© 2006 by Taylor & Francis Group, LLC

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