Scratch resistance tests, or scratch tests for short, are carried out on plastics to test the quality of adhesion of paints and coatings to plastics. To do this, a needle is passed over the surface with increasing pressure and tested to see how long the surface can resist this load.
Mechanical contact with the paint surface can lead to a change in the surface (abrasion). The Crockmeter test is a test procedure commonly used in the automotive industry for painted surfaces. This abrasion test is based on DIN EN ISO 105-X12, which is actually used to test the rub fastness 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 inspected for possible staining and the paint surface is inspected for premature wear. A Crockmeter test is also suitable for simulating a combined exposure to water or other media during the mechanical resistance test, thus considerably accelerating the ageing of a surface.
Painted keys or printed controls, on the other hand, are preferably tested with an Abrex device. Actuation is simulated by a silicone finger with defined contact pressure and lateral displacement, whereby there is always a fresh friction fabric between the silicone finger and the surface to be tested. This fabric can also be exposed to a test medium.
The resistance of automotive paintwork to stone impact is of particular interest to the automotive industry. For simulation purposes, in a stone impact test according to DIN EN ISO 20567-1, sheep-edged impact bodies (chilled cast granules or small stones) of a defined size and shape are shot onto the paint surface with compressed air. The type of impactor, the working pressure, the firing time and the angle of impact are specified in the relevant standards. Stone impact resistance is assessed by visual comparison with pictures.
The steam jet test according to DIN EN ISO 16925 is used to assess the adhesion of coatings. The steam jet test came about as a result of complaints following cleaning of the car with high-pressure cleaners or steam jet cleaners. For the test, the paint is first scratched or milled into the substrate in the form of a St. Andrew's cross. Then the edges of the St. Andrew's cross are treated with a pressurized water jet under defined conditions. The defect pattern after steam jet testing is evaluated by comparison with picture boards of different damages.