Environmental and time dependence of moisture transportation in an epoxy coating and its significance for accelerated weathering |
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Authors: | Xiaodong Shi Brian R Hinderliter Stuart G Croll |
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Affiliation: | (1) Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58102, USA; |
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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. |
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