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Description  |
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The production of polymeric materials by causing monomers to react is a
well established art. In general, such polymerizations are carried out in
one of two methods. The first of these processes is known as bulk
polymerization. In this method, the monomer is mixed with a suitable
catalyst system and allowed to react. As the polymerization proceeds, the
viscosity of the mixture increases and, if left undisturbed, a solid mass
results. This procedure is quite satisfactory where it is desired to cast
the polymer into some rigid form such as blocks, sheets or molded objects.
However, if it is desired to obtain the polymer in a form suitable for
further processing, special precautions must be taken. In general,
grinding or chopping the bulk polymer is a difficult and unsatisfactory
procedure. It is usually preferable to apply sufficient agitation to the
polymerizing mixture so that a powder or "crumb" is obtained.
Because of the difficulty encountered in processing bulk polymerization, a
second general method of conducting polymerization was developed. This
method is known as emulsion polymerization. In this method, the liquid
monomer is emulsified in an immiscible carrier liquid, usually water. The
internal dispersed monomeric phase may constitute for example from 10 -
40% by volume of the emulsion.
Emulsion polymerization has the advantage that it allows one to process a
liquid mixture even when the polymer system has proceeded to the point
where solid polymer is produced. The high heat capacity of water also
helps in controlling the temperature and in removing the heat of the
reaction.
Emulsion polymerization has its drawbacks. In order to maintain low
viscosity, the percent of, internal phase, monomer is usually kept low.
Once the polymerization is complete, the latex must be coagulated and the
polymeric material recovered and washed free of contaminants. Under
certain conditions, this may be as expensive as the grinding step in bulk
polymerization.
If the polymer is to be used in coating formulations, dissolved in
solvents, or used in plasticol formulations, it is desirable to obtain the
polymer in the form of a powder with a small particle size. Optimum
processing in the bulk polymerization method seldom yields particle size
less than about 100 microns. Emulsion polymerization can produce extremely
small particles. For example, particles in the 1 to 5 micron range.
However, the difficulty in removing surface active contaminants used in
the polymerization process from these extremely fine particles makes them
less satisfactory.
Thus, the process of emulsion polymerization comprises forming an emulsion
of monomer in an aqueous system and causing the monomer to polymerize. The
end product is generally a stable latex or suspension of finely divided
polymer particles. For example, vinyl monomers such as styrene, vinyl
chloride, vinyl acetate, methyl methacrylate, or other monomers or
mixtures of monomers are commercially polymerized by emulsion
polymerization. However, all of these emulsion polymerizations are carried
out in systems where the emulsion contains a low percent internal phase
monomer in water generally in ratios of about 20 to 40% or less.
I have now discovered a process of emulsion polymerization which comprises
forming an emulsion of monomer in an aqueous system wherein the emulsion
contains a high internal phase ratio of monomer to water, i.e., greater
than about 50%, such as greater than about 60%, for example from about
70%, but preferably about 85 to 95%.
The process of this invention combines the advantages of both bulk and
emulsion polymerization.
The polymers formed by this process are uniquely different from those
formed by the conventional emulsion process.
In practice the process of this invention is carried out by preparing a
high internal phase ratio emulsion of monomer(s) in an aqueous system and
causing the monomer(s) to polymerize by the addition of catalysts and/or
heat or ultraviolet light to form the polymer.
When the monomer is polymerized, the polymer formed is in the shape of
hollow spheres which can be crushed to produce an extremely fine powder of
unique properties.
Any monomer or mixtures of monomers heretofore polymerized by emulsion
polymerization can be polymerized in accordance with this invention
employing the same catalysts, regulators, etc. employed for low internal
phase ratio emulsions, except that the polymerization of the present
invention is carried out with a high internal phase ratio emulsion. This
process produces unique polymers.
In preparing the emulsion a suitable emulsifier must be employed which is
capable of forming a high internal phase ratio emulsion.
The emulsifiers most usually employed in the practice of this invention are
generally known as oxyalkylated surfactants or more specifically
polyalkylene ether or polyoxyalkylene surfactants. Oxyalkylated
surfactants as a class are well known. The possible sub-classes and
specific species are legion. The methods employed for the preparation of
such oxyalkylated surfactants are also too well known to require much
elaboration. Most of these surfactants contain, in at least one place in
the molecule and often in several places, an alkanol or a polyglycolether
chain. These are most commonly derived by reacting a starting molecule,
possessing one or more oxyalkylatable reactive groups, with an alkylene
oxide such as ethylene oxide, propylene oxide, butylene oxide, or higher
oxides, epichlorohydrin, etc. However, they may be obtained by other
methods such as shown in U.S. Pat. Nos. 2,588,771 and 2,596,091-3, or by
esterification or amidification with an oxyalkylated material, etc.
Mixtures of oxides may be used as well as successive additions of the same
or different oxides may be employed. Any oxyalkylatable material may be
employed. As typical starting materials may be mentioned alkyl phenols,
phenolic resins, alcohols, glycols, amines, organic acids, carbohydrates,
mercaptans, and partial esters of polybasic acids. In general, the art
teaches that, if the starting material is water-soluble, it may be
converted into an oil-soluble surfactant by the addition of polypropoxy or
polybutoxy chains. If the starting material is oil-soluble, it may be
converted into a water-soluble surfactant by the addition of polyethoxy
chains. Subsequent additions of ethoxy units to the chains tend to
increase the water-solubility, while, subsequent additions of high alkoxy
chains tend to increase the oil solubility. In general, the final
solubility and surfactant properties are a result of a balance between the
oil-soluble portions of the molecule.
In the practice of this invention it has been found that emulsifiers
suitable for the preparation of high internal phase ratio emulsions may be
prepared from a wide variety of starting materials. For instance, if one
begins with an oil-soluble material such as a phenol or a long chain fatty
alcohol and prepare a series of products by reaction with successive
portions of ethylene oxide, one finds that the members of the series are
successively more water-soluble. One finds also that somewhere in the
series there will be a limited range where the products are useful for the
practice of this invention. Similarly it is possible to start with water
or a water-soluble material such as polyethylene glycol and add,
successively, portions of propylene oxide. The members of this series will
be progressively less water-soluble and more oil-soluble. Again there will
be a limited range where the materials are useful for the practice of this
invention.
In general, the compounds which would be selected for testing as to their
suitability are oxyalkylated surfactants of the general formula
Z[(OR).sub.n OH].sub.m
wherein Z is the oxyalkylatable material, R is the radical derived from the
alkylene oxide which can be, for example, ethylene, propylene, butylene,
epichlorohydrin and the like, n is a number determined by the moles of
alkylene oxide reacted, for example 1 to 2000 or more and m is a whole
number determined by the number of reactive oxyalkylatable groups. Where
only one group is oxyalkylatable as in the case of a monofunctional phenol
or alcohol R'OH, then m=1. Where Z is water, or a glycol, m=2. Where Z is
glycerol, m=3, etc.
In certain cases, it is advantageous to react alkylene oxides with the
oxyalkylatable material in a random fashion so as to form a random
copolymer on the oxyalkylene chain, i.e. the [(OR).sub.n OH].sub.m chain
such as
--AABAAABBABABBABBA--
in addition, the alkylene oxides can be reacted in an alternate fashion to
form block copolymers on the chain, for example --BBBAAABBBAAAABBBB-- or
--BBBBAAACCCAAAABBBB--
where A is the unit derived from one alkylene oxide, for example ethylene
oxide, and B is the unit derived from a second alkylene oxide, for example
propylene oxide, and C is the unit derived from a third alkylene oxide,
for example, butylene oxide, etc. Thus, these compounds include
terpolymers or higher copolymers polymerized randomly or in a block-wise
fashion or many variations of sequential additions.
Thus, (OR).sub.n in the above formula can be written --A.sub.a B.sub.b
C.sub.c -- or any variation thereof, wherein a, b, and c are 0 or a number
provided that at least one of them is greater than 0.
It cannot be overemphasized that the nature of the oxyalkylatable starting
material used in the preparation of the emulsifier is not critical. Any
species of such material can be employed. By proper additions of alkylene
oxides, this starting material can be rendered suitable as an emulsifier
and its suitability can be evaluated by plotting the oxyalkyl content of
said surfactant versus its performance, based on the ratio of the oil to
water which can be satisfactorily incorporated into water as a stable
emulsion. By means of such a testing system any oxyalkylated material can
be evaluated and its proper oxyalkylation content determined.
As is quite evident new oxyalkylated materials will be constantly developed
which could be useful herein. It is therefore not only impossible to
attempt a comprehensive catalogue of such materials, but to attempt to
describe the invention in its broader aspects in terms of specific
chemical names would be too voluminous and unnecessary since one skilled
in the art could by following the description herein select the proper
emulsifier. This invention lies in the use of suitable oxyalkylated
emulsifiers in preparing the emulsion systems of this invention and their
individual composition is important only in the sense that their
properties can effect the preparation and polymerization of these
emulsions. To precisely define each specific oxyalkylated surfactant
useful as an emulsifier in light of the present disclosure would merely
call for chemical knowledge within the skill of the art in a manner
analogous to a mechanical engineer who prescribes in the construction of a
machine the proper materials and the proper dimensions thereof. From the
description in this specification and with the knowledge of a chemist, one
will know or deduct with confidence the applicability of oxyalkylated
emulsifiers suitable for this invention by means of the description set
forth herein. In analogy to the case of a machine wherein the use of
certain materials of construction or dimensions of parts would lead to no
practical useful result, various materials will be rejected as
inapplicable where others would be operative. One can obviously assume
that no one will wish to make a useless composition or will be misled
because it is possible to misapply the teachings of the present disclosure
in order to do so. Thus, any oxyalkylated surfactant that can perform the
function stated herein can be employed.
REPRESENTATIVE EXAMPLES OF Z
______________________________________
No. Z
______________________________________
##STR1##
2
##STR2##
3 RO
4 RS
5
##STR3##
6
##STR4##
7
##STR5##
8
##STR6##
9 Phenol-aldehyde resins.
10 O (Ex: Alkylene oxide block polymers.)
11
##STR7##
##STR8##
12
##STR9##
13 RPO.sub.4 H
14
##STR10##
15
##STR11##
16
##STR12##
17
##STR13##
18
##STR14##
19 Polyol-derived. (Ex: Glycerol, glucose,
pentaerythritol.)
20 Anhydrohexitan or anhydrohexide derived.
(Spans and Tweens.)
21 Polycarboxylic derived.
22
##STR15##
______________________________________
Although any monomer heretofore polymerized by emulsion polymerization can
be polymerized according to this invention, the preferred type of
monomer(s) is one containing a vinyl group, i.e. a group of the formula
##STR16##
where the unsatisfied valences are hydrogen, hydrocarbon such as alkyl,
aryl, cycloalkyl, heterocyclic, etc., halogen, group containing functional
groups such as hydroxy, ester, acid, ketones, etc.
The following examples are presented for purposes of illustration and not
of limitation.
EXAMPLE 1
An external phase comprising 11% of the emulsifier of Example 1, Column 7,
of U.S. Pat. No. 3,352,109 (n-decanol + 1.96 PrO + 2.61 EtO, parts by
weight); and 11% of the emulsifier of Example 20 of the same patent (crude
phenol foots + 2.2 parts EtO by weight); ethylene glycol 20%; and water
58% was prepared. 10 milliliters of this external phase was introduced
into a glass aerosol test flask. Liquid vinyl chloride monomer was added
slowly to the flask under pressure. After each addition, the flask was
shaken by hand until the mixture appeared homogenous. Incremental
additions were continued until 90 milliliters of vinyl chloride had been
added. 50 milliliters of the mixture in the flask was then transferred to
a second essentially identical flask under pressure, and 50 milliliters
more of vinyl chloride added incrementally with agitation. The two flasks
were maintained at ambient room temperature exposed to daylight for a
period of approximately 4 months. At the end of this time, the contents of
each flask was a white friable solid.
EXAMPLE 2
A procedure similar to that described in Example 1 was followed except that
the external phase was diluted with an equal amount of distilled water and
varying amounts of di-(2-phenoxyethyl)-peroxydicarbonate was added as a
catalyst. Emulsions were made with internal phase ratios of 20%, 50%, 70%,
90% and 95%. Polymerization was effected by immersing the flasks in a
water bath at a temperature of 37.degree. C. for a period of 48 hours, at
the end of which time polymerization was essentially complete, as
evidenced by the absence of free vinyl monomers. The molecular weight of
these polymers was determined, and it was found that the molecular weight
of materials in the high internal phase region was considerably higher
than the molecular weight of the materials formed by polymerization
occurring in the conventional polymerization region. This is illustrated
in Table I infra.
EXAMPLE 3
High internal phase ratio emulsions of monomeric styrene-in-water were
prepared following the procedure of Example 78 of U.S. Pat. No. 3,352,109.
Benzoyl peroxide was used as a catalyst and polymerization was carried out
at atmospheric pressure and temperature of 40.degree. C. Emulsions with
80% and 90% internal phase polymerized to friable chalky solids.
EXAMPLE 4
In commercial application, the process of this invention may be carried out
as follows. An apparatus essentially of the type described in Example 1 of
U.S. Pat. No. 3,565,817, except that the mixing chamber is 8 inches in
diameter and 12 inches in length and the pumps are scaled to allow a
through-put in the 5 to 30 gallon per minute range, is equipped so that an
appropriate aqueous external phase is metered in by one pump and the
appropriate monomer proportioned in by another pump. The equipment is
designed to operate at whatever pressure is required to maintain the
monomer as a liquid. For example, in the case of vinyl chloride pressures
above atmospheric pressure are maintained whereas when monomers such as
styrene are employed atmospheric pressure can be employed.
The polymerization catalyst is incorporated into either the external phase
or the internal phase just prior to its introduction into the
emulsification chamber.
Using this apparatus, a high internal phase ratio emulsion of a
polymerizable monomer or mixture of monomers containing a suitable
polymerization catalyst is produced and emerges from exit piping of the
mixing chamber at a rate determined by the pumping rates of the feed
pumps. This polymerizable emulsion is then conducted through the piping
system where it is maintained at the desired polymerization temperature
for a period of time long enough so that polymerization is well advanced,
but not to the point where the mixture becomes solid. When polymerization
has progressed to the desired degree, the viscous partially polymerized
emulsion is ejected into a larger chamber maintained at the appropriate
temperature and pressure for the system. Here the polymerization proceeds
to the solidification point of the emulsion and means are provided to
tumble the solidified pieces of emulsion to prevent them from
agglomerating into an intractable mass. The procedure here is essentially
similar to that employed in the bulk polymerization of monomers. When the
polymerization has proceeded to the desired point the chamber is vented,
unreacted monomer is withdrawn from the chamber and recovered, and the
chalky polymer chunks are subjected to various grinding procedures to
reduce them to the desired particulate state. Where necessary, the aqueous
external phase is then washed from the particles and the particles dried
or further processed.
In Scanning Electron Microscopy the vacuum-dried bulk polymers were
fractured and samples were mounted on copper discs prior to gold vacuum
metallization. The metallized samples were examined utilizing a JEOLCO
JSM-2 scanning electron microscope.
In order to characterize the polymers, intrinsic viscosity determinations
and gel permeation chromatography in tetrahydrofuran were performed. The
viscosity measurements were made using a No. 50 Cannon-Ubbelohde
viscometer at 30.degree. F. and a Waters ANA-PREP chromatograph was
utilized to obtain molecular weight distribution data. Before measurements
were made, the emulsified polymers were vacuum dried at
50.degree.-60.degree. C. and dissolved in tetrahydrofuran. The polymers
were then precipitated in methanol, filtered, washed with methanol,
dissolved in tetrahydrofuran and freeze dried. This procedure removed
surfactant from the polymers which would interfere with the dilute
solution measurements.
The ease of formation of the emulsions is dependent upon several
parameters. The amount of monomer added incrementally is important. For
example the addition of too much vinyl chloride causes the existing
emulsion to break and to separate into two phases. Once phase separation
had occurred, it was extremely difficult or impossible to re-establish the
emulsion. The low water phase emulsions are very sensitive to the amount
of catalyst present. It is difficult to form 90:10 and 95:5 emulsions at
high catalyst concentrations. The catalyst apparently affects the relative
solubility of the emulsifier in the two phases.
The high ratio emulsions formed rigid, friable polymer after about 8 hours
reaction time. These polymers are chalky in appearance. The 70:30 and
50:50 emulsions are also chalky in appearance, but much more easily broken
and crushed than the 90:10 and the 95:5. The 20:80 emulsions form
suspensions of polymer. The rate of polymerizations is slower at low
ratios than at high ratios of monomer:emulsifier. Polymerization carried
out in emulsions containing 70% or more vinyl chloride are essentially
quantitative.
Di-(2-Phenoxyethyl) peroxydicarbonate are chosen as the initiator because
of its low half-life temperature. When more common initiators, such as
benzoyl peroxide or AIBN* are employed, temperatures necessary for
polymerization caused breaking of the emulsion.
*2,2'-azobis (isobutyronitrile)
A GPC trace of 95:5 (monomer:water) ratio polymer with a catalyst:monomer
ratio of 6.66 mg/g was obtained. A major and a minor peak are evident and
were typical of the chromatograms obtained. The minor peak of the bimodal
distribution was more pronounced as the monomer:water ratio was increased;
approximately 3% at 50:50 and increasing to 7% at 95:5. The peak molecular
weight of the minor peak was always in the range of 1-2.times.10.sup.6.
The peak molecular weight of the major peak varied with the monomer:water
ratio at a constant catalyst concentration as shown in Table I.
TABLE I
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Peak Molecular Weight
Monomer:Water Ratio
(Major Peak)
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20:80 90,000
50:50 168,000
70:30 172,000
90:10 218,000
95:5 230,000
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The molecular weight distribution (Mw/Mn) of the major peak was always in
the range of 2.6-3.0. It was also found that the peak molecular weight
decreased as the catalyst:monomer ratio increased for a constant
monomer:water ratio. Preliminary evidence indicates that the minor high
molecular weight peak is formed after the polymer has initially gelled and
that the magnitude of the high molecular weight portion is a function of
the catalyst:water ratio (catalyst/monomer .times. monomer/water) and the
time the polymerization is allowed to continue past the gel point.
Intrinsic viscosity [.eta.] measurements were made on the series of
polymers reported in TABLE I. These data are reported only as [.eta.].
Because of the bimodal distributions, any attempt to calculate a viscosity
average molecular weight by the Mark-Houwink-Sakurada relationship would
not be completely meaningful. TABLE II shows the intrinsic viscosity and
the Huggins constant as calculated from the Huggins and Kraemer equations.
TABLE II
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Monomer:Water Ratio
[.eta.], dl/g
k' k'+k"
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20:80 0.98 0.43 0.54
50:50 1.23 0.42 0.53
70:30 1.40 0.40 0.52
90:10 1.44 0.44 0.54
95:5 1.60 0.40 0.50
______________________________________
The values for k' reported here appear high for the molecular weight ranges
under consideration. Recent evidence indicates that dissolution may not be
complete using routine procedures and that long time periods are required
to obtain acceptable scattering values at low angles less than 45.degree..
TABLE II shows how [.eta.] changes as a function of the monomer:water ratio
at a constant catalyst:monomer ratio of 6.66 mg/g. TABLE III shows the
change in [.eta.] as a function of catalyst:monomer ratio at a constant
monomer:water ratio of 70:30. These results are typical of those obtained
under other conditions. It is of interest to note that polymers having the
same [.eta.] can be produced under different conditions; e.g., polymers
having an [.eta.] of 1.58-1.60 can be produced using a monomer:water ratio
of 95:5 and a catalyst:monomer ratio of 6.66 mg/g or a monomer:water ratio
of 70:30 and a catalyst:monomer ratio of 3.33 mg/g.
TABLE III
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Catalyst: Monomer Ratio
[.eta.], dl/g
k' k"
______________________________________
8.57 1.28 0.41 0.51
6.66 1.40 0.40 0.52
4.95 1.43 0.42 0.52
3.33 1.58 0.40 0.52
______________________________________
To summarize briefly, the molecular weight of the major peak increases with
a decrease in the amount of catalyst and increases with an increase in the
internal ratio. The intensity of the secondary peak increases with an
increase in phase.
Although oxyalkylates are the preferred emulsifiers, it should be
understood that other emulsifiers besides oxyalkylates can also be
employed provided such emulsifiers do not adversely effect polymerization.
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Description |
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