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Waterborne Coatings 57 blocks and then undergo polymerization. Polyurethane dispersions, on the other hand, are produced by polycondensation of aqueous building blocks [3]. 3.2 WATER VS. ORGANIC SOLVENTS The difference between solvent-borne and waterborne paints is due to the unique character of water. In most properties that matter, water differs significant from organic solvents. In creating a waterborne paint, the paint chemist must start from scratch, reinventing almost everything from the resin to the last stabilizer added. Water differs from organic solvents in many aspects. For example, its dielectric constant is more than an order of magnitude greater than those of most organic solvents. Its density, surface tension, and thermal conductivity are greater than those of most of the commonly used solvents. For its use in paint, however, the following differences between water and organic solvents are most important: • Water does not dissolve the polymers that are used as resins in many paints. Consequently the polymers have to be chemically altered so that they can be used as the backbones of paints. Functional groups, such as amines, sulphonic groups, and carboxylic groups, are added to the resins to make them soluble or dispersible in water. • The latent heat of evaporation is much higher for water, than for organic solvents. Thermodynamically driven evaporation of water occurs more slowly at room temperature. • The surface tension of water is higher than those of the solvents commonly used in paints. This high surface tension plays an important part in the film formation of latexes (see Section 3.3). 3.3 LATEX FILM FORMATION Waterborne dispersions form films through a fascinating process. In order for crosslinking to occur and a coherent film to be built, the solid particles in dispersion must spread out as the water evaporates. They will do so because coalescence is thermodynamically favored over individual polymer spheres: the minimization of total surface allows for a decrease in free energy [5]. Film formation can be described as a three-stage process. The stages are described below; stages 1 and 2 are depicted in Figure 3.1. 1. Colloid concentration. The bulk of the water in the newly applied paint evaporates. As the distance between the spherical polymer particles shrinks, the particles move and slide past each other until they are densely packed. The particles are drawn closer together by the evaporation of the water but are themselves unaffected; their shape does not change. 2. Coalescence. This stage begins when the only water remaining is inbetween the particles. In this second stage, also called the ‘‘capillary’ stage,” the high surface tension of the interstitial water becames a factor. The water tries to reduce its surface at both the water-air and water-particle © 2006 by Taylor & Francis Group, LLC
58 Corrosion Control Through Organic Coatings interfaces. The water actually pulls enough on the solid polymer particles to deform them. This happens on the sides, above, and below the sphere; everywhere it contacts another sphere, the evaporating water pulls it toward the other sphere. As this happens on all sides and to all spheres, the result is a dodecahedral honeycomb structure. 3. Macromolecule interdiffusion. Under certain conditions, such as sufficiently high temperatures, the polymer chains can diffuse across the particle boundaries. A more homogeneous, continuous film is formed. Mechanical strength and water resistance of the film increase [5, 6]. 3.3.1 DRIVING FORCE OF FILM FORMATION A FIGURE 3.1 Latex film formation: colloid concentration (A) and coalescence (B). Note that center-to-center distances between particles do not change during coalescence. The film formation process is extremely complex, and there are a number of theories — or more accurately, schools of theories — to describe it. A major point of difference among them is the driving force for particle deformation: surface tension of the polymer particles, Van der Waals attraction, polymer-water interfacial tension, capillary pressure at the air-water interface, or combinations of the above. These models of the mechanism of latex film formation are necessary in order to improve existing waterborne paints and to design the next generation. To improve the rate of film formation, for example, it is important to know if the main driving force for coalescence is located at the interface between polymer and water, between water and air, or between polymer particles. This location determines which surface tension or surface energies should be optimized. In recent years, a consensus seems to be growing that the surface tension of water, either at the air-water or the polymer-water interface — or both — is the driving force. Atomic force microscopy (AFM) studies seem to indicate that capillary pressure at the air-water interface is most important [7]. Working from another approach, Visschers and © 2006 by Taylor & Francis Group, LLC B
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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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