ASTM D6944- Tolerance of Cured Coatings to Thermal Cycling – describes two test protocols that evaluate the effect of repeated thermal stress on the adhesion of coating systems by using heat and humidity cycling tests. In fact, this test corresponds to a specific type of accelerated burn-in test, which is somewhat similar to cyclic corrosion tests such as ASTM G85. In fact, they all use a cyclic procedure to cause deterioration of the specimen coating, but they differ by the means used to achieve this end. Finally, the test protocol needs to be paired with other coating tests to assess the properties of the sample after exposure, as the latter only provides test conditions.

Evaluating the adhesion resistance of coating systems after thermal stress application and factors to consider:

ASTM D6944-0 Standard Practice for Determination of Heat Resistance Cycling of Cured Coatings

The goal of ASTM D6944 is to determine the effect of thermal stress on the ability of coating systems to adhere to substrates by exposing them to thermal cycling tests. In practice, such testing involves exposing a sample to cyclic test conditions as a means of identifying the behavior of a property after the sample has been exposed to simulated environmental conditions for a predetermined period of time. In fact, the standard describes two different stress cycling schemes that evaluate the behavior of the coating to this thermal stress. On the one hand, method A corresponds to a three-stage cycle consisting of freezing, thawing, and soaking periods. Method B, on the other hand, is the same as method A, but does not include the immersion phase in its rotation order. Whichever method is chosen, this practice only details the test conditions required for thermal stress, but it does mention other test scenarios for evaluating certain characteristics. Therefore, this criterion needs to be combined with practices that can assess the correct attributes. Finally, the results obtained cannot be used to predict the service life of the coating system.

ASTM D6944-0 Standard Practice for Determination of Heat Resistance Cycling of Cured Coatings

The main factors to consider when performing this test are: the thickness of the film and the curing time of the coating.

Typical experimental parameters for ASTM D6944 testing:

The number of samples required is 2 and it is recommended that they be tested together with materials with known properties so that the latter can be used as a control. If the sample is concrete, a special arrangement is required to continue the method. The test specifications are listed in Table 1.

Table I: Typical test parameters