Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~1305Zt31
CONTROL METHOD OF
MOLECULAR WEIGHT OF POLYPROPYLENE
This invention relates to a process of the homo- or
co-polymerization of propylene. Specifically, the
present invention relates to a method for controlling
the molecular weight of a propylene homo- or co-polymer
which is obtained by subjecting propylene alone or a
mixture of propylene and another d -olefin copolymeri-
zable with propylene to bulk polymerization in the
presence of hydrogen as a molecular weight modifier in a
reaction tank equipped with a reflux condenser while
using the propylene or mixture itself as a liquid medium
too.
It has been well-known that upon polymerization of
propylene in the presence of a Ziegler-Natta catalyst,
the molecular weight of the resulting polypropylene can
be controlled by adjusting the volume of hydrogen to be
added during the polymerization [see, for example, J.
Polymer Sci., Ç2, 109 (1974)]. Since
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there is a certain close relationship between the
concentrations of hydrogen in vapor phases and the
molecular weights of the resulting polypropylenes tsee,
for example, J. Polymer Sci., Part AI, Vol. 8, 2717
S (1970)], polypropylene is usually prepared by
controlling the concentration of hydrogen in a vapor
phase at a constant level so that the molecular weight
of the resulting polypropylene has a desired value.
When polypropylene is prepared by bulk
10 polymerization in a large reaction tank, it is
difficult to remove the polymerization heat if the
removal of heat is effected merely through the wall of
the reaction tank or by means of a heat exchanger
provided inside the reaction tank. Accordingly, it has
lS also been known to use a reflux condenser which makes
use of the latent heat of a liquid medium.
When polypropylene is subjected to bulk
polymerlzation in a reaction tank equipped with the the~
above-mentioned reflux condenser, the concentration of
- ~ 20 hydrogen in a vapor phase however varies significantly
in accordance with the load to the reflux condenser.
It is therefore necessary to repeat the introduction or
; discharge of hydrogen frequently into or out of the
; reaction tank in order to maintain the concentration of
25 hydrogen at a constant level in the vapor phase, that
is, to control the molecular weight of the resulting
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polymer. This means that a great deal of hydrogen is
discharged and moreover, a large volume of propylene is
also discharged along with the thus-discharged hydrogen,
resulting in a problem that the above process is not
preferred economically.
The present inventors have carried out an
extensive investigation with a view toward providing a
solution to the above-described problems. The
investigation has now resulted in the finding of a
process which allows to adjust, with good controll-
ability, the molecular weight of polypropylene without
loss of hydrogen and/or propylene, leading to completion
of this invention.
The present invention is directed towards the
provision of a process for the preparation of a
propylene homo- or co-polymer of a controlled molecular
weight without 1088 of raw materials.
In one aspect of this invention, there is thus
provided a process for the preparation of a propylene
homo- or co-polymer by subjecting propylene, or a
mixture of propylene and another ~-olefin copolymeri-
zable with propylene, as a monomer or monomer mixture to
bulk polymerization at a constant temperature, in the
presence of hydrogen as a molecular weight modifier, in
a reaction tank equipped with a reflux condenser while
using the propylene or mixture itself as a liquid medium
and condensing vapor of the medium in the reflux
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condenser so as to remove at least a part of
polymerization heat. The process comprises:
(a) measuring only the heat balance of the reaction
tank in calculating the quantity of polymerization
reaction heat generated in the reaction tank based on
said measurement, not using detectable information of
the concentration of hydrogen in the vapor phase of the
reaction tank at all in this calculation and the
following calculations and determinations and
calculating the amo~nt of the monomer or monomer
mixture polymerized in the reaction tank based on the
thus-calculated quantity;
(b) determining in advance the relationship
between molecular weights and the volume of hydrogen
consumption required for the molecular weights with
respect to propylene homo- or co-polymer;
~ c) determining the volume of hydrogen consumption
required per unit amount of propylene alone or the
mixture of propylene with another ~-olefin
copolymerizable with propylene corresponding to a
desired molecular weight of the propylene homo- or
co-polymer; and
(d) reacting ~he monomer or monomer mixture while
controlling the volume of hydrogen, which is to be fed
into the reaction tank, in accordance with variations in
the volume of hydrogen required in the reaction tank as
a product of the volume of the required hydrogen
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consumption and the above-calculated amount of the
monomer or monomer mixture.
In the following description, there is reference to
the accompanying drawings, in which:
FIGURE 1 shows one example of an apparatus suitable
for use in the practice of the process of this
invention;
FIGURE 2 is a diagrammatic representation of the
relationship between the volume of hydrogen consumed in
an exemplary polymerization process at a constant
temperature and the intrinsic viscosity of the resulting
polymer measured as its tetralin solution; and
FIGURE 3 is a diagrammatic representation of the
relationship between the reaction time periods in
Examples and the concentrations of hydrogen in the
reaction vessels and the intrinsic viscosities of the
resulting polymers.
The term "another ~-olefin copolymerizable with
propylene" as used herein means at least one of
ethylene, butene-l, hexene-l, etc. and may also be
called "copolymerizable d -olefin" hereinafter. When a
propylene copolymer is prepared in accordance with the
process of this invention, no particular limitation is
imposed on the amount of the copolymerizable ~-olefin
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so long as the resulting polypropylene remains in a
slurry state. However, the upper limit of the
proportion of the copolymerizable ~-olefin other than
propylene in each resulting polymer may generally be
5 about 40 wt.% or so. For the sake of convenience in
describing the present invention, the term "propylene"
as used in the descriptive portion of the present
specification other than the Examples should be
interpreted to include not only propylene alone but
10 also a mixture of propylene and another a-olefin
copolymerizable with propylene. Correspondingly, the
term "polypropylene" as used in the descriptive portion
of the present specification other than the Examples
means not only propylene homopolymer but also the
lS c,opolymer of the mixture.
For the following reasons, the process of this
invention finds extremely important utility when
propylene is polymerized in the presence of hydrogen as
a molecular weight modifier in a reaction tank equipped
20 with a reflux condenser.
In a reaction tank having no reflux condenser,
the vapor phase and liquid phase are maintained in
vapor-liquid equilibrium and moreover, the vapor phase
is in a substantially even state. Therefore, the
25 concentration of hydrogen in the vapor phase can be
accurately determined if the gas of the vapor phase is
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sampled and its hydrogen concentration is measured. It
is hence possible to control the molecular weight of
the resulting polypropylene by comparing the thus-
detected hydrogen concentration with a desired hydrogen
5 concentration by conventionally-known desired
comparator means and on the basis of the results of the
comparison, by automatically controlling a feed valve
of hydrogen to the reaction tank and thus always
introducing a deficient volume of hydrogen into the
10 reaction vessel so as to maintain the concentration of
hydrogen in the vapor phase substantially at a constant
level.
However, the vapor phase and liquid phase are
not always maintained in vapor-liquid equilibrium when
15 a polymerization is conducted by using a reaction
vessel equipped with a reflux condenser. In addition,
the concentration of hydrogen in the vapor phase varies
considerably depending on the load to the reflux
condenser along the passage of time as mentioned above.
20 As a result, it i8 impossible to control the molecular
weight of the resulting polypropylene if a simple
automatic controlling method such as that referred to
above is relied upon.
As exemplary polymerization catalysts useful in
25 the practice of this invention, may be mentioned
catalyst systems composed of conventionally-known
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transition metal catalysts and organometallic
compounds. One or more stereoregularity improvers may
also be used in combination if necessary or desirable.
Although not limited specifically to the following
s exemplary polymerization catalysts, illustrative of the
polymerization catalyst may include titanium
trichloride obtained by reducing titanium tetrachloride
with a reducing agent such as aluminum; organoaluminum
; or organomagnesium, those obtained by subjecting
10 titanium trichloride to activation treatments such as
, . . .
its treatments with oxygen-containing organic
compounds, titanium tetrachloride and the like
,
subsequent to its grinding; those formed of titanium
trichloride or titanium tetrachloride supported on
15 carriers 9uch a9 magnesium chlorid-; etc. As exemplary
organometallic compound~, may be mentioned organo-
aluminums such as trialkylaluminums, dialkylaluminum
:
halides, alkylaluminum sesquihalides and alkylaluminum
dlhallde~ and organomagnesiums ~uch as dialkyl-
20 agnesiu0s.
One embodiment of the present invention willhereinafter be described with reference to the
accompanylng drawings.
FIGURE 1 illustrates one example of an apparatus
25 ~uitable for use in the practice of the process of this
invention,~in whlch there are illustrated an agitator-
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equipped reaction tank 1, a reflux condenser 2 in theform of a horizontal shell-and-tube heat exchanger, a
jacket 3 for the reaction tank 1 and an inlet line 5
for the introduction of a slurry into the reaction tank
5 1. Where the reaction tank 1 is employed for single-
tank polymerization or is used as the first tank upon
polymerization in a plurality of tanks connected in
series, the inlet line 5 is used for the introduction
of a catalyst slurry. Where the reaction tank 1 is the
10 second or subsequent tank in such series reaction
tanks, the inlet line 5 is employed for the introduc-
tion of a reaction slurry from the preceding reaction
tank. There are also shown a discharge line 6 for the
removal of a slurry from the reaction tank 1, a charge
15 line 7 for the introduction of propylene and a
catalyst, a sampling line 9 for the collection of gas
from the vapor phase of the reaction tank 1, and a
blower 18 adapted to recycle to the reaction tank 1
uncondensed gas which has not been condensed in the
20 reflux condenser 2 and is composed principally of
hydrogen gas. Also illustrated are a detector 4-1 for
the flow velocity and temperature of gas at the
entrance to the reflux condenser 2, another detector
4-2 for the flow velocity and temperature of a
25 condensate returning to the reaction tank 1 subsequent
to i~s recovery in the reflux condenser 2, a flow rate
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regulating valve 4-3 for hydrogen gas to be introduced
into the reaction tank 1, a further detector 4-4 for
the flow velocity and temperature of cooling (or
heating) water leaving the jacket 3, a still further
s detector 4-5 for the flow rate and temperature of
cooling (or heating) water to be introduced into the
jacket 3,
The following procedure may be followed by way
of example in order to calculate the amount of a
10 monomer or monomer mixture polymerized per unit time in
the reaction tank 1. Data signals a,b,c,d, which have
been output from the detectors 4-1,4-2,4-4,4-S respec-
tively, are input to a data processor 8, where the
quantity of heat generated per unit time in the
lS reaction tank 1 at the time of output of the data
~ignals i~ calculated by correcting the quantity of
heat removed per the ~ame unit time from the reaction
: tank 1, which has been calculated from the data signals
- a,b,c,d, in accordance with the quantity of dissipated
20 heat whlch ha~ been calculated based on the overall
: structure of the polymerization system and its
: operational conditions. Since the relation~hip between
polymerized amount of the monomer or monomer mixture
and reaction heat can be known from the compo~ition of
/
25 the thu~-polymerized monomer or monomer mixture in the
manner known per se in the art, the above-mentioned
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generated heat is converted further at the data
Qrocessor 8 into the amount of the monomer or monomer
mixture polymerized per unit time in the reaction tank
1.
Incidentally, the relationship between the
molecular weight of polypropylene of a desired
composition and the volume of hydrogen required for the
preparation of the polypropylene varies in accordance
with catalyst system, polymerization temperature and
10 the like, but as shown in FIGURE 2 by way of example,
the relationship between the intrinsic viscosity of a
polymer as measured in the form of its tetralin
solution of 135C and the volume of hydrogen
consumption per unit weight of the polymer can be
15 predetermined.
It is therefore possible to determine the volume of
hydrogen reguired per unit amount of feed propylene by
storing beforehand the above relational expression as
an equation In the data processor 8 and then inputting
20 a desired polypropylene molecular weight in the data
processor 8.
In the above-described manner, the volume of
hydrogen required in the reaction tank 1 is hence
calculated at the data processor 8 as the product of
25 the amount of the polymerized monomer or monomer
mixture, which has been calculated in advance, and the
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volume of hydrogen required for the unit amount of the
feea polypropylene. Results o~ the operation are
output as a signal e from the data processor 8. It is
therefore possible to replenish the volume of consumed
s hydrogen by changing the opening degree of the flow
rate regulating valve for hydrogen gas in accordance
with variations in the value of the signal e so as to
control the volume of hydrogen to be introduced into
the reaction tank 1, that is, to conduct the reaction
10 while maintaining the actual concentration of hydrogen
in the reaction tank 1 substantialIy at a constant
level. Accordingly, it seems to be possible to prepare
polypropylene of a uniform molecular weight.
By the way, when the present invention is
15 applied to such a reaction ~y~tem that a plurality of
tanks are connected in series to conduct continuous
polymerization therein and the molecular weight of the
resulting polymer i8 increased successively from one
tank to the next tank, hydrogen is introduced and
20 di9charged from each of the tanks along with the
:
~slurries which are introduced through the line S and
diocharged through the line 6 respectively and contains
8aid hydrogen dissolved therein. It is hence necessary
~;~ to input information on the volume of the hydrogen in
25 the data processor 8 and to perform a correction on the
-
~; basis of the information.
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On the other hand, when a single-tank polymeri-
zation process is effected in the above-described
reaction tan~ l or a polymerization process is
conducted by connecting in series a plurality of
5 reaction tanks, each, of the same type as the reaction
tank 1, each of the reaction tanks has already been
filled with a great deal of propylene not only as a
liquid medium but also as a reaction raw material at
the start-up time of the reaction. It is therefore
10 impossible to obtain a polymer of a desired molecular
weight even if hydrogen is fed in accordance with the
present invention, namely, in a volume correspond-
ing to the amount of polymerized propylene which is
calculated based on the measured and calculated
15 quantity of heat of the polymerization reaction. While
taking into consideration the volume of hydrogen to be
dissolved in the liquid propylene filled in each
reaction tank at the start-up time and the volume of
the vapor phase above the liquid medium, it is thus
20 necessary to charge at once hydrogen in a volume
corresponding to the liquid propylene at the beginning
~o that the polymerization reaction is conducted. The
molecular weight of the resulting polypropylene is then
measured and compared with a desired value. Based on
25 the results of the comparison, a small amount of
hydrogen or propylene is additionally charged in the
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reaction tank. The above-described fine correctio~
procedure is repeated until the molecular weight of the
resulting polypropylene reaches the desired value. The
reaction is thereafter allowed to proceed further in
5 accordance with this invention, whereby polypropylene
of a constant molecular weight can be prepaxed.
It is a reaction tank equipped with a reflux
condenser that can be used in the practice of the
present invention. No particular limitation is imposed
10 on the heat-removing capacity of the reflux condenser.
The present invention is particularly effective in a
steady state, that is, when it is applied to a reaction
tank the temperature of which is controlled by the
removal of heat through the reflux condenser while the
lS pregent invention is being practised.
According to the present invention, it is
po~9ible to maintain the molecular weight of the
resulting polypropylene at a constant level by
introducing hydrogen in a volume corresponding to the
20 volume of it9 consumption into the reaction tank, since
the volume of hydrogen consumption required upon
providing polypropylene of a constant molecular weight
is uniform per unit weight and the vapor phase and
liquid phase are maintained in equilibrium on average
25 although the concentration of hydrogen in the vapor
phase of the reaction tank varies depending upon the
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load to the reflux condenser and its apparent value
changes considerably.
The present invention is extremely valuable from
the industrial viewpoint because polypropylene of a
constant molecular weight can be obtained with not only
high efficiency but also good controllability by using
a reaction tank equipped with a reflux condenser and
conducting bulk polymerization of propylene in the
presence of hydrogen as a molecular weight modifier in
accordance with the process of this invention.
EXAMPLES:
Continuous bulk polymerization of liquid
propylene was conducted at 70C in the presence of a
catalyst composed of titanium trichloride and
diethylaluminum chloride in a reaction tank having the
structure shown in FIGURE 1 and an internal capacity of
40 m3 while using the liquid propylene as a medium.
Upon initiation of the polymerization, 3000 kg
of propylene and 35 Nm3 of hydrogen were first
charged in the reaction tank. Warm water was caused to
flow through the jacket so as to heat the medium up to
70C. The polymerization reaction was then initiated
while charging the catalyst and propylene at constant
feed velocities (titanium trichloride: l.0 kg/hr,
diethylaluminum chloride; 16 kg/hr, propylene: lO000
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kg/hr). During the reaction, the reaction slurry was
sampled from the reaction tank and the molecular weight
of the resultant polypropylene was measured. The thus-
measured molecular weight was compared with a predeter-
5 mined value. The molecular weight of the resultingpolypropylene was adjusted substantially to the
predetermined value by repeating several times a fine
correction procedure in which a small amount of
hydrogen was charged in the reaction tank on the basis
10 of the results of the above comparison. About 30
minutes were spent until the predetermined value was
reached.
Continuous bulk polymerization of propylene was
then conducted in accordance with the process of this
15 invention. Namely, propylene, titanium trichloride and
diethylaluminum chloride were charged at constant feed
velocities, namely, at 6000 kg/hr, 0.8 kg/hr and
8 kg/hr respectively into the reaction tank. At the
same time, a slurry was charged out at about 6000 kg/hr
20 from the reaction tank so as to maintain the level of
the slurry constant in the reaction tank. During this
polymerization, data signals a,b,c,d were input from
the detectors 4-1,4-2,4-4,4-5 into the data processor 8
to calculate the quantity of heat removed through the
25 jacket and reflux condenser. It was found to be 860
Mcal/hr. Furthermore, upon its correction by the
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quantity of heat released from the system, the quantity
of polymerization reaction heat generated in the
reaction tank was 1,006 Mcal/hr, which corresponded to
a polymerized propylene amount of 2,196 kg/hr.
The intrinsic viscosity corresponding to
polypropylene of a desired molecular weight as measured
in the form of its tetralin solution of 135C was
found to be 1.73 and the volume of hydrogen required
corresponding to the above-determined polymerized
10 propylene amount was found to be 1.152 Nm3/hr from
FIGURE 2. A correction was performed in view of the
volume of the hydrogen discharged along with the slurry
from the reaction tank which volume was 0.845 Nm3/hr.
As a consequent, hydrogen was introduced into the
~. qq7 3
15 reaction tank at the rate of 1.977 Nm /hr through the
flow rate regulating valve 4-3. By the way, the above
operation, conver~ion and correction were all performed
automatically by the data processor, and the system was
operated in such a way that the molecular weight of the
20 polypropylene in the discharged slurry was
automatically controlled by delivering the volume of
hydrogen, which was to be introduced, as the signal e
to the flow rate regulating valve 4-3.
The polymerization reaction was continued while
25 correcting the volume of hydrogen, which was to be
introduced, at intervals of five minutes in accordance
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with variations of the data signals a,b,c,d. Two hours
later, the volume of the polymerized propylene reached
2,405 kg/hr. At that point of time, the charging rate
of hydrogen was 1.323 Nm3/hr and the intrinsic
5 viscosity of the polymer sampled out from the
discharged slurry as measured in the form of its
tetralin solution of 135~C was 1.73 as desired.
The above reaction was continued for about 20
hours. FIGURE 3 diagrammatically illustrates
10 time-dependent variations in the concentration (vol.%)
of hydrogen in the vapor phase sampled out through the
line 9 as well as time-dependent variations in the
intrinsic viscosity of polypropylene in the slurry
discharged through the line 6. As understood from
15 PIG~RE 3, the intrinsic viscosity, namely, the
molecular weight was controlled at a constant level
although the concentration of hydrogen in the vapor
phase varied.
By the way, about 65~ of the total quantity of
20 heat removed through the jacket and reflux condenser in
a ~teady ~tate, namely, during the practice of the
process of this invention was accounted for on average
by reflux condenser.
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