Page 205 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.75

applies dry powder to the metal and heats it to fuse and flow into a coating. When the pow-
der is a thermosetting resin, heating continues to complete the cure reaction as well.
Powdered compound is prepared mainly by preblending the ingredients, fusing them in
an extruder or dough-mixer, crushing, and grinding to the desired particle size. For ther-
mosetting powders, heat history must, of course, be controlled to prevent premature cure.
Powders have also been prepared by ball milling and by evaporation of solutions.
The powders are expensive, but automated coating and 100 percent utilization of pow-
der are economical. Coatings are applied to metals for electrical insulation, corrosion re-
sistance, and simple decoration. Thick coatings can be applied in a single pass. There are
three specific powder coating techniques: fluid bed, electrostatic fluid bed, and electro-
static spray.
3.2.4.1 Fluid Bed. The 20 to 200 µm powder is placed in a tank with a porous bottom.
Compressed air is blown up through the porous bottom, into the bed of resin. The bed ex-
pands and rises, and behaves much like an opaque liquid. Vibrating the bed can help.
The metal object to be coated is preheated in an oven and dipped into the fluid bed. The
powder particles stick to the hot surface of the metal and flow into a continuous coat. After
the object is withdrawn from the bed, it may be heated further in an oven to reflow the
coating more uniformly and smoothly and to complete thermosetting cure.
Coating thickness is often 4 to 15 mils or more. Typical applications include electric
motors, capacitors, resistors, bus bars, and transformers; and valves, pumps, and refinery
equipment.
3.2.4.2 Electrostatic Fluid Bed. When charged air is fed up through the fluid bed, the
powder particles become charged as well. When a grounded metal object is passed over
the bed, the particles are attracted, fly up, and stick to it. Then, it is sent to an oven to fuse
and flow the particles into a continuous film and to cure the thermosetting resin.
The leading powder is epoxy resin, applied 1 to 50 mils thick; this is used for corrosion
resistance in auto underbodies, steel rebars for concrete, petroleum industry, appliances,
and citrus food cans. Epoxy-polyester powders are more resistant to yellowing and are
used 1 to 20 mils thick for aluminum wheels and architectural trim. Polyester-polyure-
thane powders are smooth, flexible, and weather resistant; 1 to 3 mil films are used for
auto trim, steel wheels, lighting fixtures, patio furniture, and appliances.
3.2.4.3 Electrostatic Spray. Powder 30 to 120 µm in size is fed from a fluid bed, into a
gun that puts an electric charge on it (high voltage and low amperage for safety), and com-
pressed air fires the charged powder toward the grounded object in a hood. The powder
collects on the object and clings to it; then it is heated in an oven to fuse the powder, flow
it into a continuous film, and cure the thermosetting resin. This applies coatings 1 to 8 mils
thick. The powder overspray is collected in the hood and recycled directly.
A less common version of this process is friction static spray, where the electrostatic
charge is produced by friction between powder particles and the spray gun itself.

3.2.5 Post-Cure
Polymerization and cross-linking of thermosetting plastics is a gradual process. It depends
on reactive groups meeting each other to complete the cure reaction. The further cross-
linking proceeds, the more the molecules are frozen in place, and the harder it is for them
to migrate and meet and react. Thus, complete cure may take very long.
Most thermosetting processes require heating in the process equipment. In a long cure
cycle, this can tie up expensive equipment for a long time, involving high capital invest-

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