is most common; the architectural market typically uses various grades of aluminum, depending on the forming and parts requirements.
In addition, pretreatment type is important. Cold-rolled steel is usually pretreated with a phosphate (iron or zinc) or in newer installations a zirconium-based product. Aluminum is typically pretreated with chromate, zirconium or sometimes zinc phosphate. For polymer development, I suggest that you obtain your substrate from a reputable test panel supplier.