Automotive
The underhood segment of the automotive industry was the first to use
powder coatings to a large extent.
During the early 1970s thin-film epoxy
powders found use as corrosion-resistant coatings on coil springs. These
springs required a coating that provided flexibility, as well as protection,
to the substrate in a hostile environment. Typical processing of coil springs
included cleaning and an ironphosphate pretreatment, after which
about 2 to 4 mils of an epoxy powder
were electrostatically applied. oFinally, a
convection cure of about 300 F for 20
minutes finished the parts. When this
process was successfully implemented,
these parts exhibited excellent field
durability.
If the pretreatment process was
compromised and the cleaning or
phosphatizing steps were ineffective,
the painted part performance would be
greatly diminished. Failures such as
gross adhesion loss and poor corrosion
resistance could occur. In the early
days, powder was positioned in the
market as a very high-quality coating.
Some might have considered it to have
such high quality that the cleaning and
pretreatment was not so crucial to the
finished part’s quality and durability.
Pretreatment of powder-coated parts is
as important to the finished-part
performance as it is for other coatings,
such as electrocoats.
Powder has been used extensively as
a protective finish to provide corrosion
and chemical resistance for many
underhood components. Stamped
steel brackets, tubing and castings of
all kinds are commonly coated. To
achieve optimum adhesion, pretreatments vary according to the type of
metal.
Perhaps one of the more surprising
uses for powder coatings is on engine
blocks and cylinder heads. For the past
five years, OEM engines have had
epoxy powder coatings applied to
cast-iron blocks and heads.
Specialized coating lines must process
these parts to handle the massive
weight of the parts and the curing
ovens used to crosslink the powder.
Even though an engine block can
weigh upwards of 300 pounds and be a
tremendous heat sink, the process is
still very economical to the engine
manufacturer.
Organic powder coatings
have been used extensively
in the automotive
industry
since the early 1970s. This
article will discuss the
growth of powder coatings in
this market from first
application s to
current and
future uses.
The old process of painting an
engine involved sending an assembled
engine through a liquid paint booth.
Much masking had to be done to prevent oversprayed paint from getting
into the intake manifold and the flywheel area. Pretreatment and cleaning
of the engine were difficult at best. The
liquid epoxy coating was a low-bake
cure product that did not have significant crosslink density. Finished-part
performance was not very good for all
of these reasons.
Powder coating these castings offers
many advantages to the engine maker.
The blocks and heads are painted
immediately after the casting process,
which helps keep the parts clean prior
to painting. Also, there is a significant
texture in the raw casting, which
greatly aids the mechanical adhesion
of the powder to the block. For these
two reasons, it is often not necessary
to chemically clean and pretreat these
castings to achieve the desired
corrosion performance.
Certainly, the powder finish has a
much higher quality than the liquid
coatings because of the much higher
molecular weight of the resins used in
the paint. That, coupled with the
crosslink density achieved in the
massive curing ovens, gives outstanding
finished-part durability. The ability to
recover the oversprayed powder during
application also results in a significant
savings over the liquid alternative.
All of the coating work takes place in
transit as the castings move from the
foundry to the engine assembly plant.
This serves to reduce transportation
costs, since the parts must move on
this path anyway. Painted parts arrive
at the engine assembly plant ready to
be machined. The coating has enough
integrity to survive this machining
process. At that point, the blocks and
heads are ready to be assembled. The
in-plant painting process can be
removed completely, eliminating
costly process equipment and reducing VOC emissions and
paint-waste disposal issues.
In modern vehicle design, the engine
compartment has been “tidied up” and
the engine itself has been decorated.
With the prevalence of fuel injection,
cast-aluminum intake manifolds
occupy a prominent place in the
engine compartment. For several
years, powder coatings have been
Paint & Coatings Industry 49
used as decorative and protective
coatings on manifolds and valve
covers. They highlight the casting and
give it a more uniform appearance
than is possible with a raw casting.
The coatings are durable enough to
have the manifold retain its
appearance for many years of field
service. Often, these coatings have a
texture that also hides defects in the
cast substrates.
Interior Trim Parts
While most automotive interior trim
materials are made of some type of
plastic, this is still a large market for
powder coatings. The primary reason
for this is that most truck interior seating requires the use of a steel platform
that raises the seat about 6 to 9 inches
above the floor. The truck and van
market currently represents 50% of the
North American automotive market.
Typically, a truck seat has a cold
rolled steel platform made from
stampings. It might require three or
more stampings to be assembled to
form one platform. These platforms
are currently coated with just a single
coat of powder, which provides
corrosion, chip and mar resistance.
The common requirements for these
finishes are uniform color and
consistent low-gloss appearance.
Typically, there are four colors involved
in a vehicle program: gray, beige, red
and blue. While the shades can vary
from program to program, the gloss is
becoming consistent throughout
the
o
industry,
averaging
about
4
gloss
on
a
60o gloss meter.
Generally speaking, polyester
chemistries have replaced epoxy
chemistry for interior applications. It
might be arguable that seat risers do
not see much UV exposure, but other
applications, such as speaker grilles,
are at or above the beltline and could
see more UV exposure.
Powders are regarded as the optimal
finishing material for interior seating
components. Powders are still limited
from the job of finishing plastic
substrates because the curing
temperatures powders need are above
the distortion temperature of plastic
substrates.
Exterior Trim Parts
Powder coatings have been used for 15
years to finish exterior trim parts for
the automotive market. A real boon to
the use of powders was the
“black out” look. Hang-on
or add-on parts such as
door handles and moldings
needed a finish that was
as durable as the body
coating materials, since
they would be added to the
vehicle in the assembly
plant. Further, these addon parts would need to be
finished at supplier or
custom-coating plants all
over the country. Polyesterurethane powders were
developed in smooth, lowgloss finishes that did two
things well: they met the
appearance requirements
and provided excellent
durability on the finished
part.
The first application
commercialized for automotive exterior trim parts
was a low-gloss, black, 30o
gloss powder for door
handles made of zinc diecastings. A finish was needed that
would be attractive and also cover the
edges and curvature of the casting in
an economical yet durable way.
One of the first specifications in this
market was FBMS l-43, a GM
specification. It called out a specific
process for cleaning, priming and
topcoating zinc castings. Polyesterurethane powders were specified as
the topcoat. For steel and aluminum
moldings, the same powder topcoats
were used, often without the need for
primers. These coatings still are in
widespread use today and represent
the best overall value for appearance,
durability and cost for exterior trim
parts finishing needs.
Currently, black is the dominant
color for powder finishes for exterior
trim parts. Different OEM’s might have
subtly different shades of black, which
vary in their degrees of jetness. They
also differ in the gloss specifications,
which range from 12 to 20 to 30 to 90+.
One of the major issues in this
market is gloss retention after weathering. While polyester-urethane coatings
provide very good overall performance,o
after about 18 to 24 months of 5
South Florida black box exposure,
these coatings lose more than 50% of
their initial gloss.
The FBMS 1-43 specification allowed
the part to be “polished black” to
restore the lost gloss. Today’s
customers want the add-on parts to
have the same durability as the newer
basecoat/clearcoat body finishes,
Customers don’t want to polish the
handles-they want them to be more
durable in the first place. Acrylic
powders have been proposed to offer
better weathering characteristics. This
is a logical progression in the powder
market since automotive basecoat/
clearcoat topcoat systems are in
acrylic chemistries.
The market for acrylic powders has
emerged in the last two years. Where
OEM customers have demanded
better performance, GMA acrylic
powders are now commercially
available for several applications,
These are predominately high-gloss
black powders. This chemistry offers
powders that have exterior weathering
properties comparable to current
BC/CC systems in a one-coat system.
The cost per pound of these materials
is approximately double that of
polyester-urethane powders, which
has caused these powders to be specified only where the increased material
costs are not an issue.
Speculation abounds that an interim
quality powder - something that is a
mid-grade in cost and performance could find a place in the market.
Acrylicurethanes should indeed fill
that gap, they offer increased Florida
gloss retention and are at a mid-point
in cost between polyester-urethane
and GMA acrylic powders. These
formulations are sometimes called
hydroxyl functional acrylics.
Wheels
Powder coatings have found an important home in the aluminum wheel
industry Since the Detroit community
first started using large quantities of
cast aluminum wheels as a design
tool, powder has grown steadily with
the market.
In 1983, most of the manufacturers
of cast aluminum wheels were located
in Southern California. At that time
the market for wheels was almost
exclusively in the
aftermarket. Those
customers took exceptional care of their
cars and were very
particular about their
appearance. The environment in which
these wheels lived was
not very severe - no
real winters, which
meant no road salt
that would accelerate
c o r r o s i o n o f the
wheels. In fact, most
wheels sold to the
aftermarket had no
protective clearcoat
at all. Powder earned
its place in this market
only when Detroit
wanted to buy millions
of wheels, put them
into service all over the
United States and
warrant them to their
customers.
The first coating
system used by
the aluminum wheel
industry as a protective clearcoating
was a solventborne
clear. One can imagine
that the Southern California Air
Resources Board would strictly limit
the use of such a paint in the Los
Angeles area. As Detroit increased
demand for these wheels, the industry
searched for an alternative coating
system to comply with these air quality
regulations.
The next step was to use a waterbased clear. While it was a reduction,
these water-based coatings still
contained VOCs. Despite the
increasing demand for wheel
production, the air quality permits
limited the number of wheels the
manufacturers could finish. Production
was actually restricted because of
these regulations,
One alternative was for manufacturers to purchase credits from an “air
broker.” An air broker obtained VOC
emission rights from companies that
were not using their full allotments.
These rights were transferred to the
purchasing company as a way to
comply with the regulations. This was
obviously not a value-added cost, but
simply a way to meet the market
demand for aluminum wheels. The
answer was to eliminate the root cause
of the problem: VOCs in clearcoating
material.
Clear powders were first used on
OEM aluminum wheels during the
1984 model year. Powders were an
answer to the wheel manufacturers’
dilemma, but there were other
advantages. The powders reduced
rejected wheels because runs and sags
were avoided on complex shaped
wheels. Also, powders could be easily
applied thicker than liquids. This
provided much better edge coverage, a
major concern in corrosion protection.
Metallic-colored powder basecoats
soon followed. Powders were able to
decorate the wheels with attractive
colors while still enhancing their
overall corrosion resistance and
durability. Even today, a powder
basecoat plus a powder clearcoat paint
system is recognized as the most
durable and highest quality paint
finish for aluminum wheels.
The primary quality issue in the
aluminum wheel finishing market today is filiform corrosion resistance.
This phenomenon is the finger-like
white corrosion usually found only on
clear-coated, bright machined surfaces.
This problem has been extensively
studied by the OEMs, and the
emerging trend is that clear acrylic
powders offer better filiform corrosion
performance than the typical polyester
TGIC clear.
5 4
S e p t e m b e r
1 9 9 4
It is further understood that the pretreatment process for the
clear machined surfaces
is critical to successful
performance against
filiform corrosion. The
cost/benefit calculations
have continued in an
attempt to determine
the best fix to this
answer. As in the exterior trim parts market,
the material cost of
switching from polyester
TGIC clear to acrylic can
mean a premium of 80%
or more.
Another major issue in
the market is the
growing trend towards
very bright metallic
wheel basecoat colors.
The aluminum pigments
used to achieve these
colors have been available only in paste form,
which is not readily
adaptable to dry powder
formulations. Therefore,
water-based liquids
have emerged as an
important technology to
achieve these bright
metallic looks.
Two factors might
change this trend. The
first - continuing VOC
reduction regulations happened 10 years ago
in the clearcoat market.
The second is new
technology to produce
these bright appearances in powder.
The end result might also allow
powder to supersede liquid coatings in
this area, as it has in the rest of the
Steel wheels are suffering in
growth due to the popularity of aluminum wheels. Most are supplied
directly to assembly plants with a
weatherable acrylic electrocoat finish,
since wheel covers will protect them
for most of their lives. About the only
other color commonly seen in the
OEM steel wheel market is a silver.
One-coat powders with sufficient
weatherability are on the horizon now.
When perfected, this market will also
discover the many benefits of a powder
topcoat to replace the liquids now in
widespread use.
Bodies
Twenty years ago in the United States,
powder was applied to automobile
bodies on a “production trial” basis.
Ten years ago, powder was used on a
“full production” basis as a powder
primer-surfacer. Today, at least three
automotive assembly plants are using
powder as both a primer-surfacer and a
black-out coating material. Numerous
others are using powder as a chip-resistant coating on rocker panels and
leading edges of sheet metal - areas
most prone to paint chipping.
The use of primer-surfacer seems to
be growing in importance and value
again. The purpose of this layer is to
protect the underlying electrocoat
primer from ultraviolet degradation
and to optimize the chip resistance of
the topcoat paint system. In a case
where this process must be added to
an assembly plant, powder can be a
very economical alternative to liquid
spray primers, due largely to the environmental friendliness of powder. it is
easier to permit an automotive assembly plant to use powder than it is to
allow liquid paint.
The ultimate automotive application
would be to use powder as the final
protective and decorative clearcoating
for the body. The trend of basecoats
followed by clearcoats has become the
state of the art in body finishing. It will
be much easier for powder to become
the substitute for liquid clears than for
the colors. Powder application systems
that can change colors rapidly and
efficiently according to the demands of
the automotive body painting market
have not yet been perfected. Therefore,
powder makes the most sense as a
topcoat, as long as every car is painted
the same color. That is the allure of a
clear powder.
Powder coatings have found a significant place in the automotive coatings
market. If the powder coatings industry
successfully addresses the issues of
appearance and economics, growth
would be sustainable at rates well
above those of other technologies.