The standard "ISO 19252 Plastics - Determination of scratch properties" published by the International Organization for Standardization (ISO) describes a method for determining the scratch properties of plastics under defined conditions. This method involves creating a scratch by moving a rigid tool (scratch tip) of specified geometry across the surface of a test specimen under specified load and speed conditions and then evaluating the result.
This method is used to investigate the behavior of specified specimen types under defined scratch conditions and to classify the scratch types of the specimens within the limitations found in the test conditions. It can also be used to determine comparative data for different types of materials by means of a scratch map in which the types of scratch behavior for each set of test load and test speed test conditions are determined using the basic method of constant load testing and also by means of the so-called critical normal load determined using the alternative method of linearly increasing load testing.
This method is suitable for use with uncoated and unpolished thermoplastic molding materials and thermoset molding materials. It also specifies the preferred dimensions and preferred scratch tip geometry for the test specimen.
For the purposes of this standard, the following terms and definitions apply.
A standard test specimen is scratched by a rigid tip that applies a load perpendicular to the test specimen and moves longitudinally along the surface at a constant speed along the centerline of the test specimen. During scratching, the horizontal force between the scratch tip and the test specimen, the vertical displacement of the scratch tip, and the scratch distance are continuously measured and recorded. The scratch force/scratch distance and tip displacement/scratch distance diagrams thus produced describe the scratch behavior of the test specimen.
Using the basic method of constant load testing, the type of scratch behavior can be determined for a range of test conditions (load and speed) and expressed as a so-called scratch map. Using an alternative method of linearly increasing the load test, the critical normal load can be determined at a given test speed. Changes in the slope or peaks in the scratch force/scratch distance curve indicate transitions from one type of scratch behavior to another.
As specimens, Type A multipurpose test specimens specified in the ISO 3167 standard are used. Test specimens shall be directly injection molded in accordance with ISO 294-1 or ISO 10724-1 standards under the conditions defined in the relevant standard for the material under examination. Specimens shall be suitably marked outside the test area to indicate the melt flow direction, the specific cavity from which the specimen originates (when using a multi-cavity mold), and the edge of the mold (cavity plate side or fixed plate side) where a specific specimen face is formed. Some types of material may exhibit different scratch behavior depending on the surface of the specimen being tested or the melt flow direction. It is essential that all conditions are strictly controlled during specimen preparation to ensure that all test specimens are in the same condition.
Test specimens shall not be bent. Surfaces and edges shall be free from scratches, dents, depressions, burrs and other defects. Test specimens shall be checked for conformity with these requirements by visual observation using straight edges, squares and flat plates and by measuring the dimensions of the parallelepiped section (thickness and width) to the nearest 0,01 mm. Specimens showing measurable or observable deviations from any of these requirements shall be rejected. In injection moulded specimens, it is not necessary to measure the dimensions of each specimen. It is sufficient to measure one specimen from each set. When multi-cavity moulds are used, the dimensions of one specimen from each cavity shall be measured. If the difference in dimensions between the mould cavities is more than 0,02 mm, the specimens from each cavity shall be treated as separate lots.
To determine the type of scratch behavior, if only one type of scratch behavior occurs during a single test, a single specimen is required for each set of test conditions. If more than one type of scratch behavior occurs during a single test, three specimens are required for each set of test conditions. To obtain a scratch map, it is recommended to use at least 25 different sets of test conditions (for example, five different constant test loads and five different test speeds). To determine the critical normal load, three specimens are required for each set of test conditions, i.e., for each test speed.
Unless otherwise specified in the relevant standard or agreed upon by the relevant parties, the samples should be conditioned for at least 291 hours in one of the standard atmospheres specified in the ISO 16 standard.
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