© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

Waterborne Coatings 59 TABLE 3.1 Estimates of Forces Operating During Particle Deformation Type of Force Operating Estimated Magnitude (N) Gravitational force on a particle 6.4 × 10 –17 Van der Waals force (separation 5 nm) 8.4 × 10 –12 Van der Waals force (separation 0.2 nm) 5.5 × 10 –9 Electrostatic repulsion 2.8 × 10 –10 Capillary force due to receding water-air interface 2.6 × 10 –7 Capillary force due to liquid bridges 1.1 × 10 –7 Reprinted from: Visschers, M., Laven, J., and Vander Linde, R., Prog. Org. Coat., 31, 311, 1997. With permission from Elsevier. colleagues [9] have reported supporting results. They estimated the various forces that operate during polymer deformation for one system, in which a force of 10 −7 N would be required for particle deformation. The forces generated by capillary water between the particles and by the air-water interface are both large enough. (See Table 3.1.) Gauthier and colleagues have pointed out that polymer-water interfacial tension and capillary pressure at the air-water interface are expressions of the same physical phenomenon and can be described by the Young and Laplace laws for surface energy [5]. The fact that there are two minimum film formation temperatures, one ‘‘wet” and one "dry," may be an indication that the receding polymer-water interface and evaporating interstitial water are both driving the film formation (see Section 3.4). For more in-depth information on the film formation process and important thermodynamic and surface-energy considerations, consult the excellent reviews by Lin and Meier [7]; Gauthier, Guyot, Perez, and Sindt [5]; or Visschers, Laven, and German [9]. All of these reviews deal with nonpigmented latex systems. The reader working in this field should also become familiar with the pioneering works of Brown [10], Mason [11], and Lamprecht [12]. 3.3.2 HUMIDITY AND LATEX CURE Unlike organic solvents, water exists in the atmosphere in vast amounts. Researchers estimate that the atmosphere contains about 6 × 10 15 liters of water [13,14]. Because of this fact, relative humidity is commonly believed to affect the rate of evaporation of water in waterborne paints. Trade literature commonly implies that waterborne coatings are somehow sensitive to high-humidity conditions. However, Visschers, Laven, and van der Linde have elegantly shown this belief to be wrong. They used a combination of thermodynamics and contact-angle theory to prove that latex paints dry at practically all humidities as long as they are not directly wetted — that is, by rain or condensation [8]. Their results have been borne out in experiments by Forsgren and Palmgren [15], who found that changes in relative humidity had no significant effect on the mechanical and physical properties of the cured coating. Gauthier and colleagues have also shown experimentally that latex © 2006 by Taylor & Francis Group, LLC
60 Corrosion Control Through Organic Coatings coalescence does not depend on ambient humidity. In studies of water evaporation using weight-loss measurements, they found that the rate in stage 1 depends on ambient humidity for a given temperature. In stage 2, however, when coalescence occurs, water evaporation rate could not be explained by the same model [5]. 3.3.3 REAL COATINGS The models for film formation described above are based on latex-only systems. Real waterborne latex coatings contain much more: pigments of different kinds (see chapter 2); coalescing agents to soften the outer part of the polymer particles; and surfactants, emulsifiers, and thickeners to control wetting and viscosity and to maintain dispersion. Whether or not a waterborne paint will succeed in forming a continuous film depends on a number of factors, including: • Wetting of the polymer particles by water (Visschers and colleagues found that the contact angle of water on the polymer sphere has a major influence on the contact force that pushes the polymer particles apart [if positive] or pulls them together [if negative] [8]) • Polymer hardness • Effectiveness of the coalescing agents • Ratio of binder to pigment • Dispersion of the polymer particles on the pigment particles • Relative sizes of pigment to binder particles in the latex 3.3.3.1 Pigments To work in a coating formulation, whether solvent-borne or waterborne, a pigment must be well dispersed, coated by a binder during cure, and in the proper ratio to the binder. The last point is the same for solvent-borne and waterborne formulations; however, the first two require consideration in waterborne coatings. The high surface tension of water affects not only polymer dispersion but also pigment dispersion. As Kobayashi has pointed out, the most important factor in dispersing a pigment is the solvent’s ability to wet it. Because of surface tension considerations, wetting depends on two factors: hydrophobicity (or hydrophilicity) of the pigment and the pigment geometry. The interested reader is directed to Kobayashi’s review for more information on pigment dispersion in waterborne formulations [16]. Joanicot and colleagues examined what happens to the film formation process described above when pigments much larger in size than latex particles are added to the formulation. They found that waterborne formulations behave similarly to solvent-borne formulations in this matter: the pigment volume concentration (PVC) is critical. In coatings with low PVC, the film formation process is not affected by the presence of pigments. With high PVC, the latex particles are still deformed as water evaporates but do not exist in sufficent quantity to spread completely over the pigment particles. The dried coating resembles a matrix of pigment particles that are held together at many points by latex particles [17]. © 2006 by Taylor & Francis Group, LLC
Page 1 and 2:
© 2006 by Taylor & Francis Group,
Page 3 and 4:
Corrosion of Ceramic and Composite
Page 5 and 6:
Published in 2006 by CRC Press Tayl
Page 7 and 8:
Preface This book has been written
Page 9 and 10:
Author Amy Forsgren received her ch
Page 11 and 12:
Contents Chapter 1 Introduction ...
Page 13 and 14:
2.4.2 Reactive Reagents ...........
Page 15 and 16:
6.2.4.1 Alkaline Blistering........
Page 17 and 18:
1 Introduction This book is not abo
Page 19 and 20:
Introduction 3 • A dissolution pr
Page 21 and 22:
Introduction 5 1.2.2 ELECTROLYTIC R
Page 23 and 24: Introduction 7 to the metal is a ne
Page 25 and 26: Introduction 9 9. Thomas. N.L., Pro
Page 27 and 28: 12 Corrosion Control Through Organi
Page 73: 58 Corrosion Control Through Organi
Page 81 and 82: 4 Blast Cleaning and Other Heavy Su
Page 83 and 84: Blast Cleaning and Other Heavy Surf
Page 99 and 100: 5 Abrasive Blasting and Heavy-Metal
Page 101 and 102: Abrasive Blasting and Heavy-Metal C
Page 113 and 114: 6 Weathering and Aging of Paint Thi
Page 115 and 116: Weathering and Aging of Paint 101 c
Page 117 and 118: Weathering and Aging of Paint 103 I
Page 119 and 120: Weathering and Aging of Paint 105 c
Page 121 and 122: Weathering and Aging of Paint 107 S
Page 123 and 124: Weathering and Aging of Paint 109 e
Page 125 and 126:
Weathering and Aging of Paint 111 I
Page 127 and 128:
7 Corrosion Testing — Background
Page 129 and 130:
Corrosion Testing — Background an
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
130 Corrosion Control Through Organ
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
Page 167:
show all

© 2006 by Taylor & Francis Group, LLC

Create successful ePaper yourself

Delete template?

Save as template?