WO2008141934A1

WO2008141934A1 - Process for the preparation of soft propylene polymer compositions

Info

Publication number: WO2008141934A1
Application number: PCT/EP2008/055587
Authority: WO
Inventors: Camillo Cagnani; Enrico Beccarini; Claudio Cavalieri
Original assignee: Basell Poliolefine Italia S.R.L.
Priority date: 2007-05-22
Filing date: 2008-05-07
Publication date: 2008-11-27
Also published as: EP2158234A1; AR066651A1; ATE525406T1; CN101679557A; KR101514375B1; RU2459836C2; TW200906861A; CL2008001466A1; US20100121000A1; US7981982B2; BRPI0811915A2; MX2009012580A; BRPI0811915B1; JP2010528136A; KR20100017209A; JP5518698B2; PL2158234T3; CN101679557B; EP2158234B1; RU2009147450A

Abstract

A process for the preparation of a propylene copolymer composition having a Flexural modulus lower than 500MPa, a total ethylene content higher than 9% and a Xylene soluble fraction at room temperature higher than 20% comprising at least one polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones said process being characterized by the fact that at least 30% by weight of said xylene soluble fraction is produced in the polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones.

Description

"PROCESS FOR THE PREPARATION OF SOFT PROPYLENE POLYMER COMPOSITIONS"

The present invention relates to a polymerization process for the preparation of soft propylene polymer compositions. The compositions obtained with this process are particularly suitable for the manufacture of films and injection-moulded items. In particular, the said articles have good flexibility, excellent impact properties even at low temperature, and a very low release of chemical substances. Hence, the said articles are particularly suitable to be used in contact with food and for biomedical applications. It is well known from the prior art that the presence of a high xylene-soluble moiety is desirable for the production of soft propylene polymer compositions. However, high values of the xylene-soluble moiety result in large quantities of the hexane extractables, so that the soft compositions often result unsuited in the field of food packaging.

The European patent application WO03/046021 discloses thermoplastic polyolefin compositions having good impact properties at low temperature and also showing low hexane-extractable moiety. Said compositions comprise a propylene copolymer having xylene-insoluble fraction of not less than 85 wt% and a propylene copolymer with 8-40 wt% of alpha-olefϊn and have a Melt Flow Rate (MFR) determined according to ISO 1133 (230⁰C, 2.16 Kg of 3-30 g/10 min. The above-mentioned values of MFR are obtained by subjecting to peroxidic degradation a precursor composition having a lower MFR.

The above mentioned compositions show a low content of hexane extractable fractions but are not entirely satisfactory in terms of softness (relatively high ratio flexural modulus/MFR) and impact properties at low and very low temperatures. On the other hand, it is possible to see that the hexane soluble fraction determined on plaque increases from 5.5% to 7.4% bw when the composition becomes softer (Flexural modulus of 370MPa in ex. 3) and the total ethylene content increases (from 8% of Ex. 1 to 9.3% of Ex. 3).. The said compositions are prepared by a sequential polymerization process comprising the preparation of component (A) in a first gas-phase polymerization apparatus comprising two interconnected polymerization zones and a second step (B) in which the propylene, ethylene-reach, copolymer is prepared by polymerizing the monomers in a separate fluidized bed reactor. The applicant has now found a process for the preparation of soft propylene polymer composition having a still improved balance between hexane soluble moiety and low flexural modulus. Therefore, it is an object of the present invention a process for the preparation of a propylene polymer composition having a Flexural modulus lower than 500MPa, a total ethylene content higher than 9% and a Xylene soluble fraction at room temperature higher than 20% comprising at least one polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones said process being characterized by the fact that at least 30% by weight of said xylene soluble fraction is produced in the polymerization step carried out in the said gas-phase polymerization reactor comprising at least two interconnected polymerization zones.

In fact, it has been surprisingly found that the so prepared compositions are characterized by a very low value of hexane extractable fraction even though they are extremely soft.

Preferably, the compositions prepared according to the process of the invention have a flexural modulus lower than 500 MPa, preferably lower than 450 MPa, most preferably lower than 400 MPa.

The fraction soluble in xylene at room temperature is higher than 20%by weight, preferably higher than

25% and more preferably higher than 30% by weight specifically ranging from 30 to 40%bw.

The total ethylene content is higher than 9%, preferably higher than 10 % and more preferably ranging from 10 to 30% by weight.

The value of the intrinsic viscosity of the xylene soluble fraction determined on the reactor grade polymer composition ranges from 0.5 dl/g to 5.0 dl/g, preferably between from 1.0 to 4.0 dl/g and more preferably from 2.0 to 4.0 dl/g.

The compositions obtained according to the process of the invention can be obtained as reactor grade with a Melt Flow Rate value according to ISO 1133 (230⁰C, 2.16 Kg) ranging from 0.1 to 50 g/10min.

Preferably, they are obtained with a MFR of less than 5 and more preferably in the range 0.5-4 g/10 min. Then, if desired they can be visbroken according to known technique in order to reach the final

MFR value suited for the selected application. The chemical degradation of the polymer (visbreaking) is carried out in the presence of free radical initiators, such as peroxides. Examples of radical initiators that can be used for this purpose are 2,5-dimethyl-2,5-di(tert-butylperoxide)-hexane and dicumyl- peroxide. The degradation treatment is carried out by using the appropriate quantities of free radical initiators, and preferably takes place in an inert atmosphere, such as nitrogen. Methods, apparatus, and operating conditions known in the art can be used to carry out this process.

The MFR after visbreaking can be in the range of from 2 to 40 g/10min and preferably from 4 to 30 g/10 min.

The so obtained propylene copolymers are characterized by an excellent IZOD Impact Strength. The samples, when tested according to ISO 180/1 A result not to be broken at 23°C. When tested at 0⁰C the impact strength ranges from 10 to 40 KJ/m² while at -20⁰C it is comprised from 5 to 10 KJ/m² thereby showing a high resilience even at low temperature. It is worth noting that softness and impact resistance are shown in the presence of very low hexane extractability, which determined on plaque, is lower than 8% and preferably lower than 7%by weight. When the hexane extractability is determined on film (lOOμm) the value is even lower than 6% by weight.

Typically the weight ratio between the xylene-soluble moiety and the hexane-extractable moiety (determined on film) is more than 4 preferably more than 5 and more preferably more than 6 even in correspondence with an amount of xylene soluble fraction higher than 30%bw and a total ethylene content of higher than 10%. This feature ensures the use of the soft compostions even in food contact applications.

As explained above the propylene copolymer compositions are produced with a process comprising at least one polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones said process being characterized by the fact that at least 30% by weight of said xylene soluble fraction is produced in the polymerization step carried out in the gas- phase polymerization reactor comprising at least two interconnected polymerization zones. Preferably, at least 50% and more preferably at least 70% by weight of said xylene soluble fraction is produced in said polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones.

Although it is possible to prepare the propylene copolymer composition disclosed above in a sequential process in which at least one polymerization step is carried out in a gas-phase reactor comprising at least two interconnected polymerization zones and another polymerization step carried out in a conventional liquid polymerization reactor or conventional fiuidized or stirred bed gas-phase reactor it is preferred that the whole composition is prepared by one or more polymerization steps all of them being carried out in a gas-phase reactor comprising at least two interconnected polymerization zones. In particular, it is preferred that all the xylene soluble fraction of the polymer composition is prepared in such a type of reactor. A polymerization process carried out in a gas-phase polumerization reactor comprising at least two interconnected polymerization zones is described in the European patent EP 782587.

The process is carried out in a first and in a second interconnected polymerization zone to which propylene and ethylene or propylene and alpha-olefins are fed in the presence of a catalyst system and from which the polymer produced is discharged. The growing polymer particles flow through the first of said polymerization zones (riser) under fast fluidization conditions, leave said first polymerization zone and enter the second of said polymerization zones (downcomer) through which they flow in a densified form under the action of gravity, leave said second polymerization zone and are reintroduced into said first polymerization zone, thus establishing a circulation of polymer between the two polymerization zones. Generally, the conditions of fast fluidization in the first polymerization zone is established by feeding the monomers gas mixture below the point of reintroduction of the growing polymer into said first polymerization zone. The velocity of the transport gas into the first polymerization zone is higher than the transport velocity under the operating conditions and is normally between 2 and 15 m/s. In the second polymerization zone, where the polymer flows in densified form under the action of gravity, high values of density of the solid are reached which approach the bulk density of the polymer; a positive gain in pressure can thus be obtained along the direction of flow, so that it becomes possible to reintroduce the polymer into the first reaction zone without the help of mechanical means. In this way, a "loop" circulation is set up, which is defined by the balance of pressures between the two polymerization zones and by the head loss introduced into the system. Optionally, one or more inert gases, such as nitrogen or an aliphatic hydrocarbon, are maintained in the polymerization zones, in such quantities that the sum of the partial pressures of the inert gases is preferably between 5 and 80% of the total pressure of the gases. The operating parameters such as, for example, the temperature are those that are usual in gas-phase olefin polymerization processes, for example between 5O⁰C and 12O⁰C. The process can be carried out under operating pressure of between 0,5 and 10 MPa, preferably between 1.5 and 6 MPa. Preferably, the various catalyst components are fed to the first polymerization zone, at any point of said first polymerization zone. However, they can also be fed at any point of the second polymerization zone. Molecular weight regulators known in the art, particularly hydrogen, can be used to regulate the molecular weight of the growing polymer.

By the use of the means described in WO00/02929 it is possible to totally or partially prevent that the gas mixture present in the riser enters the downcomer; in particular, this is preferably obtained by introducing in the downer a gas and/or liquid mixture having a composition different from the gas mixture present in the riser. According to a particularly advantageous embodiment of the present invention, the introduction into the downcomer of the said gas and/or liquid mixture having a composition different from the gas mixture present in the riser is effective in preventing the latter mixture from entering the downcomer. Therefore, it is possible to obtain two interconnected polymerization zones having different monomer compositions and thus able to produce polymers with different properties.

The above feature is particularly important for producing the propylene copolymer of the invention, because it allows to maintain different amounts of ethylene in the two zones which, in turn permit to produce in the zone less reach in ethylene a substantial amount of the propylene copolymer fraction insoluble in xylene and in the ethylene-reach zone a substantial amount of the propylene copolymer fraction soluble in xylene.

Typically, the ethylene-reach polymerization zone is the riser where the molar concentration of ethylene (expressed as molar ratio between ethylene and the total molar amount of monomers), when the whole propylene copolymer composition is produced in the gas-phase polymerization reactor comprising the two interconnected polymerization zones, ranges from 0.1 to 0.3 and preferably from 0.12 to 0.20; in the downcomer, by virtue of the barrier effect provided by the gas and/or liquid feeding described above, the composition of the gas phase is less reach in ethylene and generally in the range of from 0.01-0.05 preferably from 0.02 to 0.04. The ratio between the ethylene content in the riser and that in the downcomer is typically higher than 5 and more typically higher than 6. If an additional stage in a different type of reactor is performed, it is particularly preferred to carry out it in a conventional fluidized-bed gas-phase reactor. Preferably this stage is carried out as a second stage. The polymerization mixture is discharged from the downcomer to a gas-solid separator, and subsequently fed to the fluidized-bed gas-phase reactor operating under conventional conditions of temperature and pressure.

Each polymerization stage is carried out in presence of a highly stereospecific catalyst based on a transition metal compound. Preferably, it is an heterogeneous Ziegler-Natta catalyst. The Ziegler-Natta catalysts suitable for producing the propylene polymer compositions of the invention comprise a solid catalyst component comprising at least one titanium compound having at least one titanium-halogen bond and at least an electron-donor compound (internal donor), both supported on magnesium chloride. The Ziegler-Natta catalysts systems further comprise an organo-aluminum compound as essential co- catalyst and optionally an external electron-donor compound.

Suitable catalysts systems are described in the European patents EP45977, EP361494, EP728769, EP 1272533 and in the international patent application WO00/63261.

Preferably, the solid catalyst component comprises Mg, Ti, halogen and an electron donor selected from succinates of formula (I):

O

R₃ Il „ \ , C. . R₂

^K5 / ^ CT R₁

Re Il O wherein the radicals Ri and R₂, equal to or different from each other, are a C₁-C₂0 linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms; the radicals R3 to R₆ equal to or different from each other, are hydrogen or a Ci-C ₂0 linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms, and the radicals R₃ to R₆ which are joined to the same carbon atom can be linked together to form a cycle.

Ri and R₂ are preferably Ci-Cs alkyl, cycloalkyl, aryl, arylalkyl and alkylaryl groups. Particularly preferred are the compounds in which Ri and R₂ are selected from primary alkyls and in particular branched primary alkyls. Examples of suitable Ri and R₂ groups are methyl, ethyl, n-propyl, n-butyl, isobutyl, neopentyl, 2- ethylhexyl. Particularly preferred are ethyl, isobutyl, and neopentyl.

One of the preferred groups of compounds described by the formula (I) is that in which R3 to R5 are hydrogen and R₆ is a branched alkyl, cycloalkyl, aryl, arylalkyl and alkylaryl radical having from 3 to 10 carbon atoms. Another preferred group of compounds within those of formula (I) is that in which at least two radicals from R3 to R₆ are different from hydrogen and are selected from C₁-C₂0 linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms. Particularly preferred are the compounds in which the two radicals different from hydrogen are linked to the same carbon atom. Furthermore, also the compounds in which at least two radicals different from hydrogen are linked to different carbon atoms, that is R3 and R5 or R₄ and R₆ are particularly preferred. A particularly preferred catalyst comprises in addition to Mg, Ti, and halogen an electron donor selected from esters of phthalic acids disclosed in EP45977 and in particular of either diisobutylphathalate or dihexylphthalate or diethylphthalate and mixtures thereof.

According to a preferred method, the solid catalyst component can be prepared by reacting a titanium compound of formula Ti(OR)_n-_yX_y, where n is the valence of titanium and y is a number between 1 and n, preferably TiCU, with a magnesium chloride deriving from an adduct of formula MgCl₂^pROH, where p is a number between 0.1 and 6, preferably from 2 to 3.5, and R is a hydrocarbon radical having 1-18 carbon atoms. The adduct can be suitably prepared in spherical form by mixing alcohol and magnesium chloride in the presence of an inert hydrocarbon immiscible with the adduct, operating under stirring conditions at the melting temperature of the adduct (100-130 ⁰C). Then, the emulsion is quickly quenched, thereby causing the solidification of the adduct in form of spherical particles. Examples of spherical adducts prepared according to this procedure are described in US 4,399,054 and US 4,469,648. The so obtained adduct can be directly reacted with the Ti compound or it can be previously subjected to thermal controlled dealcoholation (80-130 ⁰C) so as to obtain an adduct in which the number of moles of alcohol is generally lower than 3, preferably between 0.1 and 2.5. The reaction with the Ti compound can be carried out by suspending the adduct (dealcoholated or as such) in cold TiCl₄ (generally 0 ⁰C); the mixture is heated up to 80-130 ⁰C and kept at this temperature for 0.5-2 hours. The treatment with TiCU can be carried out one or more times. The internal donor can be added during the treatment with TiCl₄ and the treatment with the electron donor compound can be repeated one or more times. Generally, the internal electron donor compound and also the specific succinate of formula (I) is used in molar ratio with respect to the MgCl₂ of from 0.01 to 1 preferably from 0.05 to 0.5. The preparation of catalyst components in spherical form is described for example in European patent application EP-A-395083 and in the International patent application WO98/44001. The solid catalyst components obtained according to the above method show a surface area (by B.E.T. method) generally between 20 and 500 m /g and preferably between 50 and 400 m²/g, and a total porosity (by B.E.T. method) higher than 0.2 cm³/g preferably between 0.2 and 0.6 cm³/g. The porosity (Hg method) due to pores with radius up to lO.OOOA generally ranges from 0.3 to 1.5 cm³/g and preferably from 0.45 to 1 cm³/g.

The organo-aluminum compound is preferably an alkyl-Al selected from the trialkyl aluminum compounds such as for example triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, tri-n- hexylaluminum, tri-n-octylaluminum. It is also possible to use mixtures of trialkylaluminum's with alkylaluminum halides, alkylaluminum hydrides or alkylaluminum sesquichlorides such as AlEt₂Cl and Al₂Et₃Cl₃.

Preferred external electron-donor compounds include silicon compounds, ethers, esters such as ethyl 4- ethoxybenzoate, amines, heterocyclic compounds and particularly 2,2,6,6-tetramethyl piperidine, ketones and the 1,3-diethers. Another class of preferred external donor compounds is that of silicon compounds of formula Ra⁵Rb⁶Si(OR⁷)_c, where a and b are integer from 0 to 2, c is an integer from 1 to 3 and the sum (a+b+c) is 4; R⁵, R⁶, and R⁷, are alkyl, cycloalkyl or aryl radicals with 1-18 carbon atoms optionally containing heteroatoms. Particularly preferred are methylcyclohexyldimethoxysilane, diphenyldimethoxysilane, methyl-t-butyldimethoxysilane, dicyclopentyldimethoxysilane, 2- ethylpiperidinyl-2 -t-butyldimethoxysilane and 1,1,1 ,trifluoropropyl-2 -ethylpiperidinyl-dimethoxysilane and l,l,l,trifluoropropyl-metil-dimethoxysilane. The external electron donor compound is used in such an amount to give a molar ratio between the organo-aluminum compound and said electron donor compound offrom 0.1 to 500.

The catalytic system can be formed directly in the reactor by feeding the separate components 8solid catalyst component, aluminum alkyl as cocatalyst and external donor as activator) or it can be formed beforehand by mixing the components in a separate vessel (pre-contact) and than feeding the so obtained catalyst system in the reactor. According to another preferred option, the preformed catalyst system is contacted with small amounts of olefins in order to prepare a pre -polymerized catalyst which is then sent to the main polymerization reactor. The molecular weight of the propylene polymer composition can be regulated by using known regulators, such as hydrogen.

The propylene copolymer compositions obtained by the process of the present invention may then be added with additives commonly employed in the polyolefin field, such as antioxidants, light stabilizers, nucleating agents, antiacids, colorants and fillers.

The main application of the propylene polymer compositions of the invention is the production of films, particularly soft blown films, and molded articles, particularly injection-molded items. The injection- molded articles comprising the propylene polymer compositions of the invention have good flexibility and excellent impact properties at low temperature. Due to the low hexane-extractable fraction of the polyolefin compositions of the invention the films and injection-molded articles obtained therefrom are particularly suitable for food applications. The following examples are given to illustrate and not to limit the present invention. Examples

The data of the propylene polymer materials were obtained according to the following methods: Xylene-soluble faction

2.5 g of polymer and 250 mL of o-xylene are introduced in a glass flask equipped with a refrigerator and a magnetic stirrer. The temperature is raised in 30 minutes up to the boiling pint of the solvent. The so obtained solution is then kept under reflux and stirring for further 30 minutes. The closed flask is then kept for 30 minutes in a bath of ice and water and in thermostatic water bath at 25°C for 30 minutes as well. The solid thus obtained is filtered on quick filtering paper and the filtered liquid is divided into two 100 ml aliquots. One 100 ml aliquots of the filtered liquid is poured in a previously weighed aluminum container, which is heated on a heating plate under nitrogen flow, to remove the solvent by evaporation. The container is then kept on an oven at 80⁰C under vacuum until constant weight is obtained. The residue is weighed to determine the percentage of xylene-soluble polymer. Comonomer (C2) content By IR spectroscopy.

The comonomer content of the Component B is determined on the precipitated "amorphous" fraction of the polymer. The precipitated "amorphous" fraction is obtained as follows: to one 100 ml aliquot of the filtered liquid obtained as described above 200 ml of acetone are added under vigorous stirring. Precipitation must be complete as evidenced by a clear solid-solution separation. The solid thus obtained is filtered on a metal screen and dried in a vacuum oven at 70⁰C until a constant weight is reached.

Molar ratio of feed gasses

Determined by gas-chromatography

Melt flow rate (MFR)

Determined according to ISO 1133 (230⁰C, 2.16 Kg)

Intrinsic viscosity

Determined in tetrahydronaphthalene at 135°C

Flexural modulus

Determined according to ISO 178

Stress at yield and at break

Determined according to ISO 527

Elongation at yield and break

Determined according to ISO 527

IZOD Impact Strength

Determined according to ISO 180/1 A

Ductile/Brittle transition temperature (D/B)

According to this method, the bi-axial impact resistance is determined through impact with an automatic, computerized striking hammer.

The circular test specimens are obtained by cutting with circular hand punch (38 mm diameter). They are conditioned for at least 12 hours at 23°C and 50 RH and then placed in a thermostatic bath at testing temperature for 1 hour. The force-time curve is detected during impact of a striking hammer

(5.3 kg, hemispheric punch with a ¹A" diameter) on a circular specimen resting on a ring support. The machine used is a CEAST 6758/000 type model no. 2.

D/B transition temperature is the temperature at which 50% of the samples undergoes fragile break when submitted to the above-mentioned impact test.

The plaques for D/B measurements, having dimensions of 127x127x1.5 mm are prepared according to the following method.

The injection press is a Negri Bossi™ type (NB 90) with a clamping force of 90 tons. The mould is a rectangular plaque (127x127x1.5mm).

The main process parameters are reported below:

Back pressure (bar): 20 Injection time (s): 3

Maximum Injection pressure (MPa): 14

Hydraulic injection pressure (MPa): 6-3

First holding hydraulic pressure (MPa): 4±2

First holding time (s): 3

Second holding hydraulic pressure (MPa): 3±2

Second holding time (s): 7

Cooling time (s): 20

Mould temperature (⁰C): 60

The melt temperature is between 220 and 280° C.

Hexane extractable

The hexane extractable fraction is determined according to modified FDA method (federal registration, title 21, Chapter 1, part 177, section 1520, s. Annex B) on polymer formed into 100 μm thick plaque or film. The plaque is prepared by compression molding, while the film is prepared by extrusion.

Melting temperature, melting enthalpy and crystallization temperature

Determined by DSC with a temperature variation of 20⁰C per minute

Examples 1-5

The Ziegler-Natta catalyst was prepared according to the Example 5, lines 48-55 of the European

Patent EP728769. Triethylaluminium (TEAL) was used as co-catalyst and dicyclopentyldimethoxysilane as external donor, with the weight ratios indicated in Table 1.

The propylene copolymer compositions of the examples were prepared in a single gas-phase polymerization reactor comprising two interconnected polymerization zones, a riser and a downcomer, as described in the European Patent EP782587 and WO00/02929. Triethylaluminium (TEAl) was used as co-catalyst and dicyclopentyldimethoxysilane as external donor, with the weight ratios indicated in

Table 1. The operative conditions are indicated in Table 1.

The polymer particles exiting from the polymerization step were subjected to a steam treatment to remove the unreacted monomers and dried.

The propylene polymer compositions were added with the additives indicated in Table 3 and extruded in a twin-screw extruder Berstorff (L/D=33) under the following operating conditions:

Temperature of the feeding section: 190-210⁰C

Melt temperature: 240⁰C

Temperature of the die section: 230⁰C Flow rate: 16 Kg/h

Rotational speed: 250 rpm

The properties measured on the samples are collected in Table 3.

Example 6

The process according to examples 4 is repeated with the difference that the polymer composition exiting polymerization reactor comprising two interconnected polymerization zones is sent into a conventional fiuidized bed reactor where an additional ethylene -propylene copolymer was produced.

The conditions used in said conventional fluidised bed reactor and the characterization of the final reactor-grade composition are reported in table 2. The characterization of the visbroken composition is reported in table 3.

Table 1

Table 2

Table 3

A.O. B215 -2:1 mixture of lrgafos 168 and Irganox1010 CaSt Calcium stearate

Claims

LA process for the preparation of a propylene copolymer composition having a Flexural modulus lower than 500MPa, a total ethylene content higher than 9% and a Xylene soluble fraction at room temperature higher than 20% comprising at least one polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones said process being characterized by the fact that at least 30% by weight of said xylene soluble fraction is produced in the polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones.

2. The process for the preparation of a propylene polymer composition according to claim 1 in which at least 50% by weight of said xylene soluble fraction is produced in a polymerization step carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerization zones.

3. The process for the preparation of a propylene polymer composition according to claim 1 in which the whole composition is prepared by one or more polymerization steps carried out in a gas-phase reactor comprising at least two interconnected polymerization zones.

4. Process according to claim 1 in which the reactor comprising at least two interconnected polymerization zones comprises a first of said polymerization zones running under fast fluidization conditions, and a second of said polymerization zones (downcomer) through which they flow in a densified form under the action of gravity.

5. Process according to claim 6 in which a gas and/or liquid mixture having a composition different from the gas mixture present in the first polymerization zone is introduced into the second polymerization zone thereby totally or partially preventing the gas mixture of the first polymerization zone enters the second polymerization zone.

6. Process according to claim 5 in which the gas-phase compostion of the first polymerization zone is reacher than the second.

7. The process according to claim 1 in which the propylene copolymer compositions have a flexural modulus lower than 400 MPa.

8. The process according to claim 1 in which the propylene copolymer compositions have a fraction soluble in xylene at room temperature is higher than 25%.

9. : The process according to claim 1 in which the propylene copolymer compositions have an ethylene content higher than 10 %

0. The process according to claim 1 in which the propylene copolymer compositions have hexane extractability determined on film (lOOμm) lower than 6% by weight.