U.S. patent number 3,770,482 [Application Number 05/197,559] was granted by the patent office on 1973-11-06 for electrostatic coating method of applying multilayer coating.
This patent grant is currently assigned to Beatrice Foods Co.. Invention is credited to John M. Millar, William F. Moran.
United States Patent |
3,770,482 |
Millar , et al. |
November 6, 1973 |
ELECTROSTATIC COATING METHOD OF APPLYING MULTILAYER COATING
Abstract
A process for electrostatically applying a multilayered coating
on a substrate in one operation or step is disclosed, wherein a
mixture of powders of at least two different coating materials is
used as the coating composition, each powder, in the case of
non-conducting powders, differing from the others in dielectric
constant by a factor of at least 0.1, and the powders being of
substantially different specific gravities, with the components
having the lowest dielectric constant value having the lowest
specific gravity value. At least one of the powders will be a
powder of a film-forming non-conductive organic or inorganic
polymer. Upon electrostatically applying a coating of this powdered
composition to a conductive substrate which has a neutral charge or
a charge opposite from that of the coating composition powder
particles, the powders stratify into distinct layers of different
compositions. The powders adhere to the substrate because of
contact or static electrification for a reasonable length of time
and until at least one of the powders can be cured or fused to form
the final coating. Thus, for instance, in only one pass with an
electrostatic spray gun, a protective coating of superimposed
layers of zinc, epoxy, and polyethylene can be applied to a
conductive substrate.
Inventors: |
Millar; John M. (Joppa, MD),
Moran; William F. (Randallstown, MD) |
Assignee: |
Beatrice Foods Co. (Chicago,
IL)
|
Family
ID: |
22729903 |
Appl.
No.: |
05/197,559 |
Filed: |
January 18, 1971 |
Current U.S.
Class: |
427/470; 427/473;
427/486; 428/323; 428/328; 428/413; 428/523 |
Current CPC
Class: |
B05D
7/14 (20130101); B05D 1/06 (20130101); B05D
7/542 (20130101); C23C 24/00 (20130101); Y10T
428/256 (20150115); Y10T 428/25 (20150115); Y10T
428/31511 (20150401); Y10T 428/31938 (20150401) |
Current International
Class: |
B05b 005/02 ();
B44d 001/095 (); B44d 001/12 (); B44d 001/14 () |
Field of
Search: |
;117/17,29,71R,71M,72,21,22,75,DIG.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Martin; William D.
Assistant Examiner: Sofocleous; M.
Claims
We claim:
1. A process for applying a corrosion resistant coating comprising
a plurality of distinct layers on a conductive substrate, said
process comprising electrostatically applying to said substrate a
mixture of at least two discrete powders containing up to about 96
percent by weight of one of said powders, said powders selected
from the group consisting of:
a. mixtures of at least two film-forming non-conductive polymers
selected from the group consisting of organic polymers, inorganic
polymers and mixtures thereof,
the dielectric constants of said polymers differing from one
another by a factor of at least 0.1, the specific gravity of the
polymer having the higher dielectric con-stant being at least 0.1
greater than the specific gravity of the polymer having the lower
dielectric constant;
b. mixtures of at least one film-forming non-conductive polymer
selected from the group consisting of organic polymers, inorganic
polymers and mixtures thereof with at least one conductive
metal,
the specific gravity of said metal being at least three times that
of said polymer;
c. mixtures of at least one film-forming non-conductive polymer
selected from the group consisting of organic polymers, inorganic
polymers and mixtures thereof with at least one conductive
non-metal,
the specific gravity of the conductive non-metal being at least 1.5
times that of said polymer;
d. and mixtures thereof;
the said powders being charged during the electrostatic
application, and said substrate having a neutral charge or a charge
opposite to that of said powders, whereby said powders stratify in
layers on said substrate, and thereafter curing or fusing at least
one of said powders, whereby a coating having a plurality of
different layers is produced on said substrate.
2. The process as claimed in claim 1 wherein said mixture contains
at least one conductive metal and at least one film-forming
non-conductive organic polymer.
3. The process as claimed in claim 2, wherein said conductive metal
has an average particle size of less than about 50 microns, and is
present in said mixture in the amount of about 4 to about 30
percent by weight, and said organic polymer has an average particle
size of about 10 to about 300 microns.
4. The process as claimed in claim 3, wherein said film-forming
organic polymer is a mixture of at least one thermoplastic polymer
and at least one thermosetting polymer.
5. The process as claimed in claim 4 wherein said powder mixture
contains about 5 to about 12 percent by weight of said conductive
metal, about 55 to about 75 percent by weight of said thermosetting
polymer and about 20 to about 40 percent by weight of said
thermoplastic polymer.
6. The process as claimed in claim 5 wherein said thermosetting
polymer is an epoxy polymer, and said thermoplastic polymer is an
ethylene polymer.
7. The process as claimed in claim 6 wherein said conductive metal
is zinc.
8. The process as claimed in claim 6, wherein less than 7 1/2
percent by weight of zinc is in said mixture, based on the total
weight of said mixture.
9. The process as claimed in claim 8 wherein the average particle
size of the zinc is about 4 to about 10 microns.
Description
BACKGROUND OF THE INVENTION
The electrostatic spraying of powders, and the electrostatic
fluidized bed deposition of powders, has been known to the art. The
process of electrostatically spraying powders generally involves
establishing an electrical field, within a spray gun or other
apparatus, which is capable of charging the powder particles. The
charge on the particle directs and causes the particle to deposit
on the desired object, and in many cases a completely uniform
coating is obtained over the entire object, even though the
electrostatic spray gun is directed towards only one face
thereof.
The electrical charge given to a particle during electrostatic
coating may be represented by the following general formula:
q = kE.sub.z a.sup.2
Wherein k is a factor which depends upon the nature and the shape
of the particle, E.sub.z is the electric field in the charging
zone, and a is the average radius of the particle.
The electric charge thus is dependent upon the field intensity
(E.sub.z) and on the surface area (and therefore the radius) of the
particle. The smaller the particle size, the higher the electrical
charge in relation to the particle mass (and the mass is
proportional to a.sup.3). Each charged particle during the
electrostatic coating, e.g., spraying, operation is subjected to an
electrostatic force F = qE, with E being the electric field
existing around such particle at a given moment.
With the electrostatic spraying technique, the powder is charged
and adheres to a heated or an unheated substrate for a period
generally sufficient to permit conveying the coated object to an
oven. A subsequent bake, or curing, process in the oven transforms
the powder into a smooth, uniform coating having desired
characteristics. Some of the main advantages of the electrostatic
spraying process are the fact that no solvents are used, and
therefore no solvent costs are involved and the coating operation
is much safer. Generally, any excess powder can be recovered from
the spray booth and reused, which, together with the fact that very
little overspray is encountered, results in almost negligible
powder loss. In many situations, a coating of appreciable thickness
can be built up in a single operation, as compared to the need in
conventional paint operations, to use several coatings to produce
the same thickness.
The prior art has been unable to apply, through electrostatic
means, a layer of a conductive material, e.g., a conductive metal
such as zinc, as the conductivity of the powder results in a
shorting of the electrostatic apparatus. Therefore, the prior art,
when wishing to apply superimposed layers of various materials
including at least one conductive material, has applied such layers
separately, with a non-electrostatic application means being used
to apply the conductive material.
Besides being unable to apply layers of conductive materials, the
prior electrostatic methods of coating have suffered another
distinct disadvantage in the application of a plurality of coatings
to a substrate, with baking between the coating steps. Such plural
coating operations have generally produced a resulting coating
having a plurality of layers with such poor adhesion between the
different layers that delamination may occur.
The prior art has utilized mixtures of polymeric materials, e.g.,
thermosetting polymers, and certain powdered metals for decorative
effects. For instance, furniture manufacturers frequently
electrostatically spray a mixture of powdered epoxy resin and
powdered, flaked aluminum or bronze, the powder mixture containing
about 2 percent by weight of metal, on furniture. In both cases,
the metal migrates during the baking operation to the surface of
the coating, providing an attractive metallic finish.
French patent 1,261,473 relates to the electrostatic spraying of a
polymer such as a cellulose ester. The patent discloses that
powdered aluminum may be added to the plastic powder to improve the
chargeability thereof. However, the French patent makes no mention
of the amount of aluminum powder added to the cellulose ester or
polyethylene powder, and it is clear that the patentee must be
contemplating relatively small amounts of powdered aluminum, as
very small amounts of the finely powdered aluminum should be
sufficient to change the chargeability of the polymeric powder. In
addition, even relatively low amounts of powdered aluminum, e.g.
0.5 or 1 percent by weight of the total composition, would result
in migration of aluminum to the surface of cellulose ester coatings
during the bake cycle. To avoid this metallic top coat, it is
believed clear that the French patent must be concerned with very
small amounts of powdered aluminum.
DESCRIPTION OF THE INVENTION
This invention relates to a process for electrostatically applying
a multilayer coating to substrates in one step. The coating
comprises a plurality of superimposed, distinct layers of
film-forming materials. These film-forming materials are
electrostatically applied in admixed powder form, with the
electrostatic coating apparatus applying a charge to the powders,
which, when the substrate is charged (the substrate may be
neutral), is opposite the charge of the substrate.
According to the invention, a conductive substrate is
electrostatically coated with a mixture of at least two different
powders, each powder having an average particle size of less than
about 300 microns. At least one of the powders is a powder of a
film-forming non-conductive organic or inorganic polymer. The
entire powder mixture coating composition may consist of different
film-forming non-conductive organic or inorganic polymers, or one
or more components of the coating composition may be a conductive
metal or a conductive non-metal. Preferably, at least one material
in the coating composition is highly conductive. Beause of the
charge differential between the different powders, the powders are
preferentially attracted to the substrate during the electrostatic
coating operation, with the material having the greatest charge
generally being found adjacent the substrate, and the material
having the smallest charge appearing on the outer surface of the
coating. It is extremely difficult to accurately measure particle
charges, but an approximation of the chargeability of a particular
non-conductive material will be furnished by its dielectric
constant.
Two or more powders may be utilized in the coating composition of
this invention, provided that the powders of non-conductive
materials differ from one another in dielectric constant by a
factor of at least 0.1. When mixtures of film-forming
non-conductive organic and/or inorganic polymers are utilized to
produce discrete layers of such polymers, the polymers having the
higher dielectric constant value must have a specific gravity which
is substantially higher, e.g., at least 0.1 higher, than that of
the polymer having the lower dielectric constant value. On the
other hand, when the coating composition is a mixture of said
conductive metals and such non-conductive polymers, the conductive
metal should have a specific gravity which is at least three times,
preferably four times, that of the non-conductive polymer.
Conductive non-metals may also be used in the curing compositions
of this invention, and, when used in admixture with such
non-conductive polymers, should have a specific gravity at least
1.5 times that of the non-conductive polymer.
Although it has not yet been conclusively established, it appears
that the powders used in the coating compositions of this invention
form a triboelectric series, e.g., the powders acquire different
amounts or degrees of electrostatic charge under similar charging
conditions. In the case of dielectric powders, such powders appear
to obey Coehn's Law, wherein powders of higher dielectric constant
values ar more strongly charged than powders of lower dielectric
constant values. In the case of conductive metals and conductive
non-metals, the charging mechanism may be more appropriately
described in terms of conductivity. For instance, it appears that,
when a plurality of conductive metals and/or conductive non-metals
are used in the coating composition of this invention, such
conductive materials should differ from one another, in
conductivity by a factor of at least about 10.sup.4 or thereabouts.
In any event, the powders used in the compositions of this
invention appear to form a series in which the members of such
series may be ranked in an order in which the members become
increasingly electrophilic.
After the powdered coating composition is applied to the substrate,
with the formation of stratified layers of different powders
adhering to the substrate because of the electrostatic charge, the
coated substrate is subjected to a treatment to render the coating
composition powders immobile. Such treatment generally results in
the fusion of at least one of the coating components, e.g., a
thermoplastic polymer, and/or in a chemical treatment or reaction
such as to effect at least a partial cure or conversion of at least
one of the coating components, e.g., a thermosetting polymer.
The coating components must have the above differential in
dielectric constant values, or chargeability, in order to initially
form superimposed layers when applied by electrostatic coating
methods. Thereafter, and in accordance with normal electrostatic
coating procedures, the substrate, with the charged particles of
the coating composition adhering thereto, is placed in a bake oven
until the coating composition is transformed, by curing or fusion,
into an integral coating. During this fusion or curing process, the
material having the highest dielectric constant, which will
generally be deposited in a layer adjacent the substrate, may
migrate through other coating components to the surface of the
coating if of a similar or lower specific gravity than of the upper
layers (those furthest from the substrate). The present invention
does not contemplate substantial migration of coating components
during the fusion or curing process; therefore, it is necessary to
maintain the specific gravities of the coating components within
the aforesaid ranges in order to prevent substantial migration of
one or more coating components.
Normally, the coating compositions of the present invention will
utilize two or three different components, to produce a resulting
two or three layer coating on the substrate. It will, of course, be
realized that one component or one final layer in the coating may
be itself a mixture of two or more specific materials -- e.g., tow
or more thermoplastic polymers having quite similar dielectric
constants and quite similar specific gravities. When three, four,
five or even more distinct coating components are utilized to
produce three, four, five or even more layers in the final coating,
each of the components should differ from the other components by
the differentials set forth above as to dielectric constant, or
chargeability, and specific gravity. When a plurality of
non-conductive organic polymers are utilized, the specific gravity
of each polymer should differ from the specific gravities of the
other polymers by a factor of at least 0.1, preferably by a factor
of 0.2.
The substrate may be of any conductive metal, e.g., iron, steel,
copper, aluminum and the like, or may be a conductive non-metal,
e.g. carbon, or even may be of a non-conductive material, e.g., a
wooden, glass or organic hydrocarbon polymer, which has been
rendered at least partially conductive on at least the surface
thereof, e.g., by the application of a conductive coating thereon.
Such a conductive coating could be, for instance, colloidal
graphite or silver. Such substrates are hereinafter referred to as
"conductive substrates."
In one electrostatic coating operation, e.g., by spraying or in a
fluidized bed, the powder particles will be charged, with the
charge being either positive or negative, depending upon the
equipment utilized and, to some extent, the particular nature of
the powder itself. For instance, it has been found preferable to
impart a positive charge to nylon powders. In any event, the
substrate should be neutral or of a charge opposite to the powder
to insure that the powder particles will adhere to the substrate
until the subsequent heat treatment, backing, fusing or curing
operation is completed. The substrate may be merely grounded, in
some instances, or an opposite charge may be applied thereto. It
will be generally realized, of course, that the greater the
differential in charge between the powder particles and the
substrate, the greater will be the adhesion thereinbetween, and
more material can be applied in a given pass of a spray gun, for
instance, or in a given time of immersion in a fluidized bed. In
any event, the differential between the charges on the powder
particles and on the substrate should be at least sufficient to
allow the particles to adhere to the substrate during normal
handling operations between the electrostatic coating operation and
the bake oven.
One major advantage of the present invention is in the reduction of
atmospheric pollutants and liquid polluting effluents from coating
operations. Previous procedure for producing coatings of different
components resulted in the discharge of appreciable quantities of
polluting materials into the environment, which discharges are
reduced or even eliminated by the present invention.
It is generally necessary to choose a particle size for the powders
which is not too small in order to provide a surface sufficiently
large to receive the electric charge. On the other hand, it is
generally wise not to choose too large a particle diameter for the
powders because such large diameters generally produce coatings
which are not uniformly smooth. The average particle size of the
polymeric materials in the powder admixture will generally be
within the range of 10 to 70 microns, preferably 20 to 50 microns,
and most preferably will average about 35 microns in size for
electrostatic spraying applications. For other types of
electrostatic powder applications, different powder sizes will
accordingly be used, as known to the art. For example, in an
electrostatic fluidized bed, polymer powders may be used having
particle sizes within the range of 10 to 300 microns.
As mentioned above, it is preferred that at least one powder in the
powder admixture be of a highly conductive material. For the highly
conductive material, generally a metal, the powder particle size
will normally be less than 50 microns, preferably less than 20
microns, and more preferably about 4 to 10 microns in size. With a
4 to 5 micron particle size, at least 4 percent by weight of the
metal, e.g. zinc, must be used, or else a discontinuous film of the
metal will be produced on the substrate. When using zinc or similar
metals, and multiple passes for spraying panels or the like, it is
preferred that the zinc comprise no more than about 7 1/2 percent
by weight of the powder admixture, preferably less than 6 percent,
and most preferably about 5 percent by weight of zinc is used.
However, for a single pass coating of a panel or the like, the zinc
concentration may go up as high as 20 or even 30 percent by weight
of the powder admixture.
The present invention most preferably involves a three-component
coating powder system containing from 4 to 30 percent of a metal,
i.e. zinc, 10 - 86 percent of a thermosetting material, i.e. a
thermosetting epoxy, and 10 - 70 percent of a surface layer
material, generally of a thermoplastic nature, e.g. polyethylene or
polypropylene. The preferred ranges for the above components are 5
- 12 percent, 55 - 75 percent and 20 - 40 percent respectively, all
percentages being by weight of the total composition.
The powders are sprayed while suspended in one or more fluids.
Generally, the fluid will be air or other inert gas, but it is
possible to use a non-solvent inert liquid in which the coating
powders are dispersed. The resulting suspension may be sprayed upon
the substrate, and then the non-solvent is removed during the
baking operation.
The process of the present invention produces a final coating upon
the substrate, with the final coating containing at least two
dissimilar superimposed layers. For ease and economy of operation,
it is preferred that only one coating composition be applied, with
stratification occurring between the various components thereof.
When the coating components are applied in only one operation, a
considerable cost savings will result. Even more importantly,
however, is the fact that a decided improvement in the adhesion
between the various coating layers, and, in some cases, between the
substrate and the coating layer adjacent thereto, will generally be
noted.
It is possible, however, to apply one of the coating composition
powders and thereafter apply, before any curing of the first
coating layer, a second coating composition containing, for
instance, a conductive metal. In such a situation, for instance,
wherein a first layer of powdered epoxy is applied to the substrate
in one pass, and thereafter a combination of zinc powder and epoxy
powder is applied over this first layer, without any curing of the
first layer, the metal (zinc) will penetrate through the first
layer to the substrate.
While the simple temperature fuse or cure or baking operation is
generally preferred, various other methods to cure or set organic
polymers, especially thermosetting organic polymers, or other
materials in the coating composition, may be utilized if desired.
For instance, some polyester resins are now being cured
instantaneously through the use of electron beams, as is known to
the art, and the results obtained with the simple baking operation
suggests that the electron beam process may also be used to cure
certain thermosetting polymers. Likewise, the use of organic
polymers which are cured by the action of moisture, for instance,
the moisture cured urethane systems known to the art, is suggested.
Another possibility is using a thermosetting organic polymer, e.g.
an epoxy, which has an undercatalyzed cure system therein, with a
consequential extended pot life. This type of system could function
as a simple type of time cure at room temperatures.
In most coating operations, the thermoplastic and/or thermosetting
polymers in the coating composition will be fused or cured into a
film. However, in certain situations the formation of a film may be
unnecessary and perhaps even undesired. In such situations, it may
be necessary only to fuse, for instance, thermoplastic polymer
particles to one another. In any event, the curing or heat treating
operation to which the coating compositions is subjected after the
electrostatic coating step should convert at least one component of
the coating composition into a form which adheres the coating
composition, after the electrostatic charge is dissipated, on the
substrate.
When a single pass spraying operation is utilized, a heated
substrate can be utilized. However, a heated substrate is not
preferred when multiple spray passes are utilized, as the heat from
the substrate can fuse or cure the coating material to the point
where no further penetration of various components, e.g. zinc, can
be obtained on subsequent passes.
The baking, or curing, temperature may vary widely, depending upon
the specific nature, and particle size, of the powders, as known to
the art. For instance, generally significantly different
temperature conditions will be used for thermosetting polymers as
opposed to thermoplastic polymers. Broadly, the curing temperature
will be from about 140.degree. to 1,500.degree.F, preferably from
200.degree. to 750.degree.F. The time required in the bake oven
will vary, depending upon the particular temperature utilized, and
also depending upon the nature of the powder composition. The
curing temperature may be as short as 10 seconds or even less, and
may be as long as several days or even more, but generally such
longer cure times are not preferred because of slow production
rates and adverse costs caused thereby. Preferably, the cure times
will vary from about 1 minute to about 1 hour. In any event, the
temperature-time relationship should be such as to at least partly
fuse the thermoplastic powders and/or to at least partly activate,
or cure, the thermosetting powders.
As the coating powder moves, under the influence of air pressure,
through and from the electrostatic spray gun, it is charged by
passing through a high voltage, low amperage field. The voltage
applied to the spray coating apparatus to produce such field may
vary widely, although it is generally preferred to utilize as high
a voltage spray is practicably possible. With the Ransburg
electrostatic sp.y gun utilized in the working examples herein, the
applied voltage was 90,000 volts, which is about the maximum that
can be applied with that particular electrostatic coating
equipment. Lower voltages may be used, e.g. 30,000 volts, although
it is generally preferred to use a voltage of at least 60,000
volts. There is no reason why higher voltages cannot be used if the
coating equipment is designed for same. Likewise, the pump and
motor pressures can vary considerably, but it has generally been
found suitable to have these pressures about 10 - 40 lbs per square
inch, preferably 25 - 30 lbs per square inch. Generally, the only
adverse effects noted outside the above ranges will be a slower
coating rate and some reduction in flow and in the finish gloss
appearance of the film.
While the above preferred description has been in terms of using
zinc as one of the powders in the coating composition, and while
zinc powders have been used in the working examples hereinafter, it
is to be understood that other conductive metal powders can be used
in lieu thereof. Among suitable metals are, for instance, iron,
stainless steel, zinc, copper, nickel, tin, chromium, brass,
titanium, zirconium, lead, alloys of these metals, and the like
(e.g. generally ferrous and non-ferrous conductive metals). The
coating composition may also contain conductive non-metals, such as
graphite, carbon fibers (whiskers), or the like. Various
thermoplastic polymers may be utilized, among which may be
mentioned, by way of example, polyethylene and copolymers thereof,
polypropylene and copolymers thereof, vinyl resins, nylon and other
polyamides, acrylic resins, and the like. Among thermosetting
polymers which could be used are powders of polymerizable resins
(generally resins which are heat-activated or which are used in
conjunction with catalysts) such as epoxys, polyurethanes,
polycarbonates, acrylics, crosslinkable vinyl polymers and
copolymers and the like. When various thermoplastic and
thermosetting polymers are utilized, it has been found that the
densities thereof are generally fairly close to one another, so
application conditions suitable for one polymer will generally be
fairly close to those used for another polymer. The coating
composition may also contain inorganic polymers such as silicates,
e.g., alkali metal silicates, siloxanes and boron polymers. In
addition, certain non-conductive metals which can be fused at
relatively low temperatures may also be utilized in the coating
composition.
As mentioned above, a wide variety of materials may be used in the
coating composition of the present invention. However, it is
preferred that at least one film-forming non-conductive organic
polymer, either thermoplastic or thermosetting, be included in the
coating composition, in an amount of at least 10 percent by weight.
It is also preferred that the coating composition contain two or
three components, and the remaining components are preferably
either other non-conductive organic polymers and/or conductive
metals. The coating composition may contain various fillers or
reinforcing agents, such as glass flakes or fibers, or sand or
other fine form of silica, or various other fillers commonly used
in electrostatic spraying operations.
It has unexpectedly been found that aluminum and bronze are not
suitable metal powders for the composition of this invention.
Aluminum or bronze powders, when applied in a composition at a
level of about 2 percent by weight or more and in conjunction with
an organic polymer, will generally form a metallic layer at the
substrate interface. However, upon the subsequent application of
heat, the aluminum or the bronze will migrate to the coating
surface. The exact mechanism of such migration is not now known,
but could be caused by a rapid dissipation of charge, by a leafing
effect, by a density or specific gravity effect, or a combination
of these or other factors. In any event, the present invention does
not contemplate the use of aluminum or bronze powders as the sole
conductive metal powder in the coating compositions of this
invention.
It is possible to blend the coating composition in any desired
sequence, although it has generally been found preferable to
incorporate catalysts, accelerators, and the like into
thermosetting polymers, for instance, before admixing the polymers
with other components such as thermoplastic polymers or metal
powders.
Broadly, the coating composition of the present invention, which
produces a plurality of distinct, super-imposed layers of coating
material on the substrate, may contain one or several conductive
metals or non-metals (as long as the concentration of conductive
materials in the final coating composition is such that the may be
coating apparatus is not shorted out during operation), one or
several thermoplastic polymers, one or several thermosetting
polymers, or mixtures thereof. The coating composition must contain
at least two dissimilar powders, wherein the dissimilar powders
have different dielectric constants or degrees of chargeability.
The dielectric constants of the distinct powders, in the case of
non-conductive polymers, should vary by at least 0.1 and preferably
by at least 0.2. For instance, epoxy resins generally have a
dielectric constant in the neighborhood of 4.0, with polyethylene,
polypropylene and acrylic resins having dielectric constants of
2.3, 2.75 and 2.5, respectively. In any event, there must be a
differential in the charge imparted to the respective powder
particles of dissimilar coating materials for the process of the
present invention to work. As previously mentioned, the powder may
be given either a negative or a positive charge, with the use of a
negative charge generally preferred, with the exception of certain
polymers, e.g. nylon, to which a positive charge will
preferentially be applied, as known to the art.
EXAMPLES
Example I
70 parts by weight of a black epoxy powder, 30 parts by weight of a
clear polyethylene powder, 5 parts by weight of zinc dust and 0.15
parts by weight of colloidal silica were dry blended at room
temperature until a homogenous blend was obtained.
The black epoxy powder (hereinafter sometimes called Black Epoxy
Powder No. 3) had the following composition:
Shell EPON 1004, an % by weight epichlorohydrin- bisphenol A resin
72 Dicyanamide 2 Dow XD -- 3540.03 2 amine accelerator Barium
sulfate (filler) 23 Carbon black 1.8 Monsanto PC 1344, low
molecular weight silicone oil defoamer 0.2
The epoxy powder ingredients were dispersed in a high intensity dry
blender, thereafter extruded at a temperature of 185.degree.-
200.degree.F, and then reduced to a powder in a hammer mill. The
resulting powder had the following particle size analysis:
Less than 37 micron 0.6% 38-44 micron 0.7% 45-74 micron 4.2% 75-150
micron 94.5% 151-300 micron 0.41% Over 301 micron 0.1%
The Shell EPON 1004 had a Durran softening point of 95-105, a
viscosity (in 40 percent solution in Butyl Carbitol) of 4.6 - 6.6
poises, an epoxide equivalent (grams of resin containing one
gram-equivalent of epoxide) of 875-1,025, an epoxide equivalent/100
grams of 0.11, and a hydroxyl equivalent/100 grams of 0.34. The Dow
amine accelerator XD 3540.03 was a free flowing white powder having
a total nitrogen content of 63.6 percent by weight.
The clear polyethylene powder, produced by U.S. Industries under
the trademark "Microthene FN 510" had an average particle size of
12 microns and a density of 0.924. The polyethylene appeared to
agglomerate with the colloidal silica (which had a particle size of
0.2 microns) which seemed to aid in the chargeability of the
polyethylene particles.
The zinc dust (New Jersey Zinc No. 64) was of galvanizing purity
and had an average particle size of 4.8 microns. The zinc dust
contained 95.7% metallic zinc, 4.2% ZnO, 0.04% Pb, 0.04% Cd, and
less than 0.01% Fe. 99.7 percent of the particles passed through a
325 mesh screen.
The above blended powder composition was sprayed, using a Ransburg
Model 322/8446 R-E-P Electrostatic Spray Gun, upon a mild steel
panel (6 by 12 by 1/4 inches) which had been pretreated by shot
blasting to provide a 1 mil profile (roughness). The spraying was
conducted at 78.degree.F and 40% RH. The voltage applied across the
throat of the gun was 90,000 volts and the air pump and the motor
pressures of the spray gun were 30 lbs each. The steel panel was
grounded, and the spray gun was maintained approximately 8 inches
from the panel during spraying. An effort was made to maintain only
single pass conditions of spraying, with the spray time of
approximately 4 seconds, producing an overall coating of about 2
mils on the panel.
Thereafter, the panel was carefully removed from the spray booth
and placed in a bake oven, with an effort made to keep from
disturbing the powder adhering to the panel. Th oven was maintained
at 300.degree.F for 3 minutes and thereafter the temperature was
raised, at a linear rate, for 10 minutes until the oven temperature
was 420.degree.F. At that point, the panel was removed from the
oven and allowed to cool. After cooling, the panel had a generally
flat finish, with an essentially clear coat on top overlying a
black underlayer. Zinc could not be visibly detected on the coating
surface.
The coating was scratched and indented and then examined under a
microscope (40X). Zinc was detected only at the steel-coating
interface. The black epoxy and the clear polyethylene were in
essentially separate layers over the zinc layer, with the
polyethylene layer furthest from the steel panel.
The three component powder coating composition of this example,
which is particularly preferred, is attractive for applications
wherein a protective coating having excellent corrosion resistance
is required. For instance, this coating may be used to coat the
interior of underground oil or gas pipes. The zinc layer produces a
galvanized finish on the interior of the pipe, and the epoxy layer
overlying the zinc serves to protect the zinc from abrasion, as
well as providing an integral coating of high corrosion resistance.
Finally, the polyethylene layer serves as a non-conductor of
electrical currents, preventing or minimizing electrolytic
corrosion. For some applications, the polyethylene layer provides
increased exterior durability, e.g. automobile wheel rims.
Example II
This example was generally similar to Example I, with the exception
that the coating ingredients were applied in two separate spraying
operations, with no intermediate baking.
The Black Epoxy Powder No. 3 (95 parts by weight) and the zinc dust
(5 parts by weight) of Example I were sprayed on a steel panel
under the spraying conditions described in Example I. This
composition was sprayed for 4 seconds, producing a 2.5 mil coating
on the panel. Immediately thereafter, and with no intermediate
baking or heating of the panel, a second coating was applied over
the first coating. The second coating contained 30 parts by weight
of the clear polyethylene powder and 0.15 parts by weight of the
colloidal silica of Example I. The polyethylene composition was
sprayed on the panel for a total of 5 seconds, producing a 2.0 mil
coating.
Thereafter, the coated panel was placed in an oven having an
initial temperature of 300.degree.F. The temperature was increased
at a linear rate for 10 minutes and until the temperature was
420.degree.F, at which time the panel was removed from the oven and
allowed to cool.
In visual appearance, the panel looked identical to the product of
Example I, and a microscopic examination of a scratched and
indented coating also indicated similar results.
Example III
The coating powder used in this example had the following
composition:
White epoxy powder 95 parts by weight Zinc dust 5 parts by
weight
The zinc dust was similar to that used in Example I. The white
epoxy powder (hereinafter sometimes called White Epoxy Powder No.
1) had the following formulation:
Shell EPON 1004 (similar to that of Example I) 54.4% by weight
Dicyanamide 1.3% by weight Amine accelerator (same as Example I)
1.3% by weight Titanium dioxide 43.0% by weight
The above epoxy powder ingredients were dispersed in a high
intensity dry blender until a homogeneous blend was obtained,
extruded at 185.degree. - 200.degree.F, and then reduced to a fine
powder in a hammer mill. The particle size analysis of the
resulting epoxy powder was as follows:
Less than 37 micron 0.47% 38-44 micron 0.78% 45-74 micron 2.22%
75-150 micron 49.59% 151-300 micron 16.81% Over 300 micron
0.13%
The mild steel panel was similar to that of Example I, and the same
electrostatic spray gun and spraying conditions were used, except
the temperature was 75.degree.F and the relative humidity was 42
percent. Immediately after the panel was coated, it was removed
from the spray booth, and placed in an oven at 350.degree.F for 3
minutes. Thereafter, the oven was rapidly heated to 380.degree.F
and the panel was held at this temperature for 10 minutes and then
removed and allowed to cool. The resulting panel appeared similar
in appearance to a panel coated only with White Epoxy Powder No. 1.
No zinc was visible on the surface of the panel when examined under
a microscope at 40X. One edge of the panel was sanded down and an
examination of this edge under a 40X microscope revealed a layer of
zinc at the substrate-coating interface. The total coating was
about 2 mils thick with the zinc layer about 0.2 mil thick. The
coating of this example could be used in a wide variety of coating
apparatus, such as, for instance, as a coating on auto rocker
panels, or other auto components, on tubular furniture, shelving,
tools, etc., e.g., generally for interior uses, on the interior
shell of refrigerators and other household appliances, on off-shore
drilling rigs and other applications in marine use, and the
like.
Example IV
This example was similar to Example III, except the substrate was a
glass panel. The same coating composition was utilized, and the
spray conditions were the same as Example III. The glass panel (6
by 12 inches) was coated with Ransburg's trademarked preparation
"Ransprep," a colloidal silica composition, which made the glass
surface conductive. The glass panel was grounded during the epoxy
operation.
The bake schedule used in this example was the same as that used in
Example III. The panel appeared to be generally similar to the
product produced by Example III. An examination of the film surface
next to the glass revealed the presence of a continuous zinc layer.
The outer surface of the coating appeared to be free of zinc when
viewed under a 40X microscope.
Example V
This example was similar to Example III except that a larger
particle size, and slightly different zinc powder was utilized. The
coating composition was the same as that used in Example III, with
the exception that the zinc powder (New Jersey Zinc No. 444) had an
average particle size of 6.3 microns. 99.3 percent of the zinc
passed a 325 mesh screen. The zinc powder contained 96.0% metallic
zinc, 3.9% ZnO, 0.07% Pb, 0.03% Cd, and less than 0.01% Fe.
The substrate, spray conditions, and bake schedule were the same as
Example III with the exception that the room temperature was
78.degree.F and the relative humidity was 40 percent.
The baked panel had a glossy white appearance, with no zinc visible
on the coated surface when viewed under a 40X microscope. One edge
of the panel was sanded down, and a microscopic examination (40X)
of this edge indicated the presence of a zinc layer at the
steel-coating interface.
Example VI
This example was similar to Example V, except that a higher
concentration of zinc was used. The coating system was of the
following composition:
White epoxy powder No. 1 92.5 parts by weight Zinc powder 7.5 parts
by weight
The zinc powder was the same as used in Example V. The substrate,
spraying conditions, and baking conditions were the same as Example
V.
The coated panel, after cooling, had a glossy white finish with no
appearance of zinc on the coated surface when viewed under a 40X
microscope. One edge of the panel was sanded down and microscopic
examination (40X) of this edge indicated the presence of a zinc
layer at the panel-coating interface.
Example VII
95 parts by weight of a clear epoxy powder and 5 parts by weight of
zinc powder (the same as the zinc powder used in Example I) were
dry blended at room temperature until a homogenous homogeneous was
obtained. The clear epoxy powder (hereinafter sometimes called
Clear Epoxy Powder No. 2) had the following composition:
Shell EPON 1004, an % by weight epichlorhydrin- bisphenol A resin
78.3 Trimellitic dianhydride 11.7 Stannous octoate 1.4 Silica (325
mesh) 8.4 Monsanto PC 1344 defoamer 0.2
The above ingredients, except the stannous octoate, were added to a
pebble mill and ground for 16 hours. The stannous octoate was then
added and the grinding continued for an additional 20 minutes. The
resulting powder, after screening through a 200 mesh screen, had
the following particle size analysis:
Less than 37 micron 0.0% 38-44 micron 1.1% 45-74 98.9% 75-150
micron 0.0% 151-300 micron 0.0% Over 300 micron 0.0%
The substrate was the same as in Example III. The spray conditions
were the same as Example III, with the exception that the room
temperature was 80.degree.F and the relative humidity was
40.degree.F. A simple bake schedule of 10 minutes at 380.degree.F
was used. After the baked coated panel had cooled, no zinc could be
visibly detected in the epoxy layer. The panel had a clear epoxy
film overlying the zinc layer which was next to the metal
substrate. The clear epoxy film was 2 mils in thickness, and the
zinc layer was 0.2 mils thick. The coating was scratched and
indented and then examined under microscope 40X. The zinc layer
next to the steel substrate was clearly visible.
Example VIII
This example relates to a protective coating of polyethylene and
zinc applied to a steel substrate.
5 parts by weight of zinc powder, 100 parts by weight of clear
0.924 density polyethylene powder, and 0.5 parts by weight of
colloidal silica were dry blended at room temperature until a
homogeneous blend was obtained (The zinc powder, the clear
polyethylene powder and the colloidal silica were the same as used
in Example I).
The blended powders were applied to a substrate which was similar
to that used in Example I. The spray conditions were the same as
used in Example I. Immediately after spraying, the coated panel was
baked for 12 minutes in an oven, using an initial temperature of
275.degree.F, with the temperature rising, at a linear rate, to
465.degree.F at the end of the bake cycle. The resulting coating
had a slightly textured surface, with a layer of polyethylene
overlying a layer of zinc which was adjacent the steel surface. The
polyethylene layer was about 5 mils thick and the zinc layer was
about 0.4 mil thick. It is likely that modification of the above
bake schedule would eliminate the textured nature of the coating,
if so desired.
A resulting coating should be useful in a number of applications,
including pipe coating, and the coating of metal furniture,
fencing, and the like.
Comparative Example A
This example is presented to illustrate that the present invention
requires a homogeneous blend of discrete powders.
95 parts by weight of clear epoxy powder No. 2 and 5 parts by
weight of the zinc powder used in Example I were pebble milled for
16 hours. The resulting blended powder appeared to contain
agglomerated material, quite likely because of a substantial
temperature rise during the pebble milling. The powder was
electrostatically sprayed upon a steel substrate which was similar
to the substrate of Example VII. The spraying conditions and the
bake schedule were the same as used in Example VII. The resulting
panel had a 3 mil thick clear coating thereon which contained zinc
particles dispersed regularly throughout the film. There was no
stratification of the coating components and no evidence of a
continuous layer of zinc.
Thus, this example illustrates that the powders of the different
components must be substantially discrete in order to produce a
plurality of coating material layers.
Comparative Examples B and C
These examples are presented to illustrate that the use of a
conductive metal having a specific gravity below the ranges
contemplated by the present invention results in a system wherein
the metal migrates to the outer surface of the coating -- that is,
away from the substrate-coating interface.
Both Examples B and C set forth below involve the coating of
solvent washed mild steel panels 4 .times. 6 .times. 1/8 inches,
with no preheating. The powdered coating composition was sprayed
using a Gema Gun, manufactured by Gema A. G., St. Gallen,
Switzerland, and distributed by Interrad Corporation, Greenwich,
Connecticut. The Gema Gun is basically similar to the Ransburg gun
used in the preceding examples except the charging electrode is
located in the barrel, which is made of plastic. The maximum
applied voltage, 52,000 volts, was used in each comparative
example. The pump and motor pressures were not adjustable on this
equipment. After coating the panels, a simple bake schedule of 10
minutes at 400.degree.F was used.
Comparative Example B
A dry blended mixture of a clear epoxy resin powder of less than 75
microns having the following formulation:
% By Weight Epoxy Resin (Epon 1004) 121.375 Trimellitic Anhydride
16.67 Stannous Octoate 1.97 Low molecular weight silicone oil (same
as Example I) 1.125
was blended with "Rich Gold," an Alcan Metal Products copper/zinc
alloy having a density of 2.8 g/cc. and an average particle size of
30-60 microns, in an amount of 94 percent by weight epoxy to 6
percent by weight alloy. The blended powdered composition was
sprayed using the above-described Gema Gun, upon the mild steel
panels, which were grounded. Spraying was conducted at 80.degree.F
and 50 percent RH. The resulting film was 2.9 mils thick. The
panel, after spraying and before baking, looked as though it had
been dusted with a translucent talc. After baking, the panels had a
gold metallic flake appearance with a metallic gloss.
This experiment was repeated, except the above alloy was replaced
with a corresponding amount of "Palegold 6500," made by United
States Bronze and having a similar particle size and density. A
film thickness of 3.0 mils was obtained, and after baking, the
panel had a flat gold finish having a somewhat antique appearance,
with a moderate gloss.
In both cases, substantial migration of the copper/zinc alloy to
the film surface was noted.
Comparative Example C
A dry blended (blended on a roll rack overnight) powder composition
consisting of 99.5 percent of the Black Epoxy Powder No. 3 of
Example I and 0.5 percent aluminum powder, sold under the trade
identification M224 by Alcoa and having an average particle size of
3-30 microns, a density of 2.7 g/cc and a purity of 97 percent, was
used in this example. The Gema Gun described in Comparative Example
A was used in this experiment, and the powder composition was
applied to a solvent washed steel panel, 4 .times. 6 .times. 1/8
inches, with preheating. The composition was sprayed at
82.degree.-83.degree.F and 55-56 percent RH. After coating and
before baking, a slight indication of the presence of aluminum was
noted on the surface of the panel. After the baking (10 minutes at
400.degree.F), the panel had a smooth finish with more aluminum
being visible on the surface. The film thickness was 2.8 mils.
This example was repeated with 1 percent by weight of aluminum
powder in the blended mixture. Before baking, the panel appeared
similar to that described above. After baking, the panel appeared
to definitely have more aluminum on the surface than the panel
coated with 0.5 percent aluminum powder. It was estimated that
essentially the total amount of the aluminum in the coating
composition was located in the upper surface of the film, which was
3.0 mils thick.
Comparative Example C was repeated, except 2.0 percent aluminum
powder was used in the coating composition. After baking, the film
which was 2.8 mils thick, was completely silver in color and had a
slight roughness. Before baking, the panel appeared similar to
those described above.
Comparative Example C was repeated again, this time with 4.0
percent aluminum powder in the coating composition. A film having a
coating of 3.9 mils was obtained which had an extremely rough
finish. It was estimated that essentially the total amount of
aluminum in the coating composition was on the upper surface of the
film, that is, on the side of the film furthest from the
substrate.
The explanation for the migration of the aluminum and the
copper/zinc alloys in the above comparative examples has not yet
been established, and could be due to a number of factors. In any
event, the present invention does not contemplate using powders in
the coating composition which exhibit such substantial migration
during the baking or curing operation.
The coating compositions of the above examples were applied by
means of an electrostatic spray gun. However, the results obtained
suggest the use of a number of other electrostatic coating methods,
including electrostatic fluidized bed coating and electrostatic
fluidized bed spray coating, could be used. Generally, the process
conditions (particle -- substrate charge differential, coating
powder admixture composition, and the like) will be somewhat
similar to the conditions found useable for the electrostatic spray
gun, as known to the art.
The above examples and the process conditions set forth hereinabove
are based upon the use of a Ransburg electrostatic spray gun, as
described in Example I above. Thus, these conditions are quite
specific for the Ransburg type electrostatic spraying equipment, as
is well known to the art. Naturally, other types of electrostatic
coating equipment may be utilized with corresponding changes, when
required, in the processing conditions, as is well known to those
having ordinary skill in the coating art. For instance, at least
one commercially available electrostatic spray gun has no air pump,
and thus any reference to the Ransburg air pump pressure would be
meaningless if such other equipment were used.
In most uses of the coating method of the present invention, a
final coated article, or product, will have thereon a coating
containing superimposed layers of the components of the initial
powdered coating composition. In other words, it is generally
desired to have all of the materials in the powdered coating
composition mixture appear in one or more layers of the final
coating. However, it may be desirable in some instances to remove
one or more of the components of the coating composition from the
final coating. For instance, it may be desirable to subject the
coated article to a high temperature bake to burn out the organic
polymer therein. Such a step might be used wherein an
electrostatically applied coating of zinc alone is desired (the
zinc alone could not be electrostatically sprayed due to shorting
of the electrostatic equipment, but electrostatic spraying might be
the only practical method of coating a surface which is in a
location difficult to reach).
As mentioned previously, it is exceedingly difficult, if not
impossible, to accurately measure the charge imparted to a polymer
particle during the electrostatic spraying operation, and, in any
event, it is believed that such determination is impractical for
field applications. However, a rough guideline will be the strength
of the electrical field through which the powder particles pass in
operation of the electrostatic coating apparatus, and the charge
imparted to the conductive substrate (it will be realized that the
substrate may merely be grounded, as discussed hereinabove). In
most instances, it will be sufficient to use the voltage settings
described above for operation of the Ransburg type electrostatic
spray gun, or equivalent settings on other types of electrostatic
coating apparatus, with a grounded substrate. With substrate
charging, an equivalent differential between the substrate charge
and the particle charge should be roughly observed. In any event,
the differential between the charge on the substrate and the charge
imparted to the polymer particles should be such that a substantial
portion of the powder particles are attracted to, and deposited
upon, the substrate, and thereafter remain on the substrate for a
period of time necessary to effect the at least partial fusing or
curing of at least one of the coating component materials.
* * * * *