Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
PRETREATMENT PROCESS FOR GALVANIZED PETAL PRIOR
TO ITS COATING W I T H ORGANIC POWDER
BACKGROUND OF THE INVENTION
As background to the invention, a general
discussion of metal preparation for powder coating will be
presented This will lead into a comparison of
conventional petal preparation systems and the system of
the present invention.
One skilled in the art would recognize that
achieving a goal of providing the ultimate corrosion
protection of steel with a powder coating requires that
several steps be satisfied. First, the surface must be
thoroughly cleaned of all dirt, oil, oxidation products
an any other foreign matter. Second, sites to which the
powder can bond must be available on the surface. These
are generally provided by depositing certain crystals on
the surface during the preparation operations Third, the
coating must be a functional type, which, unlike
Decorative coatings, is especially formulated to impart
corrosion resistant properties to the steel when applied
in thicknesses generally less than 0.010 inches (0.25 mm).
'hose expert in the area of powder coatings
recommend that fuzlctional coatings be applied over mild or
"black" steel. Exotic surface preparation systezns for
newlywed steel have been developed. These systems first clean
the steel, then rinse it with a solution which deposits a
microscopic layer of crystalline material, such as zinc
phosphate. The purpose of this microscopic layer is to
passivity the surface against corrosion and to provide
bonding sites for the functional powder. Galvanized steel
is not norznally recommended as the substrate, despite its
superior resistance to corrosion, resulting from the zinc
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cladding, because experience has indicated that the
functional powder does not bond as well to the zinc
cladding as to the properly prepared mild steel.
It has been determined that this incorrect
conclusion was reached because previous investigations
have applied to galvanized steel, surface preparation
systems developed for mild steel and then when the
resulting adhesion of the functional powder coating was
inferior, it was concluded that the galvanized steel was
an undesirable substrate.
An investigation leading to the subject invention
concluded that it was possible to prepare the zinc surface
in such a manner that the adhesion of the functional
powder coating to the zinc cladding is equivalent to the
adhesion achieved to properly prepared mild steel. This
means that it is possible to utilize galvanized steel as
the substrate and benefit from the superior corrosion
resistance of the zinc cladding and still achieve the
dozier outstanding adhesion of the functional powder
coating to the substrate.
A major advantage of the use of galvanized steel
over mild steel as the substrate is realized where the
power coating becomes physically damaged, as can occur,
for example, in shipping, rigging, or installing of
industrial equipment. In this situation, the corrosion
resistance is determined only by the substrate material
and the zinc cladding of the galvanized steel provides
significantly greater corrosion protection than is
possible through the use of a passivating rinse, as
typically used on mild steel.
As further background to the invention, the
presently used eight-stage or eight-step pretreatment
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process of galvanized metal prior to its organic powder
coating will be described. The present invention which is
an improved four-step pretreatment process will then be
described. The eight-stage or eight-step pretreatment
5 process is as follows:
Step 1 - The first step cleans the galvanized metal
zinc substrate surface of any grease or dirt that is
present. The grease and/or dirt is removed using an
alkaline-type cleaning solution with its pi maintained so
10 that it will not attack the zinc.
Step 2 - A water rinse is applied to remove the
alkaline cleaner from the substrate. Due to carry-over of
the alkaline cleaner from step 1, this rinse is a mild
alkaline rinse.
Step 3 - A second water rinse is utilized to remove
any of the mild alkaline residues remaining on the surface
following step 2. Thorough removal of all alkaline
residues is important because the fourth step requires a
delicate acid balance for the zinc phosphate solution. If
20 any alkalinity is left on the substrate, it will affect
the acid balance of the zinc phosphate solution.
Step q - Substrate passivation, through the spraying
ox a zinc phosphate solution on the substrate, is
accomplished in this step, which is the key to the
25 eight-step system. Reaction of this acidic solution with
the zinc substrate results in the formation and deposition
of water insoluble zinc phosphate crystals on the
surface. It is important that this zinc phosphate
solution be maintained at a pi near 3 or a powdery
30 precipitate will be deposited on the substrate. This
precipitate is undesirable, as it will significantly
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reduce coating adhesion.
The purpose of the zinc phosphate crystals is to
passivity the substrate and give irregular molecular sites
to which the powder coating can mechanically bond.
Step 5 - The fifth step is a water rinse which is
necessary to remove excess zinc phosphate and any water
soluble salts (chlorides, sulfates, or nitrates) that may
be on the surface of the substrate. These water soluble
salts have to be removed from the surface or they will
10 reduce adhesion of the coating.
Stop - The sixth step is an acidified rinse using
chromium compounds such as acid. The purpose of
this rinse is to remove the less soluble salts remaining
from the water rinse in step 5. The chromium compound is
15 used to deposit an additional barrier coat and to give the
substrate some added corrosion protection. The chrome
also fills some of the pores which exist in the zinc
phosphate crystal film, thereby enhancing the passivation
of the metal while providing additional molecular bonding
20 sites for the powder coating.
Stew - The seventh step of the pretreatment process
is a water rinse whose purpose is to remove any foreign
salts or minerals.
Step - The last step involves thoroughly drying the
I galvanized metal by the application of heat.
It should also be mentioned that in the prior art
and in previous pretreatment processes for galvanized
metal there is a so-called six-stage or six-step system.
Iris system is similar to the eight-step system except
30 that steps 3 and 7, which are respectively -the two water
rinse steps, are eliminated.
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DESCRIPTION OF THE INVENTION
This invention comprises a pretreatment process,
particularly a four-step pretreatment process for
galvanized metal prior to its coating with an organic
powder. This pretreatment process was specifically
developed for application with a galvanized metal
substrate, which does not require passivation to assure
protection of the steel from corrosion.
It is an object of this invention to provide a
pretreatment process for galvanized metal so that when
coated with a functional organic powder, the adhesion of
said coating will be superior to that possible with
conventional pretreatment systems.
It is another object of this invention to provide
a powder coating system for galvanized metal whereby the
metallic zinc layer on the steel is utilized directly to
provide sacrificial corrosion protection for the metal
rather than utilize less effective chemical rinses, such
as a zinc phosphate rinse, to deposit a corrosion
protective layer on the surface.
It is another object of this invention to provide
a fo~lr-step process for pretreating galvanized metal prior
to organic powder coating of said metal which process
results in savings in material time, and labor when
compared to the prior art pretreatment processes.
t is another object of this invention to provide
a pretreatment process for galvanized metal prior to the
organic powder coating of said metal which uses no chrome
compounds and which thus eliminates the problems of
hazardous wastes.
It is a further object of this invention to
provide a pretreatment system for galvanized metal which
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deposits minimal zinc phosphate on the metal (unlike other
systems which deposit significant amounts of zinc
phosphate on the metal) thus insuring better powder
coating adhesion to the metal. It has been found that
5 adhesion of the organic powder coating on galvanized metal
with a minimum of zinc phosphate thereon is superior to
that achieved with the prior pretreatment systems. It has
been concluded that by immediately organic powder coating
the metal, the passivating effect of the zinc phosphate
lo crystals is not required.
The four-step system of applicants' invention
follows:
Step l - The first step in the four-step pretreatment
system of this invention is a cleaning and phosphoric acid
15 etching. Grease and dirt are removed herein while at the
same time the galvanized metal is etched by the phosphoric
acid, resulting in small amounts of zinc phosphate
crystals randomly distributed over the substrate surface.
- I've second step is a water rinse, which is
20 applied to remove excess cleaning and etching solution
from step l, plus any salts (chlorides, nitrates,
sulfates, and the like) that may be on the surface of the
substrate. Due to acid carry-over from step 1, this rinse
is fly mild acid rinse.
Step 3 - The third step is a water rinse which is
utilized to fully remove any of the mild acid, salt, or
mineral residues which remain on the surface following
step 2. This step requires less time or equipment if the
water it heated.
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The last step involves thoroughly drying the
galvanized metal by the application of heat.
The following is a more detailed description of each
of the above steps, including a detailed description of
the methods for carrying out the steps.
The first step in the preparation of the
galvanized metal for coating involves cleaning and acid
etching to assure a clean and oxide-free surface. This is
accomplished through the use of a typical biodegradable
solution (liquid acid solution) containing phosphoric
acid, solvents and surfactants and which is obtainable
commercially in various formulations from, for example,
Oakite Products, Inc., in Berkeley Heights, New Jersey.
It is formulated for removing light grease, shop dirt,
welding fluxes, oxides, and mill scale from the
galvanized metal and in general microscopically etches the
metal prior to organic powder coating. Since the
cleansing and etching agent contains phosphoric acid, a
small amount of zinc phosphate crystals, resulting from
the semiweekly reaction of the phosphoric acid with the
zinc, are randomly distributed over the surface substrate
of the metal. Typically a 3-20~ solution by volume of
this acid compound at 110-150 is used. The exact
temperature and concentration must be adjusted to achieve
thorough cleaning with minimal attack of the zinc. The
solution is usually contained in a vat which is large
enough so that the entire piece of galvanized metal can be
dipped therein. It has been found that the minimum
dipping time in this solution is 1 1/2 to 3 1/2 minutes
with more time needed if the galvanized metal is heavily
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contaminated with dirt or oil or has been treated with
special rinses by the metal vendor. Sometimes, instead of
dipping, the cleaning and etching solution is sprayed onto
the galvanized metal.
The second step involves a water rinse which is
required to remove excess cleaning agent from step 1 plus
any salts, such as chlorides, nitrates or sulfates which
may be on the surface of the substrate. Typically, tap
water is used, but in areas where it contains high mineral
levels, deionized water may be more suitable. Water used
in this step becomes slightly contaminated with the acidic
cleansing agent used in step 3 as a result of carry-over
of the cleansing agent on the surface of the galvanized
steel.
lo Step 3 also involves a water rinse. As stated
previously, this water rinse is required to assure that
all of the dilute cleaning agent, plus any salts and
minerals remaining on the surface following step 2, are
removed.
In step 4, the treated galvanized metal must be
dried prior to its coating with the organic powder. This
drying is usually done by inserting the pretreated and
rinsed galvanized metal into an oven at 130-400F for
approximately 2 to 10 minutes so that it thoroughly
Aries. preferred typical condition would be inserting
the pretreated and galvanized metal in the oven at 250F
for approximately 5 minutes.
Upon completion of the four-step pretreatment
process, the galvanized metal must be powder coated within
a short period of time or at least before any amount of
zinc oxide develops on the galvanized metal as a result of
exposure to the atmosphere of other oxidizing agents.
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This is so because unlike conventional pretreatment
systems, no passivation coating is deposited in the
instant invention. Ideally, then the galvanized metal
should be powder coated immediately after it has been
5 dried. As a practical matter, however, due to equipment
limitations and location, manpower requirements, moving
needs, and the like, it has been found that the time lag
between drying (step 4) and powder coating of the
galvanized metal can range from about 5 minutes to 6 hours
lo with an average time lag of about lo minutes. Although 6
hours has been stated as an upper limit, this is an
approximation, since it has been determined that if 2 or 3
days elapse between dry-off (step 4 of this invention) and
powder coating, the adhesion of the coating will not be as
15 good as if the time lag were 5 minutes to 6 hours. Impact
of lag time on coating adhesion will vary with exposure of
the galvanized metal to temperature and humidity
conditions (i.e. inside a building storage or outside
storage and the like).
The coating to be used is a typical organic
powder such as, for example, epoxy, polyester, acrylics,
or hybrids which are mixtures of polyester and epoxy
powders Such products which have been used with great
success in the instant invention are epoxy coating powders
25 such as Pulvalure~ , Scotchkote~ , Corbel , or
Vodka , which are manufactured by and available from
various suppliers. These typical epoxy coating powders
are homogenize, melt-mixed, 100% solids designed for
application to metals.
I The organic powder coating is applied typically
by electrostatic spray although it can also be applied by
dipping the treated, rinsed, and dried galvanized metal
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into a fluidized bed of this powder. When applied by an
electrostatic spray, the powder issues from a gun which
imparts it with an electrically positive charge. Since
the galvanized metal is negatively charged, the powder is
attracted and caused to adhere to the metal The
particular thickness of the coating is not critical,
although a typical thickness which has been found to be
ideal is .004 inches (0.1) millimeters + .001 inches
(.0254 millimeters). This is the approximate thickness
which is deposited on the pretreated, rinsed, and dried
galvanized metal before putting it in an oven for curing.
Proper curing involves baking the powder coated metal in
an oven at approximately 300-550F for about 1-20
minutes Temperatures and curing times are usually given
by the manufacturer of the organic powder used.
IIaving thus described the invention with
particular reference to the preferred forms thereof, it
will be obvious to those skilled in the art to which the
invention pertains, after understanding the invention,
that various changes and modifications may be therein
without departing from the spirit and scope of the
invention and defined by the claims appended hereto.
