Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
FOAMED-WALL CONTAINER HAVING A SILVERY APPEARANCE.
FIELD OF THE INVENTION
[0001] The present invention relates generally to a foamed-wall polymer
container having a unique appearance. More particularly, the invention is
directed to a container comprising micro cellular foam, wherein the foam
micro cells contain a non-reactive gas such as nitrogen, and the container has
a silvery appearance. Also contemplated as a part of the present invention is
a method of manufacturing the foamed-wall container having a silvery
appearance.
BACKGROUND OF THE INVENTION
[0002] Biaxially oriented single and multi-layered bottles may be
manufactured from polymer materials such as, for example, polyethylene
terephthalate (PET) using a hot preform process, wherein a single or multi-
layered preform is heated to its desired orientation temperature and drawn
and blown into conformity with a surrounding mold cavity. The preform may
be prepared by any conventional process such as, for example, by extruding
a preform comprising single or multiple layers of polymer, or by injecting
subsequent layers of polymer over a previously injection molded preform.
Generally, multiple layers are used for beverage containers, to add diffusion
barrier properties not generally found in single layer containers.
pop] The various layers of polymers in the prior art multi-layered
containers are generally in intimate contact with one another, thereby
facilitating the conduct of thermal energy through the walls of the
containers.
This allows the chilled contents of the container to quickly warm to the
ambient temperature. Accordingly, such containers are often sheathed in, for
example, a foamed polystyrene shell to impart thermal insulating properties to
the container.
[0004] It would be desirable to prepare an improved plastic container
which is opaque with unique visual properties without the addition of a
coloring agent. Further, it is deemed desirable to impart thermal insulating
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properties to the improved plastic container. Also, it would be desirable to
prepare an improved plastic container having a silvery appearance without
requiring the addition of a coloring agent which would adversely effect the
recycling characteristics of the container.
SUMMARY OF THE INVENTION
[0005] Accordant with the present invention, a foamed-wall container
having a unique appearance has surprisingly been discovered. The container
comprises a micro cellular foamed polymer, and a non-reactive gas contained
within the micro cellular foam cells, wherein the container has a silvery
appearance without the addition of a coloring agent.
=
[0006] Also contemplated as an embodiment of the invention is a process
for preparing a foamed-wall container having a unique appearance. The
process comprises the steps of injection molding a polymer preform having a
non-reactive gas entrapped within the walls thereof, cooling the preform to a
temperature below the polymer softening temperature, reheating the preform
to a temperature above the polymer softening temperature, and blow molding
the preform, to prepare a container comprising a micro cellular foamed
polymer having a non-reactive gas contained within the micro cellular foam
cells, wherein the container has a silvery appearance.
[0007] The container according to the present invention is particularly
useful for packaging carbonated beverages.
DETAILED DESCRIPTION OF THE PREFERREDEMBODIMENT
[0008] The present invention is directed to a foamed-wall container having
a unique appearance, comprising a micro cellular foamed polymer, and a non-
reactive gas contained within the micro cellular foam cells, wherein the
container has a silvery appearance.
[0009] Another embodiment of the present invention is directed to a
process for making a foamed-wall container having a unique appearance,
comprising injection molding a polymer preform having a non-reactive gas
entrapped within the walls thereof, cooling the preform to a temperature below
the polymer softening temperature, reheating the preform to a temperature
above the polymer softening temperature, and blow molding the preform, to
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prepare a container comprising a micro cellular foamed polymer having a non-
reactive gas contained within the micro cellular foam cells, wherein the
container has a silvery appearance.
[0010] Suitable polymers from which the container may be prepared
include, but are not necessarily limited to, polyethylene terephthalate (PET)
and other polyesters, polypropylene, acrylonitrile acid esters, vinyl
chlorides,
polyolefins, polyamides, and the like, as well as derivatives, blends, and
copolymers thereof. A suitable polymer for commercial purposes is PET.
[0011] Polymer flakes are melted in a conventional plasticizing screw
extruder, to prepare a homogeneous stream of hot polymer melt at the
extruder discharge. Typically, the temperature of the polymer melt stream
discharged from the extruder ranges from about 225 degrees Centigrade to
about 325 degrees Centigrade. One ordinarily skilled in the art will
appreciate
that the temperature of the polymer melt stream will be determined by several
factors, including the kind of polymer flakes used, the energy supplied to the
extruder screw, etc. As an example, PET is conventionally extruded at a
temperature from about 260 degrees Centigrade to about 290 degrees
Centigrade. A non-reactive gas is injected under pressure into the extruder
mixing zone, to ultimately cause the entrapment of the gas as micro cellular
voids within the polymer material. By the term "non-reactive gas" as it is
used
herein is meant a gas that is substantially inert vis-à-vis the polymer.
Preferred non-reactive gases comprise carbon dioxide, nitrogen, and argon,
as well as mixtures of these gases with each other or with other gasses.
[0012] According to the present invention, the extrudate is injection molded
to form a polymer preform having the non-reactive gas entrapped within the
walls thereof. Methods and apparatus for injection molding a polymer preform
are well-known in the art.
[0013] It is well-known that the density of amorphous PET is 1.335 grams
per cubic centimeter. It is also known that the density of PET in the melt
phase is about 1.200 grams per cubic centimeter. Thus, if the preform
injection cavity is filled completely with molten PET and allowed to cool, the
resulting preform would not exhibit the proper weight and would have many
serious deficiencies, such as sink marks. The prior art injection molding
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literature teaches that, in order to offset the difference in the densities of
amorphous and molten PET, a small amount of polymer material must be
added to the part after the cavity has been filled and as the material is
cooling.
This is called the packing pressure. Thus, about ten Per cent more material
must be added during the packing pressure phase of the injection molding
cycle in order to insure that a preform made by injection molding is filled
adequately and fully formed. The packing pressure phase of the injection
molding operation is likewise used for polymer materials other than PET.
[0014] According to the present invention however, the polymer preform is
injection molded and simultaneously foamed using a non-reactive gas. The
gas is entrained in the material during the injection phase. Contrary to the
prior art injection molding process, wherein additional polymer material is
injected during the packing phase, the present invention utilizes minimal
packing pressure. As the polymer material is still in a molten state, the
partial
pressure of the non-reactive gas is sufficient to permit the release of the
dissolved gas from the polymer into the gas phase where it forms the micro
cellular foam structure. Thus, the preform made by the inventive process
weighs less than, but has the same form and geometry as, the polymer
preforms produced by the conventional injection molding operations that
employ the packing process.
[0015] The micro cells may contain one or more of a variety of gases
typically used in processes for making micro cellular foam structures. In one
commercially acceptable embodiment, the non-reactive gas comprises carbon
dioxide in a concentration of at least ten percent by weight of the total
weight
of the non-reactive gas. This level of carbon dioxide concentration provides
adequate partial pressure to retard the diffusion of carbon dioxide from a
carbonated beverage within the inventive container to the exterior
atmosphere. The Micro cellular foam tends to act as an effective thermal
insulator, to retard the conduct of heat energy from the atmosphere to the
chilled carbonated beverage within the container
[0016] Upon
completion of the injection molding step, the preform is cooled
to a temperature below the polymer softening temperature. For example, the
softening temperature for PET is approximately 70 degrees Centigrade.
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Thus, the entrapped non-reactive gas is retained within the walls of the
polymer preform. This cooling step conditions the polymer and preserves its
desirable properties for the successful preparation of a blow molded
container. This cooling step is also useful when employing polymers such as
polyesters, which cannot be blow molded directly from an extruded parison.
This cooling step may be effected by any conventional process used in the
polymer forming art such as, for example, by passing a stream of a cooling
gas over the surfaces of the preform, or cooling the preform while in-mold by
cooling the forming mold.
[0017] The preform is thereafter reheated to a temperature above the
polymer softening temperature. This heating step may be effected by well-
known means such as, for example, by exposure of the preform to a hot gas
stream, by flame impingement, by exposure to infra-red energy, by passing
the preform through a conventional oven, or the like. PET is generally
reheated to a temperature twenty to twenty-five degrees above its softening
temperature for the subsequent blow molding operation. If PET is reheated
too far above its glass transition temperature, or held at a temperature above
its softening temperature for an excessive period of time, the PET undesirably
will begin to crystallize and turn white. Likewise, if the preform is heated
to a
temperature above which the mechanical properties of the material are
exceeded by the increasing pressure of the non-reactive gas in the micro
cells, the micro cells undesirably will begin to expand thus distorting the
preform.
[001 8] Finally, the preform is blow molded, to prepare a container,
consisting essentially of a micro cellular foamed polymer having a non-
reactive gas contained within the micro cellular foam cells. Methods and
apparatus for blow molding a container from a polymer preform are well-
known.
[0019] The blow molded foamed-wall polymer container so produced has a
silvery appearance; as though the container were made of metal. The blow
molded container is silvery in color, and may exhibit Pantone Color Formula
Guide numbers in the range of about 420 through 425, 877, 8001, 8400, and
8420. While not wishing to be bound by any particular theory regarding the
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reason that the ultimately produced container has a unique silvery
appearance, it is believed that, as the preform cavity is being filled with
polymer, bubbles of gas are formed at the flow front of the polymer due to the
pressure drop between the dissolved gas and the relatively lower pressure in
the preform cavity. The bubbles formed at the flow front of the polymer
material as it is introduced into the preform cavity are subsequently
deposited
on the outside and inside surfaces of the preform.
[0020] From the forgoing description, one ordinarily skilled in the art can
easily ascertain the essential characteristics of the invention, and without
departing from its scope, can make various changes and
modifications to adapt the invention to various uses and conditions.
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