CROSS-REFERENCE TO RELATED APPLICATIONThis application is a continuation-in-part of and claims the benefit of priority of U.S. patent application Ser. No. 12/276,687 filed on Nov. 24, 2008 which is a continuation-in-part of and claims the benefit of priority of U.S. patent application Ser. No. 11/384,979 filed on Mar. 20, 2006.
The present invention relates generally to a foamed-wall polymer container having a unique appearance. More particularly, the invention is directed to a container having foamed and unfoamed regions, the foamed regions comprising micro cellular foam containing a non-reactive gas, and the container has one of a silvery appearance and a white appearance. Also contemplated as a part of the present invention is a method of manufacturing the foamed-wall container.
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.
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.
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 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.
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.
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.
The container according to the present invention is particularly useful for packaging carbonated beverages, such carbonated beverages typically packaged in a container having a hollow body and a threaded finish adapted to receive a corresponding threaded closure providing communication with an interior of the hollow body.
The present invention is directed to a container having foamed and unfoamed regions, the foamed regions comprising micro cellular foam containing a non-reactive gas, and the container has one of a silvery appearance and a white appearance.
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 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.
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.
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.
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.
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 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.
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. The foaming of the polymer is discussed hereinbelow in more detail. The foaming occurs without the use of a chemical blowing agent. Chemical blowing agents are frequently used by plastic container manufactures to foam a polymer melt. Blowing agents are chemicals added to plastics that generate gases and water on heating, causing the resin to assume a cellular structure. Use of chemical blowing agents typically results in an organic residue mixed with the foamed polymer. Use of a blowing agent to foam a PET polymer melt will result in the generation of water (an organic residue) in the polymer matrix of an object formed from the polymer melt. Due to the presence of water in the PET matrix, the chains of the PET polymer used to form the object will break down upon heating during processing, such as in an extruder or blow molding machine, thereby resulting in an undesired decrease in the resilience of the object. Therefore, a blow molded foamed object formed from a polymer melt foamed without a chemical blowing agent is structurally different from, and would have increased resilience as compared to, a foamed object formed from a polymer melt foamed with a chemical blowing agent.
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.
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
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. 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.
The preform is thereafter reheated to a predetermined 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 an oven having infrared heaters, or the like. It is understood that the heating step of the preform may also occur in a heated mold or with a heated fluid in a mold. By heating the preform to a desired and predetermined temperature, the translucence, and therefore appearance, of the container blow molded from the preform may be selectively varied. The translucence of the container may be selectively varied across a range until the container is opaque. In one instance, at temperatures of about 106 degrees Centigrade, the container has a silvery appearance and is translucent. At temperatures of about 112 degrees Centigrade, the container has a silvery appearance and less translucent than the container formed from the preform heated to 106 degrees Centigrade. At temperatures of about 116 degrees Centigrade, the container has a silvery appearance and is less translucent still and may be opaque. Thus, as the desired temperature for reheating the preform increases, the translucence of the container formed therefrom decreases. The desired temperature may be increased to temperatures above 116 degrees Centigrade, thereby resulting in an opaque container having a silvery appearance or an opaque container having a white appearance. Other preforms may exhibit this phenomenon but at different temperatures depending on many factors including the amount of gas bubbles entrained in cells of the micro cellular foam in the sidewall of the preform.
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. 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.
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.
If 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 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. During the filling process which can take several seconds, non-nucleated polymeric material is introduced into each injection molding cavity. The non-nucleated, molten material that contacts the chilled sidewalls of the preform cavity and the core rod therein solidifies. As the pressure of the injected material in the system is still greater than a nucleation pressure, the solidified material remains unfoamed. When the preform cavity is from about 1% to about 5% filled or greater, a pin gate for each of the preform cavities is closed and the preform cavity is no longer in fluid communication with the pressurized non-nucleated fluid. The pressure in the cavity is less than the critical pressure required for the gas in the polymer to nucleate and the sections of the preform that remain molten are caused to foam, which results in “packing out” the preform, i.e. filling the remaining portion of the preform cavity. The melted polymer flows along the central axis of the preform as the preform mold is filled with polymer solution. Thus, it is the center-most portion of the preform that remains molten at the time of closing of the pin gate. It is the center-most portion of the preform in which the foaming occurs.
In addition to the nucleation which occurs in the central portion of the preform, as described above, and while not wishing to be bound by any particular theory regarding the reason that the ultimately produced container has a unique silvery appearance, it is noted that, as the preform cavity is being filled with molten 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 localized 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. Thus, macroscopically, the cross-section of the preform shows a non-uniform structure having a foamed region disposed between two unfoamed regions, the unfoamed regions being the plastic material which hardened at the interface of the polymeric material and the cold mold surfaces of the cavity and the core rod, respectively, prior to the release of pressure in the cavity. To the inside and outside of this structure are microscopically thin regions of open-cell foam as a result of the bubbles erupting at the flow front. The preform having this foamed core section is then blow molded in a conventional fashion to produce a container having a multi-regioned structure in concordance with the multi-regioned structure of the preform.
According to another embodiment of the invention, the preform and a container blow molded from the preform may include an unfoamed region disposed between two foamed regions.
According to another embodiment of the invention, the preform is an overmolded preform suitable for blow molding an overmolded container. Either or both of the preform being overmolded or the polymer used to overmold the preform may include a foamed layer having a foamed region disposed between two unfoamed regions. Alternatively, either or both of the preform being overmolded or the polymer used to overmold the preform may include a foamed layer having an unfoamed region disposed between two foamed regions. A process for forming an overmolded preform and container having at least one foamed layer is disclosed in commonly-owned U.S. patent application Ser. No. 12/144,885 hereby incorporated herein by reference in its entirety, and in commonly-owned U.S. patent application Ser. No. 12/778,291. It is understood that the foamed layer may have a silvery appearance or a white appearance as described hereinabove. It is also understood that the foamed layer may be overmolded over or be overmolded by one of a clear polymer or a polymer having a color. If the polymer has a color, the silvery and/or white foamed layer may accentuate, enhance, and make vibrant the colored polymer, and/or the silvery or white foamed layer may give the colored polymer a metallic appearance.
From the forgoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of the invention, and without departing from its spirit and scope, can make various changes and modifications to adapt the invention to various uses and conditions.