Technology Interchange—Evaluating the Mechanical Performance of Powder Coatings
Posted on Monday, March 26, 2018
Today’s topic relates to the evaluation of the performance of a powder coated surface. Specifically, we will explore test methods used to determine the mechanical performance of the coating. By mechanical performance we’re talking about what happens when an outside force is applied to the coating, including deformation and surface attack. The mechanical performance of a coating signifies a couple properties; one is the inherent performance of the coating per the claims of the powder manufacturer, and the second is whether a coating has been properly applied and cured.
Before evaluating mechanical performance, it is important to measure the thickness of the coating. Standard film thickness gauges utilize either a magnetic measurement process (for ferrous substrates) or Eddy Current technology (for non-ferrous metals). Suppliers of these tools can be found in the Auxiliary Equipment section of PCI’s membership directory at www.powdercoating.org/directory.
Before evaluating the mechanical performance of a powder coating, it is wise to first ascertain adhesion. If the coating doesn’t adhere to the substrate, all bets are off. Impact resistance will be inferior, hardness measurement could be inaccurate and even abrasion resistance may be negatively influenced. ASTM D3359 - Measuring Adhesion by Tape Test describes a couple techniques. ASTM D3359A calls for a simple “X” cut to be made in the coating with a sharp blade. It’s important to use a clean, fresh blade to ensure a precise incision. A specified tape is then adhered to the “X” cut and rapidly pulled from the surface.
A more sophisticated adhesion test is described in ASTM D3359B. This method directs you to scribe a crosshatch pattern of cuts (either 6 x 6 or 11 x 11 cuts). The incisions can be accomplished with a sharp blade or a purposefully designed device with multiple cutting edges. An equal number of cuts are made in parallel fashion and another set of cuts are made 90° from the first set of cuts to form a lattice pattern. The incised area is cleaned with a brush and tape is applied to the crosshatch pattern. The tape is quickly removed and the tested area examined for adhesion. The ASTM method provides a scoring system, complete with images, depicting ratings ranging from 5B (100% adhesion) to 0B (0 to 35% adhesion).
ISO 2409 covers a similar test methodology as ASTM D3359B. Interestingly the rating system employed in this international standard uses values opposite to the ASTM standard. A “0” rating indicates 100% adhesion and progressively worse adhesion is depicted by numeric ratings from “1” (5% loss) through “5” (100% loss). This underscores the need to be very careful in citing your test method when reporting results.
Wet adhesion is another, more aggressive technique to consider and test methods vary depending on the application and service expectations for the coating. Pipeline coatings use a test described in the Canadian specification CSA Z245.20. This test entails submersing a coated specimen in 73° C tap water for 24 hours, then retrieving the test panel and cutting a 30 mm x 15 mm rectangle through the coating. Adhesion is measured by trying to pry the coating from the substrate with a knife. Another method tests wet adhesion by first making an “X” incision into the coating then immersing the specimen in 90° C water for 2 hours. Poor adhesion is indicated if the coating peels back from this exposure. I like to use this on particularly difficult substrates such as stainless steel or polished aluminum.
Characterizing a coating’s resistance to deformation is an interesting proposition. Some people regard impact resistance (ASTM D2794) as the gold standard in measuring flexibility. In reality, one size doesn’t always fit all. First let’s discuss factors that influence flexibility. Substrate thickness and ductility have a major influence on a coating’s flexibility. For instance, heavier gauge steel deforms less than thinner steel, hence the coating experiences less stress under the same conditions. In addition, film thickness can influence flexibility. Thinner films are nearly always more flexible than thicker ones, so it’s important to keep the coating thickness within specification. Another factor is temperature; warm coatings typically have more flexibility, while cold coatings fracture more easily under stress. Some coating specifications require sub-zero temperatures to test deformation performance. It’s important to test flexibility at a specified temperature. One other issue – different metals (alloys, really) possess different ductility. Hence, 20 gauge 3003 grade aluminum deforms significantly more than 20 gauge 1008 grade low carbon steel. So 100 inch lbs. impact deforms 3003 aluminum significantly more than the same thickness of 1008 steel.
The most common coating flexibility test is the rapid deformation caused by an impact tester (ASTM D2794 Standard Test Method for Resistance of Organic Coatings to Rapid Deformation (Impact)). In North America, a 4.0 lb. weight having a 0.625 inch radius on the contact point is raised in a calibrated tube and dropped onto a coated surface.
The distance (height) times the weight determines the force in inch-lbs. The impact creates a dimple in the substrate/coating. Failure is determined by the highest force that does not cause any cracking in the coating. Impact resistance can be measured by dropping the weight both onto the coating surface (forward impact) or from behind the coating (reverse impact). The ISO standard for impact resistance (ISO 6272) uses a 20 mm diameter ball on the impact point and a 1.0 kg. weight. Impact is quoted very nicely in Nm (Newton meters).
The GE Impact Resistance Test (Method 6226 – Federal Test Standard 141B) is more common in the aerospace industry. This test also employs a falling weight like ASTM D2794; however, the impact head has two sides that each have four impactors of varying radii. Using both sides, one can determine the effect of 0.5 to 60% elongation using 2024 grade aluminum substrate.
Impact resistance testing is useful in determining inherent resistance to a sudden mechanical shock to the surface of a coating. It also is a decent, albeit inexact, determiner of cure. Most powder coatings do not achieve ultimate impact resistance until the coating is completely crosslinked.
Measuring the elongation performance of a coating requires a slow deformation test method. These tests are usually specified if a coating will undergo some type of postforming after cure. Coated sheet stock is sometimes destined for a fastening or clinching process to adjoin sheets in assembly. Performing a slow deformation test can assure that the coating can withstand this type of stress. In other cases, slow deformation is just another characteristic that exemplifies a coating’s toughness.
“T-bend” tests are commonly specified by the precoat-postform coatings community. ASTM D4145 (ISO 17132) describes how to conduct this test. Essentially the coated test panel is bent or folded 180°. The first bend is ‘”0-T” signifying no panel thickness between the two outside faces of the coated panel. The bent edge of the coating is inspected for cracks. If no cracks appear, the coating receives a “0-T” rating. If cracked, the test panel is bent another 180° which results in one thickness of panel between the outside faces of the test panel. This condition signifies “1-T”. The coated edge is inspected. The test panel is folded over 180° repeatedly until no cracks are evident in the coated edge. Pass is stated as the number of “thicknesses” that exist between the outside faces of the panel with no cracking in the coating.
Another slow deformation technique is described in ISO 1520 Paints and Varnishes – the Cupping Test. This test method is most prevalent in Europe and involves slowly indenting a fixed coated test panel with a 20 mm diameter hemisphere. The failure point is ascertained when the coating exhibits a crack in the finish. The aforementioned issues of metal type, gauge and temperature all apply here. The test apparatus for this method comes in as either a manual or automated instrument. A magnifier is recommended to determine the initial
point of cracking.
The pipeline coating industry uses a slow deformation test that is described in, CSA Z245.20. This test calls for a 25 mm x 200 mm x 6.4 mm steel specimen to be coated with no less than 300 microns (12 mils) of epoxy powder coating. The coated test bar is cooled to -30° C for a least hour and then deformed by a mandrel in a hydraulic press. This bending process must be performed within 10 seconds of removing the cooled sample.
The ability of a finish to resist scratching and abrasion is fundamentally characterized as hardness. Measuring hardness of a powder coating can be accomplished by a variety of methods ranging from a simple pencil scratch/gouge test to the dampening effect of a weighted pendulum. The most common and simplest test is the ASTM D3363 Film Hardness by Pencil Test. This method relies on the use of artists pencils varying in hardness from 6B (softest) to 6H (hardest). The pencil lead is exposed approximately 0.25 inch and sanded to a flat edge. The lead cylinder is pressed against the coating at a 45° angle and pushed with even pressure. Pencil hardness can be rated by “scratch” or “gouge” resistance. A pass is determined by the hardest pencil that does not “scratch” or “gouge” the substrate. This test can be accomplished using either wooden or mechanical pencils. I prefer the wooden ones because the mechanical ones tend to lose their pencil lead gripping capability and give false positive results. Of all the hardness tests, this one has the greatest variability. The pressure applied by the operator can vary widely and the manufacturer and age of the pencil (lead) affects performance. It is therefore recommended to use a sled-like pencil holder that is available at a number of coatings instrument supply houses and also to store the pencils in a dry cool environment and to replace the pencils annually.
Another form of hardness measurement that involves pressure on a coated surface is micro-indentation hardness. The most common technique for powder coatings is called the Knoop Hardness Test and is described in ASTM E384. Basically a weighted (usually 100 grams) pyramidal tip is placed against a coated surface for a specified time. The indentation is then examined microscopically, and the length of the indentation is correlated to the depth and converted to Knoops.
One of the best techniques to measure surface hardness uses a pendulum to assess the dampening effect inherent to a surface. ASTM D4366 and ISO 1522 describe a test method that measures the swings of a pendulum which has a pivot point contact to a coated surface. The mechanism relies on the phenomenon that a softer surface provides more dampening and therefore fewer swings of a pendulum. The pendulum swings are counted as it swings from 6° to 3° from perpendicular (Konig) or 12° to 4° (Persoz). The Konig protocol uses a 200 gram weight and 5 mm balls as the pivot point, whereas the Persoz method employs 500 grams and 8 mm balls. This test is completely automated using photo sensors to count swings; therefore it is not influenced by test operator save the preparation of the coating.
Another type of dampening test to measure hardness is ASTM D2134, involving a Sward-Type hardness rocker. The principle of using dampening to ascertain the hardness of an organic coating is the same as the pendulum test methods; however, the Sward rocker test utilizes a precisely calibrated wheel that has two bubbles to count the number of oscillations the wheel makes. Softer surfaces dampen more quickly and therefore increasing hardness is observed with more oscillations. It is critical to ensure that the test apparatus is level and free of any scratches or other film disturbances that could influence the test.
Mechanical testing of a powder coated surface is an integral way of characterizing the toughness and expected durability in service. Careful adherence to standard test methods ensures precise qualification of performance and the delivery of the best product to your customer.