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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to thermosetting powder coating compositions and
in particular to compositions containing epoxy resins.
2. Description of the Prior Art
Epoxy resin powder coating compositions are well known, as shown by U.S.
Pat. No. 3,028,251 issued Apr. 3, 1962 to Nagel; U.S. Pat. Nos. 3,338,863
and 3,645,960 issued Aug. 29, 1967 and Feb. 29, 1972, respectively, to
Haag. However, these powder coating compositions do not meet the approval
of the Food and Drug Administration (FDA) for food contact use, such as
for protective coatings for food and beverage containers. Epoxy powder
compositions of Pregmon U.S. Pat. No. 3,882,064 issued May 6, 1975 have
FDA approval for food contact use but are not adequate for use in steel
containers for highly acidic foods such as tomato products or juices.
Therefore, there remains a need for a powder coating composition which is
approved for food contact and which is adequate for use with acidic foods.
SUMMARY OF THE INVENTION
The thermosetting powder coating composition of this invention consists
essentially of finely divided particles at least 90 percent by weight of
which have a maximum dimension not exceeding 100 microns. The particles
are a blend of a composition of:
(A) 12.50-72.95% by weight of an epoxy resin of the formula
##STR1##
where n is sufficiently large to provide a resin with a Gardner Holdt
Viscosity of L-U measured at 40% polymer solids in diethylene glycol
n-butyl ether at 25.degree. C. and having an epoxide equivalent weight of
600-1000;
(B) 20-50% by weight of an epoxy/epoxy novolac resin which is an epoxy
resin of the formula of (A) modified with an epoxy novolac resin of the
formula
##STR2##
where n of the epoxy resin and m of the epoxy novolac resin have values
sufficiently large to provide an epoxy/epoxy novolac resin having a
Gardner Holdt Viscosity of L-U measured as in (A) and having an epoxide
equivalent weight of 400-900;
(C) 5-30% by weight of a second epoxy/epoxy novolac resin of the formula of
(B) but having values for n and m sufficiently large to provide an
epoxy/epoxy novolac resin with a Gardner Holdt Viscosity of V-Z.sub.1
measured as in (A) and having an epoxide equivalent weight of 600-1000;
(D) 0.05-0.5% by weight of a catalyst of tri(dimethyl aminomethyl) phenol
or a fatty acid salt thereof; and
(E) 2.0-7.0% by weight of dicyandiamide.
Optionally, the composition can contain 45-86% by weight, based on the
weight of (A) plus (B) plus (C), of polyvinylidine fluoride in the form of
finely divided particles at least 80% by number of which have a maximum
dimension not exceeding 5 microns.
DETAILED DESCRIPTION OF THE INVENTION
The coating composition of this invention has a powder particle size such
that at least 90 percent by weight of the particles have a maximum
dimension not exceeding 100 microns. Preferably, the particles of binder
component, designated herein to be the epoxy and epoxy/epoxy novolac
resin, are 10-74 microns in maximum dimension and more preferably 20-55
microns.
About 12.5-72.9% by weight of the powder coating composition is an epoxy
resin of the epichlorohydrinbisphenol-A type of the formula
##STR3##
where n is sufficiently large to provide a resin with a Gardner Holdt
Viscosity of L-U and having an epoxide equivalent weight of 600-1000. As
used in this specification, Gardner Holdt Viscosity is based on
measurements taken at 40% polymer solids in diethylene glycol n-butyl
ether at 25.degree. C. The epoxide equivalent weight is the weight in
grams of resin that contains one gram equivalent of epoxide.
In one preferred resin which forms a high-quality coating in combination
with the other constituents of the present invention, the Gardner Holdt
Viscosity is N-R and the epoxide equivalent weight is 700-800.
About 20-50% by weight of an epoxy/epoxy novolac resin is used in the
composition. Such a resin is an epichlorohydrin-bisphenol-A resin of the
formula shown above modified with an epoxy novolac resin of the formula
##STR4##
where n of the epoxy resin and m of the epoxy novolac resin have values
sufficiently large to provide an epoxy/epoxy novolac resin having a
Gardner Holdt Viscosity of L-U and an epoxide equivalent weight of
400-900.
Epoxy/epoxy novolac resins are formed by modifying an
epichlorohydrin-bisphenol-A resin with an epoxy novolac resin of the type
shown in U.S. Pat. No. 2,685,885. It is believed that the modification is
either direct crosslinking of the two resins or indirect crosslinking
through reaction of both resins with bisphenol-A.
By varying the ratio of epichlorohydrin-bisphenol-A epoxy to epoxy novolac,
the viscosity and epoxide equivalent weight can be varied and epoxy/epoxy
novolac resins of different reactivity can be provided.
About 5-30% by weight of a second epoxy/epoxy novolac resin is used in the
composition. This resin has the same general formula as the above
epoxy/epoxy novolac but has a Gardner Holdt Viscosity of V-Z.sub.1 and an
epoxide equivalent weight of 600-1000.
A curing catalyst in the amount of 0.05-0.50% by weight is used in the
composition. Preferably, about 0.01-0.4% by weight of the catalyst is
used. The catalyst is tri(dimethyl aminomethyl)phenol or its fatty acid
salts. Typical fatty acid salts that can be used are tri(dimethyl
aminomethyl)phenol caprate, tri(dimethyl aminomethyl) caprylate,
tri(dimethyl aminomethyl)phenol isodecanoate, tri(dimethyl
aminomethyl)phenol linoleate, tri(dimethyl aminomethyl)phenol naphthenate,
tri(dimethyl aminomethyl) phenol neodecanoate, tri(dimethyl
aminomethyl)phenol octoate, tri(dimethyl aminomethyl)phenol 2-ethyl
hexoate, tri(dimethyl aminomethyl)phenol oleate, tri(dimethyl
aminomethyl)phenol palmitate, tri(dimethyl aminomethyl)phenol resinate,
tri(dimethyl aminomethyl)phenol resinoleate, tri(dimethyl
aminomethyl)phenol soyate, tri(dimethyl aminomethyl)phenol stearate,
tri(dimethyl aminomethyl)phenol tallate and the like.
One preferred catalyst which forms a high quality product is
2,4,6-tri(dimethyl aminomethyl)phenol.
The composition contains 2.0-7.0% by weight of dicyandiamide curing agent.
To improve the handling characteristics of the dicyandiamide, the compound
can be blended with finely divided silica. For example, a blend in a ratio
of 95/5 of dicyandiamide to silica can be used.
Flow control agents can be added to the powder coating composition of this
invention in amounts of about 0.01-1.0% by weight. Typical flow control
agents are polyacrylates, such as poly-2-ethylhexyl acrylate, finely
divided ethyl cellulose, and siloxanes, such as dimethyl polysiloxanes or
methyl phenyl polysiloxane.
To improve the handling properties of the powder coating composition and to
prevent caking, finely divided silica in the amounts of 0.05-0.5% by
weight can be added. The silica can be blended with any of the epoxy
resins or as mentioned before with the dicyandiamide. The silica is
preferably fumed silica. This is a very fine, amorphous silica formed from
silica spheres having an average diameter of 7-14 milimicrons. A suitable
substance for this purpose is commercially available under the trademark
Cab-O-Sil.RTM. registered to Cabot Corporation.
One particularly preferred composition that forms a high quality interior
coating for steel containers is of
(A) 33.6-55.9% by weight of an epoxy resin having the aforementioned
formula wherein the resin has a Gardner Holdt Viscosity of N-R and an
epoxide equivalent weight of 700-800;
(B) 35-45% by weight of an epoxy/epoxy novolac resin described previously
where n and m are sufficiently large to provide a Gardner Holdt Viscosity
of O-S and an epoxide equivalent weight of 500-700;
(C) 5-15% by weight of a second epoxy/epoxy novolac resin described
previously where n and m are sufficiently large to provide a Gardner Holdt
Viscosity of W-Z and an epoxide equivalent weight of 700-900;
(D) 0.1-0.4% by weight of a catalyst of tri(dimethyl aminomethyl) phenol;
and
(E) 4.0-6.0% by weight of dicyandiamide.
Another useful composition that forms a high quality finish on the interior
of steel containers is of
(A) 28.6-60.9% by weight of the epoxy resin described in the above
preferred composition;
(B) 20.0-30.0% by weight of the epoxy/epoxy novolac resin described in the
above preferred composition,
(C) 15.0-35.0% by weight of a second epoxy/epoxy novolac resin described in
the above preferred composition;
(D) 0.1-0.4% by weight of a catalyst of tri(dimethyl aminomethyl) phenol
and
(E) 4.0-6.0% by weight of dicyandiamide.
The composition can be pigmented or unpigmented but is usually pigmented
and contains pigments in a pigment/binder ratio of 2/100 to 80/100. Any of
the conventional inorganic or organic pigments, filler pigments, or dyes
can be used. Examples of the great variety of usable pigments are as
follows: metallic oxides, such as titanium dioxide, zinc oxide, iron oxide
and the like; metallic flakes such as aluminum flake; metallic powders;
metal hydroxides, "Afflair" pigments such as mica flake coated with
titanium dioxide; sulfides; sulfates; carbonates; carbon black; silica;
talc; china clay; and other extender pigments.
However, if the powder coating composition is to be in contact with foods
only those pigments and additions having Food and Drug Administration
approval are used.
One method for forming the powder coating composition of this invention is
to blend the components together and then to pass the mixture through a
conventional extruder.
The extrudate can then be reduced to a powder using conventional grinding
equipment. After grinding, the powder is passed through a sieve to remove
particle larger than 110 microns. Preferably, a seive which eliminates
particles of maximum dimension greater than 74 microns is used.
In another aspect of the present invention, it has been found that the
composition has excellent hydrophobicity, such that it is very useful as a
lining for hot water tanks, when about 45-86% by weight, based on the
weight of the binder components, of polyvinylidine fluoride is added. The
polyvinylidine fluoride can be of the type described in U.S. Pat. No.
2,435,537. Although most useful for coating the interiors of water heater
tanks, this composition is also useful to line boilers, pipe and drum
interiors, industrial containers, or any surface which contacts water at
elevated temperatures.
Preferably, the polyvinylidine fluoride is in particulate form. The
particles can have a maximum dimension up to 12-15 microns with 80% by
number, but most preferably 95% by number, having a maximum dimension of 5
microns or less. A suitable substance to use is Kynar.RTM., sold by
Pennwalt Chemical Company.
Polyvinylidine fluoride is ordinarily present in this composition in an
amount equal to 45-86%, preferably 50-75% and most preferably 57-65%, of
the weight of the binder. A preferred composition which forms a high
quality lining for the interior of a hot water tank is of
(A) 25.0-35.0% by weight of an epichlorohydrinbisphenol-A epoxy resin of
the aforementioned formula wherein the resin has a Gardner Holdt Viscosity
of N-R and an epoxide equivalent weight of 700-800;
(B) 20.0-30.0% by weight of an epoxy/epoxy novolac resin as described
previously where n and m are sufficiently large to provide a Gardner Holdt
Viscosity of O-S and an epoxide equivalent weight of 500-700;
(C) 4.0-8.5% by weight of a second epoxy/epoxy novolac resin described
previously where n and m are sufficiently large to provide a Gardner Holdt
Viscosity of W-Z and an epoxide equivalent weight of 700-900;
(D) 0.06-0.12% by weight of a catalyst of tri(dimethyl aminomethyl) phenol;
(E) 1.5-3.5% by weight of dicyandiamide; and
(F) 33.0-41.5% by weight of polyvinylidine fluoride.
The powder coating composition containing polyvinylidine fluoride can be
formed in much the same way as previously described. However, it has been
found that when the powder composition is to contain more than 20% by
weight of the fluoropolymer, the powder composition can be more easily
formed when the excess over 20% is post-added by dry blending. That is, in
such a situation, the composition which is extruded contains no more than
20% by weight, based on total composition weight, of polyvinylidine
fluoride. After the extrudate is reduced to powder form, as previously
described, an amount of particulate polyvinylidine fluoride which is
sufficient to raise the concentration to the desired level is added to,
and thoroughly mixed with, the powder.
The powder coating compositions of this invention can be applied to a
metal, glass, plastic, or a fiber-reinforced plastic substrate by
electrostatic spraying or by using a fluidized bed which can be
electrostatic. Preferably, electrostatic spraying is used in which a
voltage of 20-100 kilowatts is applied to the spray gun. The composition
can be applied in one pass or several passes to provide variable
thicknesses, after cure, of 1.5-4.0 mils, preferably 1.8-3.0 mils. After
the application of the powder, the coated article is heated at
180.degree.-235.degree. C. for 6-15 minutes to fuse and to cure the powder
particles into a substantially continuous uniform coating.
Since the binder components of the present invention are more fluid than
the polyvinylidine fluoride at the extrusion and cure temperatures, the
cured coating substantially approximates a continuous phase of crosslinked
epoxy and epoxy/epoxy novolac resins having polyvinylidine fluoride
dispersed throughout.
The powder coating composition can be applied directly to untreated metals
such as aluminum or steel. In one particular application, the powder is
applied directly to the exterior or to the interior of steel containers
which are used for aqueous acidic foods and beverages, such as tomato
products like pastes, soups, juices, and catsup.
The coating composition can also be applied over a suitably treated or
primed metal substrate. Typical conventional alkyd primers or epoxy
primers pigmented with iron oxide, carbon black, titanium dioxide and the
like can be used. Electrodeposited primers can also be used. Also, the
composition can be used directly over galvanized phosphatized steel to
form a durable coating. An electrically conductive carbon black pigment
can be added to the primer to make the surface conductive and to promote
uniform deposition of the powder while spraying.
The following Examples illustrate the invention. All quantities are on a
weight basis unless otherwise indicated.
EXAMPLE I
The following ingredients are mixed together to form powder coating
composition A.
______________________________________
Parts by
Weight
______________________________________
1. Epichlorohydrin-Bisphenol-A
type epoxy resin having the
formula described and shown
in the above specification
wherein n is sufficiently
large to provide a resin with
a Gardner Holdt Viscosity of P
and an epoxide equivalent
weight of 765. 42.5
2. Epoxy/Epoxy Novolac type resin
having the formula described
and shown in the above speci-
fication wherein n and m are
sufficiently large to provide
a resin with a Gardner Holdt
Viscosity of O-S and an epoxide
equivalent weight of 500-575.
40.0
3. Epoxy/Epoxy Novolac II resin
having the formula described and
shown in the above specification
wherein n and m are sufficiently
large to provide a Gardner Holdt
Viscosity of W-Z and an epoxide
equivalent weight of 775-850.
10.0
4. Flow agent (5% by weight polyethyl-
hexyl acrylate and 95% by weight
of the epoxy resin described as
ingredient 1.) 7.5
5. Curing agent (95/5 weight ratio
of dicyandiamide to finely divided
silica) 4.0
6. Catalyst [2,4,6-tri(dimethyl amino-
methyl)phenol] 0.15
7. Titanium dioxide pigment
50.0
Total 154.15
______________________________________
The above mixture is charged into a standard three zone melt extruder in
which zone 1 is at 50.degree. C., zone 2 is at 70.degree. C. and zone 3 is
at 100.degree. C. and which is operated at 90 revolutions per minute. The
resulting molten mixture is extruded and the resulting extrudate is cut
into pellets and charged into a grinding mill that grinds the extrudate
into a powder. The resulting powder is passed through a 140 mesh screen to
remove large particles.
The powder is sprayed onto the exterior of phosphatized cold roll steel
panels with an electrostatic powder gun and the panels are baked for 6
minutes at 205.degree. C. in a gas oven. The resulting finish on the
panels is about 1.5-2.0 mils thick and is smooth, even, glossy, flexible
and has a good appearance. The finish has a gloss measured at 60.degree.
of 96.3 and a Tukon hardness of 19.2.
Three of the above prepared panels are immersed about half way into boiling
water for one hour and then removed and examined for staining,
delamination, pinholes and other defects in the finish. No defects in the
finish on any of the exposed portions of the panels were found. After six
hours in boiling water the panels were examined and no defects were found.
Adhesion of the finish is tested by scoring the finish through to the metal
and attempting to remove finish from the panel with scotch tape. The width
of coating, measured from the center of the score line, which delaminates
is known as creepage. The creepage of the finish is minimal and therefore
its adhesion to the metal is acceptable.
Three of the above finished panels are immersed in tomato paste at
60.degree. C. for 2 weeks, tomato paste at 49.degree. C. for 1 month and
tomato paste at 38.degree. C. for 6 months, respectively. After each of
the above time periods, the panel was removed and examined for defects,
such as loss of adhesion, delaminating and staining. In each case the
finish was found to be acceptable.
Powder coating composition B is prepared identically to the above coating
composition except 25 parts by weight of the Epoxy/Epoxy Novolac resin and
25 parts of the Epoxy/Epoxy Novolac II resin are used. The resulting
powder coating composition is applied as above to phosphatized steel
substrates and baked as above. The resulting finish is about 1.5-2.0 mils
thick and is smooth, even, glossy and has a good appearance. The finish
has a Tukon hardness of greater than 16 knoops.
The panels are tested in boiling water and in tomato paste as above and
similar results were noted. None of the finishes on the panels failed the
tests but were acceptable.
EXAMPLE 2
Powder coating composition C is prepared using the same procedure and
constituents used to prepare powder coating composition A of Example 1
except the following Resin is substituted for the Epoxy/Epoxy Novolac
resin (ingredient 2):
Epoxy/Epoxy Novolac resin wherein n and m are sufficiently large to provide
a Gardner Holdt Viscosity of R to T and an epoxide equivalent weight of
700-825.
The resulting powder coating composition is applied as in Example 1 to
phosphatized steel substrates and baked as above. The resulting finish is
about 1.5-2.0 mils thick, is smooth, and glossy, even and flexible and
has a good appearance.
About half of a panel is immersed in boiling water and in tomato paste as
in Example 1. No defects in the finish such as stains, cracks, and
pinholes are noted.
EXAMPLE 3
The following ingredients are mixed together to form powder coating
composition D.
______________________________________
Parts by
Weight
______________________________________
1. The epoxy resin described as
22.06
ingredient 1 in Example 1
2. The epoxy/epoxy novolac resin
described as ingredient 2 in
Example 1. 20.76
3. The epoxy/epoxy novolac II resin
described as ingredient 3 in
Example 1 5.19
4. Flow agent (described in Example 1)
3.89
5. Curing agent (described in Example 1)
2.08
6. Catalyst [2,4,6-Tri(dimethyl amino-
methyl) phenol] 0.08
7. Titanium dioxide pigment 25.87
8. Aluminum/Cobalt Oxide Pigment
0.56
9. Kynar.RTM. 20.0
Total 100.49
______________________________________
The above mixture is charged into a standard three-zone melt extruder in
which zone 1 is at 55.degree. C., zone 2 is at 80.degree. C., and zone 3
is at 100.degree.-110.degree. C. and which is operated at 90 revolutions
per minute. The resulting molten mixture is extruded and the resulting
extrudate is broken into chips and then charged into a grinding mill where
it is ground to a fine powder. The powder is then passed through a 140
mesh screen.
This powder is then thoroughly mixed with Kynar.RTM. powder in a
concentration of 90 parts by weight of powder to 10 parts Kynar.RTM.. The
resulting powder composition is then sieved again through a 140 mesh
screen.
The powder is sprayed onto the exterior of phosphatized cold roll steel
panels with an electrostatic powder gun. The panels are heated in a gas
oven for 6 minutes at 205.degree. C. The resulting finish on the panels is
about 2.4-2.6 mils thick and is smooth and even with no popping or
cratering.
Several of the above-prepared panels are immersed in a resin kettle of
refluxing boiling water for two months. At the end of this period, the
panels are tested for adhesion, impact resistance, and extent of
blistering.
Adhesion of the finish is tested as in Example 1. The creepage is found to
be approximately 0.8 mm which is acceptable. The impact resistance is
tested with a Gardner impact tester No. 16-1120 and the coated panel is
found to have a reverse impact resistance of 100 inch-pounds. A visual
inspection is made for blistering and only slight blistering along the
panel edge, within acceptable limits, is noted.
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Description |
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