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Description  |
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BACKGROUND OF THE INVENTION
Methods of preparing crosslinked acrylic polymer microparticles commonly
referred to as microgel particles are known in the art. One such method is
disclosed in commonly-assigned copending application Ser. No. 296,700,
filed Oct. 11, 1972, now U.S. Pat. No. 3,880,796, in the name of Roger M.
Christenson et al. In this method, a non-aqueous polymer dispersion is
prepared by polymerizing an ethylenically unsaturated monomer containing
hydroxyl groups in the presence of (1) a dispersing liquid which is a
solvent for the monomer but in which the resultant polymer is insoluble
and (2) a dispersion stabilizer. The resultant non-aqueous polymer
dispersion produced by this method consists of a major proportion of
uncrosslinked acrylic polymer particles and a minor proportion (e.g., 10
percent by weight or less) of crosslinked acrylic polymer particles (i.e.,
microgel particles). Accordingly, in this method, it is necessary to
separate the microgel particles from the uncrosslinked polymer particles.
This is accomplished by the addition to the dispersion of an active
solvent for the uncrosslinked polymer particles, thereby converting the
dispersion to essentially a solution, but for the presence of the
insoluble microgel particles. The microgel particles are then separated
from the bulk of the polymer by conventional means such as centrifuging,
filtering, and the like.
The above process, while advantageous in some respects, has several serious
disadvantages. Thus, as will be apparent, the microgel particles are an
incidental by-product of the non-aqueous dispersion process and therefore
the yield is relatively low (e.g., 5 to 10 percent by weight or less).
Moreover, because of this factor, it is necessary to separate the microgel
particles from a dispersion which contains a major proportion of
uncrosslinked acrylic polymer particles by dissolving the uncrosslinked
polymer particles with an active solvent.
Still another method for producing microgel particles is disclosed in
British Pat. No. 967,051 to Bullitt et al, dated Aug. 19, 1964. In this
method, microgel particles are prepared by forming an aqueous emulsion of
monoethylenic unsaturated monomer and a crosslinking monomer containing at
least two ethylenic double bonds, heating the emulsion to a temperature of
about 40.degree. to 100.degree. C. until the reaction is substantially
complete to yield a microgel and during the reaction adding an agent to
inhibit the formation of high molecular weight substantially uncrosslinked
material. The inhibiting agent as disclosed in Bullitt et al can be an
active solvent for the monomers or a chain transfer agent. This method has
several disadvantages. Thus, the method utilizes conventional emulsion
polymerization techniques requiring careful control of the process to
prevent settling and the like. Further, the use of crosslinking monomers
containing at least 2 ethylenic double bonds (e.g., divinyl and diacrylate
monomers) has been found to lead to flocculation problems in relatively
high solids level (i.e., 40 percent by weight or higher) microgel particle
dispersions. Finally, this method requires the additional step of adding a
water-immiscible solvent or chain transfer agent to the reaction mixture.
BRIEF SUMMARY OF THE INVENTION
The method of the present invention overcomes essentially all of the
disadvantages of the prior art. Thus, the present invention provides a
method of producing crosslinked acrylic polymer microparticles in
relatively high concentrations (i.e., solids levels of 20 to 60 percent by
weight) by a process which comprises the free radical addition
copolymerization of alpha, beta-ethylenically unsaturated monocarboxylic
acid, at least one other copolymerizable monoethylenically unsaturated
monomer and crosslinking monomer selected from the group consisting of (1)
epoxy group-containing compound and (2) a mixture of alkylenimine and
organoalkoxysilane in the presence of a polymeric dispersion stabilizer
and dispersing liquid in which the cross-linked acrylic polymer particles
are insoluble. The reaction is carried out at elevated temperatures such
that the dispersion polymer first forms and then is crosslinked; usually
the temperature should be between about 50.degree. C. and 150.degree. C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Examples of alpha, beta-ethylenically unsaturated monocarboxylic acid which
may be used are acrylic acid, methacrylic acid, ethacrylic acid,
alpha-chloroacrylic acid, crotonic acid isocrotonic acid, tiglic acid and
angelic acid. The preferred alpha, beta-ethylenically unsaturated
monocarboxylic acids are acrylic acid and methacrylic acid. Methacrylic
acid is especially preferred. The amount of alpha, beta-ethylenically
unsaturated monocarboxylic acid employed in the process of the invention
is usually in the range of from about 0.5 percent to about 15 percent by
weight of the monomers used in the copolymerization process.
Various other monoethylenically unsaturated monomers may be copolymerized
with the acid monomer in the process of this invention. Although
essentially any copolymerizable monoethylenic monomer may be utilized,
depending upon the properties desired, the preferred
monoethylenically-unsaturated monomers are the alkyl esters of acrylic or
methacrylic acid, particularly those having from about 1 to about 4 atoms
in the alkyl group. Illustrative of such compounds are the alkyl
acrylates, such as methyl acrylate, ethyl acrylate, propyl acrylate, and
butyl acrylate and the alkyl methacrylates, such as methyl methacrylate,
ethyl methacrylate, propyl methacrylate and butyl methacrylate. Other
ethylenically unsaturated monomers which may advantageously be employed
include, for example, the vinyl aromatic hydrocarbons, such as styrene,
alpha-methyl styrene, vinyl toluene, unsaturated esters of organic and
inorganic acids, such as vinyl acetate, vinyl chloride and the like, and
the unsaturated nitriles, such as acrylonitrile, methacrylonitrile,
ethacrylonitrile, and the like. From about 70 percent to about 99 percent
by weight of such monoethylenically unsaturated monomers, based on the
weight of monomer solids can be utilized.
As indicated above, the crosslinking monomer employed in the process of the
invention is selected from the group consisting of (1) epoxy
group-containing compound and (2) a mixture of alkylenimine and
organoalkoxysilane.
A particularly preferred class of epoxy group-containing compounds which
may be utilized in the practice of the invention are monoepoxide compounds
which additionally contain ethylenic unsaturation. Illustrative of such
preferred compounds are, for example, glycidyl acrylate and glycidyl
methacrylate.
Various alkylenimines can be utilized in the practice of the invention
including substituted alkylenimines. The preferred class of such amines
are those of the formula:
##STR1##
where R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 are each hydrogen;
alkyl, such as methyl, ethyl, propyl, or the like, having, for example, up
to about 20 carbon atoms; aryl, such as phenyl or the like; aralkyl, such
as tolyl, xylyl or the like; or aralkyl, such as benzyl, phenethyl or the
like. R.sub.6 in the above formula is hydrogen or a lower alkyl radical
usually having not more than about 6 carbon atoms, and n is an integer
from 0 to 1.
It is intended that the groups designated by the above formula include
substituted radicals of the classes indicated where the substituent groups
do not adversely affect the basic nature of the imine in the reaction.
Such substituents can include the groups such as cyano, halo, amino,
hydroxy, alkoxy, carbalkoxy and nitrile. The substituted groups may thus
be cyanoalkyl, haloalkyl, aminoalkyl, hydroxyalkyl, alkoxyalkyl,
carbalkoxyalkyl, and similar substituted derivatives of aryl, alkaryl and
aralkyl groups where present.
A number of specific examples of alkylenimines within the class described
are as follows:
Ethylenimine (aziridine)
1,2-propylenimine (2-methyl aziridine)
1,3-propylenimine (azetidine)
1,2-dodecylenimine (2-decyl aziridine)
1,1-dimethyl ethylanimine (2,2-dimethyl aziridine)
Phenyl ethylenimine (2-phenyl aziridine)
Benzyl ethylenimine (2-phenylmethyl aziridine)
Hydroxyethyl ethylenimine (2-(2-hydroxyethyl)aziridine)
Aminoethyl ethylenimine (2-(2-aminoethyl)aziridine)
2-methyl propylenimine (2-methyl azetidine)
3-chloropropyl ethylenimine (2-(3-chloropropyl)aziridine)
Methoxyethyl ethylenimine (2-(2-methoxyethyl)aziridine)
Dodecyl aziridinyl formate (dodecyl 1-aziridinyl carboxylate)
N-ethyl ethylenimine (1-ethyl aziridine)
N-(2-aminoethyl)ethylenimine (1-(2-aminoethyl)aziridine)
N-(phenethyl)ethylenimine (1-(2-phenylethyl)aziridine)
N-(2-hydroxyethyl)ethylenimine (1-(2-hydroxyethyl)aziridine)
N-(cyanoethyl)ethylenimine (1-cyanoethyl aziridine)
N-phenyl ethylenimine (1-phenyl aziridine)
N-(p-chlorophenyl)ethylenimine (1-(4-chlorophenyl)aziridine)
Because of their availability and because they have been found to be among
the most effective, the preferred imines are hydroxyalkyl-substituted
alkylenimines, such as N-hydroxyethyl ethylenimine and N-hydroxyethyl
propylenimine.
Organoalkoxysilane monomers which may advantageously be employed in the
practice of this invention are the acrylatoalkoxysilanes,
methacrylatoalkoxysilanes and the vinylalkoxysilanes. Illustrative of such
compounds are acryloxypropyltrimethoxysilane,
gamma-methacryloxypropyltrimethoxysilane,
gamma-methacryloxypropyltriethoxysilane,
gamma-methacryloxypropyl-tris(2-methoxyethoxy)silane,
vinyltrimethoxysilane, vinyltriethoxysilane,
vinyl-tris(2-methoxyethoxy)silane and the like. Of these
organoalkoxysilanes, gamma-methacryloxypropyltrimethoxysilane is
especially preferred.
The proportion of such crosslinking monomer employed in the process of the
invention may range from 0.5 percent to 15 percent by weight of the
monomers used in the copolymerization process. When the crosslinking
monomer is a mixture of alkylenimine and organoalkoxysilane, the mole
ratio of the alkylenimine to the alpha, beta-ethylenically unsaturated
monocarboxylic acid used to prepare the polymer is generally in the range
of from 0.5:1 to 1.5:1 and the mole ratio of the organoalkoxysilane to the
alpha, beta-ethylenically unsaturated monocarboxylic acid used to prepare
the polymer is generally in the range of from 1.5:1 to 3.5:1.
The monoethylenically-unsaturated monomer, acid monomer and cross-linking
monomer are polymerized in a dispersing liquid which solubilizes the
monomers but in which the resulting polymers are essentially not soluble
and form dispersed polymer particles. The dispersing liquid is generally a
hydrocarbon medium consisting essentially of liquid aliphatic
hydrocarbons. A pure aliphatic hydrocarbon or a mixture of two or more may
be employed. To the extent that any particular polymer produced is mostly
insoluble in the hydrocarbon medium resulting, the essentially aliphatic
hydrocarbon may be modified by the incorporation of other solvent
materials such as aromatic or naphthenic hydrocarbons, and in certain
instances, the amount of such non-aliphatic component may attain as high
as 49 percent by weight of the entire dispersing liquid. However, the
dispersing liquid preferably consists essentially of aliphatic
hydrocarbons and, in general, the compositions of the present invention
contain less than 25 percent by weight based on the weight of the
dispersing liquid of an aromatic hydrocarbon and often none at all at this
stage.
It is essential that the hydrocarbon be of liquid character, but it may
have a wide boiling range from a minimum of about 30.degree. C. (in which
case high pressures may be needed in the polymerization) to a maximum
which may be as high as 300.degree. C. For most purposes, the boiling
point should be from about 50.degree. C. up to about 235.degree. C.
Examples of dispersing liquids useful herein are pentane, hexane, heptane,
octane, mixtures of the same, and the like.
Ordinarily, the polymerizable composition of monomers and dispersing liquid
should contain from about 30 to about 80 percent by weight of the
dispersing liquid. It is understood, however, that the monomeric solution
need contain only that amount of dispersing liquid necessary to solubilize
the monomers and maintain the resulting polymers in a dispersed state
after polymerization.
The monomers are polymerized in the presence of a dispersion stabilizer.
The dispersion stabilizer employed in producing the microparticles of the
invention is a compound, usually polymeric, which contains at least two
segments of which one segment is solvated by the dispersing liquid and a
second segment is of different polarity than the first segment and is
relatively insoluble (compared to the first segment) in the dispersing
liquid. Although such compounds have been used in the past to prepare
dispersions of polymer, in those instances it has been considered
necessary that the polymer produced be ungelled, film-forming and soluble
in certain solvents.
Included among such dispersion stabilizers are polyacrylates and
polymethacrylates, such as poly(lauryl)methacrylate and poly(2-ethylhexyl
acrylate); diene polymers and copolymers such as polybutadiene and
degraded rubbers; aminoplast resins, particularly highly naphtha-tolerant
compounds such as melamine-formaldehyde resins etherified with higher
alcohols (e.g., alcohols having 4 to 12 carbon atoms), for example,
butanol, hexanol, 2-ethylhexanol, etc., and other aminoplasts of similar
characteristics such as certain resins based on urea, benzoguanamine, and
the like; and various copolymers designed to have the desired
characteristics, for example, polyethylenevinyl acetate copolymers.
The presently preferred dispersion stabilizers used in this invention are
graft copolymers comprising two types of polymer components of which one
segment is solvated by the aliphatic hydrocarbon solvent and is usually
not associated with polymerized particles of the polymerizable
ethylenically unsaturated monomer and the second type is an anchor polymer
of different polarity from the first type and being relatively
non-solvatable by the aliphatic hydrocarbon solvent and capable of
anchoring with the polymerized particles of the ethylenically unsaturated
monomer, said anchor polymer containing pendant groups capable of
copolymerizing with ethylenically unsaturated monomers.
The preferred dispersion stabilizers are comprised of two segments. The
first segment (A) comprises the reaction product of (1) a long-chain
hydrocarbon molecule which is solvatable by the dispersing liquid and
contains a terminal reactive group and (2) an ethylenically unsaturated
compound which is copolymerizable with the ethylenically unsaturated
monomer to be polymerized and which contains a functional group capable of
reacting with the terminal reactive groups of the long-chain hydrocarbon
molecule (1).
Generally, the solvatable segment (A) is a monofunctional polymeric
material of molecular weight of about 300 to about 3,000. These polymers
may be made, for example, by condensation reactions producing a polyester
or polyether. The most convenient monomers to use are hydroxy acids or
lactones which form hydroxy acid polymers. For example, a hydroxy fatty
acid such as 12-hydroxystearic acid may be polymerized to form a non-polar
component solvatable by such non-polar organic liquids as aliphatic and
aromatic hydrocarbons. The polyhydroxy stearic acid may then be reacted
with a compound which is copolymerizable with the acrylic monomer to be
polymerized, such as glycidyl acrylate or glycidyl methacrylate. The
glycidyl groups would react with the carboxyl group of the polyhydroxy
stearic acid and the polymer segment (A) would be formed.
Somewhat more complex, but still useful, polyesters may be made by reacting
diacids with diols. For example, 1,12-dodecanediol may be reacted with
sebacic acid or its diacid chloride to form a component solvatable by
aliphatic hydrocarbons.
The preferred polymeric segment (A) of the dispersion stabilizer is formed
by reacting poly-(12-hydroxystearic acid) with glycidyl methacrylate.
The second polymeric segment (B) of the dispersion stabilizer is of
polarity different from the first segment (A) and, as such, is relatively
non-solvated by the dispersing liquid and is associated with or capable of
anchoring onto the acrylic polymeric particles formed by the
polymerization and contains a pendant group which is copolymerizable with
the acrylic monomer. This anchor segment (B) provides around the
polymerized particles a layer of the stabilizer. The solvated polymer
segment (A) which extends outwardly from the surface of the particles
provides a solvated barrier which sterically stabilizes the polymerized
particles in dispersed form.
The anchor segment (B) may comprise copolymers of (1) compounds which are
readily associated with the acrylic monomer to be polymerized such as
acrylic or methacrylic esters, such as methyl acrylate, methyl
methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl
methacrylate, 2-ethylhexyl acrylate, octyl methacrylate, and the like,
with (2) compounds which contain groups copolymerizable with the acrylic
monomer to be polymerized and which contain groups which are reactive with
the polymeric segment (A), such as glycidyl-containing acrylates and
methacrylates, such as glycidyl acrylate and glycidyl methacrylate. These
copolymers are further reacted with polymerizable
ethylenically-unsaturated acids, such as acrylic acid, methacrylic acid,
3-butenoic acid, crotonic acid, itaconic acid, and others mentioned
previously which contain pendant groups which are copolymerizable with the
acrylic monomer.
The preferred polymeric segment (B) is a terpolymer of methyl methacrylate,
glycidyl methacrylate, and methacrylic acid.
The segments (A) and (B) are usually combined entities, the segment (A)
being attached to the backbone of the graft copolymer and the segment (B)
being carried in or on the backbone.
The monomer solution containing the stabilizer preferably contains from
about 1 to about 25 percent by weight of the stabilizer. That is, the
amount of dispersion stabilizer used is in the range of from about 1 to
about 25 percent by weight based on the weight of monomers and dispersion
stabilizer used in the copolymerization process.
The polymerization may be carried out in a conventional manner, utilizing
heat and/or catalysts and varying solvents and techniques. Generally, a
free radical catalyst such as cumene hydroperoxide, benzoyl peroxide or
similar peroxygen compound, or an azo compound such as
azobis(isobutyronitrile) is employed.
The resultant non-aqueous acrylic dispersion consists essentially of
microgel particles (i.e., crosslinked acrylic polymer particles) dispersed
therein. These particles have particle sizes ranging from 0.1 to 10
microns. Depending upon the original concentration of monomer solids,
non-aqueous dispersions consisting essentially of the microgel particles
can be produced by the process at relatively high concentrations. The term
"relatively high concentration" as employed herein refers to the solids
level of the non-aqueus dispersion. Thus, the process of this invention
permits the production of non-aqueous dispersions of microgel particles
having solids contents of from 20 to 60 percent by weight or even higher.
The following examples are submitted for the purpose of further
illustrating the nature of the present invention and should not be
interpreted as a limitation on the scope thereof. All parts and
percentages in the examples as well as throughout the specification are by
weight unless otherwise indicated.
EXAMPLE I
To a 5-liter flask equipped with an up-and-over condenser, agitator,
thermometer, and heating mantle was charged 1900 grams of Napoleum 30 (a
medium boiling naphtha from Kerr-McGee Company), 950 grams of hexane, and
950 grams of heptane. The mixture was heated to reflux (about 85.degree.
C.) and then 200 grams of methyl methacrylate, 34 grams of a dispersion
stabilizer solution and 14.3 grams of azobis(isobutyronitrile) were added.
The dispersion stabilizer solution used contained 50.3 percent solids
(viz., dispersion stabilizer) and the dispersion stabilizer was a polymer
prepared by interpolymerizing 45.4 percent methyl methacrylate, 4.2
percent glycidyl methacrylate, 0.9 percent methacrylic acid, and 49.5
percent of a reaction product of 89.2 percent poly-12-hydroxystearic acid
and 10.8 percent glycidyl methacrylate. The solvent of the dispersion
stabilizer solution comprised 52.1 percent butyl acetate, 40.0 percent
VM&P naphtha and 7.9 percent toluene. After this addition was complete,
reflux was continued for about 20 minutes and then over a 3 -hour period
was added 4060 grams methyl methacrylate, 226 grams of
gamma-methacryloxypropyltrimethoxysilane, 595 grams of the above
dispersion stabilizer solution, 34.0 grams of methacrylic acid, 34.0 grams
of 2-hydroxyethyl ethylenimine, 18.0 grams of azobis(isobutyronitrile) and
18 grams of p-octyl mercaptan. After this addition, reflux was continued
for another 1.5 hours and the mixture was then cooled and filtered.
The resultant polymeric dispersion consisting essentially of crosslinked
acrylic polymer particles (i.e., microgel particles) had a total solids
content determined at 150.degree. C. of 54.5 percent by weight.
EXAMPLE II
To a 5-liter flask equipped with an up and over condenser, agitator,
thermometer and heating mantle were charged 1250 grams of heptane, 540
grams of Isopar H (a mixed aliphatic hydrocarbon having an initial boiling
point of 350.degree. F. and a dry point of 371.degree. F. with 90 percent
distilling between 353.degree.-357.degree. F., available from Humble Oil
and Refining Company), 50 grams of methyl methacrylate, 10 grams of the
dispersion stabilizer solution of Example I and 4 grams of
azobis(isobutyronitrile). The mixture was heated to reflux (about
103.degree. C.) and held for about 30 minutes. Then over a period of about
3 hours were added 1288 grams of methyl methacrylate, 70 grams of glycidyl
methacrylate, 42 grams of methacrylic acid, 4.2 grams of Armeen DMCD
(dimethyl cocoamine, available from Armour Chemical Company), 200 grams of
the dispersion stabilizer solution of Example I, 14 grams of octyl
mercaptan and 5.6 grams of azobis(isobutyronitrile). After this addition
was completed, reflux was continued for an additional 30 minutes and then
an additional 2.8 grams of azobis(isobutyronitrile) were added. Reflux was
then continued for another one hour and the mixture was then cooled and
filtered.
The resultant polymeric dispersion consisting essentially of crosslinked
acrylic polymer particles (i.e., microgel particles) had a total solids
content determined at 150.degree. C. of 44.9 percent by weight.
According to the provisions of the Patent Statutes, there is described
above the invention and what are now considered to be its best
embodiments. However, within the scope of the appended claims, it is to be
understood that the invention can be practiced otherwise than as
specifically described.
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Description |
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