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The scratch resistance of car interior paint

How paint and coatings used inside an automobile can be assessed.

Image © Sjoerd van der Wal |

Many people view their cars as fashion accessories. With so much choice in make, model and colours, the car itself can be a demonstration of a person’s style and personality. The interior trim of a vehicle is produced from different materials, including leather, plastic, metal, textile and sometimes even wood effect laminate. Generally, an assortment of paints, varnishes and coatings are applied to a trim material’s substrate material in order to protect the surface, which have a matt or shiny smooth finish. This article will consider materials which have a painted or lacquered finish. Typically, these trim components feature on the gear stick handle, dashboard, air vents and door panels – areas which see a lot of wear and tear throughout their lifespan. Given the importance of appearance for the interior components, it is vital that the top layer, whether paint or coating, both adheres well to the substrate to give an even smooth finish and does not come away from the substrate easily when scratched, abraded or marked in some way. Hence, testing for these properties is important in order to determine the performance level of the component.

Essentially, the scratch resistance of a material is defined as ‘the ability of the coating to resist an impression’. Scratches in the surface finish will greatly reduce the aesthetics of a component and immediately demonstrates wear and tear of the product. Laboratory tests are designed to evaluate the susceptibility of a plastic or painted surface to scratching, to ensure that a reasonable level of durability is achieved. When a scratch occurs, this is a permanent deformation of the surface, which will reduce the protective layer provided by the coating. The vulnerable substrate layer underneath will be left unprotected and exposed to potential further damage. This is why it is important that materials are scratch-resistant, so that damage in minimised in the first instance.

The scratch test

One of the most stringent scratch methods involves a movable platform, to which a test plaque (a flat plate material consisting of the composite material and coating) is positioned and moved at a set rate relative to a number of metal ‘fingers’. Scratch pins with a head diameter of 1 mm are attached to the metal fingers, and these are then loaded with varying forces – for example, 7 N, 6 N, 4.5 N, 3 N and 2 N. The scratch pins are positioned in contact with the test plaque, which is then moved for its complete distance. Once the scratch lines have been produced, they are evaluated under a light box to determine the severity of the scratch. This can be assessed using a five-point scale, ranging from ‘no scratch line at all’ through to ‘severe scratch line’. During the examination, the plaque can be angled and tilted in any direction in order to determine if a line is present.

Metal 'fingers' used in the scratch test

Different pins are used for specific tests

Micro scratch

There are other less severe scratch tests requested by original equipment manufacturers (OEMs), which call for the repeated rubbing on a sample. One in particular assesses the micro scratch performance of a sample, and has been specifically designed for high gloss, undecorated plastic and interior painted plastic components. For this particular test, SATRA makes the STM 421 rub fastness tester, which is specified in the method. During this test, a plaque is attached to the base of the machine, a felt pad is positioned into a 1 kg holder and then this is placed in contact with the test sample. The pad then moves to and fro over the plaque for a pre-determined number of cycles, which can range from 50 to 4,000, depending on the OEM’s specification. Following the test, an assessment is carried out in a controlled light box, rotating and tilting the plaque to determine if any scratch lines are visible. This method can also incorporate a recovery phase, where the samples are re-assessed after 24 hours to determine whether the coating material has recovered with time.

Mar resistance

Similar to the micro scratch test, the ‘mar test’ evaluates the performance of a material to repeated movement over the surface. During this test, a 1 mm diameter rounded ball end is attached to an arm which allows for weights to be added. The test plaque is then moved to and fro, relative to the weighted arm and the 1 mm ball stylus. Incremental loads are added to the weighted arm before assessing the severity of the marks. The lowest load at which a continuous line can be seen is often the value reported, along with comments as to whether the line is a gloss change or colour change (whitening).

In many instances, it is important that the materials chosen are able to resist both a single scratch from a relatively sharp object and a repeated scratching or rubbing action from a blunter item.

The tests mentioned in this article allow for comparisons to be made over the performance level of the coating and substrate, thus enabling evaluations of the general scratch resistance of the coating on a component. This should give manufacturers the confidence needed in the product they are supplying.

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