Degradation of a polyester‐urethane coating: Physical properties. Issue 6 (12th November 2015) - Makki, Hesam
ABSTRACT: In this article we studied the evolution of thermomechanical properties of a polyester‐urethane coating during degradation under different degradation conditions, i.e., aerobic and anaerobic conditions with and without dry/wet cycling during degradation. Dynamic mechanical and thermal analyses show that under aerobic conditions the coatings become stiffer and more brittle in the glassy state. This stiffening is probably due to the increase in the amount of hydrogen bonding and the formation of oxidized groups which increase the polarity of the material and enhance the interactions of the polymer segments. However, oxidation reactions result in a considerable decrease in cross‐link density and stiffness in the rubbery state. Both changes, in the glassy and rubbery states, give rise to development of internal stresses. These stresses increase as the degradation process proceeds. Nevertheless, for samples exposed to anaerobic conditions, the stiffness remains constant in the glassy state and the cross‐link density slightly increases as a result of degradation. This reconfirms the dominance of the effect of oxidation reactions on the mechanical failure of the coatings. Oxygen permeation measurements show a more‐or‐less time‐independent diffusion coefficient and a gradual decrease in solubility of oxygen as a function of exposure time. This results in a slight decrease in oxygen permeation (mainly in the early stage of the degradation) as degradation proceeds. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2016, 54, 659–671 Abstract : Degradation of polyester‐urethane coatings by radiation leads to changes in chemistry and crosslink density and, hence, in properties. In the glassy state, the appearance of oxidized segments dominates, resulting in stronger interactions and therefore a reduced volume and tensile stress in the coating. In the rubbery state, the lower crosslink density is the dominating factor, resulting in an increased thermal expansion coefficient so that the coating experiences a compressive stress.