Note: Descriptions are shown in the official language in which they were submitted.
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Field of the Invention
This lnvention relates to plastic food containers.
Backcround of the Invention
In vacuum packing foods and in pacXing some non-~cod
products, it is necessary to retort, or heat, the contents.
Foods must be heated during their processing to temperatures 2p-
proaching 250F to kill bacteria. Some non-food products such as
wax must be heated to keep them fluid while they are poured into
their packages.
Foods can also be reheated in their containers, in
either conventional or microwave ovens, to temperatures in the
same range as in processing or higher.
Presently, such retortable containers are made from
glass or metal. Both materials provide the gaseous barrier
necessary for long shelf life and can easily withstand high temp-
eratures without losing their rigidity, but both are becoming
increasingly expensive to manufacture. Furthermore, each has
its own disadvantages. For example, metal containers cannot be
reheated in microwave ovens, and glass containers are easily
bro~en.
Rigid plastic containers have only heretofore been
used in low-heat applications (less than about 190F), such as
in hot-filled beveraye cups or the like. And these plastic
containers have been sinyle-layer structures composed of, ~or
example, polypropylene.
Plastics have been used in some retortable packa~es.
~or example, flexlble retortable bacs have been made bv laminatin~
iuminum and plastic.
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Summary of the Invention
I have discovered that a less-expensive and generally
more desirable retortable container can be made from multiple
layers of plastic materials. A heat-tolerant structural layer
(e.g., ~ a}d~350, (Arco Polymers trademark)) is co~bined with a
gase~ barrier layer (e.g., a nitrile-based resin such as Barex
210, (Vistron trademark)) to give a plastic container that can
be retorted and that is substantially impenetrable to oxygen or
other gases, such as food aromatics, so as to prolong shelf life.
According to the present invention; there is provided a
coextruded thermoplastic sheet, suitable for forming into a
retortable container, comprising a heat-tolerant structural
layer made of Dylark , a styrene-maleic anhydride copolymer, that
remains rigid when heated to temperatures above 200F for main-
taining the rigidity of said container when heated and a gaseous
barrier layer for preventing diffusion of gases into and out of
said container, whereby said container, when forrned from said
sheet, can be heated to sterilize vacuum-packed foods and to
pack in a fluid state other products, provides a barrier to oxygen
and other gases, both incomlng and outgoing, during storage of
said container, and can be heated to prepare food for eating.
In another aspect, the invention provides a retortable
container for packaging foods and other products heated during
packaging, comprising a heat-tolerant structural layer that
remains rigid when heated to temperatures above 200F for
maintaining the rigidity of said container when heated and a
gaseous barrier layer for preventing diffusion of gases into
and out of said container, whereby said container formed can be
heated to sterilize vacuum-packed foods and to pack in a fluid
state other products provides a barrier to oxygen and other
* trademark
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gases, both incoming and outgoing, during storage of said
container, and can be heated to prepare food for eating.
In preferred embodiments, the container is thermoformed
from a three-layer coextruded sheet consisting of a Dylark 350
inner layer, a Barex 210 outer layer and an intermediate tie
layer that is a mixture of two DuPont commercial resins:
CXA 1025 and Eva 3135X, both of which are ethylene vinyl acetates;
the container has either a heat-sealed lid composed of the same
three layers or a mechanically sealed metal lid.
Preferred Embodiment
I turn now to description of the structure, manu-
facturing and use of a preferred embodiment of the invention,
after first briefly describing the drawings.
FIGURE 1 is an elevation, partially cross-sectional,
view of said preferred container embodiment, with the lid shown
raised above its installed position.
FIGURE 2 is a cross-sectional view at 2-2 of Figure 1,
showing the multilayer structure of the container walls.
FIGURE 3 is a diagrammatic view of the manufacturing
process for forming the container.
Turning to the Figures, there is shown container 10 in
which food (e.g., a ready-prepared stew) has been packed. The
container consists of bowl 14 and lid 16.
Both the lid and bowl are made from multilayer sheets
that are formed by coextrusion casting. Bowl 14 is thermoformed
from such sheets, as depicted in Figure 3. ~id 16 is cut from a
similar sheet and heat sealed to rim 20 of the bowl.
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As shown in Figure 2 the finished bowl and lid have
three layers: an inner structural layer 22 of Dylark* 350
(product of Arco Polymers) that remains rigid at temperatures up
to about 250F; an outer layer 24 of Barex* 210, (product of
Vistron), a nitrile-based resin that acts as a barrier to incom-
ing oxygen to prevent food spoilage; and an intermediate tie
layer 26 that is a mixture of two DuPont commercial resins--25%
CXA 1025, an ethylene vinyl acetate and Eva 3135X, a second
ethylene vinyl acetate.
The resin combination adheres well to nitrile layer
24 and to Dylark Layer 22, and it provides a good moisture bar-
rier, a property not adequately present in either of layers 22and
24.
The C~A 1025 (vinyl acetate) provides the necessary
adhesive qualities. The Eva 3135X dilutes the adhesive to
allow a thicker layer to be used. An undiluted CXA 1025 layer
of the desired tie layer thickness would tend to be pushed to
the lateral edges of the sheet during extrusion and to there
emerge from the sheet and stick to the rolls causing the sheet
to bind. The combination thereby solves the roll sticking
problem.
Turning to Figure 3, the coextrusion process for
forming the three-layer sheet material is shown.
Three heated containers 30, 32 and 34 serve as sources
of the Dylark 350, the mixture of DuPont resins, and Barex 210,
respectively. The DuPont resin mixture is heated in container
32 to about 390F. The Dylark 350 and Barex* 210 are heated in
containers 30 and 34 to about 400F.
*Trademark
Three conduits 36, 38 and 40 supply the heated
materials to coextrusion block 42. There the materials merge
together to form under pressure a unitary, three-layer, thick
stream 44 of generall~ circular cross-section. Stream 44
passes into extrusion die 46 (Welex standard 54 inch flex-lip
die), while maintained at a temperature of about 400F, or
slightly in excess thereof, for extrusion into continuous
sheet 48 about 36 mils
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thic~ (Dylark 23 mils; tie layer 4 mils; and Barex 210, g milJ~.
Sheet 48 then passes through a series of chill rolls 37.
The sheet may then be processed into containers or
wour.d into spools for storage (not shown).
To process sheet 48 into containers, the sheet is passed
through conventional thermoEorming apparatus 50 which impresses
the bowl shape and in doing so reduces the wall thickness by
about 503 on the average, making the finished container wall
about 18 mils thick. The individual layer thic~nesses are also
each reduced by about 50% during thermoforming, maXing the
~inished thickness about 12 mils of Dylark 350, about 2 mils of
tie layer, and about 5 mils of Barex 210.
After thermoforming, the shaped sheet 52 passes throug~
trim press 54, in which the individual bowls 14 for the containers
are separated. Thereafter, each bowl is given curled rim 20
(Fig. 1) by a conventional curling machine (not shown).
Lids 16 are separately cut from sheet 48 and heat-
sealed to curled rim 2Q by conventional heat-sealing techniques.
In use, food is packed in bowl 14, the bowl and food
are retorted, and lid 16 is heat-sealed to rim 20. The consumer
can reheat the container in an oven (conventional or microwave),
making sure that temperatures do not exceed about 250F.
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_her Embodiments
Other embodiments of the invention will occur to
those skilled in the art. For example, a metal lid could be
substituted for the plastic one and could be mechanically
sealed to rim 20. Such a lid could then be opened with a
conventional can opener, but reheating would have to be in a
conventional oven and not a microwave oven unless the metal
lid were first removed. Other heat-tolerant structural layers
could be substituted, e.g., polycarbonates or filled poly-
propylene could be used. And similarly other gaseous barrierlayers could be used, e.g., Nylon 6 or hydrolyzed polyvinyl
alcohol such as Eval , with suitable tie layers.
* trademark
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