GB2025994A - Process for producing propylene- ethylene block copolymers - Google Patents

Abstract

A process for producing propylene- ethylene block copolymers by a three-step polymerisation technique comprises (i) polymerising propylene alone, or a propylene/ethylene mixture at an ethylene/propylene molar ratio of 6/94 or less, thereby polymerising 60 to 95% by weight of said block copolymer, (ii) polymerising the product of step (i) with a propylene/ethylene mixture of molar ratio of 41 /59 to 69/31 respectively, thereby polymerising a further 1 to 20% by weight of said block copolymer, and (iii) polymerising the product of step (ii) with ethylene alone, or a propylene/ ethylene mixture of molar ratio of 90/10 or more respectively, thereby polymerising the final 4 to 35% by weight of said block copolymer, each of steps (i), (ii) and (iii) being carried out batchwise in the presence of a stereoregular polymerisation catalyst.

Description

SPECIFICATION Process for producing propylene-ethylene blocks copolymers The present invention relates to a process for producing propylene-ethylene block copolymers improved in properties, particularly, such as 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. They are a process 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 the block copolymers ar deformed by impact or bending, blushing appears at the deformed portion (referred to as "impact blushing" hereinafter), which leads to a remarkable reduction in commercial value. Accordingly, the 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 improve 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 an improvement in transparency, Japanese Patent Publication No.

24593/1 974 an improvement in impact blushing and Japanese Patent Publication (unexamined) No. 25781/1973 an improvement in impact resistance.

In general, however, these properties impact resistance, stiffness, transparency and impact blushing are in competition with one another, so that satisfactory, well-balanced polymers can not be obtained by these 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 method comprising carrying out polymerization in the absence of a molecular weight regulator in the second step and in the absence or presence of a molecular weight regulator.

In the block copolymerization, particularly in the two-step polymerization, 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, produced polymer particles have a distribution in polymerization amount per said catalyst particle. As the result, even through 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 withut mixing homogeneously.Consequently, block copolymer having well-balanced physical and optical properties can not be obtained because appearance of the molded products is remarkably damaged and impact strength, as a characteristic property of block copolymer, is reduced.

On the other hand, block copolymers obtained by the batch process have not defects described above, but impact blushing and transparency of the block copolymer are not improved still.

Further, according to investigations of the present inventors, when the continuous three-step polymerization is carried out, the molded product of the produced polymers has many fish eyes damaging 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. Further, impact blushing and transparency of the molded article are not improved still.

The inventors extensively studied to overcome these drawbacks and 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.

An object of the present invention is to provide a novel process for producing propyleneethylene block copolymers having a specified structure which comprises carrying out batchwise polymerization in three steps using a stereoregular polymerization catalyst.

Another object of the present invention is to provide propylene-ethylene block copolymers markedly well-balanced in impact resistance, stiffness, transparency and impact blushing.

Still another object of the present invention is to provide propylene-ethylene block copolymers giving a molded article having few fish eyes therein and good surface gloss.

Other objects and advantages of the present invention will be apparant from the following descriptions.

The accompanied drawing is an apparatus for continuously preparing a three-block copolymer, referring to Comparative Example 4 hereinafter described.

According to the present invention, there is provided a polymerization process for producing propylene-ethylene block copolymers, by subjecting propylene and ethylene to a three-step polymerization using a stereo-regular polymerization catalyst, characterized in that the first-step polymerization is carried out by supplying propylene alone or a propylene/ethylene mixture so that the ethylene/propylene reaction ratio (the molar ratio of ethylene to propylene which are taken into the copolymer (referred to as "ethylene/propylene reaction ratio" hereinafter)) is 6/94 or less, preferably 4.5/95.5 or less, thereby polymerizing 60 to 95% by weight, preferably 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 2 to 15% by weight, of the total polymerization amount, and the third-step polymerization is carried out by supplying ethylene alone or an ethyiene/ propylene mixture so that the ethylene/propylene reaction ratio is 90/10 or more, thereby copolymerizing 4 to 35% by weight, preferably 6 to 30% by weight, of the total polymerization amount, preferably with the polymerization amount in the second step made smaller than that in the third step, most preferably with the polymerization in the first and third steps being 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 being carried out batchwise.

The process of the present invention will be illustrated in detail hereinafter.

The propylene-ethylene block copolymerization 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 said block copolymerization is divided into many steps and that attention needs to be given to the ethylene/propylene reaction ratios and polymerization amounts in the second and third steps.

Consequently, as the stereoregular polymerization catalyst used in the present invention, there are used the well-known catalysts consisting essentially of titanium trichloride, an organoaluminum compound and optionally an electron doner.

Herein, the titanium trichloride may include its composition. As examples of the titanium trichloride there may be given, 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; titanium trichloride obtained by the method disclosed in British Patent No. 1391067; and titanium trichloride obtained by the methods disclosed in U.S. Application Ser. No. 831630 (West German Offenlegungsschrift No. 2740282).

The organo-aluminum compound includes for example dimethylaluminum chloride, diethylaluminum chloride, diisobutylaluminum chloride, diethylaluminum bromide and triethylaluminum.

Of these compounds, diethylaluminum chloride is particularly preferred.

The electron donner used as a third component of the catalyst includes for example the wellknown ones such as mines, ethers, esters, sulfur, halogen, benzene, azulene derivatives, orgnic or inorganic nitrogen compounds and organic or inorganic phosphorus compounds.

The polymerization of the present invention may be carried out in either of an inert hydrocarbon or a liquid monomer in the substantial absence of an inert hydrocarbon. Further, it may be carried out in a gaseous phase of monomer. The polymerization temperature is not particularly limited, but generally, it is within a range of 20 to 90"C, preferably 40 to 80"C.

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 the subsequent polymerization 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 copolymerization, 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 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. The reaction ratio below 41/59 or above 69/31 is not desirable because impact strength becomes poor.

The third step of the polymerization follows the second step. In this step, copolymerization is carried out by supplying ethylene alone or an ethylene/propylene mixture so that the ethylene/propylene reaction ratio is 90/10 or more. The reaction ratio below 90/10 is not desirable because impact blushing becomes poor. Further, it is further preferred that the polymerization amount in the second step is smaller than that in the third step. When the polymerization amount in the second step is smaller, the remarkable improvement in impact blushing, stiffness and transparency can be attained.

Preferably, the polymerization in the first and third steps is preferably carried out in the presence of a well-known molecular-weight regulator such as hydrogen. When the polymerization is carried out in the absence of the molecular-weight regulator, the produced polymer has sometimes 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 the 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 the polymer having higher impact strength is desired, it is not used.

Each of three steps is carried out batchwise. The batch-polymerization process 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.

Therefore, the molded article of the polymer produced according to the present invention has good appearance, and extremely excellent impact strength and impact blushing.

The present invention will be illustrated more specifically with reference to the following examples and comparative examples which are not however to be interpreted as limiting the invention thereto.

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.

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 thick) molded by pressing.

Izod impact strength ASTM D 256 Test temperature: 20"C, - 20"C Impact blushing Injection-molded sheet (1 mm thick) is placed at 20"C 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 weight (1 kg); and the 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 [71] for brevity): [71] is measured at 135"C in tetralin.

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

Example 1 TiC13 AA (a product of Toho Tianium 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 supplying propylene while maintaining the polymerization temperature at 70"C and the polymerization pressure at 9 kg/cm2G in the presence of hydrogen. The supply of propylene was stopped when the polymerization amount reached 30.4 kg, and the unreacted monomer was immediately purged.The polymer in the autoclave was sampled in a small amount and measured for [71] The second step of the polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 60"C 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 monomers were immediately purged. During this polymerization period, the ethylene concentration of the gaseous phase in the autoclave was between 1 5 and 18 mole %, and its mean value was 1 7 mole %. A small amount of the polymer was sampled and measured for [71].

The third step of the polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 60"C 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 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 material 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).

Next, Comparative example 1 will be shown in order to demonstrate that the propylene/ethylene block copolymer obtained by the method of Example 1 is markedly well balanced in the physical and optical properties as compared with polymers obtained by the well-known two-step block copolymerization techniques.

Comparative example 1 In completely the same manner as in Example 1, TiCI3 AA (a product of Toho Titanium Co., Ltd.; 32 g), diethyl aluminum chloride (1449) 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 [71] At the second step, polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 60"C and the polymerization pressure at 4 kg/cm2G in the persence of hydrogen. When the polymerization amount reached 8.7 kg, the unreacted monomers were immediately purged.During this polymerization period, the ethylene concentration of the gaseous phase in the autoclave was between 41 and 46 mole %, and its mean value was 44 mole %.

The result 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 followings are known from Tables 1 and 2: As compared with the well-known two-step polymerization technique, the process of the present invention produces the polymer which is superior in haze and impact blushing and markedly well balanced in the physical and optical properties.

Table 1 First step Second step Third step Ethylene/propylene Ethylene/propylene [71] [71] reaction ratio [71] reaction ratio dl/g dl/g molar ratio dl/g molar ratio Example 1 1.70 2.26 45/55 3.74 94/6 Comparative example 1 1.65 3.71 77/23 Table 2 Izod impact Impact Melt Brittleness Stiffness Haze strength blushing index temperature 20 C -20 C 20 cm 50 cm g/10 min C kg/cm % kg.cm/cm cm Example 1 1.8 -36 9700 78 17 5.3 1.7 3.1 Comparative example 1 1.9 -38 9500 96 13 4.9 2.4 4.8 Example 2 (1) Synthesis of catalyst Catalyst preparation 1 In a 200-l reactor with a stirrer hexane (45.5 I) and titanium tetrachloride (11.8 I) were charged.Thereafter, a solution comprising hexane (43.2 1) and diethylaluminum chloride (9.4 1) was added dropwise thereto over 3 hours with stirring. Thereafter the temperature of the reaction system maintained between - 1 0 C and 0 C for 1 5 minutes.

After the addition was finished, the temperature was raised to 65"C over 2 hours, followed by stirring for further 2 hours. The reduction product was separated from the liquid portion and washed with heptane (50 1) six times.

Catalyst preparation 2 The reduction product obtained by Catalyst preparation 1 in Example 2 was suspended in hexane (92 I), and diisoamyl ether (19.6 1) was added thereto.

After stirring at 35"C for 1 hour, the obtained solid (ether-treated solid) was separated from the liquid portion and washed with hexane (50 1) six times.

Catalyst preparation 3 A hexane solution (60 1) containing 40% by volume of TiCI4 was added to the ether-treated solid and the resulting suspension liquid was stirred at 70"C for 2 hours. The obtained solid was separated from the liquid portion, washed with hexane (50 1) ten times. The residue was then dried after removing hexane to obtain titanium trichloride solid catalyst (I).

(2) Propylene-ethylene block copolymerization Titanium trichloride solid catalyst (I) obtained in Synthesis of Catalyst of Example 2 (8.6 g), diethyl aluminum chloride (18 9) and heptane (1001) were charged in a 250-l autoclave with a stirrer.

The first step of the polymerization was advanced by supplying propylene while maintaining the polymerization temperature at 73"C 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 60"C 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 60"C and the polymerization pressure at 2 kg/cm2G in the presence of 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 paseous phase in the second and third steps were 26 mole and 73 %, respectively.

When each polymerization step was finished, the polymer produced in each step in a small amount was sampled and measured for [?7].

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 3 and 4, respectively.

Next, Comparative example 2 will be shown in order to demonstrate that, when block copolymerization is carried out by the well-known three-step polymerization technique, the polymer obtained is ill balanced in the physical and optical propeties as compared with the polymer obtained by the process 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 monomer was purged.

At the second step, polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 50"C and the polymerization pressure at 3.0 kg/cm2G in the presence of hydrogen. When the polymerization amount reached 2.8 kg, the unreacted monomers were purged.

During this polymerization step, the mean ethylene concentration of gaseous phase in the autoclave was 54 mole %.

At the third step, polymerization was advanced by supplying ethylene and propylene while maintaining the polymerization temperature at 50"C 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 concentration 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 [71].

The obtained polymer slurry was treated and dried to obtain a white 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 present method is superior in haze, impact strength, brittleness temperature and balance of physical properties to that obtained according to the wellknown three-step polymerization method shown in Comparative Example 2.

Table 3 First step Second step Third step Ethylene/propylene Ethylene/propylene [71] [71] reaction ratio [71] reaction ratio dl/g dl/g molar ratio dl/g molar ratio Example 2 1.54 1.95 59/41 3.98 94/6 Comparative example 2 1.58 3.03 83/17 4.71 95/5 Table 4 Izod impact Impact Melt Brittleness Stiffness Haze strength blushing Index temperature 20 C -20 C 20 cm 50 cm g/10 min C kg/cm % kg.cm/cm cm Example 2 2.0 -41 10100 76 12 4.9 1.6 3.0 Comparative example 2 0.8 -9 12200 87 6.3 3.2 1.5 2.8 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 - 1 0'C.

After the addition was finished, the temperature was raised to 95"C over 35 minutes, following by stirring for further 2 hours. After allowing to stand still, the reduction product was separated from the liquid portion and washed with heptane (100 cc) four times.

Catalyst preparation 2 The reduction product obtained by Catalyst preparation 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 95"C 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.

Thereafter, the titanium trichloride solid catalyst (Il) (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 70"C 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 %.

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 kg, the unreacted monomers were purged. During this polymerization 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 [71].

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 60"C for 30 minutes to make the catalyst residue in the polymer harmless. The polymer was then dried to obtain a white, powdery polymer.

The polymerization results are shown in Table 5, and the physical and optical properties of the polymer are shown in Table 6.

Next, Comparative example 3 will be shown in order to demonstrate that, when block copolymerization is carried out by the polymerization method out of scope of the present process, the polymer obtained is ill balanced in the physical and optical properties as compared with the polymer obtained by the process of the present invention.

Comparative example 3 Polymerization was initiated and three-step polymerization was carried out in the same manner as in Example 3 except that polymerization conditions were changed into those as follows.

9 First step (Liquid phase polymerization) Polymerization temperature; 70"C Polymerization amount 26.3 kg Second step (Gaseous phase polymerization) Polymerization temperature 70"C Polymerization pressure 10 kg/cm2G Polymerization amount 3.1 kg Mean ethylene concentration of gasesous phase 9 mole % Third step (gaseous phase polymerization) Polymerization temperature 60"C Polymerization pressure 4.5 kg/cm2 Polymerization amount 4.8 kg Mean ethylene concentration of gaseous phase 77 mole % 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 as in Example 3.

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

Table 5 First step Second step Third step Ethylene/propylene Ethylene/propylene [71] [Ç7] reaction ratio [71] reaction ratio dl/g dl/g molar ratio dl/g molar ratio Example 3 1.57 1.92 46/54 3.54 93/7 Comparative example 3 1.76 1.98 25/75 3.13 91/9 Table 6 Izod impact Impact Melt Brittleness Stiffness Haze strength blushing index temperature 20 C -20 C 20 cm 50 cm g/10 min C kg/cm % kg.cm/cm cm Example 3 2.5 -38 9300 79 16 5.1 1.7 3.2 Comparative example 3 2.9 -25 9700 93 9.6 3.4 1.8 3.3 Example 4 Three-step polymerization was carried out in the same manner as in Example 1 except that TiCI3 AA (produced by Toho Titanium Co., Ltd., Grade;TAC) (25 g), diethylaluminum chloride (1449) and heptane 100 1 were charged into a 250-l autoclave and polymerization conditions were changed into those as follows, First step Polymerization temperature 70"C Polymerization pressure 9 kg/cm2G Polymerization amount 29.7 kg Second step Polymerization temperature 60"C Polymerization pressure 1.6 kg/cm2G Polymerization amount 2.8 kg Mean ethylene concentration of gaseous phase 1 9 mole % Third step Poymerization temperature 60"C Polymerization pressure 2 kg/cm2G 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 polymerization, the supply of monomers was stopped and then the unreacted monomers are purped 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 will be shown in order to demonstrate that the polymer obtained by batch three-step polymerization of the present invention cause the production of fish eye extremely less than that obtained by a continous block copolymerization process and is superior in balanced physical and optical properties as compared with that obtained by the continous process.

Comparative example 4 Reference is made to the apparatus illustrated in the accompanying drawing. A1, A2, A3, A4 and A5 are a 250-l autoclave with a stirrer, respectively, which are connected in series. A1, A3 and A5 are the first, second and third steps polymerization reactors, respectively and each A2 and A4 are unreacted monomer purging vessels. Pumps (P1, P2, P3, P4 and P5) for transferring a slurry are equipped between the preceeding and succeeding reactors. A monomer and hydrogen are fed from lines F1, F2 and F4, and catalysts and heptane are fed from line F2. And the unreacted monomer and hydrogen are purged to lines Pu, and Pu2.

Heptane containing titanium trichloride AA (Toho Titanium Co., Ltd., Grade TAC) (0.25 g/l) and diethyl aluminum chloride (1.44 g/l) were supplied at a rate of 25.2 I/Hr. Propylene and hydrogen were supplied from line F, 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 Pu1, and propylene and ethylene were supplied through line F3 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 the monomers. The obtained polymer slurry was discharged through pump P5. All of those operations were continuously carried out.

Polymerization conditions in each step are shown in Table 7. [71] of the polymer sampled in small amount from each polymerization reactor (A1, A3 and A5) was measured and the ethelene/propylene reaction ratio and polymerization amount in each step were culculated from material balance. These result are shown in Table 8.

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

From Tables 7 and 8, it is appearant that the polymer obtained in Comparative Example 3 is much the same as in [71], and ethelene/propylene reaction ratio and polymerization 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 amount of fish eye on the surface. Further, when the sheet was seen through, a great number of fish eye 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.

The polymer could not be pelletized using an extruder such that melt index was less than 0.1 g/ 1 0 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 suface and in the inner body thereof.

Table 7 A, A2 A3 A4 A5 Propylene Feed Rate Kg/Hr 10.55 - 1.12 - 0.10 Ethylene Feed Rate Kg/Hr 0 - 0.26 - 1.55 Pressure Atom 10 0.5 3 0.5 3 Temperature C 70 70 60 60 60 Average Hydrogen Mol % 3.5 - 0.1 - 4.2 Concentration of or Gaseous Phase less Average Residence Time Hr 3.7 0.7 1.0 0.5 1.8 Polymerization Kg 6.1 - 0.6 - 1.5 Amount Table 8 First stage Second stage Third stage Ethylene/propylene Ethylene/propylene [71] [71] reaction ratio [71] reaction ratio dl/g dl/g mol/mol dl/g mol/mol Example 4 1.65 2.21 48/52 4.22 96/4 Comparative example 4 1.66 2.19 49/51 4.12 95/5 Table 9 Area of Brittle- Izod impact impact Sur Melt ness Stiff- Haze strength blushing face index tempera- ness gloss ture 20 C -20 C 20 cm 50 cm g/10 min C kg/cm % kg.cm/cm cm % Example 4 1.5 -32 10100 77 14 5.2 1.4 2.6 64 Comparative example 4 1.7 -25 10200 89 8.2 3.8 2.4 4.3 48

Claims (6)

1. A three-step polymerisation process for producing a propylene-ethylene block copolymer, which process comprises (i) polymerising propylene in the absence of ethylene or a propylene/ethylene mixture at an ethylene/propylene reaction ratio of 6/94 or less, thereby polymerising 60 to 95% by weight of said block copolymer.

(ii) polymerising the product of step (1) with a propylene/ethylene mixture at an ethylene/ propylene reaction ratio of 41/59 to 69/31, thereby polymerising a further 1 to 20% by weight of said block copolymer, and (iii) polymerising the product of step (ii) with ethylene in the absence of propylene or a propylene/ethylene mixture at an ethylene/propylene reaction ratio of 90/10 or more, thereby polymerising the final 4 to 35% by weight of said block copolymer, each of steps (i), (ii) and (iii) being carried out batchwise in the presence of a stereoregular polymerisation catalyst.

2. A process according to claim 1, wherein in steps (i), (ii) and (iii) 65 to 93% by weight, 2 to 15% by weight and 6 to 30% by weight, respectively, of said block copolymer is polymerised, the amount polymerised in step (ii) being less than that in step (iii).

3. A process according to claim 1 to 2 wherein steps (i) and (iii) are carried out in the presence of a molecular-weight regulator and step (ii) 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 process according to claim 1 substantially as hereinbefore described in any one of Examples t; 2(2), 3(2) or 4.

6. Shaped articles prepared from a propylene-ethylene block copolymer produced by a process as claimed in any one of the preceding claims.