Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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122
GROSSMAN
AND
PATTON
reduction. These oils act as a lubricant for the rolling process. Residual rolling oils remain-
ing on the steel surface after cold rolling can decompose when exposed to the elevated
temperatures in the annealing furnace, resulting in the deposition of carbon compounds
on the surface. The carbon is essentially baked into the surface, making it virtually impossi-
ble
to
remove without mechanical abrasion.
Since it is not really possible for
a
test panel manufacturer
to
remove surface carbon
through typical cleaning methods, it is necessary to purchase steel that is low in surface
carbon. There are effective methods for limiting surface carbon at the mill. It
is
important
to identify steel mills that are willing and able
to
enact such measures to control surface
carbon. An effective specification for steel to be used in the fabrication of test panels
should include a maximum allowable surface carbon content. Various studies have demon-
strated that surface carbon begins to have a detrimental effect on phosphatability and salt
spray resistance when present in amounts greater than
0.4
to 0.6 mg/ft’.
1.3
Surface Preparation
When preparing test panels, it is important
to
standardize not only the material used but
also on the method of surface preparation. There are a number of different surface prepara-
tion methods discussed in ASTM
D
609 and
IS0
1514.
These include alkaline cleaning,
solvent cleaning, vapor degreasing and phosphate conversion coatings.
Any of the preparation methods described in ASTM
D
609 or
IS0
15
14
is suitable
for test panel preparation. The most important consideration should be to institute tight
control
of
all process parameters for the selected preparation method. Parameters requiring
control may include cleaning time. bath temperatures, concentration, rinse water purity,
and spray pressure. Pertinent process parameters should be maintained within narrowly
defined ranges. Regardless of the preparation method selected, the panels should be free
from water break after processing. This can be determined by spraying a film of distilled
or deionized water on the cleaned surface. If the water forms a continuous unbroken film
over the entire surface, without gathering into discrete droplets or other water breaks, this
should be taken as an indication that all organic soils have been adequately removed from
the surface. A white cloth wipe test should also be conducted to indicate whether inorganic
particulate soils have been adequately removed, as the water break test is not sensitive
to
the presence of inorganic soils on the surface.
Tight control of process parameters is especially critical when preparing panels by
applying an iron or zinc phosphate conversion coating. Such characteristics as coating
weight and morphology require close monitoring in order to ensure consistent test panel
performance.
1.4
Applications
ASTM
D
609 specifies three types of steel test panel. The following is
a
description of
each type, along with some information regarding the typical applications.
Type
1
steel panels are produced from steel with a matte finish. This type of finish
is generally representative of steel used for painted surfaces on appliances and general
sheet metal applications. This type
of
surface is suitable for testing coatings systems used
in these applications, along with many automotive and general purpose applications. It is
also suitable for testing zinc and iron phosphate coating systems. However. it is important
to
remember that paint performance on this type of surface can vary significantly if factors
TEST
PANELS
123
such as surface profile, surface carbon, and the method of surface preparation are not
tightly controlled.
Type
2
steel panels have a flat polished surface. This type of finish is not available
from a mill. It is produced by removing the original mill surface in a controlled grinding
or polishing process. The purpose for removing the mill surface is to eliminate the normal
variability between different mill surfaces. The grinding process effectively eliminates
surface carbon and surface profile as variables, simply by removing the mill surface com-
pletely. This type of surface is suitable for applications in which good reproducibility is
critical and variability due
to
surface characteristics cannot be tolerated. One problem
with this type
of
surface is that, while it is highly uniform and reproducible, it is not really
representative of any surface that would be coated in a field application.
Type
3
steel panels have a smooth finish produced by rolling with polished rolls.
This type of finish is most suitable for evaluating color, gloss, distinctness
of
image, and
other appearance properties of coatings, along with flexibility and adhesion. The surface
finish on this type of steel is more easily controlled, resulting in less
lot
to lot variability
in terms of surface profile.
2.0
ALUMINUM PANELS
In many paint testing applications, it is more convenient to use aluminum test panels.
Aluminum is much lighter than steel and is
not
subject to corrosive attack
in
most environ-
ments. Aluminum panels can be prepared from many different alloys, with all kinds of
pretreatment. As it is not possible
to
describe all the different types of aluminum panels,
an effort will be made
to
describe standardized aluminum panels intended for general
purpose testing.
Aluminum panels intended for general applications should be produced from a com-
mon grade of mill finish aluminum sheet which is readily available. Aluminum Association
alloys
3003,
3105,
and
2024
have all been found suitable for test panel production. Stan-
dardized panels need
to
be cleaned in a tightly controlled process
so
that they are free
from oil and particulate soils. Solvent and aqueous cleaning are both acceptable methods.
Regardless of the cleaning method, it is essential to identify critical process parameters,
establish narrow ranges for each, and closely monitor the parameters to ensure they are
always
in
order. The cleaning process should be evaluated by conducting water break and
white cloth wipe tests at predetermined intervals
on
processed panels.
2.1
Surface
Finish
The Aluminum Association defines
mill
finish sheet as “sheet having
a
non-uniform finish
that may vary from sheet to sheet and within a sheet, and may not be entirely free from
stains or
oil.”
The standard
mill
finish produced by a given mill can vary considerably,
in terms of appearance, from the standard mill finish produced by another mill. Some
mills tend
to
roll
a
very shiny, reflective mill finish, while other mills produce
a
duller
finish with lower reflectivity. There can
also
be lot-to-lot variability in the mill finish
produced by a given mill. This surface finish variability can impact the results of some
color and appearance evaluations. To minimize this variability to some degree, aluminum
used in the production of test panels should be purchased only from prequalified mills,
which have been evaluated for surface quality and consistency.
GROSSMAN AND PATTON
2.2
Pretreatment
An untreated aluminum surface will oxidize immediately upon exposure to air or water,
resulting in the formation of a protective surface oxide layer. This oxide layer protects
the aluminum from corrosive attack in most environments. However. it also creates some
problems when the aluminum surface needs to be painted. First
of
all, the oxide layer
tends
to
be nonuniform, and it inhibits adhesive bonding between an applied coating and
the aluminum surface. In addition, the surface oxide
is
soluble in environments where the
pH falls below
4
or above
10.
For these reasons, an untreated aluminum surface is not
suitable for conducting critical performance evaluations, such as adhesion and corrosion
resistance,
of
coatings.
There are a number
of
different types of pretreatment designed
to
improve the
performance of aluminum under paint. Most aluminum pretreatment systems are essen-
tially designed to modify the surface oxide layer in such a manner as to increase its stability
in mildly acidic and alkaline environments and
to
significantly improve the paint bonding
characteristics of the surface. Several different types
of
pretreatment are described in
ASTM
D
1730
Stunclurrl
Practices
for
Preprution
of
Alunzinunz
and
Aluminunz-Alloy
Surfaces
For
Puinting.
It is not necessary
to
discuss each
of
these pretreatment methods
in
detail. It is sufficient to state that, when designing a specification for test panel pretreat-
ment, it is important to identify critical process parameters, establish narrow tolerances for
each, and institute controls to ensure that the process remains within specified tolerances.
Test panels can be supplied pretreated in accordance with any
of
the methods de-
scribed in ASTM
D
1730.
However, many of these methods are specific to certain applica-
tions or alloys. In such cases, it is not practical to use such methods
to
prepare standardized
panels for general purpose testing. On the other hand, chromate conversion coating is
easily the most widely used method to prepare aluminum surfaces for painting. A chromate
conversion coating greatly increases the paint bonding properties and the corrosion resis-
tance of an aluminum surface. Standardized test panels supplied pretreated with a chromate
conversion coating can be used to evaluate various performance characteristics of coatings,
including adhesion and corrosion resistance.
It is, however, absolutely critical that chromated panels intended for perfomlance
testing be produced
in
a tightly controlled process, with close monitoring of all important
process parameters. Variations in the chromate process conditions can result in different
chromate coatings characteristics, which in turn result in inconsistent Performance under
paint. Some of the process parameters requiring close control include the temperature and
concentration of the process baths, the contact time and the conductivity of water used in
rinse and process solutions. It is not sufficient to control these factors within normal
commercial tolerances, as this will most likely not yield the consistency in results, that is
to be expected from a standardized test substrate. For example. ASTM
B
449
Sturzdurd
Specijication
for
Clzroornutes
on
AI~rrnin~rr~
classifies a paint base chromate coating as
having a coating weight of
10-35
mglft’. This is a fairly wide range, and a panel with a
coating weight at the low end of the range can not be expected to demonstrate performance
under paint that is consistent with that of a panel with a coating weight at the higher end
of the range
in
certain types of testing. Therefore when preparing a specification for
chromating standardized test panels, a more narrow coating weight range is required,
15-20 mg/ft’ for example.
TEST
PANELS
125
2.3
Applications
Untreated aluminum panels are suitable for conducting appearance-related measurements.
such as color,
gloss,
and distinctness of image. They can also be used to evaluate pretreat-
ment and coatings systems for aluminum,
as
well as in many general purpose applications.
Aluminum panels supplied with a chromate pretreatment should be used when testing for
adhesion, flexibility, impact resistance, corrosion resistance, and similar properties.
3.0
ZINC COATED STEEL PANELS
Some paint testing applications may require steel panels with a zinc or a zinc alloy coating.
Because there are
so
many varieties of zinc and zinc alloy coatings available, these will
not be discussed
in
detail. Some examples
of
the different types of zinc coatings include
hot dipped zinc-coated steel (galvanized), electrolytically zinc coated (electrogalvanized),
zinc-iron-alloy-coated (galvanneal) and
zinc-5%-aluminum-alloy-coated
(galfan). Each of
these coatings is available
in
a wide variety
of
coating designations, as well as with
differential and one-side only coatings. The following is a set of general guidelines regard-
ing the preparation of zinc and zinc alloy coated steel panels.
When purchasing zinc- or zinc-alloy-coated steel for use in the preparation
of
test
panels. it is important
to
prepare a detailed specification for the metal to be used. Informa-
tion that should be listed
in
the purchase specification includes the type of zinc coating
required, the quality of the metal, the finish requirements, and the requirements for oiling
and chemical treatment. Seemingly minor differences in the zinc coating can significantly
impact the performance of applied coatings.
The metal used in the preparation of zinc-coated steel panels should be completely
free from wet storage stain or white rust. Wet storage stain is a deposit of white or gray
zinc corrosion products appearing on a zinc surface, which has been stored or shipped in
damp or poorly ventilated conditions. The steel mills routinely apply a chromate passivat-
ing treatment to zinc-coated steel surfaces
in
order
to
help minimize this wet storage stain.
This passivating treatment acts
as
a barrier, directly inhibiting the bonding of paints and
other coatings to the zinc surface. Zinc-coated steel. that has received a passivating treat-
ment should not be used for test panel production. It is necessary to special order the
metal without a passivating treatment, as this treatment is standard procedure at the mills.
The passivating treatment can be removed from a surface through abrasion, but this alters
the surface profile significantly.
3.1
Surface Preparation
Zinc-coated steel panels must be cleaned in order to remove any oils and particulate soils
that may interfere with successful coating of the surface. Solvent cleaning and aqueous
alkaline cleaning are both suitable methods for cleaning the panels, as described in ASTM
D
609.
It is important to note, however, that zinc, like aluminum, is subject to corrosive
attack by a strongly alkaline cleaning solution. When an alkaline cleaning process is used
to
prepare zinc coated panels for painting, take care to ensure that the cleaner concentration,
temperature, and contact time are appropriate for a zinc surface. After cleaning, zinc coated
panels should be completely free from water break and from particulate soils.
Zinc-coated steel panels may be supplied treated with a zinc phosphate conversion
coating
to
provide an improved surface for painting. It is more difficult successfully
to
126
GROSSMAN AND PATTON
phosphate a zinc-coated steel surface than it is to phosphate a cold rolled steel surface.
For this reason, close control
of
the zinc phosphate process parameters is especially impor-
tant. A zinc phosphate solution containing nickel and fluoride is most suitable for treating
a zinc-coated steel surface.
4.0
HANDLING AND STORAGE
OF
TEST PANELS
Good painting practice dictates that a test panel should be coated as soon as possible after
preparation for painting. In situations where this is not practical, test panels Inay be cleaned
and pretreated in large batches, and then stored for future use. However, once the panels
have been cleaned and pretreated,
it
is important to handle them in such a manner as
to
avoid recontamination. Precleaned panels should be handled only by the edges and only
with a gloved hand or a clean instrument, such as a pair of tongs.
Another concern is the storage
of
precleaned panels in such a manner that they are
protected from corrosive attack. This
is
especially important when dealing with steel
panels. Steel shipped from the
mill
is generally coated with a rust preventative oil. The
cleaning process removes this protective oil, making the steel panel susceptible to rusting.
If steel panels are to be stored for any period of time, they must be protected with some
type of corrosion inhibitor. The most effective method
of
accomplishing this is to wrap
the steel panels in paper that has been impregnated with a volatile corrosion inhibitor
compound (VCI). Steel panels wrapped in a suitable VC1 paper can normally be stored
for years without rusting. This is because the volatile corrosion inhibiting compounds,
including nitrites and amines. in the paper vaporize and are adsorbed onto the surface of
the steel panel. The presence of these compounds
on
a steel surface has been shown to
have a negative impact on the performance
of
applied coatings in certain types of testing.
In
such cases. the panels should be recleaned prior
to
painting in order to remove the
residual corrosion inhibitors. However, traditional cleaning methods are generally ineffec-
tive. Thorough wiping with a cloth wet with warm distilled water is the best method for
removing the VC1 compound from the surface. Wet abrasion is another effective method,
but this changes the surface profile.
VC1 papers designed for use with zinc-coated panels are also available. Aluminum
panels should be wrapped in plain kraft paper or in a plastic bag to protect them from
dust. They do not require any special packaging to prevent corrosion. It is Sufficient
to
store them
in
a dry, clean area, where they will not be exposed to moisture.
BIBLIOGRAPHY
K.
R.
Buser.
Jound
cf
Cotrtirlgs
Techrlolo,y>I,
54, 689,
29
(1982).
P.
L.
Coduti.
.Effpcts
ofSteel
Proct’ssirzg
on
the
Slrrfoce
Crrrborr
of
Cold-Rolled
Steel.
Inland Research
G.
W.
Grossman.
Oflicinl
Digest.
31, 1688
(1959).
G.
W.
Grossman.
Ptrir~t
Irldustry,
September, (1961
).
B.
S..
Skerry.
W.
J.
Culhane,
D.
T.
Smith and A. Alavi.
Jounrnl
of
Contirrgs
Techolog?.,
60,
788,
Laboratories, Chicago,
IL.
SS
September, (1990).
Part
2
Coating and Processing Techniques
This Page Intentionally Left Blank
15
Wire-Wound Rod Coating
Donald
M.
MacLeod
Industt-y
Tech,
Oklsttmr,
Floricir
1
.O
INTRODUCTION
Wire-wound metering rods have been used for more than
75
years to apply liquids evenly
to flexible materials. They were the first tools used to control coating thickness across
the full width of a moving web. The
1980s
saw a new popularity in rod use because of
improved quality and the industry trend toward shorter converting runs. Wire-wound rods
are used in a wide range
of
applications but find their greatest appeal in the manufacture
of tapes, labels, office products, and flexible packaging. The first rods were made of
ordinary carbon steel, wrapped with music wire. Today’s metering rods use precision-
ground core rods made of stainless steel, tightly wound with polished stainless steel wire at
high speeds, on custom-designed winding machines. The resulting product is
a
laboratory-
quality precision tool that can control coating thicknesses accurately within
0.0001
in.
(0.1
mil).
A
typical wire-wound rod station is shown in Figure
1.
Also called applicator rods, Mayer bars, equalizer bars, coating rods, and doctor
rods, this equipment has found uses in a wide variety
of
production applications, from
the manufacture
of
optical films to wallboard panels. Wire-wound metering rods look
deceptively simple.
A
stainless steel rod
is
wound with a tight spiral of wire, also made
of
stainless steel. The wire can be
so
small that it is almost invisible to the naked eye, or
so
large that the windings look like the coils of a hefty spring. Today, the industry has
standardized on stainless steel rods because they can be used with almost every coating
liquid. Earlier problems with rust and corrosion have been virtually eliminated. Where
abrasive wear is a problem, some converters use chrome plating to prolong the life
of
the
rod because of the hard surface presented by chromium. Chrome has its drawbacks how-
ever: it builds up unevenly at the extreme tops of the wires, changing the shape of the
wires and the resulting coating thickness. Also, if not applied properly, chrome can acquire
pitting marks
or
can flake off, contaminating the coating bath or causing uneven coating.
129
130
MACLEOD
Figure
1
Typical coating station.
Several new products introduced since
1985
have further expanded the market for rods.
Where streaking or rod cleaning is
a
problem, rods with a Teflon surface are available.
Particles that might wedge between stainless steel wires tend to slide through, preventing
buildups and subsequent streaks in the coating. Other users find the Teflon surface easier
to clean. and a few claim increased rod life up to eight times that of stainless steel rods.
Super-coat* rods are conventional rods with a second, smaller wire wound around the
primary wire of
a
normal rod. The shape of the grooves between the wires is altered
drastically, which allows certain coaters to produce controlled coatings more than twice
as thick as with conventional rods (see Fig.
2).
Others users find that for any given coating
thickness, the deeper grooves between the wires are spaced closer together and provide
a smoother finish on the product.
Today's coating machines have wider webs and run at higher speeds, outproducing
earlier equipment many times over. Meanwhile, the market for coated material requires
more and more specialized products, hence more frequent changeovers and setups. Com-
pared with other coating methods, changing coating rods requires little downtime for
l
SUPER-COAT
?-Y
CONVENTIONAL
.
~~
Figure
2
Super-coat
flow
pattern.
*
Product
of
Consler Scientific
Deslgn,
Inc.
WIRE-WOUND ROD
COATING 131
changeover and makes precise control of coating weight without changing formulations
a practical consideration for many coaters.
2.0 HISTORY
In 1905 Charles
W.
Mayer founded the Mayer Coating Machines Company in Rochester,
New York. The firm made equipment for manufacturers of carbon and wax papers, two
new and growing industries. These machines used “equalizer bars” or “doctor rods.”
the forerunners
of
today’s precision metering rods. The rods were made of carbon steel
wound with different sizes of music wire. Mayer was issued a series of patents on his
coating machines, including one in 1912 that covered his equalizer bars. Coaters of carbon
and wax papers found that they could easily change the thickness of their coating by
switching rods,
so
they began ordering rods with different wire sizes. Also, the early rods
tended
to
rust and wear out,
so
Mayer found a ready and growing market for replacement
rods, which became an important part of his business
as
more machines were shipped.
Mayer’s success in the coating machinery business became the target of federal antimono-
poly laws, and he was forced
to
release his designs and patents to others in the 1930s.
New companies sprang up, and their machines used doctor blades and roll coating methods,
in addition to wire-wound rods. Under the federal guidelines, Mayer was not required to
expose his proprietary techniques for making equalizer bars. The Mayer factory continued
to produce these rods on special winding machines, which were developed for this purpose.
As his machinery business declined, rod manufacturing continued to expand.
During this period, Mayer hired Ralph
J.
Consler on a contract basis, as
a
design
engineer to develop new equipment for the still-growing carbon paper industry and the
new “multiform” carbon coating market. Consler and another designer, Bruce DeNeve,
formed DeNeve
&
Consler Company, a partnership in Rochester, in
1940.
3.0
THEORY AND PRINCIPLE
Coating thickness is governed by the cross-sectional area
of
the grooves between the wire
coils
of
the rod, as shown in Figure 3. The geometry of this system creates a wet film
thickness directly proportional to the diameter of the wire used. For example, you can
double the coating thickness by doubling the wire size.
WIRE WOUND ROD
Diameter
of
wire deterrnlnes
amount
of
coating going
thru
this space
of
groove
1
ENLARGED DETAIL
OF
WIRE WOUND ROD
rod
/
substrate
to be coated
Figure
3
Rod
design.