Moisture in organic coatings - a review

G.K. Wel, van der, O.C.G. Adan

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267 Citations (Scopus)
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A review is given on transport and equilibrium sorption of moisture in polymer films and organic coatings. Polymeric material forms the continuous phase of a coating and is therefore important for transport properties. Besides polymer, coatings consist of pigments and fillers and various additives, each of them affecting moisture transport. Firstly, in this paper typical transport and equilibrium characteristics of pure polymer films are covered, and secondly those of coatings, assimilating the impact of pigments, fillers and additives. Discrimination between Fickian and non-Fickian effects is usually based on the appearance of kinetic absorption curves. Several types of curve are dealt with, and models that describe these curves are discussed. These include "two-stage" sorption, "sigmoidal" sorption and "Case II" sorption. The common practice of fitting Fick’s law to kinetic data by means of relaxation parameters is criticized. Transport kinetics is modelled on the basis of kinetic plots, thereby introducing relaxation parameters in Fick’s diffusion law. Although these models describe moisture transport in polymers reasonably, these models imply a lot of "curve-fitting", without a clear mechanistic foundation. Equilibrium sorption of moisture in polymeric systems is reflected by their sorption isotherms. Type II and III isotherms of the BET classification are commonly encountered, together with linear isotherms according to Henry’s law. The suitability of porous media adsorption models for polymeric systems is discussed. These models are treated in relation to interactions of water in polymers, since localized binding of water to specific groups may justify the use of these models. Finally, the — substantial — effects of pigments and fillers, as well as coating additives on moisture transport and solubility in coatings are dealt with. Pigments may lower moisture permeation, but non-ideality of pigmentation reduces this effect. Coating additives increase moisture solubility, but only little is known in detail.
Original languageEnglish
Pages (from-to)1-14
Number of pages14
JournalProgress in Organic Coatings
Issue number1-2
Publication statusPublished - 1999


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