Environmental and time dependence of moisture transportation in an epoxy coating and its significance for accelerated weathering

Abstract

It is increasingly appreciated that water is one of the most important factors in the durability behavior of coatings. The rate of water transportation and the degree of saturation of polymeric coatings depends on exposure history, exposure environment, and the nature of a coating. The focus here is on the time taken for transport of water and the variation it causes in thermal and mechanical properties, which have also been shown previously to have time-dependence that leads to discrepancies between different weathering regimes. This paper studied the effect of moisture on coatings' mechanical properties and the effect of moisture and temperature on recovery of surface indentation. The reduction of mechanical properties and the increasing rate of surface defect relaxation can be explained in terms of plasticization by water. Recovery of surface defects is related to the appearance and diffusion activity of coatings during exposure. Thermogravimetric analysis showed that the content of moisture in the sample is determined by the ambient humidity, temperature, and the rate of change in these environmental parameters. Diffusion coefficients were determined from the weight change data and an activation energy was obtained via Arrhenius equation. Changes in yellowness, as an example, and signature of chemical degradation, indicated that the cycling period in accelerated weathering may affect the degradation via the amount and distribution of moisture in coatings. Diffusion of moisture is typically slow and limits the possibility of complete drying or saturation in an exposure cycle. Results from the cumulative effect of many cycles will be different depending on the conditions.

Behavior of UV‐cured print inks on LDPE and PBAT/TPS blend substrates during curing, postcuring, and accelerated degradation

“…Radiation curing converts a reactive formulation into a highly crosslinked coating film by an irreversible reaction that provides high chemical resistance and a higher stability against degradation . The kinetics of curing depend directly on the formulation composition, and the degradation process of radiation‐curable materials requires specific conditions to initiate.…”

Effects of moisture ingressions on mechanical properties of honeycomb-structured fiber composites for aerospace applications

“…The diffusivity constant, D, is defined as the rate or speed at which the material absorbs, desorbs, or transports moisture. Moisture diffusivity is defined as liquid that is diffused and dispersed through a gas as vapor, is condensed on a surface as dew, or is transported into a material that can be quantified [5][6][7]. A common method for water absorption into polymers is the activated absorption model that can be explained in three steps [6]: (a) dissolution of water in the polymer, (b) moisture absorption onto surfaces of excess free volume elements, and (c) hydrogen bonding between polymer hydrophilic groups and water.…”

“…Shi et al explained the diffusion coefficient process of a solid using the Arrhenius equation [6]. The diffusion coefficient is determined using Eq.…”

Predicting Coating Failure using the Central Limit Theorem and Physical Modeling

“…It has been observed in measurements of the mechanical response of polymer coatings that the relaxation time of stresses is longer than the typically short cycles in accelerated exposure (36). Thus stresses that occur in each short cycle do not completely relax, and the stress remains large.…”