CA1252343A

CA1252343A - Easy open closure system

Info

Publication number: CA1252343A
Application number: CA000472460A
Authority: CA
Inventors: Shih-Lai Lu; Andrew H. Wong
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1984-02-09
Filing date: 1985-01-21
Publication date: 1989-04-11
Also published as: KR850005980A; DE3573608D1; ZA85509B; DK161840B; EP0155081A3; US4853267A; DK161840C; EP0155081A2; NO850449L; EP0155081B1; AU577084B2; ATE47149T1; DK60685D0; JPS60193838A; AU3803785A; BR8500391A; DK60685A

Abstract

EASY OPEN CLOSURE SYSTEM

Abstract An end portion for a container having a preformed opening therein and an easy-open closure system therefor, the closure system comprising an exterior tape over the opening which is manually removable, the tape being adhered by an adhesive which comprises the reaction mixture of an aryl isocyanate, an aliphatic glycol, and a defined mixture of hydroxy terminated polyesters.

Description

~S~3~3 33411 CAN SA

EASY OPEN CLOSURE SYSTEM

Technical ~icld .. . . .
This invention relates to containers having pre-formed openings or pour holes therein which are sealed by flexible, tape-based closure assemblies. The invention more specifically relates to an adhesive for use with such assemblies.
,~
Background Art There is and has been increasing use of metal containers for carbonated beverages such as soft drinks and beer. One of the earlier and most common of these containers employed an easy open closure wherein a metal ring attached to the end wall at a portion thereof which had weakened tear lines was easily lifted and pulled away therefrom, removing the portion of the end wall defined by the tear lines. Such closures have become ecologically unacceptable because the removed metal tabs are commonly discarded on the ground or in lakes. Such tabs decompose extremely slowly and, because of their sharp edges, can be dangerous.
A more recent developmcnt again ut:ilizes weakened tear lines in the end portion of a con~ainer, but is designed to retain the closure with the container.
In this instance, the mctal tab rcmain~ wilh Ihc Cll{]
portion of the can itself after manually opening.
However, any dirt and/or debris which is contained on the can end is pushed into t:he container during opening, because the portion of the end wall defined by the tear lines is in fact inserted into the can during opening, thus contacting the contents thereof.
It is known that unpressurized cans containing items such as tomato juice, oran~e Juice and similar liquids can be provided with easy open closures utilizing prcssurc-scnsitivc adhcsivc t.apc tabs, such as arc disclosed in U.S. Patent No. 3,389,827. However, such closure systems have not been foun~ acceptable for use in connection with containcrs in which are packaged 3~;3 gas-containing liquids, such as carbonated soEt drinks and beer. The pressure buildup in such systems can ordinarily cause tape closures to bulge upward and gradually peel away Erom the area immediately adjacent the preformed opening or pour hole, whereby the can seal becomes broken.
Furthermore, when utilizing containers which enclose lubricious materials, such as motor oil, the e~treme variation in temperature under which such --containers are stored and used has presented significantproblems when attempting to utilize easy open closure systems.
A number of patents purport to disclose various easy open closure systems which are asserted to be uscful in conjunction with containers having carbonated beverages therein. Examples include U.S. Patent Nos.

2,870,935, 3,292,828 and 3,339,788. An irrlprovclllcnl ~alcnl over these earlier systems is ~.S. Patent No. 3,990,603, but in all of the foregoing cases, the easy open systems disclosed therein have met with only limited commercial success.
One major problem encountered in the development of such closures is the utilization of materials having sufficient physical properties to form a seal under the conditions encountered in the filling and closing of metal containers. With any of these prior closures, especially when a~plications involving carbonated beverages, it has been difficult to obtain an adhesive capable of meeting the stringent requirements of ready application to a container end, retention of t:he elevated pressure within the container, and easy opening characteristics.
We have now discovered an adhesive system which is capable of eliminating the foregoing d~ficicncies and providing an easy open closure system which is functional both for containers containing carbonated liquids and those containing lubricious materials such as motor oils, which of necessity must be operable over wide temperature variations.

~'~5'~3~3 Summary of the Invention In accordance with the present invention, tnere is provided an end portion for a container, wherein the cnd portion has a preformed opening, i.2., a pour hole and an easy open closure system for the opening, the closure system comprising an exterior tape circumjacent, i.e., over and surrounding the opening, the tapc being peelable by hand from the exterior of the end portion surrounding thc opening, the tape typically comprising a flexible backing member and a uniform adhesive coating which is firmly anchored -to the backing member, the adhesive comprising the reaction mixture of an aryl isocyanate, an aliphatic glycol, and a mixture of hydroxy terminated polyesters, the mixture of polycstcrs comprising at least one low molecular weight hydroxy terminated polyester, the molecular weight thereof being from about 750 to about 1,250, and at least onc high molecular weight hydroxy terminated polyester, the molecular weight thereof being from about 1,700 to about 2,400, wherein the ratio of the low molecular weight polyester to the high molecular weight polycster being from about 4:1 to about 0.25:1.
In closure systems for carbonated liquids, there also is usually present an interior shect material which covers the underside of the can opening and which is firmly adhered to the bottom surface of the end portion circumjacent the opening and is also adhesively secured to the exterior tape in the area of thc opcning, the int.erior sheet material typically comprising a backing member and an adhesive layer. At least one of the cxtcrior tape or the intcrior shect: matcrial should bc moisture-impervious, and the pre-formed opening typically has a pressure-relief opening associated therewith.
The adhesive of our invention can also bc utilizcd wit:h systems such as arc disclosed in U.S.
Patent No. 4,215,791, wherein no adhesive is cont.ained on the outer backing member, only ~n the interior shcet matcrial.

3'-~

-~- 60557-2867 Accordingly, our invention thus provides an casy open closure system capable oE maintaining a scal in a pressurized container, or one containing a lubricious material, and yet allows for easy and safe manual opening of the container. Further, the closure system imparts no undesirable tastes, flavors or odors to carbonated beverages. The closure system is useful with both steel and aluminum can ends, thereby enhancing the economics of recyclability, and the system is economically competitive with existing closure systems. Furthermore, tne instant closure system is compatible with presently utilizcd canning equipment, and is also ecologically acceptablc.

Detailed Description of the Invention . _ _ The backing member of the exterior tapc has been extensively defined in other patents, notably U.S.
Patent Nos. 3,990,603, 4,215,791, 4~378,074 and

4,405,056.
Representative materials which have been found suitable as backing members include tough plastic films which have been conventionally oriented and hcat-se~ ~o impart requisite properties of toughness and tcmpcraturc resistance, such as poly-1,9-butylene terephthalate and polyethylene terephthalate. Other suitable exemplary films includc polycarbonate, composite plastic films and soft metal, an example of which is dead-soft aluminum.
Especially preferred is polycarbonate.
Those backing materials which are plas~ic can be vapor coated with a thin layer of metal, e.g., aluminum, etc. to provide opacity and can also improve ~0 ~he impermeability of some backings. Opacity ~an al~o bc ob~ained through use of a pigment, either coated onto ~he backing or blended with the backing during the manufacture thereof.
The specific adhesives which have found utility in our invention are polyurethane-based materials which comprise the reaction product of an aryl diisocyanate, an aliphatic glycol and a mixture oE hydroxy ~erminated polyesters.

3~3 While any aryl diisocyanate may be employcd as part of the reaction mixture to form our polyurethanc, diphenyl diisocyanates are preferred. For example, mixed isomeric diphenyl methane diisocyanates; diphenyl methane-p,p'-diisocyanate; dichlorodiphenyl methane diisocyanate and the like, are exemplary. Diphenyl methane-p,p'-diisocyanate provides excellent results.
In conventional fashion, an aliphatic glycol is t:ypically addcd as a chain cxtendcr. Such matcrials typically contain between 4 and 10 carbon atoms, examples of which include butane diol-1,4; hexamethylene diol-1,6;
octamethylcne diol-1,8 and the likc. In gcncral, thc chain extending glycol is preferably of the formula HO(CH
2)x~ whercin x may bc from 2 to 10 and prefcrably from 4 to 8. Butane diol-1,4 is particularly preferred.
The last necessary component is a mixture of hydroxyl terminated polyesters, such as poly(tetra-methylene adipate) glycol, poly(caprolactone) glycol, poly(hexamethylene carbonate) glycol, and the like. The first polyester should have a molecular weight of from about 750 to about 1250, with from about 900 to about 1100 being preferred, and about 1000 most preferred. The second polyester should have a molecular weight from about 1700 to about 2400, with Erom about 1800 t:o about 2200 being preferred, and about 2000 most preferred. The polyesters should be mixed at from about four parts to about 0.25 part of low molecular weight per part of high molecular weight, with from about 3:1 to 0.33:1 being preferred. The optimum ratio has been found to be 1:1.
Preferred hydroxyl terminated polyesters have an acid number of less than about 10, and prcfcrably lcss than about 3. The polyesters are conventionally prepared by an esterification reaction of an aliphatic dibasic acid or anhydride thereof with a diol. Molar ratios oE
more than one diol to acid are preferred so as to obtain linear groups containing a preponderance of terminal hydroxyl groups.
Useful polyesters include those preparcd from the esterification of dicarboxylic acids, such as adipic, ~S~

succinic, pimellc, suberic, azelaic, sebacic and the like or their anhydrides. Prcferred acids ar~ Ihosc dicarboxylic acids of the formula HOOC-R-COOH, where R is an alkylcne radical containing 2 to 8 carbon atoms. Morc preferred are those represented by the formula HOOc(cH2)xcooH
where x is a number from 2 to 10. Adipic acid is most preferred.
Diols utilized in the preparation of the polyesters are preferably straight chain diols of the formula HO~CH2)XOH, wherein x may be 2 to 10, but preferably is 4 to 8. Examples include butanediol-1,4, hexamethylenediol-1,6, octame~hylenediol-1,8 and the like, with butanediol-1,4 being particularly preferred.
The manuEacture of the polyurethanes is con-ventional, and, for example, is disclosed in U.S. Patent No. 3,493,634.
In the polyurethane manufacture, there should be present from about 1.0 to about 2.1 moles of aliphatic glycol and about 2.0 to about 3.1 moles of aryl isocyanate per mole of total polyester present, with from 1.1 to 2.0 of the glycol and 2.1 to 3.0 of the isocyanate being preEerred, and about 1.6 and 2.6 of the glycol and isocyanate, respectively, being most preferred.
As the amount of the lower molecular weight hydroxyl terminated polyster is incr~ased, the glass transition temperature (Tg) of the resultant polyurethane increases, which correspondingly reduces ~he low temperature characteristics thereof, while increasing the concentration of the higher molecular weight polyestcr will t:cnd lo improve thc cold temperaturc chnrn(~ ris~
of the polyurethane, but can increase the tendency for crystallization.
It has been found that the Tg of Lhe poly-urethane should be lower than about -30C to ensure the desired cold temperature characteristics of the adhesive.
(Tg is measured by conventional techniques, including differential scanning calorimetry and diffcrcn~ial thermal analysis.) ~, j 23~3 The crystallization temperature (Tc) should be unattainable when measured by the foreyoing analytical techniques. If a Tc is found in the thermogram, aging of the adhesive, i.e., the adhesive characteristics with age may be adversely affected.
The weight average molecular weight, as determined by gel permeation chromatography (GPC) should preferably be greater than about 90,000 to insure attainment of the desired adhesive perEormance and manufacturing characteristics.
Adhesion values of the adhesive vary depending on the enamel coating on the can end closure. Increased adhesion typically results in a reduced bonding time and temperature necessary for preparing the closure, and thus an increase in machine speed.
The adhesive is applied to the backing by conventional techniques, e.g., solvent casting, extrusion, and the like.
If an internal sheet material is utilized, the characteristics thereof are critical to proper operation of the closure system, i.e., where carbonated beverages are conccrned. In function, the interior sheet mat:crial must be capable of resisting rupture and tear propagation resulting from gas pressure in a filled containcr, while also allowing easy and complete removal t:hcrcof from thc area of the pour hole when the exterior tape is stripped from the filled container. Therefore, the interior sheet material must have properties of resisting tear and rupture when the container is filled and yet will allow easy and clean tearing around the pour hole periphery when tear is initiated by removal of the exterior tape, as are clearly disclosed in prior patents discussed above.
Materials having been found suitable as backing mcmbcrs for thc int;erior shcet mat:crial includc ~?:last ic films such as polyethylene terephthalate, unplasticized polyvinyl chloride, and films derived from a graft copolymer comprising acrylonitrile/methylacrylate -~- 60557-2867 copolymer grafted onto an acrylonitrile/butadiene copolymer backbone. ~ parlicularly uscful graft copo~ymer is formed by graft polymerizing acrylonitrile and met:hylacrylat;e in the presence oE an acryL~rlit butadiene copolymer.
One particularly useful composile Film comprises a layer of polyethylene terephthalate and a layer of polyethylene terephthalate/polyethylene iso-phthalate copolymer, preferably prepared by coex~rusion, as taught in U.S. Patent No. 3,871,947 In the manufacture of the closure systcms, metal end portions for containers, having been coated with a thin lacquer or enamel by thc can cnd manufacturer, are typically punched in order to provide a pour hole. Then, the exterior tape and the interior shcet material, iE required, arc affixed circumjacenl ~he pre-formed opening in the end portion using both heat and moderate pressure. Generally speaking, the pressurc uscd to affix the exterior tape and ~he intarior sheel material to the can end is relatively uniform around the periphery of the pour hole and in the area of the pour hole itself where the exterior tape is firmly bonded or secured to the interior sh~et material.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a container sealed by means of a tape closure, the improvement which comprises the use of a polyurethane adhesive to seal said closure, said poly-urethane adhesive comprising the non-crystallizable reaction product of an aryl isocyanate, an aliphatic glycol, and a mixture of hydroxy terminated polyesters, said mixture of polyesters comprising a low molecular weight polyester, the molecular weight thereof being from about 750 to about 1,250 and a high molecular weight polyester, the molecular weight thereof being from about 1,700 to about 2,400, the ratio of said low molecular weight polyester to said high molecular weight polyester being from about 4:1 to about 0.25:1.

2. The container of claim 1 additionally containing an interior sheet material adhesively bonded to said tape.

3. The container of claim 1 wherein said adhesive has a Tg of less than about -30°C.

4. The container of claim 1 wherein said adhesive has a weight average molecular weight of at least about 90,000.

5. The container of claim 1 wherein said ratio is from about 3:1.0 to about 0.33 to 1Ø

6. The container of claim 1 wherein said ratio is from about 1:1.

7. The container of claim 1 wherein said aryl diisocyanate is a diphenyl diisocyanate.

8. The container of claim 7 wherein said diphenyl diisocyanate is diphenyl methane-p,p'-diiso-cyanate.

9. The container of claim 1 wherein said aliphatic glycol is butane diol-1,4.

10. The container of claim 1 wherein said hydroxy terminated polysters have an acid number of less than about 10.

11. The container of claim 1 wherein at least one of said polyesters comprise the esterification product of adipic acid and butane diol-1,4.

12. The container of claim 1 wherein said adhesive has from about 1.0 to about 2.1 moles of aliphatic glycol and from about 2.0 to about 3.1 moles of isocyanate per mole of total polyester.