- Page Subtitle: Supplementary Info on Industry Techniques
To assist in better understanding the quality control tests of cured coatings, the following summary describes industry tests that are commonly used on thermoplastic and thermoset coatings. Recognize that several of these tests can be done onsite at Houston Powder Coaters’ facility; others are conducted at the original powder manufacturer. This information is to be used only as a guide for informational purposes.
Testing Methods & Techniques
MEK – Methyl Ethyl Ketone
MEK is traditionally used as a liquid solvent for surface coatings, adhesives, printing inks, and chemical intermediates. In addition, MEK can also be used as a thinner for specified coatings where fast evaporation times are needed (but not as fast as acetone).
For powder coating purposes, MEK has two purposes. 1) It is used as a medium to extract fats, resins, and oils (inclusive of machine oils) from parts not going through the media blasting process. As mineral oils and other traditional cleaners are cheaper alternatives to remove dirt, dust, and oil, MEK is used as a cleaner in rare instances. 2) Since it may also be used as a thinner of polyester and epoxy resins, MEK has limited use in “rub tests” at Houston Powder Coaters to confirm that powder coated pieces are “fully cured”.
Rub Test – Cure Test
As mentioned in the paragraph above, Rub Tests are one of the techniques used to test the final cure of a given part. After lightly rubbing a diluted MEK-soaked cotton swab on a small inconspicuous area of a cured part, no color should rub off on the cotton swab. The number of “rubs” or “passes” with the swab are dependent on the level of the test, the thickness of the coating, and the chemical make-up of the powder (hybrids will breakdown and transfer color quicker). The number of “double rubs” will typically range from 5-20.
Film Thickness – Mil Thickness Test
Once a part has been cured, a film thickness test can by conducted with a “Dry Film Thickness Gauge”. This is as simple as holding the end-reader to the metal and reading the number of mils. Certain COCs (Certificates of Compliance) require a certain number of readings per square area. The exact number is determined beforehand by the company’s engineered specification. Please consult with Houston Powder Coaters about any compliance reports prior to the onset of any job.
Holiday Test
The presence of flaws in the finished coating is collectively referred as porosity. Holiday tests are conducted to detect holes, gaps, cracks, foreign inclusions, pinholes, cavities, and contaminants within the coating (or under the coating). These discontinuities are known as “holidays”. Holiday detectors use electric charges – continuous or pulsed DC (direct currents) – to identify any inclusion, gap, or foreign matter.
As premature corrosion of a substrate can be the result of coating failure, it is important to understand where holidays may occur and why. This porosity technique can be used to test coating flaws invisible to the naked eye.
Destructive Testing – Adhesion Testing
Adhesion testing measures how well the powder sticks to the part once it is fully cured. This testing method is only conducted if the part is failing in the field and needs to come back. As an adhesion test is a destructive test, the finish of the coating will be ruined; therefore, the part will ultimately need to be run again. Most of the time, this adhesion test is performed by cutting a “cross-hatch” or “grid pattern” into the coating (to base metal). If square or sizeable pieces can be pulled or pried off, the adhesion can be suspect. If the coating comes off in a sheet, then an adhesion failure has occurred.
Formal Testing Methods for Powder Coating
Testing methods are designed by powder manufacturers to test 1) Performance, 2) Reliability, and 3) Quality Control. If requested, Houston Powder Coaters can provide data from these powder manufacturers for most powder formulations.
