While many of us do not ordinarily think of coating as a major industry,
in the same class as aerospace or automobiles, it is, in fact, a hidden giant.
Practically every manufactured product requires a coating, for decorative
and/or protective purposes.
For certain products, the "coating" is most of what you pay for.
Thus photographic emulsions, for example, are applied to the
transparent backing, or "substrate", as viscous liquids, which then dry to form the
photographic film. Various types of thin plastic sheet goods are manufactured
in a similar way. Paper is another sheet good which must be coated
using high-speed machinery.
Long lasting or time-release
pharmaceutical pills use a special coating that is
resistant to the action of stomach acid. Microelectronic fabrication
requires the application of thin layers of liquid gold, because of its
superior electrical conductivity. Architectural coatings, i.e. house paints,
are familiar to all of us. On a typical residential dwelling, the value of the paint
job can be several thousand dollars. Similarly, automotive coating is
a major component of manufacturing cost, adding about $600 per vehicle.
New base coat-clear coat systems produce a beautiful jewel-like appearance
that certainly helps to sell the car, while at the same time providing
effective rust protection. Coating
products are, in fact, the first line defense in the control of rust.
The economic importance of rust alone is amazing: a recent report from
the Brookhaven National Laboratory estimates that 2 per cent of the nation's
gross domestic product, in excess of $100 billion per year, is spent
on the prevention and remediation of rust damage.
While paints and other coatings contribute to the esthetics and utility
of many things, these
benefits often must be weighed against environmental costs. Spray coating operations
currently release pollutant gases into the environment, with resultant potential
health problems for production workers and the community as a whole.
Thus, for example,
the two auto assembly plants in Delaware produce, between them, about 40% of the
state's toxic emissions. This is due, almost entirely, to their painting operations.
About one billion pounds of organic solvents, from liquid coating
operations, are currently being released into the environment by
chemical process and manufacturing industries each year.
Government legislation has made the control of these pollutants an industrial
priority and has mandated drastic reductions in volatile organic solvent (VOC)
content. New formulations produced to meet these requirements have
far more complicated rheological and film formation behavior, and
generally exhibit less good coating performance. Candidate replacement
low-VOC coatings perform less well, however, and are more prone to defects
such as sagging (drip marks), blisters, and "orange-peel." Development of such new
coating products and processes that are economically competitive, use
minimal amounts of material, produce final defect-free coatings in
actual on-line operations, without adverse environmental
impact, is a formidable scientific and engineering challenge.
Until such time as liquid coatings dry and become immobile, their flow behavior needs to be predicted. Basic questions involve the wetting and spreading of thin liquid layers as they flow on geometrically-complex surfaces. At the same time, the important physical properties of the liquid, such as viscosity and surface tension, are continually changing, as it dries. Over the past seven years our research group has built mathematical models for these complicated processes. The resulting set of coupled nonlinear partial differential equations in space and time can then be solved numerically. Improvements in computational hardware and efficient new computer algorithms have made it possible to produce realistic flow simulations on desk-top workstations. Our computer models are continually being extended in order to include more realistic descriptions of important physical and chemical effects. Final coating defects, such as sagging, blisters and ribs, can be visualized, as they develop, in the simulation output, and their origin can be related to rheological, processing, and geometric parameters. Permissible changes in these parameters can then be investigated to identify performance improvements. Theoretical predictions are validated by microscopic and other experimental measurements. Applications include the analysis of spray, curtain, roll, and other industrial coating operations.