Field of the invention

The present invention relates to sheets and thermoformed articles comprising a polypropylene layer, which consists of a polypropylene composition comprising a polypropylene produced with a metallocene-based polymerization catalyst. The present invention further relates to a process for producing such sheets and thermoformed articles.

The technical problem and the prior art

In thermoforming, a soft polymer sheet is draped over or into a form or mold. In its basic form a thermoforming process comprises for example the steps of

(i) warming the sheet to a temperature at which it is soft,

(ii) draping the soft sheet over or into a mold, thus obtaining a formed sheet,

(iii) cooling the formed sheet to a temperature at which it can maintain its shape, and

(iv) removing the formed sheet from the mold.

In contrast to other forming processes, such as for example injection molding or blow molding, thermoforming is a low-pressure and low-temperature process.

The polymer sheet serving as feedstock for the thermoforming process may for example be produced by melt-extrusion. Thus, one often refers to "extrusion- thermoforming" to denote the complete process with the two distinct processing stages of

(i) the production of a sheet by melt-extrusion of a polymer, and

(ii) the thermoforming stage, wherein the polymer is formed or shaped. Extrusion-thermoforming comes in numerous variations. It may for example be done either in-line, i.e. the sheet coming from the sheet-extrusion stage is directly fed to the thermoforming stage, or off-line, i.e. the sheet is stored for some time before being fed to the thermoforming stage.

In extrusion-thermoforming polypropylene is of great interest because it combines low cost and good productivity with good mechanical and chemical properties. However, the difficulty lies in finding the balance between the polymer properties required to have good processability both, in the extrusion stage as well as in the thermoforming stage. These requirements may in some cases even exclude each other. For example, while high melt strength may improve the performance in the thermoforming stage, at the same time this might for example require higher extruder pressures or higher die pressures or higher melt temperatures in the extrusion stage to produce a sheet of a quality suited for the subsequent thermoforming stage.

An overview of thermoforming and extrusion-thermoforming is for example given in J.L. Throne, Understanding Thermoforming, Carl Hanser Verlag, Munich, 1999 and in J.L. Throne, Thermoforming, Carl Hanser Verlag, Munich, 1987.

Commercially available polypropylenes for thermoforming have a melt flow index (as measured according to ISO 1 133, condition L, at 230°C and 2.16 kg) of at most 6 dg/min to provide an acceptable balance between the performance in the two processing stages of extrusion-thermoforming.

However, the industry remains interested in further improving the processability in either or both of the two processing stages, preferably without loss in properties of the final article.

It is therefore an objective to provide a process for the production of an article, said article being a sheet or a thermoformed article, allowing said article to be produced with good processability in the extrusion stage or the thermoforming stage or both.

It is a further objective to provide a process for the production of a thermoformed article, wherein the processability in the extrusion stage is improved, while the processability in the thermoforming stage remains comparable.

Furthermore, it is an objective to provide sheet that can be well processed in the thermoforming stage.

Even more, it is an objective to provide sheet and thermoformed articles having good mechanical properties. In addition, it is an objective to provide sheet and thermoformed articles having good optical properties.

Brief description of the invention

We have now discovered that any of these objectives, either individually or in any combination, can be met by providing sheet or thermoformed articles comprising a polypropylene layer consisting of a polypropylene composition comprising a polypropylene produced with a metallocene-based polymerization catalyst, wherein the polypropylene has specific properties.

Thus, there is provided an article comprising a polypropylene layer, said polypropylene layer consisting of a polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene layer, of a polypropylene produced with a metallocene-based polymerization catalyst, wherein the article is a sheet or a thermoformed article, and the polypropylene has a melt flow index of at least 10 dg/min and of at most 25 dg/min, as determined according to ISO 1 133, condition L, at 230°C and 2.16 kg.

Further, there is provided a process for the production of such articles, said process comprising the steps of

(a) providing a polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene composition, of a polypropylene produced with a metallocene-based polymerization catalyst to an extruder,

(b) subsequently melting the polypropylene composition in said extruder to obtain a molten polypropylene composition;

(c) melt-extruding the molten polypropylene composition obtained in step (b) through a slit die to form an extrudate; and

(d) cooling the extrudate to obtain a sheet,

wherein the polypropylene has a melt flow index of at least 10 dg/min and of at most 25 dg/min, as determined according to ISO 1 133, condition L, at 230°C and 2.16 kg.

Furthermore, there is provided the use of a polypropylene composition comprising at least 50 wt%, relative to the total weight of said article, of a polypropylene produced with a metallocene-based polymerization catalyst in the production of a sheet or a thermoformed article, wherein the polypropylene has a melt flow index of at least 10 dg/min and of at most 25 dg/min, as determined according to ISO 1 133, condition L, at 230°C and 2.16 kg.

Detailed description of the invention

Throughout the present application the terms "polypropylene" and "propylene polymer" may be used synonymously. Throughout the present application the melt flow index, abbreviated as "MFI", of polypropylene and polypropylene compositions is determined according to ISO 1 133, condition L, at 230°C and 2.16 kg. Throughout the present application the melt index, abbreviated as "MI2", of polyethylene and polyethylene compositions is determined according to ISO 1 133, condition D, at a temperature of 190°C and a load of 2.16 kg.

Throughout the present application the terms "forming" and "shaping" may be used synonymously.

For the purposes of the present invention "sheet" is defined as having a thickness in the range from 500 pm to 2000 pm, and preferably from 700 pm to 1500 μητ

For the purposes of the present application the term "tetrahydroindenyl" signifies an indenyl group wherein the six-membered ring has been hydrogenated to form 4,5,6,7-tetrahydroindenyl. The article provided for herein is a sheet or a thermoformed article and comprises a polypropylene layer consisting of a polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene composition, of a polypropylene produced with a metallocene-based polymerization catalyst. Preferably, said polypropylene composition comprises at least 60 wt%, more preferably at least 70 wt% or 80 wt%, even more preferably at least 90 wt% or 95 wt%, and still even more preferably at least 97 wt% or 99 wt%, relative to the total weight of said polypropylene composition, of a polypropylene produced with a metallocene-based polymerization catalyst. Most preferably, said polypropylene composition consists of a polypropylene produced with a metallocene-based polymerization catalyst. Preferably, the article provided for herein further comprises a polyethylene layer, which consists of a polyethylene composition comprising at least 50 wt%, relative to the total weight of said polyethylene composition, of a polyethylene produced with a metallocene-based polymerization catalyst, wherein the polypropylene layer and the polyethylene layer are directly adjacent to one another, i.e. without a tie layer in between.

The polypropylene composition used herein may further comprise a thermoplastic polymer different from the polypropylene produced with a metallocene-based polymerization catalyst as defined in the present application. The polyethylene composition used herein may further comprise a thermoplastic polymer different from the polyethylene produced with a metallocene-based polymerization catalyst as defined in the present application. Preferred suitable thermoplastic polymers are for example propylene homopolymers, copolymers of propylene and at least one comonomer, ethylene homopolymers, copolymers of ethylene and at least one comonomer, wherein said at least one comonomer is defined as stated below. Suited propylene homopolymers or copolymers may be produced with a Ziegler-Natta polymerization catalyst. Suitable ethylene homopolymers or copolymers may be characterized by different densities and may be produced with various polymerization catalysts, such as chromium-based polymerization catalysts, metallocene-based polymerization catalysts or Ziegler-Natta catalysts, or by a radical polymerization process. With respect to the melt flow index of the polypropylene composition resp. the melt index of the polyethylene composition used herein, it is preferred that they are within the same ranges and values as defined below for the polypropylene resp. the polyethylene. POLYPROPYLENE

The polypropylene used herein has a melt flow index of at least 10 dg/min, preferably of at least 1 1 dg/min and most preferably of at least 12 dg/min. The polypropylene used herein has a melt flow index of at most 25 dg/min, preferably of at most 20 dg/min, more preferably of at most 19 dg/min and most preferably of at most 18 dg/min.

Preferably, the polypropylene used herein has a molecular weight distribution (MWD), defined as M _w /M _n , i.e. the ratio of weight average molecular weight M _w over number average molecular weight M _n , of at least 1 .0, more preferably of at least 1 .5 and most preferably of at least 2.0. Preferably, the polypropylene used herein has a molecular weight distribution, defined as M _w /M _n , of at most 4.0, more preferably of at most 3.5, even more preferably of at most 3.0, and most preferably of at most 2.8. Molecular weights can be determined by size exclusion chromatography (SEC) as described in the test methods.

The approach to use a polypropylene of a melt flow index of at least 10 dg/min, preferably in combination with a narrow molecular weight distribution, runs counter to what the person knowledgeable in extrusion-thermoforming would do. Currently available polypropylenes suitable for extrusion-thermoforming have a melt flow index of up to 6 dg/min and tend to have a rather broad molecular weight distribution so as to increase the melt strength of the polypropylene. Going to higher melt flow index, preferably in combination with a narrower molecular weight distribution, is in consequence expected to result in decreased processability and rather poor properties of sheet and articles produced therewith.

However, very surprisingly it has been found that the polypropylenes having a melt flow index of at least 10 dg/min, preferably in combination with a narrower molecular weight distribution, as defined in the present application, process well, both in the extrusion stage and in the thermoforming stage. Furthermore, the sheet and thermoformed articles produced with such a polypropylene have comparable mechanical properties as compared to sheet and thermoformed articles produced with a "conventional" polypropylene recommended for thermoforming.

Preferably, the polypropylene used herein is characterized by a high isotacticity, for which the content of mmmm pentads is a measure. Preferably, the content of mmmm pentads is at least 90 %, more preferably at least 95 %, and most preferably at least 97 %. The isotacticity may be determined by ¹³ C-NMR analysis as described in the test methods.

The polypropylene used herein preferably is a propylene homopolymer or a random copolymer of propylene and up 6.0 wt%, relative to the total weight of said random copolymer, of at least one comonomer, said at least one comonomer being different from propylene, though propylene homopolymer is preferred. The preferred random copolymer is a random copolymer of propylene and up to 5.0 wt%, more preferably up to 4.5 wt%, and most preferably up to 4.0 wt% of at least one comonomer, relative to the total weight of said random copolymer. Preferred comonomers are a-olefins having from one to 10 carbon atoms. Preferred a-olefins are ethylene, butene-1 , pentene-1 , hexene-1 , octene-1 and 3-methyl-pentene-1 . More preferred α-olefins are ethylene and butene-1 . The most preferred a-olefin is ethylene.

Preferably, the polypropylene used herein is characterized by a melting temperature T _me it of at most 160°C. The determination of melting temperatures is described in the test methods.

Without wishing to be bound by theory, the present inventors believe that the lower melting temperature of the polypropylenes used herein seems to permit easier processability in the extrusion stage and also allows reducing the cycle time in the thermoforming stage. In comparison to Ziegler-Natta polypropylenes, the polypropylene used herein is characterized by lower melting temperatures so that less energy is required in the thermoforming stage to heat the sheet to a temperature at which it can be thermoformed. Thus, in a first aspect less time is required for heating (assuming that the heaters are run at the same temperatures), and in a second aspect less time is required for cooling the formed sheet because less energy has to be removed.

Preferably, the polypropylene used herein is characterized by a percentage of 2, 1 -insertions, relative to the total number of propylene molecules in the polymer chain, of at least 0.1 %. Preferably, the percentage of 2, 1 -insertions is at most 1 .5 %, more preferably at most 1 .3 %, even more preferably at most 1 .2 %, still even more preferably at most 1 .1 %, and most preferably at most 1 .0 %. The method for determining the percentage of 2, 1 -insertions is given in the test methods. Preferably, the polypropylene used herein comprises a nucleating agent, more specifically an a-nucleating agent. For the purposes of the present application a nucleating agent is defined as a chemical compound that raises the crystallization temperature of the polypropylene. Suitable nucleating agents for use in the present invention can be selected from any of the nucleating agents known to the skilled person. It is, however, preferred that the nucleating agent be selected from the group consisting of talc, carboxylate salts, sorbitol acetals, phosphate ester salts, substituted benzene tricarboxamides and polymeric nucleating agents, as well as blends of these.

Examples for carboxylate salts are organocarboxylic acid salts. Particular examples are sodium benzoate and lithium benzoate. The organocarboxylic acid salts may also be alicyclic organocarboxylic acid salts, preferably bicyclic organodicarboxylic acid salts and more preferably a bicyclo[2.2.1 ]heptane dicarboxylic acid salt. A nucleating agent of this type is sold as HYPERFORM® HPN-68 by Milliken Chemical. Examples for sorbitol acetals are dibenzylidene sorbitol (DBS), bis(p-methyl- dibenzylidene sorbitol) (MDBS), bis(p-ethyl-dibenzylidene sorbitol), bis(3,4- dimethyl-dibenzylidene sorbitol) (DMDBS), and bis(4-propylbenzylidene) propyl sorbitol. Bis(3,4-dimethyl-dibenzylidene sorbitol) (DMDBS) and bis(4- propylbenzyhdene) propyl sorbitol are preferred. These can for example be obtained from Milliken Chemical under the trade names of Millad 3905, Millad 3940, Millad 3988 and Millad NX8000.

Examples of phosphate ester salts are salts of 2,2'-methylene-bis-(4,6-di-tert- butylphenyl)phosphate. Such phosphate ester salts are for example available as NA-1 1 or NA-21 from Asahi Denka.

Examples of substituted tricarboxamides are those of general formula (I)

wherein R1 , R2 and R3, independently of one another, are selected from C1-C20 alkyls, C5-C12 cycloalkyls, or phenyl, each of which may in turn by substituted with C1-C20 alkyls, C5-C12 cycloalkyls, phenyl, hydroxyl, C1-C20 alkylamino or C-i- C20 alkyloxy etc. Examples for C1-C20 alkyls are methyl, ethyl, n-propyl, n-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, 1 , 1 -dimethylpropyl, 1 ,2-dimethylpropyl, 3-methylbutyl, hexyl, heptyl, octyl or 1 , 1 ,3,3-tetramethylbutyl. Examples for C5- C12 cycloalkyl are cyclopentyl, cyclohexyl, cyclooctyl, cyclododecyl, adamantyl, 2-methylcyclohexyl, 3-methylcyclohexyl or 2,3-dimethylcyclohexyl. Such nucleating agents are disclosed in WO 03/102069 and by Blomenhofer et al. in Macromolecules 2005, 38, 3688-3695.

Examples of polymeric nucleating agents are polymeric nucleating agents containing vinyl compounds, which are for example disclosed in EP-A1 - 0152701 and EP-A2-0368577. The polymeric nucleating agents containing vinyl compounds can either be physically or chemically blended with the metallocene random copolymer of propylene and one or more comonomers. In physical blending the polymeric nucleating agent containing vinyl compounds is mixed with the metallocene random copolymer of propylene and one or more comonomers in an extruder or in a blender. In chemical blending the metallocene random copolymer of propylene and one or more comonomers comprising the polymeric nucleating agent containing vinyl compounds is produced in a polymerization process having at least two stages, in one of which the polymeric nucleating agent containing vinyl compounds is produced. Preferred vinyl compounds are vinyl cycloalkanes or vinyl cycloalkenes having at least 6 carbon atoms, such as for example vinyl cyclopentane, vinyl-3-methyl cyclopentane, vinyl cyclohexane, vinyl-2-methyl cyclohexane, vinyl-3-methyl cyclohexane, vinyl norbornane, vinyl cylcopentene, vinyl cyclohexene, vinyl-2- methyl cyclohexene. The most preferred vinyl compounds are vinyl cyclopentane, vinyl cyclohexane, vinyl cyclopentene and vinyl cyclohexene.

Further examples of polymeric nucleating agents are poly-3-methyl-1 -butene, polydimethylstyrene, polysilanes and polyalkylxylenes. As explained for the polymeric nucleating agents containing vinyl compounds, these polymeric nucleating agents can be introduced into the metallocene polypropylene either by chemical or by physical blending.

It is also possible to use high-density polyethylene, such as for example Rigidex HD6070EA, available from INEOS Polyolefins, or a polypropylene having a fractional melt flow, or a polypropylene that comprises a fraction of fractional melt flow.

Further, it is possible to use blends of nucleating agents, such as for example a blend of talc and a phosphate ester salt or a blend of talc and a polymeric nucleating agent containing vinyl compounds. The nucleating agent may be introduced into the metallocene polypropylene by blending it with a nucleating agent, which is either in pure form or in form of a masterbatch, for example by dry-blending or by melt-blending. It is within the scope of the present invention that the nucleating agent can be introduced into the metallocene polypropylene by blending it with a nucleated thermoplastic polymer, wherein said thermoplastic polymer is different from the metallocene polypropylene

While it is clear to the skilled person that the amount of nucleating agent to be added depends upon its crystallization efficiency, for the purposes of the present invention the nucleating agent or the blend of nucleating agents - if comprised at all - is present in the metallocene polypropylene in an amount of at least 50 ppm, preferably at least 100 ppm. It is present in an amount of at most 5000 ppm, preferably of at most 4000 ppm, even more preferably of at most 3000 ppm and most preferably of at most 2000 ppm.

The polypropylene used herein may also comprise further additives, such as by way of example, antioxidants, light stabilizers, acid scavengers, lubricants, antistatic additives, and colorants. An overview of such additives may be found in Plastics Additives Handbook, ed. H. Zweifel, 5 ^th edition, 2001 , Hanser Publishers.

The polypropylene used herein is a metallocene polypropylene, i.e. it is produced with a metallocene-based polymerization catalyst, with the metallocene-based polymerization catalyst comprising a bridged metallocene component, a support and an activating agent. Such metallocene-based polymerization catalysts are generally known in the art and need not be explained in detail. The metallocene component can be described by the following general formula

(M-R ^a )(R ^b )(R ^c )MX ¹ X ² (I) wherein R ^a , R ^b , R ^c , M, X ¹ and X ² are as defined below.

R ^a is the bridge between R ^b and R ^c , i.e. R ^a is chemically connected to R ^b and R ^c , and is selected from the group consisting of -(CR ¹ R ² ) _P - -(SiR ¹ R ² ) _p - - (GeR ¹ R ² ) _p - -(NR ¹ )p- -(PR ¹ ) _P - -(N ⁺ R ¹ R ² ) _P - and -(P ⁺ R ¹ R ² ) _P - and p is 1 or 2, and wherein R ¹ and R ² are each independently selected from the group consisting of hydrogen, d-do alkyl, Cs-Cs cycloalkyi, C6-C15 aryl, alkylaryl with C1-C10 alkyl and C6-C15 aryl, or any two neighboring R (i.e. two neighboring R ¹ , two neighboring R ² , or R ¹ with a neighboring R ² ) may form a cyclic saturated or non-saturated C4-C10 ring; each R ¹ and R ² may in turn be substituted in the same way. Preferably R ^a is -(CR ¹ R ² ) _P - or -(SiR ¹ R ² ) _p - with R ¹ , R ² and p as defined above. Most preferably R ^a is -(SiR ¹ R ² ) _p - with R ¹ , R ² and p as defined above. Specific examples of R ^a include Me ₂ C, ethanediyi (-CH ₂ -CH ₂ -), Ph ₂ C and Me ₂ Si.

M is a metal selected from Ti, Zr and Hf, preferably it is Is.

X ¹ and X ² are independently selected from the group consisting of halogen, hydrogen, C1-C10 alkyl, C6-C15 aryl, alkylaryl with C1-C10 alkyl and C6-C15 aryl. Preferably X ¹ and X ² are halogen or methyl.

R ^b and R ^c are selected independently from one another and comprise a cyclopentadienyl ring. Preferred examples of halogen are CI, Br, and I. Preferred examples of C1-C10 alkyl are methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, and tert-butyl. Preferred examples of C5-C7 cycloalkyi are cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl. Preferred examples of C6-C15 aryl are phenyl and indenyl. Preferred examples of alkylaryl with C1-C10 alkyl and C6-C15 aryl are benzyl (- CH ₂ -Ph), and -(CH ₂ ) ₂ -Ph. Preferably, R ^b and R ^c may both be substituted cyclopentadienyl, or may be independently from one another unsubstituted or substituted indenyl or tetrahydroindenyl, or R ^b may be a substituted cyclopentadienyl and R ^c a substituted or unsubstituted fluorenyl. More preferably, R ^b and R ^c may both be the same and may be selected from the group consisting of substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted tetrahydroindenyl and substituted tetrahydroindenyl. By "unsubstituted" is meant that all positions on R ^b resp. R ^c , except for the one to which the bridge is attached, are occupied by hydrogen. By "substituted" is meant that, in addition to the position at which the bridge is attached, at least one other position on R ^b resp. R ^c is occupied by a substituent other than hydrogen, wherein each of the substituents may independently be selected from the group consisting of d-do alkyl, C5-C7 cycloalkyl, C6-C15 aryl, and alkylaryl with C1-C10 alkyl and C6-C15 aryl, or any two neighboring substituents may form a cyclic saturated or non- saturated C4-C10 ring.

A substituted cyclopentadienyl may for example be represented by the general formula CsR ³ R R ⁵ R ⁶ . A substituted indenyl may for example be represented by the general formula C ₉ R ⁷ R ⁸ R ⁹ R ¹⁰ R ^{1 1} R ¹² R ^{1 3} R ¹⁴ A substituted tetrahydroindenyl may for example be represented by the general formula C9H ₄ R ^{1 5} R ¹⁶ R ¹⁷ R ¹⁸ . A substituted fluorenyl may for example be represented by the general formula Ci ₃ R ¹⁹ R ²⁰ R ²¹ R ²² R ²³ R ² R ²⁵ R ²⁶ Each of the substituents R ³ to R ²⁶ may independently be selected from the group consisting of hydrogen, C1-C10 alkyl, C5-C7 cycloalkyl, C6-C15 aryl, and alkylaryl with C1-C10 alkyl and C6-C15 aryl, or any two neighboring R may form a cyclic saturated or non-saturated C ₄ -Cio ring; provided, however, that not all substituents simultaneously are hydrogen.

Preferred metallocene components are those having C2-symmetry or those having C-i-symmetry. Most preferred are those having C2-symmetry.

Particularly suitable metallocene components are those wherein R ^b and R ^c are the same and are substituted cyclopentadienyl, preferably wherein the cyclopentadienyl is substituted in the 2-position, the 3-position, or simultaneously the 2-position and the 3-position.

Particularly suitable metallocene components are also those wherein R ^b and R ^c are the same and are selected from the group consisting of unsubstituted indenyl, unsubstituted tetrahydroindenyl, substituted indenyl and substituted tetrahydroindenyl. Substituted indenyl is preferably substituted in the 2-position, the 3-position, the 4-position, the 5-position or any combination of these, more preferably in the 2-position, the 4-position or simultaneously in the 2-position and the 4-position. Substituted tetrahydroindenyl is preferably substituted in the

2- position, the 3-position, or simultaneously the 2-position and the 3-position.

Particularly suitable metallocene components may also be those wherein R ^b is a substituted cyclopentadienyl and R ^c is a substituted or unsubstituted fluorenyl. The substituted cyclopentadienyl is preferably substituted in the 2-position, the

3- position, the 5-position or simultaneously any combination of these, more preferably in the 3-position or the 5-position or both simultaneously, most preferably in the 3-position only, with a bulky substituent. Said bulky substituent may for example be -CR ²⁷ R ²⁸ R ²⁹ or -SiR ²⁷ R ²⁸ R ²⁹ with R ²⁷ , R ²⁸ and R ²⁹ independently selected from group consisting of d-do alkyl, C5-C7 cycloalkyl, C6-C-15 aryl, and alkylaryl with C1-C10 alkyl and C6-C15 aryl, or any two neighboring R may form a cyclic saturated or non-saturated C4-C10 ring, it is preferred that R ²⁷ , R ²⁸ and R ²⁹ are methyl. Examples of particularly suitable metallocenes are:

dimethylsilanediyl-bis(2-methyl-cyclopentadienyl)zirconiu m dichloride, dimethylsilanediyl-bis(3-methyl-cyclopentadienyl)zirconium dichloride, dimethylsilanediyl-bis(3-tert-butyl-cyclopentadienyl)zirconi um dichloride, dimethylsilanediyl-bis(3-tert-butyl-5-methyl-cyclopentadieny l)zirconium

dichloride,

dimethylsilanediyl-bis(2,4-dimethyl-cyclopentadienyl)zirc onium dichloride, dimethylsilanediyl-bis(indenyl)zirconium dichloride, dimethylsilanediyl-bis(2-methyl-indenyl)zirconium dichloride, dimethylsilanediyl-bis(3-methyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(3-tert-butyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(4,7-dimethyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(tetrahydroindenyl)zirconium dichloride,

dimethylsilanediyl-bis(benzindenyl)zirconium dichloride,

dimethylsilanediyl-bis(3,3'-2-methyl-benzindenyl)zirconiu m dichloride, dimethylsilanediyl-bis(4-phenyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(2-methyl-4-phenyl-indenyl)zirconiu m dichloride, ethanediyl-bis(indenyl)zirconium dichloride,

ethanediyl -bis(tetrahydroindenyl)zirconium dichloride,

isopropylidene-(3-tert-butyl-cyclopentadienyl)(fluorenyl) zirconium dichloride isopropylidene-(3-tert-butyl-5-methyl-cyclopentadienyl)(fluo renyl) zirconium dichloride.

The metallocene may be supported according to any method known in the art. In the event it is supported, the support used in the present invention can be any organic or inorganic solid, particularly porous supports such as talc, inorganic oxides, and resinous support material such as polyolefin. Preferably, the support material is an inorganic oxide in its finely divided form.

The polymerization of propylene and the one or more optional comonomers in presence of a metallocene-based catalytic system can be carried out according to known techniques in one or more polymerization reactors. The metallocene polypropylene used in the present invention is preferably produced by polymerization in liquid propylene at temperatures in the range from 20°C to 100°C. Preferably, temperatures are in the range from 60°C to 80°C. The pressure can be atmospheric or higher. It is preferably between 25 and 50 bar. The molecular weight of the polymer chains, and in consequence the melt flow of the metallocene polypropylene, is regulated by the addition of hydrogen to the polymerization medium. Preferably, the metallocene polypropylene is recovered from the one or more polymerization reactors without post-polymerization treatment to reduce its molecular weight and/or narrow its molecular weight distribution, such as can be done by thermal or chemical degradation, and is often done for polypropylene produced with a Ziegler-Natta catalyst.

POLYETHYLENE The metallocene-catalyzed polyethylene preferably is a homopolymer of ethylene or copolymer of ethylene and at least one comonomer, said comonomer being a C3 to C10 a-olefin, such as 1 -butene, 1 -pentene, 1 -hexene, 1 -octene, 1 -methylpentene, with 1 -butene and 1 -hexene being the preferred comonomers and 1 -hexene being the most preferred comonomer.

The polyethylene used herein is a metallocene polyethylene, i.e. it is a polyethylene produced with a metallocene-based polymerization catalyst, with the metallocene-based polymerization catalyst comprising a metallocene component, a support and an activating agent. Such metallocene-based polymerization catalysts are generally known in the art and need not be explained in detail..

The metallocene component can be described by the following general formula (M-R ^a )n(R ^b )(R ^c )MX ¹ X ² (II) wherein n = 0 or 1 , and R ^a is the bridge, i.e. n = 1 , between R ^b and R ^c , i.e. R ^a is chemically connected to R ^b and R ^c , and is selected from the group consisting of -(CR ¹ R ² )p- -(SiR ¹ R ² )p- -(GeR ¹ R ² )p- -(NR ¹ ) _P - -(PR ¹ ) _P - -(N ⁺ R ¹ R ² ) _P - and - (P ⁺ R ¹ R ² )p- and p is 1 or 2, and wherein R ¹ and R ² are each independently selected from the group consisting of hydrogen, d-do alkyl, Cs-Cs cycloalkyl, C6-C-15 aryl, alkylaryl with C1-C10 alkyl and C6-C15 aryl, or any two neighboring R may form a cyclic saturated or non-saturated C4-C10 ring; each R ¹ and R ² may in turn be substituted in the same way. Preferably R ^a is -(CR ¹ R ² ) _P - or - (SiR ¹ R ² ) _p - with R ¹ , R ² and p as defined above. Most preferably R ^a is - (CR ¹ R ² ) _P - with R ¹ , R ² and p as defined above. Specific examples of R ^a include Me ₂ C, ethanediyl (-CH ₂ -CH ₂ -), Ph ₂ C and Me ₂ Si.

M is a metal selected from Ti, Zr and Hf, preferably it is Zr.

X ¹ and X ² are independently selected from the group consisting of halogen, hydrogen, C1-C10 alkyl, C6-C15 aryl, alkylaryl with C1-C10 alkyl and C6-C15 aryl. Preferably X ¹ and X ² are halogen or methyl. R ^b and R ^c are selected independently from one another and comprise a cyclopentadienyl ring, which may be substituted or unsubstituted. By "unsubstituted" is meant that all positions on the cyclopentadienyl ring, except for the one to which - if present - the bridge is attached, are occupied by hydrogen. By "substituted" is meant that, in addition to the position at which - if present - the bridge is attached, at least one position on the cyclopentadienyl ring is occupied by a substituent other than hydrogen, wherein each of the substituents may independently be selected from the group consisting of C1-C10 alkyl, C5-C7 cycloalkyl, C6-C15 aryl, and alkylaryl with C1-C10 alkyl and C6-C15 aryl, or any two neighboring substituents may form a cyclic saturated or non- saturated C4-C10 ring.

Preferred examples of halogen are CI, Br, and I. Preferred examples of C1-C10 alkyl are methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, and tert-butyl. Preferred examples of C5-C7 cycloalkyl are cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl. Preferred examples of C6-C15 aryl are phenyl and indenyl. Preferred examples of alkylaryl with C1-C10 alkyl and C6-C15 aryl are benzyl (- CH ₂ -Ph), and -(CH ₂ ) ₂ -Ph. It is, however, preferred to use a metallocene component of the following general formula, wherein

- n is 1 ;

- R ^a is -(CR ¹ R ² )p- or -(SiR ¹ R ² ) _p - most preferably R ^a is -(CR ¹ R ² ) _P - with R ¹ , R ² and p as defined above;

- M is a metal selected from Ti, Zr and Hf, preferably it is Zr;

- X ¹ and X ² are the same and are halogen or methyl, preferably chlorine or methyl, and most preferably chlorine; and

- R ^b and R ^c are selected independently from one another and comprise an indenyl or tetrahydroindenyl.

Preferably, the indenyl or tetrahydroindenyl, if substituted, is symmetrically substituted in position 2 or position 4 or both, and more preferably they are unsubstituted.

Examples of particularly suitable metallocene components include the following: bis(n-butylcyclopentadienyl)zirconium dichloride,

ethanediyl-bis(1 -indenyl)zirconium dichloride,

ethanediyl-bis(2-methyl-1 -indenyl)zirconium dichloride,

ethanediyl-bis(4-methyl-1 -indenyl)zirconium dichloride,

ethanediyl-bis(4,5,6,7-tetrahydro-1 -indenyl)zirconium dichloride.

Preferably, the polyethylene used herein has a molecular weight distribution (MWD), defined as M _w /M _n , i.e. the ratio of weight average molecular weight M _w over number average molecular weight M _n , of at least 1 .0, more preferably of at least 1 .5 and most preferably of at least 2.0. Preferably, the polyethylene used herein has a molecular weight distribution, defined as M _w /M _n , of at most 5.0, more preferably or at most 4.0, and most preferably of at most 3.5. Molecular weights can be determined by size exclusion chromatography (SEC) as described in the test methods. Preferably, the metallocene-catalyzed polyethylene used in the present invention has a melt flow index in the range from 1 .0 dg/min to 10 dg/min.

Preferably, the metallocene-catalyzed polyethylene used herein has a density of at least 0.920 g/cm ³ , more preferably of at least 0.925 g/cm ³ or 0.930 g/cm ³ , even more preferably of at least 0.935 g/cm ³ or 0.940 g/cm ³ , still even more preferably of at least 0.945 g/cm ³ and most preferably of at least 0.950 g/cm ³ . Preferably, it has a density of at most 0.970 g/cm ³ , more preferably of at most 0.965 g/cm ³ , and most preferably of at most 0.960 g/cm ³ .

The metallocene-catalyzed polyethylene used herein can be produced by methods generally known to the skilled person. The polymerization of ethylene and - if present - one or more comonomers can for example be carried out in the gas phase. It may also be carried out in a liquid polymerization medium, such as for example a hydrocarbon that is inert under polymerization conditions, such as for example alkanes such as isobutane or isopentane or butane or pentane or propane, preferably in a loop reactor.

THERMOFORMING PROCESS

The polypropylene used herein is particularly suited for the production of thermoformed articles. Examples of such articles are food storage containers, drinking cups etc.

Thermoformed articles are generally produced by a two-stage process, wherein in the first stage a sheet is produced by melt-extruding a polymer (melt- extrusion stage), and in the second stage said sheet is shaped (thermoforming stage). The two stages may either directly follow each other (in-line thermoforming) or they may not directly follow each other, in which case the produced sheet is stored for some time (e.g. a few hours, days or months) first and only later fed to the thermoforming stage. The sheet may be produced on any melt extrusion sheet line, the production process for example comprising the steps of

(a) providing a polypropylene composition to an extruder,

(b) subsequently melting the polypropylene composition in the extruder to obtain a molten polypropylene composition,

(c) melt-extruding the molten polypropylene composition obtained in step (b) through a slit die to form an extrudate, and

(d) cooling the extrudate to obtain a sheet. Alternatively, the sheet may further comprise a polyethylene layer as defined above in the present application. In this case the sheet may be produced by co- extrusion. The respective production process may for example comprise the steps of

(a) providing a polypropylene composition to an extruder,

(a ¹ ) providing a polyethylene composition to a further extruder;

(b) subsequently melting the polypropylene composition in said extruder to obtain a molten polypropylene composition;

(b ¹ ) subsequently melting the polyethylene composition in said further extruder to obtain a molten polyethylene composition;

(c) melt-extruding the molten polypropylene composition obtained in steps (b) and the molten polyethylene composition obtained in step (b ¹ ) through a slit die to form an extrudate comprising a polypropylene composition layer and a polyethylene composition layer; and

(d) cooling the extrudate to obtain a sheet.

Preferably, in step (c) the molten polypropylene composition or, when applicable, the molten polyethylene composition or, when applicable, both may be passed through a melt pump and then be melt extruded.

Melt temperatures for the polypropylene composition and, when applicable, the polyethylene composition in the sheet extrusion stage generally are in the range from 200°C to 280°C, preferably in the range from 210°C to 270°. As the process for producing sheet is well known (see for example the already cited J.L. Throne, Understanding Thermoforming, Carl Hanser Verlag, Munich, 1999 and in J.L. Throne, Thermoforming, Carl Hanser Verlag, Munich, 1987) to the skilled person no further description is deemed necessary. Exemplary sheet production conditions are given in the examples.

The polypropylene composition provided to the extruder in step (a) is as defined earlier in this application. Equally, the polyethylene composition provided to the extruder in step (a ¹ ) is as defined earlier in this application.

The second stage, the thermoforming stage, can be done on any thermoforming machine comprising a heating and a forming section, said thermoforming process comprising the steps of

(f) draping the soft sheet, which is at a temperature at which it is soft, over or into a mold, thus obtaining a formed sheet,

(g) cooling the formed sheet to a temperature at which it maintains its shape, and

(h) removing the formed sheet from the mold.

Optionally, the process may further comprise a step of warming the sheet to a temperature at which it is soft before it is draped. Such an optional step of warming the sheet may particularly be required when sheet production and thermoforming are done "off-line", i.e. not in series, and/or the sheet is stored for some time to allow it to cool to a temperature below the temperature at which it can be draped. Thus, the thermoforming stage may comprise a further step

(e) warming the sheet obtained in step (d) to a temperature at which it is soft, to obtain a soft sheet,

which in the following is used in step (f). In the thermoforming stage the propylene polymer of the present invention can be processed under conditions that are comparable to the conditions used for a polypropylene conventionally used in thermoforming, i.e. a polypropylene being characterized by lower melt flow index.

Optionally, the process may comprise a trimming step. Trimming is usually defined as the process of mechanical breaking the formed sheet into two pieces, one of which is the desired thermoformed article, the other being the edge trim. Thus, the process as defined optionally comprises the step of

(i) trimming the formed sheet obtained in step (h) to obtain a thermoformed article.

A definition of trimming and methods for trimming are given in J.L. Throne, Understanding Thermoforming, Carl Hanser Verlag, Munich, 1999, pages 93- 100, and are generally known to the person skilled in the art.

The present application also discloses the use of a polypropylene composition as defined earlier in this application in the production of a sheet or thermoformed article.

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

TEST METHODS

Melt flow index (MFI) of polypropylene and polypropylene compositions is determined according to ISO 1 133, condition L, at 230°C and 2.16 kg. Melt index (MI2) of polyethylene and polyethylene compositions is determined according to ISO 1 133, condition D, at 190°C and 2.16 kg.

Density is measured according to ISO 1 183 at 23°C.

Top load of the thermoformed cups is determined by dynamic compression of the cups at 23°C and a speed of 10 mm/min with a preload of 1 N in accordance with ISO 12048: 1994. The falling weight test was performed on the thermoformed cups following ISO 6603-2:2000 from a height of 1 m at the indicated temperatures.

Haze was measured according to ISO 14782: 1999 on samples taken from the side wall and the bottom of the thermoformed cups.

Gloss was determined in accordance with ASTM-D 2457 at an angle of 45° on samples taken from the side wall and the bottom of the thermoformed cups.

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

- 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 (log-io(Mpp) = log-io(Mps) - 0.25323 ); cut off on the low molecular weight end at M _PP = 1000.

- Calculation for polyethylene: Based on Mark-Houwink relation (logio(M _PE ) = 0.965909 ^■ log ₀ (M _PS ) - 0.28264); cut off on the low molecular weight end at M _PE = 1000.

The molecular weight distribution (MWD) is then calculated as M _w /M _n .

Xylene solubles (XS), i.e. the xylene soluble fraction, are determined as follows: Between 4.5 and 5.5 g of propylene polymer are weighed into a flask and 300 ml xylene are added. The xylene is heated under stirring to reflux for 45 minutes. Stirring is continued for 15 minutes without heating. The flask is then placed in a thermostat bath set to 25°C +/- 1 °C for 1 hour. The solution is filtered through Whatman n° 4 filter paper and 100 ml of solvent are collected. The solvent is then evaporated and the residue dried and weighed. The percentage of xylene solubles ("XS"), i.e. the amount of the xylene soluble fraction, is 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 ¹³ C-NMR analysis is performed using a 400 MHz Bruker NMR spectrometer 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 is acquired using proton decoupling, 4000 scans per spectrum, a pulse repetition delay of 20 seconds and a spectral width of 26000 Hz. The sample is 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 (CeD 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 are dissolved in 2.0 ml of TCB, followed by addition of 0.5 ml of CeD 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 2.03 ppm.

The isotacticity is 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, mmrr and mrrm are assigned using published data, for example A. Razavi, Macromol. Symp., vol. 89, pages 345-367. Only the pentads mmmm, mmmr, mmrr and mrrm are taken into consideration due to the weak intensity of the signals corresponding to the remaining pentads. For the signal relating to the mmrr pentad a correction is performed for its overlap with a methyl signal related to 2, 1 -insertions. The percentage of mmmm pentads is then calculated according to

% mmmm = AREAmmmm / (AREAmmmm + AREAmmmr + AREAmmrr +

AREA _mrrm ) · 100 Determination of the percentage of 2, 1 -insertions for a metallocene propylene homopolymer: The signals corresponding to the 2, 1 -insertions are identified with the aid of published data, for example H.N. Cheng, J. Ewen, Makromol. Chem., vol. 190 (1989), pages 1931 -1940. A first area, AREA1 , is defined as the average area of the signals corresponding to 2, 1 -insertions. A second area, AREA2, is defined as the average area of the signals corresponding to 1 ,2- insertions. The assignment of the signals relating to the 1 ,2-insertions is well known to the skilled person and need not be explained further. The percentage of 2, 1 -insertions is calculated according to

2, 1 -insertions (in %) = AREA1 / (AREA1 + AREA2) · 100

with the percentage in 2, 1 -insertions being given as the molar percentage of 2, 1 -inserted propylene with respect to total propylene. The determination of the percentage of 2, 1 -insertions for a metallocene random copolymer of propylene and ethylene is determined by two contributions:

(i) the percentage of 2, 1 -insertions as defined above for the propylene homopolymer, and

(ii) the percentage of 2, 1 -insertions, wherein the 2, 1 -inserted propylene neighbors an ethylene,

thus the total percentage of 2, 1 -insertions corresponds to the sum of these two contributions. The assignments of the signal for case (ii) can be done either by using reference spectra or by referring to the published literature.

Melting temperatures T _me it are measured on a DSC Q2000 instrument by TA Instruments based on ISO 3146. To erase the thermal history the samples are first heated to 200°C and kept at 200°C for a period of 3 minutes. The reported melting temperatures T _me it are then determined with heating and cooling rates of 20°C/min.

For the examples the following materials were used:

- PP1 and PP2 are propylene homopolymers produced in a bulk loop reactor using a bridged 2,4-disubstituted bis-indenyl zirconocene on a silica support.

- PP3 is a propylene homopolymer produced in a bulk loop reactor using a Ziegler-Natta catalyst.

- PE1 is a polyethylene produced by copolymerization of ethylene and an a-olefin in a slurry loop reactor using a bridged bis- tetrahydroindenyl zirconocene on a a silica support. It has a melt index MI2 of 4.0 dg/min, a density of 0.960 g/cm ³ , a melting temperature T _me it of 134 °C and a molecular weight distribution M _w /Mn of 2.8.

All three polypropylenes contained an additivation of antioxidants and an acid scavenger in amounts sufficient to avoid polymer degradation during processing. In addition, PP2 and PP3 contained 250 ppm of a strong nucleating agent. Further properties of PP1 , PP2 and PP3 are given in Table 1 .

Table 1 - Polymer properties

Below detection limit.

SHEET EXTRUSION Sheet having a thickness of 1 mm was produced on a 1 m wide Reifenhauser sheet extrusion line with an upward chill roll stack having three chill rolls, a 70 mm main extruder having a ratio of length to diameter (L/D) of 33, a 50 mm side extruder having a L/D of 30, a melt pump and a coathanger die. For the production of monolayer sheet, i.e. sheet consisting of a polypropylene layer, the same polymer was fed to both extruders. For the production of the co- extruded sheet having a polypropylene layer and a polyethylene layer the respective polypropylene composition was fed to the main extruder, the polyethylene composition to the side extruder, with both extruders being set to the same temperatures. The co-extruded sheet was configured such that the polypropylene layer contacted the chill roll first. Respective thicknesses were 750 pm for the polypropylene layer and 250 pm for the polyethylene layer.

Further processing conditions, such as extruder temperatures, die temperatures and chill roll temperatures, are given in Table 2. Table 2 - Sheet extrusion conditions

THERMOFORMING

After having been stored under ambient conditions for 7 days, the so-obtained sheet was thermoformed by plug-assisted pressure forming on a Gabler Swing thermoforming machine into cups having a depth of approximately 50 mm and an inner diameter of about 85 mm at the top and of 65 to 67 mm at the bottom with a rim of 5 mm at the top using a four-fold mold. Processing conditions are given in Table 3. Mechanical properties of the thermoformed cups are given in Table 4. Table 3 - Thermoforming conditions

The comparison of the thermoforming conditions of Example 2 and comparative Example 4 shows that a sheet consisting of a polypropylene produced with a metallocene-based polymerization catalyst can be thermoformed at a sheet temperature that is 10°C below that of a sheet consisting of a polypropylene produced with a Ziegler-Natta polymerization catalyst. It has come as a surprise to the present inventors that a polypropylene, which has a high melt index and was produced with a metallocene-based polymerization catalyst, and a polypropylene, which has a much lower melt flow index and was produced with a Ziegler-Natta catalyst, could be processed under similar conditions in the thermoforming stage without encountering significant processing difficulties due to the much lower melt strength of the polypropylene as defined in the present application. Table 4 - Properties of thermoformed cups

Polypropylene as internal layer, polyethylene as external layer.

Falling height: 1 m

Reported as the number of respective failure per 5 samples tested.

Surprisingly, the properties of thermoformed cups are essentially the same for the polypropylene as defined in the present application (see Example 2), and a conventional polypropylene used in extrusion-thermoforming (see Example 4), particularly as the present approach of using a high melt flow index polypropylene, preferably in combination with a narrow molecular weight distribution, runs counter to what the person skilled in the art would normally do as has been explained earlier in this application. In absence of nucleating agent, the polypropylene used in Example 1 resulted in opaque cups, as can be seen from the respective optical data in Table 4. Surprisingly these cups were also characterized by a soft touch. Such a soft touch is normally only obtainable by the addition of costly additives to the polypropylene. Thus, the present invention surprisingly allows for easier and more economic production of opaque, soft touch cups.

The results for Example 3 show that a polypropylene layer and a polyethylene layer may be co-extruded without any binding layer between and still result in good properties for the thermoformed article, and in consequence for the sheet as well. This allows for simplified production of sheet and thermoformed articles having a polypropylene layer and a polyethylene layer adjacent to one another.

The data for Example 3 also shows that the additional polyethylene layer greatly improves impact properties of the thermoformed article at lower temperatures, i.e. temperatures used in refrigeration. Based thereon it is expected that thermoformed articles produced by co-extrusion of a polypropylene layer and a polyethylene layer as defined in the present application are autoclaveable and allow the packaging of food to be stored at refrigerator or freezer temperatures and heated in the same package.