Effect of physical aging on thermal stress development in powder coatings

Thermal stress and physical aging are inherent to thennosetting systems (such as powder coatings) and may affect the coating durability leading to damage such as detachment and cracking. Both phenomena occur principally below the glass transition temperature (T_g) of the coating and affect each other. It is shown that the measurement of stress, as a function of temperature of coatings aged at different temperatures and during various times, represents a simple and interesting way to study these phenomena. The results obtained which show changes in the stress magnitude with aging are explained in terms of stress relaxation and structural recovery. The latter process is especially evident in the T_g region and can prevent the correct determination of the T_g by means of thermal stress measurements. The thermal expansion coefficient and the elastic modulus, two properties directly affecting the thermal stress magnitude were determined separately, and agree well with the proposed interpretation of experimental data. The linear dependence of thermal (compressive) stress on the logarithm of time indicates the possibility of predicting the effect of physical aging.