Uniaxially shrinkable, biaxially oriented polypropylene films
View Patent ↗Provided is a polypropylene film, and a method of making the film, the film comprising at least one core layer sandwiched between at least two skin layers, the layers comprising a core layer comprising polypropylene and within the range of from 20 wt % to 50 wt %, by weight of the materials in the core layer, of a propylene-α-olefin elastomer having within the range of from 5 wt % to 25 wt % α-olefin derived units, by weight of the copolymer; and skin layers comprising one or more polymers having a melting point within the range of from 125° C. to 160° C., a Shore D Hardness within the range of from 55 to 70, and a Flexural Modulus of at least 600 MPa; wherein the film has a Haze value of less than 10%, the film being biaxially oriented but shrinkable substantially only in the MD.
1. A film comprising at least one core layer sandwiched between at least two skin layers, the layers comprising:
(i) a core layer comprising polypropylene and within the range of from 20 wt % to 50 wt %, by weight of the materials in the core layer, of a propylene-α-olefin elastomer having within the range of from 5 wt % to 25 wt % α-olefin derived units, by weight of the copolymer; and
(ii) skin layers comprising one or more polymers having a melting point within the range of from 125° C. to 160° C., a Shore D Hardness within the range of from 55 to 70, and a Flexural Modulus (ISO 178) of at least 500 MPa;
wherein the film has a Haze value of less than 10%, and
wherein the MD dimensional stability value is less than −10% (15 sec at 135° C.), and a TD dimensional stability value within the range of from −2% to 8% (15 sec at 135° C.).
2. The film of claim 1 , wherein the polypropylene has a Vicat softening temperature (ISO 306, or ASTM D 1525) of greater than 100° C.; and the propylene-α-olefin elastomer has a Vicat softening temperature of less than 120° C.
3. The film of claim 1 , wherein the Vicat softening temperature of the polypropylene is at least 5° C. higher than the Vicat softening temperature of the propylene-α-olefin elastomer.
4. The film of claim 1 , wherein the film has an MD shrinkage of at least 10% at 143° C.
5. The film of claim 1 , wherein the film has no TD shrinkage or expands (has a dimensional stability) from −1% to 8% at 143° C.
6. The film of claim 1 , wherein syndiotactic polypropylene is absent from the core layer.
7. The film of claim 1 , wherein ethylene-based copolymers, those copolymers having at least 60 wt % ethylene derived units, are absent from the core layer.
8. The film of claim 1 , wherein the skin layers consisting essentially of one or more propylene-ethylene copolymers or propylene-ethylene-butylene terpolymers.
9. The film of claim 1 , wherein the film has a Modulus in the MD within the range of from 100 kpsi to 350 kpsi; and a Modulus in the TD within the range of from 100 kpsi to 450 kpsi.
10. The film of claim 1 , wherein the film has a Gurley stiffness in the MD within the range of from 8 mg to 24 mg; and a Gurley stiffness in the TD within the range of from 8 mg to 35 mg.
11. The film of claim 1 , wherein the film is biaxially oriented and uniaxially shrinkable.
12. An article comprising the film of claim 1 wrapped circumferentially around a container.
13. A method of forming a biaxially oriented film comprising at least one core layer sandwiched between at least two skin layers, the method comprising co-extruding:
(i) a core layer comprising polypropylene and within the range of from 20 wt % to 50 wt %, by weight of the materials in the core layer, of a propylene-α-olefin elastomer having within the range of from 5 wt % to 25 wt % α-olefin derived units, by weight of the copolymer; and
(ii) a first skin layer comprising a polymer having a melting point within the range of from 125° C. to 160° C., a Shore D Hardness within the range of from 55 to 70, and a Flexural Modulus (ISO 178) of at least 500 MPa; and
(iii) a second skin layer comprising a polymer having a melting point within the range of from 125° C. to 160° C., a Shore D Hardness within the range of from 55 to 70, and a Flexural Modulus (ISO 178) of at least 500 MPa;
forming at least a three-layer film;
stretching the film twice in the MD, the first stretch being within the range of from 2.5 to 6.5 ratio at from 80° C. to 120° C., and the second stretch being within the range of from 1.2 to 1.5 ratio at from 140° C. to 180° C.; and
stretching the film once in the TD simultaneous with or after the first MD stretch and before the second MD stretch;
wherein the MD dimensional stability value is less than −10% (15 sec at 135° C.), and a TD dimensional stability value within the range of from −2% to 8% (15 sec at 135° C.).
14. The method of claim 13 , wherein the film is stretched in the TD after the first MD stretch but before the second MD stretch at a ratio of from 4 to 12.
15. The method of claim 13 , wherein the film has an MD shrinkage of at least 10% at 143° C.
16. The method of claim 13 , wherein the film has no TD shrinkage or expands to less than 8% at 143° C.
17. The method of claim 13 , wherein syndiotactic polypropylene is absent from the core layer.
18. The method of claim 13 , wherein each skin layer independently consist essentially of one or more propylene-ethylene copolymers or propylene-ethylene-butylene terpolymers.
19. The method of claim 13 , wherein the film has a Modulus in the MD within the range of from 100 kpsi to 350 kpsi; and a Modulus in the TD within the range of from 100 kpsi to 450 kpsi.
20. The method of claim 13 , wherein the film has a Gurley stiffness in the MD within the range of from 8 mg to 24 mg; and a Gurley stiffness in the TD within the range of from 8 mg to 35 mg.
21. The method of claim 13 , wherein the polypropylene has a Vicat softening temperature (ISO 306, or ASTM D 1525) of greater than 100° C.; and the propylene-a-olefin elastomer has a Vicat softening temperature of less than 120° C.
22. The method of claim 13 , wherein the film is biaxially oriented.
23. The method of claim 13 , wherein the film is simultaneously stretched in the MD and the TD at a ratio of from 4 to 12, followed by stretching again in the MD.
24. A method of making an article comprising wrapping a film made in accordance with the method of any one of numbered claim 13 around a container.