Coatings put to the acid test

Colour and gloss

The assessment of colour and gloss is often done purely visually by comparison with a master sample or a grey scale. The correct lighting is very important here. Additional conspicuous features (e.g. blistering, paint peeling) are included in the inspection. Alternatively, colour changes during batch changes or due to environmental influences can be quantified by colour and gloss measuring devices. The measuring geometries are standardised in order to obtain comparable values. 

Paint adhesion (cross and cross-cut test)

The cross-cut test is used to determine the adhesion of a single-coat or multi-coat paint to the substrate. This shows how a painted surface behaves against injuries. In the cross-cut test, the surface is scratched in a cross shape with a sharp blade. The adhesion of the lacquer is then checked by means of a peel test and the quality of the adhesion is evaluated according to DIN EN ISO 2409 with the cross-cut characteristic values GT0 to GT5 or according to ASTM D 3359-02 with the characteristic values 5B to 0B. Adhesion tests are carried out as standard before and after climate storage.

Scratch test

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.

Environmental simulation and weathering

Most paints are directly exposed to the environment. The most important ageing parameters are temperature, light and humidity, which can be simulated individually or in combination over time. It is also possible to change the conditions or sprinkle within predefined cycles. Depending on the type of paint, different influences can have a particular effect on paint changes. In the automotive sector, measuring conditions of the type Floridest or Kalahari test are frequently tested. After storage, changes in paint adhesion or colour are examined. A special test that is mainly used for painted metal parts is the salt spray test.

Chemical resistance

During everyday use, the paint comes into contact with a variety of chemicals. These are typically sweat, hand cream, sun cream, various solvents, cleaning agents and disinfectants, but also cola or fuel, depending on the area of use. Chemical resistance is usually tested by direct contact with the test medium. If necessary, the test is intensified by simultaneous exposure to friction or temperature. The changes in the paint are evaluated according to special criteria. 

Abrasion resistance, rub resistance or colour fastness

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 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-type 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.  

Stone chip test

The resistance of automotive paintwork to stone impact is of particular interest to the automotive industry. In a stone impact test, 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 to simulate the impact. 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. 
 

Steam jet test

The steam jet test according to DIN EN ISO 16925 is used to assess the adhesion of paints and coatings. The steam jet test came into being as a result of complaints following cleaning 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. The edges of the St. Andrew's cross are then treated with a pressurized water jet under defined conditions. The defect pattern after steam jet testing is evaluated by comparison with image plates of different damage.  
 

Salt spray test

The salt spray test according to DIN EN ISO 9227 is used for painted metal parts. It simulates both the influence of salty air near the sea and the exposure of a component to road salt. A neutral pH salt solution (NSS) is atomised in a special chamber. This precipitates on the component and covers the surface with a corrosive film of salt water. After the salt spray test, which lasts several days, the component is rinsed with distilled water to remove loosely adhering residues. Afterwards, it can be checked whether the corrosion protection effect of a paint is fulfilled at all points of a component or whether red rust becomes visible on the surface. The salt spray test is well suited to compare the corrosion protection effect of different protective coatings.