Testing is an important part of the operation of a paint system. Testing is done to monitor the system and to confirm that the finish meets established quality standards and the expectations of the customer. The end use of a painted product shoulddetermine what tests are important.
The incoming paint material is sometimes tested to confirm that the color and gloss are correct and the paint is free of defects. In some cases, the paint supplier will certify that the paint they are providing meets all required physical properties.
The paint should also be tested to confirm that it is ready to spray. Some sprayed sample panels can be used to confirm this before the paint is put into production.
Viscosity, a measurement of the flow characteristics of the paint, should always be tested before using a fresh batch of coating.
10.2 PAINT VISCOSITY
Viscosity is a very important paint property, usually determined by measuring the time required for a given quantity of paint to flow through a hole in the bottom of a metal cup. A number of cup sizes and drain hole diameters are available for use with different viscosity paints. Three widely used viscosity cups are the Fisher, Ford, and Zahn viscometer. Table 10.1 gives conversion values for use with these cups.
It is important to correct the viscosity cup flow-out times for the temperature of the paint. The recommended spray viscosity is typically based on a temperature of 70◦F. If the viscosity is measured at a colder or warmer temperature, the actual viscosity could be too low or too high.Apaint supplier can provide the appropriate corrections for their product. Be sure that the type of Zahn cup or other viscosity cup used is consistent. In addition, be sure that the method of measurement is consistent. Some operators consider the first break in the paint to be the stop point, while others may wait until the cup is virtually empty.
10.3 FINISHED FILM TESTING
Finished film testing is used to confirm physical properties of the coating after it is applied and cured. Daily tests might include such things as gloss, color, hardness, and adhesion. The particular tests used depend on the properties that the coating is supposed to provide and the established quality standards.
Finishing tests are an important task in confirming that the finish meets the standards and can provide the necessary performance characteristics.
10.3.1 PAINT TEST STANDARDS
Most of the finished film tests used by industrial painters are based on the American Society for Testing Materials (ASTM) international standards (originally known as the American Standards for Testing and Materials). ASTM has several publications that provide the test methods and how to interpret the results. Some performance tests are based on other sources such as the American Architectural Manufacturers Association (AAMA) or European standards.
It is important to have a precise test method and consistent standard to provide meaningful data and consistent results. A good test method describes how the test panel will be prepared, how the test will be conducted, and how the results will be reported. If the test frequency is too random or the test methods are inconsistent, the results will not be as useful.
10.3.2 TYPES OF TESTS
There are standard tests that cover most properties of the finished film that are relatively simple to do as part of a daily operation. Some of the more complicated tests, or tests that involve expensive, specialized equipment, can be conducted by testing labs.
Tests can also be created that will help to determine if the part will meet a particular real-world condition. For example, if the part will be subjected to brake fluid, then a chemical test that subjects it to brake fluid is useful.
10.3.2.1 Film Thickness
The thickness of a dry paint film is important to the performance of the coating. Thin films may appear to be the wrong color, or they may not provide the necessary level of performance. Water can more easily penetrate to the substrate if the film is too thin. Thick films can be a problem too. Thicker films are more likely to crack in use or they may create problems when two mating parts must be fitted together.
Wet-film thickness is measured for process control. It can be measured with a handheld immersion gauge that has a series of teeth along the edge of different lengths. The gauge is placed on the freshly painted surface, and the edge of the gauge is supported by two teeth of the same length for measurement control. All the other teeth are different in length and shorter than the control teeth. The wet thickness is measured as the last tooth that leaves a mark in the paint film. Therefore, if the
wet gauge reads 3 mil and the coating is 50% solids, then the dry-film will be about 1.5 mil. This is a rough method of predicting the final dry-film thickness, but it does give the painter an indication of whether or not he/she is applying the correct amount of paint without waiting until parts come out of the oven. This method of measuring leaves marks in the paint, requiring the part to be reworked.
The dry film thickness of paint on iron or steel surfaces is easily determined with a pull-off gauge. Pull-off gauges reflect the force required to pull a magnet suspended on a spring inside a metal case away from the painted surface. The force will decrease as the paint thickness increases, because the paint keeps the magnet further away from the metal. Pull-off gauges may be graduated in microns, mil (thousandths of an inch), or arbitrary units. They are direct measurement devices.
Dry-film thickness can be measured much more accurately with an electronic instrument. If the base material is metallic, a device is used that measures the strength of eddy currents induced in the metal by a probe containing a conducting coil.
The strength of the eddy currents decreases as the paint thickness increases. The read-out meter for eddy current strength can be graduated in any desired thickness units.
If the dry paint is on a nonmetallic surface, film thickness can be measured with a beta ray back scattering gauge. This instrument emits low-energy radiation in the form of beta rays (electrons) that pass through the paint and are reflected by the more dense material beneath it. The quantity of beta rays that are reflected back to the gauge decreases as the film thickness increases. These gauges also work when the painted surface is metallic.
Dry-film thickness on nonmetallic surfaces is sometimes determined by cutting a “V” shaped trough through the paint and measuring the width of the cut at the top of the trough. A thicker film will produce a wider “V” at the top. A low-power magnifying glass with internal calibrations is often used to make this measurement. It may be calibrated directly in thickness units.
10.3.2.2 Film Hardness
The hardness of the paint film is important, because it is related to brittleness and water permeability. Films that have been over-cured or under-cured can often be detected by hardness testing. Pencil hardness and indentation hardness are two widely used testing techniques.
Pencil hardness tests measure resistance to indentation by a series of increasingly hard pencils that have been sharpened to a chisel point. The higher the hardness of the pencil lead required to make a gouge into the paint, the harder the film.
Indentation hardness testers measure film penetration, in a specified time, by a metal tip under certain mechanical loading conditions. The time factor is important, since many paints deform gradually under load.
10.3.2.3 Impact Resistance
Resistance to impact damage (chipping and cracking) is an important property for nearly every paint film. It is difficult to have both high impact resistance and high hardness. This is because impact energy is best absorbed when the film is softer and able to deform upon impact. However, hardness is desirable, because it normally means better gloss retention and weatherability. Most paints are formulated with a compromise between hardness and impact resistance.
Impact resistance testing usually involves striking a painted panel with a hard object, such as a steel ball or hammer and measuring the indentation that results. Another test (Society of Automotive Engineers Test Procedure V400) is called a gravel damage test. The gravel damage test uses stones hurled against the panel to inflict damage.
The most common way to measure impact resistance is ASTM D 2794-93. A standard weight is dropped from a height onto a coated panel. The indentation is inspected to detect if the coating has cracked. The weight can be dropped from different heights, and the results are then measured in inch-pounds. The goal is to see how many inch-pounds the coating can take without cracking.
10.3.2.4 TapeAdhe sion
Adhesion of a paint film to its substrate is often measured by jerking the paint away from a scribed “X” or grid with a strip of tape. Three M Company’s #600 Crystal brand transparent tape or another tape with strong adhesive properties will work for this test. The tape is jerked back upon itself as nearly in the plane of the painted surface as possible. A numerical rating system from a 0 for total failure to a 5 for 100% adhesion with no loss may be used to evaluate tape adhesion test results. If a
scribed grid has been used, the failure of adhesion may be expressed as the percentage of squares that have some loss of paint (D 3359-97).
A more severe test of adhesion is sometimes used on aluminum substrates. The standardASTM D 3359 test is used, but the crosshatch pattern is subjected to boiling demineralized water for 20 min before being tested with the tape pull. This test, AAMA2604, 184.108.40.206, is useful on aluminum substrates that will be used in an outdoor environment where the paint film will be subjected to water permeability and possible oxidation under the film.
10.3.2.5 Humidity Testing
Water vapor is one of the most severe agents to which a paint film is exposed. Because water molecules are small and binder molecules are constantly vibrating, moisture can easily penetrate lightly pigmented paints. Water also penetrates heavily pigmented primer films, although at a much slower rate.
Moisture in a paint film can be warmed by sunlight or other sources of energy. When warmed, it will tend to vaporize and exert a pressure that causes the film to swell. If the film is flexible or if the water occurs near a spot of grease or salt crystal under the paint, the swelling will be more severe, possibly causing adhesion failure or discoloration of the film.
The condensing humidity test is widely used to measure humidity resistance. In this test, water vapor is allowed to condense on sample panels. The condensed water drips off the panel and is revaporized by means of an evaporative heater in the bottom of the test chamber. A typical test involves water at 60◦C (140◦F) for 24 h. After testing, panels are checked for blistering, color changes, and loss of gloss.
Another humidity test involves exposing the panels to 100% relative humidity at 38◦C (100◦F) for 24 h.
Neither of these tests can provide service life predictions. However, they are useful for determining the best paint from a series of formulations.
10.3.2.6 Salt Spray Testing
The use of salt solution spray testing is an attempt to accelerate the corrosion process and cause early paint failure. Panels are usually exposed for up to 14 days to a mist of 5% (w/v) sodium chloride solution at 33–36◦C (92–97◦F). The mist is produced by blowing hot saturated air through a 5% salt solution.
The panels are evaluated for two types of corrosion:
1. Rust-through—the percentage of the surface, which has rust visible through the paint
2. “Creep”—the distance in 1/32 of an inch (0.8 mm) from the center of the scribe line that the paint film breaks down and separates from the substrate The results are measured on scale with a predetermined point described as failure in number of hours of exposure. Sometimes, acetic acid is added to the salt spray solution to accelerate the corrosion.
Salt spray testing has been used as a standard for performance by many coaters from all different types of industry. It is poorly understood and not necessarily reliable
as a predictor of field service. The most important value of salt spray testing is comparison of different pretreatment methods and coatings to see what appears to be the best combination for corrosion resistance. In all cases, the salt spray comparison should be done with the same steel panels and the same test cabinet.
One weakness with the results of a salt spray test is the substrate itself. Steel quality varies substantially and the failure in the salt spray test may be due to poor steel, not pretreatment or the paint film. For this reason, it is very important to always run standard q-panels as a control.
Another factor to consider when evaluating the pretreatment or paint film is the cure cycle. If the paint film is not fully cured, it can cause failure in the salt spray test that may be blamed on the pretreatment process or the paint. The cure of the film should be confirmed before the panels are tested in salt spray.
10.3.2.7 QUV Testing
AQ-LAB Ultraviolet (QUV) test chamber reproduces the damage caused by sunlight, rain, and dew. Coated parts or test panels are placed inside the chamber and subjected to alternating cycles of light and moisture at controlled, elevated temperatures.
Parts can be measured for resistance to chalking, fading, color fastness, cracking, blistering, embrittlement, strength loss, and oxidation.
TheQUVtest chamber provides a more accurate comparison of different processes and materials than salt spray testing alone. It is more helpful in evaluating the affect of outdoor exposure on different coating materials and a somewhat better predictor of actual field life.
10.3.2.8 Outdoor Exposure
Outdoor exposure tests are slow, but they are the best way to predict weatherability. Three months of exposure takes a while, but it does provide good information about how well the paint film will last in an outdoor environment. Long-term exposure is not very useful for solving transient production problems, but it is a good way to qualify a paint material for use in the sun.
The most common defects turned up by exposure testing are fading, cracking, checking, chalking, blistering, and peeling. Often a portion of the panel is buffed after testing to see to what extent the original appearance can be restored.
Exposure tests are usually conducted in a sunny climate such as Florida orArizona to get the maximum ultraviolet radiation effect. Panels are usually exposed 5◦ from the horizontal, facing south. Florida is a preferred location in this country because of the sudden and frequent humidity changes that occur there. The test panel is held in a clamp that shields the top portion of the panel from exposure. Mirrors and mechanical devices are sometimes used to accelerate the exposure and produce earlier results.
10.3.2.9 Temperature–Humidity Testing
Temperature changes, especially if they are rapid, present a severe challenge to paint films. Cracking is the most common failure related to temperature change.
The film fails because of stress that develops when pigments, binders, additives, water vapor, and micro-air bubbles in the film expand and contract at different rates as the temperature changes. Since expansion coefficients for trapped air, water vapor, pigments, and other paint components may be quite different, large internal stresses
Testing Paint Materials 175
can develop at various locations within the film. The stress created in the film can cause it to crack.
A typical temperature–humidity test cycle is
• 24 h at 38◦C (100◦F) and 100% relative humidity
• 20 h at –23◦C (–10◦F)
• 4 h at ambient temperature
This cycle is often repeated 12–15 times. The panel is rated by evaluating the percent of the surface area that has cracked.
10.3.2.10 RinseBliste ring
A rinse blister test helps to expose any unreacted salts left on the surface after evaporation of a water rinse. The process water is applied as droplets to a panel and allowed to dry. After the droplets have dried, the panel is top-coated and subjected to humidity testing. Blisters often occur where water-soluble salts have been left from the droplets. This is because moisture from the humidity chamber moves through the topcoat, collects around the salt crystals, dissolves them, and exerts a swelling pressure against the overlying film. Panels are usually evaluated against a control after 24, 48, and 96 h. Sometimes a tape adhesion test is also run without a scribe pattern being cut in the film.
10.3.2.11 Corrosion Cycling
Paint films in use are always exposed to a number of cyclic environmental factors. A corrosion cycle test measures the behavior of a film under a combination of test conditions.
Usually, a combination of salt, humidity, and changing temperature is used for corrosion cycle testing. A typical sequence is listed below:
• 4 h at 5% neutral salt spray
• 18 h at 38◦C (100◦F) 100% relative humidity
• 2 h at –23◦C (–10◦F)
The resulting panels are rated against a control on the basis of the percent of rusted area. Cycle testing can be a more accurate way of predicting comparative qualities of coatings than salt spray alone.
10.3.2.12 Color Matching
Color matching is a very important step in the process of manufacturing. Colors must be consistent so that the product is identical and recognizable from part to part.
In addition, many products have multiple components, sometimes with different substrates, and each component must be the same color.
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