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Testing Equipment ......

Adhesion

In order to perform satisfactorily, coatings must adhere to the substrates on wh ich they are applied. In practice, three different test procedures are used to assess the resistance of paints and coatings to separation from substrates:

• Cross-Cut Test

This test method specifies a procedure for assessing the resistance of paints and coatings to separation from substrates when a right-angle lattice pattern is cut into the coating, penetrating through to the substrate.

The method may be used for a quick pass/fail test. When applied to a multi-coat system, assessment of the resistance to separation of individual layers of the coating from each other may be made.

• Scrape Adhesion

This test method covers the determination of the adhesion of organic coatings such as paint, varnish, and lacquer when applied to smooth, flat (planar) panel surfaces. It has been found useful in providing relative ratings for aseries of coated panels exhibiting significant differences in adhesion.

The materials under test are applied at uniform thickness to flat panels, usually sheet metal of uniform surface texture. After drying, the adhesion is determined by

pushing the panels beneath a rounded stylus or loop that is loaded with increasing amounts of weight until the coating is removed from the substrate surface.

• Chipping Test

Resistance to chipping of multi-coat systems is an important factor in automotive finishing. For the protection of auto body parts, the automotive industry applies multicoat systems wh ich have to be resistant to corrosion and mechanical stress. Experience has solved the problem of testing corrosion-resistance to a large extent. Resistance to mechanical stress, however, has not yet been satisfactorily determined.

The complete structure of a coating is decisive for its resistance to chipping. Changing the material or the coating process can have an effect on the extent of damage. The front and side body parts of the car are particularly exposed to chipping caused by roadway stones.

The multi-impact test method - sharp-edged, chilled casting pieces are thrown against a test panel by compressed air - simulates the actual stress as closely as possible. However, multi-impact test results often deviate extremely due to the statistical distribution of particles and the impact density. To combat this insufficiency in an otherwise practical test method, an instrument with a single impact mechanism was developed.