Note: Descriptions are shown in the official language in which they were submitted.
~t~56t~
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a high-solids
film-forming composition of a low molecular weight
polyester with controlled hydroxyl functionality,
a polyisocyanate or aminoplast resin, and optionally
an epoxy resin or the ester of an epoxy resin and
monocarboxylic acid.
Description of the Prior Art
Conventional polyester-based coating com-
positions are well known in the finishes art, often
comprising one or more hydroxyl-functional components
which co-react with a suitable curing agent to form a
polymeric paint film. For example, U.S. Patent
15 3,994,851 issued November 30, 1976 to Chang shows
a specific polyester polyol which is cured with an
amine-aldehyde condensation product. U.S. Patent
3,535,287 issued October 20, 1970 to Wynstra shows
a 3-component polyester oligomer that is cured with
20 a polyisocyanate.
A study of this and related prior art would
make it evident, however, that in the field of
polyester coatings, it is often necessary to sacrifice
one desirable property to enhance another. For
25 example, it is often difficult to obtain a coating
composition, applicable at high solids levels, that
is also tough, flexible, and durable, or that does
not contain a resin which crystallizes out of solution
at rocm temperature.
Therefore, with the current emphasis on
reduction of solvent emissions, there is a continued
need for coatings that not only can be applied at
relatively high solids levels but also can be cured
at commercially acceptable temperatures to produce
35 a durable, flexible but hard, weather-resistant finish.
5~
SUMMARY OF THE INVENTION
There is provided by the present invention,
a coating composition of a film-forming blend and a
solvent for the blend wnerein the blend is at least
40% by weight of the composition and consists
essentially of
(a) a polyester polyol that is the
reaction product of
(1) neopentyl glycol and at least
one other hindered diol con-
taining two methylol groups wherein
each methylol group is attached
directly to a cycloaliphatic
or aromatic structure or to a
tertiary carbon atom, the molar
ratio of neopentyl glycol to
hindered diol being 2:1 to 6:1,
and
(2) a mixture of an aromatic and
an aliphatic dicarboxylic acid
wherein the molar ratio of
aromatic acid to aliphatic acid
is from 2:1 to I0:1,
wherein the molar ratio of (1) to (2) is from
1.3:1 to 1.9:1 and wherein the polyol has a
hydroxyl content of about 3.0-10.0~ by weight;
(b) a curing agent for the polyol; and
(c) 0-50% by weight, based on the
weight of (a) plus (b), of an epichlorohydrin-
bisphenol-A epoxy resin, or the esterification
product of said resin with a monocarboxylic
acid, or mixtures of these.
DETAILED DESCRIPTION OF THE INVENTION
The polyester coating composition of the
present invention, quite llseful as a finish ~or
s~
automobiles, appliances, steel furniture, or even
general industrial use, is composed primarily of a
film-forming blend and a solvent for the blend. It
can, however, also contain pigments, a reaction
catalyst to decrease the curing time, and any of the
various additives that are advantageously used in coating com-
positions for industrial or automotive finishes. The
film-forming blend consists essentially of a polyester
polyol, a polyisocyanate or aminoplast curing agent
for the polyol, and optionally, an epoxy resin or
epoxy-resi~/acid ester. The film-forming blend con-
stitutes 40-90~, preferably 55-90%, of the combined
weight of the blend and the solvent.
The polyester polyol used in the present
invention constitutes 55-80% by weight of the film-
forming blend. This polyol is the condensation-
reaction product of neopentyl glycol, at least one
other hindered diol, and an aromatic and an aliphatic
dicarboxylic acid.
The alcoholic components used to form the
relatively specific polyester polyol of the present
invention are neopentyl glycol and at least one other
hindered, diprimary diol. It has been found that this
combination of difunctional alcohols imparts both stain
resistance and hardness to the final coating. The
hindered diprimary diols that are usable in this
invention are those having two methylol groups,
each of which is attached directly to an aromatic
or cycloaliphatic hydrocarbon structure or to a
tertiary carbon atom. Examples of two preferred
su^h diols are cyclohexane dimethylol and the monoester of
neopentyl glycol and hydroxypivalic acid. This ester can be
formed according to U.S. Patent 3,057,911. .~ par-
ticularly useful polyol is formed when the molar
'7~
ratio of neopentyl glycol to other hindered diprimary
diol is from 2:1 to 6:1.
The dicarboxylic acids useful in the for-
mation of the polyester polyol have the general
5 formula
O O
,. ..
HO-C-R -C-OH
where R is aliphatic or aromatic. Of the aliphatic
structures, the most useful are where R is alkylene,
vinylene, or cycloaliphatic.
Preferred acids when R is alkylene are
those in which R has 2-10 carbon atoms. Most pre-
ferred of these are succinic acid, glutaric acid,
adipic acid, and pimelic acid. When R is a mono-
unsaturated aliphatic, the most useful acids are those
in which R has 2-8 carbon atoms with the preferred
acids being maleic and itaconic acids. The aromatic
dicarboxylic acids that are preferred are phthalic,
iso-phthalic, terephthalic, uvitic, and cumidic
acids. When R is cycloaliphatic, preferred are
cyclohexane or cyclohexene dicarboxylic acids,
although other such dicarboxylic acids could also
be used.
Mixtures of these aromatic acids and
aliphatic acids can be used, but at least one
of each kind of acid must be present. Whether
mixtures of each kind of acid are used, or
whether only one of each kind of acid is used,
the molar ratio of aromatic diacids to aliphatic
diacids should have a range of akout 2:1 to 10:1. A ratio of 2:1
to 6:1 is preferred and a ratio of akout 4:1 is most preferred
It is to be further understood that the lower alkyl mono or
di-esters of these acids and the anhydrides,
where applicable, of these acids can also be used
i7~3
in place of the acids themselves with equivalent
results. If the above-mentioned ester are used,
the alkyl groups preferably have no more than 5
carbon atoms.
A particularly useful polyester polyol
is formed when the molar ratio of the alcoholic
components to the dicarboxylic acid components is
from 1.3:1 to 1.9:1, with about 1.6:1 being preferred.
The polyester polyol can typically be formed
by charging the reactants, a suitable solvent, and
optionally a reaction catalyst into a reaction vessel
that is usually equipped with a condenser and
agitator. Useful solvents are, for example, xylene,
toluenel other substituted benzenes, napthalene
and substituted naphthalenes. The reaction catalysts
can be present in the usual amounts and include, for
example, dibutyl tin oxide, dibutyl tin dilaurate,
sulfuric acid, or one of the sulfonic acids.
The reaction mixture is heated to its
reflux temperature, usually 100-300C, and there
maintained for a period of 1-8 hours. During ihis
period, the esterification by-products are with-
drawn. The reaction product, the polyester polyol,
should have a number average molecular weight
(determined by gel permeation chromatography based
on polystyrene standards) of 3U0-lS00, preferably
~00-700. The reactants should be chosen also so
that the polyester polyol has a hydroxyl content
of 3-10% by weight, preferably about 5.5-7.5% by
weight.
As the curing or crosslinking agent for
the polyol, an organic polyisocyanate or an amino-
plast resin is used. The organic polyisocyanates,
present in the coating composition in a stoichio-
metric amount for the other constituents of the
film-forming blend, can be aliphatic, cycloaliphatic,
alkaryl, aralkyl, heterocyclic, and aryl di- or
triisocyanates. Oligomers of these can also be used.
Typically useful polyisocyanates are, for example,
diphenylmethane-4,4'-diisocyanate,
diphenylene-4,4'-diisocyanate,
toluene-2,4-diisocyanate,
toluene-2,6-diisocyanate,
3,3'-dimethoxy-4,4'-diphenylene diisocyanate
methylene-bis-(4-cyclohexyl isocyanate)
tetramethylene diisocyanate,
hexamethylene diisocyanate,
decamethylene diisocyanate,
ethylene diisocyanate,
ethylidene diisocyanate,
propylene-1,2-diisocyanate,
cyclohexylene-1,2-diisocyanate,
m-phenylene diisocyanate,
p-phenylene diisocyanate,
1,5-naphthalene diisocyanate,
3,3'-dimethyl-4,4'-biphenylene diisocyanate,
3,3'-dimethoxy-4,4'-biphenylene diisocyanate,
3,3'-diphenyl-4,4'-biphenylene diisocyanate,
4,4'-biphenylene diisocyhanate,
3,3'-dichloro-4,4'-biphenylene diisocyanate,
furfurylidene diisocyanate,
bis-(2-isocyanatoethyl)fumarate,
1,3,5-benzene triisocyanate,
para,para',para''-triphenylmethane
triisocyanate,
3,3'-diisocyanatodipropyl ether,
xylylene diisocyanate,
15~i70
diphenyl propane~4,4'-diisocyanate, and
isophorone diisocyanate.
Preferred among these are hexametnylene
diisocyanate, methylene~bis-(4-cyclohexyl isocyanate),
and isophorone diisocyanate. Particularly preferred
polyisocyanates are biurets of the formula
C-NH-R2-NCO
OCN-R2-N
\ 2
C-NH-R -NCO
o
where R2 is an aliphatic or aromatic hydrocarbon
group having 1-12 carbon atoms. These biurets can
be made according to Wagner et al. U.S. Patent
4,015,165, issued September 27, 1977. In a most
preferred biuret, R2 is -(CH2)6-. This biuret is
a trimex of hexamethylene diisocyanate (HM~I), which
is obtained by reacting three moles of HMDI with one
mole of water.
The aminoplast resins that are useful
are well known as crosslinking or curing agents.
Particularly useful are the alkylated products of
aminoplast resins, the resins themselves being
prepared by the condensation of at least one
aldehyde with at least one of urea, N,N-ethyleneurea,
dicyandiamide, and aminotriazines such as melamines
and guanamines~ ~mong the aldehydes that are
suitable are formaldehyde, revertable polymers
thereof such as paraformaldehyde, acetaldehyde,
crotonaldehyde, and acrolein. Preferred are for-
maldehyde and revertable polymers thereof. The
aminoplast resins can be alkylated with at le~st one
and up to six alkanol molecules containing 1-6
carbon atoms. The alkanols can be straight chain,
branched, cyclic, or mixtures of these. Preferred
are aminoplast resins that have been alkylated with
methanol, butanol, or a mixture of these two. Most
preferred are the methylated melamine-formaldehvde
resins such as hexamethoxymethylmelamine. In the
coating composition, the weight ratio of polyester
polyol to aminoplast resin will normally be about
1.5:1 to 4:1.
The film-forming blend of the present
i.nvention may also contain up to 50~, preferably
5-20% and more preferably 5-10~ 7 by weight,
based on the combined weights of the polyol and
curing agent, of an epoxy resin or the esteri-
fication product of the epoxy resin with a
monocarboxylic acid.
The epoxy resin is itself the reaction
product of epichlorohydrin and bisphenol-A and has
the general formula
CH~2--`~ c~ t ~ c_~o~C~2-cH-cH2~ o
CH3 n
CH3 /' \
----C~2 CH--CH2
c~3
where n is sufficiently large to provide an epoxy
resin having an epoxide equivalent weight of 450-
2000, preferably 850-1050. The epoxide eauivalent
weight is the unit weight of epoxy resin that
contains one unit equivalent of epoxide.
The epoxy resin/acid e~ster, which may be
used instead of, or in a mixture with, the epoxy
resin, is formed by reacting the above epoxy resin
with a monocar~oxylic acid. Although the choice
of the acid is not critical, most preferred are
benzoic acid, para-t-butyl benzoic acid, and the
higher fatty aclds, those having 12-18 carbon atoms.
The esters can be prepared by reacting
the acid and epoxy resin together in a closed
vessel equipped with agitator, thermocouple, and
condenser. The temperature is raised gradually by
heat application, with agitation starting as soon
as the epoxy resin melts, to about 230-270C over a
1-2 hour period. This temperature is maintained
until the resultant ester attains the desired
functionality, which can be determined by intermittent
sampling to measure the acid number of the ester.
At this point, the reaction mixture is cooled and can
be thinned with an appropriate organic solvent.
The polyester polyol, epoxy resin, and
epoxy-resin~acid ester are normally each in solution
and are suitable for direct use to form the coating
composition of this invention by blending with each
other and with the curing agent. ~When the curing
agent is a polyisocyanate, however, a two-component
sy tem should be used. That is, a solution of
polyisocyanate is in one package, and a solution of
the polyester polyol and, optionally, the epoxy
resin or epoxy-resin/acid ester is in a separate
package. The two solutions are thoroughly mixed
just before applying the coating composition.
Separation of the two solutions is usually necessary
since the "pot life" of the composition is short -
the polyisocyanate reacts with the hydroxyl groups
~G ~ L?~
of the polyol at a rapid rate, even at roomtemperature.
Instead of the two-component, "two-
package", system described above, a "one package"
coating composition can be prepared if the reactive
groups of the polyisocyanate are blocked with a
blocking agent such as methyl ethyl ketoxime. This
eliminates the need for keeping the polyester
polyol apart from the polyisocyanate until just before
use. When the coating composition, with the blocked
polyisocyanate, is applied and heated to 150-160C,
the blocking agent is released, permitting the poly-
isocyanate to react with the polyester.
Regardless of the method by which the
final coating composition is mixed, the comFosition contains
40-90~ by weight of the filmrforming blend and 10~60% by
weight of a solvent for the blend, these percentages being
based on the ccmbined weight of the blend and the solvent.
One of the useful aspects of the present invention is that it
can be conveniently spray-applied even at these high weight-
solids levels. The solvent of the final composition can be
a mixture of the organic solvents in which the constituents
of the film-forming blend are each formed.
The composition can also contain mixtures of various
diols, preferably hindered, diprimary diols of the kind dis-
cussed earlier, as reactive diluents. These materials will
advantageously act as a solvent for the film-forming blend,
and upon application, will not volatilize but will rather
beccme crosslinked into the final coating.
The coating composition of this invention may con-
tain about 0.01-2.0% by weight, base~ on the wieght of the
film-forming blend, of a curing catalyst. When the curing
agent is a polyisocyanate, the catalysts are usually organo
metallics such as dibutyl tin dilaurate and zinc octoate,
which are preferred, dibutyl tin di-2-ethylhexoate, stannous
5~'7~
-
11
octoate, stannous oleate, zinc naphthenate, vanadium
acetyl acetonate, or zirconium acetyl acetonate. Also useful
as catalysts are tertiary amines, such as, for example,
triethyle~e dia~ine, heptametllyliosbiguanide, triethylanune,
pyridine, di~ethylaniline, and methyl morpholine. When a
two-com~onent system is used, the catalyst can be added to
either the polyisocyanate solution or the solution of the
other constituents of the fi ~ forming blend.
When the curing agent is an aminoplast,
the catalyst can be an acid catalyst such as para-
toluenesulfonic acid. Other suitable catalysts
include acid phosphates such as methyl and butyl
acid phosphate, acid pyrophosphates such as dimethyl
acid pyrophosphates, organic acid sulfate esters,
and other organic sulfonic acids. The sulfonic
acids can be neutralized with an amine, preferably
a tertiary amine.
The coating composition of the invention
can be pigmented, containing an amount of pigment
in a pigment/film-former weight ratio of about
.005/1 to 100/1. Useful pigments are, for example,
metallic oxides, such as titanium dioxide or zinc
oxide; metal hydroxides; metal flakes; sulfides;
sulfates; carbonates; carbon black; silica; talc;
china clay; and organic dyes.
The pigments can be introduced into the
coating composition by first forming a mill base
with the polyester polyol. The mill base can be
formed, for example, by conventional sand-grinding
or ball-milling techniques, and then can be blended,
by simple stirring or agitation, with the other
constituents of the coating composition.
The coating composition can further optionally
contain, as a durability enhancer, an ultraviolet light sta-
bilizer, an antioxidant, or both. The ~ltraviolet light
5~
12
stabilizer can be present in an amount of 1-20%
by weight, based on the weight of the film-forming
blend; the antioxidant can be present in an amount
of 0.1-5~ by weight, based on the weight of the
film-forming blend.
Typical ultraviolet light stabilizers are
benzophenones, triazoles, triazines, benzotriazoles, benzoates,
lower alkyl thicnethylene-containing phenols, substituted
benzenes, organophosphorous sulfides, and sub-
stituted methylene malonitriles. Particularly usefulare the hindered amines and nickel compounds shown
in U.S. Patent 4,061,616 (December 6, 1977).
Typical antioxidants are tetra-kis
alkylene (di-alkyl hydroxy aryl) alkyl ester
alkanes, reaction product of p-amino diphenylamine
and glycidyl methacrylate, and alkyl hydroxyphenyl
groups bonded through carboalkoxy linkages to a
nitrogen atom of a heterocyclic nucleus containing
an imidodicarbonyl group or an imidoditniocarbonyl
group.
One preferred combination of ultraviolet
light stabilizer and antioxidant is 2-hydroxy-4-
dodecyloxy benzophenone or a substituted 2(2'-
hydroxyphenyl) benzotriazole and tetra-kis
methylene 3(3',5'-dibutyl-4'hydroxyphenyl) propionate
methane.
The coating composition can be applied
to a variety of substrates by any of the conven-
tional application methods such as spraying, dipping,
brushing, or flow coating. Substrates that can be
advantageously coated with the present composition
are, for example, metal, steel, wood, glass, or
plastics such as polypropylene, polystyrene,
copolymers of styrene, and the like. The coating
is particularly suited for application over primed
12
~Z~5~
13
or unprimed metal or steel. Typical uses are for
coating steel that has been treated with zinc
phosphate or iron phosphate, metal substrates pre-
coated with conventional alkyd or epoxy prlmers, and
galvanized steel.
When the curing agent is polylsocyanate,
the coating can be cured at ambient temperalures.
Also, regardless of which curing agent used,
-the composition can be dried (cured) by heating
at 120-210C ~or 15-30 minutes, although if a
blocked isocyanate is used, the temperature should
be at least 150C. Either method, however, ultimately
produces a coating that is hard, durable, scratch and
stain resistant, weather resistant, and chemical
resistant. The composition is suitable, for example,
for coating automobile or truck bodies, railroad
equipment, appliances, and any industrial equipment.
In an additional aspect, it is possible
to apply the present coating composition as a two-
coat system in which a first, pigmented coat isapplied as previously described over the substrate
and is then overlaid with a second, unpigm~nted
coat. This can impart to the finish a gloss or
appearance that is improved over that attainable
when a single coat system is used. This is par-
ticularly desireable when the composition is used as
an automotive coating. When such a two-coat system
is employed, however, the first coat should be
allowed to cure to a point where it is tack-free
before the second coat is applied. This will
normally prevent the solvent in the second coat
from attacking the first coat. This attack, or
strike-in, can cause the polymers of the two coats
to combine at their interface, negating the improve-
ment in the gloss or appearance.
~2~`~5~ 3
E ~PLE
The foL owlng 3 ingredients are pxepared
as follows:
1. Polyest~r Polyol Solution
Parts by
Portion I Weight
~Ionoes.er o~ neo~entyl glycol652.8
and hydxo~y valic acia
Neopentyl glycol 1331.2
Phthalic znhydride 592.0
Isophthalic acid 664.0
Adipic acid 292.0
Dibutyl 'in oxide 3 3
Xylene 125.0
Portion 2
15 Xylene 78.0
Portion 3
2-Ethyl hexanol ~203.0
Portion 1 is charged into a reaction
vessel equipped with an agitator and vapor condenser,
and is heated to r~flux, approximately 210C. This
temperature is maintained until the reaction is com-
pleted, determined by monitoring the flow of the
water of esterification from the condenser and by
intermittent samplins to determine when the acid
number reaches 5(completion). Total water collected
is 288 grzms. Portion 2 is added to the mixture,
which is then allowed to cool to about 125C, after
which portion 3 is added. This mixture is then
agitated an~ filtered. The resulting reaction
product, the polyester polyol, has a hydroxyl con-
tent of about 6.3~ by weight (based on product solids
weight) and a number averag~ molecular weight ~gel
permeation chromatography) of about 570-620; The
polyester polyol solution has a Gardner-Holdt
viscosity of ~-6-1/4 and a solids content of about
87% by weight.
14
~45~
2. Mill Base
Parts by
Weight
Polyester polyol solution 34.5
~in~redient 1~
Amyl acetate 18.0
Pigment dispersant (copolymer3.0
of methyl met;hacrylate/
2-ethylhexyl acrylate in
62.5/37.5 weight ratio in a
toluene/methyl isobutyl ketone/
methyl ethyl ketone solvent,
copolymer/solvent weight ratio
of 1/1)
TiO2 White pigment 10~.0
The constituents are added to a mixing
vessel and mixed for about 1 hour. The mixture
is then charged into a sand mill and ground at a
temperature of about 35C.
A coating composition is then prepared
with the following constituents:
Parts by
Portion 1 Weight
Polyester polyol solution40.2
(ingredient 1)
Epoxy resin ~60~ weight 8.3
solution of an epichlorohydrin-
bisphenol A epoxy resin, having
an epoxide equivalent weight of
875-1025, in a methyl ethyl
ketone/xylene solvent)
Mill base (ingredient 2) 155.5
Ethyl acetate 8.2
Portion 2
-
Dinonylnaphthalene disulfonic2.0
acid (40~ by weight in isobutanol)
Portion 3
. .
Hexamethoxymethylmelamine 30.0
Portion 1 is charged into a stainless
steel vessel and mixed for 15 minutes after which
time portion 2 is added, with mixing continuing
~ 3'7
16
for an additional 5 minutes. Portion 3 is then
thoroughly mixed into the vessel, giving a coating
composition in which the Eilm-forming blend (polyester
polyol, epoxy resin, and hexa-methoxymethylmelamine)
is about 70~ of the comhined weights of the
film-forming blend and solvent. Including pigments,
the coating composition is approximately 83~ solids by
weight.
The composition is sprayed (airless spray
using a pressure of 2400 pounds per square inch) onto
BONDERITE 1000* panels (cold rolled steel with iron
phosphate layer) and the panels thus coated are baked
for 30 minutes at 135C. When then tested, the
coating has a pencil hardness of 3H, a Tukon hardness
of 21.3 at 25C and of 7.0 at 70C.
The coated steel panels of this example have
a reverse impact of 80 inch-pounds (GARDNER* impact
tester) and the coating exhibits no visible cracks
when the panel is bent 180 around a conically-shaped
mandrel varying from 1/8 inch to 1-1~2 inches diameter
over an 8-inch length.
Several of the coated panels are scored to
the metal with a nail and placed in a salt-spray
cabinet where they are exposed to a mist of a solution
of NaCl (5~ by weight) in water. After 300 hours, the
coating creepage from the score line is 4 mm. After
500 hours, the creepage is 7 mm.
The coatings on such panels are found to be
resistant to stains from such common substances as
mustard, lipstick, and orange dye and to be immune
from attack by common solvents like toluene, xylene,
and methyl ethyl ketone.
* - denotes trade mark
16
