Package making – Methods – With contents treating
Reexamination Certificate
2000-10-25
2003-01-07
Sipos, John (Department: 3721)
Package making
Methods
With contents treating
C053S084000, C053S275000, C053S361000, C053S467000, C053S485000
Reexamination Certificate
active
06502369
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method of supporting a plastic container exposed to the combination of internal pressure variations and elevated temperatures during product filling and packaging thereby eliminating distortion induced by the heat and internal pressure.
BACKGROUND OF THE ART
Plastic containers for packaging foods have been widely accepted in some applications, such as soft drinks, bottled water and juices, due to their well known advantages over conventional glass and metal containers. Substantial reductions in weight and the low cost of using plastic containers creates circumstances where it is highly desirable to expand the application of plastic containers if possible to other areas in packaging. However, in many cases, conventional use of plastic containers results in problems when the plastic containers are exposed to an elevated temperature which reduces the plastic strength, internal vacuum which collapses a container, or high internal pressures combined with heat which tends to bulge or distort the plastic walls of the container to an acceptable degree. To date plastic containers have not replaced conventional use of glass bottles or jars for packaging many food products, which are pasteurized or retort processed after filling the container.
Thermal distortion of the container may cause unacceptable bulging of the side walls, bottom surface distortion can cause the container to lean to one side, distortion of the bottle neck area can create problems in sealing the containers after hot filling, expansion of the side walls can cause difficulty in attaching labels and any distortion of the container detracts from the aesthetic appeal of the packaging itself.
There have been attempts to modify blow moulded polyester containers or PET bottles in order to enable hot filling or retort processing with limited success. For example, heat setting of the plastic container or co-extrusion of heat resistant materials with other less costly resins have been applied however at significantly increased cost and increased manufacturing cycle time. In heat treating, the thermoplastic material is subjected to heat wherein the crystal structure is changed to increase the heat resistance to heat distortion of the final package.
In general however, heat setting and addition of heat resistant materials involve unacceptable increases in costs that detract from the principle advantages of using plastic containers.
In the case of hot filling of plastic containers, the conventional manner of dealing with a resultant negative or vacuum pressure within the plastic container is to use specially designed vacuum panels in the lateral sides of the bottle or in the bottom surface which bow inwardly to deform and accommodate the product shrinkage and negative internal pressure. Large collapsible panels in the sidewalls of PET containers severely restrict the design of the package itself and limit the application of labels. Also the expense of specially designed dies, maintenance of separate bottle inventory for hot fill applications and moulding machine change over costs result from using a different bottle design for different product processing methods.
As is known in the art, hot fill applications involve heating of comestible products to a temperature approximately 140° F. to 205° F. (60° C. to 96° C.), placing the hot product in the container and sealing the container. During cooling of the product however, the hot product and hot gas in the head space shrink in volume. Cooling after sealing therefore creates a negative internal pressure or vacuum within the final filled container. Further shrinkage of the product occurs if the package is refrigerated below ambient temperature for storage. Without collapsible vacuum panels in the side of the plastic container, the resulting pressure differential creates a net external pressure causing the container to buckle or collapse inwardly, sometimes referred to as “paneling”.
Therefore, conventional methods of adapting plastic containers to products which are heated during processing and packaging have met with limited success. Disadvantages include the risk of heat distortion which can be addressed by specially designed vacuum collapsing panels or relatively expensive heat set and heat resistant containers.
In the case of hot filled aluminium cans, it is well-known that negative internal pressure caused by hot filling can be counteracted by adding liquefied nitrogen gas or dry ice immediately before sealing the aluminium container. During this process the nitrogen or carbon dioxide gas created on contact with the hot product creates a positive pressure within the sealed aluminium container. The relatively high strength aluminium container can resist a high internal pressure during processing. When the product cools, the shrinkage of the product and negative pressure resulting is countered by a greater positive pressure created by the gas within the sealed container to produce a residual net positive pressure in the final container. Examples of this prior art process are described in U.S. Pat. No. 4,703,609 to Yoshida et al. and U.S. Pat. No. 4,662,154 to Hayward.
Attempts have been made to apply the same technology to plastic containers with limited success. U.S. Pat. No. 5,251,424 to Zenger et al. describes essentially the same process applied to hot filling of a plastic PET container. In Zenger, the hot filled product is poured into a plastic bottle, liquid nitrogen is dosed into the hot product immediately before closing the container and the container is permitted to cool to storage temperature. Normally, a hot filled product will shrink and create a vacuum within the plastic container which conventionally has been addressed with vacuum panels. However, Zenger et al. describes a method of increasing internal positive pressure through use of liquid nitrogen gas which counteracts the negative pressure created on cooling of the hot filled product.
In theory, the Zenger et al. method permits a conventional PET plastic container to be used for hot filling applications, which results in cost savings. However, in practice it has proved extremely difficult to implement. The accurate dosing of liquid nitrogen or solid dry ice to the precision required for use of plastic containers has proved illusive.
As is apparent to those skilled in the art, by nature a heat formable blow-moulded plastic bottle is very sensitive to variations in the heat absorbed by the material. The uniformity of plastic container composition and the uniformity of heat of the hot filled product are such that it is extremely difficult to predict with sufficient accuracy the performance of the hot filled plastic container. Variations in product density, heat distribution, and physical forces applied to the product filled plastic container during handling and packaging operations can have significant effect on the performance when dosed with liquid nitrogen to increase the internal pressure.
Further, the accurate dosing of liquid nitrogen gas or solid dry ice into the product prior to capping is extremely difficult to accomplish with the required accuracy. In the case of liquid nitrogen, the size of a liquid drop can vary significantly and the volume of liquid nitrogen required is in the order of one or two drops only. The inherent inaccuracy is not a particular difficulty when the packaging has a high margin of safety in its strength such as for example in the dosing of product packed in aluminium cans.
In the case of PET plastic containers however the packaging when heated is at a significantly reduced structural strength due to the heat sensitivity of plastic materials. As well, the dosed product when capped subjects the packaging to the most extreme internal pressure that it will experience in its service life. When the gas forms to create a high internal pressure, the packaging is heated and has a reduced strength, the container is also subjected to hydrostatic forces from the liquid product within the container and is usually in transit on conveyors or otherwise s
Andison David
Scheffer Steven
Amcor Twinpak-North America Inc.
Desai Hemant M.
Jaffe Michael A.
Kusner Mark
Sipos John
LandOfFree
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