Quality Control

Once a coating has been chosen for production, it is necessary to ensure that future supplies of the coating are consistent from batch to batch, maintaining satisfactory application properties, appearance, stability, and performance characteristics. Only by so doing can production quality be maintained. Testing can be costly; consequently, quality control programs should be designed using the simplest test methods and the least number of tests necessary to ensure essential quality levels. Numerous tests and equipment for evaluating and monitoring of coatings are available, most of which are described in the following publications:

• Annual Book of ASTM Standards, Vol 06.01, Paint--Tests for Formulated Products and Applied Coatings; Vol 06.02, Paint--Pigments, Resins, and Polymers; Cellulose; Vol 06.03, Paint--Fatty Oils and Acids, Solvents, Miscellaneous; Aromatic Hydrocarbons

• Federal Test Method Standard No. 141a, "Paint, Varnish, Lacquer and Related Materials; Methods of Inspection, Sampling and Testing"

• Paint Testing Manual, Physical and Chemical Examination of Paint, Varnishes, Lacquers, and Color, STP 500, American Society for Testing and Materials

• Physical Testing for Paint Finishes, NCCA Technical Manual, National Coil Coaters Association

Table 8 lists a number of the most common tests for paint.

Table 8 Selected test methods for paint and painted surfaces

Test

ASTM

Federal method 141

Wet or liquid tests

Nonvolatile (solids content)

D 2369

4041

Vehicle solids

D 2698

4052

Pigment content

D 2698

4022

Weight per gallon

D 1475

4184

Flash point

D 56, D 93, D 92

4291, 4293, 4294

Viscosity

Ford cup

D 1200

4282

Brookfield

D 2196

4287

Stormer

D 562

4281

Zahn

D 3794

Gardner Holt tubes

D 1545

4271

Fineness of grind

D 1210

4411

Reducibility and dilution stability

4203

Drying time

4061

Dry film performance tests

Hiding power

D 344, D 2805

Color (pigmented coatings)

D 1729

4250

Specular gloss

D 523

6101

Abrasion resistance

Falling sand

D 968

6191

Taber abraser

D 1044

6192

Adhesion

D 2197

6303

Dry film thickness

Magnetic gage

D 1186

6181

Mechanical gage

D 1005

6183

Hardness

Pencil

D 3363

Sward Roeker

D 2143

Indentation

D 1474

6212

Humidity resistance

D 2247

6201

Salt spray resistance

B 117

6061

Viscosity. Controlling paint viscosity is necessary to maintain the desired properties of the coating and to ensure that the process operates at the maximum possible efficiency. Pigmented materials require close control of solids content to eliminate that influence on viscosity readings. Close control of the temperature is also necessary, because viscosity varies inversely with temperature.

Viscosity may be checked by:

• The efflux cup method

• The torsional method

• The bubble viscometer

Commercial equipment is available to assist in performing any of these tests for viscosity.

The efflux cup method uses containers closely controlled in size having a precise orifice in the bottom. After the cup is filled with paint to be checked, paint is permitted to drain through the orifice. The length of time, in seconds, to the first break in the flow stream of the paint is the viscosity. Several commercial cups are available, including the Zahn and Ford cups, designed to permit rapid testing of viscosity on the production line.

The torsional method measures the resistance of paint to rotation of a disk immersed in the paint. Several instruments for measuring the rotational resistance require that the paint be placed in a special container, and other instruments permit the viscosity to be measured in the shipping container.

With the bubble viscometer, the viscosity of the liquid is measured by the speed with which a bubble of air rises in the liquid. The material is confined in a glass tube, which is completely filled, except for a small bubble, and stoppered. The viscosity determination may be made in two ways:

• Comparing the rate of rise with that of a material of known viscosity contained in a tube of the same size

• Measuring the time required for the bubble to travel between two marks on the tube, which must have been calibrated with one or more liquids of known viscosity

Color of paint is most often controlled by visual comparison against a reference standard. One deficiency of this type of control, however, is the difficulty of retaining permanent color standards. Because of this difficulty, stabilized dry drift control panels have come into use. These panels can be either paper chips coated with paints designed to have a minimum of color change on aging, or porcelain or ceramic panels properly coated with the appropriate paint. Each control has its advantages; however, both are subject to soiling. All comparisons should be made under a standardized source of light to eliminate extraneous influences of various sources of light.

Visual comparisons do not permit the assignment of numerical values to differences and are subject to wide variations of opinion. It is possible, however, to analyze color on various instruments, assign numerical tolerances, and eliminate subjective judgment to some degree.

Colorimeters measure the three attributes of color: hue, saturation, and brightness. Colorimeters require the use of some reference standard, although not necessarily of exactly the same color as the paint being tested. Because the numerical values established with the colorimeter must still be related to a reference standard, they cannot be considered as absolute.

Spectrophotometers, which measure and analyze color throughout the entire visual spectrum of 400 to 700 nm, come nearest to being absolute measuring devices. Curves obtained on the spectrophotometer are used as a permanent, reproducible reference. Figure 13 shows spectrophotometry recordings of red, gray, and blue flat paints prepared by extending cadmium red, ivory black, and Prussian blue, respectively, with zinc white. These curves indicate the variation in the light reflectance of the three colors across the entire visible spectrum.

400 450 5GD 550 fiDG 650 700

Wave length, nm

400 450 5GD 550 fiDG 650 700

Wave length, nm

Fig. 13 Spectrophotometry curves of gray, red, and blue flat paints

Gloss. The procedure used for measuring gloss of a painted surface is by visual comparison with a known standard. By comparing the sharpness of an image reflected by a sample surface with the image reflected by a standard surface, even relatively small differences can be detected. However, because this procedure is based on human judgment and does not lend itself to the assignment of numerical values, instruments are often used for the measurement of gloss.

Photoelectric glossmeters measure gloss from various fixed angles. The viewing angle used most often is 60° from the vertical, but a viewing angle of 85° is more sensitive for low-gloss paints. The scale of glossmeters is calibrated from 0 to 100 with the higher numbers indicating higher gloss. Readings of 0 to 15 are generally considered flat, 15 to 80 are semigloss, and 80 to 100, high gloss. Instruments accurate to less than one unit are available. However, it is difficult to apply paint films to this degree of accuracy with any consistency, and a five-unit variation in gloss is acceptable.

Abrasion resistance of organic films may be determined by test methods using either falling sand or an abraser.

The falling-sand method uses a funnel-shaped hopper, which feeds sand to a vertical tube of 19 mm (4 in.) ID and 915

mm (36 in.) long. Sand is permitted to flow down the tube and impinge on the test panel, which is placed at a 45° angle beneath the tube. The test is complete when the sand abrades through the paint film, exposing a spot of bare metal 4 mm

(32 in) in diameter. The abrasion coefficient in liters per mil is found by dividing the volume (in liters) of sand used by thickness of the paint film (in mils).

The Taber Abraser method uses abrasive wheels of various grits, a method of applying loads of 250, 500, or 1000 g on the wheels, and a turntable to which the test panel can be clamped. This test can be used to obtain either the wear index (rate of wear) or the wear cycles (amount of wear) of the paint film.

Elongation properties of an applied organic film may be measured by bending a test panel over a tapered cone and measuring the length of the first continuous crack. The apparatus and test methods used are described in detail in ASTM D 522 ("Test Method for Elongation of Attached Organic Coatings with Conical Mandrel Apparatus").

Blistering. A water immersion test may be conducted to determine the resistance of organic films to failure when immersed in water in an accelerated manner. Distilled water is used to eliminate the influence of any chemicals contained in tap water. The test procedure is set forth in ASTM D 870 ("Practice for Testing Water Resistance of Coatings Using Water Immersion"). The method for evaluating degree of blistering is given in ASTM D 714 ("Method for Evaluating Degree of Blistering of Paints").

Environmental tests may be required to evaluate paint films in a particular service environment. For example, a detergent immersion test is used to determine the suitability of a particular paint for use on a home laundry machine. Tests for resistance to acids, alkalis, industrial fumes, and other corrosive media may be established, with the criterion for failure being predetermined by agreement.

Salt spray (fog) tests (ASTM B 117) are arbitrary performance tests useful in establishing and maintaining certain standards of quality for the organic finish, particularly when correlated with field tests. For example, if spring clips, phosphate coated and painted with two coats of phenolic-based zinc chromate primer, can withstand 100 h in salt spray before failure, they may be expected to last 5 years or more in applications such as license plate brackets, molding retainers, and wire retainers on automobiles.

The test consists of placing parts or panels to be tested in a chamber in which a 5 wt% solution of sodium chloride is atomized. The exposure zone of the salt spray chamber must be maintained at 33 to 36 °C (92 to 97 °F).

Exterior-exposure tests may be conducted in accordance with ASTM D 1014 ("Method for Conducting Exterior Exposure Tests of Paints on Steel") to determine the resistance of a paint film to exposure. These tests are usually conducted in specified areas to obtain information on the influence of various atmospheres, such as industrial fumes, arid but intensely sunny climates, or salt air.

Artificial weathering tests using apparatus for exposing specimens to water and carbon-arc light are detailed in ASTM G 23, "Practice for Operating Light- and Water-Exposure Apparatus (Carbon-Arc Type) for Exposure of Nonmetallic Materials." Fluorescent UV-condensation type tests are described in G 53, "Recommended Practice for Operating Light- and Water-Exposure Apparatus (UV-Condensation Type) for Exposure of Nonmetallic Materials." These test methods simulate conditions of atmospheric exposure that act in a highly accelerated manner on the test panels. ASTM D 822, "Recommended Practice for Operating Light- and Water-Exposure Apparatus (Carbon-Arc Type) for Testing Paint, Varnish, Lacquer, and Related Products," is concerned with the variations in test conditions and the evaluation of test results. This test predicts the results of more time-consuming exterior-exposure tests with some degree of reliability.

Dry film thickness bears a direct relation to product cost and product performance. Film must be measured accurately for control of cost and performance. Several types of equipment and methods may be used for measuring the thickness of dry paint films on ferrous and nonferrous metals. Among these are dial micrometers, eddy current and magnetic thickness gages, and penetration and microscopic methods.

Dial micrometers are accurate depth-measuring devices with a calibrated dial and a pressure foot with a maximum diameter of 3.2 mm (- in). The test panel is clamped firmly to a base. The pressure foot of the dial comparator is brought 8

into contact with the paint film, and a reading is taken. Without disturbing the panel, the paint film is carefully removed from the panel where the reading was taken. The pressure foot is then brought into contact with the panel at the point of the previous reading. The difference in gage readings is the thickness of the paint film. This procedure, set forth in ASTM D 1005 ("Test Methods for Measurement of Dry-Film Thickness of Organic Coatings Using Micrometers") is not adaptable to paint films thinner than 13 pm (0.5 mil) or unusually soft films.

Eddy current thickness gages work on the principle of induced current changes in a high-frequency alternating current coil in a probe held in close proximity to a metal surface. Thickness of nonconductive films may be measured on any metal substrate.

Magnetic thickness gages are available in several types, the most useful being the portable gage using permanent magnets. These instruments measure the reduction of magnetic forces by a nonmagnetic coat of paint between a permanent magnet and the magnetic base to which the paint is applied. This reduction in magnetic force is calibrated in terms of paint-film thickness. This method is suitable for films from 13 pm to 6.3 mm (0.5 to 250 mils) thick. The instrument accuracy is generally 5% of the thickness measured.

The penetration method is practical where the paint film is applied to a surface that conducts electricity. This method measures the depth of travel of a small drill. The drill and the painted metal are connected electrically to a signal light that lights when the drill tip touches the metal panel. The drill is set with the tip just touching the surface of the paint film. The drill is then rotated and advanced slowly into the paint film until the signal light indicates contact. The measured distance of travel of the drill is a direct measurement of the paint film thickness. The accuracy of this method is approximately 10% of the film thickness. A portable scratch thickness gage also is available for determining the thickness of a dry paint film by the penetration method.

The microscopic method of measuring dry film thickness is the most accurate of the methods described. A cross section of the painted panel or part is mounted and polished. Using a calibrated eyepiece or screen, the magnified image of the paint film is measured. This method has the disadvantages of being a destructive test as well as requiring more specimen preparation than other methods. The accuracy of the microscopic method is limited only by the optical equipment used and the care exercised in preparing the specimen.

A portable microscopic method uses a cutting tip of a precise angle to slice through the coating. The exposed cross section is then measured microscopically. This instrument is accurate for thicknesses ranging from 2.5 to 1250 pm (0.1 to 50 mils). However, it is not suitable for brittle or rubbery coatings unless a power-driven cutting tip is used.

Wet Film Thickness. Inspection gages may be used to measure the thickness of wet paint films. With these gages, it is possible to determine whether the wet film is of adequate thickness to develop the desired dry film thickness.

Hardness of a paint film may be approximately determined by scratching it with pencils of different hardness sharpened in a mechanical sharpener. This test does not reveal a specific hardness, but enables one paint film to be compared either to another paint film or to an acceptable standard film. The test is run to determine the softest lead that can penetrate the surface of the paint film. If only one manufacturer's pencils are used, more consistent comparisons are obtained. The disadvantage of this test is the possible variability of the force exerted by the operator. It is a useful test, however, when one skilled operator is making empirical comparisons of two panels side by side.

Coating hardness can also be measured by using a spring-loaded penetrating needle instrument. The depth of penetration, or pressure required for penetration, provides an indication of coating hardness. Instruments of this type are available for a range of coating types, from soft elastomeric coatings to hard metallic ones.

Adhesion of the paint film can be measured by a test described in ASTM D 3359, Method A or B. Method A involves cutting an X scribe through the coating to the substrate; Method B requires a cross-hatch cutting pattern. After the cut is made, a special tape is pressed over the cut area and then briskly removed. Adhesion is determined by the amount of delamination that occurs.

Adhesion testing can be simplified and standardized by using commercially available instruments, which have a solid cutting head that makes several parallel lines with one pass. Cutting guides are also available with various spacings to enable the user to cut a precise cross-hatch pattern with a sharp utility knife. The spacing of scribed lines is usually correlated to total film thickness, with thicker films requiring spacing of cuts to be wider.

Impact resistance may be determined by the use of an instrument that consists of a 25 mm (1 in.) diam impact rod that tapers to an impact nose with a 6.4 mm (-4 in.) spherical radius, a tube 26 mm in.) inside diameter (ID) that serves to guide the impact rod in its downward fall, a base plate with a 13 mm (2 in.) diam hole through it, and a bracket to support the tube and position the base plate. The tube is graduated in inch-pounds and is slotted, so that a pin, protruding from the impact rod, can be used to raise the rod to a specific inch-pound location. The base is positioned so that its 13 mm (1 in.) diam hole can engage the nose of the rod at the bottom of its fall.

To test the impact characteristics of a paint film, a test panel, or an actual part, if made of sheet metal, is placed over the base, and the impact rod is permitted to fall from a height that generates the desired force. Direct impact is obtained by facing the paint film toward the falling rod; reverse impact is obtained by facing the paint film away from the rod. The impact makes a 6.4 mm (4 in.) spherically radiused impression in the test panel. Results are measured by the force the paint film can withstand without cracking, chipping, or flaking.

The Gravelometer (Fig. 14) is another device that is used to compare impact resistance of organic films against an accepted standard. Using air at 70 kPa (10 psi), this device propels 14 kg (30 lb) of steel shot a distance of 685 mm (27 in.) against a painted area 125 by 150 mm (5 by 6 in.). The amount of paint that is retained on the panel is indicative of the impact resistance and the adhesion of the paint film. This is a more severe test than the rod impact tester, but the Gravelometer is more revealing, because of the greater area involved. The SAE J400 test uses graded gravel instead of steel shot and is gaining wide acceptance as a standard test.

Fig. 14 Gravelometer. Used to measure impact resistance of paint films

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Fig. 14 Gravelometer. Used to measure impact resistance of paint films

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