Plastic container having a crystallinity gradient

Details Plastic container having a crystallinity gradient Plastic container having a crystallinity gradient

2000-06-30

2003-09-30

Yu, Mickey (Department: 3728)

Receptacles

End wall structure

One-piece side and end wall

C220S608000

Reexamination Certificate

active

06626324

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention generally relates to plastic containers for retaining a commodity during a pasteurization or retort process and during subsequent shipment and use. More specifically, this invention relates to plastic containers that minimize flavor scalping of commodities inside the plastic containers.
BACKGROUND
Recently, manufacturers of polyethylene terephthalate (PET) containers have begun to supply plastic containers for commodities that were previously packaged in glass containers. The manufacturers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities. Manufacturers currently supply PET containers for various liquid commodities, such as juices. They also desire to supply PET containers for solid commodities, such as pickles. Many solid commodities, however, require pasteurization or retort, which presents an enormous challenge for manufactures of PET containers.
Pasteurization and retort are both methods for sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above 70° C., for duration of a specified length. Retort differs from pasteurization in that it also applies overpressure to the container. The pressure is necessary because a hot water bath is often used and the overpressure keeps the water in liquid form above its boiling point temperature. These processes present technical challenges for manufactures of PET containers, since new pasteurizable and retortable PET containers for these food products will have to perform above and beyond the current capabilities of conventional heat set containers. Quite simply, the PET containers of the current techniques in the art cannot be produced in an economical manner such that they maintain their material integrity during the thermal processing of pasteurization and retort and during subsequent shipping.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. Crystallinity is characterized as a volume fraction by the equation:
%
⁢
 
⁢
Crystallinity
=
ρ
-
ρ
a
ρ
c
-
ρ
a
×
100
where &rgr; is the density of the PET material; &rgr;
a
is the density of pure amorphous PET material (1.333 g/cc); and &rgr;
c
is the density of pure crystalline material (1.455 g/cc). The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves streching a PET container along a longitudinal axis and expanding the PET container along a transverse axis. The combination promotes biaxial orientation. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having roughly 20% crystallinity (average sidewall crystallinity).
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. Used by itself on amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque (and generally undesirable in the sidewall of the container). Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a heated blow mold, at a temperature of 120-130° C., and holding the blown container for about 3 seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85° C., currently use heat setting to produce PET juice bottles having a range of up to 25-30% crystallinity.
Although heat set PET bottles perform adequately during hot fill processes, they are inadequate to withstand a pasteurization or retort process. PET containers also suffer from flavor scalping. Although not fully understood, flavor scalping includes a transfer of the flavor of a commodity to the walls of a plastic container. This phenomenon, though not harmful, dulls the flavor of the commodity, thereby reducing the overall satisfaction of the customer.
Thus, the manufacturers of PET containers desire to produce a PET container that maintains material integrity during any subsequent pasteurization or retort of the contents in the PET container, and during subsequent shipment of the PET container. Further, the manufacturers of PET containers desire to produce a PET container that minimizes flavor scalping of the commodities inside the PET containers. It is therefore an object of this invention to provide such a container that overcomes the problems and disadvantages of the conventional techniques in the art.
SUMMARY OF THE INVENTION
Accordingly, this invention provides for a plastic container which maintains material integrity during any subsequent pasteurization or retort process, and during subsequent shipment and use. Additionally, this invention provides for a plastic container which minimizes flavor scalping of the commodities inside the plastic container.
Briefly, the plastic container of the invention includes a portion of a sidewall having an interior surface with an interior crystallinity, and an exterior surface with an exterior crystallinity. The interior crystallinity is greater than the exterior crystallinity. The higher crystallinity on the interior surface provides increased resistance to flavor scalping along the surface contacting the commodity, thereby minimizing flavor scalping of the commodities inside the plastic container.
Another object is to provide a container with high crystallinity, namely a sidewall crystallinity of greater than 30%.
By using the fluid cycle process, the plastic container
10
can be produced having a sidewall
16
with a sidewall density greater than 1.375 g/cc. This density roughly corresponds to a 34.4% crystallinity and will allow the plastic container
10
to maintain its material integrity during a pasteurization or retort process of the commodity in the plastic container
10
, and during subsequent shipment of the plastic container
10
. As used herein, crystallinities greater than 30% are considered “high crystallinities”. Other densities greater than 1.375 g/cc, including 1.38 g/cc (roughly corresponding to 38.5% crystallinity), 1.385 g/cc (roughly corresponding to 42.6% crystallinity), and even 1.39 g/cc (roughly corresponding to 46.7% crystallinity) are possible with the fluid cycle process, without significantly impacting the visually perceptible transparency or clarity of the plastic container
10
.


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The Crystallization of Oriented Poly(ethyleneterephthalate), F

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