This invention relates to polypropylene ultra-lightweight containers. More specifically, this invention relates to polypropylene ultra-lightweight containers that can be used for packaging refrigerated or frozen food products. The use of polypropylene having relatively high levels of polyethylene to prepare such containers allows for significant reduction in the thickness of container components without significant reduction in impact strength.
Containers for retail food products must meet a number of requirements. First, they must not significantly modify taste, odor, texture, or other organoleptic properties of the food product stored therein. They must also be able to consistently survive shipping and/or handling from the manufacturer to the retail outlet as well as display at the retail outlet and transportation from the retail outlet to the home. Thus, such containers must have significant impact resistance. For frozen or refrigerated food products, significant impact resistance must be maintained at low temperatures. In many cases, the container (or at least certain components such as, for example, the lid) must also remain flexible at such low temperature.
Typically, such food containers and lids are prepared from high density polyethylene (HDPE), low density polyethylene (LDPE), or high flow linear low density polyethylene (LLDPE). Polypropylenes having relatively low levels of polyethylene (i.e., generally below about 12 molar percent) have also been used to prepare food containers (bowls only) which can be used at refrigeration temperatures (i.e., about 35 to about 40xc2x0 F.); polypropylene has generally not been used successfully to prepare corresponding lids. Although such materials posses reasonable properties, relatively thick walls (and, thus, relatively high weights) are generally required to obtain containers having the required flexibility, impact resistance, and the like. Moreover, it is difficult to prepare thin-wall containers using these polyethylenes and polypropylenes with injection molding techniques to provide good impact strength at freezer temperatures (below 0xc2x0 F.); moreover, polypropylenes do not provide good impact strength at freezer temperatures (below 0xc2x0 F.). Generally, such polyethylenes and polypropylenes will not consistently fill the injection molds if wall thickness is below about 0.024 inches.
It would be desirable, therefore, to provide food containers having significantly reduced wall thicknesses and, thus, overall weight, without significantly reducing impact resistance. It would also be desirable to provide such reduced wall thickness containers which maintain their impact resistance at frozen and/or refrigeration temperatures. The present invention provides such food containers. By reducing the wall thickness, significant cost savings can be achieved.
This invention relates to polypropylene ultra-lightweight containers. More specifically, this invention relates to polypropylene ultra-lightweight containers that can be used for packaging refrigerated or frozen food products. The use of polypropylene to prepare such containers allows for significant reduction in the thickness of container components without significant reduction in impact strength. The polypropylene ultra-light containers of this invention are especially preferred for use with refrigerated or frozen food products. For example, the present food containers (generally consisting of a lid and a bowl) can be used to package Cool Whip(trademark) and similar food products which will be sold and stored at refrigeration or frozen temperatures.
The polypropylene ultra-lightweight containers of the present invention are prepared by injection molding, preferably using injection molds with multiple gates (i.e., multiple inlets for the injected polypropylene). Initially it was thought that such multiple gate molds would be necessary to allow-the mold to completely fill. It has surprisingly been found that such multiple gate molds are not required. Thus, a single gate mold (wherein the gate is generally located at the bottom of the bowl mold and at the center of the lid mold) can be used. The ability to use such single gate molds is a significant advantage since multiple gate molds are more costly and complex to make and use. The polypropylene useful in this invention contains about 15 to about 25 molar percent polyethylene and has a melt flow index of about 30 to about 50 g/10 min. (ASTM 1238). More preferably, the polypropylene useful in this invention contains a relatively high amount of polyethylene (i.e., about 17 to about 19 molar percent) and has a melt flow index of about 30 to about 40 g/10 min (ASTM 1238). An especially preferred polypropylene for use in the present invention is a polypropylene copolymer resin sold under the tradename Pro-fax(trademark) SG-802N by Montell North America Inc. and having about 17.7 molar percent polyethylene, about 11 weight percent ethylene, and a melt flow index of about 35 g/10 min (ASTM 1238). This particular polypropylene from Montell North America has been used mainly for automotive interior trim and seating.
The use of polypropylene as described herein allows the production of bowls and lids having significantly reduced wall thicknesses (generally less than about 0.018 inches thick) and reduced overall weights without sacrificing impact resistance, especially at low temperatures (i.e., about xe2x88x9210 to about 40xc2x0 F.). Moreover, the use of the present polypropylene allows the production of thin-walled bowls and lids using injection molding techniques.
Using such polypropylenes to prepare, for example, lids and bowls similar to those used for packaging Cool Whip(trademark), allows for weight reductions and wall-thickness reductions of about 20 to 40 percent without significantly reducing impact resistance as compared to conventional polyethylene containers. Such thin-walled polypropylene containers maintain sufficient impact resistance and flexibility at low temperatures (i.e., about xe2x88x9210xc2x0 F.) to allow their use for refrigerated and/or frozen food products.