EP0680447A4

EP0680447A4 - Peelable heat seal.

Info

Publication number: EP0680447A4
Application number: EP94911374A
Authority: EP
Inventors: Ihab Hekal
Original assignee: Polystar Packaging Inc
Current assignee: Polystar Packaging Inc
Priority date: 1993-01-29
Filing date: 1994-01-31
Publication date: 1996-09-18
Also published as: WO1994016963A1; EP0680447A1; AU6391494A

Abstract

A closure part (20) for use in a two part closure. The surfaces of the parts are heat sealed (28). The closure part (20) comprises a material (38) that when heat sealed (20) to the other part (22) forms a weakened portion under the heat sealed portion with less cohesion than the remainder of the closure part.

Description

PEELABLE HEATSEAL

RELATED APPLICATIONS

This application is related to United States Patent Applica¬ tion Serial No. 07/741,235 filed August 5, 1991, of William H. Heyn and Donald J. Roth, entitled "Container Closure." Said application is assigned to the same assignee as the present ap¬ plication and is incorporated herein by reference.

_ ? -

TECHNICAL FIELD

This invention relates to heat sealed closures employing multi-phase polymer blends. More particularly, it relates to closures providing a peel-away lid such as disclosed in the above-identified patent application, and in U.S. Patent Nos. 3,946,872, 4,890,758, 5,069,355 and 5,125,528.

In all but the latter patent a breakaway heat seal is pro¬ vided so that a closure or lid may be removed from a package, particularly for food. In Patent Nos. 3,946,872 and 5,069,355, the breakaway feature is provided by defoliation of plastic mate¬ rial in either the lid or in an end-member attached to the pack¬ age.

In Patent No. 4,890,758, the special peel layer is provided in the end panel, which adheres to the lid when it is removed to show where they have been sealed together.

In an earlier U.S. patent which I assigned to Continental Can Company, I disclosed an end panel or package to which a lid was heat sealed which, being of polypropylene loaded with talc, is weakened so that it cohesively breaks below the seal when the lid is peeled away. The above-identified patent application also discloses cohesive failure in a peel-away seal.

Patent No. 5,125,528 discloses flexible end panels which are heat sealed to end rings during the molding process of the end rings. The end rings are then applied to the opening of a pack¬ age. The lids in this patent are removed by tearing along pre- imprinted score lines.

The prior art additionally discloses adhesively bonding the flexible lid to a container rim or to an end panel which is then affixed to the container with the lid being torn off at the adhe¬ sive, rupturing it during the process.

Each prior art package has disadvantages. The pre-scored systems do not provide any indication of the integrity of the seal achieved between the flexible lid and the end panel or lip of the container. The systems employing cohesive rupturing pro¬ vide a ragged rupture which is uncontrolled in thickness and extent, and leaves varying thicknesses of material on the non- cohesively rupturing part. The containers employing defoliation either are complicated to assemble or do not provide for precise defoliation at the seal, and again, none of these systems provide a means for easily testing the integrity of the seals of the flexible lid to the end panel or rim of the container.

DISCLOSURE OF THE INVENTION

According to my invention, multi-phase polymer blends are utilized in either the flexible lid or an end panel to which the lid is heat-sealed. The first of the polymers employed has an affinity for the material to which it is to be heat sealed during the heat-sealing process. The second polymer is chosen such that it moves away from the surface being heat sealed and forms a region adjacent to the heat seal in which the second polymer predominates. The two polymers are chosen such that the second polymer concentration forms a weakened portion such that upon opening the container, the sealed surface of the polymer blend pulls away from the concentrated portion adjacent the seal in a controlled fashion, depositing a rather precise thickness layer of the first polymer onto the lid, or the end panel, or the con¬ tainer lip to which the multi-polymer blend has been heat sealed.

I disclose the use of such seals in closures of the type disclosed in the above-identified U.S. Patent Application and in U.S. Patent No. 5,125,528, wherein preferably, the ring to which the flexible lid is sealed during the molding process of the ring, is made of the multi-phase polymer blend, according to my invention. Alternatively, I disclose that the multi-phase poly¬ mer blend may be employed in the surface of the lid itself, and I disclose that such lids may be applied directly to packages, either having polymer or metallic surfaces at their lip openings.

Preferably, I employ a blend of polypropylene and poly¬ ethylene where the polypropylene is sealed to a polypropylene or metal surface. The polypropylene concentrates at the heat seal and displaces the polyethylene which concentrates adjacent the heat seal and provides the weakened portion. To provide affinity to metal of the polypropylene, a polar polymer is added such as ethylene vinyl acetate and the polypropylene is treated with maleic anhydride so that it will bond to the metal.

The invention contemplates the use of other incompatible polymer mixtures such as polypropylene and nylon, styrene and polyethylene, polyethyleneterephthalate (PET) or PETG and poly- propylene. Any incompatible mixture wherein a first component selectively diffuses to the seal and forms a stronger bond there than the cohesion of the second component or its adhesion to the first component is within the invention.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide im¬ proved heat-sealed closures for packages of all kinds, including those subject to cooking.

Another object of the invention is to provide such closures providing a unique telltale when opened, showing that they have been completely sealed to the container or to a frame member or end panel attached to the container.

Still another object of the invention is to provide such closures that may be applied directly to metal and plastic by heat sealing.

Still another object of the invention is to provide such closures of the type disclosed in the above-identified U.S. Pat¬ ent Application and U.S. Patent No. 5,125,528.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of con¬ struction, elements, and arrangements of parts, articles of manu¬ facture possessing the features, properties and relations of elements, and several steps and the relation of one or more of said steps with respect to each of the others, and possessing the characteristics, properties, and the relations of components which will be exemplified in the article, constructions and meth¬ ods hereinafter described. The scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIGURE 1 is a top view of a container closure according to my invention;

FIGURE 2 is a partial cross-sectional and diagrammatic view taken along line 2-2 of Figure 1 ;

FIGURE 3 is a cross-sectional view of a mold in which a solid polymer ring, according to the invention, is molded and heat sealed to a peelable lid;

FIGURE 4 is a diagrammatic partial cross-sectional view of a heat seal according to the invention;

FIGURE 5 is a cross-sectional view, partially cut away of a closure, according to my invention, applied to a prior art con¬ tainer having a plastic sealing surface;

FIGURE 6 is a cross-sectional view, partially cut away of a lid, according to my invention, applied to a container having a metallic surface.

The same reference characters refer to the same elements throughout the several views of the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIGURE 1, a closure, according to the in¬ vention, is generally indicated at 20. It comprises a ring por¬ tion 22, and a lid portion 24. The lid portion 24 is provided with a ring pull 26, which may be temporarily affixed to the lid portion 24 at heat seals 28. Now referring to FIGURE 2, the lid portion 24 is sealed against the ring portion 22, at a heat seal indicated at 30. The ring part 22 is provided with a annuler portion generally indicated at 32 for sealing to a metal can of aluminum of tinned steel. This is preferably accomplished by means of induction heating of the metal can. The ring portion 22 is provided with an upstanding portion, generally indicated at 34, which provides for stacking of the cans after the ring por¬ tion 22 is applied to the cans. Ribs, 36, are provided so that the assembled closures 20 may be stacked before application to the cans.

For a fuller understanding of the details of this closure, reference should be had to the above-identified application Serial No. 07/741,235.

According to my invention, when the ring part 22 is injec¬ tion-molded against the lid part 24 of the two incompatible poly¬ mers forming the ring part 22, concentrates under the seal 30, as generally indicated at 38, and the other component polymer con¬ centrates at portion 40, indicated below portion 38. The rela¬ tive sizes of the portions 38 and 40 to the parts 22 and 24, are greatly exaggerated in FIGURE 2 for the purpose of illustrating same. The portion 38 has a high proportion of the polymer which strongly seals to the bottom surface of part 24 with an adhesion which is either greater than the cohesion of the portion 40 wherein the second polymer component of ring 22 is concentrated or the adhesion at interface between portions 38 and 40.

Thus, when the lid 24 is peeled away from the ring 22, the portion 38 remains sealed along seal 30 to the lid 24. If por¬ tion 38 is a different color than the lid 24, and if the seal between parts 24 and 22 is hermetic, a colored ring telltale v/ill be formed at the seal 30 at the bottom of lid 24 indicating that a good seal was achieved. This can be used for testing a certain portion of the sealed containers for integrity of their seals.

As indicated in the above-identified patent application, the ring 22 is injection-molded against the lid 24 in a mold 42 shown in FIGURE 3.

In order to allow for the migration and concentration of the polymer component having the greatest affinity for the lid 24 onto the portion 38 under the lid (see FIGURE 2) , the injection molding process must be such that it does not stir the two compo¬ nents together during the flow of the plastic melt of the ring 22 within the mold 42. I have found that laminar flow is required of the liquid plastic melt in the mold 22 so that mixing of the portions 38 and 40 does not occur, which would destroy the integ¬ rity of the telltale portion 38.

Those skilled in the art will realize that the lid 24 in many instances, is comprised of multiple layers of polymer mate¬ rials providing vapor, gas barriers, etc. As more fully pointed out below, I prefer that the lower surface 44 of the lid 24, be of the same plastic material as that component of the ring 22 which migrates and concentrates in the telltale portion 38.

In FIGURE 4 plastic parts 46 and 48 are shown which have been heat-sealed together in an area under the arrow 50 in the conventional manner. Again, shown exaggerated in size, a portion 52 adjacent the interface 54 between parts 46 and 48 contains a concentration of the polymer having the greatest affinity for part 48 and a surrounding higher concentration indicated at 56 of the other polymer comprising part 46. The other polymer or the interface between the other polymer and the polymer having a higher concentration in portion 52, is weaker and when parts 46 and 48 are peeled apart, the portion 52 will stick to part 48, providing a telltale.

In another embodiment of the invention indicated in FIGURE 5, a lid, generally indicated at 58 is heat sealed to a metal can, generally indicated at 60. The metal can comprises, for example, a 4 mil aluminium portion 62 to which polypropylene has been applied in a 2 mil layer, as indicated at 64. The peelable lid 58 comprises an upper portion 66 which may be one or more layers provide vapor barrier, gas barrier, strength, etc. to the lid 58. The lower surface layer 68 comprises polypropylene and another incompatible polymer such as polyethylene, in accordance with the invention. The lid 58 is sealed to the can body 60 as generally indicated at 70. In accordance with the invention, when the lid 58 is peeled away from the can body 60, a portion of the lower surface of the layer 68 will remain adhered to the polypropylene layer 64 at the seal 72.

In another embodiment of the invention showed in FIGURE 6, a peelable lid, generally indicated at 74, may be directly heat- sealed to a can body 76 of metal, either by heat-sealing or by induction heating of the can body 76, by spin sealing, by ultra sonic sealing, or the like. The lid 74 is again formed of upper layers, generally indicated at 78, appropriate for the use of the container, and a lower surface layer 80 of two incompatible plas¬ tics, one of which has an affinity for the can 76 during heat sealing or during induction heating of the can 76, again forming a concentrated layer 82 which will remain adhered to the can 76 when the lid 74 is peeled away.

In applying the invention to the closure illustrated in FIGURES 1 and 2 that can be molded in the manner illustrated in FIGURE 3, for use on containers for foods that must be cooked in order to preserve them, the following composition of materials for the ring part 22 is preferred. It consists essentially, by weight, of about 30% polypropylene copolymer, about 36% polypro¬ pylene homopolymer, about 9% low density polyethylene, about 15% high-density polyethylene, about 9% ethylene vinyl acetate and about 1% green color concentrate. The part 22 has a nice appear¬ ance and provides a telltale that contrasts nicely with a white polypropylene lid surface. The homo- and co-polymer polypro¬ pylene has been reacted with maleic anhydride. The concentration of maleic anhydride in the total mixture is less than .8% the FDA limit, in the range of about .2 to about .4% and preferably about ^**(&

For the lid 58, illustrated in FIGURE 5, the polypropylene layer 68, which seals to the polypropylene layer 64 on the can 60 and leaves a telltale thereon when peeled away, two compositions work well. The preferred one consists essentially, by weight, of about 8.75% ethylene vinyl acetate, about 15% high density poly¬ ethylene, about 8.75% low density polyethylene, about 40% poly¬ propylene homo-polymer, and about 27.5% polypropylene co-polymer. Another consists essentially, by weight, of about 7% ethylene vinyl acetate, 13.5% high density polyethylene, about 7% low- density polyethylene, about 35% polypropylene homo-polymer and about 37.5% polypropylene co-polymer. The latter formula is a little harder to peel, but provides a stronger seal. A minor amount of colorant and filler may be added as desired.

Sheet material of either of these latter compositions may be directly sealed to a can body 76 as shown in FIGURE 6, by re¬ acting the polypropylene with maleic anhydride prior to mixing.

The polypropylene layer surface 44 of the lid 24 in FIGURES 1 and 2 may be polypropylene co-polymer supplied by Exxon Chem¬ ical Company as Escorene^ PD7341N.

While I do not wish to be bound by any particular theory as to why the above materials provide the features of my invention, or for that matter to any particular theory as to why the above mixtures provide a good seal and a telltale, it is my opinion that during the injection molding of the ring 22 against the lid 24, illustrated in FIGURES 1 and 2, in the mold illustrated in FIGURE 3, as long as strong turbulence is avoided, as the mate¬ rial of the ring 22 moves against the lid surface 44, since the surface 44 is polypropylene, the polypropylene in the melt 22 tends to wet the polypropylene surface 44, concentrate there at the exclusion of the polyethylene and ethylene vinyl acetate, which move away from the surface 44 and concentrate there. In peeling, the polypropylene that has wet the polypropylene surface is cohesively bonded thereto with a stronger bond than either exists in the volume of higher concentration of EVA and polyeth¬ ylene or at the interface between this concentration and the polypropylene adhered to the surface 44.

The formulas given above provide a telltale layer of poly¬ propylene which is preferably colored, on the surface of the peeled-away lid 24, which has a thickness of about _ mil. The ring 22, which may be any closed shape to conform to the can body, adheres very nicely to a metal can of aluminum or steel coated with tin, even if the surfaces are further coated with epoxy, since the polar EVA is attracted to the metal of the can, which is preferably heated by induction. The EVA synergistically cooperates with the maleated polypropylene . The EVA remains attached to the metal when the seal is broken, thus providing a telltale that a hermetic seal has been achieved.

In reducing my invention to practice, I looked for plastic materials which could be processed in direct contact with food up to 270 degrees F. and would provide good adhesion to metal, whether steel, aluminum or tin, even if coated with epoxies. Since polypropylene has been approved for food and is extensively used in peeling lids, polypropylene was chosen as one of the polymer materials. Clearly, other polyolefins could be used in the application.

Although maleated polypropylene itself exhibits fair adhe¬ sion to metal, I found that the addition of an elastic polymer to the mixture reduced the stress between the metal and the maleated polypropylene. Since this material (ethylene vinyl acetate) is also polar, it accelerates the bonding of the maleated polypro¬ pylene to the metal and provides good adhesion.

By combining polypropylene homo- and co-polymers, an excel¬ lent compromise may be achieved between the need for the polypro¬ pylene to be treated at a high temperature during canning and to be perhaps kept in cold storage or left out in the cold during winter transportation. The co-polymer contains a rubber such as ethylene propylene rubber (EPR) .

By adding non-compatible polymers (ethylenes) to the poly¬ propylene, I provide for weakening material below the seal.

Polyethylene is excellent for this purpose since it reduces the cohesive strength of the material and causes the peel value to drop, as does the ethylene vinyl acetate. The proportions are adjusted to achieve the appropriate peel value.

High density polyethylene stiffens the plastic, and this makes it more stable at high temperatures, while low density polyethylene makes it more flexible. The ratio is adjusted ap- propriately for the intended use.

When the ethylene vinyl acetate is blended with the maleated polypropylene, the ethylene vinyl acetate tends to gather around the maleic anhydride groups. When the material is melted and comes in contact with a metal surface, the ethylene vinyl acetate wets the polar surface and the anhydride and the carboxy groups react with the metal surface to form a chemical bond. When the metal and the material is cooled, the shrinkage of the plastic creates tension on the bond and tries to dislodge the bond. The presence of ethylene vinyl acetate because of its resilience, minimizes the tension on the bond. Additionally, the rubber, such as ethylene propylene rubber which is present in the poly¬ propylene co-polymer, improves the bond by minimizing the stresses on the bond. Other rubbers, such as polyisobutalin or polybutadyene can be used in addition to ethylene vinyl acetate or in place of it, to improve the adhesion.

It should be noted that not only does the polyethylene in the mix reduce the cohesion of the material under the polypro¬ pylene layer 38 shown in FIGURE 2, 52 in FIGURE 4, and 82 in FIGURE 6, the ethylene vinyl acetate also adds to this function, since the adhesion of ethylene vinyl acetate to polypropylene, and polyethylene to polypropylene is less than the cohesion of polypropylene to polypropylene.

Although in the material utilized in the ring 22 illustrated in FIGURES 1 and 2, both the homo- and co-polymers of polypro¬ pylene were maleated, success has been achieved where only the homo-polymer or the co-polymer polypropylene is maleated.

Other polar polymers containing ethylene, besides ethylene vinyl acetate could be used, such as ethylene acrylic acid or ethylene methacrylic acid.

A wide variety of commercially available materials may be utilized successfully, over wide ranges of compositional strength of the various elements as illustrated in the following table to practice the invention.

RUNS 1-20 are suitable for the ring 22 in Figures 1 and 2. RUN 21 is suitable for the lid surface 68 in Figure 5. (C) indi¬ cates co-polymer; (H) indicates homo-polymer.

All percentages are by weight.

Suppliers are identified by the following prefixes: X = EXXON CHEMICAL COMPANY (U.S.) EX = EXXON CHEMICAL COMPANY (EUROPE) D = DUPONT CHEMICAL COMPANY EASTMAN ^**** EASTMAN CHEMICAL COMPANY ARIS = ARISTECH CHEMICAL CORPORATION H = HIMONT INC.

The rest of the identification of type is the identification used by the supplier.

Thus, the compositions of the invention comprise about 50% to 80% of a first polymer, preferably polypropylene, of about 20% to about 50% of a second polymer, preferably polyethylene, and about 10% of polymers comprising ethylene and acetate groups. The polypropylene comprises from about 20% to about 80% polypro¬ pylene homo-polymers. The composition comprises up to about 40% low-density polyethylene and up to about 40% high-density poly¬ propylene.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are effi¬ ciently attained, and certain changes may be made in the above constructions, articles, and methods without departing from the scope of the invention, is it intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.

Particularly, it is to be understood that in said claims, ingredients or compounds recited in the singular are intended to include compatible mixtures of such ingredients wherever the sense permits .

It is also to be understood that by "ring" as used in the following claims, I mean any closed loop of material of a shape conforming to the shape of the package body to which it is to be affixed.

Claims

Having described by invention, what I claim as new and de¬ sired as secured by Letters Patent is:

1. A closure part for use in a two-part closure wherein the surfaces of said parts are adhered together by the applica¬ tion of heat to form a heat seal; said closure part comprising a material that when heat sealed to said other part forms a weak¬ ened portion under said seal having less cohesion than the re¬ mainder of said closure part and the adhesion of said parts at said seal.

2. A closure part as defined in claim 1, wherein said closure part, comprises an incompatible mixture of a first poly¬ mer which migrates and concentrates at the surface thereof at said seal, and a second polymer which migrates and concentrates under said seal during the application of heat.

3. A closure part as defined in claim 2, wherein the sur¬ face of the other part comprises a third polymer.

4. A closure part as defined in claim 3, wherein said first and third polymers are the same.

5. A closure part as defined in claim 4, wherein said first and third polymers are polyolefins.

6. A closure part as defined in claim 5, wherein said polyolefins are polypropylene.

7. A closure part as defined in claim 6, wherein said polypropylene comprises homo- and co-polymers.

8. A closure part as defined in claim 1, wherein said closure part is a solid polymer ring adapted to be bonded to the rim of an open-mouthed container.

9. A closure part as defined in claim 8, wherein said ring comprises a polar polymer. 10. A closure part as defined in claim 8, wherein said ring comprises a maleic anhydride modified polymer.

11. A closure part as defined in claim 10, wherein said ring comprises additional polar polymers.

12. A closure part as defined in claim 11 comprising, by weight:

A. about 66% polypropylene;

B. about 24% polyethylene; and

C. about 9% of a third polymer comprising ethylene and acetate groups.

13. A closure part as defined in claim 12 wherein said polypropylene comprises, by weight about 30% rubber copolymer and about 36% homopolymer.

14. A closure part as defined in claim 13, wherein said polyethylene comprises by weight about 9% low density polyeth¬ ylene and about 15% high density polyethylene.

15. A closure part as defined in claim 14, wherein said third polymer is ethylene vinyl acetate.

16. A closure part as defined in claim 12, wherein said polyethylene comprises by weight about 9% low density polyeth¬ ylene and about 15% high density polyethylene.

17. A closure part as defined in claim 12, wherein said third polymer is ethylene vinyl acetate.

18. A closure part as defined in claim 12, wherein at least a portion of said polypropylene has been reacted with maleic anhydride.

19. A closure part as defined in claim 13, wherein at least a portion of said polypropylene has been reacted with maleic anhydride. 20. A closure part as defined in claim 2, wherein said first polymer comprises homo-polymer and rubber co-polymers.

21. A closure part as defined in claim 20, wherein said second polymer comprises ethylenes.

22. A closure part as defined in claim 21 wherein said ethylenes are of the high density and low density types.

23. A closure part as defined in claim 1, wherein said closure is formed of a polymer mixture comprising, by weight: about twenty percent (20%) to about fifty percent (50%) of polymers, each comprising ethylene; and about fifty percent (50%) to about eighty percent (80%) polymers each comprising polypropylene.

24. A closure part as defined in claim 23, wherein said polymer mixture further comprises, by weight: up to about ten percent (10%) of polymers each com¬ prising ethylene rubbers.

25. A closure part as defined in claim 23, wherein said polymer mixture comprises, by weight up to about forty percent (40%) low-density polyethylene.

26. A closure part as defined in claim 23, wherein said polymer mixture comprises up to about forty percent high-density polyethylene.

27. A closure part as defined in claim 23, wherein said polymer mixture comprises of about twenty percent (20%) to about eight percent (80%) polypropylene homo-polymer.

28. A closure part as defined in claim 23, wherein said polymer mixture comprises up to about fifty percent (50%) poly¬ propylene rubber co-polymer. 29. A closure part as defined in claim 23, wherein said polymer mixture comprises, by weight, up to about forty percent

(40%) low-density polyethylene, up to about forty percent (40%) high-density polyethylene, from about twenty percent (20%) to about eighty percent (80%) polypropylene homo-polymer, up to about fifty percent (50%) polypropylene rubber co-polymer and up to about ten percent (10%) of an ethylene-containing polymer comprising about twenty-five percent (25%) vinyl acetate.

30. A closure part as defined in claim 23, wherein said polymer comprises up to about ten percent (10%) of an ethylene containing polymer, containing about twenty-five percent (25%) vinyl acetate.

31. A closure part as defined in claim 23, wherein said propylene polymers have been reacted with less than .8% by weight of maleic anhydride.

32. A closure part as defined in claim 23, wherein said mixture comprises by weight about 9% ethylene vinyl acetate, about 9% low-density polyethylene, about 15% high-density poly¬ ethylene, about 30% polypropylene homo-polymer, about 30% poly¬ propylene rubber co-polymer.

33. A closure part as defined in claim 32, wherein at least some of said polypropylene has been reacted with maleic anhy¬ dride, which comprises less than .8% of said mixture.

34. A closure part as defined in claim 23, wherein said polymer mixture comprises by weight about 7% ethylene vinyl ace¬ tate, about 13.5% high-density polyethylene, about 7% low-density polyethylene, about 35% polypropylene homo-polymer and about 37.5% polypropylene co-polymer.

35. A closure part as defined in claim 34, wherein at least some of said polypropylene has been reacted with maleic anhydride which comprises less than .8% of said mixture. 36. A closure part as defined in claim 35, wherein said maleic anhydride concentration in said mixture is less than about

4%

37. A closure part as defined in claim 33, wherein said maleic anhydride comprises less than about .4% of said mixture.

38. A closure comprising:

A. a container part providing a sealing surface;

B. a pliable closure, the surface of said closure part adhereable to the sealing surface of said container part on the application of heat to form a peelable seal, the adhesion of said surfaces together at said seal being greater than the cohe¬ sion at a weakened portion formed during said application of heat under said seal in one of said parts.

39. A closure as defined in claim 38, wherein said con¬ tainer part is a solid polymer ring adapted to be bonded to the rim of an open-mouthed container.

40. A closure as defined in claim 39, wherein said weakened portion forms in said polymer ring during heat sealing to said closure.

41. A closure as defined in claim 40, wherein said ring is injection-molded against said closure and said weakened portion is formed during said injection molding.

42. A closure as defined in claim 40, wherein said ring comprises a polar polymer.

43. A closure as defined in claim 40, wherein said ring comprises a maleic anhydride modified polymer.

44. A closure as defined in claim 43, wherein said ring comprises additional polar polymers. 45. A closure as defined in claim 42, wherein said ring is heat sealed to a metal container.

46. A closure as defined in claim 43, wherein said ring is heat sealed to a metal container.

47. The method of making a closure part for sealing to another closure part to provide a peelable seal providing a tell¬ tale, comprising:

A. combining first and second incompatible polymers;

B. forming said polymers into a closure part;

C. sealing said closure part to the surface of an¬ other closure part, said first polymer migrating to the surface of said other closure part and displacing said second polymer therefrom during said sealing operation.

48. The method defined in claim 47, wherein said sealing step is accomplished by heat sealing.

49. The method defined in claim 48, wherein the surface of said other closure part comprises the same polymer as said first polymer.

50. The method defined in claim 47, wherein said forming step is accomplished by injection molding said closure part against the surface of said other closure part.

51. The method defined in claim 50, wherein said other closure part forms a peelable lid.

52. The method defined in claim 50, wherein said sealing is accomplished by laminar flow of said polymer against said other closure part during said injection-molding step.

53. A closure part as defined in claim 1, wherein said closure part forms the upper surface of a container. 54. A closure part as defined in claim 53, wherein said closure part is a polymer layer adhered to a metal surface.

55. A closure part as defined in claim 3, wherein said mix¬ ture forms the upper surface of a container and said other part forms the lid of said container.

56. A closure part as defined in claim 55, wherein said mixture is formed in a layer on a metal surface.