Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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USE OF TETRAFUNCTIONAL INITIATORS TO
IMPROVE THE RUBBER PHASE VOLUME OF HIPS
Field of the Invention
[0001] The present invention is related to methods and compositions useful to
improve the manufacture of copolymers of vinyl aromatic, monomers such as
styrene. It
relates more particularly to methods of copolymerizing vinyl aromatic monomers
with
multifunctional initiators in the presence of diene polymers.
l0 Background of the Invention
(0002] The polymerization of styrene is a very important industrial process
that
supplies materials used to create a wide variety of polystyrene-containing
articles. This
expansive use of polystyrene results from the ability to control the
polymerization
process. Thus, variations in the polymerization process conditions are of
utmost
importance since they in turn allow control over the physical properties of
the resulting
polymer. The resulting physical properties determine the suitability of
polystyrene for a
particular use. For a given product, several physical characteristics must be
balanced to
achieve a suitable polystyrene material. Among the properties that must be
controlled
and balanced are average molecular weight (Mw) of the polymer, molecular
weight
distribution (MWD), melt flow index (MFI), and the storage rriodulus (G'). For
rubber
toughened materials, such as high impact polystyrene, which is composed of
rubber
particles in a polystyrene matrix, factors that influence rubber morphology,
such as
rubber particle size, rubber particle size distribution, swell index,
grafting, and the
rubber phase volume, as measured by the ratio of the % gel to % rubber (G/R),
are also
critical to balance physical and mechanical properties.
[0003] Methods for preparing branched polymers are well-known in the art. For
example, the preparation of branched polystyrene by free radical
polymerization has
been reported in various patents. The polymerization of branched polystyrenes
in the
presence of elastomers to produce HIPS, however, presents various challenges,
since
branching reactions can lead to crosslinking of the matrix and also of the
rubber phase.
[0004] A wide variety of peroxy compounds is known from the literature as
initiators
for the production of styrenic polymers. Commercially available initiators for
polymer
production may be classified in different chemical groups, which include
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diacylperoxides, peroxydicarbonates, dialkylperoxides, peroxyesters,
peroxyketals, and
hydroperoxides.
[0005] Mono- and bifunctional peroxide initiators are commonly used in the
manufacture of rubber-modified polystyrene (PS), and peroxides have been used
to
increase the rate of polymerization and to modify the degree of chemical
grafting
between polystyrene and the elastomer (typically polybutadiene rubber) used to
modify
PS. Increasing the rate of polymerization by using initiators causes the
molecular
weight of the PS matrix to decrease; chemical grafting may or may not increase
depending on the levels and the temperature at which the initiator is used.
Thus, the
use of initiators to manufacture high impact polystyrene (HIPS) requires an
optimization
of rate, temperature, molecular weight, chemical grafting, as well as other
parameters.
[0006] Commercial polystyrene made by the conventional free-radical process
yields
linear structures. As noted, methods to prepare branched polystyrenes,
however, are
not easily optimized and few commercial non-linear polystyrenes are known.
Studies of
branched polymers show that these polymers possess unique molecular weight-
viscosity relationships due to the potential for increased molecular
entanglements.
Depending upon the number and length of the branches, non-linear structures
can give
melt strengths equivalent to that of linear polymers at slightly higher melt
flows.
[0007] U.S. Pat. No. 6,353,066 to Sosa describes a method of producing a co-
polymer by placing a vinylbenzene (e.g. styrene) in a reactor, placing a cross-
linking
agent (e.g. divinylbenzene) in the reactor, and placing a chain transfer agent
(e.g.
mercaptan) in the reactor and forming a polyvinylbenzene in the presence of
the cross-
linking agent and chain transfer agent.
[0008] It would be desirable if methods could be devised or discovered to
provide
vinylaromatic polymers with increased branching, such as branched polystyrene
for the
manufacture of HIPS. It would also be helpful if a method could be devised
that would
help optimize the physical properties of rubber- toughened vinylaromatic
polymers
having increased branching, while maintaining production rates and molecular
weight
properties. Such materials may have a higher melt strength than those having
linear
chains, and may improve processability and mechanical properties of the final
product.
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Summar~~ of the Invention
[0009] There is provided, in one form, a method for producing an improved
copolymerized product that involves copolymerizing at least one vinylaromatic
monomer
with at least one diene polymer in the presence of at least one
multifunctional initiator.
The multifunctional initiator may be a trifunctional or tetrafunctional
peroxide. A
copolymerized product is recovered that has a ratio of %~ gel to % rubber (G/R
or rubber
phase volume) that increases as swell index increases.
[0010] In another embodiment of the invention, there is provided an improved
copolymerized product made by copolymerizing at least one vinylaromatic
monomer
with at least one diene polymer in the presence of at least one
multifunctional initiator.
The multifunctional initiator may be a trifunctional or tetrafunctional
peroxide. A
copolymerized product is recovered that has a G/R that increases as swell
index
increases.
[0011] In still another embodiment of the invention, there is a resin that
includes at
least one vinylaromatic monomer, at least one diene polymer, and at least one
multifunctional initiator. The multifunctional initiator is either a
trifunctional or tetra-
functional peroxide, and the amount of multifunctional initiator is sufficient
to produce a
copolymerized product that has a G/R that increases as swell index increases.
[0012] In yet another embodiment of the invention, there are provided articles
made
from the resins and copolymerized products of this invention.
Brief Description of the Drawings
[0013] FIG. 1 is a graph of % polystyrene v. time in hours for equivalent
peroxide
functionalities, where the feed is styrene;
[0014] FIG. 2 is a graph of % polystyrene v. time in hours for equivalent
peroxide
functionalities, where the feed is styrene but contains 7% Diene 55;
[0015] FIG. 3 is a graph of Mw in thousands as a function of % conversions for
isothermal polymerization at 110°C for equivalent peroxide
functionalities;
[0016] FIG. 4 is a plot of % solids as a function of time for various levels
of JWEB 50
tetrafunctional initiator for a feed of styrene including 4% Bayer 380;
[0017] FIG. 5 is a plot of G/R ratio v. swell index for commercial FINA HIPS
materials;
and
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[0018] FIG. 6 is a plot of gel/rubber ratio vs. swell index for experiments
with
tetrafunctional initiator (JWEB50) and various commercial grades.
Detailed Description of the Invention
[0019] The inventors have explored the potential for providing branched
polystyrene
having at least some increased branching by using tetrafunctional initiators
or
trifunctional initiators. The invention concerns initiating polymerization of
a vinyl
aromatic monomer such as styrene in various solvents and in the optional
presence of
a polydiene, such as polybutadiene, with a multifunctional initiator (e.g. tri-
or
tetrafunctional) and to use the multifunctional initiator to obtain branched
structures.
[0020] For conventional HIPS resins, the rubber phase volume is a key
parameter
that can be estimated from solution properties. The rubber phase volume refers
to the
rubber particles or discontinuous phase, which consists of rubber, trapped
polystyrene
(occlusions) and grafted polymer. A convenient way to classify HIPS materials
is by
calculating the dry gel obtained for a given rubber level. For commercial HIPS
materials, the gel/rubber ratio (G/R) can vary from 1 to 4 for swell indices
of 10 -12, and
as the swell index increases the G/R ratio decreases. The G/R ratio is the
ratio of the
gel to % rubber, and is also termed the rubber phase volume (RPV). This ratio,
the G/R,
is important in the manufacture of HIPS materials because it represents the
"rubber
efficiency" of the process, i.e., how much rubber must be used to obtain
similar product
quality. The less rubber needed to produce a set of desired properties in a
HIPS
material, the more efficient the process. The G (percent gel) is measured by
first
dissolving the resin in toluene, separating the gel fraction by
centrifugation, and then
drying the wet gel. The percent gel is then calculated from this dried residue
by the for-
mula: Percent Gel = 100×dried gel weight, divided by the initial weight
of the
sample. The percent rubber is measured by the well-known Iodine Monochloride
(I-CI)
titration method.
[0021] It has been surprisingly discovered herein that contrary to
conventional HIPS
resins, with multifunctional peroxide initiators, the opposite trend is seen,
that is as the
level of multifunctional initiator is increased, G/R increases even though the
swell index
of these materials is very high.
[0022] Generally, for this invention, the G/R increases from about 1 to about
4 as the
swell index increases from about 8 to about 20. Alternatively, in another non-
limiting
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embodiment of the invention, the G/R ranges from about 1 to about 3 while the
swell
index ranges from about 12 to about 20. In one particular non-limiting
embodiment of
the invention, the G/R ranges from about 1.5 to about 3.0 while the swell
index ranges
from about 10 to about 14. This unexpected phenomenon is discussed further
with
respect to the data below.
[0023] In one non-limiting embodiment of the invention, the melt flow index
(MFI) for
the resins of this invention range from about 2 to about 7. In another non-
limiting
embodiment of the invention, the MFI range from about 3 to about 5.
[0024] In theory, tetrafunctional materials can be schematically represented
by the
shape of a cross. If at the end of each arm of the cross, the potential for
initiation or
chain transfer exists, it is possible to envision polystyrene molecules that
will have
higher molecular weight than by using bifunctional initiators only. Similarly
to
tetrafunctional initiators, trifunctional initiators simply have three "arms"
or starting
points instead of the four found in tetrafunctional initiators.
[0025] In the present case, relatively small levels of the tetrafunctional
initiators are
used to optimize the melt properties resulting from the formation of branched
structures.
With the tetrafunctional initiator, four linear chains for one branched
molecules are
formed. At high levels of initiators the amount of linear chains, initiated by
the alkyl radi-
cals, will lower the effect brought by the branched chains, initiated by the
tetrafunctional
radicals. Further, multifunctional peroxides can be used to increase
polymerization
rates and chemical grafting, while maintaining or increasing PS matrix
molecular weight.
The potential use of these multifunctional initiators in the production of
HIPS allows
higher production rates while maintaining molecular weights and improving
rubber
phase volume.
[0026] The composition of the invention can include a polydiene-modified
monovinyl
aromatic polymer, and can include a rubber (polybutadiene)-modified
polystyrene.
Styrene monomer can be polymerized in the presence of from about 2 to about 15
weight percent rubber to produce a copolymer having impact resistance superior
to that
of polystyrene homopolymer. A rubber that can be used in making the subject
compositions is polybutadiene. The resultant thermoplastic composition, which
can be
made with these materials, is high impact polystyrene, or HIPS. The
predominant
morphology of the polymer made from embodiments of the invention is cell or
"salami"
with some core-shell structure, meaning that the continuous phase of
polystyrene
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comprises a plurality of dispersed structures in which polystyrene is trapped
within
rubber particles having a distinct membrane and small quantities of
polystyrene are
occluded inside single cell polybutadiene shells grafted to the aromatic
polymer.
[0027] Styrene polymerization processes are well known. The compositions of
the
invention can be made by batch polymerization in the presence of from about 2
to 15,
and in some embodiments can be from about 4 to about 12, weight percent
polybutadiene using multifunctional initiators at concentrations of from about
50 to
about 1200 ppm and using a solvent. In another non-limiting embodiment of the
invention the concentration of multifunctional initiator may range from about
100 to
about 600 ppm.
[0028] For comparison, monofunctional and bifunctional initiators are also
used in the
Examples of this Description. The structures of some of the initiators are
shown below:
TRIGONOX 42S Peroxide (Monofunctional):
CH3 O CH3 CH3
CH3-C-O-O-C-CH2-CH-CHZ-~-CH3
CH3 CH3 CH3
LUPERSOL 331 Peroxide (Bifunctional):
i Ha
- i -CH3
O CH3
~ H3
O- i -CH3
CH3
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LUPERSOL 531 Peroxide (Bifunctional):
i Hs
- i -CH2CH3
~~ a
H2CH3
PERKADOX 12-AT25 (Multifunctional):
CH3 i H3
CH3- i - ~ ~ - i -CH3
CH3 n CH3 a CH3
CH3 /0 CH3 ~ i H3
CH3-C-O/ O- i -CH3
CH3 CH3
[0029] In one non-limiting embodiment of the invention, the multifunctional
initiator is
a trifunctional or tetrafunctional peroxide and is selected from the group'
consisting of tri-
or tetrakis t-alkylperoxycarbonates, tri- or tetrakis-(t-
butylperoxycarbonyloxy) methane,
tri- or tetrakis-(t-butylperoxycarbonyloxy) butane, tri- or tetrakis (t-
amylperoxycarbonyloxy) butane, tri- or tetrakis (t-C4_6 alkyl
monoperoxycarbonates) and
tri- or tetrakis (polyether peroxycarbonate), and mixtures thereof. In one non-
limiting
embodiment of the invention, the tetrafunctional initiator has four t-alkyl
terminal groups,
where the t-alkyl groups are t-butyl and the initiator has a poly(methyl
ethoxy) ether
central moiety with 1 to 4 (methyl ethoxy) units. This molecule is designated
herein as
LUPEROX~ JWEB 50 and is available from Atofina Petrochemicals, Inc. Another
commercial product suitable as a multifunctional initiator is 2,2 bis(4,4-di-
(tert-butyl-
peroxy-cyclohexyl)propane) from Akzo Nobel Chemicals Inc., 3000 South
Riverside
Plaza Chicago, Illinois, 60606. Another commercial product is 3,3',4,4' tetra
(t-butyl-
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peroxy-carboxy) benzophenone from NOF Corporation Yebisu Garden Place Tower,
20-3 Ebisu 4-chome, Shibuya-ku, Tokyo 150-6019.
[0030] Monofunctional peroxide initiators can undergo hemolytic cleavage to
produce
monoradicals, each of which can initiate a chain. Bifunctional initiators,
depending on
the breakdown patterns, can cause chain extension if biradical formation is
possible
from a fragment. Tri- and tetrafunctional initiators can also cause chain
extension.
Because of the possible and various complex decomposition patterns, it is not
easy to
determine a priori how a given initiator will decompose under a given set of
conditions;
however, by measuring the molecular weight of the resultant polymer, it is
possible to
determine if the initiators are able to produce chain extension.
[0031] Suitable optional solvents for the polymerization include, but are not
nec-
essarily limited to ethylbenzene, xylenes, toluene, hexane and cyclohexane.
Chain
transfer agents and crosslinking agents can be used in applications of this
invention as
taught by art.
[0032] It has been discovered that multifunctional initiators can be used
together with
chain transfer agents and cross-linking agents to manufacture polystyrene and
HIPS
that is more highly branched. The chain transfer agent and/or cross-linking
agent may
be added prior to, during or after the initiator is added to the monomer.
[0033] It has also been discovered that the polymerization of a vinyl aromatic
monomer such as styrene carried out in the presence of divinylbenzene (DVB)
and n-
dodecyl mercaptan (NDM) to produced branched structures as disclosed in U.S.
Pat.
No. 6,353,066 (incorporated by reference herein) can be improved by using a
tetrafunctional initiator in combination with DVB and NDM. Extensive studies
have been
done to determine the conditions suitable for optimizing the melt rheology,
however, it
has been surprisingly found that an increase in rate can be produced while
obtaining
the desired molecular parameters.
[0034] Grafting is also favored by using polybutadiene having a medium or high-
cis
isomer content. Polybutadiene useful in making the composition of the
invention is
produced, for example, by known processes by polymerizing butadiene in either
a
hexane or cyclohexane solvent to a concentration of about 12 weight percent,
and
flashing off the solvent at a temperature ranging from about 80° to
100°C. to further
concentrate the polybutadiene solution to about 24 to 26 weight percent, the
approximate consistency of rubber cement. The polybutadiene is then
precipitated from
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the solution as a crumb using steam, then dried and baled. Commercially
available
rubbers suitable for producing HIPS are available from several suppliers such
as Bayer
380, 550, and 710 (Bayer Corporation, Orange, Texas) and Firestone Diene 35,
55 and
70 (Firestone Polymers, Akron, Ohio).
[0035] In one non-limiting embodiment of the invention, the copolymerized
products
of this invention may have a polydispersity of from about 2.2 to 4.5. In
another non-
limiting [preferred] embodiment, the copolymerized products of this invention
may have
a polydispersity ranging from about 2.3 to 4Ø In another non-limiting
embodiment the
polydispersity may range from about 2.3 to 3.2.
[0036] Not only has it been surprisingly discovered that G/R increases as the
swell
index increases using the multifunctional initiators of this invention, but it
has also been
found that acceptable G/R can be achieved at increased polymerization rates
using
these initiators in polymerizations of styrene. The rate of polymerization
styrene is about
10%/hr at 130°C from 10 to about 50% solids (no initiator). As the
level of ]WEB
increases, the rate (slope of the line) can be increased by a factor of 2 to 7
times that of
pure styrene (no initiator) in the range of 10 to 50% PS conversion as the
level of
initiator increases. Compared to pure styrene, the slopes are 2.3, 4.3 and 6.6
times that
of pure styrene for 200, 400 and 600 PPM of ]WEB, respectively as will be seen
in FIG.
4.
[0037] In making the certain compositions of the invention, batch or
continuous
polymerizations can be conducted in 97:3 to 91:9 styrene to rubber, 85:15 to
80:20
typical styrene solvent mixtures to 60-80% styrene conversion to polystyrene
and then
flashing off the unreacted monomer and the solvent. In a non-limiting, typical
preparation, 3-12% of rubber is dissolved in styrene, then about 10%
ethylbenzene is
added as 90:10 styrene:ethylbenzene. The ethylbenzene is used as a diluent.
Other
hydrocarbons can also be used as solvents or diluents. A possible temperature
profile
to be followed in producing the subject compositions is about 110°C for
about 120
minutes, about 130°C for about 60 minutes, and about 150°C for
about 60 minutes, in
one non-limiting embodiment. The polymer is then dried and devolatilized by
conventional means. Although batch polymerizations are used to describe the
invention, the reactions described can be carried out in continuous units, as
the one
described by Sosa and Nichols in US 4,777,210, incorporated by reference
herein. In
another non-limiting embodiment of the invention, the copolymerizing may be
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conducted at a temperature between about 80°C to about 200 °C;
in an alternate
embodiment of the invention from about 110°C to about 180°C.
[0038] It will be appreciated that other components may be added during or
prior to
the polymerizations described herein that would be within the scope of the
invention.
Such components include, but are not necessarily limited to, chain transfer
agents,
cross-linking agents, accelerators, lubricants, and diluents and the like.
[0039] The invention will now be described further with respect to actual
Examples
that are intended simply to further illustrate the invention and not to limit
it in any way.
[0040] Studies have been done to determine the conditions suitable for
optimizing the
melt rheology of a branched polystyrene system using multifunctional
initiators,
however it has also been surprisingly discovered that an increase in rate can
be
obtained while producing the desired molecular parameters, particularly an
improvement in the gel to rubber ratio. Laboratory polymerization studies were
conducted using the peroxide initiators described in Table I. The structural
repre-
sentations for some of these peroxides were given previously.
TABLE I
Initiators Used in Styrene Polymerization Studies
Peroxide Class Type 1 hr. T~,2,
C
TRIGONOX 42S Peroxyester Monofunctional 110
LUPERSOL 331 Peroxyketal Bifunctional 112
LUPERSOL 531 Peroxyketal Bifunctional 112
PERKADOX 12- Peroxyketal Multifunctional 112
AT25
JWEB 50 Peroxyketal Multifunctional 119 (in ethylbenzene)
121 (in dodecane)
[0041] The first four initiators were chosen for study due to their
similarities in half-
life temperatures and differences in peroxide functionalities. The
polymerizations were
performed isothermally (110°C), as well as non-isothermally
(temperature ramp
process), for both crystal and HIPS systems. Further, initiator concentrations
were
varied to assess rate and molecular weight effects.
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Isothermal Polyrmerization Studies - Crystal Polystyrene
[0042] Isothermal polymerizations were conducted at 110°C to monitor
conversion
and molecular weight as a function of reaction time. The chosen reaction
temperature
of 110°C is essentially that of the one-hour half-life temperatures of
the initiators.. The
polymerization rate increased with increasing initiator concentration [I],
generally
following the expected square root relationship.
[0043] From the well-known kinetic expressions, the degree of polymerization
(molecular weight) is inversely proportional to the rate of polymerization.
The molecular
weight decreased with increasing initiator concentration. Further, the
molecular weight
obtained at a given initiator concentration becomes relatively constant after
20-30%
conversion.
[0044] The molecular weight behavior for styrene polymerization using
bifunctional
initiators (LUPERSOL 331, LUPERSOL 531) was different. Initially, a decrease
in
polymer molecular weight was obtained with increased initiator concentration
due to
increased polymerization rate. However, rather high molecular weights were
seen at
higher conversions. Several researchers have attributed this molecular weight
enhancement to "chain-extension" polymerization. Basically, the high molecular
weight
is due to the initiation of undecomposed peroxides on the polymer chain ends,
followed
by chain propagation reactions. Thus, the polymerization characteristics
observed for
the bifunctional initiator systems indicate that both high rates and molecular
weights can
be obtained simultaneously. Such a desirable rate/molecular weight
relationship is even
more evident with the tetrafunctional initiator (PERKADOX 12-AT25). It was
seen that
the polymerization rates and polymer molecular weights were significantly
higher than
those from the bifunctional systems.
[0045] The bifunctional initiators yielded a significantly higher
polymerization rate
than did the monofunctional initiator, but similar molecular weights (at
conversions of
greater than 35%). The tetrafunctional initiator gave an extremely rapid
polymerization
rate and superior molecular weights when compared to the bifunctional
peroxides.
Similar effects were noted when the initiators are compared on an equi-
peroxide
functionality basis.
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Non-Isothermal Polymerization Studies - Crystal PS
[0046] Non-isothermal polymerization studies were conducted to assess the
effects
of initiator type/functionality on crystal PS properties, particularly on
molecular weight.
The reaction profile was 2 hours at 110°C, 1 hour at 130°C, 1
hour at 150°C, followed
by devolatilization at 240°C for 0.5 hours (<2 mmHg; <267 Pa).
[0047] A tetrafunctional initiator gave a significantly higher polymerization
rate than
did any of the other peroxides. LUPERSOL 531, a t-amyl peroxyketal, yielded a
more
rapid rate than does the t-butyl derivative (LUPERSOL 331 ). Interestingly,
the
tetrafunctional initiator yielded the highest molecular weight crystal PS
(about 20%
higher Mw). The higher molecular weight fraction obtained, however, led to an
increased polydispersity (about 3.5). The bifunctional initiators yielded
similar molecular
weights and higher rates than does the monofunctional peroxide. Similar
results were
obtained when the initiators are compared on an equi-peroxide functionality
basis. The
results further supported the mechanism of polymer chain extension via
decomposition
of end-group peroxides, followed by propagation.
Non-Isothermal Polymerization Studies - HIPS
[0048] Laboratory HIPS materials were prepared with the initiators using 7%
Diene
55 via a temperature ramp process. Diene 55 is a polybutadiene available from
Firestone Polymers. The results were similar to those obtained in the crystal
PS
polymerization studies. A rapid polymerization rate was obtained with the
tetrafunctional
peroxide; however, the resulting "pellet" molecular weight (particularly Mw)
was still
quite high. Again, a broadening of the molecular weight distribution was
noted. Further,
it is seen that the bifunctional initiators also lead to superior
polymerization
rate/molecular weight relationships when compared to the monofunctional
peroxide.
The advantages of the tetrafunctional initiator in terms of molecular weight
were readily
apparent.
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TABLE II
Comparison of Molecular Weights and Polydispersities
for PS and HIPS Products with Different Initiators
Feed Parameter Lup 331 Lup 531 Perk 12 Tria 42S
Example 1 2 3 4
Styrene Mn in thousands 82 100 100 75
Mw in thousands 250 256 352 220
Polydispersity 3.0 2.6 3.5 2.9
Example 5 6 7 8
7 % Diene 55 Mn in thousands 92 110 100 110
Mw in thousands 250 260 320 240
Polydispersity 2.7 2.4 3.2 2.2
[0049] The effects of initiator type and concentration on rubber phase
properties
must also be considered; these results are given in Table IV. In these
Examples, the
amount of rubber is dependent on the conversion in the fourth series reactor,
i.e. no
recycle. Note that polydispersities for the multifunctional initiators Perk 12
and Trig 42S
range from 2.2 to 3.2.
[0050] FIG. 1 presents graphs of % polystyrene as a function of time for
equivalent
peroxide functionalities for the four initiators of Table II where the feed is
styrene, such
as for Examples 1-4. Generally, the plots are roughly equivalent. FIG. 2
provides plots
of % polystyrene as a function of time for equivalent peroxide functionalities
for the four
initiators of Table II where the feed is styrene and 7% Diene 55, such as for
Examples
5-8. Again the results are comparable except that after about two hours the
polystyrene for Perkadox 12-AT25 is somewhat higher. The data in FIGS. 1 and 2
are
from ramp processes.
[0051] FIG. 3 is a plot of Mw (in thousands) as a function of % conversion for
isothermal polymerization at 110°C for equivalent peroxide
functionalities for the four
initiators of Table II. Interestingly, the monofunctional Trigonox 42S gave
relatively
lower conversions and somewhat higher molecular weights as compared with the
bifunctional Lupersol initiators. The multifunctional Perkadox 12-AT25
provided
relatively higher conversions and higher Mw indicative of the greater
functionality.
[0052] FIG. 4 is a plot of % solids vs. time for various levels of JWEB 50
tetra-
functional initiator for a styrene feed having 4 % Bayer 390 rubber. It may be
seen that
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as the amount of JWEB 50 tetrafunctional initiator is increased, the steeper
the plot of
solids v. time indicating rapid polymerization with increasing tetrafunctional
initiator.
Molecular weight data for polymerizations conducted using JWEB 50
tetrafunctional
initiator are summarized in Table III, below. An initiator proportion of 400
ppm JWEB
gave a polymerization rate of about 4.3 times that of thermal polymerization
in the
absence of peroxide, while a level of 600 ppm JWEB gave a polymerization rate
of
about 6.6 times that of pure styrene. These rates are very unusual,
particularly consid-
Bring that acceptable G/R values are obtained. It may also be seen that Mp
decreases
and Mz increases with increasing initiator proportion in Table III.
Table III
Summary of Mol. Wt. Data for Polymerizations with JWEB-50
Ex. Sample Mn Mw Mp Mz Mz+1 MWD
9 4% Bayer380 133145 323895 329801 49727 667047 2.43
0
10 200 ppm JWEB 140409 327845 302749 53557 784150 2.33
6
11 400 ppm JWEB 134609 326436 273374 58609 926299 2.43
7
12 600 ppm JWEB 115929 320599 256155 62593 1046255 2.76
8
[0053] As seen in Table IV, the rubber chemistries are generally similar for
the
initiators. Of interest however, are the relatively high grafting or
gel/rubber values
obtained with the tetrafunctional peroxide. These results indicate that
"normal" rubber
phase properties are attainable at high polymerization rates with PERKADOX 12-
AT25.
[0054] It may also be seen in Table IV, in Examples 17 and 18 using a
tetrafunctional
initiator, that as the swell index increased from 11.0 to 14.3, the ratio of
%gel/%rubber
increased from 2.76 (26.8/9.7 for Example 17) to 2.84 (23.9/8.4 for Example
18). This
trend follows an increase in the PERKADOX 12-AT25 concentration.
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TABLE IV
Effect of Initiators on HIPS Properties
Ex, fll ppm Initiator G/R Ratio SI RPS (vol. med.,,u)
13 152 L331 2.4 12.3 1.51
14 303 ~ L331 2.8 8.7 2.44
15 168 L531 2.8 8.7 2.61
16 335 L531 2.6 9.6 3.05
17 163 P 12 2.8 11.0 1.34
18 326 P12 2.8 14.3 2.25
19 134 T42S 2.4 9.9 1.19
20 268 T42S 2.9 8.8 1.96
NOTES:
1. SI is swell index.
2. RPS is volume median rubber particle size measured by a Malvern Analyzer in
methyl ethyl
ketone.
3. A grafting percent can be obtained as follows: % grafting = 100 (% gels - %
rubber)/%
rubber. This is the same as 100 (G/R - 1 ).
Molecular Weight Stability Studies
[0055] Previous laboratory studies showed that polystyrene produced via
peroxide
initiation exhibited similar levels of thermal degradation (i.e., chain
scission) to those of
thermally polymerized polystyrene. Further work was conducted to compare the
thermal
stability of polymers prepared with a bifunctional initiator (168 LUPERSOL 531
) to that
of polystyrene prepared with the tetrafunctional initiator (163 and 326 ppm
PERKADOX
12-AT25).
[0056] The samples were heated isothermally at 270 °C for 1 hour in a
differential
scanning calorimeter (DSC). Molecular weights were then obtained via gel
permeation
chromatograph (GPC). The results are summarized in Table V.
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TABLE V
Effects of Heat Treatment on Molecular Weight
%Mw % Mn
Ex. fll ppm Initiator Mw11000 Decrease Mn/1000 Decrease
21 168 L531 263 - 116 -
22 168 L531-H 224 14.8 92 20.7
23 163 P12 309 - 112 -
24 163 P12-H 240 22.3 80 28.6
25 326 P12 314 - 82 -
26 326 P12-H 282 10.2 77 6.1
NOTE: The "-H" designation indicates after heat treatment.
[0057] As seen from Table V, the molecular weight decreases after heat
treatment
ranged from 10-22% for Mw and 6-29% for Mn. The degree of thermal degradation
for
the tetrafunctional initiator-produced PS was within the general range for
that of the
bifunctional initiator-produced PS.
[0058] It may be concluded that:
~ The utility of monofunctional initiators is limited in terms of increasing
polymerization productivity due to kinetic constraints.
~ Bi- or multifunctional initiators offer superior rate/molecular weight
relationships.
~ The developmental tetrafunctional initiator (e.g. PERKADOX 12) yielded
significantly higher polymerization rates and molecular weights (particularly
Mw)
than did LUPERSOL 331 or 531.
It is apparent that proper selection and usage of bi- or multifunctional
initiators may
yield the optimum balance of polymerization rate and molecular weight.
Improvement of Rubber Phase Volume of HIPS
[0059] It has been discovered that tetrafunctional initiators, such as
alkylperoxy-
carbonates, for instance JWEB50 tetra t-butylperoxycarbonate available from
ATOFINA
Petrochemicals, Inc., can be used to improve the rubber phase volume of HIPS
products, as measured by the ratio of % gels/% rubber.
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[0060] FIG. 5 shows the relationship of % gels/% rubber vs. swell index for
commercial products. The % gels was used a measure of rubber phase volume and
was measured by dissolving HIPS in toluene, separating the insoluble gel phase
by
centrifugation and then reporting the % of insoluble gel of the total sample.
Swell index
(SI) is measured in the same experiment. After separating the insoluble gel
phase by
centrifugation, the swollen gel is weighed, dried under vacuum and then the
weight of
the dry gel is obtained. The swell index is the ratio of the weight of swollen
gel to dry
gel, and it is a measure of the degree of crosslinking of the rubber phase.
[0061] It is well known that the impact properties of HIPS are determined by
the
properties of the rubber phase volume; thus, an improvement in the % gel/%
rubber
ratio (G/R) is highly desirable.
[0062] FIG. 5 shows that some commercial resins have a G/R of 2.2-3.0 at a
swell
index of 13-9. Note particularly that as the swell index increases the G/R
decreases. In
one non-limiting explanation, this may be because at higher swell indices the
solvent
expands the rubber network more and the polystyrene that is trapped inside
migrates or
diffuses out of the rubber particles, which leads to lower gel values.
[0063] Table VI shows the data obtained as the level of tetrafunctional
initiator is
increased. Batch syntheses were carried out isothermally at 127°C.
[0064] FIG. 6 compares the results of Examples 27, 28, 29 and 30 of this
invention
with some of the commercial grades from FIG. 5. It may be noted that JWEB50
shows
a surprising, opposing trend that as the level of JWEB50 is increased, the G/R
ratio
increases, even though the swell index of these materials is very high. The
trend of the
commercial materials is indicated by the lighter dashed descending line, and
this is the
trend commonly observed. The trend shown by the darker, ascending line for
JWEB50
is surprising and quite unique. Without wishing to be bound to any particular
explanation, it is not clear if this effect is due to the potential for
forming branched
structures exhibited by multifunctional initiators. The extent of branching
can be
measured by the rheological technique used in L. Kasehagen, et al., "A New
Multifunctional Peroxide Initiator for High Molecular Weight, High
Productivity, and
Long-Chain Branching," Society of Plastics Engineering, ANTEC, Paper 99, 2000,
incorporated by reference herein.
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TABLE VI
Effect of JWEB on G/R Ratio
Ex. Formulation, ppm JWEB50 Swell Index Gel/Rubber
27 0 16.1 1.14
28 200 19.0 1.52
29 400 19.7 1.70
30 600 20.5 2.30
[0065] The resins of this invention are expected to produce HIPS with higher
rubber
efficiencies, improved impact strength and ductility.
[0066] The styrene-based polymers of the present invention are expected to
find use
in other injection molded or extrusion molded articles. Thus, the styrene-
based
polymers of the present invention may be widely and effectively used as
materials for
injection molding, extrusion molding or sheet molding. It is also expected
that the
polymer resins of this invention can be used as molding material in the fields
of various
different products, including, but not necessarily limited to, household
goods, electrical
appliances and the like.
[0067] In the foregoing specification, the invention has been described with
reference
to specific embodiments thereof, and has been demonstrated as effective in
providing
methods for preparing polymers using multifunctional peroxide initiators.
However, it will
be evident that various modifications and changes can be made thereto without
departing from the scope of the invention as set forth in the appended claims.
Accordingly, the specification is to be regarded in an illustrative rather
than a restrictive
sense. For example, specific combinations or amounts of vinylaromatic
monomers,
diene .polymers, multifunctional peroxide initiators, and other components
falling within
the claimed parameters, but not specifically identified or tried in a
particular polymer
system, are anticipated and expected to be within the scope of this invention.
Further,
the methods of the invention are expected to work at other conditions,
particularly
temperature, pressure and proportion conditions, than those exemplified
herein.
18
