Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
Appendix E Testing
A variety of quality assurance tests are used on cured paints for characteristics including thickness, adhesion, chemical resistance, color match and cure. Companies generally select tests based on customer requirements. For example, applications that involve high exposure to water and/or weather require certain performance standards from the coating. Whether the coating supplier provides this information or the manufacturer does the tests at their facility, the customer must be assured that the coating can perform according to specifications. This appendix provides brief descriptions of tests that are commonly performed by coaters.
The thickness of both wet and dry films are often measured.
The purpose of measuring wet films is to determine if they are sufficiently thick to develop the required thickness when dry. Gauges used to measure the thickness of wet coatings cut through the film. The two most extensively used gauges include a wheel gauge and a tooth gauge. Wheel gauges are rolled through the wet film to contact the base material. A tooth gauge is simply pressed into the wet coating to measure the thickness
A wide variety of gauges are used to determine the thickness of dry films. Thickness measurements can be performed on substrates containing iron by using a magnetic "pull-off" type gauge. Magnetic attraction decreases in proportion to the coating thickness. Pencil and banana gauges are two types of pull-off gauges. For other substrates, micrometers can be used to measure coating thickness. Destructive thickness methods include placing a piece of tape on the substrate prior to painting, removing it, and measuring the difference between the tape thickness before and after painting.
Adhesion is defined in ASTM Designation D907 as the state in which two surfaces are held together by interfacial forces which consist of either valence forces or interlocking action, or both. It would be difficult or even impossible to measure these forces. Often it is difficult to determine the true adhesion of a coating due to issues such as voids in the surface profile, improper surface preparation and surface contamination. Many factors other than substrate and paint properties may influence adhesion. As a result, the type of test used should be selected according to the modes of failure observed in service. The most common adhesion tests include film removal and cross-hatch. Inertia tests that use vibration to lift the coating are rarely used.
Tools used to test film removal vary from pocket knives to mechanically operated cutting edges, blades, or points. Gauges are used on some devices to measure the force needed to remove the coating.
This test requires that two sets of parallel cuts are made at 90 degree angles to each other, forming a checker-board grid. The percent of paint remaining on the substrate is estimated. In some cases, additional 45 degree cuts are made.
Several properties are involved in the measurement of abrasion resistance. These include mar resistance, hardness, elasticity and tensile strength.
Flexibility (bend or impact) is usually measured by removing a piece of tape applied prior to painting. ASTM D-3359 provides details about this simple test, including a rating scale for evaluating results.
It is critical to perform this test consistently. The actual method may vary, but the procedure must be performed identically every time.
A wide variety of devices are used to measure the hardness of paint films, including scratch or pendulum mechanisms.
Scratch tests on paint films maybe performed using mechanically operated styli or knives. The pencil hardness test is also widely used. In this test, pencil lead with specified hardness is pushed against the paint. The hardest lead that does not mar the paint is considered the paint hardness. Pencils are available in 17 different grades of hardness ranging from 9H, the hardest, to 6B, the softest (SME, p. 30-3). As with other subjective tests, procedures must be followed consistently so test results are meaningful.
Extent of Cure (Solvent Resistance)
Extent of cure can be determined via hardness testing or a "solvent rub" test. The solvent rub test involves rubbing the cured coating a prescribed number of times with a cloth saturated with a specific solvent. If no color appears on the cloth, the paint is considered cured.
Water and weather resistance can be measured in a variety of ways. Immersion, humidity resistance, and accelerated weathering tests are typical methods. Accelerated weathering tests combine UV light exposure with elevated temperatures and humidity or salt sprays. In addition to predicting field performance, the accelerated weathering test used evaluates different coatings' performance. It is an effective screening tool for choosing alternate formulations.
Color matching is challenging because it requires the manipulation of many variables which contribute to the test's outcome. Reflected light is the basis for interpreting color. Light sources (sunlight or specific artificial sources) vary in intensity, thus the amount of reflected light may vary. Conduct color matching, whether visual or instrumental, under several light sources. The Munsell system and the CIE systems are commonly used and employ three different light sources to determine color ( KSBEAP, p. 26-27). For a summary of various tests, see the table below.
Summary of Paint Tests (KSBEAP, p. 27)