What is coatings mar resistance

In many applications, coatings require resistance to accidental damage, such as scratches or dents caused by moving objects on them. One of such applications are coatings of furniture paints, which, in addition to a number of properties such as blocking resistance or resistance to household chemicals, also need resistance to accidental scratches caused by moving various objects.

However, this is not about damage causing complete destruction of the coating, such as cutting through the entire thickness of the coatings by scratching to the substrate, but damage causing visually unsightly changes, e.g. not scratching through the entire thickness of the coating. The resistance of coatings to such damage is called mar resistance.

Stylus loop based on a coating prior to the mar resistance test

In accordance with ASTM D16: mar resistance, (1) ability of a coating to resist visual damage caused by light abrasion, impact, or pressure. (2) resistance of the surface of the coating to permanent deformation resulting from the application of a dynamic mechanical force.

Below, schematically shows the differences between the classic scratch to the substrate, which determines the load caused by the cut of the coating to the substrate (on the right), and the mar, which refers to only partial damage to the coating (on the left). Mar resistance refers to coatings that are really hard. In water-based paints intended for furniture, it is particularly important, especially when selecting polymer dispersion for one-component acrylic systems.

Difference between mar resistance and scratch resistance

The mar resistance test is performed in accordance with ASTM D5178 using a balanced beam equipped with a U-loop stylus which, when loaded, exerts pressure on the coating and which is moved over it.

In the Spektrochem paints and coatings laboratory, this method is used for tests of various polymer dispersions for coating systems that require mar resistance, as well as for case studies for manufacturers of various additives, which we check in terms of their impact on improving the mar resistance of coatings, e.g. for waxes, silicone slip additives, functional fillers, matting agents, as well as colloidal nano-silica, etc.

The ASTM D5178 method is usually complementary to additional tests such as scratch resistance, hardness (pencil and pendulum), scrape adhesion and others relating to the resistance of coatings to damage due to scratching and abrasion.

Balanced beam – an instrument for determining the mar resistance of coatings

The graph below shows the results of the mar resistance test of a clear wood coating based on a 1K acrylic polymer dispersion, in which two different additives were tested to increase the mar resistance.

The graph shows examples of two doses of 0.5 % and 1.0 % active in total formulation. As you can see, the additives used allow for a significant increase in mar resistance, especially in the case of slip additive Y, which already at a dose of 0.5 % allows to increase the mar resistance by 100%. Such case studies are created in the Spektrochem laboratory in various formulations and are dedicated to technical marteting departments of raw material producers who want to enrich their technical materials with case studies typically dedicated to tests on formulations and coatings dedicated to various regions of the world.

Mar resistance test results – two slip additives in two doses

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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