EP4731686A1

EP4731686A1 - Process for the preparation of a heterophasic polypropylene composition

Info

Publication number: EP4731686A1
Application number: EP24734921.0A
Authority: EP
Inventors: Jingbo Wang; Markus Gahleitner; Karlheinz Friedrich; Pauli Leskinen
Original assignee: Borealis GmbH
Current assignee: Borealis GmbH
Priority date: 2023-06-26
Filing date: 2024-06-26
Publication date: 2026-04-29
Also published as: CN121419999A; WO2025003221A1

Abstract

The present invention relates to a process for the preparation of a heterophasic polypropylene composition by a multi-stage polymerization process using a Ziegler-Natta catalyst. The prepared heterophasic polypropylene composition comprises ethylene and 1-butene comonomer units. The process is characterized by an efficient usage of 1-butene comonomers during preparation of the heterophasic polypropylene composition.

Description

Process for the preparation of a heterophasic polypropylene composition

The present invention relates to a process for the preparation of a heterophasic polypropylene composition by a multi-stage polymerization process using a Ziegler-Natta catalyst. The prepared heterophasic polypropylene composition comprises ethylene and 1 -butene comonomer units. The process is characterized by an efficient usage of 1 -butene comonomer during preparation of the heterophasic polypropylene composition.

Background

Plastic packaging is widely used in daily life due to its good cost/performance balance. Polyolefin polymers based on polypropylene and/or polyethylene are often used in these applications. For adjusting the properties of these polymers, other comonomers of higher alpha-olefins, such as 1 -butene and 1 -hexene, are employed. Employment of these comonomers allows for excellent flexibility in improving the properties of choice in the final polymer.

Currently, the majority of the polyolefin polymers is produced in heterogeneous processes comprising multiple steps. On the one hand, these processes are advantageous due to their positive economic aspects. On the other hand, they face some challenges, in particular, when high alpha-olefins are employed in the processes. These comonomers normally have much higher solubility in the materials and higher boiling points, which makes it difficult to remove residual comonomers after the polymerization process. The difficulties increase with the C-content of the comonomer.

The residual comonomers do not only impair the product quality (e.g. leading to unacceptable taste and/or odor), but also safety issues are concerned such as the risk of explosion.

Currently the common practice to remove residual comonomers is post-reactor treatment, like aeration. This additional treatment of the polymer composition is very cost-consuming as it requires extra energy and time, leaving a high CO2 footprint. Further, removing residual comonomers in this way is also a waste of raw materials.

Further, development of suitable aeration processes is challenging as polyolefin polymers have an intrinsic stickiness and a tendency to agglomerate above a relatively low temperature, as described in e.g. European patent application EP- A-3647349. Thus, finding a more sustainable and economic solution would be highly appreciated.

Object of the invention

It is thus an object of the present invention to provide a more sustainable and economically beneficial process for the preparation of polyolefin polymers comprising higher alpha-olefins, particularly for use in packaging applications. Further, it is an object of the present invention to provide respective polyolefin polymers that do not suffer from the drawbacks connected with the presence of residual comonomers.

Summary of the invention

It has been surprisingly found that the above-described object can be achieved by a process for the preparation of a heterophasic polypropylene composition by a multi-stage polymerization process, comprising

I.) preparing a first propylene polymer fraction (A1 ) being a propylene copolymer of propylene with 1 -butene comonomer units in a content in the range of from 2.0 to 10.0 wt.-% and ethylene comonomer units in a content in the range of from 0.1 to 5.0 wt.-% in a first reactor, preferably a loop reactor, in a first polymerization stage in the presence of a Ziegler-Natta catalyst, wherein the first propylene polymer fraction (A1 ) has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.1 to 10.0 g/10 min;

II.) transferring the first propylene polymer fraction (A1 ) to a second polymerization stage and preparing a second propylene polymer fraction (A2) being a propylene copolymer of propylene with 1 -butene comonomer units and ethylene comonomer units in the presence of a Ziegler-Natta catalyst in a second reactor, preferably a first gas phase reactor (GPR1 ); wherein the first propylene polymer fraction (A1 ) and the second propylene polymer fraction (A2) together form a propylene copolymer (A) of propylene with ethylene comonomer units in a content in the range of from 0.5 to 2.5 wt.-% and 1 -butene comonomer units in a content in the range of from 4.0 to 15.0 wt.-%, wherein the propylene copolymer (A) has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.3 to 10.0 g/10 min; and

III.) transferring the propylene copolymer (A) to a third polymerization stage and preparing a propylene ethylene elastomer (B) as a third propylene polymer fraction in the presence of a Ziegler-Natta catalyst in a third reactor, preferably a second gas phase reactor (GPR2), to obtain the heterophasic polypropylene composition; wherein the heterophasic polypropylene composition has: a) a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.5 to 10.0 g/10 min; b) a melting temperature T_m, determined according to differential scanning calorimetry (DSC) described in the specification, in the range of from 125 to 145 °C; and c) a content of xylene soluble fraction (XS), based on the total weight of the heterophasic polypropylene composition and determined at 25 °C according to ISO 16152, of from 10 to 45 wt.-%.

The process is characterized by a high usage of comonomers, and thus low contents of residual comonomers that have not been integrated in the polymer.

It has further been found that the above-described object can be achieved by a heterophasic polypropylene composition prepared by this multi-stage polymerization process. The prepared heterophasic polypropylene composition is of high quality having low contents of residual comonomers, and thus low contents of volatile organic compounds (VOC). No further treatment is required to remove the comonomers.

Description of the invention

Process for the preparation of the heterophasic polypropylene composition

The present invention relates to a process for the preparation of a heterophasic polypropylene composition, and to the heterophasic polypropylene composition obtained or obtainable by the process.

The term ‘heterophasic polypropylene composition’ as used herein means that at least two distinct phases are present in the composition, i.e., a propylene copolymer (A) and a propylene ethylene elastomer (B). The propylene copolymer

(A) forms a crystalline matrix of the heterophasic polypropylene composition and the propylene ethylene elastomer (B) is dispersed in the crystalline matrix. Presence of the distinct phases is easily detectable via high resolution microscopy like scanning electron microscopy (SEM) or atomic force microscopy (AFM), but also by DSC analysis: the crystalline matrix propylene copolymer (A) will show a melting point higher than that of the propylene ethylene elastomer

(B). The term ‘propylene copolymer" as used herein denotes a polymer comprising, based on the total weight of the polymer, at least 50 wt.-% of propylene monomer units and further alpha-olefin comonomer units different from propylene.

The wording ‘alpha-olefin’ comonomer units (e.g., propylene, ethylene, 1 -butene units etc.) used herein for contents of the polymer is to be understood to teach these units for preparing the polymer. The resultant polymer contains the respective units derived from these alpha-olefin monomer units.

Unless otherwise indicated, percentages throughout this disclosure are percentages by weight (wt.-%) based on the total weight of the respectively described enclosing entity.

The process according to the present invention leads to a product containing a low content of residual comonomers. The 1 -butene comonomers are usually present with a content of less than 1 mg per kg of the heterophasic polypropylene composition. This indicates that the usage of the 1 -butene comonomers is very high and efficient.

The process preferably leads to a heterophasic polypropylene composition that has a percentage of the lower explosive limit (LEL), determined as described in the method section herein, in the range of from 1 to 15 vol.-%, more preferably from 2 to 13 vol.-%.

The lower explosive limit (LEL) is the lowest concentration (percentage) of a gas or a vapor in an atmosphere capable of producing a flash of fire in the presence of an ignition source (arc, flame, heat). At a concentration in the atmosphere below the LEL, there is not enough “fuel” to continue an explosion. Concentrations lower than the LEL are generally too lean to explode. Often gases have an LEL value of between 1 and 5 vol.-%. For example, if a gas has an LEL value of 1 .0 vol.-%, and the atmosphere contains less than 1 .0 vol.-% of this gas, an explosion is not likely to occur even if a source of ignition is present. Further, 50 vol.-% of LEL of this gas corresponds to half of the LEL value, i.e. 0.5 vol.-% of this gas in the atmosphere. Lower percentages of LEL are desirable in order to minimize the risk of explosion.

The process is a multi-stage polymerization process comprising at least three stages of polymerization in steps I.), II.) and III.), wherein the polymers (A1 ), (A2) and (B) are prepared, respectively, in this order.

The process may comprise one or more further polymerization stage(s)/step(s). Particularly preferred is a prepolymerization step, which precedes the polymerization step I.), as described further below.

Accordingly, the process at least comprises I.) preparing a first propylene polymer fraction (A1 ) being a propylene copolymer of propylene with 1 -butene comonomer units in a content in the range of from 2.0 to 10.0 wt.-% and ethylene comonomer units in a content in the range of from 0.1 to 5.0 wt.-% in a first reactor, preferably a loop reactor, in a first polymerization stage in the presence of a Ziegler-Natta catalyst, wherein the first propylene polymer fraction (A1 ) has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.1 to 10.0 g/10 min;

II.) transferring the first propylene polymer fraction (A1 ) to a second polymerization stage and preparing a second propylene polymer fraction (A2) being a propylene copolymer of propylene with ethylene comonomer units and 1 -butene comonomer units in the presence of a Ziegler-Natta catalyst in a second reactor, preferably a first gas phase reactor (GPR1 ); wherein the first propylene polymer fraction (A1 ) and the second propylene polymer fraction (A2) together form a propylene copolymer (A) of propylene with 1 -butene comonomer units in a content in the range of from 4.0 to 15.0 wt.-% and ethylene comonomer units in a content in the range of from 0.5 to 2.5 wt.-%, wherein the propylene copolymer (A) has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.3 to 10.0 g/10 min; and

III.) transferring the propylene copolymer (A) to a third polymerization stage and preparing a propylene ethylene elastomer (B) as a third propylene polymer fraction in the presence of a Ziegler-Natta catalyst in a third reactor, preferably a second gas phase reactor (GPR2), to obtain the heterophasic polypropylene composition.

Preparation of the first propylene polymer fraction (A1)

Step I.) of the process comprises preparing the first propylene polymer fraction (A1 ) that preferably consists of a single propylene polymer.

The propylene polymer of the first propylene polymer fraction (A1 ) is a propylene copolymer of propylene with 1 -butene comonomer units in a content in the range of from 2.0 to 10.0 wt.-%, preferably from 3.0 to 8.0 wt.-% and more preferably from 3.5 to 7.5 wt.-%, and ethylene comonomer units in a content in the range of from 0.1 to 5.0 wt.-%, preferably from 0.3 to 3.0 wt.-%, more preferably from 0.5 to 2.0 wt.-% and most preferably from 0.7 to 1.5 wt.-%, based on the total weight of the first propylene polymer fraction (A1 ). The comonomer contents may be determined by ¹³C-NMR spectroscopy, as described in the method section. The first propylene polymer fraction (A1 ) has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.1 to 10.0 g/10 min, preferably from 0.3 to 5.0 g/10 min and more preferably from 0.7 to 3.0 g/10 min.

Preferably, the first propylene polymer fraction (A1 ) has a xylene soluble fraction (XS), determined at 25 °C according to ISO 16152, in the range of from 1.0 to 8.0 wt.-%, more preferably from 1.5 to 7.0 wt.-%, and even more preferably from 2.0 to 6.5 wt.-%, based on the total weight of the first propylene polymer fraction (A1 ). The xylene soluble fraction (XS) preferably amounts to at most 7.0 wt.-% of the first propylene polymer fraction (A1 ).

Step I.) of the process preferably takes place in a bulk phase reactor, more preferably a loop reactor. This first polymerization stage is carried out in the presence of a Ziegler-Natta catalyst.

Preferably, the Ziegler-Natta catalyst comprises: a) compound(s) of a transition metal of Group 4 to 6 of IIIPAC; b) a Group 2 metal compound; c) an internal donor, wherein said internal donor is a non-phthalic compound, preferably is a non-phthalic acid ester; d) a co-catalyst; and e) optionally an external donor.

The internal donor is preferably selected from (di)esters of non-phthalic carboxylic (di)acids, 1 ,3-diethers, derivatives and mixtures thereof. Most preferred internal donors are (di)esters of citraconic acid.

Preferably, the Ziegler-Natta catalyst does not comprise phthalic acid esters, and the heterophasic polypropylene composition is free of phthalic acid esters and their respective decomposition products.

Suitable Ziegler-Natta catalysts for use in the above-described polymerization reactions and preparation thereof are described in the European patent application EP-A-3562850.

As co-catalysts, commonly aluminium alkyl compounds of the general structure AIR3 or aluminum halo-alkyl compounds of the general structure AIR_nX(3-n) with R being a linear or branched alky group with 2 to 8 carbon atoms and X being a halogen, preferably chlorine, are employed. The number n is preferably 2, and R is preferably an ethyl, butyl or isobutyl group. According to one preferred embodiment, the co-catalyst is triethyl aluminium, AI(C2Hs)3. As external donors, commonly organosilanes of the general structure SiRl 4 or alkoxysilanes of the general structure SiR1 n(OR2)(4-n) with R1 and R2 being independently selected from linear or branched alky groups, substituted or nonsubstituted cycloalkyl groups, as well as substituted or non-substituted aryl groups. The number n is preferably 2 or 3. Preferred examples of external donors are cyclohexyl trimethoxysilane (commonly called ‘donor C’) and dicylopentyl dimethoxysilane (commonly called ‘donor D’).

Preparation of the second propylene polymer fraction (A2) and propylene copolymer (A)

Step II.) of the process comprises transferring the first propylene polymer fraction (A1 ) to a second polymerization stage and preparing a second propylene polymer fraction (A2).

The first propylene polymer fraction (A1 ) and the second propylene polymer fraction (A2) together form a propylene copolymer (A). The propylene copolymer (A) forms the crystalline matrix of the heterophasic polypropylene composition.

The second propylene polymer fraction (A2) is different from the first propylene polymer fraction (A1 ). Preferably, the second propylene polymer fraction (A2) has a higher total content of comonomers than the first propylene polymer fraction (A1 ). More preferably, the second propylene polymer fraction (A2) has a higher content of ethylene comonomer units and 1 -butene comonomer units than the first propylene polymer fraction (A1 ).

The second propylene polymer fraction (A2) preferably consists of a single propylene polymer.

The propylene polymer of the second propylene polymer fraction (A2) is a propylene copolymer of propylene with ethylene comonomer units and 1 -butene comonomer units.

The second propylene polymer fraction (A2) preferably has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.1 to 10.0 g/10 min, more preferably from 0.3 to 5.0 g/10 min and most preferably from 0.5 to 3.0 g/10 min.

The MFR2 of the second propylene polymer fraction (A2), produced in the second polymerization stage is determined according to equation (1 ): equation (1 ) wherein MFR(A) is the MFR2 of the propylene copolymer (A) w(A 1) and w(A2) are the weight fractions of the propylene polymer fractions (A1 ) and (A2) in the propylene copolymer (A)

MFR(A 1) is the MFR2 of the propylene polymer fraction (A1 ) produced in the first polymerization stage.

Respectively adapted equations may be used to determine the parameters of the second propylene polymer fraction (A2) or of polymers prepared in further stages of a preparation process.

The propylene copolymer (A) is a copolymer of propylene with 1 -butene comonomer units in a content in the range of from 4.0 to 15.0 wt.-%, preferably from 4.5 to 12.0 wt.-% and more preferably from 5.0 to 10.0 wt.-%, and ethylene comonomer units in a content in the range of from 0.5 to 2.5 wt.-%, preferably from 0.7 to 2.4 wt.-% and more preferably from 1.0 to 2.3 wt.-%, based on the total weight of the propylene copolymer (A). The comonomer contents may be determined by ¹³C-NMR spectroscopy, as described in the method section.

Preferably, propylene, 1 -butene and ethylene are the only comonomers present in the propylene copolymer (A). It is preferred that the propylene copolymer (A) is substantially free of units derived from other alpha-olefin monomers (i.e., Cs to Cs alpha-olefin monomers), e.g., it comprises less than 0.1 wt.-%, preferably less than 0.05 wt.-%, and more preferably less than 0.01 wt.-% of other alphaolefin monomers such as Cs to Cs alpha-olefin monomer units.

The propylene copolymer (A) is preferably a multimodal polymer. The term ‘multimodal polymer" as used herein denotes a polymer composition comprising at least two polymer fractions, which have been produced under different polymerization conditions resulting in different (weight average) molecular weights and/or molecular weight distributions for the fractions and/or different comonomer contents. The prefix ‘multi’ relates to the number of different polymer fractions the polymer consists of. The term ‘multimodal polymer" comprises bimodal, trimodal, tetramodal etc. polymers.

More preferably, the propylene copolymer (A) is a bimodal polymer, i.e., it consists of two different polymer fractions, i.e., the first propylene polymer fraction (A1 ) and the second propylene polymer fraction (A2).

Preferably, the propylene copolymer (A) comprises, or consists of, from 30 to 70 wt.-%, based on the total weight of the propylene copolymer (A), of a first propylene polymer fraction (A1 ); and from 30 to 70 wt.-%, based on the total weight of the propylene copolymer (A), of a second propylene polymer fraction (A2). In other words, the weight ratio of the first propylene polymer fraction (A1 ) to the second propylene polymer fraction (A2) in the propylene copolymer (A) is preferably in the range of from 30:70 to 70:30, more preferably from 35:65 to 65:35 and most preferably from 40:60 to 60:40.

Preferably, the propylene copolymer (A) has a molecular weight distribution Mw/Mn, being the ratio of the weight average molecular weight M_w and the number average molecular weight M_n, in the range of from 3.0 to 10.0, more preferably from 4.0 to 8.0 and most preferably 5.5 to 7.0.

The propylene copolymer (A) has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.3 to 10.0 g/10 min, preferably from 0.5 to 5.0 g/10 min and more preferably from 0.7 to 3.0 g/10 min.

Preferably, the propylene copolymer (A) has a xylene soluble fraction (XS), determined at 25 °C according to ISO 16152, in the range of from 1.0 to 15.0 wt.- %, more preferably from 2.0 to 14.0 wt.-%, and even more preferably from 3.0 to 13.0 wt.-%, based on the total weight of the propylene copolymer (A).

Step II.) of the process preferably takes place in a first gas phase reactor (GPR1 ). This second polymerization stage is also carried out in the presence of a Ziegler-Natta catalyst, as described for the first polymerization stage.

Preparation of the propylene ethylene elastomer (B)

Step III.) of the process comprises transferring the propylene copolymer (A) to a third polymerization stage and preparing a propylene ethylene elastomer (B) as a third propylene polymer fraction (B).

The propylene ethylene elastomer (B) forms the dispersed phase that is dispersed in the crystalline matrix.

The propylene ethylene elastomer (B) is a copolymer of propylene and ethylene. Preferably, the ethylene content in the propylene ethylene elastomer (B), based on the total weight of the propylene ethylene elastomer (B), is in the range of from 20 to 60 wt.-%, more preferably from 22 to 50 wt.-%, and most preferably from 25 to 45 wt.-%. The ethylene content is determined by ¹³C-NMR spectroscopy, as described in the method section.

The propylene ethylene elastomer (B) may comprise 1 -butene comonomer units. The content of 1 -butene comonomer units in the propylene ethylene elastomer (B) is preferably in the range of from 0 to 5.0 wt.-%. Preferably, the propylene ethylene elastomer (B) does not comprise 1 -butene comonomer units.

Preferably, propylene and ethylene and optionally 1 -butene are the only monomers present in the propylene ethylene elastomer (B). It is preferred that propylene ethylene elastomer (B) is substantially free of units derived from other alpha-olefin monomers (i.e. , Cs to Cs alpha-olefin monomers), e.g., it comprises less than 0.1 wt.-%, preferably less than 0.05 wt.-%, and more preferably less than 0.01 wt.-% of other alpha-olefin monomers such as Cs to Cs alpha-olefin monomer units.

Preferably, propylene ethylene elastomer (B) has a molecular weight distribution Mw/Mn, being the ratio of the weight average molecular weight M_w and the number average molecular weight M_n, in the range of from 3.0 to 10.0, more preferably from 4.0 to 8.0 and most preferably 5.5 to 7.0.

Step III.) of the process preferably takes place in a second gas phase reactor (GPR2). This third polymerization stage is also carried out in the presence of a Ziegler-Natta catalyst, as described for the first polymerization stage.

Heterophasic polypropylene composition

After step III.) the heterophasic polypropylene composition is obtained.

The heterophasic polypropylene composition comprises propylene, 1 -butene and ethylene comonomer units. Thus, the heterophasic polypropylene composition comprises or consists of a terpolymer of propylene, ethylene and 1 -butene.

The heterophasic polypropylene composition has a melt flow rate MFR2 (230 °C), determined according to ISO 1133, in the range of from 0.5 to 10.0 g/10 min, preferably from 0.5 to 5.0 g/10 min and more preferably from 0.5 to 3.0 g/10 min.

The heterophasic polypropylene composition has a melting temperature T_m, determined according to differential scanning calorimetry (DSC) described in the specification, in the range of from 125 to 145 °C, preferably from 128 to 143 °C and more preferably from 130 to 140 °C.

Preferably, the heterophasic polypropylene composition has a crystallization temperature T_c, determined according to differential scanning calorimetry (DSC) described herein, in the range of from 90 to 100 °C, more preferably from 91 to 99 °C and most preferably from 92 to 98 °C.

The heterophasic polypropylene composition has a content of xylene soluble fraction (XS), based on the total weight of the heterophasic polypropylene composition and determined at 25 °C according to ISO 16152, of from 10 to 45 wt.-%, preferably from 1 1 to 40 wt.-% and more preferably from 12 to 35 wt.- %.

Preferably, the heterophasic polypropylene composition comprises, based on the total weight of the heterophasic polypropylene composition and determined by ¹³C-NMR spectroscopy, a content of 1 -butene comonomer units in the range of from 3.0 to 14.0 wt.-%, more preferably from 4.0 to 12.0 wt.-% and most preferably from 5.0 to 10.0 wt.-%.

Preferably, the heterophasic polypropylene composition comprises, based on the total weight of the heterophasic polypropylene composition and determined by ¹³C-NMR spectroscopy, a content of ethylene comonomer units in the range of from 3.0 to 15.0 wt.-%, more preferably from 4.0 to 13.0 wt.-% and most preferably from 5.0 to 11 .5 wt.-%.

Preferably, propylene and 1 -butene and ethylene are the only comonomers present in the heterophasic polypropylene composition. It is preferred that the heterophasic polypropylene composition is substantially free of units derived from other alpha-olefin monomers (i.e. , Cs to Cs alpha-olefin monomers), e.g., it comprises less than 0.1 wt.-%, preferably less than 0.05 wt.-%, and more preferably less than 0.01 wt.-% of other alpha-olefin monomers such as Cs to Cs alpha-olefin monomer units.

The comonomer contents are determined by ¹³C-NMR spectroscopy, as described in the method section.

Preferably, the propylene copolymer (A) and the propylene ethylene elastomer (B) are the sole propylene polymer components in the heterophasic polypropylene composition.

The weight ratio of the propylene ethylene elastomer (B) to the propylene copolymer (A) in the heterophasic polypropylene composition is preferably in the range of from 1 :99 to 40:60, more preferably from 2:98 to 35:65 and most preferably from 3:97 to 30:70.

It is to be understood herein that the heterophasic polypropylene composition may comprise further components such as additives, which may optionally be added in a mixture with or without a carrier polymer (e.g., in a master batch).

Suitable additives include fillers, lubricants, processing aids, antioxidants, UV absorbers, light stabilizers, nucleating agents, foaming or blowing agents, clarifiers and pigments.

The additives may be present in contents in the range of from 0.1 to 10.0 wt.-%, preferably from 0.3 to 5.0 wt.-%, more preferably from 0.5 to 3.0 wt.-%, based on the total weight of the heterophasic polypropylene composition.

Preferably, the heterophasic polypropylene composition comprises, based on the total weight of propylene polymer components in the heterophasic polypropylene composition: i.) from 25 to 60 wt.-%, more preferably from 30 to 55 wt.-%, of the first propylene polymer fraction (A1 ), ii.) from 30 to 50 wt.-%, more preferably from 35 to 50 wt.-%, of the second propylene polymer fraction (A2) and iii.) from 1 to 40 wt.-%, more preferably from 2 to 30 wt.-% of the third propylene polymer fraction (B), wherein the sum of all three fractions amounts to 100 wt.-% of propylene polymer components in the heterophasic polypropylene composition. As described above, the heterophasic polypropylene composition may comprise additives - in addition to the propylene polymer components.

The present invention also relates to the heterophasic polypropylene composition obtained and/or obtainable by the above-described process, in any of the embodiments and with any of the features described above.

The heterophasic polypropylene composition is characterized by good mechanical properties.

Preferably, the heterophasic polypropylene composition has a flexural modulus, determined according to ISO 178, in the range of from 250 to 550 MPa, more preferably from 260 to 530 MPa and most preferably from 270 to 500 MPa.

Preferably, the heterophasic polypropylene composition has an impact strength at +23 °C, determined according to ISO 179/1 eA, in the range of from 30.0 to 100.0 kJ/m², more preferably from 40.0 to 95.0 kJ/m².

Further preferably, the heterophasic polypropylene composition has an impact strength at -20 °C, determined according to ISO 179/1 eA, in the range of from 1 .5 to 7.0 kJ/m², more preferably from 1 .7 to 6.0 kJ/m².

The heterophasic polypropylene composition may be used for the manufacture of articles, preferably including films and molded articles, as well as a modifier polymer for other polymers.

The films may be prepared by any method known in the art, such as casting or extrusion. The films may be multilayer or monolayer films, but are preferably monolayer films. Preferably, the films are non-oriented films. The films may consist of heterophasic polypropylene composition as the sole polymer component.

As used herein the term 'molded article’ denotes an article that is produced by any conventional molding technique, e.g., extrusion blow molding, injection molding, stretch molding, compression molding, rotomolding or injection stretch blow molding. The molded articles can be containers, such as bottles, cups, buckets, beakers, trays or parts of such articles, such as see-through-windows, lids, or the like.

The articles may be employed in a number of end applications, in particular thin wall packaging applications and food packaging applications.

Further description of the process

The process according to the present invention is a multi-stage process wherein the propylene polymers are produced in an order of subsequent stages.

Ideally, the process produces a homogenous mixture of the various components. Typically, compounding is employed. Compounding usually involves mixing or/and blending the various components in a molten state, often by extrusion.

The process according to the present invention comprises at least three stages of polymerization in steps I.), II.) and III.), wherein the polymers (A1 ), (A2) and (B) are prepared, respectively, in this order.

Accordingly, in step I.) of the process, propylene, 1 -butene and ethylene are fed into a first reactor, preferably a bulk phase reactor such as a loop reactor, to prepare the first propylene polymer fraction (A1 ).

In step II.) of the process, propylene, 1 -butene and ethylene are fed into a second reactor, preferably a first gas phase reactor (GPR1 ), where the first propylene polymer fraction (A1 ) has been previously placed, to prepare the second propylene polymer fraction (A2). The two propylene polymer fractions (A1 ) and (A2) form the propylene copolymer (A).

The propylene copolymer (A) is placed in a third reactor, preferably a second gas phase reactor (GPR2), and propylene, ethylene and optionally 1 -butene are added in step III.) of the process, to prepare the heterophasic polypropylene composition. Additional steps such as blending with additives or pelletizing of the polymer may be included in the process.

Generally, polymers produced in a multi-stage process are also designated as ‘in-situ’ blends. The resulting end product consists of an intimate mixture of the polymers from the three or more reactors. These three or more polymers may have different molecular weight distribution curves, and/or they may differ in terms of comonomer content or type. The end product thus contains a mixture of three or more polymers with differing properties, i.e. , it is a multimodal polymer mixture.

The first polymerization stage for producing the first propylene polymer fraction (A1 ) is preferably a slurry polymerization step. The slurry polymerization usually takes place in a liquid mixture of the employed monomers without the presence of an inert diluent, like a hydrocarbon diluent.

The temperature in the first polymerization stage is typically from 60 to 100 °C, preferably from 60 to 80 °C. An excessively high temperature should be avoided to prevent partial dissolution of the polymer into the liquid phase, fouling of the reactor and early deactivation of the catalyst. The pressure is typically from 1 to 150 barg, preferably from 40 to 80 barg.

The slurry polymerization may be conducted in any known reactor used for slurry polymerization, such as any bulk phase reactor. Such reactors include a continuous stirred tank reactor and a loop reactor. It is especially preferred to conduct the polymerization in a loop reactor. In such reactors the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in US-A-4582816, US-A-3405109, US-A-3324093, EP-A-479186 and US-A-5391654. It is thus preferred to conduct the first polymerization stage as a slurry polymerization in a loop reactor.

The slurry may be withdrawn from the reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in the patent applications US-A-337421 1 , US-A-3242150 and EP-A- 1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007/025640. The continuous withdrawal is advantageously combined with a suitable concentration method, as disclosed in EP-A-1310295 and EP-A-1591460. It is preferred to withdraw the slurry from the first polymerization stage continuously.

Hydrogen is typically introduced into the first polymerization stage for controlling the MFR2 of the polymer. The amount of hydrogen needed to reach the desired MFR2 depends on the catalyst used and the polymerization conditions, as will be appreciated by the skilled worker.

The average residence time in the first polymerization stage is typically from 20 to 120 minutes, preferably from 30 to 80 minutes. As it is well known in the art the average residence time T can be calculated from equation (2) below:

T = — equation (2)

Qo wherein VR is the volume of the reaction space (in case of a loop reactor, the volume of the reactor, in case of the fluidized bed reactor, the volume of the fluidized bed)

Q_o is the volumetric flow rate of the product stream (including the polymer product and the fluid reaction mixture).

The production rate is suitably controlled with the catalyst feed rate. It is also possible to influence the production rate by suitable selection of the monomer concentration. The desired monomer concentration can then be achieved by suitably adjusting the propylene feed rate.

The first propylene polymer fraction (A1 ) is transferred to the second polymerization stage for producing the second propylene polymer fraction (A2) preferably in a gas phase polymerization step, i.e., carried out in a first gasphase reactor (GPR1 ). Any suitable gas phase reactor known in the art may be used, such as a fluidized bed gas phase reactor.

For gas phase reactors, the reaction temperature used will generally be in the range 60 to 115 °C (e.g. 70 to 1 10 °C), the reactor pressure will generally be in the range 10 to 25 barg, and the residence time will generally be 1 to 8 hours. The gas used will commonly be a non-reactive gas such as nitrogen or low boiling point hydrocarbons such as propane together with monomer.

A chain transfer agent (e.g., hydrogen) is typically added to the second polymerization stage for controlling the molecular weight.

In the second reactor, preferably the first gas-phase reactor (GPR1 ), a mixture of the first propylene polymer fraction (A1 ) and the second propylene polymer fraction (A2) is formed being the propylene copolymer (A).

The propylene copolymer (A) is transferred to the third polymerization stage for producing the propylene ethylene elastomer (B) in a third reactor, preferably a gas phase polymerization step, i.e., carried out in a second gas-phase reactor (GPR2). Any suitable gas phase reactor known in the art may be used, such as a fluidized bed gas phase reactor. The condition range described above for the first gas phase reactor are similarly used for the second gas phase reactor.

A preferred multistage process is the above-identified slurry-gas phase process, such as developed by Borealis and known as the Borstar® technology. In this respect, reference is made to the European patent applications EP-A-0887379 and EP-A-0517868.

The process may comprise one or more further polymerization stage(s)/step(s). Particularly preferred is a prepolymerization step, which precedes the polymerization step I.). The purpose of the prepolymerization is to polymerize a small amount of polymer onto the catalyst at a low temperature and/or a low monomer concentration. By prepolymerization, it is possible to improve the performance of the catalyst in slurry and/or modify the properties of the final polymer. The prepolymerization step is typically conducted in slurry.

Thus, the prepolymerization step may be conducted in a loop reactor. The prepolymerization is again preferably conducted in liquid propylene without the presence of an inert diluent, like a hydrocarbon diluent.

The temperature in the prepolymerization step is typically from 0 to 60 °C, preferably from 10 to 50 °C and more preferably from 15 to 40 °C.

The pressure is not critical and is typically from 1 to 150 barg, preferably from 40 to 80 barg.

The amount of monomer is typically such that from 0.1 to 1000 grams of monomer per one gram of solid catalyst component is polymerized in the prepolymerization step. As the person skilled in the art knows, the catalyst particles recovered from a continuous prepolymerization reactor do not all contain the same amount of prepolymer. Instead, each particle has its own characteristic amount which depends on the residence time of that particle in the prepolymerization reactor. As some particles remain in the reactor for a relatively long time and some for a relatively short time, then also the amount of prepolymer on different particles is different and some individual particles may contain an amount of prepolymer which is outside the above limits. However, the average amount of prepolymer on the catalyst typically is within the limits specified above.

The molecular weight of the prepolymer may be controlled by hydrogen as it is known in the art. Further, antistatic additives may be used to prevent the particles from adhering to each other or the walls of the reactor, as disclosed in WO-A- 96/19503 and WO-A-96/32420.

The catalyst components are preferably all introduced to the prepolymerization step when a prepolymerization step is present. However, where the solid catalyst component and the cocatalyst can be fed separately it is possible that only a part of the co-catalyst is introduced into the prepolymerization stage and the remaining part into subsequent polymerization stages. In such cases it is necessary to introduce as much co-catalyst into the prepolymerization stage that a sufficient polymerization reaction is obtained therein. It is understood within the scope of the invention, that the amount of polymer produced in the prepolymerization typically lies within 1 .0 to 5.0 wt.-%, based on the total weight of the heterophasic propylene composition.

It will be appreciated that the propylene polymers may contain standard polymer additives as described above.

After preparation, the obtained polymer is typically extruded and pelletized. The extrusion may be conducted in the manner generally known in the art, preferably in a twin screw extruder. One example of suitable twin screw extruders is a corotating twin screw extruder. Those are manufactured, among others, by Coperion or Japan Steel Works. Another example is a counter-rotating twin screw extruder. Such extruders are manufactured, among others, by Kobe Steel and Japan Steel Works. Before the extrusion at least part of the desired additives, as mentioned above, are preferably mixed with the polymer. The extruders typically include a melting section where the polymer is melted and a mixing section where the polymer melt is homogenized. Melting and homogenization are achieved by introducing energy into the polymer. Suitable level of specific energy input (SEI) is from about 150 to about 450 kWh/ton polymer, preferably from 175 to 350 kWh/ton.

Measurement methods

All parameters mentioned in the description of the invention or the examples were measured according to the methods described below.

Melt Flow Rate

The melt flow rate (MFR) was determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR is determined at 190 °C for PE and 230 °C for PP. The load under which the melt flow rate is determined is usually indicated as a subscript, for instance MFR2 is measured under 2.16 kg load (condition D).

Description of microstructure quantification by NMR spectroscopy

Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of the polymers.

Quantitative ¹³C{¹ H} NMR spectra were recorded in the molten-state using a Bruker Avance III 500 NMR spectrometer operating at 500.13 and 125.76 MHz for ¹ H and ¹³C respectively. All spectra were recorded using a ¹³C optimised 7 mm magic-angle spinning (MAS) probe head at 180°C using nitrogen gas for all pneumatics. Approximately 200 mg of material was packed into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz. This setup was chosen primarily for the high sensitivity needed for rapid identification and accurate quantification {klimke06, parkinson07, castignolles09}. Standard single-pulse excitation was employed utilising the NOE at short recycle delays of 3s {pollard04, klimke06} and the RS-HEPT decoupling scheme {fillip05,griffin07}. A total of 1024 (1 k) transients were acquired per spectra.

Quantitative ¹³C{¹ H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals. All chemical shifts are internally referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm and assignments done according {brandoliniOl , randall89, resconiOO}.

Characteristic signals corresponding to the incorporation of 1 -butene were observed and the comonomer content quantified in the following way. The amount isolated 1 -butene incorporated in PBP sequences was quantified using the integral of the aB2 sites at 43.6 ppm accounting for the number of reporting sites per comonomer:

B = laB2 I 2

If observed the amount consecutively incorporated 1 -butene in PBBP sequences was quantified using the integral of the aaB2 site at 40.6 ppm accounting for the number of reporting sites per comonomer:

BB = 2 * laaB2

In case of presence of consecutively incorporated 1 -butene (BB) the isolated incorporated butene (B) needs to be corrected due to influencing signals by subtracting BB/2.

Characteristic signals corresponding to different incorporations of ethylene were observed and the comonomer content quantified utilising following assignments and equations for the respective sequences:

The amount of P was quantified based on the Saa methylene site including additional propene units not covered by Saa:

P= Isacc + B + (0.5*BB) + E + (0.5*EE)

The total amount of comonomers was then calculated as follows:

Btotai = B + B B Etotai = E + E E + E E E Ptotai = P

B[mol%] = 100* Btotai I ( Btotai + Etotal + Ptotai)

E[mol%] = 100* Etotal I ( Btotai + Etotal + Ptotai)

P[mol%] = 100* Ptotai I ( Btotai + Etotal + Ptotai)

The weight percent comonomer incorporation was calculated from the mol%:

B [wt%] = 100*(B[mol%]*56.11 ) / ((E[mol%] *28.05) + (B[mol%] *56.1 1 ) + (P[mol%]*42.08))

E [wt%] = 100*(E[mol%]*28.05) / ((E[mol%] *28.05) + (B[mol%] *56.1 1 ) + (P[mol%]*42.08))

For e.g. xylene soluble fraction where butene was not observed the related equations collapse to the following:

P= Isacc + E + (0.5*E E)

The total amount of comonomer was then calculated as follows:

Etotal = E + E E + E E E Ptotai = P

E[mol%] = 100* Etotal I ( Etotal + Ptotai)

P[mol%] = 100* Ptotai I ( Etotal + Ptotai)

The weight percent comonomer incorporation was calculated from the mol%:

E [wt%] = 100*(E[mol%]*28.05) / ((E[mol%] *28.05) + (P[mol%]*42.08))

References klimke06 : Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys. 2006;207:382. parkinson07: Parkinson, M., Klimke, K., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys.

2007;208:2128. pollard04: Pollard, M., Klimke, K., Graf, R., Spiess, H.W., Wilhelm, M., Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004;37:813. filip05: Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239 griffin07: Griffin, J.M., Tripon, C., Samoson, A., Filip, C., and Brown, S.P., Mag. Res. in Chem. 2007 45, S1 , S198 castignolles09: Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M., Gaborieau, M., Polymer 50 (2009) 2373 resconiOO: Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253 randall89: J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201. brandoliniOl : A. J. Brandolini, D.D. Hills, “NMR spectra of polymers and polymer additives”, Marcel Deker Inc., 2000 abis86: L. Abis, Mackromol. Chem. 187, 1877-1886 (1986)

Molecular weight and molecular weight distribution

Molecular weight averages (Mz, Mw and M_n), Molecular weight distribution (MWD) and its broadness, described by polydispersity index, PDI= M_w/M_n (wherein M_n is the number average molecular weight and Mw is the weight average molecular weight) were determined by Gel Permeation Chromatography (GPC) according to ISO 16014-1 :2003, ISO 16014-2:2003, ISO 16014-4:2003 and ASTM D 6474- 12 using the following formulas:

For a constant elution volume interval AV,, where At, and Mi are the chromatographic peak slice area and polyolefin molecular weight (MW), respectively associated with the elution volume, Vi, where N is equal to the number of data points obtained from the chromatogram between the integration limits.

A high temperature GPC instrument, equipped with either infrared (IR) detector (IR4 or IR5) from PolymerChar (Valencia, Spain) or differential refractometer (Rl) from Agilent Technologies, equipped with 3x Agilent-PLgel Olexis and 1x Agilent- PLgel Olexis Guard columns was used. As the solvent and mobile phase 1 ,2,4- trichlorobenzene (TCB) stabilized with 250 mg/L 2,6-Di-tert-butyl-4-methyl- phenol) was used. The chromatographic system was operated at 160 °C and at a constant flow rate of 1 mL/min. 200 pL of sample solution was injected per analysis. Data collection was performed using either Agilent Cirrus software version 3.3 or PolymerChar GPC-IR control software.

The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with 19 narrow MWD polystyrene (PS) standards in the range of 0.5 kg/mol to 11500 kg/mol. The PS standards were dissolved at room temperature over several hours. The conversion of the polystyrene peak molecular weight to polyolefin molecular weights is accomplished by using the Mark Houwink equation and the following Mark Houwink constants:

Kps = 19 x 10-³ mL/g, aps = 0.655

K_PE = 39 x 10-³ mL/g, a_PE = 0.725

KPP = 19 x 10-³ mL/g, a_PP = 0.725 k third order polynomial fit was used to fit the calibration data.

All samples were prepared in the concentration range of 0.5 to 1 mg/ml and dissolved at 160 °C for 2.5 hours (PP) or 3 hours (PE) under continuous gentle shaking.

Flexural Modulus (FM)

The flexural modulus was determined in a 3-point-bending test at 23 °C according to ISO 178 on 80x10x4 mm³ test bars injection molded in line with ISO 1873-2.

Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC) analysis, melting temperature (Tm), melt enthalpy (H_m) and crystallization temperature (Tc)were measured with a TA Instrument Q200 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC was run according to ISO 11357 I part 3 /method C2 in a heat/cool/heat cycle with a scan rate of 10 °C/min in the temperature range of -30 to +225 °C. Crystallization temperature (T_c) was determined from the cooling step, while melting temperature (Tm) and melt enthalpy (Hm) were determined from the second heating step.

Throughout the present description, the term Tm is understood as peak temperature of melting as determined by DSC at a heating rate of 10 K/min.

Throughout the present description, the term T_c is understood as peak temperature of crystallization as determined by DSC at a cooling rate of 10 K/min. Notched Impact Strength (NIS)

The Charpy notched impact strength (NIS) was measured according to ISO 179 1 eA at +23 °C or at -20 °C, respectively, using injection molded bar test specimens of 80x10x4 mm³ prepared in accordance with ISO 1873-2.

Xylene soluble fraction

The xylene soluble fraction (XS) and xylene insoluble fraction (XI) were determined according to ISO 16152 at 25 °C.

Lower explosive limit (LEL)

The LEL determination method is based on gas chromatographic determination of volatile compounds in polyolefins with a static headspace technique directly from pellets and powders. Estimated lower explosive limit (LEL) can be calculated from volatile organic compounds, the respective calibration being based on external standard technique (ESTD).

The external standard is prepared by diluting n-octane to 0.1 -1 vol.-%. Based on the measurement results, a factor can be calculated for the n-octane in the reference material.

Rf(faktor ) = CIA

C = known concentration of the standard [mg]

A = the measured area of the peak

An average is calculated from the three determined factors, which is used to calculate the results. The data system automatically calculates the gas chromatograph analyzes with the parameters according to the calculation data, if it finds peaks at the right time intervals.

The volatile compounds contained in the sample [mg/kg] are calculated from the formula:

Volatile compounds [mg/kg] = (Sum of the peak areas of the sample Rf I Weight of the sample [m#]) x 1 000 000.

Examples

Preparation of the heterophasic polypropylene composition

Two heterophasic polypropylene compositions were prepared as Inventive Examples IE1 and IE2 according to the process of the invention in a multi-stage polymerization process as described above, under the conditions outlined in Table 1. The catalyst was a Ziegler-Natta catalyst prepared as described in EP- A-3562850. The solid catalyst component was used along with triethylaluminium (TEAL) as co-catalyst and dicyclopentyl dimethoxy silane (donor D) as external donor.

Additionally, Comparative Example CE1 was prepared in a process which did not comprise the third polymerization stage (i.e. in GPR2). Accordingly, Comparative

Example CE1 does not contain the propylene ethylene elastomer (B).

Table 1: Process conditions

The polymers prepared above were mixed with 0.25 wt.-% of Irganox B215 (a synergistic 2: 1 blend of antioxidants Irgafos 168 (tris(2,4- ditertbutylphenyl)phosphite, CAS No: 31570-04-4) and Irganox 1010 (pentaerythritol tetrakis[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionate], CAS No: 6683-19-8, commercially available from BASF SE, Germany) and 0.05 wt.- % of CEASIT Fl (Ca-stearate, CAS No: 1592-23-0, commercially available from Baerlocher GmbH, Germany), and then compounded in a twin screw extruder ZSK 18, with melt temperature of 200-220 °C and throughput of 7 kg/h.

Properties of the heterophasic polypropylene composition Properties of the polymer compositions were measured and are summarized in Table 2 below.

Table 2: Properties of the polymer compositions

It is noted that the polymer compositions prepared by the process according to the invention, i.e. Inventive Examples IE1 and IE2, have a much lower percentage of the lower explosive limit (LEL) than the composition prepared in Comparative Example CE1.

Table 3: Volatile compounds (VOC) determined for the polymer compositions

Claims

1 . A process for the preparation of a heterophasic polypropylene composition by a multi-stage polymerization process, comprising

I.) preparing a first propylene polymer fraction (A1 ) being a propylene copolymer of propylene with 1 -butene comonomer units in a content in the range of from 2.0 to 10.0 wt.-% and ethylene comonomer units in a content in the range of from 0.1 to 5.0 wt.-%, based on the total weight of the first propylene polymer fraction (A1 ) and determined by 1³C-NMR spectroscopy as described in the specification, in a first reactor, preferably a loop reactor, in a first polymerization stage in the presence of a Ziegler-Natta catalyst, wherein the first propylene polymer fraction (A1 ) has a melt flow rate MFR2, determined according to ISO 1133 at 230 °C, in the range of from 0.1 to 10.0 g/10 min;

II.) transferring the first propylene polymer fraction (A1 ) to a second polymerization stage and preparing a second propylene polymer fraction (A2) being a propylene copolymer of propylene with 1 -butene comonomer units and ethylene comonomer units in the presence of a Ziegler-Natta catalyst in a second reactor, preferably a first gas phase reactor (GPR1 ); wherein the first propylene polymer fraction (A1 ) and the second propylene polymer fraction (A2) together form a propylene copolymer (A) of propylene with 1 -butene comonomer units in a content in the range of from 4.0 to 15.0 wt.-% and ethylene comonomer units in a content in the range of from 0.5 to 2.5 wt.-%, based on the total weight of the propylene copolymer (A) and determined by ¹³C-NMR spectroscopy as described in the specification, wherein the propylene copolymer (A) has a melt flow rate MFR2, determined according to ISO 1133 at 230 °C, in the range of from 0.3 to 10.0 g/10 min; and

III.) transferring the propylene copolymer (A) to a third polymerization stage and preparing a propylene ethylene elastomer (B) as a third propylene polymer fraction in the presence of a Ziegler-Natta catalyst in a third reactor, preferably a second gas phase reactor (GPR2), to obtain the heterophasic polypropylene composition; wherein the heterophasic polypropylene composition has: a) a melt flow rate MFR2, determined according to ISO 1133 at 230 °C, in the range of from 0.5 to 10.0 g/10 min; b) a melting temperature Tm, determined according to differential scanning calorimetry (DSC) described in the specification, in the range of from 125 to 145 °C; and c) a content of xylene soluble fraction (XS), based on the total weight of the heterophasic polypropylene composition and determined at 25 °C according to ISO 16152, of from 10 to 45 wt.-%.

2. The process according to claim 1 , wherein the heterophasic polypropylene composition comprises, based on the total weight of propylene polymer components in the heterophasic polypropylene composition: i.) from 25 to 60 wt.-% of the first propylene polymer fraction (A1 ), ii.) from 30 to 50 wt.-% of the second propylene polymer fraction (A2) and iii.) from 1 to 40 wt.-% of the third propylene polymer fraction (B), wherein the sum of all three fractions amounts to 100 wt.-% of propylene polymer components in the heterophasic polypropylene composition.

3. The process according to claim 1 or claim 2, wherein the Ziegler-Natta catalyst comprises: a) compound(s) of a transition metal of Group 4 to 6 of IIIPAC; b) a Group 2 metal compound; c) an internal donor, wherein said internal donor is a non-phthalic compound, preferably is a non-phthalic acid ester; d) a co-catalyst; and e) optionally an external donor.

4. The process according to claim 3, wherein the internal donor is preferably selected from (di)esters of non-phthalic carboxylic (di)acids, 1 ,3-diethers, derivatives and mixtures thereof.

5. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition is free of phthalic acid esters and their respective decomposition products.

6. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition comprises, based on the total weight of propylene polymer components in the heterophasic polypropylene composition: i.) from 30 to 55 wt.-% of the first propylene polymer fraction (A1 ), ii.) from 35 to 50 wt.-% of the second propylene polymer fraction (A2) and iii.) from 2 to 30 wt.-% of the third propylene polymer fraction (B), wherein the sum of all three fractions amounts to 100 wt.-% of propylene polymer components in the heterophasic polypropylene composition.

7. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition has: a) a melt flow rate MFR2, determined according to ISO 1133 at 230 °C, in the range of from 0.5 to 5.0 g/10 min; and/or b) a melting temperature Tm, determined according to differential scanning calorimetry (DSC) described in the specification, in the range of from 130 to 140 °C; and/or c) a content of xylene soluble fraction (XS), based on the total weight of the heterophasic polypropylene composition and determined at 25 °C according to ISO 16152, of from 12 to 35 wt.-%.

8. The process according to any one of the preceding claims, wherein the first propylene polymer fraction (A1 ) and/or the propylene copolymer (A) has/have a melt flow rate MFR2, determined according to ISO 1133 at 230 °C, in the range of from 0.5 to 3.0 g/10 min.

9. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition has a flexural modulus, determined according to ISO 178, in the range of from 250 to 550 MPa.

10. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition has an impact strength at +23 °C, determined according to ISO 179/1 eA, in the range of from 30.0 to 100.0 kJ/m², and/or an impact strength at -20 °C, determined according to ISO 179/1 eA, in the range of from 1.5 to 7.0 kJ/m².

11. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition has a crystallization temperature T_c, determined according to differential scanning calorimetry (DSC) described in the specification, in the range of from 90 to 100 °C.

12. The process according to any one of the preceding claims, wherein the heterophasic polypropylene composition has a percentage of the lower explosive limit (LEL), determined as described in the specification, in the range of from 1 to 15 vol.-%.

13. The process according to any one of the preceding claims, wherein the process further comprises a prepolymerization step that precedes the process step I.) and that is preferably performed in a loop reactor.

14. The process according to any one of the preceding claims, wherein the first polymerization stage of the process step I.) is a slurry polymerization, preferably performed in a liquid mixture of the monomers used in the polymerization reaction.

15. A heterophasic polypropylene composition obtainable by the process according to any one of the preceding claims.