Chapter 13
Downloaded by UCSF LIB CKM RSCS MGMT on November 23, 2014 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch013
Transport Properties of Waterborne Polymeric Coatings
H. K. Tay and J. M. Sykes
Department of Materials, Oxford University, Parks Road, Oxford ΟΧ1 3PH, England
Gravimetric sorption curves for water in a water-borne vinyl chloride/ vinylidene chloride copolymer coating showed that paint pH had a profound effect on the water uptake. At pH 3, water uptake increases markedly with pigment volume concentration but not forfilmscast from pH 5 paint. By calculating the amount of water adsorbed on the surface of the pigments, it was found that the water content was influenced by different pigments for different paint pH. pH did not affect water uptake infilmscastfromlatex. The pigment content in the paint influences the rate of and the activation energy for water transport. Keywords: gravimetric, pH, pigment volume concentration, diffusion coefficient, latex. When painted metals are exposed to wet corrosive environments the first process leading to the onset of corrosion will be uptake of water by the coating and transport of water to the coating-metal interface. At the interface water will reduce the adhesion of the coating and may lead to development of blisters. The failure of adhesion and development of a separate aqueous phase in contact with the metal is considered by some workers (1) to be essential for the development of corrosion. Water in the coating will also influence the solubilisation of ions and ionic diffusion. Water and oxygen are consumed in the corrosion process, but most paintfilmsare so permeable to water and oxygen that their transport is not usually rate-limiting (2). It may however be an important factor in controlling the deterioration of coatings, for instance by blistering. This work examines factors influencing water transport in a water-borne coating based on a vinyl chloride / vinylidene chloride copolymer latex (3,4) and the part played by pigments added to make a primer. It seeks to understand controlling factors in water absorption and transport in pigmented water-borne systems. The latex was based on this chlorinated polymer because of its remarkably low permeability to water and oxygen diffusion. It had been found that (5) among typical paint resins (eg. acrylic primer, epoxy/polyamide), 1-coat free films of VC-VDC copolymer latex has the lowest
©1998 American Chemical Society
In Organic Coatings for Corrosion Control; Bierwagen, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
161
Downloaded by UCSF LIB CKM RSCS MGMT on November 23, 2014 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch013
162 permeability to water vapour, even when compared to some solvent-borne resins like pigmented alkyd and chloropolymers. The paint is normally formulated at about pH 5 to avoid dehydrocMorination of the polymer (6). Primer formulated at this pH with zinc phosphate pigments provides (5) good corrosion protection. The influence of pigments on permeability can vary and will depend upon the volumefraction(PVC) added, butflake(or "lamellar") pigments, such as micaceous iron oxide or metallic aluminium, that align themselves parallel to the substrate are commonly added to improve the barrier properties of coatings. Water will be forced to traverse a longer and tortuous path through the pigmented coating as compared to the water transport in an unpigmented coating (7). However, pigment-polymer adhesion is also a crucial factor in determining water transport, because the pigment polymer interface provides a possible easy path for water diffusion. If the particles make contact with one another (the content exceeds the percolation threshold), creating connected pathways, then transport should be facilitated (8). At high volumefractions(exceeding the critical PVC) the polymer matrix fails to fills all the voids between the pigment (9) and barrier properties are gravely impaired. A convenient way to quantify water transport in polymers is through the determination of sorption curves forfreefilmsof paint immersed in water. Interpreta tion of the results requires solution of the diffusion equation under non-steady state conditions. This is not difficult provided that the diffusion coefficient D can be treated as a constant independent of water concentration in the polymer. Typically sorption curves show a linear water uptake which increases with V*time, butfinallylevels off when the polymer is saturated with water. The Fourier transformed solutions for the Fick's equations in the case of sorption into both sides of a thin sheet with D treated as constant and for small times is (10), ^
κ
-
4
Ν
UJ 2
L-H ^
+
2
Τ(-\γ ierfc A) 2(Dtt
(1)
where, M - mass of water absorbed or desorbed at time t, M = mass of water absorbed or desorbed at time °°, / = thickness of paint film, ierfc = /«&ξ
