WO2012066118A1

WO2012066118A1 - Improved adhesion between polypropylene and polyethylene layers

Info

Publication number: WO2012066118A1
Application number: PCT/EP2011/070444
Authority: WO
Inventors: Olivier Lhost; Pascal Navez
Original assignee: Total Petrochemicals Research Feluy
Priority date: 2010-11-18
Filing date: 2011-11-18
Publication date: 2012-05-24
Also published as: TW201231274A

Abstract

The present invention relates to articles comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein the polypropylene layer consists of a polypropylene composition comprising a polypropylene produced with a metallocene-based polymerization catalyst. Said articles are characterized by excellent adhesion between the polypropylene layer and the polyethylene layer. Further, the present application relates to the manufacture of such articles.

Description

IMPROVED ADHESION BETWEEN POLYPROPYLENE AND

POLYETHYLENE LAYERS

Field of the invention

The present application relates to articles comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein the polypropylene layer consists of a polypropylene composition comprising a polypropylene produced with a metallocene-based polymerization catalyst. Said articles are characterized by excellent adhesion between the polypropylene layer and the polyethylene layer. Further, the present application relates to the manufacture of such articles.

The technical problem and the prior art

Polypropylene and polyethylene are used in a wide range of applications having widely varying property requirements. Generally, polyethylene is preferred when the required properties include for example flexibility, toughness, impact resistance or tear resistance. Polypropylene is preferred when the required properties include for example stiffness and chemical resistance.

While polypropylene as well as polyethylene, individually taken, have properties that allow producing pure polypropylene or polyethylene articles, for example films or sheet, such articles tend to be deficient in at least one property. For example, a polyethylene film has good sealing properties but might lack in rigidity, while a polypropylene film has good rigidity but might lack in sealing properties. Knowing that the properties of polypropylene and polyethylene complement each other, it seems attractive to consider multilayered articles with a polypropylene and a polyethylene layer. However, the thermodynamical incompatibility of polypropylene and polyethylene generally requires such multilayered articles to have a tie layer between the polypropylene layer and the polyethylene layer, thus leading to added difficulties in the production of such articles. Due to their incompatibility multilayer articles with a polypropylene layer adjacent to a polyethylene layer without any tie layer between tend to delaminate.

Recent results have shown that the tendency to delaminate can be reduced and the adhesion between a polypropylene layer and a polyethylene layer improved if the polyethylene is produced with a metallocene-based polymerization catalyst.

However, in spite of the progress made, there remains further interest to find solutions that would further increase the adhesion between a polypropylene layer and polyethylene layer, without necessarily having to revert to the use of a tie layer in between.

It is therefore an object of the present application to provide an article comprising a polypropylene layer and a polyethylene layer adjacent to one another having good adhesion between each other.

It is also an object of the present application to provide an article comprising a polypropylene layer and a polyethylene layer adjacent to one another having good adhesion between each other without necessarily having to use a tie layer in between. It is a further object of the present application to provide articles having good mechanical properties, particularly impact performance, rigidity or both.

Furthermore, it is an object of the present application to provide articles having good optical properties.

Additionally, it is an object of the present application to provide articles having good sealing properties. It is a further object of the present application to provide a method for producing such an article.

Brief description of the invention

The presently named inventors have surprisingly found that any one of these objects can be attained individually or in combination by providing an article comprising a polypropylene layer and a polyethylene layer adjacent to each other, wherein the polypropylene layer consists of a well-defined polypropylene composition.

Thus, the present application provides for an article comprising a polypropylene layer and a polyethylene layer adjacent to each other, wherein said polypropylene layer consists of a polypropylene composition comprising at least 70 wt%, relative to the weight of said polypropylene composition, of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer. Preferred articles provided for in the present application are selected from the group consisting of sheet, film, thermoformed article, blow-molded article, rotomolded article and pipe. Further, the present application provides for a process for the production of the above article, wherein the article is selected from the group consisting of sheet, film, thermoformed article, blow-molded article and pipe, said process comprising the steps of

(a) polymerizing propylene and at least one comonomer with a metallocene- based polymerization catalyst to obtain a metallocene random copolymer of propylene and at least one comonomer, wherein the random copolymer comprises at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer,

(b) polymerizing ethylene or copolymerizing ethylene and at least one comonomer in presence of a polymerization catalyst to obtain an ethylene polymer,

(c) preparing a polypropylene composition, which comprises at least 70 wt%, relative to the total weight of said polypropylene composition, of the random copolymer of propylene and at least one comonomer obtained in step (a),

(d) preparing a polyethylene composition, which comprises at least 70 wt%, relative to the total weight of said polyethylene composition, of the ethylene polymer obtained in step (b),

(e) co-extruding the polypropylene composition prepared in step (c) and the polyethylene composition prepared in step (d) to prepare an article comprising a polypropylene layer and a polyethylene layer adjacent to one another.

Additionally, the present application provides for a process for the production of the above article, wherein the article is a rotomolded article, said process comprising the steps of (A) - polymerizing propylene and at least one comonomer in presence of a metallocene-based polymerization catalyst to obtain a random copolymer of propylene and at least one comonomer, wherein the random copolymer comprises at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer,

- preparing a polypropylene composition, which comprises at least 70 wt%, relative to the total weight of said polypropylene composition, of the metallocene random copolymer of propylene and at least one comonomer,

- feeding said polypropylene composition into the mold of a rotomolding machine,

- heating said polypropylene composition in the mold to a temperature above its melting temperature while continuously rotating the mold, and

- subsequently cooling the mold;

(B) - polymerizing ethylene or copolymerizing ethylene and at least one comonomer in presence of a polymerization catalyst to obtain an ethylene polymer,

- preparing a polyethylene composition, which comprises at least 70 wt%, relative to the total weight of said polyethylene composition, of said ethylene polymer,

- feeding said polyethylene composition into the mold of a rotomolding machine,

- heating said polyethylene composition in the mold to a temperature above its melting temperature while continuously rotating the mold, and

- subsequently cooling the mold; and

(C) removing the rotomolded article from the mold,

wherein steps (A) and (B) directly follow each other in either order.

Furthermore, the present application provides for the use of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, in an article comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of said random copolymer of propylene and at least one comonomer, to improve the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, between the polypropylene layer and the polyethylene layer to at least 120 % of the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, for the same article wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of a random copolymer of propylene and at least one comonomer produced with a Ziegler-Natta catalyst, wherein the random copolymer of propylene and at least one comonomer produced with a metallocene-based polymerization catalyst and the random copolymer of propylene at at least one comonomer produced with a Ziegler-Natta catalyst have melt flow indices, determined according to ISO 1 133, condition L, at 230°C and 2.16 kg, differing by at most 15 % relative to the higher melt flow index, and have comonomer contents differing by at most 10 %, relative to the higher comonomer content.

Use of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at least 0.5 wt% and at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, in an article comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of said random copolymer of propylene and at least one comonomer, to improve the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, between the polypropylene layer and the polyethylene layer to at least 120 % of the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, for the same article wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of a propylene homopolymer produced with a metallocene-based polymerization catalyst, wherein the random copolymer of propylene and at least one comonomer and the propylene homopolymer have melt flow indices, determined according to ISO 1 133, condition L, at 230°C and 2.16 kg, differing by at most 15 % relative to the higher melt flow index.

Detailed description of the invention

Throughout the present application the terms "polypropylene" and "propylene polymer" or "propylene copolymer" may be used synonymously.

Throughout the present application the term "propylene random copolymer" may be used to denote a "random copolymer of propylene and at least one comonomer". The term "metallocene propylene random copolymer" may be used to denote a "random copolymer of propylene and at least one comonomer having been produced with a metallocene-based polymerization catalyst".

Throughout the present application the terms "polyethylene" and "ethylene polymer" or "ethylene copolymer" 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 190° and 2.16 kg. Throughout 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. In general terms, the present application provides for an article comprising a polypropylene layer and a polyethylene layer adjacent to each other, wherein the polypropylene layer consists of a polypropylene composition and the polyethylene layer consists of a polyethylene composition. Preferably, the articles is selected from the group consisting of sheet, film, thermoformed article, blow-molded article, rotomolded article and pipe.

POLYPROPYLENE COMPOSITION The polypropylene composition, of which the polypropylene layer consists, comprises at least 70 wt%, relative to the total weight of the polypropylene composition, of a random copolymer of propylene and at least one comonomer. For the purpose of the present application it is essential that said random copolymer of propylene and at least one comonomer has been produced with a metallocene-based polymerization catalyst, i.e. is a metallocene propylene random copolymer. It is further essential that said random copolymer of propylene and at least one comonomer comprises at most 6.0 wt%, relative to the total weight of the random copolymer, of the at least one comonomer. Preferably, the polypropylene composition comprises at least 80 wt%, more preferably at least 90 wt% or 95 wt%, even more preferably at least 97 wt%, and still even more preferably at least 99 wt%, relative to the total weight of said polypropylene composition, of the metallocene propylene random copolymer. Most preferably, said polypropylene composition consists of the metallocene propylene random copolymer. Preferably, the metallocene propylene random copolymer comprises at least 0.5 wt%, relative to the total weight of said metallocene propylene random copolymer, of the at least one comonomer. Preferably, the metallocene propylene random copolymer comprises at most 5.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, more preferably at most 4.5 wt%, even more at most 4.0 wt%, still even more preferably at most 3.5 wt% or 3.0 wt%, and most preferably at most 2.5 wt%.

Preferably, the metallocene propylene random copolymer of propylene and at least one comonomer is a random copolymer of propylene and at least one alpha-olefin different from propylene. 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. Most preferably, the at least one comonomer is ethylene.

Preferably, the metallocene propylene random copolymer used herein has a melt flow index of at least 0.5 dg/min, and most preferably of at least 1 .0 dg/min. Preferably, the metallocene propylene random copolymer has a melt flow index of at most 20 dg/min, more preferably of at most 18 dg/min, even more preferably of at most 16 dg/min or 14 dg/min, still even more preferably of at most 12 dg/min, and most preferably of at most 10 dg/min.

Preferably, the metallocene propylene random copolymer 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 metallocene propylene random copolymer 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. Preferably, the metallocene propylene random copolymer 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 ¹³C-NMR analysis as described in the test methods.

Preferably, the metallocene propylene random copolymer 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. The metallocene propylene random copolymer used herein is obtained by polymerizing propylene and at least one comonomer with a metallocene-based polymerization catalyst. Preferably the metallocene-based polymerization catalyst comprises 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)MX¹X² (I) wherein R^a, R^b, R^c, M, X¹ and X² 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 -(CR¹R²)_P- -(SiR¹R²)_p- - (GeR¹R²)p- -(NR¹)_P- -(PR¹)_P- -(N⁺R¹R²)_P- and -(P⁺R¹R²)_P- and p is 1 or 2, and wherein R¹ and R² are each independently selected from the group consisting of hydrogen, d-do alkyl, Cs-Cs cycloalkyl, C6-C15 aryl, alkylaryl with d-C-io alkyl and C6-C15 aryl, or any two neighboring R (i.e. two neighboring R¹, two neighboring R², or R¹ with a neighboring R²) may form a cyclic saturated or non-saturated C4-C10 ring; each R¹ and R² may in turn be substituted in the same way. Preferably R^a is -(CR¹R²)_P- or -(SiR¹R²)_p- with R¹, R² and p as defined above. Most preferably R^a is -(SiR¹R²)_p- with R¹, R² and p as defined above. Specific examples of R^a include Me₂C, ethanediyl (-CH2-CH2-), Ph₂C and Me₂Si.

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

X¹ and X² 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 X¹ and X² 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 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 (- CH₂-Ph), and -(CH₂)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 CsR³R R⁵R⁶. A substituted indenyl may for example be represented by the general formula C₉R⁷R⁸R⁹R¹⁰R^{1 1} R¹²R^{1 3}R¹⁴ A substituted tetrahydroindenyl may for example be represented by the general formula C9H₄R^{1 5}R¹⁶R¹⁷R¹⁸. A substituted fluorenyl may for example be represented by the general formula Ci₃R¹⁹R²⁰R²¹ R²²R²³R² R²⁵R²⁶ Each of the substituents R³ to R²⁶ 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 C₄-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 -CR²⁷R²⁸R²⁹ or -SiR²⁷R²⁸R²⁹ with R²⁷, R²⁸ and R²⁹ 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 R²⁷, R²⁸ and R²⁹ are methyl.

Examples of particularly suitable metallocenes are:

dimethylsilanediyl-bis(2-methyl-cyclopentadienyl)zirconium dichloride, dimethylsilanediyl-bis(3-methyl-cyclopentadienyl)zirconium dichloride, dimethylsilanediyl-bis(3-tert-butyl-cyclopentadienyl)zirconium dichloride, dimethylsilanediyl-bis(3-tert-butyl-5-methyl-cyclopentadienyl)zirconium

dichloride,

dimethylsilanediyl-bis(2,4-dimethyl-cyclopentadienyl)zirconium 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)zirconium dichloride, dimethylsilanediyl-bis(4-phenyl-indenyl)zirconium dichloride, dimethylsilanediyl-bis(2-methyl-4-phenyl-indenyl)zirconium 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)(fluorenyl) 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 metallocene propylene random copolymer used herein is produced by polymerizing propylene and at least one comonomer in presence of a metallocene-based polymerization catalyst to obtain a metallocene random copolymer of propylene and at least one comonomer. The polymerization of propylene and the at least one comonomer in presence of a metallocene-based polymerization catalyst can be carried out according to known techniques in one or more polymerization reactors. The metallocene propylene random copolymer used herein is preferably produced by polymerization in liquid propylene at temperatures in the range from 20°C to 100°C. More preferred 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 resulting metallocene propylene random copolymer, is regulated by the addition of hydrogen to the polymerization medium.

Preferably, the metallocene propylene random copolymer is recovered from the one or more polymerization reactors without post-reactor treatment, such as thermal or chemical degradation, to reduce its molecular weight and/or narrow the molecular weight distribution, as is often done for polypropylene produced with a Ziegler-Natta catalyst.

POLYETHYLENE COMPOSITION

The polyethylene composition, of which the polyethylene layer consists, preferably comprises at least 70 wt%, relative to the total weight of said polyethylene composition, of at least one polyethylene. Said polyethylene composition more preferably comprises at least 80 wt%, even more preferably at least 90 wt% or 95 wt%, still even more preferably at least 97 wt% or 99 wt%, and most preferably consists of the at least one polyethylene.

The at least one 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. Preferably, the at least one polyethylene used herein has a density of at least 0.920 g/cm³, more preferably of at least 0.925 g/cm³ or 0.930 g/cm³, even more preferably of at least 0.935 g/cm³ or 0.940 g/cm³, still even more preferably of at least 0.945 g/cm³ and most preferably of at least 0.950 g/cm³. Preferably, it has a density of at most 0.970 g/cm³, more preferably of at most 0.965 g/cm³, and most preferably of at most 0.960 g/cm³.

Preferably, the melt index (MI2) of the at least one polyethylene is at least 0.5 dg/min, most preferably at least 1 .0 dg/min. Preferably, the melt index (MI2) of the at least one polyethylene is at most 20 dg/min, more preferably of at most 18 dg/min, even more preferably of at most 16 dg/min or 14 dg/min, still even more preferably of at most 12 dg/min, and most preferably of at most 10 dg/min. Preferably, the at least one 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. The at least one polyethylene used herein can be produced in a polymerization process in presence of a polymerization catalyst 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.

Suitable polymerization catalysts for the polymerization of ethylene and - if present - one or more comonomers are chromium-based polymerization catalysts, Ziegler-Natta polymerization catalysts and metallocene-based polymerization catalysts. Of these, Ziegler-Natta polymerization catalysts and metallocene-based polymerization catalysts are preferred; and metallocene- based polymerization catalysts are most preferred.

The metallocene component used in the production of the at least one polyethylene used herein can be described by the following general formula

(M-R^a)_n(R^b)(R^c)MX¹X² 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 -(CR¹R²)p- -(SiR¹R²)p- -(GeR¹R²)p- -(NR¹)_P- -(PR¹)_P- -(N⁺R¹R²)_P- and - (P⁺R¹R²)p- and p is 1 or 2, and wherein R¹ and R² are each independently selected from the group consisting of hydrogen, d-do alkyl, Cs-Ce cycloalkyl, C6-C15 aryl, alkylaryl with d-do alkyl and C6-C15 aryl, or any two neighboring R may form a cyclic saturated or non-saturated C4-C10 ring; each R¹ and R² may in turn be substituted in the same way. Preferably R^a is -(CR¹R²)_P- or - (SiR¹R²)_p- with R¹, R² and p as defined above. Most preferably R^a is - (CR¹R²)_P- with R¹, R² and p as defined above. Specific examples of R^a include Me₂C, ethanediyl (-CH₂-CH₂-), Ph₂C and Me₂Si.

M is a metal selected from Ti, Zr and Hf, preferably it is Zr. X¹ and X² are independently selected from the group consisting of halogen, hydrogen, d-do alkyl, C6-C15 aryl, alkylaryl with d-do alkyl and C6-C15 aryl. Preferably X¹ and X² 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 d-do alkyl, C5-C7 cycloalkyl, C6-C15 aryl, and alkylaryl with d-do 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 d-do 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 d-C-i o alky I and C6-C15 aryl are benzyl (- CH₂-Ph), and -(CH₂)₂-Ph.

It is, however, preferred to use a metallocene component of the following general formula, wherein

- n is 1 ;

- R^a is -(CR¹R²)p- or -(SiR¹R²)_p- most preferably R^a is -(CR¹R²)_P- with R¹, R² and p as defined above;

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

- X¹ and X² 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.

The polypropylene composition used herein may further comprise a thermoplastic polymer different from the random copolymer of propylene and at least comonomer produced with a metallocene-based polymerization catalyst as defined in the present application. The polyethylene composition may further comprise a thermoplastic polymer different from the at least one polyethylene as defined in the present application.

Preferred suitable thermoplastic polymers are for example propylene homopolymers, copolymers of propylene and at least one comonomer, different from the random copolymer as defined above (e.g. produced with a Ziegler- Natta catalyst instead of a metallocene-based polymerization catalyst, or with a different metallocene-based polymerization catalyst, or with a different 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 (MI2) of the polyethylene composition used herein, it is preferred that they within the same ranges and values as defined above for the random copolymer of propylene and at least one comonomer, produced in presence of a metallocene-based polymerization catalyst, as defined in the present application, resp. the at least one polyethylene as defined above. The polypropylene composition as well as the polyethylene composition 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. ARTICLES AND THEIR PRODUCTION

The above defined article comprising a polypropylene layer and a polyethylene layer adjacent to each other is preferably an article selected from the group consisting of sheet, film, thermoformed article, blow-molded article, rotomolded article and pipe. These articles may be produced by generally known methods, which are briefly described in the following.

Sheet and film are typically produced by melt-extruding a polymer by a process comprising for example the following steps

(i) feeding a polymer composition to an extruder;

(ii) subsequently melting the polymer composition in the extruder to obtain a molten polymer composition;

(iii) melt-extruding the molten polymer composition obtained in step (ii) through a slit die to form an extrudate; and

(iv) cooling the extrudate to obtain a sheet or a film.

Thermoformed articles are typically produced by a process comprising the steps of

(i) feeding sheet to a thermoforming machine, wherein the sheet has a preferred thickness in the range from 500 pm to 2000 pm;

(ii) optionally warming the sheet to a temperature at which it is soft, to obtain a soft sheet;

(iii) draping the soft sheet over or into a mold, thus obtaining a formed sheet; (iv) cooling the formed sheet to a temperature at which it maintains its shape; and

(v) removing the formed sheet from the mold.

The sheet used in step (i) of the process for the production of thermoformed articles may either be directly coming from a sheet production line ("in-line thermoforming") or may have been stored for some time (e.g. a few hours, days or months) before being fed to the thermoforming line. Blow-molded articles are generally formed by extrusion blow molding, the extrusion blow molding process comprising the steps of

(i) feeding a polymer composition to an extruder;

(iii) melt-extruding, preferably in a downward direction, the molten polymer composition obtained in step (ii) through a circular die to obtain a hollow extrudate (parison);

(iv) clamping the hollow extrudate into a mold so that the extrudate is closed off at both ends;

(v) injecting a gas under pressure into the clamped-off parison to obtain an expanded parison having the shape of the mold;

(vi) cooling and ejecting to obtain the blow-molded article.

Pipes are generally produced by a process comprising the following steps:

(i) feeding a polymer composition to an extruder;

(iii) melt-extruding, preferably horizontally, the molten polymer composition obtained in step (ii) through a circular die to obtain a hollow extrudate;

(iv) cooling the hollow extrudate to obtain the pipe.

The sheet, film, thermoformed article, blow-molded article and pipe of the present application may be produced by co-extrusion or by coating.

In co-extrusion, the polypropylene composition and the polyethylene composition are fed to separate extruders, molten to form the respective molten compositions, which are then extruded from a die, whereupon the still molten extrudates are combined and cooled. Hence, the process for the production of an article selected from the group consisting of sheet, film, thermoformed article, blow-molded article and pipe comprises the steps of (a) polymerizing propylene and at least one comonomer with a metallocene- based polymerization catalyst to obtain a metallocene random copolymer of propylene and at least one comonomer as defined above;

(b) polymerizing ethylene or copolymerizing ethylene and at least one comonomer in presence of a polymerization catalyst to obtain an ethylene polymer as defined above;

(c) preparing a polypropylene composition, which comprises at least 70 wt%, relative to the weight of said polypropylene composition, of the metallocene random copolymer of propylene and at least one comonomer obtained in step (a);

(d) preparing a polyethylene composition, which comprises at least 70 wt%, relative to the total weight of said polyethylene composition, of the the ethylene polymer obtained in step (b); and

(e) co-extruding the polypropylene composition prepared in step (c) and the polyethylene composition in step (d) to prepare an article comprising a polypropylene layer and a polyethylene layer adjacent to one another.

The co-extrusion step (e) may comprise the following steps:

(e1 ) feeding the polypropylene composition prepared in step (c) to a first extruder, and melting the polypropylene composition to obtain a molten polypropylene composition;

(e2) feeding the polyethylene composition prepared in step (d) to a second extruder, and melting the polyethylene composition to obtain a molten polyethylene composition;

(e3) extruding the molten polypropylene composition and the molten polyethylene composition from a die such that the respective extrudates are combined to form an article comprising a polypropylene layer and a polyethylene layer adjacent to one another.

Coating is a process comprising first the production of an intermediate article, to which later one or more further layers are applied to. In coating, the article is preferably selected from the group consisting of sheet, film and pipe. Hence, the process for the production of an article selected from the group consisting of sheet, film, thermoformed article, blow-molded article and pipe comprises the steps of

(a¹) polymerizing propylene and at least one comonomer with a metallocene- based polymerization catalyst to obtain a metallocene random copolymer of propylene and at least one comonomer as defined above;

(b¹) preparing a polypropylene composition, which comprises at least 70 wt%, relative to the weight of said polypropylene composition, of the metallocene random copolymer of propylene and at least one comonomer obtained in step (a¹);

(c¹) feeding the polypropylene composition prepared in step (b¹) to an extruder and melting it to obtain a molten polypropylene composition;

(d¹) extruding the molten composition of step (c¹) through a die to form an extrudate;

(e¹) cooling the extrudate to form an intermediate article;

(f) polymerizing ethylene or copolymerizing ethylene and at least one comonomer in presence of a polymerization catalyst to obtain an ethylene polymer as defined above;

(g¹) preparing a polyethylene composition, which comprises at least 70 wt%, relative to the total weight of said polyethylene composition, of the the ethylene polymer obtained in step (f );

(h¹) feeding the polyethylene composition prepared in step (g¹) to an extruder and melting it to obtain a molten composition;

(i¹) extruding the molten composition of step (h¹) through a die onto the intermediate article obtained in step (e¹) to form an article such that the article comprises a polypropylene layer and a polyethylene layer adjacent to one another.

For the purposes of the present application the intermediate article my either lack the polypropylene layer or the polyethylene layer, which is then applied later so as to obtain the article as defined previously in the present application. Thus, the process may also be conducted in such a way that the order of steps is (f), (g¹), (h¹), (d¹), (e¹), (a¹), (b¹), (C), and (i¹).

Co-extrusion and coating are well known and widely used in the industry so that the processes for production of co-extruded or coated sheet, film, thermoformed article, blow-molded article and pipe need not be discussed in more detail so that the skilled person can reproduce the articles and processes provided for in the present application. The rotomolded article provided for in the present application is produced by first rotomolding from a first polymer composition an intermediate single layered rotomolded article in a mold, which is subsequently cooled, whereupon a second polymer composition is fed to the mold, i.e. the inside of the rotomolded article. For the present application either the polypropylene layer or the polyethylene layer may be made first; it is, however, preferred that the polypropylene layer is produced first, and the polyethylene layer second. In any case, it is important that the two steps directly follow each other. Hence, the process for the production of a rotomolded article, said article having been defined earlier in this application, comprises the steps of

(A) - polymerizing propylene and at least one comonomer in presence of a metallocene-based polymerization catalyst to obtain a metallocene random copolymer of propylene and at least one comonomer as defined above,

- preparing a polypropylene composition, which comprises at least 70 wt%, relative to the total weight of said polypropylene composition, of said metallocene random copolymer of propylene and at least one comonomer,

- subsequently cooling the mold; (B) - polymerizing ethylene or copolymerizing ethylene and at least one comonomer in presence of a polymerization catalyst to obtain an ethylene polymer as defined above,

- feeding said polyethylene composition into the mold of a rotomolding machine,

- subsequently cooling the mold; and

(C) removing the rotomolded article from the mold,

wherein steps (A) and (B) directly follow each other in either order. Preferably step (A) is followed by step (B). Because steps (A) and (B) directly follow each other, the resulting rotomolded article has a polypropylene layer directly adjacent to a polyethylene layer. Depending upon the order of steps (A) and (B) the exterior of the rotomolded article may either be formed by the polyethylene layer or the polypropylene layer. The presently named inventors have been very surprised to see that the adhesion is dramatically increased when a metallocene random copolymer of propylene and at least one comonomer as defined previously in the present application is used in the polypropylene layer of an article comprising a polypropylene layer and a polyethylene layer adjacent to one another.

Hence, the present application also provides for the use of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, in an article comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of said random copolymer of propylene and at least one comonomer, to improve the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, between the polypropylene layer and the polyethylene layer to at least 120 %, preferably to at least 130 %, even more preferably to at least 140 %, and most preferably to at least 150 %, of the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, for the same article wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of a random copolymer of propylene and at least one comonomer produced with a Ziegler-Natta catalyst, wherein the random copolymer of propylene and at least one comonomer produced with a metallocene-based polymerization catalyst and the random copolymer of propylene at at least one comonomer produced with a Ziegler-Natta catalyst have melt flow indices, determined according to ISO 1 133, condition L, at 230°C and 2.16 kg, differing by at most 15 % relative to the higher melt flow index, and have comonomer contents differing by at most 10 %, relative to the higher comonomer content.

It has also come as a complete surprise that the use of a metallocene random copolymer comprising from 0.5 wt% to 6.0 wt%, relative to the total weight of said random copolymer, of at least one comonomer leads to a further drastic increase in adhesion as compared to the adhesion achieved with a propylene homopolymer instead. In consequence, the present application further provides for the use of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at least 0.5 wt% and at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, in an article comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of said random copolymer of propylene and at least one comonomer, to improve the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, between the polypropylene layer and the polyethylene layer to at least 120 %, preferably to at least 130 %, even more preferably to at least 140 %, and most preferably to at least 150 %, of the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, for the same article wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of a propylene homopolymer produced with a metallocene-based polymerization catalyst, wherein the random copolymer of propylene and at least one comonomer and the propylene homopolymer have melt flow indices, determined according to ISO 1 133, condition L, at 230°C and 2.16 kg, differing by at most 15 % relative to the higher melt flow index.

Examples

The following examples illustrate the advantages of the present invention. While the examples have been performed using films, the presently named inventors firmly believe that the respective results can directly be applied to other articles such as blow-molded articles, rotomolded articles, thermoformed articles and pipes.

TEST METHODS

The 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. The 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.

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 Mp_P = 1000.

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

The molecular weight distribution (MWD) is then calculated as M_w/M_n.

The ¹³C-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 (C6D₆, 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 CeD₆ 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 ¹³C-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_meit 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_meit are then determined with heating and cooling rates of 20°C/min. 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.

Haze was measured according to ISO 14782: 1999 on samples taken from the side wall of the extrusion blow-molded bottles.

Gloss was determined in accordance with ASTM-D 2457 at an angle of 20° on samples taken from the side wall of the extrusion blow-molded bottles. Drop tests were carried out using water-filled bottles by dropping the bottles from a height, which was increased until 50 % of the bottles failed, i.e. exhibited a crack. The reported height is then reported. POLYPROPYLENES

Table 1 shows the properties of the polypropylenes used in the examples.

All polypropylenes were produced in a bulk loop reactor using a commercially available Ziegler-Natta catalyst in the production of PP-01 to PP-04 and a metallocene-based polymerization catalyst with a dimethylsilyl-bridged bis(indenyl)zirconium dichloride derivative as metallocene component in the production of PP-05 to PP-10.

All polypropylenes contained a sufficient amount of antioxidants and acid scavengers to reduce their degradation during processing.

Table 1

POLYETHYLENES Table 2 shows the properties of the polyethylenes used in the examples. Table 2

Polyethylene PE-01 was produced in a slurry loop reactor using a supported metallocene-based polymerization catalyst with ethanediyl- bis(tetrahydroindenyl)zirconium dichloride as metallocene component. Polyethylene PE-02 was produced in a gas-phase reactor using a Phillips-type chromium-based polymerization catalyst.

All polyethylenes contained a sufficient amount of antioxidants and acid scavenger to reduce their degradation during processing.

FILM

Cast films having a thickness of about 200 pm were produced from the polypropylenes PP-01 to PP-10 as well as from the polyethylenes PE-01 and PE-02 using a Brabender single-screw extruder with a diameter of 19 mm and a ratio length over diameter (L/D) of 25, equipped with a flat sheet die (150 mm wide; 0.5 mm die gap) and a Brabender Univex chill roll and take-off system. Extruder temperature was set at 220°C. Screw speed was 70 rpm. Chill roll temperatures were maintained at 20°C for the extrusion of the polypropylenes PP-01 to PP-10 and at 60°C for the extrusion of polyethylenes PE-01 and PE- 02. Take-off speed, determined as the chill roll speed, was ca. 10 m/min.

ADHESION

A polypropylene film and a polyethylene film produced as described above were sealed together on a Brugger HSG-C 951 heat-sealing machine. Sealing was performed under the following conditions: - Temperature: 180°C

- Pressure: 10 N, imposed on the jaw of 150 mm by 10 mm

- Sealing time: 3.5 s The sealed films were stored for 24 hours at a temperature of 23°C and a humidity of ca. 50 %.

Adhesion was determined by pulling apart the seal in a Zwick Z 2.5/TH1 S at a speed of 200 mm/min following standard ASTM D 88 and recording the maximum force necessary to pull the seal apart. The results in N are given in Table 3.

Table 3

Table 3 clearly shows the significant advantages of the articles provided for in the present application. The use of a polypropylene produced with a metallocene-based polymerization catalyst drastically improves in an absolutely surprising degree the adhesion between a polypropylene layer and a polyethylene layer, without having to revert to the use of a tie layer between the two. The effect is even more pronounced when metallocene random copolymers of propylene and one or more comonomers are used in the polypropylene layer. This latter behaviour for metallocene polypropylene, i.e. that the adhesion is much improved for random copolymers, came as a complete surprise in view of the inverse behavior for Ziegler-Natta polypropylene.

EXTRUSION BLOW-MOLDED ARTICLE To further show the advantages of the present invention extrusion blow-molded 430 ml bi-layered bottles were produced by co-extruding PE-01 as external layer with PP-1 1 as internal layer on a Battenfeld VK extrusion blow molding machine. PP-1 1 was produced with metallocene-based polymerization catalyst with a dimethylsilyl-bridged bis(indenyl)zirconium dichloride derivative as metallocene component, was used. PP-1 1 had a melt flow index of 6.0 dg/min, an ethylene content of 2.0 wt%, a value M_w/M_n of 2.7, a melting temperature of 138°C, 0.7 % of 2, 1 -insertions and comprised 1400 ppm of ADK Na-21 , available from Ashai Denka. Blow molding conditions are indicated in Table 4. Representative properties of the extrusion-blow molded bottles are indicated in Table 5.

Table 4

Table 5

The results show that the bi-layer extrusion blow-molded bottle has improved drop test performance as well as better optical properties as evidenced by the haze and gloss data, all the while keeping the top load performance. Thus, the present invention allows to produce bottles comprising a polypropylene layer and a polyethylene layer adjacent to one another, said bottles being characterized by good optical and mechanical properties.

Claims

1 . Article comprising a polypropylene layer and a polyethylene layer adjacent to each other, wherein said polypropylene layer consists of a polypropylene composition comprising at least 70 wt%, relative to the weight of said polypropylene composition, of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer.

2. Article according to claim 1 , wherein the random copolymer of propylene and at least one comonomer comprises at least 0.5 wt% and at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer.

3. Article according to any of the preceding claims, wherein the random copolymer of propylene and at least one comonomer is a random copolymer of propylene and at least one alpha-olefin different from propylene, said a-olefin preferably being selected from the group consisting of ethylene, butene-1 , pentene-1 , hexene-1 and 4-methyl- pentene-1 , said a-olefin being most preferably ethylene.

4. Article according to any of the preceding claims, wherein the random copolymer of propylene and at least one comonomer has a melt flow index of at least 0.5 dg/min and of at most 20 dg/min, determined according to ISO 1 133, condition L, at 230°C and 2.16 kg.

5. Article according to any of the preceding claims, wherein the article is selected from the group consisting of sheet, film, thermoformed article, blow-molded article, rotomolded article and pipe. Process for the production of the article of claims 1 to 5, wherein the article is selected from the group consisting of sheet, film, thermoformed article, blow-molded article and pipe, said process comprising the steps of

(a) polymerizing propylene and at least one comonomer with a metallocene-based polymerization catalyst to obtain a metallocene random copolymer of propylene and at least one comonomer, wherein the random copolymer comprises at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer,

Process for the production of the article of claims 1 to 5, wherein the article is a rotomolded article, said process comprising the steps of

(A) - polymerizing propylene and at least one comonomer in presence of a metallocene-based polymerization catalyst to obtain a random copolymer of propylene and at least one comonomer, wherein the random copolymer comprises at most 6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, - preparing a polypropylene composition, which comprises at least 70 wt%, relative to the total weight of said polypropylene composition, of the metallocene random copolymer of propylene and at least one comonomer,

- subsequently cooling the mold;

- feeding said polyethylene composition into the mold of a rotomolding machine,

- subsequently cooling the mold; and

(C) removing the rotomolded article from the mold,

wherein steps (A) and (B) directly follow each other in either order.

8. Process for the production of the article according to claims 6 and 7, wherein the random copolymer of propylene and at least one comonomer is further defined as in any of claims 2 to 4. 9. Use of a random copolymer of propylene and at least one comonomer, said random copolymer having been produced with a metallocene-based polymerization catalyst, and said random copolymer comprising at most

6.0 wt%, relative to the total weight of said random copolymer, of the at least one comonomer, in an article comprising a polypropylene layer and a polyethylene layer adjacent to one another, wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of said random copolymer of propylene and at least one comonomer, to improve the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, between the polypropylene layer and the polyethylene layer to at least 120 % of the adhesion, defined as the maximum force necessary to pull the seal apart, determined following ASTM D 88, for the same article wherein said polypropylene layer comprises at least 70 wt%, relative to the total weight of said polypropylene layer, of a random copolymer of propylene and at least one comonomer produced with a Ziegler-Natta catalyst, wherein the random copolymer of propylene and at least one comonomer produced with a metallocene-based polymerization catalyst and the random copolymer of propylene at at least one comonomer produced with a Ziegler-Natta catalyst have melt flow indices, determined according to ISO 1 133, condition L, at 230°C and 2.16 kg, differing by at most 15 % relative to the higher melt flow index, and have comonomer contents differing by at most 10 %, relative to the higher comonomer content.

Use according to claims 8 and 9, wherein the random copolymer of propylene and at least one comonomer is further defined as in any of claims 2 to 4.