Chemistry Reference
In-Depth Information
Before considering process details, a short digression is worthwhile to com-
pare alternative means for applying an organic coating, say, an acrylic copolymer,
to a metal substrate. Save for powder coatings, which are outside the scope of
this text, the film former must be diluted before application, which will be by
spraying on an industrial scale. Solution coatings have viscosities dependent on
polymer molecular weight and concentration. By contrast, the viscosities of dis-
persion coatings are independent of polymer molecular weight (viscosities of sus-
pensions depend on the size and concentration of the particles, Chapter 3) and are
lower than those of solutions with practical polymer levels. A third alternative
involves use of aqueous emulsions. Here again, there are advantages in terms of
low application viscosities and high polymer concentrations. The emulsion system
has the further benefit of containing low levels of volatile organic materials.
Emulsion-based coatings contain higher polymer concentrations than NADs,
where the solute levels are limited by the formation of some soluble low-
molecular-weight polymer. The cost of drying the coating is in favor of the NAD,
however, since the boiling point and heat of vaporization of a low-molecular-
weight hydrocarbon are appreciably lower than those of water. In both the NAD
and emulsion systems, the polymer is made in the diluent in which it will be
applied; solution-polymerized acrylics require the use of expensive solvents like
ketones and esters. In industrial spray applications, NADs have the advantages of
rapid release of diluent between the substrate and the spray nozzle, better resis-
tance to sagging (the polymer contains less solvent and has a lower tendency to
flow) and fewer problems with solvent popping (caused by release of the retained
diluent during drying). In summary, then, while the chemical nature of the coating
will be approximately the same in all three cases, the choice of polymerization
process can have profound effects on the subsequent performance of the material.
During dispersion polymerization polymer particles are formed from an ini-
tially homogeneous reaction mixture by polymerization in the presence of a poly-
meric steric stabilizer. The process is applicable to monomers that yield polymers
that are insoluble in a solvent for the monomer. Styrene has been polymerized in
alcohols, with steric stabilizers such as poly(
-vinylpyrrolidone) (see Fig. 1.4 for
monomer structure) or hydroxypropyl cellulose. Hydrocarbon solvents are used
for the polymerization of methacrylic esters, using steric stabilizers like poly
(12-hydroxy stearic acid), polyisobutene, poly(dimethyl siloxane) (1-44), and
other polymers. The dispersions are suitable for use as surface coatings and the
polymer particles themselves have applications as toners in xerography and as
chromatographic packing materials. Particle sizes can be produced with narrow
distributions, when necessary, and diameters between 0.1 and about 15
N
m are
attainable. Other features of NADs include their ability to make dispersions of
polymers like polyacrylonitrile that are insoluble in common commercial solvents.
By controlling the monomer feed in a semicontinuous polymerization, certain
monomers may be concentrated toward the interior or surfaces of the polymer
particles, for example, so that particle fusion is favored during the baking cycle
after a coating has been applied to a substrate. This is illustrated in the two-stage
μ
