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Description  |
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BACKGROUND OF THE INVENTION
This invention pertains to a method of coating substrates with aliphatic
and cycloaliphatic liquid epoxy resin compositions and particularly to
placing a tin salt of trifluoromethanesulfonic acid, as a polymerization
catalyst on the substrate.
High solids compositions must contain low molecular weight components in
order to impart low viscosity to them. Because of this low molecular
weight, these components must be highly active so that they react rapidly
during cure and develop physical and mechanical properties rapidly. One of
the major disadvantages of high solids coatings, in many instances, is the
very limited pot-life due to the highly reactive components. This
necessitates a two-package application where complicated metering
equipment, mixing valves and controls are required to properly mix the
catalysts and coatings intermediates just prior to their application to a
suitable substrate. This also limits the application techniques which can
be used. However, the advantages of high solids coatings, such as
low-energy demand and low solvent emission, make these systems very
attractive from both an economical and ecological point of view.
It is an object of this invention to provide a system which eliminates the
problem of pot-life.
It is another object to provide a system for preparing high solids curable
aliphatic or cycloaliphatic liquid epoxy systems which cure rapidly
without sacrificing physical or mechanical properties in the cured
product.
Other objects will become apparent to those skilled in the art upon a
further reading of the specification.
SUMMARY OF THE INVENTION
In the method of fabricating cured epoxy-coated substrates employing a
two-package technique wherein an epoxy resin composition and catalyst are
stored separately until use and mixed soon before application to said
substrates, an improvement has been discovered which comprises first
treating said substrates by adhering thereto a curing amount of a sulfonic
acid salt having the formula:
M(R.sub.x SO.sub.3).sub.y
wherein R.sub.x is a fluoroalkyl having 1 to about 18 carbons, M is
selected from the group consisting of ammonium (NH.sub.4.sup.+) cation,
quaternary ammonium cations, cations of protonated amines, a mono- or
polyvalent cation of a metal selected from the group consisting of metals
of Group I to V inclusive and VIII and subgroups VIB and VIIB of the
Periodic Table and y is an integer equal to the valence of M, thereafter
applying an aliphatic or cycloaliphatic liquid epoxy high solids resin
composition containing sufficient amounts of monofunctional carboxylic
acids or sufficient amounts of polyols to provide an epoxy/carboxy or
epoxy/hydroxy ratio respectively of from about 0.5 to about 5 to said
treated substrates and heating the resultant composite at a temperature of
about 50.degree.-165.degree. C. until a cured resin coating is obtained.
In the field of solvent coatings, efforts have been made to reduce the
amount of volatile solvent present and to increase the amount of reactive
components that will react to produce the coatings on the substrate. At a
sufficiently high concentration of such components, one has what is known
as a high solids coating composition.
DESCRIPTION OF THE INVENTION
The epoxy high solids resin compositions of this invention contain
aliphatic or cycloaliphatic liquid epoxy resins well known to those
skilled in the art as described fully in U.S. Pat. Nos. 3,027,357,
2,890,194, 2,890,197, 3,117,099, 3,031,434, 3,125,592 and 3,201,360
incorporated herein by reference.
Of particular interest is that portion of U.S. Pat. No. 3,027,357 extending
from Column 4, line 11 to Column 7, line 38 and of U.S. Pat. No. 3,201,360
extending from Column 2, line 60 through Column 4, line 43. Among some of
the specific illustrative epoxy resins disclosed therein are:
3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate
bis(3,4-epoxycyclohexylmethyl)adipate
vinyl cyclohexane dioxide
bis(2,3-epoxycyclopentyl)ether
epoxidized linseed oil
epoxidized soybean oil
methyl epoxy linseedate
butyl epoxy soyate
octyl epoxy linseedate
epoxidized polymers and copolymers of butadiene, and the like.
The liquid epoxy resins of this invention contain the epoxy
##STR1##
group. They are thermosetting resins but are not self-curing or
self-hardening and must be cured with curing agents or catalysts to effect
molding to a hardened state.
The aliphatic and cycloaliphatic liquid epoxy resins are formed by reacting
peracetic acid with olefinic esters of cycloaliphatic compounds. Some
preferred cycloaliphatic epoxy resins include the following:
##STR2##
3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate
##STR3##
bis(2,3-epoxycyclopentyl) ether
##STR4##
bis(3,4-epoxycyclohexyl methyl) adipate, and
##STR5##
2-(3,4-epoxycyclohexyl-5,5-spiro)-(3,4-epoxy) cyclohexane-m-dioxane.
The term "epoxide equivalent" is defined herein as in Epoxy Resins by H.
Lee and K. Neville, page 21, McGraw-Hill Book Co., Inc. NYC (1957), viz.,
the weight of epoxy resin in grams which contains 1 gram equivalent of
epoxy, i.e.,
##STR6##
Epoxide equivalents are determined by reacting a known quantity of resin
with a known quantity of hydrochloric acid and back-titrating the
remaining acid to determine its consumption.
The following tests and definitions were used in evaluating the use of the
instant invention.
TESTS
Solvent resistance is a measure of the resistance of the cured film to
attack by acetone and is reported as the number of rubs or cycles of
acetone soaked material required to remove one-half of a film from the
test area. The test is performed by stroking the film with an acetone
soaked cheesecloth until that amount of film coating is removed. The
number of cycles required to remove this amount of coating is a measure of
the coating solvent resistance.
Reverse or face impact measures the ability of a given film to resist
rupture from a falling weight. A Gardner Impact Tester using an eight
pound dart is used to test the films cast and cured on the steel panel.
The dart is raised to a given height in inches and dropped on to the
reverse or face side of a coated matal panel. The inches times pounds,
designated inch-pound, absorbed by the film without rupturing is a measure
of the films reverse or face impact resistance.
Pencil hardness is a measure of film hardness. The adhesion and cohesive
strength of the film also influences pencil hardness. Pencils of known
lead hardness are shaped to a cylindrical point with a flat tip. The
pencils are manually pushed into the coating surface at a 45.degree.
angle. Pencil hardness is recorded as the hardest pencil which does not
cut the coating.
Crosshatch adhesion--The coated substrate is cut with a series of 10
parallel razor blades 1/8 inch apart in a crosshatch pattern. Adhesion of
the coating to the substrate is tested by firmly applying high tack tape
and pulling the tape off with a quick pull. The percent coating remaining
within the crosshatch pattern is recorded as the crosshatch adhesion.
Spot Test--Five days after the coating is oven cured it is contacted with a
caustic solution for 16 hours. Then the panel is washed, dried and rated
on a 1 to 10 basis with 10 representing no visible failure and 1
representing complete failure.
Wet Crosshatch Adhesion, Wet Pencil Hardness--Five days after the coating
is oven cured the panel is immersed in a 55.degree. C. water bath for 16
hours after which the property is determined.
The high solids epoxy resin composition in addition to aliphatic or
cycloaliphatic liquid epoxy resins also contains monofunctional carboxylic
acids or polyols. The former are represented by the generic formula:
HOOC-R-COOR').sub.z
wherein z is an integer having a value of from 0 to 2, preferably 0 or 1; R
is an alkyl group having from 6 to 24 carbon atoms, preferably from 12 to
18 carbon atoms when z is 0 and from 1 to 12 carbon atoms, preferably from
2 to 6 carbon atoms, when z is 1 or 2; a cycloalkyl group having 5 or 6
ring carbon atoms; a phenyl group, a naphthyl group or --CH.dbd.CH-- when
z is 1; R' is an alkyl group having from 1 to 8 carbon atoms, preferably 1
to 3 carbon atoms, or a
--C.sub.n H.sub.2n (OC.sub.n H.sub.2n).sub.m OC.sub.p H.sub.2p+1
group where n is 2 to 4 and preferably 2; m is 0 to 10 and preferably 2 to
7; and p is 1 to 15.
The most preferred carboxylic acids have a pKa value of less than 4.
Illustrative carboxlic acids defined by the generic formula above when z=0
include:
caprylic acid
capric acid
hendacanoic acid
lauric acid
tridecanoic acid
pentadecanoic acid
stearic acid
arachidic acid
behenic acid
cerotic acid
2-ethylhexoic acid
9-methyl-decanoic acid
benzoic acid
naphthoic acid, and the like.
When z is 1 or 2, the monocarboxylic acids are the partial esters (having
one free carboxyl group) of di- or tri-carboxylic acids or the anhydrides
thereof. Illustrative thereof are the partial esters of: oxalic, malonic,
succinic, adipic, suberic, azelaic, sebacic, brassylic, maleic, fumaric,
itaconic, phthalic, isophthalic, terephthalic, trimellitic, tartaric,
1,2-cyclo-hexanedicarboxylic, 1,4-cyclohexanedicarboxylic acids, and the
like. Alkyl esters are preferred wherein the alkyl contains 1 to about 20
carbons, e.g., methyl, butyl, decyl, lauryl, octadecyl and the like.
Suitable polyols for use in this invention include: alkane diols, triols,
and tetraols; aliphatic ether containing diols, triols and tetraols;
cycloaliphatic containing diols, triols, and tetraols; aromatic containing
diols, triols, and tetraols, and the like. Specific examples of polyols
include: ethylene glycol, diethylene glycol,
2,2,4-trimethyl-1,3-pentanediol, propylene glycol, dipropylene glycol,
2,2-dimethyl-1,3-propanediol, polypropylene glycol having an average
molecular weight of about 150 to about 600, and having 2 to 4 terminal
hydroxyl groups, triethylene glycol, 1,4-cyclohexanedimethanol,
2,2-dimethyl-3-hydroxypropyl 2,2-dimethyl-3-hydroxypropionate,
1,3-butanediol, tetraethylene glycol, 2,2-bis(4-hydroxyphenyl) propane and
the ethylene and propylene oxide adducts of 2,2-bis(4-hydroxyphenyl)
propane, pentaerythritol, erythritol, glycerine, trimethylol-propane,
1,4-butanediol, 1,6-hexanediol, tripropylene glycol,
2,2-bis(4-hydroxycyclohexyl)propane, 1,2,6-hexanetriol, 1,3-propanediol,
and the polycaprolactone ester of a polyol in which from about 1 to about
5, preferably from about 1.5 to about 4.0 moles of caprolactone are
esterified with a polyol, such as trimethylol propane or diethylene
glycol. Preferably the polycaprolactone ester of a polyol is the
polycaprolactone ester of trimethylol propane in which about 1.5 moles of
caprolactone are reacted with trimethylol propane or the polycaprolactone
ester of trimethylol propane where about 3.6 moles of caprolactone are
esterified with trimethylol propane, and the like. Polycaprolactone
polyols are described in U.S. Pat. No. 3,169,945, for example. Also ester
diols and ester diol alkoxylates produced by the reaction of an ester diol
and an alkylene oxide, as described in U.S. Pat. No. 4,163,114, are
suitable for use herein.
It is believed, although applicants are not bound by such, that the acids
and the polyols initiate polymerization of the epoxy resins by a ring
opening step in the presence of the sulfonic acid salt which also
propagates said polymerization. Accordingly with monoepoxides the
initiation would then take place as represented below:
##STR7##
Propagation of I or II with the sulfonic acid salt would be represented as:
##STR8##
In the case of polyepoxides crosslinking occurs, as is known in the art, by
initiation and propagation from more than one epoxy site on the
polyepoxide molecule.
The preparation of the sulfonic acid salt catalysts is described in U.S.
Pat. No. 3,842,019 incorporated herein by reference.
The preferred curing agent is Sn(CF.sub.3 SO.sub.3).sub.2 and is available
commercially from the Minnesota Mining and Manufacturing Co. It can be
dissolved in various solvents, such as, acetone 50/50 (Vol./Vol.) mixtures
of Carbitol (Union Carbide Corporation trademark for monoalkyl ethers of
diethylene glycol) and water and the like. This facilitates applying the
curing agent to the substrate of choice.
This invention is unique in that it provides a method of catalyst
application to the substrate first followed by application of the high
solids liquid epoxy composition so that the two streams are not mixed and
the pot-life problem is eliminated. The prior art method of curing liquid
or solid epoxy resins has normally necessitated intimately mixing the
curing agent with the epoxy resin in order to harden the resin. In this
invention, the catalyst curing agent and epoxy composition are maintained
apart, the catalyst on the substrate and the epoxy composition in a
separate mode. They come in contact only when the epoxy composition is
coated on the surface of the curing agent. There is no need for mixing of
this heterogeneous composite.
The curing agent catalyst can be applied to the substrate as a solution in
a suitable catalyst such as ketones, or mixtures of glycol monoethers and
water. This catalyst solution can be rolled on, sprayed, painted, or
applied by any conventional coating technique known to those skilled in
the art. The solution coated substrate can be dried at ambient
temperatures or heated if desired to drive off the solvent. The substrate
now coated with catalyst can be coated immediately with the liquid epoxy
composition or stored and coated with the epoxy at some later time, since
the catalyst stays active. In either case, curing of the epoxy system
takes place upon application of heat. This provides an unexpected latitude
of operation conditions never developed before. Temperatures required for
curing extend from a minimum of about 100 to a maximum of about
200.degree. C. A preferred curing temperature will vary depending on the
particular liquid epoxy resin used. For example, with BAKELITE.RTM.
Cycloaliphatic Epoxide ERL-4221, the preferred curing temperature is about
300.degree. F. (150.degree. C.).
The coatings, after curing are solvent resistant and exhibit good physical
properties.
Since the method of this invention provides an epoxy coating system which
is both very reactive and free of pot-life problems, low molecular weight
epoxy resins can be used having low viscosities. In some instances epoxy
systems containing 100% solids and yet displaying viscosities of less than
200 cps, as determined by Brookfield RVT viscometer measurements, can be
cured with this technique.
The invention is further described in the examples which follow. All parts
and percentages are by weight unless otherwise specified.
EXAMPLE 1
A 0.1 percent solution of Sn(CF.sub.3 SO.sub.3).sub.2 dissolved in
CELLOSOLVE.RTM. Acetate was applied by drawn down to Bonderite 37 cold
rolled steel panels with a #60 wire wound rod. These panels were allowed
to air dry at room temperature and then coated by drawn down with a
mixture of 7.5 grams of ethylene glycol and 37.68 grams of BAKELITE.RTM.
Cycloaliphatic Epoxide ERL-4221 (3,4-epoxycyclohexyl
methyl-3,4-epoxycyclohexyl carboxylate) (epoxide/hydroxyl ratio of 1.1/1).
This panel was baked 5 minutes at 250.degree. F. (120.degree. C.) and gave
a slightly tacky coating which, on further heating for 5 minutes at
300.degree. F. (150.degree. C.), produced a hard, glossy coating. The same
formulation applied to an untreated panel showed no cure after baking at
300.degree. F. (150.degree. C.).
EXAMPLE 2
A 0.1 percent solution of Sn(CF.sub.3 SO.sub.3).sub.2, dissolved in
acetone, was applied by draw down to Bonderite 37 cold rolled steel panels
and air dried at room temperature. Panels were coated with a formulation
consisting of 29.41 grams ethylene glycol, 72.81 grams triethylene glycol,
and 150.7 grams of BAKELITE.RTM. Cycloaliphatic epoxide ERL-4221
(epoxide/hydroxyl ratio of 1.1/1) and baked 10 minutes at 300.degree. F.
(150.degree. C.). A hard, glossy, clear coating was obtained. This
formulation was then applied to treated panels which had been aged for
various time periods. The properties on these coatings are listed in the
following Table 1.
TABLE 1
__________________________________________________________________________
Impact Passed
Treated
Cure Strength 100
Run
Panel Panels
Cycle
(in-lbs)
Pencil
Acetone
No.
Treatment
Aged, days
Min./.degree.F.
Reverse
Face
Hardness
Rubs
__________________________________________________________________________
1 Sn(CF.sub.3 SO.sub.3).sub.2
Initial (0)
5/300
300 300
3H Yes
2 SN(CF.sub.3 SO.sub.3).sub.2
1 5/300
75 75
4H Yes
3 SN(CF.sub.3 SO.sub.3).sub.2
4 5/300
25 200
4H Yes
4 SN(CF.sub.3 SO.sub.3).sub.2
15 5/300
<5 35
5H Yes
5 None Initial (0)
5/300
No Cure
__________________________________________________________________________
EXAMPLE 3
Example 2 was repeated using a 75/25 mixture of triethylene glycol/ethylene
glycol with BAKELITE.RTM. cycloaliphatic epoxide ERL-4221
(epoxide/hydroxyl ratio of 1.1/1). This formulation was applied to treated
Bonderite panels which had been aged for five and nine weeks prior to
coating. These coatings were cured 5 minutes at 300.degree. F.
(150.degree. C.) and showed pencil hardness of 4H and 5H, respectively.
Both coated panels passed 100 acetone rubs; and the coating on the treated
panel, aged five weeks, showed reverse impact strength of 50 inch-pounds
with face impact strength of 50 inch-pounds while the panel aged nine
weeks had reverse and face impact strengths of 175 and 200 inch-pounds,
respectively.
EXAMPLE 4
A 100 percent solids coating was formulated with 10.19 grams triethylene
glycol, 5.54 grams trimethylolpropane, and 41.45 grams of BAKELITE.RTM.
ERL-4221 and was applied to Bonderite 37 steel panels which were
previously treated with 0.1 percent solutions of Sn(CF.sub.3
SO.sub.3).sub.2 in acetone. These coatings were baked 5 minutes at
300.degree. F. (150.degree. C.). The coatings passed 100 acetone rubs,
showed 2H pencil hardness, and had reverse and fact impact strengths of 5
and 50 inch-pounds, respectively.
EXAMPLE 5
Example 4 was repeated using a formulation of 22.62 grams of Caprolactone
Polyol A (A polycaprolactone triol having an average molecular weight of
540 and an average hydroxyl number of 310.) and 20.72 grams of
BAKELITE.RTM. ERL-4221. The coating cured 15 minutes at 300.degree. F.
(150.degree. C.), passed 100 acetone rubs, showed 320 inch-pounds reverse
and face impact strength, and had pencil hardness of B.
EXAMPLE 6
Bonderite 37 cold rolled steel panels were treated with 0.1 percent
solution of Sn(CF.sub.3 SO.sub.3).sub.2 in acetone and with 0.25 percent
solution of stannous octoate in acetone. Other panels were untreated.
These panels were dried at room temperature and then coated with a mixture
of 12.5 grams of Resin A. Resin A is a polycarboxylic acid prepared from
.about.3 equivalents of phthalic anhydride with 3 equivalents of a 50/50
mixture of Caprolactone Polyol A and Caprolactone Polyol B (A
polycaprolactone polyol having an average molecular weight of 300 and an
average hydroxyl number of 560.) and 15.2 grams of BAKELITE.RTM.
Cycloaliphatic epoxide ERL-4221 (epoxy/carboxyl ratio of 3.3). The panels
were placed in a forced draft oven for 20 minutes at 350.degree. F.
(175.degree. C.). All coatings were hard and glossy, but only the panels
treated with Sn(CF.sub.3 SO.sub.3).sub.2 produced cured coatings with good
solvent resistance, as seen in Table 2.
TABLE 2
______________________________________
100 Impact Strength
Run Catalyst Acetone (in-lbs) Pencil
No. Treatment Rubs Reverse Face Hardness
______________________________________
1 Sn(CF.sub.3 SO.sub.3).sub.2
Passed <5 50 2H
2 Stannous Octoate
Failed 15 50 2H
3 None Passed <5 5 HB
______________________________________
EXAMPLE 7
A panel that had been treated with catalyst Sn(CF.sub.3 SO.sub.3).sub.2 and
stored under ambient conditions for about 5.5 months had the Example 3
formulation applied to it. The coated panel was then cured for 5 minutes
at 300.degree. F. (150.degree. C.) in a forced air oven. After cooling to
room temperature, the coating had the following properties--Hardness-5H,
Acetone Rubs-100, Reverse Impact-25 in-lbs., Face impact-50 in-lbs.
EXAMPLES 8-12
When the procedure described in Example 1 is repeated with the exception
that caprylic, lauric, stearic, arachidic or benzoic acids are substituted
for the ethylene glycol, a comparable cured epoxy coating is obtained.
EXAMPLES 13-16
When the procedure described in Example 2 is repeated with the exception
that monomethyl maleate, monodecyl sebacate, or monooctadecyl phthalate
are substituted for the ethylene glycol and monomethyl isophthalate,
monobutyl itaconate or monolauryl adipate for the triethylene glycol, a
comparable cured epoxy coating is obtained.
MODIFICATIONS
These examples are made with specific hydroxy and epoxy compounds. Other
materials suitable in this reaction are hydroxyl compounds such as
polyether and polyester polyols, 1,4-butanediol, 1,3-propanediol,
1,6-hexanediol, Bisphenol A, diethylene glycol, polymeric polyols such as
styrene/allyl alcohol, and acrylic polyols, and others as well as carboxyl
compounds such as liquid polycarboxylic acids, acrylic polymers containing
carboxyl groups, and others reacted with various epoxy derivatives such as
cycloaliphatic epoxides, aliphatic epoxides, and glycidyl-type epoxides.
Catalyst concentration can be varied to increase or decrease reaction
rate, and bake temperatures and cycles can be increased or decreased to
control rate of cure.
Although the invention has been described in its preferred forms with a
certain degree of particularity, it is understood that the present
disclosure has been made only by way of example and that numerous changes
can be made without departing from the spirit and the scope of the
invention.
* * * * *
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Description |
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