Powder coating quality is tested at every step of the process
Through powder manufacture, application, and coating performance, testing is an important part of ensuring powder coating quality control. Testing by powder manufacturers is important because once the powder reaches the applicator it cannot be adjusted. Testing by applicators is important to guarantee a successful coating application, thereby providing a coating that will perform at its best. High quality protection and decoration mean high quality control.
In this article we look at the variety of powder coating quality control measures, from manufacture to coating, and the international standards that apply to them.
What makes powder coating quality control important
“It’s a poor craftsman who blames his tools” may be how the saying goes, but no one working with substandard tools can ever expect to reach the same performance levels as someone using only the highest quality tools and materials. In order to ensure consistency, manufacturers employ a series of quality control measures, checking the colour, gloss, and texture of the powder against a master batch standard. Manufacturers are responsible for the quality of the powder particles, the pigmentation, the resin, and the additives included for hardness, matting, and texturing. All of these elements affect the final powder and its suitability for certain applications.
Applicators also need to test powder coating to check their own processes for possible problems. Testing for powder coating functionality includes tests for film thickness, adhesion, uniformity, curing, impact resistance, as well as tests for appearance properties like haze and gloss. Deviation in these tests can indicate a problem with the substrate preparation, the equipment, application technique, or maybe indicate a larger issue with the intended coating.
Powder coating quality standards and measures
Both manufacturers and applicators perform certain tests to ensure that the powder they provide or the coating they apply is of the highest quality and conforms to industry standards. Below we look at the various tests carried out for both powder and cured finish.
Powder coating quality measures for the manufacturer
These tests examine various qualities of the dry powder, including particle size and colour. Each of these properties will affect the final finish, so it is vital for quality assurance that they comply to standards.
- Particle Size Distribution
Having the right powder particle size and distribution for an application is one of the main physical requirements that a powder needs to meet. The particle size affects the powder’s ability to take on the electrostatic charge, the electrostatic charge needs to be sufficient for the powder to attach to the surface, and then the powder needs to encourage film buildup without becoming insulating. The tests for particle size and distribution include sieve analysis (the simplest technique where powder is passed through a series of sieves), sedimentation techniques (such as the Andreasean pipette, Backman and Shimadzu sedimentation balance and photosedimentometer), the Coulter counter (a process where powder is dispersed in an electrolyte), and laser light diffraction.
- Powder Dry Flow
The dry flow of a powder affects how it handles and transports through feed lines during application and recovery. A powder needs to have reasonable flow characteristics, and the wider the particle size distribution the worse the flow of the powder. Tests for this include angle of repose (analysing the angle formed as the powder gathers in a piles on a collecting plate), and the SAMES flowmeter (measuring powder behaviour in a ‘fluidised’ clear plastic tube).
- Moisture Content
Moisture content can cause powder to clag, affecting distribution and flow. The most direct method of determining moisture content is by heating a quantity of powder in an oven at 150°C. A more accurate method is to heat a weighed amount for 15 minutes in a 200°C oven before allowing it to cool in a desiccator. Weight loss is then determined.
- Specific Gravity
The specific gravity of a powder coating is its density relative to water. Water has a specific density of 1. This is an important measure because it helps calculate the coverage of the powder and so the cost as a function of coverage. It is measured through either displacement or a gas pycnometer. In the displacement method, much like Archimedes in his bath, the amount of fluid displaced by a known weight of powder is used to determine its specific gravity. A gas pycnometer measures the volume of air displaced by a known weight of powder, and is the more accurate method.
- Gel Time
The gel time of a powder coating is the length of time it takes for a thermosetting powder to gel on a polished metal surface at a specified temperature. It is a useful indication of curing speed and the consistency of powder quality.
The powder coating inspection methods for cured powder
Manufacturers also perform a plethora of cured coating tests for quality assurance before sending products out to the applicator. Applicators perform these tests too in order to ensure the highest quality from the coatings they produce. To perform the tests, the powder is spray applied electrostatically and cured at the appropriate temperature for the specified time at a controlled film thickness. The substrate is usually untreated steel, lightweight iron phosphated steel, or lightweight zinc phosphated steel.
- Crosshatch Adhesion Test
An easy and popular test for adhesion, this test is performed by making a series of cuts through the powder coating in a crosshatch formation, five times up and five times down. Adhesive tape is then applied to the area and removed. If large amounts of paint come off with the tape, the coating fails the test.
- Pencil Hardness Test
This test, as the name suggests, takes a pencil and pits it against the coating. Pencils are measured by their hardness, so are a good measure of the hardness of a coating. The pencil is sharpened, and pressed into the coating at a 45-degree angle. It is then pushed, and if it pushes down into the metal, the coating failed. If there is no scratch, the coating passed.
- Cure Testing
The common cure test is the solvent rub test. Acetone and methyl ethyl ketone (MEK) are the two solvents most commonly used, with the MEK test being the ASTM standard. A q-tip or cotton swab is doused in solvent being being rubbed firmly on the coating surface. In general, 25 double rubs are performed. If the coating is coming off onto the cotton, that is a fail. There may be some de-glossing but that is often still a pass.
- Film Thickness
A Film Thickness Meter (or gauge) is used to determine the film thickness of a coating. With most, they are simply pressed to the coating and will give a readout of the thickness at that point. The test should be repeated in various point across the surface.
Powder coating quality standards in the UK
Of course, all of the tests conform with various international and national standards for powder coating. There are many standards relating to measuring a coating’s performance, safe application, end use application, and other aspects. The standards include:
- BS 3900 Methods of Test for Paints
- BS EN 12206-1:2004 Paints and varnishes – Coating of aluminium and aluminium alloys for architectural purposes. Coatings prepared from coating powder
- AAMA 2603, 2604 & 2605 – Voluntary Specification, performance requirements and test procedures for superior performing organic coatings on aluminium extrusions and panels (with coil coating appendix)