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| Corrosion
Resistance - Powder coatings act as a barrier to corrosive chemicals
and moisture that are essential components of the corrosion process.
Chemical
Resistance - Chemicals are not limited to chemical manufacturing
plants. They can be cleaners used around the home and office; lubricating
oils, gasoline and anti freeze used in the garage as well as many
other compounds which may come in contact with a coating during
a manufacturing process or in subsequent end use. All should be
identified during the coating selection process to insure that the
substrate is adequately protected.
Abrasion
Resistance - Powder coatings generally provide outstanding abrasion
resistance. Store and library shelving, home and office furniture
are just a few of the markets where powder's superior abrasion resistance
has been recognized. As with many properties, standard tests, such
as Taber Abrasion, or in-house developed tests can be used to define
the degree of performance required. Your coating supplier, however,
is more likely to have actual data or a close approximation for
industry wide test methods such as those provided by the American
Society for Testing and Materials (ASTM).
Impact
Resistance - Like abrasion resistance and hardness, impact resistance
is a measure of the coating's toughness. Powder coatings are formulated
to withstand blows from hammers and wrenches on an oil rig, stone
damage to lawn mowers and automotive components as well as the everyday
wear and tear of children's toys, furniture and playground equipment.
Mar
Resistance - Unlike abrasion resistance which usually involves a
loss in coating weight due to repetitive motion, mar resistance
is usually associated with a single incident and is frequently described
by a Pencil Hardness test. Other test methods are used as well,
including several on-the-spot varieties such as fingernail mar and
nickel scratch tests. However, both the test method and interpretation
of results tend to be subjective. Some very high gloss and some
low gloss coatings can mar due to reorientation of components on
the surface so it is very important to accurately define the performance
needs and to communicate these to the coating supplier via a reproducible
test method. |
Appearance
characteristics can be equally important to the coater and OEM, providing
brand name type recognition, styling options and quality that can be immediately
seen. These properties are often best defined with a physical standard,
although instrument based test methods have become very reliable for some
characteristics. Powder coatings are available to cover a wide variety
of appearance needs including the following:
| Smooth
coatings which rival liquid finishes are available in a variety
of gloss ranges. Smooth, high gloss coatings can offer high distinctness
of image (DOI), which translates to an illusion of depth usually
achieved by multiple liquid coats. Smooth matt finishes approach
anodized aluminum or black oxide processes in appearance.
Textured
coatings are often employed to hide substrate irregularities that
may "telegraph" to the surface of a smooth finish. They
can also be effective for hiding fingerprints and giving a distinctive
feel to a product. They are also used for their non-slip characteristics.
Wrinkle
finishes are a special class of textures that offer styling variation
and a consistent appearance. As with color, there is no substitute
for a physical standard when defining the desired type of texture.
Coating suppliers often have a library of textures available and
one of these may serve as the standard to describe your needs.
Metallic
coatings, which add sparkle highlights to a base color, are very
popular styling tools. Solid metallic that reproduce the appearance
of the base metal are used as alternatives to plating and add richness.
Hammertones
and Veins give antique or distressed looks which are widely used
on metal furniture.
Clear
coatings provide protection for plumbing fixtures and other hardware.
They also serve as topcoats over metallic basecoats to bring out
the luster and to provide protection for these pigment rich surfaces.
Reduced gloss versions are available to give a satin look.
Color
variety is almost limitless and new special effects are continuously
developed to meet the demands of stylists and designers. Many coating
suppliers offer a pallet of off-the-shelf colors as well as custom
color match services. It is impossible, however, to match an "idea"
due to differences in color perception from person to person. Even
computer generated reflectance or chromaticity data cannot be used
to provide an exact match. There is no suitable substitute for a
good physical standard when it comes to defining color requirements.
Gloss
ranges from flat to high gloss are available in most powder chemistries.
Even though reliable gloss measuring instruments are available,
and a number therefore more easily describes gloss, it may be important
to consider a physical standard in the proper range due to the impact
of gloss on perceived color. For example, a bright, high gloss red
may appear to be off-color when reformulated at a 60 o gloss of
ten, even though the same pigmentation is used. Gloss and color
are inseparable.
Opacity
governs the hiding power of a coating and is directly related to
the type and amount of pigments used. In detailing your needs it
is important to consider the minimum film thickness that will be
required to completely hide the substrate. A test in which the coating
is applied over panels that are half black and half white can be
used to develop a numerical value known as contrast ratio. |
With
weatherability, functional and appearance requirements considered, the
performance characteristics that are affected by the application system
still remain to be defined. The following needs must be effectively communicated
to your coating supplier:
| Cure
Cycle: To provide effective and reproducible performance in nearly
all of the preceding properties, the proper degree of cure is essential.
Cure is best described by the amount of time the substrate is at
a given temperature. Air temperatures, oven set points, total time
in the oven or line speed do not effectively convey this information.
The efficiency of the oven and part mass make it necessary to conduct
a complete oven survey to identify temperature profiles that can
differ from part to part and top to bottom of the oven. With this
information in hand your coating manufacturer should be able to
supply a coating which meets your needs or advise you of any inadequacy
of the oven. For new installations the coating manufacturer can
supply a cure schedule for a preselected coating and this can be
used to define the oven requirements. The type of heat also is an
important factor that must be communicated. Combustion products
from natural or synthetic gas or propane, and the rapid heat up
rate from infrared are just a few examples of conditions which can
impact the coating and thus must be discussed at the earliest time
in the process.
Overbake
Resistance: Excessively high peak metal temperature or extended
time at temperature can cause coating surface degradation which
may initially be noted as gloss reduction or color change. High
temperatures will be apparent in the oven survey while extended
times usually result from line shut down for maintenance, breaks,
etc. Some degree of overbake resistance is generally built into
custom formulated products if the condition is communicated up front.
The best solution however, is to prevent overbake conditions in
the first place.
Intercoat
Adhesion: While the powder coating process is generally very forgiving
some rejects do occur (see above) due to lack of adequate process
controls. These parts are frequently recovered by recoating. In
other cases, clear topcoats or color coats over a primer may require
good intercoat adhesion. Some powder chemistries and some formulation
additives do not lend themselves to recoating, therefore it is essential
that these possibilities or plans are discussed during the coating
selection process.
Particle
Size: Powder particle size plays an important role in the application
process. Size distribution affects no fewer than 10 other properties
critical to consistent performance of the coating system, including:
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Powder Movement in Air |
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Electrostatic Charging |
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Faraday Cage Penetration |
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Deposition and Build Rate |
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It
is important to note that other conditions also influence these properties
and particle size is not the sole determining factor. Also noteworthy
is the fact that the coating process, recovery system, frictional particle
breakdown and in-line sieving systems make particle size a dynamic issue
which must be managed on line. The size distribution produced by the powder
manufacturer must take the coating system design and anticipated changes
into account to provide a suitable product.
| Substrate:
The composition and condition of the substrate must be communicated
to the coating supplier to prevent potential problems. Porous, low
quality castings, for example, tend to out gas during cure resulting
in bubbles and blisters. Depending on the severity, coatings are
available which help to overcome these problems.
Film
Thickness: As mentioned in the discussion of opacity, film thickness
will guide the amount of pigmentation necessary for hiding. Low
film thickness may also require special resins or additives for
smoothness while the same is true in preventing drips and sags from
heavy films. |
Finally,
requirements driven by any subsequent processing subsequent processing
steps must be identified. These include:
| Formability:
Bending, crimping, punching, drilling, etc., all require a high
degree of film flexibility. Some powder chemistries, such as TGIC
polyesters, are readily post-formable mentioned in the discussion
of opacity; film thickness will guide the amount of pigmentation
necessary for hiding. Low film thickness may also require special
resins or additives for smoothness while the same is true in preventing
drips and sags from heavy films.
Printability:
As in the case of recoatability some coatings do not easily accept
over printing. This is frequently due to the excellent solvent resistance
provided by many powder coatings, coupled with the fact that printing
inks are moving toward less aggressive solvents. Decal adhesion
can have similar problems and solutions. The proper combination
of coating and ink or adhesive must be selected. |
In
some instances it will be impossible to have optimum performance for every
property identified and compromise of a sort will become necessary. Realistically,
the formulation characteristics needed to achieve the highest level of
performance with one property may be completely in opposition to the direction
needed to achieve the same level of performance with another. Thus a simple
ranking of properties, in order of importance to you, may help form the
basis of this compromise without sacrificing overall performance.
Powder coatings used on reinforcement bar (rebar) for concrete highways
exemplify the above. These coatings must be extremely tough, corrosion
and chemical resistant, as well as capable of achieving cure very rapidly.
During road construction they are bent beyond a 90-degree angle so the
coating must therefore exhibit good flexibility and adhesion with minimal
substrate preparation. To achieve the rapid cure with high corrosion and
chemical resistance specific resin and cure systems are required. These
generally result in cured films with very high cross link density. This
degree of structure in the polymer usually translates to brittleness and
tends to bind the sites on the molecule that provide adhesion to metal.
On the other hand flexibility and adhesion can be provided by different
chemistries which do not offer adequate cure, corrosion and chemical resistance.
With an understanding that first you must be capable of coating the rebar
and second, it's basic function is to protect the metal from corrosion;
it follows that flexibility must be optimized in a highly cross-linked
system. To do this it is necessary to define the exact degree of flexibility
needed. This will minimize any sacrifice of corrosion resistance. The
fact that the relationship between the two properties is nonlinear forms
the basis for a potential solution. That is, a 50% increase in flexibility
does not require a 50% reduction in corrosion resistance.
With
some order of importance and well-defined critical needs it is possible
to fully satisfy the overall performance objective as in the above example.
The tendency to concentrate too heavily on single properties and loose
sight of the two questions initially posed usually results in an over
engineered or over specified product. This does not provide the degree
of insurance that may be intended. Instead over engineering tends to add
cost and blur the real objective.
With the performance criteria selected it now becomes a matter of defining
test methods and establishing target values. With these tools your powder
supplier can provide samples for laboratory tests or trial runs at an
equipment vendor. The most meaningful results, however, will be achieved
with parts cleaned, coated and cured on the actual line that will be used.
Results from this series of events can be organized into a material specification
developed from proven performance.
Based upon the preceding discussion it should be clear that a great deal
of planning and communication can be invested in selecting the right powder
coating for the job at hand. It is a process that can take minutes to
months depending upon the criticality of the application, the level of
understanding of all parties involved and the status of the coating line.
e.g., an existing line versus an installation in-process versus the planning
stage. Like all good investments, the time spent in making a careful selection
will usually result in dividends down the road. In this case that could
translate to a smooth running, cost effective coating operation as well
as satisfied customers.
Define Quality Requirements
With a material specification and physical coating standard in place it
becomes possible to discuss quality. Quality is a net result of the entire
coating system running effectively. Some film properties such as dielectric
strength are heavily dependent upon the coating material or formulation.
However, all film properties are dependent upon proper application. Table
II attempts to quantify application dependence on a scale of one to ten
and it also lists examples of key process control points that affect each
property.
A handful of characteristics may be used to define the quality of the
coated part. These, in general, have implications far beyond the property
itself and they therefore become good tools to use to evaluate film quality.
They include the following:
| Coverage
- The ability to examine a part and see first hand that all areas
in general, and critical areas specifically, have adequate coverage
is an easy step to achieving the desired protection or appearance.
Although typically done after cure it is not uncommon, especially
in problem solving situations, to examine the powder coated part
before the coating is fused. This may allow a simple blow off and
recoat to recover reject parts. On line visual inspection may be
aided by spotlights to help illuminate recesses, etc. Continuity
testers or pinhole detectors can be used on or off line to check
for voids in the coating.
Off line microscopic evaluation is useful in sensitive areas such
as the evaluation of coating on the sharp edges of an electric motor
armature to insure adequate insulation. Insufficient coverage may
be disguised or not easily detected. However, a well encapsulated
part offers the best protection from the environment. Textures offer
particular challenges because peaks and valleys often mislead thickness
measurements. Textures may have limited flow to "heal"
surface defects. Any pinholes that result could readily become a
site for corrosion.
Thickness
- A thickness gauge offers another easy quality check that can be
done on or off line. Proper thickness is critical to many final
properties. Inadequate film thickness can adversely affect corrosion,
chemical in heat resistance as well as several appearance characteristics.
Excessive film thickness may reduce impact results and post powder
fabrication performance and flexibility as well as overall appearance
characteristics
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Appearance
Defects - Any coated part should look equal to or better than the physical
standard. The presence of excessive orange peel, protrusions or seeds,
fish eye craters, bubbles or blisters, contamination, sag, color or gloss
problems can be readily checked by visual on-line inspection. A variety
of instruments are available for more discriminating or critical needs.
These include:
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profile-o-meters and optical instruments to measure orange
peel or the scattering of light caused by orange peel.
spectrophotometers and calorimeters to measure color difference.
Note that different instruments, measuring parameters and
color space or mathematical models give different results
so it is important to stick to the system used in the specification.
gloss
meters must be specified at 20ƒ , 60ƒ , or 85ƒ . Calibration
of all instruments is very important. |
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Appearance
defects, too, are indicators that other problems may be present in the
system. Craters may indicate insufficient cleaning, flash rusting, oil
in air lines or contamination from some other source.
| Cure
- The importance of complete cure cannot be overstated. It is vital
to most film properties of thermosetting powders. Completeness of
cure is often indirectly determined by optimizing a critical physical
property by way of a time or temperature ladder. In an example,
direct impact on a shelving bracket could be checked after incremental
increases in oven temperature. The maximum impact is achieved at
full cure due to the fact that un-crosslinked resins are relatively
brittle and overcure can degrade flexibility. If the maximum impact
turns out to be 100 inch-pounds that result becomes the target for
full cure. This may even be beyond the impact required in the material
specification since many end products are not subjected to such
force.
Other examples of physical properties used to determine cure are
flexibility (mandrel bend test), adhesion (cross hatch test) and
solvent resistance. A solvent rub test seems to be the most popular,
with the degree of softening or rub through as well as the amount
of coating which transfers to the cotton swab, good indicators of
cure. It is very helpful to standardize this test as much as possible
to achieve meaningful results. Even though 50 double rubs is widely
used in the industry some people may use 50 seconds of rubbing.
The amount of pressure exerted on the swab also adds subjectivity.
Complete cure can also be more directly determined with sophisticated
instruments such as a Differential Scanning Calorimeter that will
detect residual exothermic energy as a result of incomplete cure
in a film sample. All of the above cure tests are destructive in
nature.
Adhesion
- Impact, mandrel bends and cross hatch tests are also used to determine
adhesion. Perhaps this is why solvent rub is so popular for cure.
If properly cured, poor adhesion is a strong indicator of problems
with the substrate. This can often by verified by examining the
underside (facing the substrate) of the removed film. The presence
of dirt, rust or other residue is a sign that cleaning may be insufficient.
Excessive amounts or degraded conversion coating can also result
in an unbound transition layer that interferes with adhesion. Adhesion
problems are a leading cause of corrosive failure.
Other characteristics important to end use or film quality may also
be selected to define quality. In cases where weatherability is
a critical property, test coupons may be cut from raw stamped metal
stock and hung on line for cleaning, coating and cure. These can
act as control pieces for further evaluation, including accelerated
or outdoor weathering. Log sheets which contain test results, operating
parameters and other pertinent information can help assure that
the process is under control. Changes in results then become important
indicators that something is wrong. When this occurs the log sheets
will help to connect cause and effect, thus becoming an important
tool in problem solving and prevention. During system start up logs
can help to define process capability with ranges used to establish
product or process specifications as well as providing feedback
into powder, pretreatment chemical or other supplier specifications |
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