Polypropylene film with improved balance of mechanical properties

Abstract

The present invention relates to a blown film made of a composition comprising a) a propylene heterophasic copolymer (A) with a matrix propylene polymer and an ethylene-propylene-rubber characterized in that the film has been monoaxially oriented in the machine direction with a stretch ratio of 1:1.1 to 1:10, to a process for making such a film, to the use of said composition in making such a film, and to an article comprising such a film.

Claims

1. A blown film made of a composition comprising 50 to 90 wt. % of a propylene heterophasic copolymer (A) having a MFR(230 C./2.16 kg) from 0.5 to 15 g/10 min, with a matrix propylene polymer and 10 to 20 wt. % of an ethylene-propylene-rubber, wherein the matrix propylene polymer of propylene heterophasic copolymer (A) is a propylene copolymer comprising ethylene as comonomer in an amount of from 0.1 to 10 wt. % and wherein the ethylene-propylene-rubber has a propylene content from 40 to 80 wt. % of the total weight of the ethylene-propylene-rubber, an ethylene homo- or copolymer (B), having a density greater than 0.923 and up to 0.950 g/cm.sup.3, and optionally less than 10 wt. % or less of additives including antioxidants, neutralizers, inorganic fillers, antiblocking agents, nucleation agents, lubricants or antistatic agents, wherein the film is monoaxially oriented only in the machine direction with a stretch ratio of 1:1.1 to 1:10, and has a thickness of more than 70 m to about 150 m, said film having a tensile impact strength in the machine direction of at least 1300 kJ/m.sup.2 to about 3500 kJ/m.sup.2, and a tensile impact strength in the transverse direction of at least 250 kJ/m.sup.2 to about 4000 kJ/m.sup.2.

2. A film according to claim 1, wherein the film is monoaxially oriented in the machine direction with a stretch ratio of 1:1.2 to 1:4.

3. A film according to claim 1 wherein the composition comprises 60 to 80 wt. % of propylene heterophasic copolymer (A).

4. A film according to claim 1 wherein the ethylene homo-or copolymer (B) is present in an amount of 50 wt. % to 10 wt. %.

5. A film according to claim 1 wherein the matrix propylene polymer of the propylene heterophasic copolymer (A) is a homopolymer.

6. A film according to claim 1 wherein the matrix propylene polymer of propylene heterophasic copolymer (A) is a propylene copolymer comprising ethylene as a comonomer.

7. A film according to claim 1 wherein the content of the ethylene-propylene-rubber in the propylene heterophasic copolymer (A) is about 15 wt. % based on the total weight of polymer (A).

8. A film according to claim 1 wherein base propylene heterophasic copolymer (A) has a MFR (230 C./2.16 kg) from 0.1 to 15 g/10 min.

9. A film according to claim 1 wherein the ethylene homo- or copolymer (B) is a linear ethylene copolymer or a low density ethylene homo- or copolymer.

10. A film according to claim 1 wherein the ethylene homo- or copolymer (B) is a linear ethylene copolymer or a low density ethylene homo- or copolymer and wherein the linear ethylene copolymer is a C.sub.4 to C.sub.8 alpha-olefin.

11. A film according to claim 1 wherein the ethylene homo- or copolymer (B) has a density greater than 0.910 to 0.930 g/cm.sup.3.

12. A film according to claim 1 wherein the ethylene homo- or copolymer (B) has an MFR (190 C./2.16 kg) of 0.1 to 10 g/10 min.

13. A film according to claim 1 wherein the film has an elongation at break in transverse direction of at least 100% before sterilization.

14. A film according to claim 1 that is oriented and sterilized.

15. A sterile film according to claim 1 that has an elongation at break in transverse direction of at least 20%.

16. A process for the production of monoaxially oriented polypropylene blown film wherein a composition comprising (A) 50 to 90 wt. % of a propylene heterophasic copolymer (A) having a MFR(230 C./2.16 kg) from 0.5 to 15 g/10 min, with a matrix propylene polymer and 10 to 20 wt. % of an ethylene propylene rubber, wherein the matrix propylene polymer of propylene heterophasic copolymer (A) is a propylene copolymer comprising ethylene as comonomer in an amount of from 0.1 to 10 wt. % and wherein the ethylene-propylene-rubber has a propylene content from 40 to 80 wt. % of the total weight of the ethylene-propylene-rubber, (B) an ethylene homo- or copolymer (B), having a density greater than 0.923 up to 0.950 g/cm.sup.3, and (C) optionally less than 10 wt. % of additives including antioxidants, neutralizers, inorganic fillers, antiblocking agents, nucleation agents, lubricants or antistatic agents, wherein said composition is formed into a film having a thickness of more than 70 m to about 150 m in a blowing process which film then is monoaxially oriented in the machine direction in a stretch ratio of 1:1.1 to 1:10, said film having a tensile impact strength in the machine direction of at least 1300 kJ/m.sup.2 to about 3500 kJ/m.sup.2, and a tensile impact strength in the transverse direction of at least 250 kJ/m.sup.2 to about 4000 kJ/m.sup.2.

17. A process according to claim 16 wherein the film after orientation is sterilized.

18. An article comprising a film according to claim 1.

19. An article comprising a film according to claim 1 which is a stand-up pouch or an adhesive tape.

Description

EXAMPLES

1. Measurement Methods

(1) a) Tensile Impact Strength/Elongation

(2) The tensile impact strength is measured by the tensile impact test according to the standard EN ISO 8256:1996 on blown films with 50 m. Measurements of the test specimens were: 80 mm10 mm. For each tensile impact strength value in tables 1 and 2 an average from 10 determinations was calculated.

(3) Elongation at break was measured manually on destroyed specimens from tensile-impact strength testing by dividing the total length of the specimen parts after tensile-impact strength testing by the length of the specimen from before tensile-impact strength testing.

(4) b) Melt Flow Rate

(5) The melt flow rate is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and thus the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is measured with a load of 2.16 kg at 230 C. for polypropylene or 190 C. for polyethylene.

(6) c) Density

(7) The density is determined according to ISO 1183 and is indicated in g/cm3 or kg/m3. The density is an indication of the crystallinity of the polymer

2. Experiments

(8) a) Materials

(9) The starting materials for the films produced in the present examples are commercially available and had the following properties:

(10) Polymer (A) was used in an amount of 70 wt % based on the total polypropylene composition. It was a heterophasic propylene copolymer having a MFR (230 C., 2.16 kg) of 0.85 g/10 min, with 85 wt % of polypropylene homopolymer as matrix polymer and 15 wt % ethylene-propylene rubber which contained 50 wt % ethylene.

(11) Polymer (B) was used in an amount of 30 wt % based on the total polypropylene composition. It was a linear low density polyethylene with a density of 0.923 g/cm3, a MFR (190 C., 2.16 kg) of 0.2 g/10 min and 7 wt % butene comonomer.

(12) b) Blown Film

(13) Blown films were produced on a single screw extruder with barrel diameter of 70 mm and a round-section die of 200 mm with 1.2 mm die gap in combination with a monolip cooling ring and internal bubble cooling (IBC). Melt temperature was 220 C. in the die; the temperature of the cooling air was kept at 15 C. and the blow up ratio (BUR) 2.5:1. A film thickness of 100 m was adjusted through the ratio between extruder output, take off speed and BUR.

(14) c) Orientation Process

(15) Orientation is achieved by reheating the non-oriented polypropylene film at a temperature of 135 C., stretching into the machine direction and then after an annealing step the film is cooled. The films are stretched in a ratio of 1:2.

(16) d) Sterilisation Process

(17) The films are stored for 2 hours in a heated oven at 121 C. In between the film layers a paper is given in order to prevent sticking together of the film layers.

(18) TABLE-US-00001 TABLE 1 Tensile-impact Elongation at Film Material strength [kJ/m.sup.2] break [%] thickness 50 m MD TD MD TD Example 1 Blown 1364.6 2156 16.7 146.1 (not Film ori. sterilised) 1:2 Comparative Blown 886.9 261.9 30 0 Example 1 Film non- (not ori. sterilised)

(19) TABLE-US-00002 TABLE 2 Tensile-impact Elongation at Film Material strength [kJ/m.sup.2] break [%] thickness 50 m MD TD MD TD Example 1a Blown 2283.9 2122.4 45.4 58.7 (after Film ori. sterilisation) 1:2 Comparative Blown 1975.4 403.1 84.9 0 Example 1a Film non- (after ori. sterilisation)

(20) As can be seen from the results of Tables 1 and 2 the mechanical properties of the present films in both machine and in transverse direction are balanced before and after sterilisation.