CA1126899A - Process for producing propylene-ethylene block copolymers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A novel process is proved herein for producing propylene-ethylene block copolymers by a three-step polymerization technique using a stereoregular catalyst. The first-step polymerization is carried out by supplying propylene alone or a propylene/ethylene mixture so that the ethylene/
propylene reaction ratio is 6/94 or less, thereby polymerization 60 to 95%
by weight of the total polymerization amount. The second-step polymerization is carried out by supplying a propylene/ethylene mixture so that the ethylene/
propylene reaction ratio is 41/59 to 69/31, thereby polymerizing 1 to 20% by weight of the total polymeriztion amount. Finally, the third-step polymerization is carried out by supplying ethylene alone or a propylene/ethylene mixture so that the ethylene/propylene reaction ratio is 90/10 or more, thereby polymerizing 4 to 35% by weight of the total polymerization amount. Each polymerization step of the first to third steps is carried out batchwise. The process described above can given propylene-ethylene block copolymers which are markedly well balanced in impact strength, stiffness, transparency and impact blushing.

Description

The present invention relates to a process for producing propylene-ethylene block copolymers having improved properties, particularly, e.g., improved impact resistance, stiffness, transparency, impact blushing and surface gloss.
Crystalline polyolefins have been produced on a commercial basis since a stereoregular catalyst was invented by Ziegler and Natta. Particularly, crystalline polypropylene attracts attention as a general-purpose resin having excellent stiffness and heat resistance. Crystalline polypropylene, however, has the drawback that it is brittle at low temperatures, so that it is not suitable for usages requiring impact resistance at low temperature. Many improvements have already been proposed as a result of extensive studies to ; overcome this drawback. Of these improvements, those disclosed in Japanese Patent Publication Nos. 14834/1963, 1836/1964 and 15535/1964 are particularly :
~ useful from the industrial point of view. These publications disclose processes . ~
including the block copolymerization of propylene and other olefins, particularly, ethylene.
~ But, block copolymers produced by these well-known methods also ; have drawbacks. For example, they are inferior to the crystalline polypropylene in the stiffness and transparency of molded or fabricated products. Further, when block copolymers are deformed by impact or bending, blushing appears at the ~;~ deformed portion treferred to as "impact blushing" hereinafter?, which leads to a remarkable reduction in commercial value. Accordingly, block copolymer having good impact blushing, in other words, showing a blushed area as small as possible, even if blushed, is required.
In order to overcome such drawbacks, there have been proposed : .:
many processes in which the block copolymerization is carried out in three steps.

Specifically, Japanese Patent Publication No. 20621/1969 discloses a process : - . . : -, . - : .

Z68~9 which results in an improvement in transparency, Japanese Patent Publication No.
24593/1974 discloses a process which results in an improvement in impact blushing and Japanese Patent Publication (unexamined) No. 25781/1973 discloses a process which results in an improvement in impact resistance.
In general, however, these properties, i.e., impact resistance, stiffness, transparency and impact blushing are in competition with one another, so that satisfactory, well-balanced polymers can not be obtained by those well-known processes.
; Further, Japanese Patent Publication (unexamined) No. 8094/1977 disclosed a process for improving impact strength and stiffness of polymers by a -continuous three-step polymerization procedure comprising carrying out poly-merization in the absence of a molecular weight regulator in the second step and in the absence or presence of a molecular weight regulator.
In block copolymerization, particularly in the two-step poly-merization, both batch and continuous processes are well-known. However, in the continuous two-step polymerization process, using two or more reactors connected seriesly, catalyst particles have a residence time distribution in the each reactor. Therefore, the produced polymer particles have a distribution in polymerization amount in relation to such catalyst particles. As a result, even though the polymer particles are melt-mixed with an extruder or molding machine, ^ non-dispersed polymer particles (referred to as "fish eye" for brevity hereinafter) are present in the melt-mixed product without mixing homogeneously. Consequently9 block copolymer having well-balanced physical and optical properties can not be obtained because the appearance of the molded products is remarkably damaged and impact strength, as a characteristic property of block copolymer, is reduced.
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On the other hand, block copolymers obtained by the batch process do not have the defects described above, but impact blushing and transparency of the block copolymer are still not improved.
Further, when the continuous three-step polymerization is carried out, the molded product of the produced polymers has many fish eyes, thus damaging the appearance of the molded product similar to that of the block copolymer obtained by the continuous two-step polymerization process, and has relatively low impact strength and poor surface gloss.
Fuzther, impact blushing and transparency of the molded article are still not improved.
An object of one aspect of the present invention is to provide a novel process for producing propylene-ethylene block copolymers having a - specified structure.
An object of another aspect of the present invention is to provide propylene-ethylene block copolymers markedly well-balanced in impact resistance, stiffness, transparency and impact blushing.
An object of still anobher aspect of the present invention is ` to provide propylene-ethylene block copolymers giving a molded article having few fish eyes therein and good surface gloss.
The inventors have extensively studied means to overcome these drawbacks and have found that block copolymers having not only impact ;~ strength, stiffness and surface gloss but also improved impact blushing and - transparency, that is, well balanced in these properties can be obtained by a specified batch three~step polymerization process.
According therefore, to aspects of the present invention, a polymerization process is provided for producing propylene-ethylene block copolymers, ~y subjecting propylene and ethylene to a three-step polymerization using a stereoregular polymerization catalyst. The process comprises CarryiDg out the first-step polymerization by supplying propylene alone or a propylene/

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ethylene mixture so that the ethylene/propylene reaction ratlo~the molar ratlo of ethylene to propylene which are taken into the copolymer (referred to as "ethylene/propylene reaction ratio" hereinafter)~ is 6/94 or less, preferably by a variant thereof, 4.5/95.5 or less, thereby polymerizlng - 60 to 95% by weight, preferably by a variant thereof, 65 to 93% by weight, of the total polymerization amount. The second-step polymerization is carried out by supplying a propylene/ethylene mixture so that the ethylene/propylene reaction ratio is 41/59 to 69/31, thereby polymerizing 1 to 20% by weight, preferably, by a variant thereof, 2 to 15% by weight, of the total polymerization a unt. The third-step polymerization is carried out by supplying ethylene alone or an ethylene/propylene mixture so that the ethylene/propylene reaction ratio is 90/10 or more, thereby copolymerizing 4 to 35% by weight, preferably, by a variant thereof, 6 to 30% by weight, of the total polymerization amount.
Preferably, by a variant thereof, the polymerization amount in the second step is made smaller than that in the third step. Most preferably by another variant, the polymerization in the first and third sLeps is carried out in the presence of a molecular-weight regulator, and the polymerization in the second step being carried out in the presence or absence of the molecular-weight regulator, and further the polymerization in each step is carried out batchwise.
By a variation thereof, the molecular-weight regulator is hydrogen.
The accompanied drawings is a schematic form of an apparatus for continuously preparing a three-block copolymer, referring to Comparative Example 4 hereinafter described.

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The propylene-ethylene block copolymerization of an aspect of the present invention can be carried out in substantially the same manner as in the conventional polymerization for producing isotactic polypropylene using a stereoregular polymerization catalyst, except -that the block copolymerization is divided into ~ny steps and that attention needs to be given to the ethylene/propylene reaction ratios and polymerization a~ounts in the second and third steps.
Consequently, as the stereoregular polymerization catalyst used in the process of aspects of the present invention, there are used the well-known catalysts consisting essentially of titanium trichloride, an organo-aluminum compound and optionally an electron donor.
Examples of the titanium trichloride included in the catalyst include, for example, titanium trichloride produced by the reduction of titanium tetrachloride with a metal or organo-metallic compound, or, further, the activation of the reduction product' products obtained by the pulverization of the foregoing substances, and titanium trichloride obtained by the procedure disclosed in British Patent No. 1391067.
The organo-aluminum compound includes, for example, dimethyl-aluminum chloride, diethylaluminum chloride, diisobutylalumi-num~chloride, diethylaluminum bromide and triethylaluminum. Of these compounds, -`~ diethylaluminum chloride is particularly preferred.
The electron donGr used as a third component of the catalyst includes, for example, the well-kno n ones, e g., ~ :

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The polymerization of aspects of the present invention may be carried out either in an inert hydrocarbon or in a liquid monomer in the substantial absence of an inert hydrocarbon. Further, it may be carried out in a gaseous phase of the monomer. The polymerization temperature is not particularly limited, but generally, it is within a range of 20 to 90C, preferably 40 to 80C.
At the first step of the polymerization, propylene alone ''` is polymerized, or a propylene/ethylene mixture is polymerized so that the ethylene/propylene reaction ratio is 6/94 or less, preferably 4.5/95.5 or less. In the case of the polymerization of propylene, polymers having the physical properties markedly well balanced can be obtained by carrying out tne subsequent polymerization according to the process of an aspect of the present invention. When improvements in transparency, impact blushing and impact strength are desired at a little sacrifice of stiffness if necessary, a small amount of ethylene is additionally added in the copolymerization. In the copoly-merization, propylene and a small amount of ethylene may be polymerized at the same time in a mixed state, or propylene alone may be first polymerized followed by copolymerization of a mixture of propylene and a small amount of ethylene.
In either case, almost the same effect can be obtained. When the ethylene/
propylene reaction ratio exceeds the scope set forth in the process of an aspect of the present invention, stiffness is extremely lowered.
The second step of the polymerization follows the first step.
In this step, copolymerization is carried out by supplying a propylene/ethylene mixture so that the ethylene/propylene reaction ratio is 41/59 to 69/31. ~eaction ratios below 41/59 or above 69/31 are not desirable because impact strength becomes poor.

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The third step of the polymerization follows the second step. In this step, copolymerlzation is carried out by supplying ethylene alone or an ethylene/propylene mixture so that the ethylene/propylene reaction ratio is 90/10 or more. ~eaction ratios below 90/10 are not desirable because impact blushing becomes poor. Further, it is preferred that the polymerization ; amount in the second step be smaller than that in the third step. When the ; polymerization amount in the second step is smaller, a remarkable improvement in impact blushing, stiffness and transparency can be attained.
The polymerization in the first and third steps is preferably carried out in the presence of a well-known molecular-weight regulator, e.g., hydrogen. When the polymerization is carried out in the absence of the molecular-weight regulator, the produced polymer sometimes has poor processability.
Accordingly, in this case, it is difficult to apply the polymer to a usual ~; molding.
The polymerization in the second step may be carried out in the presence or absence of the molecular weight regulator. When a polymer having good processability and giving a molded article of good surface gloss, is particularly required, the molecular weight regulator is used. Other hand, `~ when a polymer having higher impact strength is desired, it is not used.
~ 20 Each of three steps is carried out batchwise. The batch--~ polymerization process of aspects of the present invention can give a polymer which cause extremely less occurrence of fish eyes in the comparison with the polymer produced by the conventional continuous polymerization process. There-fore, the molded article of the polymer produced according to the process of aspects of the present invention has good appearance, and extremely excellent impact strength and impact blushing.

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The present invention in its various aspects will now be described more specifically with reference to the following exa~ples and comparative examples.

The results of the examples are shown in Tables 1 to 9. The values of physical properties in the tables were measured by the following testing methods.

10 Melt index : ASTM D 1238-57T
Brittleness temperature : ASTM D 746 -; Stiffness : ASTM D 747-58T
Haze : ASTM D 1003 Test sample : Sheet (1 mm -; 15 thick) molded by pressing.
Izod impact strength : ASTM D 256 Test temperature : 20C, . -20C
~ Impact blushing : Injection-molded sheet ; 20 (1 mm thick) is placed at 20C on a Du Pont ~ impact tester; the hemi-spherical tip (radius 6. 3 -~ mm) of the dart is contacted with the sheet;
impact is given to the top of the dart with the 20 cm and 50 cm natural fall of a welght (1 kg); and th~
area of the blushed portion is measured.
~`~ Surface gloss : ASTM D 523 Test sample : Injection-molded sheet (1 mm thick) Intrinsic viscosity (referred to as [n] for brevity) :
~: 30 [n] iS measured at 135C in tetralin.

These values were measured using test samples ~ prepared as follows : The polymer particles obtained by the ,~ examples were mixed with well-known additives, e.g., an antioxidant, formed into pellets through an extruder and ~` then pressed or injection-molded.

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;26899 g Example 1 TiCQ3 AA (a product of Toho Titani~m Co., Ltd.;
32 g), diethylaluminum chloride (144 g) and heptane (100 liters) were charged in a 250-:Liter autoclave with a stirrer.
The first step of the polymerization was advanced by supply-ing propylene while maintaining the polymerization tempera-ture at 70C and the polymerization pressure at 9 kg/cm2G
in the presence of hydrogen. The supply of propylene was stopped when the polymerization amount reached 30.~ kg, and the unreacted monomer was immediately purged. The polymer in the autoclave was sampled in a small amount and measured for 1~]-; The second step of the polymerization was advanced by supplying ethylene and propylene while maintaining thepolymerization temperature at 60C and the polymerization pressure at 2.5 kg/cm2G in the presence of hydrogen. The supply of ethylene and propylene was stopped when the polymerization amount reached 3.4 kg, and the unreacted nomers were immediately purged. During this polymerization period, the ethylene concentration of the gaseous phase in the autoclave was between 15 and 18 mole ~, and its mean value was 17 mole %. A small amount o~ the polymer was sampled and measured for cn~ .
The third step of the polymerization was advanced by supplying ethylene and propylene while maintaining the ~ polymerization temperature at 60C and the polymerization ;~` pressure at 2.7 kg/cm2G in the presence of hydrogen. The supply of ethylene and propylene was stopped when the polymerization amount reached 8.5 kg, and the unreacted monomers were immediately purged. During this polymerization period, the ethylene concentration of the gaseous phase in the autoclave was between 71 and 77 mole %, and its mean .
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value was 74 mole %.

n-Butanol was added to the resulting polymer slurry to decompose the catalyst, and the slurry was filtered and dried to obtain a white, powdery polymer.

The ethylene/propylene reaction ratio in the second and third steps were calculated from the materlal balance.
The calculated values and the polymerization results are shown in Table 1. The physical and optical properties of the polymer obtained are shown in Table 2.

Further, the ethylene/propylene reaction ratios were obtained using the well-known infrared absorption spectra, and it was found that the values obtained were almost the same as those obtained from the material balance (this is also the same in the following examples and comparative examples).
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~- Next, Comparative example 1 is provided in order `~ 20 to demonstrate that the propylene/ethylene block copolymer obtained by the process of Example 1 is markedly well balanced in physical and optical properties~as compared with polymers obtained by the well-known two-step block copolymeri-~; zation technique.
Comparative example 1 In completely the same manner as in Example 1, TiCQ3 AA (a product of ~oho Titanium Co., Ltd., 32 g), diethyl aluminum chloride (144 g) and heptane (100 liters) were charged in a 250-liter autoclave with a stirrer,-~; followed by the first-step polymerization. The supply of propylene was stopped when the polymerization amount reached 30.1 kg, and the unreacted monomer was immediately purged.
- A small amount of the polymer was sampled and measured for [~].

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8~9 At the second step, po]ymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 60C and the polymerization pressure at 9 kg/cm2G in the persence of hydrogen. When S the polymerization amount reached 8.7 kg, the unreacted monomers were immediately purged. During this polymerization period, the ethylene concentrat:ion of the gaseous phase in the autoclave was between 41 and 46 mole ~, and its mean value was 44 mole %.
The resulting polymer slurry was treated in the ` same manner as in Example 1 to obtain a white, powdery -~ polymer.

The polymerization results are shown in Table 1, and the physical and optical properties of the polymer obtained are shown in Table 2. The following are seen from Tables 1 and 2:

As compared with the well-known two-step polymerization technique, ; 20 the process of an aspect of the present invention produces the polymer which is superior in haze and impact blushing and markedly well bala~nced in physical and optical properities.

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" ' : ' ' ' ' ~:126899 Example 2 (1) Synthesis of catalyst Catalyst preparation 1 In a 200-Q reactor with a stirrer hexane (45.5 Q) and titanium tetrachloride (11.8 Q) were charged. There-after, a solution comprising hexane (43.2 Q) and diethyl-aluminum chloride t9.4 Q) was added dropwise thereto over

3 hours with stirring. Thereafter the temperature of the reac-tion system maintained between -10C and 0C for 15 minutes.
After the addition was finished, the temperature was raised to 65C over 2 hours, followed by stirring for - further 2 hours. The reduction product was separated from the liquid portion and washed with heptane (50 Q) six times.
Catalyst preparation 2 The reduction product obtained by Catalyst prepara-tion 1 in Example 2 was suspended in hexane (92 Q), and di-isoamyl ether (19.6 Q) was added thereto.
After stirring at 35C for 1 hour, the obtained solid (ether-treated solid) was separated from the liquid portion and washed with hexane (50 R ) six times.
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Catalyst preparation 3 A hexane solution (60 Q) containing 40 ~ by volume of TiCQ4 was added to the ether-treated solid and ~he ; resu~ting suspension liquid was stirred at 70C for 2 hours.
The obtained solid was separated from the liquid portion, washed with hexane (50 Q) ten times. The residue was then dried after removing hexane to obtain titanium trichloride solid catalyst (I).
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(2) Propylene-ethylene block copolymerization Titanium trichloride solid catalyst (I) obtained in Synthesis of Catalyst of Example 2 (8.6 g), diethyl aluminum chloride (128 g) and heptane (100 Q) were charged in a 250-Q
; autoclave with a stirrer.

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-, .~- ~ ': -: ' ' ~ILlZ6899 ~14-The first step of the polymerization was advanced by supplying propylene while maintaining the polymerization temperature at 73C and the polymerization pressure at 7 kg/cm2G in the presence of hydrogen. The supply of propylene was stopped when the polymerization amount reached 28.4 kg, and the unreacted monomer was purged.

The second step of the polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 60C and the polymerization pressure at 2 kg/cm2G. The supply of ethylene and propylene ~ was stopped when the polymerization amount reached 1.5 kg, `~ and the unreacted monomers were purged.
, The third step of the polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 60C and the polymerization pressure at 2 kg/cm2G in the presence o~ hydrogen. The supply of ethylene and propylene was stopped when the polymerization amount reached 7.9 kg, and the unreacted monomers were purged.
' ~ During the polymerization, the mean ethylene ; concentrations of the gaseous phase in the second and third steps were 26 mole and 73 mole %, respectively.

When each polymerization step was finished, the polymer produced in each step in a small amount was sampled and measured for [n ~ The obtained polymer slurry was treated with the ;~ alcohol in the same manner as in Example 1 to obtain white powdery polymer. The polymerization results and physical and optical properties of the polymer were shown in Tables : 35 3 and 4, respectively.

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;8~9 Next, Comparative example 2 is provided in order to demonstrate that, when bloc]c copolymerization is carried out by the well-known three-step polymerization techni~ue, the polymer obtained i5 ill baLanced in physical and optical properties as compared with the polymer obtained by the process of aspects of the present invention.

Comparative example 2 In the same manner as in Example 2, the catalyst comprising the solid catalyst (I), diethylaluminum chloride and heptane were charged in a 250 liter autoclave with a stirrer, and the first step of polymerization was carried out supplying propylene in the presence of hydrogen. When the polymerization amount reached 20.6 kg, the unreacted lS monomer was purged.
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At the second step, polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 50C and the polymerization ; 20 pressure at 3.0 kg/c~2G in the presence of hydrogen. When the polymerization amount reached 2.8 kg, the unreacted , .
monomers were purged.

During this polymerization step, the mean ethyIene concentration of gaseous phase in the autoclave was 54 mole %.
' ~ At the third step, polymerization was adva,lced by -~ supplying ethylene and propylene while maintaining the ~` polymerization temperature at 50C and the polymerization pressure at 5.5 kg/cm2G in the presence of hydrogen. When the polymerization amount reached 4.8 kg, unreacted monomers were purged. During this step, the mean ethylene concentra-tion of gaseous phase in the autoclave was 81 mole %.

And, in each step, when the polymerization was finished, the polymer produced in each step in a small amount was sampled and measured for [n].

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The obtained polymer slurry was treated and dried to obtain a whlte powdery polymer in the same manner as in Example 2. The polymerization results and physical and optical properties of obtained polymer are shown in Tables 3 and 4, respectively. It is found from Table

4 that the polymer obtained according to the process of an aspect of the present invention is superior in haze, impact strength, brittleness temperature and balance of physical properties to that obtained according to the well-known three-step polymeri~ation method shown in Comparative Example 2.

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- l~Z~899 Example 3 (1) Synthesis of catalyst Catalyst preparation 1 Atmosphere in a 500-cc reactor was replaced with argon, and heptane (80 cc) and titanium tetrachloride (20 cc) were added thereto. Thereafter, a solution comprising heptane (100 cc) and ethylaluminum sesquichloride (41.2 cc) was added dropwise thereto over 3 hours with stirring while maintaining the temperature of the reaction system at -10C.
After the addition was finished, the temperature was raised to 95C over 35 minutes, followed by stirring for further 2 hours. After allowing to stand still, the reduc-tion product was separated from the liquid portion and washed with heptane (100 cc) four times.

Catalyst preparation 2 The reduction product obtained by Catalyst prepara-tion 1 in Example 3 was suspended in toluene (250 cc), and iodine and di-n-butyl ether were added thereto so that the molar ratios of the both to titanium trichloride in the reduction product were 0.1 and 1.0, respectively. Reaction was then carried out at 95C for 1 hour.

After the reaction was finished, the supernatant liquor was removed, and the residue was washed with toluene (30 cc) three times and then with heptane (30 cc) two times.
The residue was then dried to obtain titanium trichloride solid catalyst (II).
(2) -Propylene-ethylene block copolymerization After a 200-liter autoclave with a stirrer was evacuated, propylene was charged under pressure to 300 mmHg (gauge pressure), and then the pressure in the autoclave was reduced to -500 mmHg (gauge pressure). This operation was repeated three times.

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~26~399 Thereafter, the titanium trichloride solid catalyst (II) (2.6 g) and diethylaluminum chloride (51 g) were charged in the autoclave.

The first step of the polymerization was advanced by supplying liquid propylene (51 kg) and maintaining the polymerization temperature at 70C in the presence of hydrogen. When the polymerization amount reached 26.7 kg, the unreacted monomer was purged.
At the second step, polymerization was advanced in a gaseous phase by supplying ethylene and propylene while maintaining the polymerization temperature at 70~C and the polymerization pressure at 10 kg/cm2G in the presence of hydrogen. When the polymerization amount reached 2.6 kg, the unreacted monomers were purged. During this polymerization . period, the mean ethylene concentration of the gaseous phase ; in the autoclave was 20 mole %.
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; 20 At the third step, polymerization was advanced in a gaseous phase by supplying ethylene and propylene while maintaining the polymerization temperature at 60~C and the polymerization pressure at 4.5 kg/cm2G in the presence of hydrogen. When the polymerization amount reached 7.3 ky, the unreacted monomers were purged. During this polymeri-zation period, the mean ethylene concentration of the ~` gaseous phase in the autoclave was 81 mole %. At the end `~ of each polymerization step, a small amount of the polymer was sampled and measured for [n]
The polymer obtained was transferred to a 200-liter autoclave with a stirrer, and after adding propylene oxide (180 g), the polymer was stirred at 60C for 30 minutes to make the catalyst residue in the polymer harmless. The polymer was then dried to obtain a white, powdery polymer.

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The polymerization results are shown in Table 5, and the physical and optical properties of the polymer are shown in Table 6.
Next, Comparative E~ample 3 is provided in order to demonstrate that, when block copolymeriæation is carried out by a polymerization process outside of ~he scope of the process of aspects of the present invention, the polymer obtained is ill balanced in physical and optical properties as compared with the polymer obtained by the . ~ 10 process of aspects of the present invention.
Comparative example 3 Polymerization was initiated and three-step poly-merization was car~ied out in the same manner as in Example 3 except that polymerization conditions were changed into those as follows.

First step (Liquid phase polymerization) ~ Polymerization temperature; 70C
: 20 Polymerizatlon amount 26. 3 kg ~ Second step (Gaseous phase polymerization) `~ Polymerization temperature 70C
Polymerization pressure 10 kg/cm G
~ Polymerization amount 3.1 kg~
.` 25 Mean ethylene concentration of gaseous phase 9 mole %
Third step (gaseous phase polymerization) Polymerization temperature 60C
~:: Polymerization pressure 4. 5 kg/cm2 Polymerization amount 4.8 kg Mean ethylene concentration of gaseous phase 77 mole %
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And, the polymerization in each step was carried out adding hydrogen. The obtained polymer was treated and : dried to obtain a white powdery polymer in the same manner : 35 as in Example 3.

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The polymerization results and the physical and optical properties of the polymer are shown in Tables 5 and 6, respectively. From the comparison of Example 3 with comparatiVe Example 3, it is apparent that the polymer obtained by process of aspects of the present invention is superior in haze, impact strength, brittleness temperature and markedly well balanced in physical and optical properties to that obtained in Comparative Example 3.

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1~12~i8~39 Example 4 Three-step polymerization was carried out in the same manner as in Example 1 except that TiCQ3 ~A (produced by Toho Titanium Co., Ltd., Grade; TAC) (25 y)9 diethyl-aluminum chloride (144 g) and heptane 100 ~ were chargedinto a 250-Q autoclave and polymerization conditions were changed into those as follows, First step Polymerization temperature 70~C
Polymerization pressure 9 kg/cm G
Polymerization amount 29.7 kg Second step Polymerization temperature 60C
Polymerization pressure 1.6 kg/cm G
Polymerization amount 2.8 kg Mean ethylene concentration of gaseous phase 19 mole %
Third step Polymerization temperature 60C
Polymerization pressure 2 kg/cm G
Polymerization amount 6.8 kg ~ Mean ethylene concentration of gaseous phase 81 mole %
:' The polymerization was carried out in the presence of hydrogen in the first and third steps and in the absence of hydrogen in the second step. After finishing the poly-merization, the supply of monomers was stopped and then the unreacted monomers are purged immediately. The obtained polymer slurry was treated to obtain a white powdery polymer in the same manner as in Example 1.

The polymerization results and the physical and optical properties of the polymer are shown in Tables 8 and 9, respectively.
Next, Comparative Examples 3 and 4 are provided ` in order to demonstrate that the polymer obtained by batch :
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three-step polymerizati~n of aspects of the present lnven~lon ~re much less likely to ca~lse the production ~f fish eye than that obtained by a continuous block copolymerization process, and the product obtained according to the process of aspects ~f this invention is superior in balanced physical and optical properties as compared with that obtained by the continuous process of the prior art.

Comparative example 4 Reference is now made to the apparatus illustrated in the accompanying drawing. Al, A2, A3, A4 and A5 are a 250-Q
autoclave with a stirrer, respectively, which are connected in series. Al, A3 and A5 are the first, second and third steps polymerization reactors, respectively and each A2 and A4 are unreacted monomer purging vessels. Pumps (Pl, P2, P3, P4 and P5) for transferring a slurry are equipped between `~ 15 the preceeding and succeeding reactors. A monomer and ;~ hydrogen are fed from lines Fl, F2 and F4, and catalysts and heptane are fed from line F2. And the unreacted monomer and hydrogen are purged to lines Pul and Pu2.

Heptane containing titanium trichloride AA (Toho Titanium Co., Ltd., Grade TAC) (0.25 g/Q) and diethyl aluminum chloride ~1.44 g/Q)were supplied at a rate of 25.2 Q/Hr. Propylene and hydrogen were supplied from line Fl to polymerize propylene.
Then, the obtained polymer slurry was transferred to reactor A2, the slurry was transferred to reactor A3 after purging the unreacted monomer and hydrogen through line Pu ; and propylene and ethylene were supplied through line F3 ; 30 thereby carrying out polymerization. Further, the slurry obtained in reactor A3 was transferred to reactor A4, the slurry was transferred to reactor A5 after purging the ~`( unreacted monomers and hydrogen and ethylene, propylene and ~; hydrogen were supplied through line F2 thereby copolymerizing `` 35 the monomers. The obtained polymer slurry was discharged through pump P5. All of those operations were continuously `~ carried out.
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Polymerization conditions in each step are shown in Table 7. [n] of the polymer sampled in small amount from each polymerization reactor (Al, A3 and A5) was measured and the ethylene/propylene reaction ratio and polymerization amount in each step were culculated from material balance.
These results are shown in Table 8.

After decomposing the catalyst with the addition of butanol, the slurry was filtered and dried to obtain a white powdery polymer. Physical properties of the obtained polymer are shown in Tabl~ 9.

From Tables 7 and 8, it is appearant that the polymer obtained in Comparative Example 3 is much the same as in ~], and ethylene/propylene reaction ratio and poly-merization amount in each step as in Example 3.

A sheet of 1 mm thick was prepared by injection - molding in the same manner as in Example 3, but the sheet had a large amGunt of fish eye~on the surface. Further, when the sheet was seen through, a great number of fish eyeS
was observed in the inner body of the sheet, and therefore the sheet had a markedly poor appearance. The sheet was inferior in impact strength and surface gloss to that of polymer obtained in Example 3. Further, the sheet had a problem in great impact blushing, because places around fish eyes of the sheet were blushed with impact blushing test.

Comparative example 5 Polymerization was carried out in the same manner as in Example 4 except that hydrogen was not supplied to the reactor (A5) in third step. As the results, the obtained polymer was much the same in polymerization amount in each step and ethylene content of polymer polymerized in each ~` step as in Example 4.

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The polymer could not be pelletized using an extruder such that melt index was less than 0.1 g/10 min.
Therefore, injection molding of the powdery polymer was tried, but the molding was difficult due to incomplete injection of the polymer into a mold.

The obtained shaped article had markedly many fish eyes on the surface and in the inner body thereof.

- 10 Table 7 _ = A1 A2A3 A4 A5 Propylene Feed Rate Kg/Hr 10.55 _1.12 _ 0.10 Ethylene Feed Rate Kg/~r o _0.26 _ 1.55 Pressure Atom 10 0.5 3 0~5 3 Temperature C 70 70 60 6060 Average Hydrogen Mol % 3.5 _ 0.1 _ 4.2 20 Concentration of or Gaseous Phase less Average Residence Hr 3.7 0.7 1.0 0.5 1.8 Time Polymerization Kg 6.1 _ 0.6_ 1.5 Amount ~

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a propylene-ethylene block copolymer by subjecting propylene and ethylene to three-step polymerization using a stereroregular polymerization catalyst, comprising: carrying out the first-step polymerization out by supplying propylene alone or a propylene/
ethylene mixture so that the ethylene/propylene reaction ratio is 6/94 or less, thereby polymerizing 60 to 95% by weight of the total polymerization amount;
carrying out the second-step polymerization by supplying a propylene/ethylene mixture so that the ethylene/propylene reaction ratio is 41/59 to 69/31, there-by polymerizing 1 to 20% by weight of the total polymerization amount; and carrying out the third-step polymerization is carried out by supplying ethylene alone or a propylene/ethylene mixture so that the ethylene/propylene reaction ratio is 90/10 or more, thereby polymerizing 4 to 35% by weight of the total poly-merization amount; each polymerization step of the first to third steps being carried out batchwise.

2. A process according to claim 1, wherein each polymerization amounts in the first, second and third steps are 65 to 93% by weight, 2 to 15%
by weight and 6 to 30% by weight of the total polymerization amount, respectively;
and wherein the polymerization amount in the second step is less than that in the third step.

3. A process according to claim 1, wherein the polymerizations in the first and third steps are carried out in the presence of a molecular-weight regulator; and wherein the polymerization in the second step is carried out in the presence or absence of the molecular-weight regulator.

4. A process according to claim 3, wherein said molecular-weight regulator is hydrogen.

5. A block having improved well balanced impact strength, stiffness, transparency and impact blushing, whenever prepared by the process of claims 1 or 2 or by their obvious chemical equivalents.

6. A block having improved well balanced impact strenght, stiffness, transparency and impact blushing, whenever prepared by the process of claims 3 or 4 or by their obvious chemical equivalents.