Thermosetting
powders are much more widely used than thermoplastics.
They are primarily
composed of relatively high molecular weight
solid resins and a crosslinker.
Thermoset powders
are used for a wide variety of decorative and protective applications.
They are applied by the electrostatic spray process, heated to the necessary
temperature
and cured.
The primary resins
used in the formulation of thermosetting powders
are:
• Epoxy
• Polyester
• Acrylic
These primary resins are used with different crosslinkers to produce a
variety of powder materials. Many crosslinkers, or cure agents, are used
in powder coatings, including amines, anhydrides, melamines, and blocked
or non-blocked isocyanates.
Some materials
also use more than one resin in hybrid formulas.
When a thermoset powder is applied and subjected to heat it will melt,
flow and chemically crosslink to form a finished film.
The chemical
reaction in the cure cycle creates a polymer network that provides excellent
resistance to coating breakdown. A thermoset powder that has cured and
crosslinked will not melt and flow again if subjected to heat a second
time.
Epoxy
Epoxy powders were the first commercially available thermoset materials
and they are the most commonly used of the thermosetting powders today.
They are available
in a wide range of formulations for thick film functional applications
and thin film decorative applications.
They provide excellent toughness, chemical resistance, corrosion resistance
and flexibility.
The primary drawback
with epoxies is that they will chalk when subjected to UV radiation. For
this reason they are rarely used for outdoor applications.
Functional Epoxy Powder Applications
Functional epoxies are commonly used for electrical insulation and
corrosion protection.
Some typical
applications for functional epoxies
are electric motors, alternators and electrical control junction boxes,
taking advantage of its insulating properties.
Because functional
epoxies also have very good chemical resistance, they are used in a variety
of applications where corrosion resistance is required, such as piping,
under the hood automobile components and concrete ”rebar.“
Decorative Epoxy Powder Applications
Epoxies are often used for decorative applications. They can be formulated
to provide a variety of glosses, colors and textures and applied in relatively
thin films of 0.5 to 3 mil, still providing the toughness and durability
of the functional coatings.
Typical applications
include metal office furniture, shelving, interior car parts, and toys.
Epoxy Polyester Hybrids
Epoxy-Polyester ”Hybrids“, like the name implies, combine
epoxy
resins with polyester resins to form a powder with most of the same properties
as epoxies.
Although some
hybrids are less resistant to chemicals and solvents, they are tough,
flexible and competitively priced.
Hybrids are likely to be used in many of the same applications as
epoxies. In addition to the slight improvement in weatherability, the
polyester resin sometimes provides some improvement in charging characteristics
for electrostatic application.
Polyester Powder
Polyester resins are used to formulate urethane polyesters and polyester
triglycidyl isocyanurate (TGIC) materials.
Urethane Polyesters
Urethane cured polyester powders have excellent resistance to outdoor
environments, toughness and very good appearance characteristics at 1
to 2 mil film thickness.
A smooth, thin
film that resists weathering and physical abuse makes the urethane polyesters
a popular finish for high quality products.
It is common to block the crosslinker in urethane polyesters with ecaprolactam.
To begin the crosslinking process, the material must reach
a temperature above the blocking agent threshold. With e-caprolactam,
unblocking occurs at approximately 360 °F (182 °C). Therefore,
temperatures must be higher than 360 °F to start the melt phase of
the cure cycle.
Powders containing e-cap are usually sensitive to films thicker than 3
mils. Thicker films with these urethanes may lose some of their mechanical
properties and they may exhibit outgassing effects due to ecaprolactam
evolution.
They are used for exterior applications such as patio furniture, automotive
wheels and trim, lawnmowers and a wide range of other products requiring
high quality, decorative finishes comparable to wet coatings.
Polyester TGIC
Polyester TGIC coatings use the epoxy functional crosslinker triglycidyl
isocyanurate (TGIC).
TGIC’s have
very good adhesion characteristics, corrosion resistance and exterior
durability. They can typically be cured
at lower temperatures than urethanes and/or have shorter cure cycles.
In the cure cycle, they have good overbake characteristics and
they are less sensitive to outgassing. They also provide good edge
coverage and tough, thick films (3–5 mil/75–100 m) due to
the inherent higher melt viscosity of the non-blocked TGIC crosslinker.
They have equal corrosion resistance to urethane polyesters but they are
somewhat less resistant to chemicals.
Some typical applications are automotive wheels, air conditioners,
lawn furniture, and air conditioner cabinets.
Acrylic Powders
Like the polyesters, acrylics give excellent exterior durability.
Common acrylic-based
materials include urethane acrylics (hydroxyl functional resins), acrylic
hybrids (acid functional resins) and glycidyl methacrylate acrylics (GMA)
(epoxy functional resins).
Urethane acrylics require cure temperatures of 360 °F
(182 °C). Like urethane polyesters, they may exhibit problems with
outgassing at thicker films (3 mils/75 m). They offer excellent thin film
appearance, good chemical resistance and hard films. Flexibility and impact
resistance is usually poor.
GMA acrylics can be cured in less time or lower temperatures
than the urethane acrylics and they also can provide superior weathering
characteristics. They make excellent clear coats over brass or chrome
due to their exceptional clarity.
Like the urethane
acrylics, flexibility is somewhat
limited. They can be applied in films greater than 3 mils (75 m)
without the risk of outgassing that is characteristic with the urethanes.
GMA acrylics are typically not compatible with other resins and are run
in systems that are isolated from other powders.
Acrylic hybrids combine the acrylic resin with an epoxy binder.
They are somewhat
better than a polyester/epoxy hybrid but still not considered acceptable
for outdoor use. The mechanical properties that are characteristic in
epoxies are a benefit of these materials and theyhave much better flexibility
than other acrylics.
Because of their good appearance, tough surface, exceptional weatherability,
and excellent electrostatic application characteristics, acrylics are
frequently used for applications on products that have very high quality
standards.
Appliances, automobiles
and other products that require durability and long life in a harsh environment
are good candidates
for acrylic powder coatings. Typical applications include automotive wheels,
plumbing fixtures and vending machines.
Research has been conducted to determine the suitability of acrylic powder
coating as a clear coat on automotive bodies. While the United States
automobile manufacturers continue to evaluate this application, one European
manufacturer is using it in production.
Powder
Coater's Manual 1/98
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