Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~j BACKGROU~D OF THE I~VE~TIO~
Isotactic polypropylene, as defined in ~atta et al
SP 3,112,300, is a polypropylene which consists essentially of
isotactic macromolecules, i.e., macromolecules having substantiall
the isotactic structure and being insoluble in (non-extractable
i with) boiling n-heptane.
While said polypropylene is adapted to use in many
commercially important applications, its impact strengih at .
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temperatures of 0C or less is rather low, particularly for so-
called commercial grade polypropylene.
Different ways of improving the impact strength of the
l polypropylene at low temperatures without unacceptable adverse
1 affect on its other properties, including its flexural rigidity
and thermal resistance have been proposed.
The technique which is most widely used for achieving
that objective consists in polymeriæing propylene in contact with
¦ a Ziegler/Natta stereospecific catalyst until most of the propy-
1 lene is polymerized and then, during the final stage of the
propylene polymerization feeding a different olefin, in
particular ethylene, to the polymerization zone and continuing
'1 the polymerization until the amount of the added olefin, e.g.,
¦¦ ethylene, polymerized is from 1% to 20% of the total (final)
1I polymeric composition obtained.
USP 3,624,184 discloses a typical method which is
widely followed. According to that method, propylene is first
polymerized in an inert hydrocarbon solvent such as n-heptane and
in the presence of a stereospecific polymerization catalyst
~0 prepared by mixing a Ti trihalide with a dialkyl Al monohalide
to obtain a poly~erization slurry, i.e., a slurry of polypropylene
in the n-heptane. After "flash-off" of unreacted propylene until
the slurry comprises a controlled amount of unreacted propylene,
the slurry is preferably transferred to a second reactor, a
mixture of ethylene and propylene in a molar ratio ranging, in
general, from 1 to 6 is introduced into the second reactor, and
the polymerization is continued until the amount of polymerized
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ethylene reaches a prefixed value which, generally, i9 rom 5%
to 2~/o by weight.
The main disadvantage of that process - which in
practice prevents conducting the polymerization continuously and
involves many difficulties in batch polymerization - is that
during the polymerization of ethylene in the presence of propylene
Ij dissolved in the reaction medium, or in the presence of propylene
I¦ fed in with the ethylene, considerable amounts of rubbery
¦¦ ethylene/propylene copolymers soluble in the reaction medium are
j formed, which give rise to considerable difficulties in the heat
¦ exchange and in transfer of the polymerization slurry.
I
THE PRESE~T INVENTION
One object of this invention is to provide a new method
for improving the low~temperature characteristics of the composi-
¦¦ tions based on isotactic polypropylene which avoids or minimizes
¦¦ the problems encountered in the widely used prior art process
¦I discussed supra.
~¦ That and other objects are achieved by the method of
the invention which is a two-step method with critical modifica-
tions which make it possible to avoid, or to substantially
minimize, the disadvantages of the known two-step method.
The present method for making compositions which, while
containing at least 50~/O by weight of isotactic polypropylene,
have a high resistance to impact at temperatures of 0C and below,
comprises the step of (1) polymerizing propylene in an inert
liquid hydrocarbon medium or diluent and in the presence of a
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stereospecific catalyst obtained by mixing a Ti trihalide, such
as crystalline TiCl3, with a dialkyl Al monohalide, such as die-
thyl Al Chloride, and the step (2) of feeding ethylene or a mix-
ture of ethylene and propylene to the polymerization slurry obtai-
ned in step ~1) and continuing the polymerization until the amount
of polymerized ethylene is, at most, 20% of the total polymeric
composition.
~owever, in the present method, the second step comprises
one or more ethylene/propylene copolymerization steps in which the
ethylene/propylene molar ratio is at least one of the following:
(A) the ratio of ethylene fed to propylene present
in the system ranges from 1/99 to 40/60, preferably
from 5/95 to 30/70, and the amount of copolymer
formed is comprised between 3% and 8% by weight
of the total (final) polymeric composition;
and/or
(B) the ethylene/propylene molar ratio in the ethylene/ .
propylene mixture fed to the polymerization slurry
ranges from 60/40 to 85/15, and the amount of copo-
lymer formed is comprised between 3% and 18% of the
total (final) polymeric composition obtained.
Said copolymerizations carried-out in accordance wlth
(A) or (B) are the essence of the present invention and process
in that they make possible composition which, although consist- .
ing for at least 50% by weight of isotactic polypropylene, ne-
vertheless have high impact resistance at low temperatures,
~ithout any appreciable deterioration of the other valuable
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properties of compositions the essential constituent of which is
isotactic polypropylene while, at the same time, avoiding the
problems encountered with respect to heat exchange and transfer
of the polymerization slurry, even when the final polymerization
product has a content of combined ethylene as high as 2~/o~
In the practice of this invention the inert liquid
hydrocarbon polymerization medium or diluent is an aliphatic,
cycloaliphatic, aromatic or alkyl-substituted aromatic hydro-
carbon, such as, for example, hexane, cyclohexane, heptane,
xylene or the like; the stereospecific catalyst is of the Ziegler/,
Natta type and obtained by mixing a titanium trihalide (for
example TiC13 obtained by reduction of TiC14 with Al or with
organometallic compounds of Al preferably complexed with electron-
donor compounds) with dialkyl Al monohalides (for example
(C2H5)2 AlCl).
Step (1) of the process is generally carried out in the
presence of hydrogen as molecular weight regulator.
A presently preferred embodiment of the process
comprises the following operations:
~a) producing isotactic polypropylene by polymerizin~
propylene in a hydrocarbon solvent (e.g., heptane)
at a temperature of from 50C to 80-C and at a
pressure between 3 and 10 kg~cm gauge, in the
presence of hydrogen as molecular weight modifier,
and of a catalyst obtained by mixing TiC13 (or
TiC13 complexed with electron-donors) with
(C2H5)2 AlCl; a suspension of substantially
isotactic polypropylene in the hydrocarbon solvent
is thus obtained (and referred to herein as the
polymerization slurry of the first step);
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(b) regulation of the propylene concentration in th~
slurry, generally by reducing the pressure to even
lower than 0.2 Xg/cm gauge (flashing) and, prefe-
rably but not necessarily, raising the temperature
of the system;
(c) introduc-tion of ethylene into the propylene contai-
ning slurry, in an amount such as to obtain a mo-
lar ratio of the fed ethylene to propylene existing
in the system of from 1/99 to 40/60, allowing
the mixture of the two monomers to polymerize at a :
temperature of from 60C to 80C and at a pressure
generally lower than 10 kg/cm gauge and usually
comprised between 0.2 and 2.0 kg/cm gauge, until
the resulting ethylene/propylene copolymer consti-
tutes 3% to 8~, preferably 4% to 6% by weight of the
total polymeric composition and finally, preferably
but not necessarily
(d) introducing into the polymerization slurry of (c)
ethylene or an ethylene/propylene mixture very rich
in ethylene and preferably containing more than
80~ by moles of ethylene, allowing such mixtures
to polymerize at a temperature of from 60C to 80C .
. and at a pressure generally lower than 10 kgjcm2
gauge, usually comprised between 0.2 and 2.0 kg/cm2
gauge.
, .
. In an.equally advantageous alternative procedure, ~c)
and/or (d) are replaced by
. (c') which consists in feeding into the slurry of ~b)
andjor that of ~c) ethylene continously and pro-
pylene discontinously to obtain, during the pXopy-
lene feeding, a molar ratio of ethylene to propylene
comprised between 60/40 and 85/15, and in the fed
as a whole a molar ratio of at least 80~20 between
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the total amount of ethylene fed and the total
amount of propylene fed; both monomers are poly-
merized at a temperature of from 60C to 80C and
l at a pressure generally lower than 10 kg/cm gauge,
usually between 0.2 and 2.0 kg/cm2 gauge, until
the resulting copolymer makes up 3% to l~/o~
preferably 4% to ~/0 by weight of the total
composition.
After such operations [(a) to (d)] or [~a), (b) and
(c')], the catalyst is deactivated, if necessary, by the addition~
of lower aliphatic alcohols and washed. Thereafter, t.e poly- ¦
meric material is separated from the hydrocarbon solvent and the
resulting polypropylene composition is dried.
Operations (a) to (d) and (c') may be carried out in
one reactor or in more than one reactor. In the latter case,
the production of isotactic polypropylene ~operation (a)] occurs
in a reactor (primary reactor) and operations (b), the propylene
concentration regulation, and (c), ethylene feeding, occur in a
flashing apparatus. Finally, operation (d) or (c') is conducted
in another reactor (secondary reactor).
Transfer of the polymerizatlon slurry from one reactor
to another can be effected without difficulty by using the
systems known to those skilled in the art. Furthermore, it is
particularly easy to secure effective thermal control of ~he
2S polymerization reactions during all operations of the process.
In practice, the present process can be carried out continuously
without having to use very complicated and expensive devices for
the transfer of the slurries or having to stop the reactor~ for
assuring thermal control of the polymerization reactions.
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The polypropylene compositions obtained by the process
of this invention exhibit melt index values (g/10') comprised
between 0.1 and 10 and have a number of excellent characteristics,
in particular:
¦ ~ modulus of ela~ticity to flexure comprised between 750~ and
13,000 kg/cm;
~ emhrittlement temperature comprised between -15 and -60C;
o resilience at 0C comprised between 5 and 20 kg/cm/cm; and
~ transition temperature D/F comprised between 0C and -50C.
l The following,examples are given to illustrate the
¦ essential features of the invention, and are not intended to be
limiting.
EXAMPLE 1
~ This example illustrates the preparation of polypropy- ¦
lene compositions according to a continuous process comprlsing,
in the order given, operatibns (a), (b), (c)-and (d) as defined
hereinabove. The operating conditions for each single step are
described hereinafter.
I OPeratiOn la) ,
A 4 m (primary) reactor was continuously fed with:
~ hydrocarbon solvent (technical heptane) - 250 l/hour
o polymerization catalyst 3 TiC13.AlC13
~obtained from TiC14 by reduction with
aluminum and successive activation by
25' dry-grinding, complexed with methyl
benzoate (MB); TiC13/MB molar ratio =
0.1) in the form of heptane solution
containing 7 g/l of TiC13 ' - 40 l/hour
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molecular weight regulator: hydrogen - 120 l/hour
~ propylene - 150 kg/hour
The reaction conditions were:
o temperature - 60C
o manometric pressure - 5-6 kg/cm gauge
o residence time - 4 hours.
By operating as descri~ed hereinabove, it was possible
to obtain 136 kg/hour of substantially isotactic polypropylene,
suspended in the hydrocarb~n solvent, that contained still active
10- I catalyst and unreacted propyiene. This suspension is referred~to,
for simplicity, as polymerization slurry.
OPeration (b)
The polymerization slurry was transferred to another
reactor (flashing apparatus) of 1.8 m , by pressure difference
lS (from 5-6 kg/cm g. in the primary reactor to 0.2 kg~/cm2 g. in
the flashing apparatus), bringing the temperature to 70C and
allowing the unreacted propylene in excess to flash; a propylene-
saturated slurry (at the pressure and temperature indicated
hereinabove~ was thus obtained.
Operation tc)
The flashing apparatus was fed with 150 l/h of technical
heptane and with 1 kg/h of ethylene so as to obtain an ethylene/
propylene molar ratio equal to 25/75, whereupon the monomeric
mixture was polymerized at a temperature of 70C, at a pressure of
0.2 kg/cm g and with a residence time in the flashing apparatus
of 1 hour.
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A rubbery copolymer in an amount of 6.8 kg/h corres-
ponding to 4. ~/o of the total composition, was thus produced.
OPeration (d)
The polymeri~ation slurry of (c) was transferred by
means of pumps to another reactor (secondary) of 1.5 m , into
which an ethylene/propylene mixture in a molar ratio of 97/3, ¦ -
corresponding to a feeding of 30 kg/h of ethylene and of 1,86 kg/h
of propylene, was introduced; the whole was allowed to polymerize
at a temperature of 70C, at a pressure of 0.7 kg/cm g., with a
residence time of 2 hours. By this procedure, a crystalline
copolymer containing a high percentage of ethylene was produced
in an amount of 29.9 kg/h corresponding to 18.4% by weight of the
total composition. Still operating continuously the polymeriza-
tion slurry of the secondary reactor was transferred at first into
a reactor wherein it was treated at 85C with n-butanol, in order i
to deactivate the catalyst, then subjected to washing with water,
centrifuged at 50C and dried at a maximum temperature of 125C.
The resulting polypropylene composition having a final
content of combined ethylene equal to 12% by weight, exhibited the
following physical-chemical and technical characteristics:
o melt index ~g/10') 0.5
e melting point C 172 ,1
o elasticity modulus kg/cm 8,000
o resilience Xg/cm/cm 11.8
D embrittlement temp. C -55
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Tensile test: ¦
~ max. tensile strength kg/cm 260
e elongation % 570
o yield strength kg!cm 234
o transition temp. DF C -25.5
0 viscosity dl/g 3.4
This test for preparing polypropylene compositions was
conducted continuously for 44 days without meeting with any
difficulties regarding the heat exchange and the transfer of the
slurry from one reactor to another.
By way of comparison, the test was repeated, but t
without operation (c), i.e., without the feeding of ethylene into
the flashing apparatus. In this case, a polymeric product
exhibiting characteristics similar to those of the product
according to this invention was obtained, but, due to troubles
with respect to the heat exchange and slurry transfer, the
reactor run had to be stopped after about 24 hours.
EXAMPLE 2
This example illustrates the preparation of polypropy-
lene compositions according to a continuous process comprising in
the order given, operations ~a), ~b), (c) and Ic') as described
hereinabove. Thc operating conditions in the primary reactor and
in the flashing apparatus - operations (a), (b) and (c) were the
same as in Example 1. Operation (d) was rcplaced by (c'), during
which ethylene was fed in continuously and propylene was fed i~
, discontinuously.
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The polymerization slurry of (c) was transferred by
means of pumps into another 1.5 m reactor (secondary), to which
ethylene and propylene were fed continuously and discontinuously
respectively, operating at a temperature of 70C and at a
pressure of 0.7 - 1 kg/cm g., with a residence time of 2 hours.
More particularly in a first test, A, an ethylene/propylene
mixture having a molar ratio = 80/20 was fed in first for 10
minutes, then ethylene alone was fed in for 20 minutes, after
which an ethylene/propylene mixture (80/20) was fed in for
another 10 minutes and finally ethylene alone for 20 minutes,
repeating this type of feeding for all the time required for test !
A.
In a second test, B, the type of hourly feeding was
similar to that of test A, the only difference consisting in that,
during the propylene feeding, the ethylene/propylene molar ratio
. was 70/30 instead of 80.~20.
In test A, the ethylene/propylene total molar ratio was
92.5/7.5, while in test B it was 88/12.
Unlike the preceding example, in operation (c') of this
Example a predominantly rubbery copolymer was formed during the
feeding of propylene, and a predominantly crystalline copolymer
. was formed during the feeding of ethylene only.
~y operating successively according to example 1, poly-
propylene compositions were obtained having contents of combined
total ethylene, of isotac~ic polypropylene, of rubbery and
, crystalline copolymer as specified below:
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Test A Test B
Content of substantially isotactic
polypropylene, % by weight 80.8 79.8
total content of combined
ethylene, % by weight 12.0 9.9
rubbery copolymer operation (c)
% by weight 4.3 4.3
rubbery copolymer operation (c')
% by weight 6.1 7.2
crystalline copolymer operation (c')
% by weight 8.8 8.7
The physical-mechanical and thermal characteristics of the two
polypropylene compositions are listed below: ¦
Test A Test B
melt index g/10' 0.93 1.8
melting point C 169 168
elasticity modulus kg/cm - 8100 9500
resilience (at 0C) kg/cm/cm 10.9 7.9
embrittlement temp. C -40.5 -21.5
Tensile test:
¦ o max. tensile strength Xg/cm2 260 266
¦ ~ elongation % 650 766
e yield strength kg/cm
transition temp. DF C -12.5 -7
I viscosity 3 2.7
Tests A and B were conducted continuously for a tim~
period of 44 days without encountering any difficulty.
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By way of comparison, tests A and B were repeated, but
replacing (c') with (d), during which ethylene and propylene
were fed continously in molar ratios equal to those found on
the whole in (c') of tests A and B according to this invention.
In both comparison tests, polymeric products were ob-
tained exhibiting properties analogous with those of the pro-
ducts of tests A and B, but, owing to troubles with the heat
exchange and slurry transfer, the reactors run had to be stopped
after about 30 hours.
EXAMPLE 3
This example illustrates the preparation of polypropy-
lene compositions employing operations, (a), (b), (c) and (d) as
defined herein, all conducted in the same reactor. To such pur-
pose, a 20 l reactor was charged with:
3 TiCl3.AlCl3 8.0 g
Al~C2H5)2Cl 16.0 g
technical heptane 10 l
propylene up to 4 kg/cm2 g.
The reactor was maintained at 65 C and in a time of
1.5 hours 2.5 kg of substantially isotactic polypropylene were-
obtained Coperation ~a)~. The isotacticity index of tha
polypropylene was between 90 and 93.5, depending on the tests.
The pressure in the reactor was successively reduced to 1 kg/~m2
g., thus obtaining a propylene-saturated (130 g) polymeriza-
tion slurry[ operation (b)~; then ethylene was fed in until
the slurry had absorbed it up to a pressure of 0.5 kg/cm
g; subsequently, by feeding in additional ethylene,
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the pressure was brought again to 1 kg/cm g., allowing the
ethylene/propylene mixture to polymerize [operation tc)]; finally
ethylene/propylene mixtures very rich in ethylene (at least 9~/0
by moles) were continuously fed in and allowedto polymerize in
¦ the reaction slurry. At the conclusion of the polymerization the
¦ catalyst was deactivated and the polypropylene composition thus
I obtained was separated and purified as described in Example 1.
¦ The following Table I gives the characteristics of the
polypropylene compositions obtained as a function of the operating
conditions in (c) and (d) and of the polypropylene type
~isotacticity index) produced in operation (a).
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TABLE I
Test 1 Test 2 Test 3 Test 4 Test 5 ¦
OPeration ta)
Isotacticity index: % 93.5 93.5 90.5 92 90
Operation (c)
Residual propylene: g 130 130130 130 130
Ethylene fed: g 10 10 30 10 10
Ethylene/propylene
molar ratio: 10/90 10/90 25/75 10/90 10/90 !
Op,eration (d)
Etilylene fed: g 450 450 4S0450 450
Propylene fed: g 50 20 70 50 20
Ethylene/propylene
molar ratio: 94/6 97/3 90/10 94/6 97/3
Ethylene final
content: % by weight 14.9 17.5 11 10.4 12~5
Melt flow index: g/10' 0.57 0.45 3.5 2.4 1.9
Embrittlement
temperature: C -43 -41 -44 -34 -28
. Flexural rigidity kg/m 11,100 11,050 10,400 ~1,050 11,750
Resilience at OC
. kg/cm/~m 9.2 8.2 8.8 8.2 8.0
Tensile tests
Max. tensile strength: 2
kg/cm 302 304 257 280 268
Elongation at break: % 240 161 660 690 490
Yield stress: kg/cm 302 304 257 280 268
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EXAMPLE 4
This example illustrates the preparation of the poly-
propylene compositions by a continuous process that comprises, in
the order given, operations (a), (b) and (c') as defined herein.
The operating conditions for (a) and (b) were the same as in
Example 1: in (c') the temperature was 70C, the pressure was
0.7 - 1 kg/cm gauge and the residence time in the reactor
(secondary) was 2 hours. The feeding modali-ties of ethylene and
propylene during (c') and the characteristics of the polypropylene
compositions thus obtained are reported in Tables II and III.
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Tests l,to 10 were conducted 'continuously,for a 44-day
period without meeting with any difficulty. Test 10 was repeated ¦ ,,
for comparative purposes, but replacing tc') with a (d) in which
ethylene and propylene were fed continuously in molar ratios equal
to those generally used in (c') of test 10 according to the
present invention.
In the comparative test, a polymeric material was
obtained exhibiting characteristics rather similar to those of
the product of test 10 but, due to troubles with heat exchange
and slurry transfer, the run of the reactors had to be stopped
~ after abo 30 hours.
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