Coating Testing

Color and Gloss

The assessment of color and gloss is often carried out purely visually by comparison with a master sample or a gray scale. Correct lighting is very important here. Additional irregularities (e.g., blistering, coating delamination) are included in the test. Alternatively, color changes due to batch changes or environmental influences can be quantified using color and gloss measuring devices. The measurement geometries are standardized to obtain comparable values.

Coating Adhesion (Cross-Cut and Cross-Hatch Test)

The cross-hatch test is used to determine the adhesion of a single-layer or multi-layer coating to the substrate. This reveals how a coated surface behaves against damage. In the cross-cut or cross-hatch test, the surface is scored in a cross-hatch pattern with a sharp blade. Afterwards, the adhesion of the coating is checked using a pull-off test, and the quality of adhesion is evaluated according to DIN EN ISO 2409 with cross-hatch classifications GT0 to GT5 or according to ASTM D 3359-02 with classifications 5B to 0B. Coating adhesion tests are routinely performed before and after climatic aging.

Scratch Test

Scratch resistance tests, or simply scratch tests, are performed on plastics to test the adhesion quality of paints and coatings on plastics. For this purpose, a needle is guided over the surface with increasing pressure, and it is checked how long the surface can withstand this load.

Environmental Simulation and Weathering

Most coatings are directly exposed to the environment. The most important aging parameters are temperature, light, and humidity, which can be simulated individually or in combination in an accelerated manner. Changes in conditions or spraying within predefined cycles are also possible. Depending on the type of coating, different influences can particularly affect coating changes. In the automotive sector, test conditions such as the Florida test or Kalahari test are frequently tested. After aging, changes in coating adhesion or color are investigated. A special test primarily used for coated metal parts is the salt spray test.

Chemical Resistance

In everyday use, coatings come into contact with various chemicals. These typically include sweat, hand cream, sunscreen, various solvents, cleaning agents, and disinfectants, but also cola or fuel depending on the application area. Chemical resistance is usually tested by direct contact with the test medium. The test may be intensified by simultaneous friction or temperature stress. Changes in the coating are evaluated according to specific criteria.

Abrasion Resistance, Rub Fastness, or Color Fastness

Mechanical contact with the coated surface can lead to a change in the surface (abrasion). The Crockmeter test is a common test method for coated surfaces in the automotive industry. This rub test is based on DIN EN ISO 105-X12, which is actually used to test the color fastness to rubbing of textiles. A standardized standard fabric or a special felt is rubbed over the surface for a defined number of strokes. At the end of the test, the fabric is visually examined for possible staining and the coated surface for premature wear. A Crockmeter test is also suitable for simulating a combined effect of water or other media during mechanical resistance testing, thereby significantly accelerating the aging of a surface.

Coated buttons or printed control elements, however, are preferably tested with an Abrex type device. The actuation is simulated by a silicone finger with defined contact pressure and lateral displacement, with a fresh rubbing fabric always located between the silicone finger and the surface to be tested. This fabric can also be treated with a test medium.

Stone Chip Test

The resistance of automotive coatings to stone chipping is of particular interest to the automotive industry. For simulation, in a stone chip test, sharp-edged impact bodies (hard cast granulate or small stones) of defined size and shape are shot onto the coated surface with compressed air. The type of impact bodies, the operating pressure, the impact time, and the angle of impact are specified in relevant standards. The assessment of stone chip resistance is carried out by visual comparison with images.

Steam Jet Test

The steam jet test according to DIN EN ISO 16925 serves to assess the adhesion of paints and coatings. The steam jet test originated from complaint cases after cleaning with high-pressure cleaners or steam jet cleaners. For the test, the coating is first scored or milled down to the substrate in the shape of a St. Andrew’s cross. Subsequently, the edges of the St. Andrew’s cross are treated with a pressurized water jet under defined conditions. The damage pattern after the steam jet test is evaluated by comparison with image panels of different damage levels.

Salt Spray Test

The salt spray test according to DIN EN ISO 9227 is used for coated metal parts. It simulates both the influence of saline air near the sea and the stress on a component from road salt. In a special chamber, a pH-neutral salt solution (NSS) is atomized. This settles on the component and covers the surface with a corrosive saltwater film. After the several-day salt spray test, the component is rinsed with distilled water to remove loosely adhering residues. Afterwards, it can be checked whether the corrosion protection effect of a coating is fulfilled at all points of a component or if red rust becomes visible on the surface. The salt spray test is well suited for comparing the corrosion protection effect of various protective coatings.