This invention relates to a peelable film structure. More particularly, the invention relates to a peelable film structure that can be used in packaging applications requiring easy opening or as a lid in plastic containers. It is known to heat seal a film to the top of a container in such a manner that the film acts as a lid for the container. The container to which the lid film is heat sealed can be any type of heat sealable material, typically polyethylene or polypropylene or polypropylene homopolymers or copolymers.

The lid film typically comprises a multilayer film which includes a thin layer of polyethylene or a lacquer. The purpose of the thin layer of polyethylene or lacquer is to enable the lid film to be heat sealed to the container. The heat seal is often very difficult to break, which makes it hard to open the container without tearing the lid. One solution to this problem has been to apply special adhesives onto the lid film. Another solution involves the use of multilayer peelable films, in which the peeling occurs within the body of the multilayer film.

It is an object of the present invention to provide a film structure which overcomes this problem.

According to one aspect of the present invention, there is provided a film structure comprising:

(a) a core layer comprising an olefin polymer; and

(b) a heat sealable layer on the core layer comprising a blend of LDPE (low density polyethylene) and a material incompatible with LDPE.

The film structure can be heat sealed, by layer (b) , to a plastic container to form the lid of the container, or to itself to form a package. When used with a plastic container, the film structure can be readily peeled from the container in order to open it.

The material incompatible with LDPE is preferably a heat sealable material.

It is preferred that the incompatible material is an olefin polymer, particularly homopolymers or copolymers of ethylene, propylene, or butylene. It is especially preferred that the olefin polymer in the heat sealable layer is an ethylene-propylene copolymer, a propylene- butylene copolymer, or an ethylene-propylene-butylene terpolymer. A particularly preferred ethylene-propylene copolymer contains 1 to 30 wt.%, more preferably 2 to 7 wt.%, most preferably 3 to 5 wt.%, ethylene. The ethylene-propylene copolymer preferably has a melt flow rate at 230*C from 2 to 15, more preferably from 3 to 8; a crystalline melting point from 125 to 150°C; a number average molecular weight from 25000 to 100000; and a density from 0.89 to 0.9 g/cm ³ . The preferred propylene-butylene copolymers contain up to 30 wt.% butylene. The preferred ethylene-propylene- butylene terpolymers contain 1 to 10 wt.%, more preferably 2 to 6 wt.%, ethylene; 80 to 97 wt.%, more preferably 88 to 95 wt.%, propylene; and 1 to 20 wt.%, more preferably 2 to 15 wt.%, butene-1.

The heat sealable layer preferably contains 30 to 70 wt.% LDPE, more preferably 40 to 60 wt.% LDPE, and most preferably 50 wt.% LDPE.

Preferably the core layer comprises polypropylene. One alternative material for the core layer is HDPE. The core layer may comprise voids. More specifically, the core layer may be cavitated. Suitable procedures for cavitating the core layer are disclosed in EP-A-0083495. A preferred cavitating agent is polybutylene terephthalate, especially as spherical particles.

Desirably the film structure is biaxially oriented. The film structure may include a skin layer disposed on the opposite side of the core layer to the heat sealable layer. It is possible for the skin layer to be heat

sealable too. If desired, the skin layer may have the same composition as the heat sealable layer. Preferably the skin layer is an olefin polymer.

When the skin layer is used, its surface may be corona treated, flame treated, or treated by chemicals. This surface treatment is most beneficial if the skin layer is to be printed on or laminated to another film.

The heat sealable layer of the film structure of this invention may be coated with a heat-sealable coating, such as, for example, acrylic or polyvinylidine chloride (PVdC) . Heat sealing the film (i.e., acrylic to acrylic or PVdC to PVdC) provides seals which are peelable.

Further, the heat sealable layer of the present film structure may be coated with a lacquer which acts normally as an adhesive between surfaces, e.g., a plastic container and its lid. Due to the cohesive failure of the heat sealable layer of the film structure, any resulting seals, such as, for example, between a plastic (e.g., PVC or polystyrene) container and the present film structure are peelable.

According to another aspect of the invention, there is provided a container comprising a plastics body and a film structure as described above, wherein the film structure is heat sealed to the body in order to provide a lid for the container.

Preferably the container has an outer heat sealable layer to which the film structure is heat sealed. The outer layer is preferably an olefin polymer, more preferably polypropylene or polyethylene. When the incompatible material of the heat sealable layer is heat sealable, it is preferred that the material of the outer layer of the container is the same as the incompatible material.

Reference is now made to the accompanying drawing which shows a film structure according to the invention being peeled from a container.

In the drawing a film structure generally designated 10 comprises core layer 12, a heat sealable layer 14, and a skin layer 16. The core layer 12 comprises 100% isotactic polypropylene; the heat sealable layer 14 comprises a blend of 50 wt.% ethylene-propylene-butylene terpolymer with 50 wt.% LDPE; and the skin layer 16 has the same composition as the heat sealable layer 14. The film structure 10 has been biaxially oriented.

The film structure 10 is shown being peeled from a container 18 to which it had been heat sealed by heat sealable layer 14. The container 18 has a heat sealable outer layer (not shown specifically in the drawing) to which the heat sealable layer 14 has been heat sealed.

The film structure 10 can readily be peeled from the container 18, as shown in the drawing. This may leave parts (e.g., part 20 shown in the drawing) of the heat sealable layer 14 on the container 18. After opening, the film exhibits stress whitening in the sealed area, which can provide evidence of tampering. This feature is particularly useful when the film is used with medical or food packaging.

Examples Comparative Example 1

Film A was prepared by coextruding a heat sealable layer of 50% ethylene-propylene-butylene terpolymer with 50% high density polyethylene (HDPE) , having 0.75 micron thickness, and a polypropylene support. The total film thickness was 25 microns.

Comparative Example 2 Film B was prepared by coextruding a heat sealable layer of 50% ethylene-propylene-butylene terpolymer with 50% high density polyethylene (HDPE), having 1.5 microns thickness, and a polypropylene support. The total film thickness was 25 microns.

Comparative Example 3

Film C was prepared by coextruding a heat sealable layer of 50% ethylene-propylene-butylene terpolymer with 50% linear low density polyethylene (LLDPE) , having 0.75 micron thickness, and a polypropylene support. The total film thickness was 25 microns.

Comparative Example 4

Film D was prepared by coextruding a heat sealable layer of 50% ethylene-propylene-butylene terpolymer with 50% linear low density polyethylene (LLDPE), having 1.5 microns thickness, and a polypropylene support. The total film thickness was 25 microns.

Example 1

Film E was prepared by coextruding a heat sealable layer of 50% ethylene-propylene-butylene terpolymer with

50% LDPE, having 0.75 micron thickness, and a polypropylene support. The total film thickness was 25 microns.

Example 2

Film F was prepared by coextruding a heat sealable layer of 50% ethylene-propylene-butylene terpolymer with

50% LDPE, having 1.5 microns thickness, and a polypropylene support. The total film thickness was 25 microns.

Example 3

The films A to F are all heat sealable; films E and F are examples of film structures according to the invention. The following tests were carried out to demonstrate that the films E and F, which can be used in the invention, have superior peelability to the films A to D.

Each film A to F was heat sealed onto a rigid homopolymer sheet of 240 microns thickness. The sealing was carried out on an Otto Brugger (at 5 bars for 0.5

second) at various temperatures. The seal strengths were then measured. The results are set out in Table 1.

Table 1 Seal Strenσth/σ/25 cm

Film 120-C 130-C 140'C 150'C 160"C 170'C

A 0 0 125 165 450 900 ^*

B 0 100 230 150 510 900 ^*

C 0 225 220 465 525 800 ^*

D 0 115 285 350 610 ^* 800 ^*

E 0 60 300 255 350 350

F 0 250 350 400 345 350

tearing observed

These results show that the films E and F possess lower seal strengths than the other films, particularly at high temperatures. The seal strengths attainable with films E and F are sufficient to provide a good heat seal, but at all temperatures the films E and F were peelable without tearing.

The results also show that films E and F produced seals having nearly constant peel strength over an extended heat seal temperature range when heat sealed onto a polypropylene sheet. This is advantageous over other film structure that demonstrate only a narrow temperature range where the seals do not tear.

Example 4

The films E and F were heat sealed onto rigid polyethylene sheets of LDPE or MDPE, and the same tests as in Example 3 were carried out. The results are shown in Table 2.

Table 2

Seal Strenσth/σ/25 cm

Film 130 ' C 140 ' C 150 ' C 160 ' C 170-C

Film E:

LDPE 500 1250 1315 >2000 >2000

MDPE 0 0 360 435 410

Film F:

LDPE 385 1100 1525 >2000 >2000

MDPE 0 0 255 380 430

No tearing was observed in any of the results of Example 4. The results in Table 2 show that films E and F provide peelable seals when they are heat sealed onto a polyethylene sheet and can provide seals with very high seal strengths without tearing, depending on the type of polyethylene.