Degradation of Polymer Coatings in Service: How Properties Deteriorate Due to Stochastic Damage

Abstract

Degradation of polymer coatings in service occurs due to the cumulative effect of a vast number of aggressive photons, molecules, or other assaults that occur at random locations and random intervals such that the Central Limit Theorem can be applied. This permits use of the Normal distribution to describe the extent and variation in the damage that occurs. In addition, initial network formation is also governed by stochastic processes as small reactive molecules react to form the polymer. Molecular dynamics simulations show that extensive variation is possible in the connectedness and local topology of a crosslinked polymer, so there are statistically occurring flaws and inconsistencies, even before exposure in service. The original defects, together with the damage caused by degradation, determine the performance of coatings. For deterioration in service, accumulation of damage is modelled as a stochastic process with the properties of the Normal distribution, using the mean to represent the most likely state of the material. Inserting this in very well-known theories of material properties such as gloss and toughness shows how the kinetics of failure of coatings may arise. Models include basic physical property values of materials and show how they affect the outcome. This approach also shows how nonsimultaneous degradation leads to an apparent induction period in the change of properties that may not originate with autocatalytic degradation chemistry. Equations generated in this approach demonstrate that different properties are sensitive in different ways to the degradation process and thus decay with a different rate, even though the basic mechanism of deterioration is identical.