Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PREPARING COPOLYMERS ~F ALPHA-METHYLSTYRENE
AND ACRYLONITRILE
The present invention relates to a process for
preparing copolymers of alpha-methylstyrene and
acrylonitrile. More particularly the present invention
reIates to an emulsion process for preparing such
copolymers having improved~uniformity of monomer content
thereby producing polymers improved physica1 properties.
Batch, emulsion polymerizations o~ alpha-
methylstyrene and acrylonitrile wherein both monomers
are added initially or by means of one or two additions
during the course of the the polymerization result in
compositions having monomer contents that vary due to
the variation in monomer content experienced during the
polymerization. Such variation or compositional drift
is the result of a disparity in the reactivity of the
two monomers. In particular, it has been found that
acrylonitrile~is more reactive under skandard
polymerization conditions and is consumed faster Shan is
alpha-methylstyrene monomer. Consequently, copolymers
prepared from an initial charge of monomers or otherwise
utilizing reaction conditions that are not designed to
account for the differing reactivities of the two
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monomers comprise a mixture of polymers having differing
monomer content.
Optimal polymer properties are believed to
result ~rom copolymers comprising alpha-methylstyrene
and acrylonitrile having a weight ratio of polymerized
monomers from 70/30 to 85/15. Copolymerîc products
comprising even small amounts of polymer product of a
composition outside of this range tend to be deficient
in desirable physical properties such as toughness or
impact strength.
In USP 4,526,928 there is disclosed a process
for the preparation of copolymers of alpha-methylstyrene
and acrylonitrile wherein the ratio of
alpha methylstyrene to acrylonitrile is maintained at
greater than 90/10 at all times during the
polymerization. While such process results in the
preparation of a copolymer having a high glass
transition temperature, it has now been discovered that
processes involving the polymerization of monomers
maintainin~ such a high ratio of alpha-methylstyrene to
acrylonitrile disa~vantageously are characterized by
lengthy reaction times due to the excessive amount of
alpha-methylstyrene which has a reduced reaction rate
compared to acrylonitrile Consequently, in order to
provide a commerically acceptable polymerization process
it would be desirable to achieve a reduction of reaction
time. Moreover, it has now been discovered that glass
transition temperatures of polymers prepared in the
a~orementioned manner are not necessarily better than
those prepared utilizing reduced ratios and that the
toughness of the resulting polymer (as measured by
notched Izod impact values) is o~ten inferior. This
undesired result is believed to be due to compositional
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drift, i.e. the preparation of small amounts of polymer
having inferior properties due to variation in monomer
ratio during the polymerization.
In USP 4,294,946 there is also disclosed a
proce~s for the preparation of copolymers of
alpha-methylstyrene and acrylonitrile. In the disclosed
process the reaction is not terminated until conversion
of monomer exceeds 99 percent~ In the process, alpha-
methylstyrene is batch loaded and acrylonitrile is added
to the reactor by a secondary addition step. Although
the reference suggests that the process may be operated
in a continuous manner~ no teachings as to the relative
rates of addition of the various monomers for such a
continuous process were suggested. Furthermore, the
reference provided no teaching as to the existence and
effect of compositional drift in the copolymers or for a
method for alleviating the same.
It would be desirable if there were provided an
impro~ed process for preparing copolymers of
alpha-meth~lstyrene and acrylonitrile resulting in
poLymers of improved uniformity and physical properties.
Summary of the ~nvention
According to the present invention there is now
provided a process for preparing copolymers of
alpha-methylstyrene and acrylonitrile having an overall
monomer weight ratio (alpha-methylstyrene/acrylonitrile)
from a~out 70/30 to about 85/15 comprising the steps Gf:
1) charging a first monomer feed stream
eompri~ing alpha-methylstyrene and acrylonitrile in a
weight ratio of from 75/25 to 85/15, in a continuous or
semicontinuous manner to a well mixed reactor operating
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under emulsion polymerization conditions and initially
comprising from 0.05 to 5.0 percent by weight based on
final copolymer weight of a polymeric seed latex, until
the amount of unreacted alpha-methylstyrene plus
acrylonitrile in the reactor is from 20 to 45 percent
based on total monomer and polymer weight;
2) thereafter, continuously or semicontinuously
charging to the reactor with mixing under emulsion
polymerization conditions a second monomer feed stream,
comprising acrylonitrile in the substantial absence of
alpha-methylstyrene, until the amount of unreacted
alpha-methylstyrene plus acrylonitrile in the rPactor is
from 5 to 15 percent base~ on total monomer and polymer
weight;
3) thereafter, continuing the emulsion
polymerization until total monomer conversion reaches at
least 90 weight percent; and
4) recovering the copolymer o~
alpha methylstyrene and acrylonitrile.
Copolymers prepared according to the above
described process possess an improved uniformity of
composition and a concomitant improvement in physical
properties, particularly improved toughness as measured
by Izod impact compared to polymers prepared without
regard to the above constraints. Consequently,
formulated copolymer products prepared utilizing the
present invented compositions are able to achieve
improved levels of performance without concomitant
sacrifice of alternative physical properties. More
particularly, melt flow rate that normally is adversely
affected by increasing levels of rubbery additives, may
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be maintained at elevated levels by utilizing reducedamounts of impact modifying additives due to the
inherent improved toughness of compositions prepared
according to the present invented process.
The process of the present invention utilizes
known emulsion technology including the use oE
emulsifiers such as sodium lauryl sulfate, potassium
oleate, sodium oleate, potassium or sodium salts of
fatty acids, potassium or sodium salts of rosin acids
and sodium alkylbenzene sulfonates. A particularly
desired emulsifier is sodium n-dodecylbenzene sulfonate.
All ingredients employed in the polymerization are
purified prior to use. Moreover, the reaction is
conducted under an inert atmosphere such as nitrogen,
preferably at a slightly elevated pressure from 5 to 20
psig (140-240kPA). The reaction ls conducted at
elevated temperatures on the order of 35 to ~5C,
preferably from 50 to 90C. Agitation is employed in
order to maintain a homogeneous reaction mixture.
free radical initiator, such as the well known sodium
peroxydisulfate or potassium peroxydisulfate initiators,
may be employed. A molecular welght regulator such as a
mercaptan or other suitable chain transfer agent may
also be employed.
In a preferred embodiment of the present
invention, alpha-methylstyrene and acrylonitrile are
initially added in a monomer weight ratio of
alpha-methylstyrene to acrylonitrile from 80/20 to
85/lS. While the reaction is preferably conducted in a
continuous manner, a semicontinuous addition of monomers
may also be employed. By semicontinuous addition is
meant addition of the monomer charge in ali~uots
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followed by reaction periods on the order of about one
tenth of the total reaction time or less.
Typically, alpha-methylstyrene and
acrylonitrile are continuously added over a period of
from l to 6 hours under polymerization conditions. As a
general rule, the initial charging of a first monomer
feed stream attains an excess of u~reacted alpha-
methylstyrene monomer of from 600 to 500(), preferably
670 to 5000 weight percent based on unreacted
acrylonitrile. Addition of acrylonitrile monomer only
starts soon after cessation of comonomer addition and is
preferably continued under polymerization conditions for
l to 4 hours. Addition of acrylonitrile usually results
in the lowering of total quantity of unconverted
monomers, although there generally remains an excess of
unreacted alpha-methylstyrene of from 600 to 5000,
preferably 670 to 5000 weight percent based on the
weight of unreacted acrylonitrile. After complete
addition of monomers, the polymerization is generally
allowed to continue for from lS minutes to one hour.
In a highly preferred embodiment, the initial
monomer concentrations and the rate of addition of
acrylonitrile monomer are adjusted so as to yield a
polymer product having a monomer weight ratio (alpha-
methylstyrene/acrylonitrile) from 75/25 to 78/22~ In
order to achieve commercially practical reaction times,
conversions of 90% to 96 percent based on total monomer
weight are generally employed. The product is recovered
by normal recovery techniques including steam stripping
of the resulting mixture to remove volatile components,
coagulation, and pelletizing of the resulting product.
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In applications wherein additional modification
of impact properties is desired, a rubber such as a
styrene- or styrene/acrylonitrile- ~rafted polybutadiene
rubber or a styrene/butadiene copolymer rubber may be
employed. In a preferred embodiment a rubber latex is
added to the alpha-methylstyrene/acrylonitrile copolymer
latex prior to coagulation and pelletizing thereof. The
resulting rubber modified polymer product may be dried
and employed as an injection molding resin.
As previously noted, the addition of
acrylonitrile monomer in a continuous or semicontinuous
manner after an initial period of copolymer formation is
conducted in a manner such that the monomer
concentration in the reaction mixture is maintained
between the aforementioned limits. Addition of
acrylonitrile in a too rapid manner results in
compositional drift due to the preparation of copolymers
containing excess amounts of acrylonitrile and having
inferior impact properties.
The seed latex employed in the initial
polymerization preferabIy comprises any compatible
polymeric latex having a particle size from lO0 to l,000
Angstroms. Highly preferred are polymeric seed latices
comprising polystyrene, polybutadiene, copolymers of
styrene and butadiene, or copolymers of
alpha-methylstyrene and acrylonitrile.
Having described the new invention the
Eollowing examples are provides as further illustrative
and are not to be construed as limiting.
Example l
A one-gallon (0.004 M3), glass-lined latex
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reactor was charged with 1147 g of deionized water, 107
g of polystyrene seed latex (400 Angstrom number average
particle size) (30 percent active, thereby providing 3
weight percent seed based on final polymer weight), 0.3
g of disodium ethylene diaminetetraacetic acid, 1.0 g of
sodium bicarbonate, and 20 g of 43 percent aqueous
sodium n-dodecylbenzene sulfonate. The mixture was
purged with nitrogen, and the reactor was then evacuated
with a water aspirator three times at room temperature.
After the final evacuatîon, the reactor was pressurized
with 10 psi (69 KPa) of nitrogen. The reactor was
heated to 85C and stirred at 2000 rpm. An aqueous feed
consisting of 141 g of water, 41.8 g of 43 percent
aqueous sodium n-dodecylbenzene sulfonate, and 1.4 g of
sodium persulfate and a monomer feed consisting of 750 g
of alpha~methylstyrene, 187 g of acrylonitrile,
(providing a ratio alpha-methylstyrene/acrylonitrile of
80/20) and 2.0 g of n-octyl mercaptan (first monomer
feed stream) were started simultaneously and added at
rates of 37.4 and 234.6 g per hour to the reaction,
respectively. After four hours, addition of first
monomer feed stream was halted. The percentage of
unreacted monomers based on total monomers and polymer
was 31.4 percent. The amount of unreacted alpha-
methylstyrene compared to acrylonitrile was 2070percent.
A second monomer feed stream consisting of 41 g
of acrylonitrile and 0.59 g of n-octyl mercaptan was
started at a rate of 10.4 g per hour. The aqueous feed
rate was charged at 7.8 q per hour. The additions of
aqueous feed and second monomer stream were continued
for four additional hours. Total monomer feed resulted
in a ratio of alpha-methylstyrene/-acrylonitrile, of
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77/23. The amount of unreacted monomers compared to
total monomers and polymer was 13.6 percent. The amount
of alpha-methylstyrene based on acrylonitrile was 650
percent. The second monomer feed was then halted, the
aqueous feed continued for an additional 15 minutes, and
the mixture was retained at 85C Eor 15 minutes. An
antifoaming agent (antifoam H-10, 15 percent active,
available from Dow Corning Corporation) was added and
volatile components were steam distilled from the
mixture. The reaction went to 90 percent conversion.
The ratio of polymerized alpha-
methylstyrene/acrylonitrile in the finished latex was
76/24~
Examples 2 - 4 and Comparative
Additional latex samples were prepared
substantially according to the techniques of Example 1
utilizin~ various ratios oE monomers in the first
monomer feed stream. The latexes were blended with a
bimodal styrene/acrylonitrile grafted butadiene rubber
prepared according to the following rubber preparation.
~ubber Preparation
A twenty-gallon (0.07 M3), glass-lined latex
reactor was charged with 61,979 g of bimodal butadiene
rubber latex (75 percent 1400 Angstroms, 25 percent-8000
Angstromsl 33 percent active). The mixture was purged
with nitrogen, then evacuated with a water aspirator
three times at room temperature. The reactor was heated
to 80C and stirred at 150 rpm. An aqueous feed~
consisting oE 6618 g of deionized water, 22.1 g oE
sodium persulfate, and 529.4 g of aqueous 43 percent
sodium n-dodecylbenzene sulfonate, and a monomer feed,
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consisting of 8603 g of styrene and 2427 g of
acrylonitrile, were started simultaneously and added to
the reactor at 1792 and 2757 g per hour, respectively.
The additions were completed in four hours and the
reaction product was retained at 80C for an additional
30 minutes. An anti-foaming agent (Antifoam H-10, 15
percent active) was added and 650 ml of volatile
reaction by products were steam stripped from the
mixture. The reaction ran to 92 percent conversion.
The blends comprising 15 percent by weight
rubber are prepared by latex blending, coagulating,
dewatering, and pelletizing the resin. Test specimens
are prepared by injection molding the resulting
products. Tests on such specimens are conducted
according to ASTM standards. Resulting are contained in
Table I.
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By examining Table I it may be seen that
compositions having improved toughness and equivalent or
improved melt flow rates are prepared by observing the
processing conditions specified for the present
invention.
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