WO2009077464A1

WO2009077464A1 - Process for the production of a bimodal polypropylene having low ash content

Info

Publication number: WO2009077464A1
Application number: PCT/EP2008/067458
Authority: WO
Inventors: Alain Standaert; Jérôme GROMADA
Original assignee: Total Petrochemicals Research Feluy
Priority date: 2007-12-14
Filing date: 2008-12-12
Publication date: 2009-06-25
Also published as: JP2011506666A; PL2220131T3; KR20100074338A; WO2009077460A1; US20140051315A9; US20110034645A1; BRPI0820910A2; CN101896511A; EA201000765A1; EP2220132A1; US20110086568A1; EP2220131B1; EP2220131A1; DK2220131T3; EP2070956A1

Abstract

The present invention relates to a process for making low ash propylene homo- or copolymers with improved properties, with 'ash' denoting aluminium as well as residues of catalyst, cocatalyst or any additive, such as titanium (Ti) and silicium (Si) derivatives, used in the production of propylene polymers. The propylene polymers of the present invention are useful to make films, such as capacitor films, as well as fibers and nonwovens, such as for example staple fibers, spunbond nonwovens or meltblown nonwovens.

Description

Process for the Production of a Bimodal Polypropylene Having Low Ash

Content

Field of the invention

The present invention relates to a process for making low ash propylene homo- or copolymers with improved properties, with "ash" denoting aluminium as well as residues of catalyst, cocatalyst or any additive, such as titanium (Ti) and silicium (Si) derivatives, used in the production of propylene polymers. The propylene polymers of the present invention are useful to make films, such as capacitor films, as well as fibers and nonwovens, such as for example staple fibers, spunbond nonwovens or meltblown nonwovens.

The technical problem and the prior art

Propylene homo- and copolymers are produced in the presence of

(a) a Ziegler-Natta catalyst comprising a titanium compound, which has at least one titanium-halogen bond, and an internal electron donor, both supported on a magnesium halide in active form,

(b) an organoaluminium compound, such as an aluminium alkyl compound, and

(c) an optional external electron donor (ED).

As internal electron donors, mention may be made of compounds selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids. Particularly suitable internal electron donors are succinates, diethers, such as 1 ,3-diethers, and phthalic acid esters, such as diethyl, diisobutyl, di-n-butyl, dioctyl, diphenyl and benzylbutyl phthalate. As the Ziegler-Natta catalyst, the organoaluminium compound and the optional external electron donor (ED) are not removed after the polymerization but rather left in the polymer, propylene polymers comprise residues of the catalytic system, such as aluminium (Al), titanium (Ti), magnesium (Mg) and chlorine (Cl). The total of these residues is called "ash".

High levels of ash in a propylene polymer may lead to plate-out and in consequence necessitate frequent cleaning of down-stream processing equipment, for example of a film or sheet extrusion line or of a fiber or nonwoven production line. In order to reduce the ash content the propylene polymer may be washed. However, such a washing process is energy intensive, as the polymer needs to be dried afterwards, and therefore also expensive.

EP-A-O 449 302 discloses a process for the production of polypropylene with less than 15 ppm of ash content. Such a polypropylene is for example particularly suitable for use in capacitor films. In said process, the catalyst is a Ziegler-Natta catalyst, the internal electron donor is 2-isopropyl-2-isoamyl-1 ,3- dimethoxypropane or 2-cyclohexyl-2-isopropyl-1 ,3-dimethoxypropane and the organoaluminium compound is Al-triisobutyl. There is no external electron donor. Operating conditions are as follows: the molar ratio Al/Ti is 30, the average residence time is 6 hours, polymerization is carried out at 70 °C in liquid propylene in a 950 I loop reactor, - propylene feed is 88.5 kg/h, polypropylene production is 46 kg/h, and the yield is 150 kg of polypropylene per g of catalyst, corresponding to 25 kg /g catalyst /h.

The resulting Al residue in the polypropylene is between 4.5 and 4.8 ppm. Due to the low yield and low productivity this process is not of commercial interest anymore. Polypropylenes having a low ash content are of interest in applications requiring very clean propylene polymers, for example in capacitors as a dielectric material. The capacity C of an electric plate capacitor is defined as

C = ε - A - d^"1 (eq. 1 ) with ε being the dielectric constant, which is determined by the material, A being the area of the conductive plates, and d being the distance between the conductive plates. In principle, the distance d between the conductive plates corresponds to the thickness of the dielectric material that may be selected in accordance with e.g. applied voltage and desired lifetime of the capacitor. In order to compensate for any faults in the dielectric material, such as a polypropylene film, one either has to use a film that is thicker than is necessary under eq. 1 or one has to use a number a thinner films on top of each other.

Capacitor manufacturers have a strong interest in reducing the distance d between the conductive plates in order to reduce the costs as well as the sizes of the capacitors; in other words the capacitor manufacturers are interested in having for example very thin polypropylene film. For these to be processable and at the same time to comply with the requirements for use in capacitors such polypropylene films need to have improved mechanical properties in combination with a low total ash content.

It is therefore an object of the present invention to provide propylene polymers that fulfill these requirements.

In particular, it is an object of the present invention to provide propylene polymers characterized by improved mechanical properties and a low ash content.

Further, it is an object of the present invention to provide propylene polymers having improved processability. Furthermore, it is an object of the present invention to provide propylene polymers characterized by improved mechanical properties, a low ash content and improved processability.

In particular, said films may be used as capacitor films. Or they may be used in packaging applications.

In particular, said fibers and nonwovens may be used in hygiene applications.

Brief description of the invention

We have now discovered a polymerization process that allows the production of propylene polymers that fulfill at least one of the above objectives.

Thus, the present invention provides a process for the production of propylene polymers with improved properties and a low ash content by polymerization of propylene and one or more optional comonomers in at least two sequential polymerization reactors in presence of (a) a Ziegler-Natta catalyst comprising a titanium compound having at least one titanium-halogen bond, and an internal electron donor, both supported on a magnesium halide in active form,

(b) triethyl aluminium,

(c) an optional external electron donor (ED), and (d) hydrogen, wherein the internal electron donor comprises at least 80 wt% of a diether, the molar ratio AI/TΪ is at most 40, the propylene polymers are recovered from the polymerization reactor, without any washing, as a powder and optionally converted to pellets, and wherein the propylene polymer fractions produced in the at least two of the sequential polymerization reactors differ in average molecular weight such that the ratio of the melt flow indices of the propylene polymer fractions with the highest and the lowest melt flow index is at least 10. In addition, the present invention provides propylene polymers obtained by said process. Said propylene polymers are further characterized by a low ash content and a broad molecular weight distribution. Preferably, said propylene polymers have a low xylene solubles content.

Further, the present invention provides films, fibers and nonwovens made with the propylene polymers produced by said process as well as the use of said films, fibers and nonwovens.

Detailed description of the invention

For the purpose of the present invention the term "ash" means aluminium as well as residues of catalyst, cocatalyst or any additive, such as Ti and Si derivatives, used in the production of propylene polymers.

For easier understanding the terms "diether catalyst", "succinate catalyst" and "phthalate catalyst" are used to denote a Ziegler-Natta catalyst with a diether compound as internal electron donor, resp. a Ziegler-Natta catalyst with a succinate compound as internal electron donor, resp. a Ziegler-Natta catalyst with a phthalate compound as internal donor.

For the purposes of the present invention the terms "propylene polymer" and "polypropylene" may be used interchangeably.

The propylene polymers of the present invention are homopolymers or random copolymers of propylene and one or more comonomers. Said one of more comonomers can be ethylene or a C₄-C₂O alpha-olefin, such as 1 -butene, 1 - pentene, 4-methyl-1 -pentene, 1 -hexene, 1 -octene. The random copolymers of the present invention comprise at least 0.1 wt%, preferably at least 0.2 wt%, and most preferably at least 0.5 wt% of comonomer(s). They comprise at most 2 wt% of comonomer(s). Preferably, the random copolymers are copolymers of propylene and ethylene.

The melt flow index of the propylene polymers of the present invention is in the range from 1 to 2000 dg/min as measured according to ISO 1 133, condition L, at 230°C with a load of 2.16 kg. If used for films the propylene polymers of the present invention preferably have a melt flow index in the range from 1 to 10 dg/min, more preferably in the range from 1 to 4 dg/min and most preferably in the range from 1 .5 to 4 dg/min. When used in fiber and nonwoven applications the propylene polymers of the present invention have a melt flow index in the range from 5 dg/min to 2000 dg/min (as measured according to ISO 1 133, condition L, at 230°C under 2.16 kg). When used for fiber spinning the melt flow of the propylene polymers is in the range from 5 dg/min to 40 dg/min. When used in the spunbonding process the melt flow of the propylene polymers is at least 10 dg/min, preferably at least 15 dg/min, and most preferably at least 20 dg/min. When used in the spunbonding process the melt flow of the propylene polymers is at most 300 dg/min, preferably at most 200 dg/min, more preferably at most 150 dg/min, even more preferably at most 100 dg/min and most preferably at most 60 dg/min. When used in the melt blown process the melt flow of the propylene polymers is at least 100 dg/min, preferably at least 150 dg/min, more preferably at least 200 dg/min, even more preferably at least 250 dg/min and most preferably at least 300 dg/min. When used in the melt blown process the melt flow of the propylene polymers is at most 2000 dg/min, preferably at most 1800 dg/min, more preferably at most 1600 dg/min, and most preferably at most 1400 dg/min.

The Ziegler-Natta catalyst comprises a titanium compound, which has at least one titanium-halogen bond, and an internal donor, both supported on magnesium halide in active form. The internal donor used in the present invention is a diether or a blend of a diether and one or more internal donors different from diether, provided that such a mixture shows polymerization behavior comparable to a Ziegler-Natta catalyst with only diether as internal donor. A mixture of internal donors could for example comprise a diether and a phthalate, or a diether and a succinate.

Alternatively to a Ziegler-Natta catalyst comprising a mixture of internal donors as described above it is also possible to employ a mixture of a diether catalyst and one or more Ziegler-Natta catalysts comprising an internal donor other than diether, provided that such a mixture shows a polymerization behavior that is comparable to that of a pure diether catalyst. For example, it is possible to employ a mixture of a diether catalyst and a phthalate catalyst or a mixture of a diether catalyst and a succinate catalyst.

Independently of whether one Ziegler-Natta catalyst with a mixture of internal donors or a mixture of Ziegler-Natta catalysts, each with a different electron donor, is used, the diether comprises at least 80 wt%, preferably at least 90 wt%, more preferably at least 95 wt%, and even more preferably at least 99 wt% of the total weight of the electron donors. It is most preferred that the internal donor essentially consists of a diether.

Ziegler-Natta catalysts comprising a diether as internal donor are well known in the art and can for example be obtained by reaction of an anhydrous magnesium halide with an alcohol, followed by titanation with a titanium halide and reaction with a diether compound as internal donor. Such a catalyst comprises about 2 - 6 wt% of titanium, about 10 - 20 wt% of magnesium and about 5 - 30 wt% of internal donor with chlorine and solvent making up the remainder.

Particularly suited as internal donors are 1 ,3-diethers of formula

R¹ R²C(CH₂OR³XCH₂OR⁴) wherein R¹ and R² are the same or different and are CrCi₈ alkyl, C₃-Ci₈ cycloalkyl or C₇-Ci₈ aryl radicals; R³ and R⁴ are the same or different and are CrC₄ alkyl radicals; or are the 1 ,3-diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6 or 7 carbon atoms and containing two or three unsaturations. Ethers of this type are disclosed in published European patent applications EP-A-O 361 493 and EP-A-O 728 769. Representative examples of said diethers are 2-methyl-2-isopropyl-1 ,3- dimethoxypropane; 2,2-diisobutyl-1 ,3-dimethoxypropane; 2-isopropyl-2-cyclo- pentyl-1 ,3-dimethoxypropane; 2-isopropyl-2-isoamyl-1 ,3-dimethoxypropane; 9,9-bis(methoxymethyl)fluorene.

Suitable succinate compounds have the formula

wherein R¹ to R⁴ are equal to or different from one another and are hydrogen, or a CrC₂O linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms, and R¹ to R⁴, being joined to the same carbon atom, can be linked together to form a cycle; and R⁵ and R⁶ are equal to or different from one another and are a linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms.

Suitable phthalates are selected from the alkyl, cycloalkyl and aryl phthalates, such as for example diethyl phthalate, diisobutyl phthalate, di-n-butyl phthalate, dioctyl phthalate, diphenyl phthalate and benzylbutyl phthalate. Ziegler-Natta catalysts comprising a diether, a succinate or a phthalate as internal donor are commercially available for example from Basell under the Avant ZN trade name.

In the polymerization process of the present invention the external electron donor (ED) is optional. It is nevertheless preferred to perform the polymerization in presence of an external electron donor (ED). Suitable external electron donors (ED) include certain silanes, ethers, esters, amines, ketones, heterocyclic compounds and blends of these. It is preferred to use a 1 ,3-diether as described above or a silane. It is most preferred to use silanes of the general formula

R p R _qSi(OR )(₄-p-q) wherein R^a, R^b and R^c denote a hydrocarbon radical, in particular an alkyl or cycloalkyl group, and wherein p and q are numbers ranging from 0 to 3 with their sum p + q being equal to or less than 3. R^a, R^b and R^c can be chosen independently from one another and can be the same or different. Specific examples of such silanes are (tert-butyl)₂Si(OCH₃)₂, (cyclohexyl)(methyl) Si(OCHs)₂ (referred to as "C donor"), (phenyl)₂Si(OCH₃)₂ and (cyclopentyl)₂ Si(OCHs)₂ (referred to as "D donor").

The organoaluminium compound used in the process of the present invention is triethyl aluminium (TEAL). Advantageously, the triethyl aluminium has a hydride content, expressed as AIH₃, of less than 1 .0 wt% with respect to the triethyl aluminium. More preferably, the hydride content is less than 0.5 wt%, and most preferably the hydride content is less than 0.1 wt%. It would not depart from the scope of the invention if the organoaluminium compound contains minor amounts of other compounds of the trialkylaluminium family, such as triisobutyl aluminium, tri-n-butyl aluminium, and linear or cyclic alkyl aluminium compounds containing two or more Al atoms, provided they show polymerization behavior comparable to that of TEAL. In the process of the present invention the molar ratio Al/Ti is at most 40, preferably it is in the range from 10 to 40, and more preferably it is in the range from 15 to 35.

If an external electron donor (ED) is present, it is preferred that the molar ratio AI/ED is at most 120, more preferably it is in the range from 5 to 120, and most preferably in the range from 5 to 80.

Before being fed to the first polymerization reactor the catalytic system preferably undergoes a premix and/or a pre-polymerization step. In the premix step, the triethyl aluminium (TEAL) and the external electron donor (ED) - if present -, which have been pre-contacted, are mixed with the Ziegler-Natta catalyst at a temperature in the range from 0°C to 30⁹C, preferably in the range from 5°C to 20°C, for up to 15 min. The mixture of TEAL, external electron donor and Ziegler-Natta catalyst is pre-polymerized with propylene at a temperature in the range from 10°C to 100°C, preferably in the range from 10⁹C to 30 °C, for 1 to 30 min, preferably for 2 to 20 min.

The polymerization of propylene and one or more optional comonomers can be carried out according to known techniques. The polymerization can for example be carried out in liquid propylene as reaction medium. It can also be carried out in a diluent, such as a hydrocarbon that is inert under polymerization conditions (slurry polymerization). It can also be carried out in the gas phase.

For the present invention propylene homopolymers and random copolymers are preferably produced by polymerization in liquid propylene at temperatures in the range from 20⁹C to 100°C. Preferably, temperatures are in the range from 60⁹C to 80 °C. The pressure can be atmospheric or higher. Preferably the pressure is between 25 and 50 bar.

Hydrogen is used to control the chain lengths of the propylene polymers. For the production of propylene polymers with higher MFI, i.e. with lower average molecular weight and shorter polymer chains, the concentration of hydrogen in the polymerization medium needs to be increased. Inversely, the hydrogen concentration in the polymerization medium has to be reduced in order to produce propylene polymers with lower MFI, i.e. with higher average molecular weight and longer polymer chains.

For the present invention it is essential that the production of the propylene polymers is done in at least two sequential polymerization reactors, i.e. at least two polymerization reactors in series, wherein the propylene polymer fractions produced in the at least two of the sequential polymerization reactors differ in average molecular weight and melt flow index. Expressed differently, the propylene polymers of the present invention comprise at least two propylene polymer fractions, with each of said propylene polymer fractions being produced in a separate polymerization reactor. The ratio of the melt flow indices of the propylene polymer fractions with the highest and the lowest melt flow index is at least 10. The resulting propylene polymer is therefore characterized by a broader molecular weight distribution (MWD) or higher polydispersity index (Pl).

It is clear to the person skilled in the art that the production of the propylene polymers can be done for example in three, four or five sequential polymerization reactors. However, it is preferred that the production of propylene polymers is done in two or three sequential polymerization reactors, for which case it is preferred that the contribution of the first polymerization reactor is in the range from 35 wt% to 65wt% of the total weight of the propylene polymer. In the case of three sequential polymerization reactors it is preferred to have two sequential loop polymerization reactors followed by a gas- phase polymerization reactor.

The propylene polymers produced according to the present invention are characterized by a polydispersity index (Pl) in the range from 5 to 7. The polydispersity index (Pl) has first been described by Zeichner and Patel in the Proceedings of the 2^nd World Congress of Chemical Engineering, Montreal, Canada, 6, 373 (1981 ). The polydispersity index (Pl) is defined as

with Gc, which is expressed in Pa, being the modulus at the intersection of the storage modulus G' and loss modulus G", i.e. G_c is the modulus when G' = G". In the literature this intersection is often called "the cross-over point". Storage modulus G' and loss modulus G" can be obtained for a molten polymer sample from dynamic rheology measurement.

Propylene, one or more optional comonomers and hydrogen are fed to each polymerization reactor. In order to obtain a propylene polymer having the necessary polydispersity index, the hydrogen to propylene feeding ratios to the reactors are adjusted. Polymerization catalyst, triethyl aluminium (TEAL) and the optional external electron donor (ED), all of which may optionally have been premixed and/or pre-polymerized, are fed to the first of the sequential polymerization reactors only

Polymerization medium comprising the propylene polymer is withdrawn from the polymerization reactor using suitable means for withdrawal and is fed to the subsequent polymerization reactor. From the last of the sequential polymerization reactors, the propylene polymer is recovered without any washing as a powder and is optionally converted to pellets in a peptization step.

The residence time in each polymerization reactor is at most 2 hours, more preferably at most 1 .5 hours, and most preferably at most 1 .25 hours. The residence time in the polymerization reactor is at least 0.25 hours, preferably at least 0.5 hours, and most preferably at least 0.75 hours.

The productivity of the polymerization catalyst is equal to or higher than 30 kg of propylene polymer per g catalyst. Preferably, it is higher than 35 kg, more preferably higher than 40 kg, even more preferably higher than 45 kg of propylene polymer per g catalyst.

The propylene polymers of the present invention are characterized by a low titanium content in combination with a low aluminium content.

The titanium content of the propylene polymers of the present invention is at most 2 ppm, preferably at most 1 .5 ppm, more preferably at most 1 .25 ppm and most preferably at most 1 ppm.

The propylene polymers of the present invention comprise at most 30 ppm of aluminium, preferably at most 25 ppm, more preferably at most 20 ppm, even more preferably at most 15 ppm, and most preferably at most 10 ppm.

The propylene polymers of the present invention comprise at most 15 ppm of chlorine, more preferably at most 12 ppm and most preferably at most 10 ppm.

The propylene polymers of the present invention comprise at most 5 ppm of magnesium, more preferably at most 4 ppm and most preferably at most 3 ppm.

The propylene polymers of the present invention comprise a total ash content of at most 50 ppm, preferably of at most 45 ppm, more preferably of at most 40 ppm, and most preferably of at most 35 ppm.

The propylene polymers of the present invention are characterized by a very low xylene solubles content of at most 2.5 wt% of the total propylene polymer, preferably of at most 2.4, 2.3, 2.2 or 2.1 wt% and most preferably of at most 2.0 wt%, relative to the total weight of the propylene polymer.

It has been surprisingly found that propylene polymers characterized by such a low level of xylene solubles can be produced with good productivity. It has also come as a surprise that the propylene polymers of the present invention are characterized by good processability and good mechanical properties of the final articles, such as films, fibers and nonwovens.

The propylene polymers of the present invention are characterized by very high isotacticity, for which the content of mmmm pentads is a measure. The content of mmmm pentads is at least 97.0 %, preferably at least 97.5 % and most preferably at least 98.0 %. The isotacticity is determined by NMR analysis according to the method described by GJ. Ray et al. in Macromolecules, vol. 10, n° 4, 1977, p. 773-778.

The propylene polymers of the present invention may contain additives such as, by way of example, antioxidants, light stabilizers, acid scavengers, lubricants, antistatic additives, nucleating/clarifying agents, colorants. An overview of such additives may be found in Plastics Additives Handbook, ed. H. Zweifel, 5^th edition, 2001 , Hanser Publishers.

The propylene polymers of the present invention are specifically suited for film applications, such as cast films, blown films, bioriented films. Such films in turn are well-suited for packaging applications. The propylene polymers of the present invention are particularly suited for capacitor films.

When films are made with the propylene polymers of the present invention it is preferred that such films comprise the propylene polymer of the present invention in at least 50 wt%, more preferably in at least 70 wt% or 80 wt% or 90 wt%, even more preferably in at least 95.0 wt% or 97.0 wt% and still even more preferably in at least 99.0 wt%, relative to the total weight of the film. It is most preferred that the film consists of the propylene polymers of the present invention.

For use in film applications the preferred polymer is a homopolymer. Preferably it is characterized by a melt flow in the range from 1 to 10 dg/min, preferably in the range from 1 to 4 dg/min and most preferably in the range from 1.5 to 4 dg/min. Further the preferred polymer for film applications is characterized by a xylene solubles fraction of at most 2.5 wt%, preferably of at most 2.2 wt%, more preferably of at most 2.1 wt% and most preferably of at most 2.0 wt%, relative to the total weight of the propylene polymer. Further it is characterized by low contents in chlorine, magnesium, aluminium, titanium and total ash as described before.

The propylene polymers of the present invention are specifically suited for fiber and nonwoven applications, such as staple fibers, spunbond nonwovens, meltblown nonwovens. Staple fibers in turn can be used for making thermal bonded nonwovens. Thermal bonded nonwovens and spunbond nonwovens can be used in hygiene applications, such as diapers or feminine hygiene articles, in construction applications or geotextiles; potentially in combination with one or more meltblown nonwovens. Meltblown nonwovens are particularly suited for filter applications.

When fibers and nonwovens are made with the propylene polymers of the present invention it is preferred that such fibers and nonwovens comprise the propylene polymer of the present invention in at least 50 wt%, more preferably in at least 70 wt% or 80 wt% or 90 wt%, even more preferably in at least 95.0 wt% or 97.0 wt% and still even more preferably in at least 99.0 wt%, relative to the total weight of the fiber or nonwoven. It is most preferred that the fiber or nonwoven consists of the propylene polymers of the present invention.

For use in fiber and nonwoven applications the preferred propylene polymer is a homopolymer.

Examples

The following examples show the advantages of the present invention. Propylene polymers according to the present invention as well as comparative propylene polymers were produced according to the following pilot plant polymerization procedure:

Triethyl aluminium (TEAL) as solution in hexane and cyclohexylmethyldimethoxysilane (donor C) or di-cyclopentyl-di-methoxysilane (D donor) as solution in hexane were pre-contacted for about 1 min at room temperature, followed by addition of the polymerization catalyst in form of an oily slurry with 17 g of catalyst per liter of slurry. The resulting blend was mixed at room temperature for about 5 min and injected into a prepolymerization loop reactor, which was kept at a temperature of 15°C. The pre-polymerized catalytic system was then fed into the first of two serially connected 150 I loop reactors thermoregulated at given temperatures, to which also propylene and hydrogen in quantities sufficient to obtain the targeted MFIs were added continuously. The Al/Ti molar ratio was kept in the range from 25 to 35. The propylene flow rate was regulated in such a way that the polymer concentration in the reactor was kept constant with the density of the polymer slurry in the reactor being higher than 0.40 kg/I. The average residence time in the reactors was from 70 to 90 minutes (In an industrial propylene polymerization plant the residence time would be shorter than this.).

The polymerization catalysts used herein are commercially available from Basell. Avant ZN126 is a Ziegler-Natta catalyst containing 3.5 wt% titanium, 14.4 wt% magnesium and a diether compound as internal donor. Avant ZN 127 is a Ziegler-Natta catalyst containing 3.2 wt% titanium, 13.0 wt% magnesium and a diether compound as internal donor.

The propylene polymers produced in the polymerization pilot plant were additivated with a sufficient amount of antioxidants and acid scavengers to reduce their degradation during further processing. The additivated propylene polymers were pelletized using a melt extruder. The melt flow index (MFI) was measured according to ISO 1 133, condition L, at 230⁰C with a load of 2.16 kg.

The melt flow index (MFI) produced in the second reactor is calculated using : Log(MFI,,nai) = Wi • L-Og(MFh) + W₂ • L-Og(MFI₂), wherein MFh and MFI₂ are the melt flow indices of the propylene polymer fractions produced in the first resp. the second polymerization loop reactor, and W₁ and w₂ are the respective weight fractions of the propylene polymer fractions produced in the first resp. the second polymerization loop reactor as expressed in wt% of the total propylene polymer produced in the two polymerization loop reactors. These weight fractions are commonly also described as the contribution by the respective loop.

Xylene solubles (XS) were determined as follows: Between 4.5 and 5.5 g of propylene polymer were weighed into a flask and 300 ml xylene were added. The xylene was heated under stirring to reflux for 45 minutes. Stirring was continued for 15 minutes exactly without heating. The flask was then placed in a thermostated bath set to 25 °C +/- 1 °C for 1 hour. The solution was filtered through Whatman n° 4 filter paper and exactly 100 ml of solvent were collected. The solvent was then evaporated and the residue dried and weighed. The percentage of xylene solubles ("XS"), i.e. the percentage of the xylene soluble fraction, was then calculated according to

XS (in wt%) = (Weight of the residue / Initial total weight of PP) ^* 300 with all weights being in the same unit, such as for example in grams.

The aluminium, magnesium and titanium contents of the propylene polymer were determined by an inductively coupled plasma technique with atomic emission spectroscopy using a polymer sample of 1 O g. The aluminium, magnesium or titanium contents ("Al", "Mg" or "Ti" in tables 2A and 2B) are given in ppm based on the total weight of the propylene polymer. Total ash content is measured as follows: 10 g of a PP sample is charred in a platinum crucible till total carbon disappearance. After cooling, the crucible is weighed and the amount of ash determined by difference and reported to 10 g. Ash content is expressed in ppm.

Chlorine content in PP is determined on a 10 g sample by a calibrated WD-XRF method. It is given in ppm.

Molecular weight distribution is determined by Size Exclusion Chromatography (SEC) at high temperature (145⁰C). A 10 mg PP sample is dissolved at 160⁰C in 10 ml of TCB (technical grade) for 1 hour. The analytical conditions for the Alliance GPCV 2000 from WATERS are :

- Volume: +/- 400μl

- Injector temperature: 140⁰C - Column and detector: 145⁰C

- Column set: 2 Shodex AT-806MS and 1 Styragel HT6E

- Flow rate: 1 ml/min

- Detector: Refractive index

- Calibration: Narrow standards of polystyrene - Calculation: Based on Mark-Houwink relation (log(M_PP) = log(M_PS) -

0.25323 )

The polymer index (Pl) is given as Pl = 10⁵ Pa ^■ G_c ^~1. G_c is the cross-over modulus in Pascal determined at 230⁰C using a dynamic rheometer in frequency sweep with a strain of 20% on an ARES from Tainstrument, branch Of WATERS.

The isotacticity (mmmm %) is determined by NMR analysis according to the method described by GJ. Ray et al. in Macromolecules, vol. 10, n° 4, 1977, p. 773-778. It is performed on the dried product resulting of the extraction by boiling heptane of the xylene insoluble PP fraction. For this, the xylene insoluble fraction as obtained above is ground up into small bits, of which 2 g are weighed into a Soxhlet cartridge. The extraction is then performed in a Soxhlet apparatus for 15 hours with heptane as solvent. The heptane insoluble fraction is recovered from the Soxhlet cartridge and dried in air for a minimum of 4 days. Said heptane insoluble fraction may then be used for the determination of the isotacticity by NMR.

The pelletized propylene polymers were used for the production of a monolayer cast film having a thickness of about 0.4 mm on a Welex 1 .25" (ca. 3.2 cm) extruder with a 10" (ca. 25.4 cm) die cast film line.

After aging for 1 week, squares of 85 mm by 85 mm were cut from the so- produced cast film. They were then stretched on a KARO IV laboratory stretcher manufactured by Bruckner Maschinenbau GmbH. The squares were stretched at a temperature of 145°C after a preheating time of 25 s. Simultaneous stretching ratio at 30 m/min in machine direction was 6, in cross-machine direction 6.

The stretched film samples were tested for 1 % secant modulus according to ASTM D882-02 at room temperature.

Comparative examples 1 to 5

Comparative examples 1 to 5 were produced with Avant ZN 126 resp. ZN 127 as indicated in table 1 A. All of the comparative examples 1 to 5 were produced with the melt flow indices of the propylene polymers produced in the two loop reactors being essentially the same. Further, comparative examples 1 to 2 were produced with a Al/Ti ratio higher than required by the present invention. Polymerization conditions are given in table 1 A, with catalyst productivity given in g of propylene polymer per g catalyst. For the production of these samples the polymerization pilot plant was run in such a way that the melt flow indices of the propylene polymers produced in the two loop reactors were within error of measurement the same, i.e. the molecular weight distribution of the propylene polymer being recovered after the second polymerization reactor was monomodal. Properties of the propylene polymers are given in table 2A.

Examples 1 to 2 and comparative examples 6 to 8 Examples 1 to 3 and comparative examples 6 to 7, which have a AI/TΪ ratio higher than required by the present invention, were produced with Avant ZN126 resp. ZN127 as indicated in table 1 B. Polymerization conditions are given in table 1 B, with catalyst productivity given in g of propylene polymer per g catalyst. For the production of these samples the polymerization pilot plant was run in such a way that the melt flow indices of the propylene polymers produced in the two loop reactors were different, i.e. the molecular weight distribution of the propylene polymer being recovered after the second polymerization reactor was bimodal. Properties of the propylene polymers are given in table 2B.

The results in tables 2A and 2B show the advantages of the propylene polymers produced in accordance with the present invention. Compared to the propylene polymers of comparative examples 1 to 8, which were produced in accordance with prior art polymerization conditions, the propylene polymers of examples 1 and 2, which were produced in accordance with the present invention, show a very low content of total ash, a low level of residual aluminium and at the same time excellent mechanical properties. For the mechanical properties the increase in secant modulus has been particularly surprising.

Further, it has been very surprising that the productivity of the polymerization catalyst can be maintained in two sequential polymerization reactors, even when performing the polymerization using the low TEAL/propylene ratio of the present invention.

Even more surprising the propylene polymer produced according to the present invention are characterized by excellent extrudability as compared to propylene polymers produced in accordance with the prior art. The comparison of the extrusion pressure at 250 °C in tables 2A and 2B clearly demonstrates this advantage.

Table 1 B

Table 2A

Claims

1. Process for the production of propylene polymers with improved properties and a low ash content by polymerization of propylene and one or more optional comonomers in at least two sequential polymerization reactors in presence of

(a) a Ziegler-Natta catalyst comprising a titanium compound having at least one titanium-halogen bond, and an internal electron donor, both supported on a magnesium halide in active form, (b) triethyl aluminium,

(c) an optional external electron donor (ED), and

(d) hydrogen, wherein the internal electron donor comprises at least 80 wt% of a diether, the molar ratio AI/TΪ is at most 40, the propylene polymers are recovered from the polymerization reactor, without any washing, as a powder and optionally converted to pellets, and wherein the propylene polymer fractions produced in the at least two of the sequential polymerization reactors differ in average molecular weight such that the ratio of the melt flow indices of the propylene polymer fractions with the highest and the lowest melt flow index is at least 10.

2. Process according to claim 1 , wherein there is an external electron donor (ED).

3. Process according to claim 2, wherein the molar ratio AI/ED is at most 120.

4. Process according to claim 2 or claim 3, wherein the external electron donor (ED) comprises a silane.

5. Process according to any of the preceding claims, wherein the polymerization of propylene and one or more optional comonomers is done in two sequential polymerization reactors.

6. Process according to claim 5, wherein the contribution of the first polymerization reactor is in the range from 35 wt% to 65 wt% of the total weight of propylene polymer.

7. Propylene polymer made according to the process of claims 1 to 6.

8. Propylene polymer according to claim 7 comprising at most 2 ppm of titanium.

9. Propylene polymer according to claim 7 or claim 8 comprising a total ash content of at most 50 ppm.

10. Propylene polymer according to any of claims 7 to 9 comprising at most 2.5 wt% of xylene solubles, relative to the total weight of the propylene polymer.

1 1 . Propylene polymer according to any of claims 7 to 10 having a polydispersity index (Pl) in the range from 5 to 7.

12. Propylene polymer according to any of claims 7 to 1 1 having an isotacticity of at least 97.0 % mmmm.

13. Film comprising the propylene polymer of any of claims 7 to 12.

14. Fiber or nonwoven comprising the propylene polymer of any of claims 7 to 12.

15. Use of the film of claim 13 as capacitor film.