Field of the invention

The present application discloses a process for polymerizing propylene or copolymerizing propylene and one or more comonomers in presence of a metallocene-based polymerization catalyst comprising a specifically selected metallocene to produce a polypropylene having improved elongational properties. Further, the present application discloses such a polypropylene having improved elongational properties.

The technical problem and the prior art

Polypropylene offers good optical, chemical, thermal and mechanical properties in combination with low cost. This is the reason for polypropylene having become one of the most widely used commodity polymers. With regards to optical properties, polypropylene is characterized by good transparency, which can further be enhanced by the addition of a nucleating or clarifying agent. With regards to chemical properties, polypropylene shows good resistance to a wide range of chemicals, thus rendering it one of the materials of choice for the packaging of chemicals or cleaning products. With regards to thermal properties, polypropylene is characterized by a melting temperature well above that of water, thus rendering it suitable for use in hot-fill applications or microwave cooking. With regards to mechanical properties, polypropylene is characterized by good stiffness and - through the addition of rubber - impact strength both at ambient as well as freezing temperatures. All of these properties as well as the described uses are purely given as examples; actually many more uses than the ones described can be envisaged.

In addition, polypropylene has the advantage of being easily processed in a wide range of transforming technologies, such as for example in injection molding, extrusion blow molding, injection blow molding, injection stretch blow molding, fiber extrusion, film and sheet extrusion, pipe extrusion, and thermoforming. At least to some extent, the good properties of polypropylene are due to its semicrystalline nature. However, polypropylene has poor elongational properties. This is most evident in transforming technologies requiring melt strength, such as for example blow film molding, extrusion blow molding or thermoforming, to name a few only.

In view of continuing efforts in the transformation industry to downgauge, i.e. to reduce the weight of the resulting polypropylene articles, there is a need for polypropylenes having improved elongational properties, thus at the same time allowing for improved processability in the transformation process. Considering for instance blown film applications, improved elongational properties induces a film thickness repartition improvement. Several properties associated to the film thickness such as bubble stability and resistance to perforation are thereby improved. An improvement of the film thickness repartition induces a reduction of thin parts of the film. Such improvement could be used to reduce the mean thickness of the film while keeping good properties. As a whole, this induces downgauging.

Hence, it is an objective of the present application to disclose a polypropylene having improved elongational properties.

It is a further objective of the present application to disclose a polypropylene that can be produced easily in existing polymerization plants. It is also an objective of the present application to disclose a polymerization process yielding such a polypropylene.

Brief description of the invention

Any of these objectives can be attained either singly or in any combination by the polypropylene as defined below and by the following process for polymerizing propylene or copolymerizing propylene and one or more comonomers in presence of a metallocene-based polymerization catalyst comprising a specifically selected metallocene. Thus, the present application provides a polypropylene comprising at least 95.0 wt% propylene, relative to the total weight of said polypropylene, said polypropylene having

a) a ¹³ C-NM R spectrum, recorded in l,2,4-trichlorobenzene/C ₆ D ₆ at 130 °C, wherein essentially no peaks at δ 44.88 ppm, 44.74 ppm, 44.08 ppm and 31.74 ppm are present, said ¹³ C-NMR spectrum is determined as described in the test methods, and

b) a value of G' of at least 10 Pa at a value of G" of 100 Pa, (G'(G"= 100 Pa) > 10 Pa) with G' and G" being determined as disclosed in the test methods.

Further, the present application provides a process for producing a polypropylene according to an embodiment of the present invention. Said process comprises the step of polymerizing propylene or copolymerizing propylene and at least one comonomer in presence of a metallocene-based polymerization catalyst comprising a metallocene of general formula (I) or (II)

^ ² α(3-^-5-^-α ₅ Η ₂ )(3,6-Ρ ⁵ -ΡΙυ)ΜΧ ₂ (I)

^ ² α(3-^-5-^-α ₅ Η ₂ )(2,7-Ρ ⁵ -ΡΙυ)ΜΧ ₂ (Π) wherein

- R ¹ is hydrogen or a hydrocarbyl group having from 1 to 20 carbon atoms,

- R ² , R ³ , R ⁴ and R ⁵ are each, independently from one another, a hydrocarbyl group having from 1 to 20 carbon atoms,

- M is selected from the group consisting of titanium, zirconium and hafnium, and

- X is selected from the group consisting of F, CI, Br, I, and hydrocarbyl groups having from 1 to 10 carbon atoms.

Furthermore, the present application provides for the use of such a metallocene to produce the above polypropylene. Brief description of the drawings

Figure 1 shows a plot of G' as x-value versus G" as y-value for the polypropylene samples of Comparative Example 1, Example 1 and Example 2, with G' and G" given in Pa.

Figure 2 shows ¹³ C-NMR spectra obtained as described in the test method. A: ¹³ C- NMR spectrum of example 1 (PP2), B: ¹³ C-NMR spectrum of example 2 (PP3) and C: ¹³ C-NMR spectrum of a long chain branched isotactic polypropylene similar to the one described in the publication of Weng et al. (macromolecules 35, 3838 (2002)).

Detailed description of the invention

The polypropylene of the present application comprises at least 95.0 wt%, preferably 96.0 wt%, more preferably 97.0%, most preferably 98.0 wt% of propylene, relative to the total weight the monomeric units of which said polypropylene consists. More preferably, the polypropylene comprises at least 99.0 wt% of propylene. Most preferably, the polypropylene consists only of propylene, i.e. is a propylene homopolymer.

In case the polypropylene is not a propylene homopolymer, the polypropylene comprises at least one, preferably a single, comonomer in such an amount that the weight percentages of propylene and comonomer(s) add up to 100 %. Preferably the at least one comonomer is an alpha-olefin having from 1 to 10 carbon atoms. More preferably, the at least one comonomer is selected from the group consisting of ethylene, butene-1, pentene-1, hexene-1 and 4-methyl- pentene-1. Even more preferably, the at least one comonomer is selected from the group consisting of ethylene, butene-1 and hexene-1. The most preferred comonomer is ethylene.

The present polypropylene has a ¹³ C-NMR spectrum (recorded in 1,2,4- trichlorobenzene/C ₆ D ₆ at 130 °C), wherein essentially no peaks at δ 44.88 ppm, 44.74 ppm, 44.08 ppm and 31.74 ppm are present. The ¹³ C-NMR spectrum may be determined as described in the test methods. The term "essentially no peaks" is meant to understand that the intensity of these peaks is below the detection limit. The present polypropylene is further characterized by a value of G' of at least 10 Pa at a value of G" of 100 Pa (G'(G"= 100 Pa) > 10 Pa), with G' and G" being determined as disclosed in the test methods. Preferably, said polypropylene has a value of G' of at least 10 Pa, preferably at least 20 Pa, more preferably at least 50 Pa at a value of G" of 100 Pa. Said polypropylene has a value of G' of at most 1000, preferably at most 800, more preferably at most 500 Pa, even more preferably at most 300 Pa and at most 100 Pa at a value of G" of 100 Pa.

The present polypropylene exhibit rheological features, namely G' and G" values, highlighting an elasticity increase at low frequency compared to linear polypropylene. This is classically ascribed to the presence of long chain branching structures. Said long chain branching structures are however not detectable by ¹³ C- NMR spectrum. Hence, the polypropylene according to an embodiment of the invention has a new molecular architecture.

Preferably, the present polypropylene has a weight average molecular weight M _w of at least 100000 g mol ^"1 , preferably at least 110000 g mol ¹ , more preferably at least 115000 g mol ^"1 , with M _w determined by size exclusion chromatography, frequently also referred to as gel permeation chromatography (GPC), as disclosed in the test methods.

Preferably, the present polypropylene has a molecular weight distribution, defined as the ratio M _w /M _n of weight average molecular weight M _w to number average molecular weight M _n , of at least 1.0 and of at most 3.5, preferably at least 1.5 and at most 3.3, more preferably at least 1.8 and at most 3 with M _n and M _w determined as disclosed in the test methods.

Preferably, the present polypropylene has an isotacticity of at least 90 % mmmm pentads, preferably at least 95% mmmm pentads, more preferably at least 97% mmmm pentads, most preferably at least 99% mmmm pentads with the percentage of mmmm pentads determined as disclosed in the test methods.

The present polypropylene may be produced by polymerizing propylene or copolymerizing propylene and at least one comonomer in presence of a metallocene-based polymerization catalyst comprising a specifically selected metallocene. Preferably, the metallocene-based polymerization catalyst comprises a metallocene and an activating agent, or alternatively the metallocene-based polymerization catalyst may comprise a metallocene, an activating agent and a support. Such metallocene-based polymerization catalysts are generally known in the art. This metallocene used in the present application comprises a "bridge", a substituted cyclopentadienyl ligand and a substituted fluorenyl ligand.

The term "bridge" is widely used in metallocene chemistry to denote a chemical group that "bridges", i.e. is bonded simultaneously to, for example two substituted cyclopentadienyl ligands or two substituted fluorenyl ligands or a substituted cyclopentadienyl ligand and a substituted fluorenyl ligand.

In the metallocenes used in the present application the "bridge" is a chemical group having the general formula R ¹ R ² C.

Throughout the present application, the following numbering scheme will be adhered to for describing the positions of substituents on the substituted cyclopentadienyl ligand and the substituted fluorenyl ligand :

The metallocene used in the present process is a metallocene of general formula (I) or (II)

^ ² α(3-^-5-^-α ₅ Η ₂ )(3,6-Ι1 ⁵ -ΡΙι_)ΜΧ2 (I) ^ ² α(3-^-5-^-α ₅ Η ₂ )(2,7-Ι1 ⁵ -ΡΙι_)ΜΧ2 (II) wherein

- R ¹ is hydrogen or a hydrocarbyl group having from 1 to 20 carbon atoms,

- R ² , R ³ , R ⁴ and R ⁵ are each, independently from one another, a hydrocarbyl group having from 1 to 20 carbon atoms,

- M is selected from the group consisting of titanium, zirconium and hafnium, and

- X is selected from the group consisting of F, CI, Br, I, and hydrocarbyl groups having from 1 to 10 carbon atoms. It is noted that for both metallocenes, i.e. of formula (I) and (II), the substituted cyclopentadienyl ligand is substituted in positions 3 and 5.

The substituted fluorenyl ligand is substituted in positions 3 and 6 for the metallocene of formula (I) and in positions 2 and 7 for the metallocene of formula (II).

Preferred hydrocarbyl groups having from 1 to 20 carbon atoms are selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, tert- butyl, n-pentyl, n-hexyl and phenyl, each of which may also be substituted with one or more of the preferred hydrocarbyl groups, provided that the total number of carbon atoms does not exceed 20.

With regards to R ¹ , hydrogen and methyl are preferred; hydrogen is most preferred.

With regards to R ² , tert-butyl and phenyl are preferred; phenyl is most preferred.

With regards to R ³ , iso-propyl, sec-butyl, tert-butyl and phenyl are preferred; tert- butyl and phenyl are more preferred; tert-butyl is most preferred.

With regards to R ⁴ , methyl, ethyl, n-propyl, n-butyl and phenyl are preferred; methyl and ethyl are most preferred.

With regards to R ⁵ , iso-propyl, tert-butyl and phenyl are preferred; tert-butyl is most preferred.

With regards to M, zirconium is preferred. With regards to X, the hydrocarbyl group having from 1 to 10 carbon atoms is preferably an alkyl or a benzyl, and more preferably is selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, tert-butyl, n- pentyl, n-hexyl and benzyl (-CH ₂ Ph). It is most preferred that X is either CI or methyl.

Non-limitative examples of specific metallocenes are the following :

Preferably, the activating agent used in the present application is methylalumoxane (MAO) or an organoaluminum compound defined by general formula R ₃ AI, wherein each R is independently selected from alkyl having from one to ten carbon atoms. Preferably, R is an alkyl having from one to six carbons atoms, such as for example methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, tert-butyl, n-pentyl and n- hexyl. Most preferably, R is ethyl. Other activating agent such as, but not limited to, bore based ionic activating agent including triphenylcarbenium tetrakispentafluorophenyl borate, Ν,Ν-dimethylanilinium (tetrakis pentafluorophenyl) borate, tris(perfluorophenyl)boron, etc, can also be used in the process according to the present application.

If MAO is used, the activating agent is preferably used in such an amount that the molar ratio Al/metallocene is at least 50, preferably at least 70, more preferably at least 100, even more preferably at least 500 and most preferably at least 1000. Preferably said molar ratio is at most 10000, more preferably at least 8000 and most preferably at least 6000. When bore based ionic activating agent are used, the amount of ionic activating agent is such that the molar ratio B/Zr is ranging between 0.8 and 5, preferably between 1 and 2.

In addition, an organoaluminum compounds is used as alkylating agent and as poison scavenger. This organoaluminium compound is preferably selected from trimethylaluminum (Me ₃ AI, frequently referred to as "TMA"), triethylaluminum (Et ₃ AI, frequently referred to as "TEAL"), tri-n-propylaluminum (nPr ₃ AI), tri-iso- propylaluminum (iPr ₃ AI), tri-n-butylaluminum (nBu ₃ AI), tri-iso-butylaluminum (iBu ₃ AI, frequently referred to as "TIBAL"), and tri-sec-butylaluminum (secBu ₃ AI). Compared to the minimum quantity required for the alkylation of the metallocene, the alkylating agent is used in excess.

The metallocene may either be used as dissolved in an inert hydrocarbon diluent (homogeneous polymerization) or supported on an inert support material, such as for example silica (heterogeneous polymerization). Both methods are known to the person skilled in the art and need not be described in more detail.

Preferred inert hydrocarbon diluents are propane, n-butane, iso-butane, n-pentane, n-hexane, hexanes (mixture of isomers), n-heptane, n-octane, n-nonane, n- decane, toluene and xylene. More preferred diluents are n-hexane, hexanes (mixture of isomers) and toluene. Toluene is most preferred.

Preferably, the present polymerization process is performed at temperatures of at least 20 °C, more preferably of at least 30 °C, even more preferably of at least 40 °C and most preferably of at least 50 °C. The present polymerization process is preferably performed at temperatures of at most 120 °C, more preferably of at most 110 °C and most preferably of at most 100 °C. Preferably, the present polymerization process is a slurry polymerization process. While the pressure of propylene may be chosen as seen fit for the respective polymerization reactor used, it is nevertheless preferred that the propylene pressure be at least 1 bar, more preferably at least 2 bar, and most preferably at least 3 bar. It is preferred that the propylene pressure be at most 46 bar, more preferably at most 40 bar or 30 bar, even more preferably at most 25 bar or 20 bar, still even more preferably at most 15 bar and most preferably at most 10 bar. Propylene pressure is given as absolute pressure in the polymerization reactor. Compared to linear polypropylene grades, the polypropylene obtained by the present polymerization process is characterized by an improved elasticity as shown by the G' storage modulus values. Said polypropylene is hence characterized by improved elongational properties. It is expected that these improved elongational properties will result in improved processability in applications and transformation methods which necessitate a certain level of melt strength in the polypropylene. This is specifically the case in extrusion applications, such as for example blown films, foamed blown films, extrusion blow molding, fiber extrusion or film extrusion, as well as in applications where an intermediate article is transformed into a final article at a temperature at which the polypropylene is soft, such as for example in thermoforming, injection stretch blow molding, the production of BOPP film, or in fiber production with an additional drawing step.

The improved elongational properties of the present polypropylene are expected to allow easier processing on the one hand, while on the other hand also resulting in improved properties, particularly in improved mechanical properties such as for example in improved toughness, of the final polypropylene articles.

Test methods

Molecular weights were determined by Size Exclusion Chromatography (SEC) at high temperature (145 °C). A 10 mg polypropylene or polyethylene sample was dissolved at 160 °C in 10 ml of trichlorobenzene (technical grade) for 1 hour. Analytical conditions for the GPCV 2000 apparatus from WATERS were:

- Injection volume: +/- 400μΙ

- Automatic sample preparation and injector temperature: 160 °C

- Column temperature: 145 °C

- Detector temperature: 160 °C

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

- Flow rate: 1 ml/min

- Detector: Infrared detector (2800-3000 cm ^"1 )

- Calibration : Narrow standards of polystyrene (commercially available)

- Calculation for polypropylene: Based on Mark-Houwink relation (logi ₀ (M _PP ) = logio(Mps) - 0.25323 ); cut off on the low molecular weight end at M _PP =

1000.

The molecular weight distribution (MWD) was then calculated as M _w /M _n . Melting temperatures T _me i _t and crystallization temperatures T _{cr St} were determined according to ISO 3146 on a DSC Q2000 instrument by TA Instruments. To erase the thermal history, the samples were first heated to 200 °C and kept at 200 °C for a period of 3 minutes. The reported melting temperatures T _me i _t and T _cryst were then determined with heating and cooling rates of 20 °C/min.

The ¹³ C-NMR analysis was performed at an operative frequency of 125 MHz using a 500 MHz Bruker NMR spectrometer with a high temperature 10mm cryoprobe under conditions such that the signal intensity in the spectrum is directly proportional to the total number of contributing carbon atoms in the sample. Such conditions are well known to the skilled person and include for example sufficient relaxation time etc. In practice, the intensity of a signal is obtained from its integral, i.e. the corresponding area. The data were acquired using proton decoupling, 240 scans per spectrum, a pulse repetition delay of 11 seconds and a spectral width of 26000 Hz at a temperature of 130 °C. The sample was prepared by dissolving a sufficient amount of polymer in 1,2,4-trichlorobenzene (TCB, 99%, spectroscopic grade) at 130 °C and occasional agitation to homogenize the sample, followed by the addition of hexadeuterobenzene (C ₆ D ₆ , spectroscopic grade) and a minor amount of hexamethyldisiloxane (HMDS, 99.5+ %), with HMDS serving as internal standard. To give an example, about 200 mg of polymer were dissolved in 2.0 ml of TCB, followed by addition of 0.5 ml of C ₆ D ₆ and 2 to 3 drops of HMDS.

Following data acquisition the chemical shifts are referenced to the signal of the internal standard HMDS, which is assigned a value of δ 2.03 ppm.

The isotacticity was determined by ¹³ C-NMR analysis on the total polymer. In the spectral region of the methyl groups, the signals corresponding to the pentads mmmm, mmmr, rmmr, mmrr, rmrr+mrmm, mrmr, rrrr, mrrr and mrrm were assigned using published data, for example A. Razavi, Macromol. Symp., 1995, vol. 89, pages 345-367. . Some area correction were performed in case of overlap with signals related to 2,1-insertions, 1,3 additions, nporpyl chain ends, etc. The percentage of mmmm pentads was then calculated by normalization of all the methyl pentads area according to

% mmmm = AREA _mmmm / (AREA _mmmm + AREA _mmmr + AREA _mmrr + AREA _mrrm ) ·

100

The ¹³ C-NMR detection limit in these conditions is about 0.6/10 000 C. G' (storage modulus) and G" (loss modulus) were measured at a temperature of 230 °C using a dynamic rheometer in a frequency sweep with a strain of 20% on an ARES-G2 instrument from TA, branch of WATERS.

Examples

The advantages of the present invention are illustrated by the following representative examples.

Comparative polypropylene PP1 was a commercial linear polypropylene having conventional elongational properties. PP1 was obtained using a metallocene precursor dimethylsilylbis(2-methyl-4-phenylindenyl)zirconium dichloride activated with methylalumoxane (MAO, 10 wt% in toluene).

PP2 was a polypropylene obtained with a metallocene-based polymerization catalyst comprising metallocene (III). PP3 was a polypropylene obtained with a metallocene-based polymerization catalyst comprising metallocene (IV).

PP2 and PP3 were produced in a laboratory polymerization glass reactor equipped with a Pelton-type turbine in 150 ml of toluene as diluent. The metallocene-based polymerization catalyst was used in such an amount that the concentration of metallocene in the toluene was 10 μιτιοΙ ¹ . The metallocene-based polymerization catalyst was activated in situ by the addition of 5000 molar equivalents, respective to mol of metallocene, of MAO (as a 10 wt% solution in toluene, Albermale Co., containing ca. 10% free TMA). Polymerization temperature was initially set to 60 °C by means of thermostated water circulated through the double wall of the reactor. Propylene pressure was kept constant at 5 bar. Polymerization was continued for 30 min and then stopped by the removal of propylene from the polymerization reactor followed by adding methanol to deactivate the polymerization catalyst. The resulting polypropylene powder was filtrated, washed and dried at room temperature under vacuum.

Properties of the polypropylenes PP1, PP2 and PP3 are shown in Table 1. Table 1

A plot of G' as x-value versus G" as y-value is shown in Figure 1. The graph for Comparative Example 1 clearly shows a relationship between G' and G" that is linear (in a log-log representation) over the range of G' measured. However, the respective graphs for Example 1 and Example 2 show that from a certain G'-value downwards the curves start to deviate from linear behavior, thus showing improved elongational properties.

It is noted here that the ¹³ C-NMR spectra of Example 1 and of Example 2 did not show any peaks at δ 44.88 ppm, 44.74 ppm, 44.08 ppm and 31.74 ppm (Figure 2, A and B). This is not the case for long chain branched isotactic polypropylene for which the ¹³ C-NMR spectrum shows peaks at δ 44.88 ppm, 44.74 ppm, 44.08 ppm and 31.74 ppm (Figure 2, C). The spectrum C shown in Figure 2 is the ¹³ C-NMR spectrum of the polypropylene described in Weng et al. 2002 (macromolecules, 35, 3838-3843). Said polypropylene is a long chain branched isotactic polypropylene obtained by polymerization of propylene and ethylene using a metallocene precursor dimethylsilylbis(2-methyl-4-phenylindenyl)zirconium dichloride activated with methylalumoxane (MAO, 10 wt% in toluene). The presence of long chain branching is witnessed in the ¹³ C-NMR spectra by the presence of peaks having essentially the same intensity (peaks with an * in Figure 2, C). The absence of peaks at δ 44.88 ppm, 44.74 ppm, 44.08 ppm and 31.74 ppm in the ¹³ C-NMR spectra of the polypropylene polymers according to the present invention (Figure 2, A and B) indicates the absence of long chain branching. However, said polypropylene exhibit rheological features, namely G' and G" values, indicating the presence of long chain branching structures. Hence, the polypropylene according to an embodiment of the invention has a new molecular architecture associated with improved elasticity at low frequency compared to linear polypropylene.

The improved elasticity and hence, improved elongational properties are expected to translate into improved processability as well as into improved mechanical properties in the final polypropylene articles.