CA1213534A - Thermoplastic food container - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

A container and its method of manufacture are disclosed in which a multi-layered thermoplastic material is utilized to form a container, which container is particularly advantageous for packaging foods which are susceptible to spoilation by the permeation of oxygen through the wall of the container. The invention also discloses a container having means for hot-filling with food products.

Description

~L3534 THERMOPLASTIC FOOD CONTAINER

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BACKGROUND OF THE INVENTION
~' The present invention generally relates to ` thermoplastic containers for receiving food products and ~ more particularly involve~ a thermoplastic container made - from a multi-layered thermoplastic material.
; In the current state of food packaging many food products such as soups and juices are still being packaged primarily in metal cans. Because of the high acidity and high susceptibility to spoilation from oxygen encroachment, many food products must be packaged in metal containers which exhibit some added chemical resistance to the acidity - as well as being tightly and hermetically sealed. This involves, in the case of metal cans r some interior coating on the metal can to prevent the acidity in the ~ood product from attacking the metal in the can.

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Other types of food packaging containers involve glass containers with metal caps. The glass container offers both chemical resistance to acidity and a highly impermeable barrier to oxygen ingress into the container.
Unfortunately, the glass container is expensive to manufacture and is subject to breakage and the accompanying danger of glass injuries to the consumers. Both the metal can with interior coating and the glass container are measurably more expensive to manufacture and less desirable than thermoplastic containers for the same food products~
Current food product packaging which utilizes plastic containers suffers from the disadvantage that the majority of plastic materials accepted by the FDA for contact with products for human consumption do not offer all the desired characteriistics for containers. For example, one highly desirable material for food product containers i5 polyethylene terephthalate (PET). This polymer is a tough clear plastic material having good strength characteristics and moderate barrier characteristics. Unfortun-ately, the material does not offer the impermeability needed to protect products which are susceptible to oxygen encroachment and spoilation therefrom. For example, soups and juices suffer rapid spoilation when placed in straight PET containers because of the relative porosity of PET to the ingress of oxygen molecules. Thus, PET as a food container package leaves much to be desired.
A~licant is aware of the manufacture of a laminated container having ex-terior and interior layers of a s~

structural polymer with a sandwiched barrier layer therebetween. Whereas this container is an advancement over the prior art its use is limited in the area of hot-fill and retort since it has upper temperature limits which, in some cases, would prevent hot filling and retorting. In some cases where a high hot fill or retort temperature is required, followed by chilling of the product for retail sale, the polymers in the structural layers become susceptible to trauma fracture which might occur during packaging, shipping, stocking, or even by dropping on the floor.
The present invention overcomes the disadvantages of prior art containers by providing a multi-layered, multi-segmented container which is highly impervious to the transmission of gasses su~h as C02 and oxygen therethrough, is tough and resilient, is inexpensive to manufacture, and allows hotfilling and retorting without degradation of the container's desirable qualities.

SUMMARY OF TE~E INVENTION

The present invention discloses a two-piece food packaging container made of a multi-layered thermoplastic material with an interior layer comprising a desirable package polymer such as polyethylene terephthalate or polypropylene, a layer of a barrier polymer such as ethylene vinylalcohol (EVAL), and an exterior "girdle"
layer having enhanced fracture resistance. The container of this invention may be manuEactured by coextruding the barrier polymer inside a "sandwich" of the two structural layers into a sheet material and then thermoforming the

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container in two separate sections which are then joined by spin-welding.
In accordance with the present teachings, a composite thermoplastic container for receiving hot-filled material therein is provided. The container comprises a tapered upper body section containing a filler opening at the top end thereof and an open bottom end larger than the filler section; and a tapered lower body section having a bottom closure and an open upper end adapted for snug-fitting relationship with the upper body portion. At least one of the body sections comprises a multi-layered thermoplastic material, at least one layer of which comprises a barrier polymer having a low gas permeability and at least one other layer comprises an impac-t copolymer; and, wherein the body sections are friction-welded together to form a single container.
In accordance with a further aspect of the present teachings, a method is provided of forming a barriered, nestable, composite thermoplastic container for receiving hot-filled products and having vacuum compensating ability.
The method comprises coextruding a composite thermosplastic sheet having at least one layer of a heat-resistant structural polymer material, at least one impact layer of a tough copolymer, and at least one layer of a barrier polymer. A
tapered upper body section having an opened larger end and a narrower end with a filler opening is thermoformed from the sheet. A papered lower body section having ~ larger open upper end and a bot-tom closure is thermoformed from the sheet with the bottom closure being thermoPormed to contain a flanged, inwardly flexible diaphragm section arranged to flex inward in response to a vacuum in the container. The upper and lower body sections are friction-welded together to form an integral container.

~LZ~35~4 -4a-BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is partial cross-sectional ~ide view of one embodiment of the container made according to the present invention.

Figure 2 is an axial end view of the container of Figure 1 taken at line 2-2 thereof.

Figure 3 is an axial bottom view of the container of Figure 1.

Figure 4 is a partial cross-sectional side view of the container of Figure 1 after the container has been filled.

Figure 5 is a cross-sectional side view of an alternate embodiment lid closure for the container of Figure 1 and;

Figure 6 is a cross-sectional side view of the container of Figure 5 after hot filling.

Figure 7 is a partial cross-sectional view of a barriered container according to the present invention.

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DESCRIPTION OF THE PREFER~ED EMBODIMENTS

Referring to Figure 1, there is illustrated a thermoplastic container 101 which is particularly advantageous for packaging hot food products such as vegetable juice. The container 101 comprises an upper body portion 102 and a bottom portion 103. Upper body portion 102 comprises a tapered frustoconical wall section having at its upper end an indented neck section 104 and a peripheral rim 105. Rim 105 has located therein an indented lip section 106 defining a fill opening 107.
Base cup 103 comprises an upwardly extending frustoconical wall section 108 having at the upper end thereof a peripheral spin-weld flange 109 integrally formed thereon. Cup 103 has a bottom support rim 110 and a diaphragm bottom section 111.
Diaphragm bottom 111 comprises a central diaphragm disk 111 separated from rim 110 by a pair of alternating flex-grooves 112 and 113. The provision of disk 111 and grooves 112 and 113 allows a certain amount of flexibility in the volume content of the container 101.
A flexible metal cap 114 is provided for tight-fitting engagement over fill rim 105. Top 114 preferably is made of a ductile material such as aluminum foil. A collapsable center section 115 slightly raised from top 114 is formed therein to provide flex space above the filled product in the container. A plurality of indentations 116 are formed in the upper portion o~ the conical section of body 102 and preferably at least three indentations 116 are formed in the wall of the upper body ~353~

portion. Likewise a set of indentations 117 are formed in lower cup portion 103 and comprises at least three indentations therein. The indentations 116 and 117 are adapted for nested stack control so that several nested components of the same configuration do not jam or lock together prior to denesting and application.
The material preferred for forming the present container consists of a multi-layered thermoplastic sheet having an interior layer of polypropylene, an exterior layer of a propylene/ethylene copolymer, and a central layer of EVAL bonded therebetween. This sandwich effect results in a waterproof layer on the interior of the container formed of polypropylene and a tough, fracture-resistant "girdle" layer ~f copolymer, with a captured or "sandwiched" layer of EVAL between the two structural layers. The layer of EVAL serves to provide an oxygen barrier for the container to prevent the ingress of oxygen and to protect the food product ~rom early spoilage. The copolymer layer, comprising a propylene/ethylene copolymer, acts as a reinforcement or "girdle" around the structural barrier layers. The properties of the copolymer add the extra toughness associated with the ethylene elastomer to provide greatly enhanced fracture resistance to the container, especially at low temperatures and after heating. The inner layer of a homopolymer such as polypropylene provides rigidity of the container during the elevated temperatures of hot-filling or retorting while the outer layer becomes rather soft. The copolymer outer layer then provides toughness at the lower temperatures where the homopolymer mlght be fracture-prone. In one preferred 1;2~35~3~
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embodiment the multi-layered thermoplastic material was extruded in a sheet coextrusion die such as those disclosed in U. S. Patent 4,100,237 ,;
, and was thereafter thermoformed by means :. such as blow molding into the final upper and lower body portion. One method of thermoforming such containers which also offers the additional advantage is obtaining biaxial orientation in the containers ';

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.~ , .. Other methods of thermoforming the container ~- portions such as vacuum molding and more conventional - methods of blow forming can be utilized to form these container portions. After the upper portion 102 is -~ thermoformed a stamping machine can be utilized to cut top opening 107 thereinO
After the upper and lower container portions are ~ formed from the composite sheet, each section is placed in .' its respective spin-welding mandrel and they are joined~ together by spin-welding means to form a hermetically :. sealed integral container 101. It is preferred that a slight interference fit be designed into the container ~; sections between flange 109 and the lower skirt portion of : conical wall 102. This allows for slight variations in . sizing between the two respective container portions and ., .

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~ Z~35~4 further aids in their joining by spin-welding techniques.
The provision of polypropylene or polyethylene terephthalate outer and inner layers in the two container sections insures a good solid friction weld between the two sections due to the susceptibility of these polymers to spin-welding techniques.
Referring now to Figure 4, a container 101 is illustrated in its filled and cooled state. One problem with prior art containers is the shrinkage involved when material which has been heated or retorted to sterilization temperatures is placed in the food container ~a process called "hot-filling"). The problem generally occurs when hot food products at temperatures in excess of 200 F are placed in the containers and the containers are then sealed. As the heated material and the air above it in the container cools it naturally contracts due to the change in temperature. In many instances this could cause buckling of the external walls or top and bottom of the containers.
This is particularly troublesome in thermoplastic containers which have higher flexibility than glass and metal. In the present invention this is overcome by the provision of the unique diaphragm bottom section 111 containing expansion grooves 112 and 113 to allow flexing upward of the bottom portion. Likewise the top seal 114 has a provision for compensating for the shrin~age of the headspace air and of the products. As shown in Figure ~, section 116 of top 114 is ductile enough to be drawn downwardly into the container opening as the product shrinks in response to being cooled. Thus as the product :12:~3534 shrinks, the crown portion 115 of upper seal 114 is drawn downward into the lip area 106, 107 of top portion 104.
Thus in Figure 4 it can be seen how top seal 114 has been drawn downward by cooling of the product and bottom diaphram section 111 has flexed upward into the container to of~set the shrinkage of the liquid. One of the principal advantages of the present invention is that these two effects are not obvious to the consumer. One of the significant disadvantages of previous thermoplastic container designs is that the shrinkage due to cooling usually caused an uneven flexing inward of the side walls or of the top/bottom sections. This flexing or l'buckling"
of the plastic container was often mistakenly perceived by the consumer as indicating a spoiled product. Thus the present invention, by providing the flex diaphram bottom section and the collapsible top seal, compensates for the effect of the shrinkage in the container contents. The flexing of the bottom is indistinguishable to the average consumer and the downward drawing of the top seal appears to be the natural sealed condition of the top.
Figures 5 and 6 illustrate the preferred embodiment of the invention wherein a different construction for the metal foil lid is utilized. In the lid illustrated in Figure 5, lid 214 comprises an outward circular flange section 215 and a concentric ribbed diaphram section 216. The rib 216 and the dome 217 extend downward toward the container to eliminate most of the headspace above the material in the container. This large reduction in the headspace gives a reduction in the vacuum effect when the contents and trapped air cool. This in 1~13S34 turn will provide a more controlled downward flexure of the foil lid as the product in the container cools. Figure 6 illustrates the ~inal downward position of the seal after total collapse into the fill opening of the container.
Thus the present invention discloses a thermoplastic food and beverage container manufactured from a composite thermoplastic sheet material having a barrier buried therein, which container is formed by thermoforming an upper and a lower section from the multi-layered sheet and then spin-welding the two sections into a single integral container. The bottom section of the container comprises a disguised flexible diaphram section and the top section receives a collapsible foil seal to compensate for the volume shrinkage due to cooling of the headspace air and of the hot-filled product.
In addition to the advantages of the present invention enumerated above, the design of the container disclosed herein is particuarly advantageous for shipping.
Whereas most prior art conventional containers generally are formed in a single operation and must be either formed on location or shipped empty, the present invention discloses a container which is nestable and can be shipped at a fraction of the cost of conventional containers.
Since most conventional containers which are shipped must be shipped empty the truck or railcar containing the conventional containers is filled primarily with air.
For example, the average semi-trailer load of normal products generally weighs about 20,000 pounds. A filled semi-trailer load of empty plastic containers might weigh

3~4 only 2,000 pounds. Thus the shipper of the containers is paying for the cost of an entire truck load of containers but is primarily shipping air. With the present invention the containers may be shipped in separate parts, which due to their frusticonical nature can be nested in a very compact arrangement. The top sections 102 are nested together and shipped in one container and the bottom sections 103 may be nested together and shipped in a separate container. Thus the user of the containers realizes tremendous economies in shipping by the nestability of the various components. The ~iller or canner receives the nested portions and places them in a relatively simple and inexpensive spin-welding device to form the containers sectiotns into a single sealed integral container. Spin-welding systems whlch are particularly useful in welding the present container are disclosed in U.S. Patents 3,800,40 to Mistarz et al; 3,799,821 to Jones;
3,708,376 to Mistarz; and RE 29,448 to Brown et al.
Although a specific perferred embodiment of the present invention has been described in the detailed ~*scription above, the description is not intended to limit the invention to the particular forms or embodiments disclosed therein since they are to be recognized as illustrative rather than restrictive and it will be obvious to those skilled in the art that the invention is not so limited. For example, whereas the container materials were cited as being polyethylene terephthalate (PET) or polypropylene, it is obvious that other known thermoplastic container materials can be utilized. For example, polyethylene, polyethylene terephthalate glycol (PETG), 35~34 polyvinyl chloride, polystyrene, and various other poly olefins and polyesters can be utilized for the container material. Likwise other barriers can be utilized in the sandwich between the wall polymers; for example such as saran (PVDC) and barex. In one particular preferred embodiment, a five layer coextruded sheet was manufactured having a polypropylene such as Shell Chemical's 5225 homopolymer as the interior layer. The exterior layer was formed from an impact copolymer manufactured by Northern Petrochemical and designated as a reactor copolymer of ethylene and propylene, product code 151OL~. The barrier layer was EVAL "F" manufactured by Kuraray Chemical of Japan and it was surrounded on each side by an adhesive manufactured by Mitsui Chemical Industries of Japan and designated as Admer QF-50~. Another copolymer which may be utilized as the exterior layer is a propylene/ethylene copolymer manufactured by Shell Chemical Co. and designated as Shell 7921. One important criteria in selecting the polymers for the copolymer appears to be that their melt indices be very close together. In the preferred embodiment above the polymer melt indices of the copolymer are both both 0.6. In addition, whereas the container is depicted as being generally cylindrical and is spin~welded together, it is also possible to use other geometrical cross-sectional shapes such as oval, elliptical or polygonal, and to friction-weld them by means other than spin-welding, i.e. by oscillatory bonding or welding. Thus the ~3 ~ ~

invention is declared to cover all changes and modifications of the specific example of the invention herein disclosed for purposes of illustration, which do not .constitute departure from the spirit and scope of the invention.

Claims (14)

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

1. A composite thermoplastic container for receiving hot-filled material therein, said container comprising:
a tapered upper body section containing a filler opening at the top end thereof and an open bottom end larger than said filler section; and a tapered lower body section having a bottom closure and an open upper end adapted for snug-fitting relationship with said upper body portion;
wherein at least one of said body sections comprises a multi-layered thermoplastic material, at least one layer of which comprises a barrier polymer having a low gas permeability; at least one other layer comprises an impact copolymer; and, wherein said body sections are friction-welded together to form a single container.

2. The composite container of claim 1 wherein said thermoplastic material comprises polypropylene said impact copolymer comprises ethylene/propylene, and said barrier material comprises an ethylene-vinyl-alcohol.

3. The composite container of claim 1 wherein said container comprises interior and exterior layers of a tough thermoplastic coextruded with a barrier layer surrounded by an adhesive therebetween.

4. The composite container of claim 1 further comprising vacuum compensating means at one end of said container adapted to flex inward in response to vacuum created in said container by cooling therein.

5. The composite container of claim 4 wherein said vacuum compensating means comprises a flexible flanged diaphragm formed in said thermoplastic body closure.

6. The composite container of claim 4 wherein said vacuum compensating means comprises a foil cap sealed on said filler opening, said cap having a collapsible diaphragm section formed therein.

7. The composite container of claim 4 wherein said vacuum compensating means comprises a foil cap sealed on said filler opening and having a collapsible diaphragm section, and a flexible flanged diaphragm section formed in said bottom closure and arranged to flex inward in response to vacuum in said container.

8. A method of forming a barriered, nestable, composite thermoplastic container for receiving hot-filled products and having vacuum compensating ability, said method comprising:
coextruding a composite thermoplastic sheet having at least one layer of a heat-resistant structural polymer material, at least one impact layer of a tough copolymer, and at least one layer of a barrier polymer;

thermoforming from said sheet a tapered upper body section having an open larger end and a narrower end with a filler opening;
thermoforming from said sheet a tapered lower body section having a larger open upper end and a bottom closure, said bottom closure being thermoformed to contain a flanged, inwardly flexible diaphragm section arranged to flex inward in response to a vacuum in said container;
and, friction-welding said upper and lower body sections together to form an integral container.

9. The container-forming method of claim 8 further comprising the step of providing a sealable top closure for said container, having flexure means formed therein adapted to flex inwardly in response to a vacuum in said container.

10. The container-forming method of claim 8 wherein said coextrusion step comprises coextruding at least five layers of polymer, with outer and inner layers of a structural polymer, a central layer of a barrier polymer, and tie layers of adhesive between said barrier layer and said outer and inner layers.

11. The container-forming method of claim 10 wherein said structural polymer is selected from the group comprising polypropylene, propylene, polyethylene, polyethylene terephthalate, polyethylene terephthalate glycol, polystyrene, polyvinylchloride, and copolymers of ethylene and propylene.

12. The container-forming method of claim 10 wherein said barrier polymer is selected from the group comprising ethylene vinyl alcohol, polyvinyl alcohol, polyvinylidene chloride, PVC/PVdC copolymer, and styrene-acrylonitrile copolymer.

13. The container-forming method of claim 8 wherein said friction welding step comprises spin-welding.

14. The method of claim 8 wherein said friction-welding step comprises oscillatory welding.