Stress-opacifiable tamper indicator

Abstract

A container is hermetically sealed with an easy opening tape closure, at least a portion of which opacifies when stressed, providing a visual indication that the closure has been tampered with.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending application Ser. No. 397,896 filed Sept. 17, 1973, now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to an improved container closure.

In recent years, many containers have been hermetically sealed with easy opening tape closures which are affixed over a hole in the top of the container by a heat-sealing or pressure-sensitive adhesive; see Abere and Brochman, U.S. Pat. No. 3,389,827. A typical closure has a free end which is easily gripped by the fingers to facilitate removal, after which the contents, e.g., fruit juice, oil, salt, etc., can be removed via the exposed hole.

Although tape closures of the Abere et al type provide an effective and easily opened seal, they are subject to tampering. It is possible to carefully break the seal, remove or adulterate the contents of the container and then replace the tape without leaving any visible traces.

A tape closure which changes color when an attempt has been made to remove it with the aid of an organic solvent is disclosed in U.S. Pat. No. 3,680,236. This closure incorporates a dye-containing layer which dissolves and stains the tape when contacted with an organic solvent, e.g., kerosene or toluene. Unfortunately, however, most tampering is purely mechanical in nature.

A further type of tape closure is the heat shrunk sleeve found around the neck of many bottles containing barbecue sauce, wine and other comestibles. This type of closure is generally not resealable once it has been opened, but each type of bottle requires a custom sized sleeve which is useful only for a particular size and configuration. In the past, such sleeves have been mechanically held in position before and during heat shrinking.

The present invention provides a simple way of detecting whether any of several types of tape closure seal has been broken. In each case, the closure visually indicates when it has been mechanically tampered with, thereby providing increased safety and product confidence to the ultimate consumer. In making heat shrunk closures, the sleeve is held in place by adhesive until shrinkage has occurred, eliminating the need for a separate holding means.

This invention comprises, in one aspect, an improved form of previously known tape closures. The invention provides a closure having, at an exposed surface, a layer of material which is originally transparent or translucent but which opacifies when stressed (as occurs when the closure is removed to open the container). Before the stress-opacifying material is stressed, any underlying substrate is visible; after stressing; however, the material becomes opaque white or pastel. Any underlying surface or material which was previously visible is thereby obscured, resulting in a change of appearance which indicates that the tape closure has been tampered with. The evidence of tampering remains even if the closure is thereafter resealed. The entire closure can be formed from a stress-opacifying material; alternatively, the stress-opacifying material can be part of a separate indicator layer disposed on top of an otherwise conventional tape closure.

The tape closure chosen can be of the type described in the aforementioned U.S. Pat. No. 3,389,827, the disclosure of which is incorporated herein by reference. The closure of this invention can also be used in the form of an elongated pressure-sensitive adhesive coated strip to seal the joint between a can and lid in slip-cover or collar cans of the type commonly used to package coffee, canned ham, etc.; see U.S. Pat. No. 3,516,852. The strip is also useful in holding conventional fitted caps or tops on plastic or cardboard food containers (e.g., of yogurt or cottage cheese). In such cases, the closure of this invention can be formed by wrapping a strip of heat-shrinkable tape around the container overlying the joint between the body and the tape and extending below, and optionally above the rim. The application of heat shrinks the tape around the rim, thereby forming a tight tape seal. The heat shrunk tape collar may thus be utilized either primarily as a seal (as in the slip-cover cans) or primarily as a tamper indicator (as in bottles, etc.) providing an air tight secondary seal in the latter case. Thus, if the primary seal should fail as may occur if the sealing lip of a bottle is chipped, the tape will protect the container's contents.

Where the indicator layer is a separate layer attached to the closure used, at least a portion of the indicator layer is partially disposed on the free end of the tape closure so that it is flexed and begins opacifying before any substantial portion of the closure is removed, thus insuring that the contents of the sealed container cannot be altered without detection.

The stress-opacifying indicator layer is advantageously formed from a 1-10 mil (25-250 micron) film of normally transparent stress-opacifying unplasticized polyvinyl chloride, other suitable stress-opacifying materials including films of polybutadiene/polystyrene, polyvinyl chloride/polyvinyl acetate and isotactic polypropylene/butyl rubber blends.

Where the stress-opacifying film constitutes the entire tape closure, it will generally be thicker than where it appears only in a layer on top of a supporting tape closure. The thickness of the films used will necessarily depend on the internal pressure of the container being sealed. Cans containing carbonated beverages, for example, will naturally require a thicker, stronger, closure than packages of unpressurized dry materials, such as breakfast cereals.

When a separate indicator layer is used, it is advantageously secured to the tape closure by pressure-sensitive adhesive, although other thermosetting and thermoplastic adhesives may be used. The adhesive should, of course, bond the indicator to the tape closure so firmly and adherently that the indicator strip will be stressed beyond its opacification point when the tape closure is flexed. Adhesives which do not sufficiently firmly bond the indicator layer to the tape closure will allow the indicator to buckle or separate from the tape closure; therefore, the indicator will not be flexed and no visual change will occur to indicate tampering. Functionally stated, the adhesive should be so firmly bonded to both the indicator layer and the tape closure that the force required to peel the indicator layer from the closure exceeds the force required to stress-opacify the indicator layer.

The peel adhesion of the adhesive can be measured using a modified version of the Pressure-Sensitive Tape Council's 180.degree. peel adhesion test PSTC-1. In this modification, a strip is formed by coating the adhesive to be measured on a 1-3 mil (25-75 micron) biaxially oriented polyethylene terephthalate film at a dry coating weight of 10-12 grains per 24 square inches (about 40-50 grams/m.sup.2). A strip 10 inches (about 25 cm) long and 1 inch (2.5 cm) wide is cut from the tape and applied to a clean 2-inch .times. 5-inch (about 5 .times. 12.5 cm) steel plate so that 5 inches (about 12.5 cm) extends beyond one end and rolled to insure that no air bubbles are entrapped. The test piece is allowed to age for 24 hours at room temperature, after which the free end of the tape strip is doubled back over the adhered portion, pulled beyond the end of the panel and clamped in one pair of jaws of an Instron tensile testing machine. The exposed end of the steel panel, adjacent the doubled-back end of the tape, is then clamped in the second pair of jaws of the tensile tester, so that when the two pairs of jaws are separated, the tape is stripped free from the steel panel. The jaws are separated at the rate of 12 inches (about 30.5 cm) per minute, the force required giving a measure of peel adhesion. Adhesives which have a peel adhesion of at least about 50 ounces per inch of width (about 560 grams per centimeter of width) have been found generally suitable for securing an indicator layer to a tape closure to insure sufficient stress to opacify the indicator. Beneath the stress-opacifying layer is a colored material. When the stress-opacifying layer is unflexed and therefore transparent, the colored layer is visible along the entire closure. When the stress-opacifying layer is flexed, it becomes translucent or opaque, blocks out the underlying color and thus creates a different visual impact, indicating tampering with the closure in which the stress-opacified layer is incorporated.

It is also possible to print indicia, such as the words "open" or "opened" on the upper surface of the stress-opacifying layer, the indicia being the same color as the underlying colored layer. Such indicia are invisible before the closure has been flexed but show up as colored words or patterns on the opaque white or pastel background after the closure has been flexed.

It is also within the purview of this invention to prepare closures which incorporate colored layers both above and below the stress-opacifiable layer, the exposed colored layer being transparent so that the indicator initially has an appearance which is the sum of the two separate colors. Upon opacification, only the top color will be visible. Further information on such colored systems and materials for forming them can be found in U.S. Pat. No. 3,468,744, the disclosure of which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention may be had by referring to the accompanying drawing in which like numbers refer to like parts in the several views and in which:

FIG. 1 is a top plan view of a can lid having an aperture covered by the tape closure of this invention;

FIG. 2 is a cross-sectional view of the can lid of FIG. 1 taken along section line 2--2 and looking in the direction of the arrows;

FIG. 3 is a perspective view of a roll of tape suitable for making can closures of this invention;

FIG. 4 is a perspective view of a slip cover can sealed by a closure of this invention;

FIG. 5 is a partial elevation of a food container in partial section with its fitted cover in place showing a heat shrinkable tape in position for shrinking; and

FIG. 6 depicts the container of FIG. 5 after the tape has been heat shrunk.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a can lid 10 having aperture 12 covered and sealed by tape closure 14, transparent stress-opacifiable indicator layer 16 being firmly adhered to the upper surface thereof.

Indicator strip 16 comprises stress-opacifying layer 20 overlying and bonded to colored layer 22, the latter being firmly attached to tape closure 14 by means of adhesive layer 24. Tape closure 14 comprises a layer of strong film 26 attached to can top 10 in the area surrounding aperture 12 by adhesive 28.

In an alternative embodiment, film 26 of closure 14 is itself a strong stress-opacifying material, thereby eliminating the need for a separate indicator layer. In this embodiment, colored layer 22 is disposed between adhesive 28 and film 26, and closure 14 itself opacifies when flexed giving a visual change which shows the closure has been tampered with.

As shown in FIG. 3, tape closure 14 of FIGS. 1 and 2, with its indicator strip 16, is conveniently died from an elongated strip unwound from a convolutely wound roll 30 of material. Film 32, suitable for forming a tape closure, has a continuous indicator layer 16 of stress-opacifying material adhesively bonded to its exposed surface. A layer of adhesive 33, suitable for attaching the closure to a can top, is firmly adherently bonded to the surface of film 32 opposite indicator strip 16. Tape closures are conveniently die-cut from the strip of roll 30 to the desired shape, such as that shown in FIG. 3. (It will be understood, of course, that roll 30 will conventionally be wound upon itself about a core, adhesive side in, and is shown otherwise in FIG. 3 for convenience in understanding.)

In FIG. 4, slip cover can 35, comprising cover 36 and body 38, is sealed with strip closure 34, which covers the line (not shown) along which the lower edge of cover 36 overlaps the upper edge of body 38. Closure 34 has pull tab 40 with indicator 41 disposed between tab 40 and the point 42 where the closure 34 overlaps and is adhered to itself slightly to insure a hermetic seal. A portion of cover 36 and container 38 is recessed slightly so that strip closure 34 does not protrude beyond the major circumferential surface of the can, thereby protecting strip 34 from accidental damage during shipping and handling. Further details of this type of sealing construction can be found in aforementioned U.S. Pat. No. 3,516,852, the disclosure of which is incorporated herein by reference.

In FIGS. 5 and 6, food container 44 is sealed with easily removable depressed center cover 46. A strip of heat-shrink tape 47, having pressure-sensitive adhesive 48 disposed on one side, wound around and adhered to rim 49 formed by the cover and container overlying the line of abutment between the two. As shown in FIG. 5, tape 47 is applied along rim 49 of the container and its ends overlapped so that the film will form a complete sleeve when heat shrunk. After being attached to rim 49, tape 47 is heated, e.g., by means of a jet of air at a temperature of about 100.degree.-200.degree.C., causing the film to shrink and enclose the rim as shown in FIG. 6.

The invention is further described with reference to the following nonlimiting examples. Parts and percentages are by weight unless otherwise noted.

EXAMPLE 1

A 7.5 mil (about 190-micron) film of oriented rigid unplasticized polyvinyl chloride (commercially available from B.A.S.F. under the trade designation "Luvitherm") was reverse roll coated on one side with a black vinyl ink comprising carbon black pigment in a phthalate-plasticized polyvinyl chloride:polyvinyl acetate binder resin solution. The coated film was then placed in a 65.degree.C. warm air oven for 15 minutes to drive off the solvent and leave a dry coating weight of 0.25 to 0.30 gram ink per 24 square inches (about 50 mg/cm.sup.2). A suitable ink can be made by diluting MRX-9243 black vinyl ink, sold by Crescent Ink and Color Company, from its normal 39% solids content to 24% solids content, using a blend of equal volumes of toluene and methyl ethyl ketone.

A thin primer layer of 7.5% solids dimer based polyamide (obtainable from General Mills under the trade designation "Versalon" 1140) in 2:1 isopropanol:toluene solvent was coated over the ink. A pressure-sensitive adhesive was made by dissolving (1) 100 parts of an ABA block copolymer where A is 15,000 molecular weight polystyrene and B is 30,000 molecular weight polybutadiene (available from Shell Chemical Company under the trade designation "Kraton" 1101) and (2) 95 parts of .alpha.-terpene in toluene. The blend was diluted with toluene to 40% solids and knife coated on the primed, inked surface. The film was dried at 65.degree.C. for 20 minutes to evaporate the toluene, leaving 12 grains of adhesive per 24 square inches (about 50 grams/m.sup.2). The resulting strip was laminated to the upper surface of a container sealing closure similar to those described in U.S. Pat. No. 3,389,827, Example II, the closure being formed of an aluminum vaporcoated biaxially oriented polyethylene terephthalate film to form a laminated structure similar to FIG. 2.

When viewed from its exposed side, after being applied to the sealing closure, the indicator strip originally appeared black, since the black ink printed on the nonexposed side of the transparent "Luvitherm" polyvinyl chloride film was visible. When the closure was flexed during removal, however, the polyvinyl chloride film attained an opaque white appearance blocking the black color from view and thereby indicating that the closure had been partially removed.

Example 2 ______________________________________ A polyvinyl chloride based film was made by mixing: Parts polyvinyl chloride resin available from Union Carbide Co. 100 acrylate-butadiene styrene copolymer available from Rohm and Hass as "Acryloid" KM 228 9 Montan wax acid derivative available as "E" wax from B.A.S.F. Wyandotte Corp. 5 Barium/cadmium stabilizer available as "Mark 99" from Argus Chemical Co. 3 Acrylic ester polymer available as "Acryloid" K-120N from Rohm and Hass 2 Epoxy stabilizer available as "Drapex" 3-2 from Argus Chemical Co. 2 Phosphite chelate available as "Mark C" from Argus Chemical Co. 1 ______________________________________

The mixture was calendered to form a 7-mil (190-micron) thick polymeric film. The calendered film was stretched in the longitudinal direction at a 2.5 to 1 stretch ratio, reducing the film's thickness to about 2.75 mils and making it heat shrinkable.

The stretched film was reverse roll coated with a black vinyl ink as in Example 1. A thin primer layer was coated over the ink. A pressure-sensitive adhesive was made by dissolving 100 parts of the ABA block copolymer of Example 1 and 95 parts of alpha-pinene resin in toluene (commercially available from Pennsylvania Industrial Chemical Co. under the trade designation "Piccolyte".alpha.135. The blend was diluted with toluene to 40% solids, knife coated on the primed inked surface, and dried at 65.degree.C. for about 20 minutes to evaporate the toluene, leaving 8 grains of adhesive per 24 square inches (about 27.5 grams/m.sup.2).

A 2-inch (about 5 cm) strip of the tape was formed into a band with its ends overlapping about 5/8 inch (about 9 cm) and placed around the neck of a ketchup bottle with about 1/16 inch (about 0.16 cm) extending beyond the cap. The bottle with the tape held in position was rotated in a stream of hot air at about 300.degree.F. (about 160.degree.C.) for about 3 seconds. The tape shrunk conforming to the bottle and cap providing an air tight seal.

When the end of the tape is pulled away from the bottle, the film stress opacifies changing the appearance of the film surface from black to white.

Claims

What we claim is:

1. In a container sealed by means of a tape closure, the improvement which comprises:

at least a portion of the closure surfaced with an indicator strip comprising a stress-opacifying colored backing;

whereby when said indicator layer is flexed to open said closure, said backing will become opaque and display a color difference between the flexed and unflexed portions indicating said flexing even if said covering is returned to its previous sealing position.

2. The sealed container of claim 1 where said indicator strip comprises a heat-shrinkable tape which has been exposed to heat whereby the tape forms a tight seal.

3. The sealed container of claim 1 wherein said indicator strip bears on its exposed surface printed indicia of the same color as the colored backing, whereby when the indicator is unflexed, the indicia are invisible and when the indicator is flexed, the indicia will be visible against an opaque background.

4. The sealed container of claim 1 further characterized by said container having a rigid wall with formed aperture therein, the cover over said aperture being a tape closure having a free end, said closure comprising a sheet material removably adhered directly to a portion of said wall which surrounds said aperture, said closure capable of being grasped by said free end and pulled back upon itself without rupture to expose the underlying material and said indicator layer being adhered to said closure by an adhesive.

5. The sealed container of claim 4 wherein the adhesive which bonds the strip to the closure has a peel force of at least 50 ounces per inch of width.

6. The sealed container of claim 1 where said stress opacifying material is an oriented polyvinyl chloride polymeric film.

7. The sealed container of claim 1 where said container is a slip cover can sealed by a length of tape having a free end suitable for gripping having said indicator layer adhered to said closure so that at least a portion of the indicator layer is flexed before any portion of said closure is removed from said can.

8. A container sealed with a tape closure, said closure comprising a sheet material coated over one face with an adhesive which was at least originally tacky and pressure-sensitive, at least a portion of the opposite face of said tape being surfaced with an indicator comprising a stress-opacifying backing, whereby when said closure is flexed, the backing will opacify and display a color difference between the flexed and unflexed portions, even if the tape is thereafter returned to its previous sealing position.