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Description  |
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BACKGROUND OF THE INVENTION
The invention concerns aqueous dispersions of copolymers based on
(meth)acrylic acid derivatives, their preparation and use, for example, in
the production of thermosetting or stoving enamels. Enamels produced this
way yield coats of increased surface hardness, with increased elasticity,
extensibility and substrate adhesion.
It is known that aqueous dispersions of (meth)acrylic ester copolymers,
which contain, optionally, other monomer units derived, for example, from
styrene, and which are thermosettable by the incorporation of comonomers
containing hydroxyl groups, such as hydroxypropyl methacrylate, and by the
addition of a cross-linking agent, like hexamethoxymethyl melamine, can be
used as binders for stoving enamels. Enamels containing these binders
frequently have a high surface hardness, but they are generally either
extremely brittle, or they are flexible so that the undercoat can still be
deformed, for example, without the enamel chipping off, but their surface
hardness is then insufficient.
OBJECTS OF THE INVENTION
An object of the present invention is therefore to provide aqueous
dispersions of copolymers which are particularly suitable for use as
binders for stoving enamels which yield coats of improved surface hardness
and increased flexibility.
Another object of the present invention is the obtaining of an aqueous
dispersion containing from 30% to 70% by weight of a cross-linkable
copolymer based on (meth)acrylic acid and its derivatives, said
cross-linkable copolymers consisting essentially of monomer units of
(meth)acrylic acid and/or its derivatives, other ethylenically-unsaturated
compounds copolymerizable therewith, and from 1% to 15% by weight of the
total weight of said copolymers, of monomer units of the formula:
##STR1##
wherein R represents a straight-chained alkylene having 2 to 10 carbon
atoms, optionally substituted with up to three methyl and/or ethyl groups,
and R' represents a member selected from the group consisting of hydrogen
and methyl, where the hydroxy group is linked to a carbon atom having at
least one hydrogen attached thereto.
A further object of the present invention is the development of a process
for the production of the above aqueous dispersion.
A yet further object of the present invention is the development of a
stoving enamel comprising the aforesaid aqueous dispersion and a
cross-linking agent.
These and other objects of the present invention will become more apparent
as the description thereof proceeds.
DESCRIPTION OF THE INVENTION
The drawbacks of the prior art were overcome and the above objects were
achieved by the development of aqueous dispersions containing from 30% to
70% by weight, based on the total weight of the dispersion, of copolymers
of (meth)acrylic acid and/or its derivatives and, optionally, other
ethylenically-unsaturated compounds that are copolymerizable therewith, as
well as customary additives. These dispersions are characterized in that
the copolymers contain from 1% to 15% by weight, based on the total weight
of the copolymers, of units of the general Formula I:
##STR2##
where R=straight-chained hydrocarbon radicals with 2 to 10 carbon atoms,
which can be substituted, if necessary, by up to three methyl and/or ethyl
radicals;
R'=H or CH.sub.3
and where the hydroxy group is linked to a carbon atom of the radical R to
which at least one additional hydrogen is linked directly.
More particularly, the present invention relates to an aqueous dispersion
containing from 30% to 70% by weight of a cross-linkable copolymer based
on (meth)acrylic acid and its derivatives, said cross-linkable copolymers
consisting essentially of monomer units of (meth)acrylic acid and/or its
derivatives, other ethylenically-unsaturated compounds copolymerizable
therewith, and from 1% to 15% by weight of the total weight of said
copolymers, of monomer units of the formula:
##STR3##
wherein R represents a straight-chained alkylene having 2 to 10 carbon
atoms optionally substituted with up to three methyl and/or ethyl groups,
and R' represents a member selected from the group consisting of hydrogen
and methyl, where the hydroxy group is linked to a carbon atom having at
least one hydrogen attached thereto.
The dispersions according to the invention can be prepared by emulsion
polymerization in the presence of customary initiators forming free
radicals and/or catalysts and, optionally, customary additives where a
monomer mixture, dispersed in the aqueous phase, is copolymerized which
contains from 1% to 15% by weight, based on the total weight of the
comonomers, of compounds of the general Formula II:
##STR4##
wherein R and R' have the same meaning as in Formula I.
More particularly, the process of the invention is a process for the
production of the above aqueous dispersions consisting essentially of the
steps of polymerizing, under polymerizing conditions, an aqueous
dispersion containing from 30% to 70% by weight of said dispersion of
monomers selected from the group consisting of (meth)acrylic acid and/or
its derivatives, other ethylenically-unsaturated compounds copolymerizable
therewith and from 1% to 15% by weight of said monomers of (meth)allyl
esters of the formula:
##STR5##
wherein R represents a straight-chained alkylene having 2 to 10 carbon
atoms, optionally substituted with up to three methyl and/or ethyl groups,
and R' represents a member selected from the group consisting of hydrogen
and methyl, where the hydroxyl group is linked to a carbon atom having at
least one hydrogen attached thereto, and recovering said aqueous
dispersion containing from 30% to 70% by weight of cross-linkable
copolymers.
Suitable monomers, in addition to the compounds of Formula II, for the
preparation of the dispersions according to the invention, are
particularly derivatives of acrylic acid and of methacrylic acid, like
alkyl esters having from 1 to 18 carbon atoms in the alkyl of
(meth)acrylic acid, and styrene. The term "(meth)acrylic acid" relates to
both methacrylic acid and acrylic acid. The term "(meth)allyl" relates to
both allyl and methallyl. The mention of acrylic acid derivatives
generally also applies to methacrylic acid derivatives.
The preferred aqueous dispersions contain copolymers which are composed of:
(a) from 30% to 70% by weight of monomer units selected from the group
consisting of alkyl (meth)acrylates or styrene which form homopolymers
with a high glass transition temperature, particularly a glass transition
temperature of 80.degree. C. or over,
(b) from 19% to 69% by weight of monomer units selected from the group
consisting of alkyl (meth)acrylates which form homopolymers with a low
glass transition temperature, particularly a glass transition temperature
of 10.degree. C. or less,
(c) from 0 to 20% by weight of monomer units of other
ethylenically-unsaturated monomers, and
(d) from 1% to 15% by weight of monomer units of the general Formula I, all
weights based on the total weight of the copolymers.
These preferred dispersions are prepared by polymerization in an aqueous
emulsion, as described above, of a monomer mixture of 30% to 70% by weight
of monomers which yield in the polymerization the units described under
(a), 19% to 69% by weight of monomers which yield the units described
under (b), 0 to 20% by weight of additional monomers yielding the units
described under (c), and 1% to 15% by weight, based on the total weight of
the monomer mixture, of the compounds of the general formula II, described
under (d).
Suitable monomers which yield homopolymers with a high glass transition
temperature (T.sub.g equal to or greater than 80.degree. C.) are, for
example, alkyl esters with alkyl radicals of 1 to 4 carbon atoms of
methacrylic acid, particularly methyl methacrylate, ethyl methacrylate,
propyl methacrylate and the butyl methacrylates, but also styrene.
Suitable monomers which yield homopolymers with a low glass transition
temperature (T.sub.g equal to or less than 10.degree. C.) are, for
example, alkyl esters with alkyl radicals of 1 to 8 carbon atoms of
acrylic acid, particularly butyl acrylate and octyl acrylate.
Other suitable ethylenically-unsaturated monomers which are or can be
polymerized up to 50% by weight, preferably up to 20% by weight, into the
copolymers according to the invention are, for example, (meth)acrylic acid
and its salts, (meth)acrylamide, (meth)acrylonitrile, N-methylol
(meth)acrylamide and its derivatives, for example, etherified with
customary alkyl radicals, N-vinyl-2-pyrrolidone, N-vinyl lactams,
butadiene, isoprene, divinylbenzene, vinyl esters of saturated carboxylic
acids, particularly vinyl lower alkanoates, such as vinyl acetate, vinyl
and vinylidene halides, ethylene, unsaturated dicarboxylic acids, like
maleic acid, fumaric acid, and itaconic acid, as well as their salts,
their (half) esters and their (half) amides.
The concentration of these other ethylenically-unsaturated monomers in the
monomer mixture or the units in the copolymer derived therefrom preferably
do not exceed 3% by weight, based on the total copolymer weight.
More particularly, therefore, the present invention involves an aqueous
dispersion containing from 40% to 60% by weight of a cross-linkable
copolymer having:
(a) from 30% to 70% by weight of said copolymer of monomer units which form
homopolymers having a glass transition temperature of 80.degree. C. or
higher selected from the group consisting of alkyl esters with alkyl
having from 1 to 4 carbon atoms of methacrylic acid and styrene,
(b) from 19% to 69% by weight of said copolymer of monomer units which form
homopolymers having a glass transition temperature of 10.degree. C. or
lower selected from the group consisting of alkyl esters with alkyl having
from 1 to 8 carbon atoms of acrylic acid,
(c) from 0 to 20% by weight of said copolymer of monomer units of other
monomers selected from the group consisting of (meth)acrylic acid and its
salts, (meth)acrylamide, (meth)acrylonitrile, N-methylol-(meth)acrylamide,
N-alkoxymethyl-(meth)acrylamide, N-vinyl-2-pyrrolidone, N-vinyl-lactams,
butadiene, isoprene, divinylbenzene, vinyl lower alkanoates, vinyl
halides, vinylidene halides, ethylene, alkenedioic acids having from 4 to
8 carbon atoms, their salts, mono-lower alkyl esters, di-lower alkyl
esters, monoamides and diamides, and
(d) from 1% to 15% by weight of said copolymer of monomer units of
(meth)allyl esters of the formula
##STR6##
wherein R represents a straight-chained alkylene having 2 to 10 carbon
atoms, optionally substituted with up to three methyl and/or ethyl groups,
and R' represents a member selected from the group consisting of hydrogen
and methyl, where the hydroxy group is linked to a carbon atom having at
least one hydrogen attached thereto.
The units of Formula I are derived from the (meth)allyl esters of Formula
II, preferably from (meth)allyl .alpha.-hydroxyalkanoates. These compounds
and their preparation are disclosed in the published German Application P
28 22 436, which corresponds to commonly assigned U.S. patent application
Ser. No. 37,749, filed May 10, 1979, now U.S. Pat. No. 4,245,076. Therein
is disclosed a process of reacting (meth)allyl alcohol with the free
.omega.-hydroxycarboxylic acids or the lactones derived therefrom in the
presence of acid catalysts, such as sulfuric acid, p-toluene sulfonic
acids and acid ion-exchangers.
The following examples of preferred compounds of Formula II are mentioned:
the allyl and methallyl esters of
5-hydroxyvaleric acid,
6-hydroxycaproic acid,
6-hydroxymethylcaproic acid,
6-hydroxydimethylcaproic acid,
7-hydroxyenantic acid, and
11-hydroxyundecanoic acid.
The copolymers contained in the dispersions according to the invention have
preferably K-values (according to Fikentscher) of about 40 to 140,
particularly about 60 to 120.
Particularly preferred are dispersions which contain 40% to 60% by weight
of these copolymers, which are composed, based on the total weight of the
copolymers, of:
(a) 50% to 60% by weight of monomer units derived from from methyl
methacrylate or styrene,
(b) 24% to 47% by weight of monomer units derived from butyl acrylate,
(c) 0 to 3% by weight of monomer units derived from acrylic acid and/or
acrylamide, and
(d) 3% to 13% by weight of monomer units derived from allyl
6-hydroxycaproate.
For the preparation of these particularly preferred dispersions, the
above-mentioned monomers are polymerized under free-radical polymerization
conditions in an aqueous emulsion in the percentages indicated for the
copolymer composition.
The polymerization for the preparation of the aqueous dispersions according
to the invention is effected according to the generally known methods of
emulsion polymerization with water as the second phase, using customary
additives, like emulsifiers and, optionally, protective colloids, as well
as initiators and/or catalysts, and, optionally, reducing agents or
activators and buffer substances.
It is possible to charge the reaction mixture with the exception of the
initiators, catalysts, reducing agents and/or activators, jointly and to
start the polymerization by adding these free-radical-supplying
components.
Frequently it is more advantageous to add gradually to a charge of water,
emulsifier and, optionally, protective colloids, the total amount or a
part of the monomers and/or of the free-radical supplying components
during the polymerization.
Naturally it is also possible to charge individual monomers, and to add the
other monomers gradually, or to proceed in the reverse order. In
particular, it was found expedient in many cases to charge the total
amount or a major part of the (meth)allyl ester of the hydroxycarboxylic
acid in mixture with a smaller portion of the other monomers, and to add
the remaining amount of the monomers gradually during the polymerization.
The initiator system can naturally also be added at once, but frequently it
is found of advantage to dose it over the polymerization period.
The same dispersion aids can be used in the preparation of the dispersions
according to the invention which have been used heretofore in the
preparation of aqueous dispersions of emulsion polymerization. Thus, for
example, ionic and nonionic emulsifiers can be used alone or together with
protective colloids. Examples of ionic emulsifiers are particularly the
anionic alkylsulfonic acids, arylsulfonic acids, alkarylsulfonic acids,
and aralkylsulfonic acids, such as tetradecanesulfonic acid, as well as
their salts, particularly with the metals of the first main group of the
periodic system, or with ammonia or with amines.
Examples of nonionic emulsifiers are the addition products of ethylene
oxide onto alkylphenols, fatty alcohols, fatty acids, fatty amines, etc.
Examples of protective colloids are fully or partly saponified polyvinyl
acetate and cellulose derivatives, like methyl cellulose and hydroxyethyl
cellulose.
The catalysts or initiators for the preparation of the dispersions
according to the invention are the same substances that are generally used
in emulsion polymerization, particularly compounds forming free radicals,
like peroxide or azo compounds, such as ammonium or potassium peroxide or
redox catalysts, which are composed of peroxide compounds and reducing
agents as activators.
If necessary, buffer substances, like alkali metal bicarbonates, alkali
metal phosphates, etc., can also be used.
The dispersion aids and the polymerization initiators and catalysts and
buffer substances are used in the amounts customary in emulsion
polymerization. The amount of the compounds forming free radicals is
generally preferably in the range of from 0.001% to 1% by weight, based on
the total weight of the comonomers; the amount of the dispersion agents is
generally preferably in the in the range of from 0.1% to 10% by weight,
based on the total weight of the comonomers.
The dispersions according to the invention can be used preferably for the
production of thermosetting or stoving enamels. These stoving enamels are
particularly suitable for coating solid surfaces, preferably metal
objects, such as casings or parts of machines or refrigerators, since they
yield coats of great surface hardness and do not chip off on impact
stresses, due to their elasticity. Beyond that, these coats also are
suitable for the "coil-coating" methods, since the shaping of the metal
after the coating has practically no effect on the quality of the coat.
The thermosetting enamels are produced by adding in known manner to the
dispersions according to the invention, pigments, dyes, fillers and,
optionally, other customary aids, such as leveling aids, dispersion
agents, wetting agents, aids to influence the rheological properties of
the enamels, as well as by the addition of cross-linking agents.
The cross-linking agents, such as melamine resins, urea-formaldehyde
resins, phenol-formaldehyde resins and blocked isocyanates, are added
preferably in amounts of from 1% to 10%, particularly from 2% to 5%, by
weight, based on the weight of the binder, hence of the copolymers
contained in the dispersions according to the invention.
The pigments, dyes and/or fillers, as well as leveling aids, film-forming
aids, dispersion agents, wetting agents and/or aids to influence the
rheological properties used in customary amounts are those substances
which have already been used in stoving enamels based on aqueous polymer
dispersions.
Pigments or fillers, like metal oxides or metal sulfide compounds, carbon
black, sulfates, silicates, chromates, or organic coloring pigments, metal
powders, like aluminum, bronze, etc., talc, barite, kaolin, quartz, mica,
diatomaceous earth, PVC powder, etc., are preferably used in amounts of
from 0 to 100%, particularly from 20% to 80% by weight, based on the
binder content.
The additional wetting and dispersion agents, like alkali metal
metaphosphate and alkali metal polyphosphate compounds, can be used
preferably in amounts of up to 2% by weight, based on the weight of the
binders. The leveling and film-forming agents, like polyhydric alcohols,
high-boiling hydrocarbons, particularly aromates, and high-boiling esters,
can be used preferably in amounts up to 10% by weight, particularly from 0
to 3% by weight, based on the binder, and aids to influence the
rheological properties, like higher-boiling alcohols, can be used
preferably in amounts of up to 20% by weight, based on the total weight of
the dispersion.
The stoving enamels thus produced can be hardened at temperatures of
80.degree. to 300.degree. C., preferably at 100.degree. to 200.degree. C.,
if necessary under pressure. The required heating time is generally 5 to
60 minutes, but preferably not more than 30 minutes. The stoving time can
be reduced by the use of suitable catalysts, such as acid compounds
(toluene-sulfonic acid) with formaldehyde condensation products as
cross-linking agents, or organic tin compounds (dibutyl tin dilaurate)
with the use of blocked diisocyanates as cross-linking agents. With them,
completely homogeneous coats which are resistant to most solvents are
obtained.
The following examples illustrate the invention without being limitative.
EXAMPLES
Examples 1 to 5 illustrate the preparation of the dispersions according to
the invention.
EXAMPLE 1
Into a two-liter glass vessel with stirrer and reflux condenser were
charged: 330 gm of water and 9 gm of a 30% aqueous alkyl sulfonate
solution (Mersolat.RTM.K 30, Bayer AG), and heated to 80.degree. C. After
adjusting the solution to a pH value of 3.3, the following monomer
emulsion was added within two hours at 80.degree. C. under stirring:
______________________________________
Amount
in Grams
______________________________________
550.0 Water
4.5 Mersolat.RTM. K 30 (30%)
4.5 K.sub.2 S.sub.2 O.sub.8
9.0 Acrylic acid
18.0 Acrylamide
495.0 Methyl methacrylate
350.0 Butyl acrylate
57.0 Allyl 6-hydroxycaproate.
______________________________________
Subsequently a pH of 8 was established with ammonia, and the polymerization
was continued for one hour at 80.degree. C. under stirring. The dispersion
had then a solids content of 50.2% by weight and a viscosity of 765 mPa.s
(Brookfield RVT-viscosimeter, 10 min.sup.-1).
EXAMPLE 2
Example 1 was repeated but the parts by weight in the monomer emulsion were
varied as follows for the last three monomers:
______________________________________
Grams
______________________________________
540.0 Methyl methacrylate
250.0 Butyl acrylate
114.0 Allyl 6-hydroxycaproate.
______________________________________
EXAMPLE 3
Example 1 was repeated and the following changes were made in the monomer
composition:
______________________________________
Grams
______________________________________
Methyl methacrylate
540.0 (Instead of 495 grams)
Ethylhexyl acrylate
305.0 (Instead of butyl acrylate)
______________________________________
EXAMPLE 4
Example 1 was repeated, but instead of 495 gm of methyl methacrylate, 495
gm of styrene were employed.
COMPARISON TEST A
Example 1 was repeated but instead of allyl 6-hydroxycaproate, the same
molar amount of 2-hydroxypropyl methacrylate was employed. The monomer
composition of the emulsion contains instead of the amounts indicated in
Example 1:
______________________________________
Grams
______________________________________
Butyl acrylate 361.0 (Instead of 350 gm)
2-Hydroxypropyl methacrylate
46.0 (Instead of allyl 6-
hydroxy caproate)
______________________________________
EXAMPLE 5
The copolymer dispersions of Examples 1 to 4 and of Comparison Test A were
mixed according to the following formula to a white glossy stoving enamel
(Table 1).
TABLE 1
______________________________________
Parts by
Weight Components
______________________________________
100.0 Copolymer dispersion (Examples 1 to 4
and Comparison Test A)
8.0 Hexamethoxymethyl melamine (25% in
water/isopropanol 93:7)
0.2 Sodium metaphosphate (Calgon.RTM. N, 10%
in water)
0.2 Polyphosphate (dispersion agent PA 30,
Hoechst AG)
15.0 Titanium dioxide Kronos.RTM. RN 45
1.0 Propylene glycol.
______________________________________
The mixtures were applied in a thickness of 20 .mu.m on Erichsen cupping
plates (DIN 1624, Deutsche Industrienorm, German Industrial Standard) and
stoved at 150.degree. C. for ten minutes.
In all cases, firmly adhering glossy coats were obtained which were
subjected to the tests listed in Table 2.
In addition, Table 2 contains values from the determination of the breaking
elongation according to DIN 53 504. To this end, unpigmented films were
produced and annealed for ten minutes at 150.degree. C.
TABLE 2
______________________________________
Dispersion Compari-
from Example No.
1 2 3 4 son Test
______________________________________
Surface
hardness according
to Koenig (s)
(DIN 53 157)
137 166 105 162 97
Breaking
elongation (%)
(DIN 53 504)
330 140 370 130 130
Ball impact
test (inch-pound)
(ASTM D 2749)
70 2 -- -- <2
Erichsen
cupping (mm)
plate plate plate plate 8
(DIN 53 156)
crack crack crack crack
Mandrel
bending text (mm)
(DIN 53 152)
<2 <2 <2 <2 <2
Degree of
cross-linkage (%)*
>99 >99 >99 >99 >99
______________________________________
*Determined by extraction tests with boiling ethyl acetate.
These tests show that the stoving enamels of the invention had a higher
surface hardness and good flexibility without loss of other properties.
The preceding specific embodiments are illustrative of the practice of the
invention. It is to be understood, however, that other expedients known to
those skilled in the art or disclosed herein may be employed without
departing from the spirit of the invention or the scope of the appended
claims.
* * * * *
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Description |
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