2. Description of the Prior Art The present invention is in the same general field as the microwaveable, thermoplastic container disclosed in U. S. Patent No. 5,219,628 to Hathaway et al.

This patent is directed to an improved container which is resistant to heat deformation and environmental stress crack resistance. Prior to Hathaway et al.'s container it was well known to fabricate microwaveable containers from polystyrene or high impact polystyrene ("HIPS") in foamed and non-foamed conditions.

Hathaway et al. disclose a container having a substrate layer of a thermoplastic polymer and an inner protective layer that contacts food. The inner layer includes a protective polymer that is a blend of styrene/maleic anhydride copolymer and another polymer selected from a group consisting of polystyrene, rubber modified polystyrene, polymethyl methacrylate, polypropylene, and mixtures thereof. As stated above, the multilayered construction of Hathaway et al.'s container improved the durability and heat resistance of the container.

However, the container disclosed in Hathaway et al. does have several drawbacks. Even though Hathaway et al.'s container does possess improved heat

resistance and stress crack resistant properties, it is subject to failure after relatively short time intervals within a high powered microwave oven. Reference is made to Table 1 of Hathaway et al. which illustrates that a dual layer container still failed after only five minutes of heating stew. Due to safety risks involved when consumers handle very hot containers, there is presently a need for microwaveable containers that are capable of withstanding significant overexposure to heat.

Summary of the Invention In general, the present invention is directed to a multilayer, microwaveable container comprising a substrate layer fabricated from thermoplastic polymeric material having low heat deflection temperature and low environmental stress crack resistant characteristics. The substrate is constructed to have opposing surfaces. A protective layer is applied over one surface of the substrate layer, the protective layer being fabricated from a blend of polyphenylene oxide and high impact polystyrene. The container further comprises a finish layer applied over the protective layer. The finish layer is preferably fabricated from high gloss thermoplastic material.

More specifically, the thermoplastic polymeric material of the substrate layer is selected from a group consisting of polystyrene, rubber-modified polystyrene and polymers miscible with polyphenylene oxide. In addition, the thermoplastic polymeric material of the substrate layer has reground polyphenylene oxide and high impact polystyrene mixed therein. The protective layer has 25 to 75 percent by weight polyphenylene oxide and 25 to 75 percent by weight

high impact polystyrene, and preferably, approximately 25 percent by weight polyphenylene oxide and approximately 75 percent by weight high impact polystyrene. The high gloss thermoplastic material of the finish layer is selected from a group consisting of general purpose polystyrene and high impact polystyrene.

Accordingly, among the several objects of the present invention are: the provision of an improved multilayer, microwaveable container that is capable of withstanding excessive exposure to heat generated from a microwave oven without deforming or cracking; the provision of such a container having a high quality, attractive finish; the provision of such a container which can withstand the stresses of storing food in the refrigerator and/or freezer and placing it directly into a microwave oven for heating; and the provision of such a container that is easy and cost-efficient to manufacture.

Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.

Brief Description of the Drawings In the drawings which illustrate the best mode presently contemplated for carrying out the present invention: Fig. 1 is a perspective view of a multilayer, container of the present invention; Fig. 2 is a cross-sectional view of the container; and

Fig. 3 is a cross-sectional view similar to Fig. 2 illustrating a container of another preferred embodiment.

Corresponding reference numerals designate corresponding parts throughout the several views of the drawings.

Detailed Description of the Preferred Embodiments Referring now to the drawings, and more particularly Figs. 1 and 2, there is generally indicated at 10 a container of the present invention which is especially suited for heating and/or reheating food in a microwave oven. As shown, the container 10 illustrated in the drawings is that of a three-compartment plate. However, it should be readily appreciated that the general principles used to construct the container 10 can be used for other microwaveable items as well, such as bowls, coffee cups and the like. The container 10 is especially suited for withstanding extremely to high temperatures caused by food being exposed for extensive lengths of time in a microwave oven.

Turning now exclusively to Fig. 2, there is illustrated a cross section of container 10. As shown, the container 10 includes three layers--namely, a substrate layer 12, a protective layer 14, and a finish layer 16.

These layers are preferably fabricated and bonded to one another by extrusion processing equipment that is well known in the art of thermoplastic manufacturing. As detailed in the Hathaway et al. patent, other processes can be used as well and still fall within the scope of the present invention.

As its name implies, the substrate layer 12 is the main structural layer of the container 10. It is fabricated from thermoplastic polymeric material having a low heat deflection temperature, along with a low environmental stress crack resistant characteristic.

Preferably, the heat deflection temperature is no greater than 210*F. Without the protective layer 14, the substrate layer 12 is subject to deforming and cracking when exposed to heated food, especially food having a high content of fats and oils.

The thermoplastic polymeric material of the substrate layer 12 is selected from a group consisting of polystyrene, rubber-modified polystyrene or high impact polystyrene ("HIPS") and any other polymers that are miscible with polyphenylene oxide ("PPO"). HIPS is generally the preferred material because it is easy to process in both extrusion and thermoforming. Also, it does not require specially designed equipment and is the most cost-effective material of the group listed above.

The protective layer 14, as illustrated in Fig. 2, overlies one of the surfaces of the substrate layer 12. Preferably, the protective layer 14 is extruded over the substrate layer 12 by conventional extruder processing apparatus. The protective layer 14 prevents fats and oils created by heated and/or cooked food from penetrating into the substrate layer 12, thereby preserving the structural integrity of the substrate layer 12.

The material used in the protective layer 14 is a blend of polyphenylene oxide ("PPO") and HIPS. Depending upon the temperature protection wanted, or thermal performance, the ratio of PPO is increased or decreased.

For example, preferably, the material consist of approximately 25 percent PPO and 75 percent HIPS. This material can be purchased from General Electric Company under the registered trademark NORYL, which consists of 50 percent PPO and 50 HIPS. The ratio of PPO and HIPS can be modified for a specific application by adding HIPS material with a gravimetric blender as the material is fed to the extruder processing apparatus. In any event, it is desirable that the layer has 25 to 75 percent by weight PPO and 25 to 75 percent by weight HIPS.

The PPO/HIPS blend of the protective layer 14 will protect the container 10 in its ability to retain its stiffness at elevated temperatures when used to reheat or cook a variety of prepared and/or pre-cooked foods in a microwave oven. PPO is especially suited for raising the heat deflection temperature of the protective layer 14, thus protecting the container 10 from softening with hot foods during reheating or cooking. As stated above, many of the prior art thermoplastic materials, especially those disclosed in the Hathaway et al. patent, are susceptible to cracking when contacted with fats or oils contained in foods. The PPO of the PPO/HIPS blend will improve the environmental stress crack resistance, thus making the container 10 suitable for storing food in the refrigerator and/or freezer. In fact, the container 10 fabricated along the principles of the present invention can be used directly from the refrigerator/freezer to the microwave oven.

Another particular benefit of using a blend of PPO and HIPS is that these materials have the same shrinkage factor which makes the multi-layer construction of the container 10 feasible because existing tooling and product

design features can still be utilized, thus avoiding having to re-tool and redesign the product. Moreover, since the substrate layer 12 can be fabricated from re-grind containing PPO/HIPS material, the substrate layer 12 is further enhanced. In this regard, the thermoforming process typically generates approximately 50 percent re-grind material which can be recycled. All of this re-grind material is preferably reconstituted into the substrate layer 12 wherein virgin HIPS is added to achieve the desired thickness.

The finish layer 16 is primarily provided for giving the container 10 a high gloss finish that is attractive in appearance. The substrate and protective layers 12,14 both have a flat-type finish so it is important that the container 10 be provided with the high gloss finish layer 16 for enhancing the overall appearance of the container. Preferably, the finish layer 16 is also extruded onto the exposed side of the protective layer 14 by conventional apparatus and methods. The finish layer 16 can be fabricated from a high gloss thermoplastic material, such as general purpose polystyrene and HIPS. Both of these materials are compatible with the PPO/HIPS blend of the protective layer 14. If another non-compatible material is chosen, then a bonding layer can be applied between the protective and finish layers 14,16 for ensuring the finish layer 16 is attached to the protective layer 14.

Tests have been performed on a container similar to container 10 using a 1000 watt microwave oven at full power. A number of prepared foods, such as chili with beans containing 27 percent saturated fat, have been tested in a three-compartment plate. In a particular test, 16 ounces of

food were placed within a container, the large compartment containing eight ounces and the two smaller compartments containing four ounces each. During this test, food temperature and re-heating time in excess of ten minutes was recorded until the food started to dry. During the re- heating cycle, the plate was observed for deformation and rigidity. The plate performed well.

If the container 10 requires any additional thermal insulation, then the substrate layer 12 or the protective layer 14 may be foamed with known existing methods by adding a blowing agent during its manufacture.

In addition, to further enhance the overall appearance of the container 10, color additives can be applied to any or all of the layers to achieve a certain color.

Referring now to Fig. 3, there is generally indicated at 20 a container having two additional layers.

This container is especially suited for situations where both sides (i. e., inner and outer surfaces) must have a finished appearance. For example, some disposable food containers oftentimes require that its outer surfaces all be glossy.

More specifically, as with container 10, the container includes substrate, protective and finish layers 12,14, and 16, respectively, but it also includes a second protective layer 22 and a second finish layer 24. As shown, the second protective layer 22 is disposed over the other surface of the substrate layer 12 (opposite protective layer 14) and the second finish layer 24 is disposed over the second protective layer 22. The second protective and finish layers 22,24 are fabricated from identical materials as the protective and finish layers 14,16, respectively,

and applied onto the substrate layer 12 in an identical manner.

Thus, it should be observed that the containers 10 and 20 disclosed herein is especially suited for cooking and/or reheating food in microwave ovens without cracking or deforming as a result of being exposed to excessive heat. It can therefore be seen that for these reasons, the instant invention is believed to represent a significant advancement in the art which has substantial commercial merit.

While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.