Because powder coatings are difficult to adjust toward
the end of manufacture, the utmost care must be taken
to ensure the batch is correct right from the start.
By Kevin Biller
The purpose of a quality control
is two-fold. First of all, it provides
a check on performance,
ensuring that the product shipped to
your customer meets their requirements,
and secondly, it is a means to
ensure that you are producing a consistent
powder, batch to batch.
In powder coating manufacture,
raw materials are dedicated and subsequently
processed to meet a customer’s
requirement. Unlike paint manufacture
where the batch is processed
and eventually adjusted near the end to
meet quality requirements (i.e., shade,
viscosity, etc.), powder coatings are very
difficult to adjust toward the end of
manufacture. The utmost in care must
be taken to make sure the batch is correct
right from the start. Consequently
quality control really should emphasize
raw material and process control rather
than assessing properties after the batch
is made.
Control of Incoming Raw Materials
This can be accomplished by guarantying
that your raw materials are consistent
and within specification. Reputable
raw material suppliers will gladly furnish
a certificate of analysis (COA) for the
products that they sell to you. The COA
should demonstrate performance for the
critical features of the product. A basic
compendium is listed below.
Resins and Crosslinkers
• Functionality: Acid Value, Hydroxyl
Number or Epoxide Number.
• Melt Viscosity (ASTM 4287)
• Color (ASTM D1544)
• Cleanliness
Pigments
• Color (ASTM D2244)
• Oil Absorption
• % Water
Extender Pigments
• Oil Absorption
• Particle Size
• Color
Additives
8 Non-volatile Content
• Viscosity
• Melt Point or Softening Point
Larger powder manufacturers may
have the luxury, if they choose, to staff
a raw material quality assurance laboratory
that can evaluate the properties of
incoming raw materials. However, most
powder makers must rely upon the conscientiousness
and integrity of their suppliers.
It pays to be aware of how tight
a raw material specification can be and
if you, as a powder producer, can specify
how tightly your supplier can control
their product.
Resins and Crosslinkers
In general, resins and crosslinkers
COAs should report a measure of functionality
of their product. Acid value is
the preferred assessment of the functionality
of a carboxyl polyester or carboxyl
acrylic resin. Accordingly hydroxyl number
is a good indicator of functionality
for hydroxyl polymers. Epoxide value or
epoxide equivalent weight (EEW) is the
best means to represent the functionality
of an epoxide containing resin. Crosslinker
quality needs to be cited specific
to the functional group on the product.
Blocked isocyanates are characterized
by % NCO (isocyanate), whereas TGIC
(triglycidyl isocyanurate) is controlled
by EEW. The quality of amine functional
crosslinkers is measured by amine
equivalent weight whereas phenolic curing
agents are evaluated by phenolic hydroxyl
number.
The molecular weight of resins and
most crosslinkers is related to melt viscosity.
Viscosity measurement is dependent
upon the instruments and
techniques developed by your supplier.
Regardless of specific technique, it entails
assessing the stress caused to a sample
when shear is applied at a temperature
that allows the material to be fluid.
Most common in the industry is the use
of a cone and plate roto-viscometer. The
sample is placed on the heated platen of
the roto-viscometer. The platen is heated
to a prescribed temperature typically between
150 and 200°C. The resistance to
rotation is measured and converted into
a viscosity unit.
The color of a resin or crosslinker is
important for two reasons. First it will affect
the final color of a clear coat or light
colored powder coating, and second it
is an indication of how careful a batch
was made. Most polymers tend to yellow
when oxygen is introduced during polymerization.
This is an indication of lack
of tight process control. Your incoming
resin or crosslinker should be very nearly
water white. Historically resin suppliers
used the Gardner Color scale and
specified a “2” maximum. This equates
to a very slight yellow cast. Improved
process control has compelled suppliers
to graduate to an APHA scale of color.
This scale is much more sensitive than
the Gardner scale and is now employed
by most raw material suppliers.
It is incumbent for a resin manufacturer
to produce clean, bit-free product.
Resin makers can ensure the cleanliness
of their products by filtration as the material
is discharged from their reaction
vessels. You know what size screens your
suppliers use when making their resins.
Some resin manufacturers filter product
through screens as fine as 10 microns.
This provides the cleanest resins
available.
Pigments
Pigments are controlled by their color.
This is typically reported in the coloristic
values of “L” (lightness-darkness),
Manufacturing
powder coatings of
consistent quality
requires diligent
incoming raw
material control
and conscientious
process control.
“a” (red-green), and “b” (yellow-blue).
These aspects are usually stated as a
delta value (i.e., ΔL, Δa and Δb), which
denotes the divergence versus a targeted
value. The oil absorption of a pigment is
also important as it influences the melt
viscosity of the finished powder. It is defined
as how many grams of oil (similar
in nature to resin) can be absorbed by
100 grams of the pigment. Details can be
found in ASTM D285 or ASTM D1483
test methods.
Extender Pigments
Extender pigments are meant to be
incorporated as a means to lower overall
raw material cost while causing no or
minimal impact on other properties. Critical
to an extender’s quality is particle size,
color and oil absorption. The particle size
is usually quoted per sieve analysis. Crucial
to the powder coating manufacturer
is not only the particle size distribution
but also the top end particle size. Large
particles (e.g. greater than 40 microns)
will protrude through the powder finish
if applied at a thin film thickness. Most
extender pigment suppliers will state
% retained on a 325 mesh (44 micron)
sieve. Greater detail is needed to properly
assess an extender’s particle size. It is recommended
to require the median particle
size and the absolute top end particle size.
Oil absorption is also important for the
same reason expressed above.
Additives
Qualifying the suitability and consistency
of additives is very specific to
each material. Viscosity, melt point and/
or softening point are all good places to
start. Some additives are relatively low
in molecular weight and therefore your
supplier should provide a statement of a
minimum of nonvolatile content. The reactive
additives should also be controlled
by some characteristic chemical measurement
related to functionality (e.g., equivalent
weight) or reactivity.
It is important to establish a good
relationship with your raw material suppliers.
The success of your operation and
the quality of your products is contingent
upon the quality of your incoming raw
materials. When choosing a raw material
supplier, economics are certainly crucial,
however value is also measured in the
quality and consistency of their products
and most important the confidence and
trust you have in the way they do business
with you.
Process Control Rather than
Quality Control
After establishing consistent, trusted sources of raw materials
it is critical to institute strict process control of your manufacturing
functions. It is very difficult to “fix” a poorly manufactured
powder coating. After extrusion your powder coating
formula is “locked-in.” The components, their concentrations
and their ratios cannot be adjusted. It is therefore paramount to
ensure that your processes are in control.
The Formula. This starts with guarantying that your formulas
are correct before they are introduced to your plant. It is best to
have an independent party review formulas as they are inputted
into your production system. This should be someone knowledgeable
in formulating, but not necessarily one of your chemists.
The quality control supervisor may fit this role. This review
should make sure that only preferred raw materials are used.
Additionally ratios and concentrations of raw materials should
be scrutinized. Important analysis should include: Are the resin
to crosslinker ratios at or near stoichiometric? Is the pigment
concentration high enough to provide adequate hiding? Are the
additives incorporated at reasonable levels?
Weigh-Up. A well-designed production weigh-up system helps
to minimize errors that can occur during weigh-up. Small quantities
of critical raw materials (i.e., less than 100 grams) should
be weighed on appropriately sensitive balances. These should
be at least accurate to 0.10 grams. On the other hand, large
quantities of less critical raw materials should not be weighed
to inordinately precise levels.Extremely small quantities can be
better incorporated by using a master batch technique. This
entails preparing an “intermediate” comprised of a higher concentration
of additive in a common raw material such a filler
pigment or resin. This allows the manufacturer to weigh a more
easily managed quantity of material.
The weighing of a batch should be endeavored by a welltrained
operator; an individual with a good sense of numbers
and familiarity of the scales used. Scales should be well maintained
and regularly calibrated by an outside party on at least
a quarterly basis. The weighing process should include the reporting
of the batch or lot number of each raw material. The
weigh-up record should have a placeholder for not only the
operator to initial as each material is weighed, but also a space
for another individual to double check the weighing operation.
Premixing. The order of addition of raw
materials into a premixing vessel is an
important aspect of process control. Minor
additions of raw materials should be
“sandwiched” between bulk materials to
ensure best distribution during mixing.
Taking a small sample of premixed material
and melting it on a hot plate allows
you to make a quick qualitative assessment
of viscosity and color.
Extrusion. After premixing is complete
a small amount of material should be
processed through the extruder before
sampling this process. Extruded material
should be collected and made into a
powder in your laboratory. It should then
sprayed to a relevant film thickness onto
suitable substrate and baked according to
conditions representative of your customer’s
process. This extruder check should
be evaluated for color, gloss, mechanical
performance and cleanliness. If the extruder
check passes these tests, then the
extrusion process can recommence. This
extruder check should be made for every
distinct mixture that is introduced to the
extruder. For example, if your mixing operation
utilizes 500 lbs. mixes then every
new 500 lbs. mix should be checked at
the extruder.
Grinding and Classification. Extruded
material is cooled then comminuted into
flake. This flake is subsequently introduced
into the grinding/sifting process
and the finished powder is packaged.
The finished powder should be checked
for particle size distribution and rheology.
It should also be applied to a suitable
substrate at a specified film thickness
and then tested for color, gloss, cleanliness,
and mechanical performance. It
is important to evaluate your powder at
a meaningful film thickness. Too thin a
film build can introduce texture (which
affects gloss) and can create inadequate
hiding (which affects color). Too thick
a coating minimizes texture and can
erroneously provide hiding. Additionally,
film thickness sometimes affects
mechanical performance. Thicker films
tend to be less flexible. The film thickness
specified should correlate to the
minimum expected on your customer’s
finishing line.
Particle size distribution can be
measured by either sieve analysis or
laser particle diffraction techniques.
Sieve analysis involves passing a sample
through sieves of prescribed openings.
Fractions are weighed before and after
sieving. Results are quoted as percent retained
on a given mesh. Analysis should
include measurement of the large particle
size and small particle size components
of a sample. Percent retained on a
90 micron mesh sieve is a good means to
evaluate the concentration of large particles.
This fraction should be minimized
mainly through efficient pulverization
during manufacture. A level of <0.5% is
a good standard for a typical industrial
grade powder coating. Sample retention
on a finer mesh sieve such as a 45 micron
screen provides a good assessment
of the smaller particle size portion of
your material. This fraction should range
from 22% to 28%. Higher levels of fine
particles cause handling and application
problems in the field due to excessive
agglomeration and poor electrostatic
charging.
Particle size measurements using laser
diffraction are much more sophisticated
than sieve analysis. Instruments
using this technique can be quite expensive.
These devices delineate the particle
size distribution into a large number
of narrow fractions. Moreover they can
provide important statistical analysis of
the particle size distribution such as:
particle size average, median, mode, a
variety of percentile data and analysis
of the particle size distribution curve.
These instruments are quite useful when
producing high quality powder coatings
for very demanding applications such as
the automotive industry.
Finished Powder Testing
The specification should embody
critical tests that correspond to the performance
needed by your customer.
Sometimes specification authors include
exhaustive batteries of tests that exceed
any practical correlation to what is needed
by the manufacturer. This is to be
avoided. It is important for you to ascertain
the performance your customers
need, and then set out to create a quality
specification for the coatings. Here are a
few tests to consider.
The three key performance areas to
be regularly ascertained are appearance,
cure and application.
Appearance
Appearance requirements involve
color (both visual and instrumental),
gloss, smoothness and cleanliness. Color
should be measured with an instrument
capable of delineating lightness-darkness
(“L” value), red-green (“a” value)
and blue-yellow (“b” value). Your instrument
should also be capable to compare
one color to another. This comparison is
typically expressed as “ΔE”. “ΔE” is described
as the mathematical relationship
of the square root of (ΔL2 + Δa2 + Δb2).
Color tolerance should be dictated by
your customer’s requirements. Generally
the lighter the color, the lower the
tolerance. The color of whites and pastel
shades should within a ΔE of 1.0 or less.
On the other hand, bright reds, yellow
and oranges can sometimes be visually
acceptable even though they may far
off as measured instrumentally. Additionally,
powders that contain metallic,
pearlescent or bronze pigments are not
easily measured using most conventional
color instrumentation. The color of
these types of products must usually be
assessed visually.
Gloss is simply measured
by an instrument
that projects a beam of
light onto the coating surface.
A sensor located in
the instrument measures
the amount of incident
light reflected off the coated
surface and compares it
to a black glass standard.
The amount of light is reported
as a percentage of
incident light reflected at
a given angle from the perpendicular.
The angle of
illumination has an influence
in comparing surfaces
of differing glosses. Very
high gloss coatings should be compared
at an illumination angle of 20° whereas
low to medium gloss coatings differentiate
better at an illumination angle of 60°.
Cure/Viscosity
The cure of a finished powder can
be measured by means of a simple gelation
test. This test measures the time it
takes for a sample to gel or crosslink at
a prescribed temperature. The temperature
should somewhat coincide with the
recommended baking temperature. The
sample is placed on a heated surface and
manually agitated until it hardens. The
end point is relatively subjective and
has to be fairly well defined in the test
method.
A very simple yet effective melt viscosity
test measures involves measuring
the distance a compressed pellet of powder
travels down an inclined plate at an
elevated temperature. This test known
as the pellet flow or inclined plate flow
test uses a small sample of powder
formed into a pill or pellet. The powder
can be compressed into a pellet using
a commercially available hand press.
The pellet is then placed on a horizontal
preheated surface (can be metal or
glass) and allowed to melt for a minute
or less to provide adhesion to the surface.
The surface is then tilted to a prescribed
angle and the pellet is allowed
to flow down the surface. 65 degrees
from the horizontal is a good angle for
this test. The apparatus is retrieved after
a prescribed duration that should somewhat
coincide with the recommended
bake time and the temperature of the
powder coating. The distance of flow
is measured after the plate has cooled.
Consistently formulated and processed
powders should perform within a
range of +/- 10%.
Mechanical Performance
Powder coated test panels should be
prepared and assessed for mechanical
performance. Quick tests include crosshatch
adhesion, impact resistance and
mandrel bending. Crosshatch adhesion
involves cutting into the cured coating
film with a suitable blade such as a razor
or craft knife. A number of parallel
incisions are made to the metal. These
are then intersected by perpendicular
incisions. Distance between parallel cuts
should be consistent. A distance of 1.0
to 2.0 mm is acceptable. The adhesion
is then evaluated by placing a suitable
strong tape to the crosshatch and rapidly
pulling it away from the coating. Adhesion
to the substrate should 95% or
greater.
Impact resistance involves rapid deformation
caused by dropping a weighted
ball onto the coated surface at a prescribed
film thickness. The diameter of
the ball is critical. The impact resistance
of the coating should be known through
laboratory testing prior to introducing
the formula to production. The results
of this test are quoted as weight times
distance. Inch pounds or centimeter kilograms
are typically the units quoted.
Impact resistance is affected not only
coating type and degree of cure but also
by coating and substrate thickness. Undercured
powders possess inferior impact
resistance. Thicker coatings tend
to be less flexible. Thinner substrate deforms
more than thicker substrate. It is
important to measure impact resistance
at a consistent film thickness and over a
standardized substrate.
Mandrel bending quantifies slow deformation
of a powder coating. This test
can be as simple as manually bending
a coated test panel. Apparatus is available
that will allow the tester to deform
the coated panel in a tightly controlled
orientation. This test can be conducted
with a conical mandrel, which provides
deformation over an ever-increasing
mandrel. The performance is measured
as the smallest diameter bend that the
coating survives. Film thickness and
substrate thickness both affect mandrel
flexibility. Thicker films tend to be less
flexible. Thicker substrates tend to provide
more coating flexibility.
Application Performance
Application performance is dominated
by particle size distribution. A
tightly controlled particle size distribution
will usually yield a powder coating
with sufficient application performance.
Particle size can be measured using laser
diffraction with a sophisticated instrument.
These instruments measure the
diffraction of light as particles are passes
through a laser. The degree of diffraction
coincides with particle diameter. The
particle size distribution is reported in
a series of channels representing small
particle size ranges. Mean, median and
mode are all calculated as are other descriptions
of the distribution (percentiles,
etc.).
Powder fluidity can be measured if
a manufacturer feels this is a necessary
test. Fluidity can be assessed by placing
a suitably large sample of powder (50 to
100 grams) into a lab size fluidization
hopper and measuring the expansion of
the sample after a prescribed amount of
fluidizing air is introduced to the hopper.
Fluidity can also be measured by placing
a sample of powder onto a surface and
measuring how well it flows peripherally.
This is quantified by measuring the
angle of repose of the powder sample.
Good fluidization id earmarked by a low
angle of repose.
Chemical Resistance
The cure of a powder coating can be
measured by evaluating its resistance to
a strong solvent. Usually a solvent rub
test is sufficient. This can be accomplished
by rubbing a saturated soft cloth
or cotton swab against the surface of the
coating. Undercured powders will soften
or solvate. Methyl ethyl ketone or acetone
are commonly used. More exotic
chemical resistance testing is usually
outside the realm of quality control testing
unless specifically required by your
customer.
In summary, manufacturing powder
coatings of consistent quality requires
diligent incoming raw material control
and conscientious process control. Eliminating
problems before they appear is
the mantra of a good powder manufacturer.
The focus of the quality team must
always be consistency in processing, not
finding problems after large amounts of
time and money are committed. By paying
most attention to the front end of the
process you will avoid costly errors and
the generation of non-conforming powder
coatings.
Kevin Biller is technical editor of
Powder Coated Tough and the president
of The Powder Coating Research Group.
He can be reached at:
kevinbiller@yahoo.com.
