GB2439051A

GB2439051A - Voided biaxially oriented polypropylene seal film

Info

Publication number: GB2439051A
Application number: GB0611942A
Authority: GB
Inventors: Chris Konkel
Original assignee: Innovia Films Ltd
Current assignee: Innovia Films Ltd
Priority date: 2006-06-16
Filing date: 2006-06-16
Publication date: 2007-12-19
Also published as: GB0611942D0

Abstract

The 50-250 žm thick biaxially stretched polypropylene (BOPP) film has a bulk density = 0.2-0.9 g/cc, yield / 40 cm<2>/g, light transmission <20%, tensile strength at break / 50 MPa, elongation at break / 50% and 1% secant modulus / 1000. Preferably the polypropylene containing void initiating particles is co-extruded with a printable ethylene propylene butylene co-polymer, and simultaneously drawn in the machine (MD) and transverse (TD) directions. Preferably the film includes additives or blending polymers. Preferably the film is included in an innerseal laminate, including metallic (aluminium foil), induction sealing and polyester (polyethylene terephthalate, PET) layers. Preferred values and MD:TD ratios are given for the parameters. The laminate is used to seal containers.

Description

SEAL FILM

The present invention relates to a seal film, in particular to a cap or container innerseal film, and to a manufacturing method for such films.

Innerseal films are commonly used on packaging containers such as bottles, tubs, cartons, jars, pots, tubes and other types of container to provide a substantially airtight or perhaps even hermetic seal to protect the product inside the container from exposure to the atmosphere and provide tamper evidence in its pre-sale condition. Commonly such a seal is located beneath or within an openable closure of the container such as a lid or cap.

The innerseal desirably has properties such as sealability, peelability, printability, moisture transfer prevention, vapour transfer prevention, mechanical strength and cuttability, although the precise requirements of any particular innerseal are determined at least to some extent by its end application.

EP-B-0680890 relates to container innerseals which are used to provide an airtight seal for containers. The exemplified innerseals of this disclosure have a laminate structure including a medium or high density polyethylene layer.

It is an object of the invention to provide an improved sealable polymer film suitable for innerseaj purposes.

According to the present invention there is provided a voided biaxially oriented polypropylene film having a thickness of from 5Opm to 250pm, a density in the range of from 0.2 to 0.9 g/cm3, and at least one other property selected from: 1. A yield of at least about 40cm2/g 2. Light transmission of less than about 20% 3. Tensile strength (at break) of at least about 5OMPa in one or both of the machine and transverse directions 4. Elongation (at break) of at least about 50% in one or both of the machine and transverse directions 5. 1% Secant modulus of at least about 1000 in one or both of the machine and transverse directions.

The preferred films of this invention are opaque, voided, oriented BOPP films prepared using a simultaneous draw process. However, sequentially drawn films may also be used in accordance with the invention.

It has been discovered that the inclusion of relatively thick, voided, biaxially oriented polypropylene films in accordance with the invention as part of an innerseal laminate are effective in innerseal applications and provide in at least some respects improved innerseal films. For example, the films of the invention display well balanced mechanical properties which facilitate downstream conversion of the film into an innerseal composite. The voided films of the invention are consistently smooth in their surface finish, and when laminated as part of an innerseal composite structure form even, good quality laminates. The films of the invention also have excellent temperature stability.

The melting point of films in accordance with the invention is preferably above 100 C, more preferably above 120 C and most preferably above 140 C.

Below the melting point, at temperatures such as 60 C which might be encountered during final packaging and processing of the end product, the films of the invention remain stable. The films of the invention also display good tear resistance and good cuttability, especially die cuttability.

The film of the invention preferably comprises particles in at least one layer thereof, said particles incompatible with said layer to cause the initiation of voids therein when the cast polyolefin is stretched simultaneously in both the MD and ID.

These void-initiating particles may have a mean aspect ratio x/y of at least 2 (e. g. long and thin) and a mean size of the longest particle dimension greater than about 3 microns (preferably about 6 microns); and/or may have a mean aspect ratio of about 1 (e. g. spherical or boulder-like), with a narrow size distribution, a mean particle size of from about 3 to about 10 microns, (preferably about 6 microns), and which are substantially free of particles above about 12 microns in size and optionally also substantially free of particles below about 3 microns in size.

Optionally the particles are present in an amount from about 5% to about 40% by weight of said layer.

One suitable type of void-initiating particle is a calcium carbonate type filler, which may be used to initiate the formation of voids in the simultaneous draw process. Other suitable voiding agents include by way of example only mica, barium sulphate, glass beads & flakes, polymer beads, salt, silica, metallic particles & flakes, and mixtures of two or more thereof.

The film of the invention will generally be incorporated into a laminate structure forming an innerseal. Accordingly, the invention provides an innerseal laminate incorporating the film of the invention as part of the laminate structure. The laminate structure may also include a metallic layer, for example a metallic foil layer such as an aluminium foil layer. It may also include a sealing layer, for example an induction sealing layer. It may also include one or more polyester layers, for example polyethylene terephthalate layers.

Thus, the invention provides an innerseal laminate comprising a biaxially oriented, voided polypropylene film as hereinbefore described, an aluminium foil layer, a polyethylene terephthalate layer and an induction sealing layer.

A composite innerseal structure in accordance with the invention typically includes an induction heatable layer (generally a metallic foil layer) and a sealing layer, and may also include a heat sink layer therebetween to distribute heat evenly from the induction heatable layer to the sealing layer.

Typically the heat sink layer when present comprises a polyester such as polyethylene terephthalate.

The invention also provides a container sealed with the innerseal laminate of the invention.

In preferred embodiments of the invention, the film may possess at least two, preferably at least three, more preferably at least four, and most preferably five of the other properties stipulated above.

The thickness of the film of the invention is preferably from 110pm to 240pm, more preferably from 120pm to 230pm, most preferably from 130pm to 220pm.

The density of the film of the invention is preferably from 0.55 to 0.75 glcm3, more preferably from 0.57 to 0.74 g/cm3, most preferably from 0.59 to 0.72 g/cm3.

The yield of the film of the invention is preferably at least about 55cm2/g, more preferably at least about 60cm2/g, and most preferably at least about 65cm2/g.

The light transmission of the film of the invention is preferably less than about 15%, more preferably less than about 10% and most preferably less than about 5%.

The tensile strength (at break) of the film of the invention is preferably at least about 6OMPa, more preferably at least about 7OMPa and most preferably at least about 8OMPa in one or both of the machine and transverse directions. It is desirable that the films in accordance with the invention exhibit well balanced mechanical properties in the machine and transverse directions.

Therefore, preferably the ratio of the tensile strength of the film measured in the MD with respect to the TD is at least about 0.6, preferably from about 0.75 to 1.5, more preferably from about 0.8 to about 1.35, most preferably from about 0.85 to about 1.3.

The elongation (at break) of the film of the invention is preferably at least about 60%, more preferably at least about 65% and most preferably at least about 70% in one or both of the machine and transverse directions.

Preferably the ratio of the elongation of the film measured in the MD with respect to the TD is at least about 0.8, preferably from about 0.85 to 1.9, more preferably from about 0.9 to about 1.75, most preferably from about 0.95 to about 1.7.

The 1% Secant modulus of the film of the invention is preferably at least about 1050, more preferably at least about 1100 and most preferably at least about 1150 in one or both of the machine and transverse directions. Preferably the ratio of the 1% Secant modulus of the film measured in the MD with respect to the TD is at least about 0.75, preferably from about 0.8 to 1.8, more preferably from about 0.85 to about 1.7, most preferably from about 0.9 to about 1.6.

The film of the invention (before incorporation in any innerseal laminate structure) may itself be a single layer film or a multilayer film. A multilayer film in accordance with the invention may be formed as a coextrudate having a core or substrate layer at least predominantly of biaxially oriented polypropylene, and a least one coextruded skin layer of a different material, for example a copolymeric material comprising two or more of polypropylene, polyethylene and polybutylene.

In a multi-layer film in accordance with the invention having at least a substrate later and a skin layer, the skin layer is preferably ink printable. The skin layer has a thickness of from about 0.O5pm to about 2pm, preferably from about Olpm to about 1.5pm, more preferably from about O.2pm to about 1.25pm, most preferably from about O.3pm to about O.9pm.

The film or the skin layer of the film may comprise additional materials such as anti-block additives, opacifiers, fillers, UV absorbers, cross-linkers, colourants, waxes and the like. Where high opacity voided film is required, titanium dioxide powder or other finely ground mineral fillers can be added. The combination of voiding and opacifying agents leads to a film with higher opacity than can be achieved using either of these techniques alone.

Pigments or dyes can be incorporated in to the structure to produce coloured voided film. Use of metallic voiding agents can give a metallic effect voided film. When metallic platelet particles are used the particles orient in the plane of the film and give an enhanced metallic effect. Films of the invention may contain Ti02 to give enhanced whiteness and higher opacity. Ti02 may be present in the film in an amount of greater than about 5%, for example from about 9% to about 10% by weight.

The film of the invention may be further treated, by corona discharge treating for example, further to improve ink receptivity of the film or of the skin layer of the film.

The film substrate may comprise a blend of biaxially oriented polypropylene with one or more further materials selected for example from homopolymeric materials block copolymeric materials and random copolymeric materials and combinations of two or more thereof.

The polymeric film can be made by any process known in the art, including, but not limited to, cast sheet, cast film, or blown film. The voided (or cavitated) polypropylene films of the invention may be provided with a homopolymer polypropylene core and skin layers with a thickness substantially below that of the core layer and formed for example from random co-polymers of ethylene and propylene or random terpolymers of propylene, ethylene and butylene. The film comprises biaxially orientated polypropylene (BOPP), which may be prepared as balanced films using substantially equal machine direction and transverse direction stretch ratios, or can be unbalanced, where the film is significantly more orientated in one direction (MD or TD). Sequential stretching can be used, in which heated rollers effect stretching of the film in the machine direction and a stenter oven is thereafter used to effect stretching in the transverse direction. Alternatively, simultaneous stretching, for example, using the so-called bubble process, or simultaneous draw stenter stretching may be used.

The invention will now be more particularly described with reference to the

following example.

EXAMPLE

Preparation of Film Polypropylene was blended with calcium carbonate at a level of 15.4% by weight calcium carbonate and titanium dioxide at a level of 9.8% by weight of titanium dioxide. The calcium carbonate was supplied by Ampacet in the from of a masterbatch called Pearl 70. This core polymer blend and a terpolymer of propylene, ethylene and butylene as the skin polymers were coextruded from a triple channel annular die to form a polypropylene film having a core layer and skin layers on either surface thereof. The resultant polypropylene tube was then cooled by passage over a mandrel within the tube, and externally quenched by passage through a water bath surrounding the mandrel, heated to stretching temperature, expanded by internal gas pressure, and withdrawn from the expansion zone at a rate greater than that at which it was fed thereto, so that the tube was stretched to seven times its original dimensions in both the direction of extrusion and in a direction transverse thereto. The stretched tubular film was then opened out to form flat film which was subsequently heat-set at a temperature of 120 C. on a mall-surfaced roller heat-setter of the kind described in GB-A-1124886 to form an opaque voided BOPP film suitable for use as a substrate in the example herein.

The opaque voided film compositions are presented in Table 1:

Table I

Property Example I Film (Innovia TB22C 160) Thickness l6Opm Density 0.7gIcm3 Yield 89cm21g Light Transmission 4% Tensile Strength @ break (MD) 95MPa Tensile Strength @ break (ID) 84.1 MPa Elongation @ break (MD) 76% Elongation @ break (TD) 81% 1% Secant Modulus (MD) I29OMPa 1% Secant Modulus (ID) 11 9OMPa Melting Temperature 170 C The film exhibits satisfactory characteristics for innerseal application when laminated in an innerseal structure comprising a sealing layer, an aluminium foil layer, a polyester layer, and a layer of the thick, opaque voided film of the

example. -10-

Claims

CLAIMS

A voided biaxially oriented polypropylene film having a thickness of from 5Opm to 250pm, a density in the range of from 0.2 to 0. 9 g/cm3, and at least one other property selected from: a. A yield of at least about 4Ocm2Ig b. Light transmission of less than about 20% c. Tensile strength (at break) of at least about 5OMPa in one or both of the machine and transverse directions d. Elongation (at break) of at least about 50% in one or both of the machine and transverse directions e. 1% Secant modulus of at least about 1000 in one or both of the machine and transverse directions.

2. A film according to claim 1, being an opaque, voided, oriented BOPP film prepared using a simultaneous draw process.

3. A film according to claim 1 or claim 2 comprising void-initiating particles in at least one layer thereof.

4. A film according to any one of claims 1 to 3 possessing two, three, four or five of the other properties stipulated in claim 1.

5. A film according to any one of claims 1 to 4 having a thickness of from 110pm to 240pm.

6. A film according to claim 5 having a thickness of 120pm to 230pm.

7. A film according to claim 6 having a thickness of from 130pm to 220pm.

8. A film according to any one of claims 1 to 7 having a density of from 0.55 to 0.75 g/cm3.

9. A film according to claim 8 having a density of from 0.57 to 0.74 g/cm3.

10. A film according to claim 9 having a density of from 0.59 to 0.72 g/cm3.

11. A film according to any one of claims I to 10 having a yield of at least about 55cm2/g.

12. A film according to claim II having a yield of at least about 60cm2/g.

13. A film according to claim 12 having a yield of at least about 65cm2/g.

14. A film according to any one of claims I to 13 having light transmission of less than about 15%.

15. A film according to claim 14 having light transmission of less than about 10%.

-12 - 16. A film according to claim 15 having light transmission of less than about 5%.

17. A film according to any one of claims 1 to 16 having a tensile strength (at break) of at least about 6OMPa in one or both of the machine and transverse directions.

18. A film according to claim 17 having a tensile strength (at break) of at least about 7OMPa in one or both of the machine and transverse directions.

19. A film according to claim 18 having a tensile strength (at break) of at least about 8OMPa in one or both of the machine and transverse directions.

20. A film according to any one of claims 1 to 19 wherein the ratio of the tensile strength of the film measured in the MD with respect to the ID is at least about 0.6.

21. A film according to claim 20 wherein the ratio of the tensile strength of the film measured in the MD with respect to the ID is from about 0.75 to 1.5. -13-

22. A film according to claim 21 wherein the ratio of the tensile strength of the film measured in the MD with respect to the TD is from about 0.8 to about 1.35.

23. A film according to claim 22 wherein the ratio of the tensile strength of the film measured in the MD with respect to the ID is from about 0.85 to about 1.3.

24. A film according to any one of claims 1 to 23 wherein the elongation (at break) of the film is at least about 60% in one or both of the machine and transverse directions.

25. A film according to claim 24 wherein the elongation (at break) of the film is at least about 65% in one or both of the machine and transverse directions.

26. A film according to claim 25 wherein the elongation (at break) of the film is at least about 70% in one or both of the machine and transverse directions.

27. A film according to any one of claims 1 to 26 wherein the ratio of the elongation of the film measured in the MD with respect to the ID is at least about 0.8. -14-

28. A film according to claim 27 wherein the ratio of the elongation of the film measured in the MD with respect to the ID is from about 0.85 to 1.9.

29. A film according to claim 28 wherein the ratio of the elongation of the film measured in the MD with respect to the ID is from about 0.9 to about 1.75.

30. A film according to claim 29 wherein the ratio of the elongation of the film measured in the MD with respect to the ID is from about 0.95 to about 1.7.

31. A film according to any one of claims ito 30 wherein the 1% Secant modulus of the film is at least about 1050 in one or both of the machine and transverse directions.

32. A film according to claim 31 wherein the 1% Secant modulus of the film is at least about 1100 in one or both of the machine and transverse directions.

33. A film according to claim 32 wherein the 1% Secant modulus of the film is at least about 1150 in one or both of the machine and transverse directions.

34. A film according to any one of claims 1 to 33 wherein the ratio of the 1% Secant modulus of the film measured in the MD with respect to the ID is at least about 0.75.

35. A film according to claim 34 wherein the ratio of the 1% Secant modulus of the film measured in the MD with respect to the ID is from about 0.8 to 1.8.

36. A film according to claim 35 wherein the ratio of the 1% Secant modulus of the film measured in the MD with respect to the TD is from about 0.85 to about 1.7.

37. A film according to claim 36 wherein the ratio of the 1% Secant modulus of the film measured in the MD with respect to the TO is from about 0.9 to about 1.6.

38. A film according to any one of claims 1 to 37 formed as a coextrudate having a core or substrate layer at least predominantly of biaxially oriented polypropylene, and a least one coextruded skin layer of a different material.

39. A film according to claim 38 wherein the skin material comprises a copolymeric material comprising two or more of polypropylene, polyethylene and polybutylene. -16-

40. A film according to claim 38 or claim 39 wherein the skin layer is ink

printable.

41. A film according to any one of claims 38 to 40 wherein the skin layer has a thickness of from about 0.O5pm to about 2pm.

42. A film according to claim 41 wherein the skin layer has a thickness of from about O.lpm to about 1.5pm.

43. A film according to claim 42 wherein the skin layer has a thickness of from about 0.2pm to about 1.25pm.

44. A film according to claim 43 wherein the skin layer has a thickness of from about 0.3pm to about 0.9pm.

45. A film according to any one of claims I to 44 wherein the film or the skin layer of the film comprises one or more additional materials selected from anti-block additives, opacifiers, fillers, UV absorbers, cross-linkers, colourants, waxes and other functional materials.

46. A film according to any one of claims 1 to 45 comprising a blend of biaxially oriented polypropylene with one or more further materials.

47. A film according to claim 46 wherein the one or more further materials is or are selected from homopolymeric materials, block copolymeric materials, random copolymeric materials and combinations of two or more thereof.

48. An innerseal laminate incorporating the film of any one of claims I to 47.

49. An innerseal laminate according to claim 48 comprising a metallic layer.

50. An innerseal laminate according to claim 49 comprising a metallic foil layer.

51. An irinerseal laminate according to claim 50 comprising an aluminium foil layer.

52. An innerseal laminate according to any one of claims 47 to 51 comprising a sealing layer.

53. An innerseal laminate according to claim 52 wherein the sealing layer is an induction sealing layer.

54. An innerseal laminate according to any one of claims 47 to 53 comprising one or more polyester layers. -18-

55. An innerseal laminate according to claim 54 wherein the polyester comprises polyethylene terephthalate.

56. A container sealed with the innerseal laminate of any one of claims 47 to 55. -19-