Polypropylene-based article having an increased surface tension retention

1 . A process for the production of a polypropylene based article having improved surface tension retention, the process comprises:

A) providing a metallocene-catalysed propylene-ethylene random copolymer (RACO) having

(a) an ethylene content in a range of from 1.0 to 10.0 wt % based on a total weight of the metallocene-catalysed propylene-ethylene random copolymer (RACO);

(b) a melt flow rate MFR 2 (230° C./2.16 kg) measured according to ISO 1133 in a range of from 1.0 to 20.0 g/10 min;

(c) a melting temperature Tm as determined by DSC according to ISO 11357 of from 115 to 150° C.;

(d) a xylene cold soluble (XCS; ISO 16152, 2005; 25° C.) fraction of from 0.1 wt % to 5.0 wt % and

(e) a content of 2,1 regio-defects in a range from 0.05 to 1.5 mol % as determined by 13C-NMR spectroscopy

B) forming a polypropylene based article out of the metallocene-catalysed propylene-ethylene random copolymer (RACO) with a thickness in a range of from 1.0 to 1000 μm and

C) subjecting one or more surface(s) of the polypropylene based article to a treatment that increases the surface tension, to yield a polypropylene based article having improved surface tension retention,

wherein the polypropylene based article having improved surface tension retention has a dyne retention ratio (DRR 7 ) of at least 76% or more, wherein dyne retention ratio (DRR 7 ) indicates the dyne retention of the polypropylene based article having improved surface tension retention after 7 days of storage, in relation to an original dyne retention value of the polypropylene based article having improved surface tension retention, determined 6 hours after the treatment that increases the surface tension, multiplied by 100, and

wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) is the sole polymer used for forming the polypropylene based article.

2 . The process according to claim 1 , wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) furthermore has a crystallization temperature Tc as determined by DSC according to ISO 11357 in a range of 85° C. to 120° C., and/or a flexural modulus (determined according to ISO 178 on injection molded specimens) of 600 to 1400 MPa.

3 . The process according to claim 2 , wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) has a crystallization temperature Tc as determined by DSC according to ISO 11357 in a range of 90° C. to 115° C. and/or a flexural modulus (determined according to ISO 178 on injection moulded specimens) in a range of 700 to 1400 MPa.

4 . The process according to claim 1 , wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) is unimodal or multimodal in view of the comonomer content and/or MFR 2 and comprises polymer fractions (RACO-1) and (RACO-2).

5 . The process according to claim 4 , wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) is unimodal or multimodal in view of the comonomer content and/or MFR 2 and consists of polymer fractions (RACO-1) and (RACO-2).

6 . The process according to claim 4 , wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) comprises

30.0 to 70.0 wt % of polymer fraction (RACO-1) having

(i) an ethylene content in a range of from 1.5 to 5.5 wt % and

(ii) a melt flow rate MFR 2 (230° C./2.16 kg) measured according to ISO 1133 in a range of from 2.0 to 15.0 g/10 min and

70.0 to 30.0 wt % of polymer fraction (RACO-2) having

(i) an ethylene content in a range of from 2.2 to 8.0 wt % and

(ii) a melt flow rate MFR 2 (230° C./2.16 kg) measured according to ISO 1133 in a range of from 4.0 to 12.0 g/10 min.

7 . The process according to claim 1 , wherein the treatment that increases the surface tension is a corona-treatment or a flame-treatment.

8 . The process according to claim 1 , wherein the treatment that increases the surface tension is a corona-treatment.

9 . The process according to claim 1 , wherein the metallocene-catalysed propylene-ethylene random copolymer (RACO) is produced by a process comprising:

a) polymerizing in a first reactor (R1) propylene and ethylene, obtaining polymer fraction (RACO-1) of the propylene-ethylene random copolymer (RACO),

b) transferring the polymer fraction (RACO-1) and unreacted comonomers of the first reactor into a second reactor (R2),

c) feeding to the second reactor (R2) propylene and ethylene,

d) polymerizing in the second reactor (R2) and in the presence of the polymer fraction (RACO-1) propylene and ethylene and obtaining polymer fraction (RACO-2),

the polymer fraction (RACO-1) and the polymer fraction (RACO-2) forming the propylene-ethylene random copolymer (RACO),

wherein the polymerization takes place in the presence of a metallocene catalyst comprising (a) a complex of formula (I):

wherein

M is zirconium or hafnium;

each X is a sigma ligand;

L is a divalent bridge selected from —R′ 2 C—, —R′ 2 C—CR′ 2 —, —R′ 2 Si—, —R′ 2 Si—SiR′ 2 —, —R′ 2 Ge—, wherein each R′ is independently a hydrogen atom, C 1 -C 20 -hydrocarbyl, tri(C 1 -C 20 -alkyl)silyl, C 6 -C 20 -aryl, C 7 -C 20 -arylalkyl or C 7 -C 20 -alkylaryl;

R 2 and R 2 ′ are each independently a C 1 -C 20 hydrocarbyl radical optionally containing one or more heteroatoms from groups 14-16;

R 5 ′ is a C 1-20 hydrocarbyl group containing one or more heteroatoms from groups 14-16 optionally substituted by one or more halo atoms;

R 6 and R 6 are each independently hydrogen or a C 1-20 hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16;

R 7 is hydrogen or C 1-20 hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16;

R 7 ′ is hydrogen;

Ar and Ar′ are each independently an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R 1 ;

each R 1 is a C 1-20 hydrocarbyl group or two R 1 groups on adjacent carbon atoms taken together can form a fused 5 or 6 membered non aromatic ring with the Ar or Ar′ group, said ring being itself optionally substituted with one or more groups R4;

each R 4 is a C 1-20 hydrocarbyl group; and

(ii) a cocatalyst comprising at least one or two compounds of a group 13 metal.

10 . The process according to claim 1 , wherein the polypropylene based article formed in step B) is a cast film or a blown film, wherein the cast film or blown film is a non-oriented film, a machine direction oriented (MDO) film or a biaxially oriented film.

11 . The process according to claim 10 , wherein the cast film or blown film has a film thickness of 5 to 500 μm.

12 . The process according to claim 11 , wherein the cast film or blown film has a film thickness of 15 to 150 μm.

13 . The process according to claim 10 , wherein the polypropylene based article is a non-oriented cast film or blown film.

14 . The process according to claim 13 , wherein the polypropylene article is a non-oriented cast film.

15 . A polypropylene based article obtained by the process according to claim 1 , wherein the one or more surface(s) subjected to a surface energy increasing treatment has/have a dyne level of at least 45 dyne/cm, measured according to ASTM D2578-09 directly after the treatment.

16 . The polypropylene based article according to claim 15 , wherein the one or more surface(s) subjected to a surface energy increasing treatment maintains a dyne level of at least 38 dyne/cm measured according to ASTM D2578-09 for a period of 7 days after the treatment.

17 . The polypropylene based article according to claim 15 , wherein the dyne retention ratio (DRR 7 ) is at least 78.0% or more.

18 . The polypropylene based article according to claim 15 , wherein the polypropylene based article has a haze value measured on a 50 μm cast film determined according to ASTM D1003-00 in a range of from 0.05 to 1.0%.

19 . The polypropylene based article according to claim 15 , wherein the polypropylene based article has a tensile modulus in machine and transverse direction, determined according to ISO 527-3 at 23° C. on cast films of 50 μm thickness in a range of from 300 to 1000 MPa.

20 . The polypropylene based article according to claim 15 , wherein the one or more surface(s) subjected to a surface energy increasing treatment has/have a dyne level of 48 to 60 dyne/cm measured according to ASTM D2578-09 directly after the treatment.