© 2006 by Taylor & Francis Group, LLC

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