Corrosion Control Through Organic Coatings Part 3 potx

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3 Waterborne Coatings

Most of the important types of modern solvent-borne coatings — epoxies, alkyds,

acrylics — are also available in waterborne formulations. In recent years, even

urethane polymer technology has been adapted for use in waterborne coatings [1].

However, waterborne paints are not simply solvent-borne paints in which the organic

solvent has been replaced with water; the paint chemist must design an entirely new

system from the ground up. In this chapter, we discuss how waterborne paints differ

from their solvent-borne counterparts.

Waterborne paints are by nature more complex and more difficult to formulate

than solvent-borne coatings. The extremely small group of polymers that are soluble

in water does not, with a few exceptions, include any that can be usefully used in

paint. In broad terms, a one-component, solvent-borne coating consists of a polymer

dissolved in a suitable solvent. Film formation consists of merely applying the film

and waiting for the solvent to evaporate. In a waterborne latex coating, the polymer

particles are not at all dissolved; instead they exist as solid polymer particles dis￾persed in the water. Film formation is more complex when wetting, thermodynamics,

and surface energy theory come into play. Among other challenges, the waterborne

paint chemist must:

• Design a polymer reaction to take place in water so that monomer building

blocks polymerize into solid polymer particles

• Find additives that can keep the solid polymer particles in a stable, even

dispersion, rather than in clumps at the bottom of the paint can

• Find more additives that can somewhat soften the outer part of the solid

particles, so that they flatten easier during film formation.

And all of this was just for the binder. Additional specialized additives are

needed, for example, to keep the pigment from clumping; these are usually

different for dispersion in a polar liquid, such as water, than in a nonpolar organic

solvent. The same can be said for the chemicals added to make the pigments

integrate well with the binder, so that gaps do not occur between binder and

pigment particles. And, of course, more additives unique to waterborne formu￾lations may be used to prevent flash rusting of the steel before the water has

evaporated. (It should perhaps be noted that the need for flash rusting additives

is somewhat questionable.)

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