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Waterborne Coatings 61 FIGURE 3.2 Pigment and binder particle combinations. The polymer particles are black, and the pigment particles are white or striped (representing two different pigments). Top: High PVC, with binder particles aggregated between pigment particles. Middle: High PVC and dispersed binder particles. Bottom: Low PVC and enough binder to fill all gaps between pigment particles. The problems of PVC-pigment dispersion imbalance are shown in Figure 3.2. In the top part of Figure 3.2, the PVC is very high and the binder particles have flocculated at a limited number of sites between pigment particles. When they deform, the film will consist of pigment particles held together in places by polymer, with voids throughout. The middle section of Figure 3.2 shows the same very high PVC, but here the binder particles are dispersed. The binder particles may form a continuous film © 2006 by Taylor & Francis Group, LLC
62 Corrosion Control Through Organic Coatings around the pigment particles, but voids still occur because there simply is not enough binder. The bottom part of Figure 3.2 shows the ideal scenario: the PVC is lower, and the surrounding black binder is able to not only cover the pigment particles but also leave no void between them. 3.3.3.2 Additives In real waterborne paints, the film formation process can result in a nonhomogeneous layer of cured paint. Tzitzinou and colleagues, for example, have shown that the composition of a cured paint layer can be expected to vary through the depth of the coating. They studied an anionic surfactant in an acrylic latex film. Using AFM and Rutherford backscattering spectrometry on cured films, they found a higher concentration of surfactant at the air surface than in the bulk of the coating [18]. Wegmann has also studied the inhomogeneity of waterborne films after cure, but attributes his findings mainly to insufficient coalescence during cure [19]. The chemistry of real latex formations is complex and currently defies predictive modeling. A reported problem for waterborne modelers is that an increase in curing temperature can affect various coating components differently. Snuparek and colleagues added a nonionic emulsifier to a dispersion of copolymer butyl methacrylate/butyl acrylate/acrylic acid. When cure took place at room temperature, the water resistance of the films increased with the amount of emulsifier added. When cure happened at 60°C, however, the water resistance of the films decreased with the amount of emulsifier added [4]. 3.4 MINIMUM FILM FORMATION TEMPERATURE Minimum film formation temperature (MFFT) is the minimum temperature needed for a binder to form a coherent film. This measurement is based on, although not identical to, the glass transition temperature (T g) of the polymer. If a coating is applied below the MFFT, the water evaporates as described for Stage 1 (see Section 3.3). However, because the ambient temperature is below the MFFT, the particles are too hard to deform. Particles do not coalesce as the interstitial water evaporates in stage 2. A honeycomb structure, with Van der Waals bonding between the particles and polymer molecules diffused across particle boundaries, does not occur. The MFFT can be measured in the laboratory as the minimum temperature at which a cast latex film becomes clear. This is simply because if the coating has not formed a coherent film, it will contain many voids between polymer particles. These voids create internal surfaces within the film, which cause the opacity. Latexes must always be applied at a temperature above the MFFT. This is more difficult than it sounds, because the MFFT is a dynamic value, changing over time. In a two-component system, the MFFT begins increasing as soon as the components are mixed. Two-component waterborne paints must be applied and dried before the MFFT has increased enough to reach room temperature. When © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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7 Corrosion Testing — Background
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Corrosion Testing — Background an
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