Electric heating – Microwave heating – Cookware
Reexamination Certificate
2001-11-27
2003-07-29
Van, Quang T. (Department: 3742)
Electric heating
Microwave heating
Cookware
C426S243000, C099SDIG014
Reexamination Certificate
active
06600143
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a foamed polypropylene resin container for microwave cooking excellent in flexural rigidity under high temperature conditions.
BACKGROUND OF THE INVENTION
Foamed polypropylene resin products are excellent in heat insulating ability and light weight and have been used as construction/earthmoving materials or packaging containers. Japanese Patent Application Laid-Open No. 264938/1987 discloses a multi-layered container for microwave cooking made from a foamed polypropylene resin.
Recently, foods such as curry packed in a container preserved at room temperature become popular and they are heated as packed by a microwave oven. Thus it need to be cooked for 3 minutes or longer in the case of the use of a microwave oven having a power output of 500 Watt. It results in that the temperature of the container after the heating is immediately in some cases raised up to as high as about 120 to about 140° C. and there arises the problem that the container heated may easily be deformed.
SUMMARY OF THE INVENTION
The inventors of the present invention made intensive studies to solve the above-mentioned problem and finally have found that providing a non-foamed layer made of a polypropylene resin containing a specific amount of talc on at least one side of a foamed polypropylene resin layer prevents the resulting container from being deformed upon cooking by a microwave oven and consequently it provides an foamed polypropylene resin container for microwave cooking excellent in heat resistance.
Thus, the present invention provides a foamed polypropylene resin container for microwave cooking having a non-foamed layer containing 100 parts by weight of a polypropylene resin and 40 to 100 parts by weight of talc on at least one side of an foamed polypropylene resin layer.
DETAILED DESCRIPTION OF THE INVENTION
The foamed polypropylene resin container for microwave cooking of the present invention is characterized in that it comprises a non-foamed layer comprising a polypropylene resin and 40 to 100 parts by weight of talc per 100 parts by weight of the polypropylene resin on at least one side of an foamed polypropylene resin layer.
The present invention relates to a foamed polypropylene resin container for microwave cooking which is excellent in rigidity under high temperature conditions, e.g., when heated in a microwave oven with curry, rice, or other foods packed therein, and therefore is hardly deformed.
The foamed polypropylene resin container is light and highly heat insulating, but its flexural rigidity is deteriorated and thus the container tends to be deformed, when the food contained therein is cooked in a microwave oven and the entire container is heated to high temperatures.
Accordingly, in the present invention, at least one side of a foamed polypropylene resin layer is provided with a non-foamed layer containing 40 to 100 parts by weight of talc per 100 parts by weight of a polypropylene resin. In view of flexural rigidity upon heating at high temperatures, the amount of the talc per 100 parts by weight of the polypropylene resin is preferably not less than 40 parts by weight. From the viewpoint of moldability of the resulting container, the amount of the talc per 100 parts by weight of the polypropylene resin is preferably not more than 100 parts by weight. For higher flexural rigidity under high temperature conditions, it is more preferred that the amount of the talc is not less than 55 parts by weight, and from the viewpoint of moldability, it is more preferred that the amount of the talc is not more than 85 parts by weight.
Moreover, incorporation of talc into the foamed layer hinders the growth of foam and results in collapsing foam during the production of a foamed polypropylene resin container for microwave cooking of the present invention or a base sheet being a constituent thereof.
Moreover, when blending talc into the foamed layer, a much larger amount of talc is required to impart the same flexural rigidity as that in the case of the non-foamed layer, leading to a large increase in the weight of the resulting container. In contrast, incorporation of talc only to the non-foamed layer makes it possible to improve flexural rigidity without making the resulting foamed polypropylene resin container for microwave cooking much heavier.
From the viewpoint of flexural rigidity, the particle size of talc dispersed in the polypropylene resin forming the non-foamed layer is preferably about 0.1 to about 10 &mgr;m, more preferably about 1 to about 5 &mgr;m. A foamed polypropylene resin container for microwave cooking having an higher flexural rigidity may be obtained by setting the particle size so as to fall within the range mentioned above.
In the present invention, the particle size of the talc dispersed in the polypropylene resin forming the non-foamed layer is figured out in the following manner.
A cross-section of the non-foamed layer taken in the thick direction was examined by a scanning electron microscope (SEM) at a sufficient magnification to resolve the talc particles clearly. Regarding the talc particles observable through the microscope, the maximum length of each of at least twenty talc particles was measured and the average of the values obtained was employed as the particle size of the talc dispersed in the polypropylene resin forming the non-foamed layer. Usually, the talc particles are clearly resolved at a magnification of from 5,000 to 60,000.
In the present invention, from the viewpoint of heat insulating ability of the foamed container to be obtained, the foaming ratio of the foamed layer is preferably about two times or more, more preferably about three times or more. It is preferred that, in view of the strength of the foamed container produced, the foaming ratio is about 40 times or less, more preferably about 10 times or less. The foaming ratio of the foamed layer is adjustable by controlling the amount of a foaming agent to be added or physical conditions in the molding.
Moreover, in the present invention, the term “non-foamed layer” refers to a layer with a foaming ratio of from about 1.0 to about 1.5 times, preferably about 1.0 to about 1.1 times.
The thickness of the foamed polypropylene resin container for microwave cooking of the present invention is preferably from about 0.1 to about 3 mm. Containers having such thickness as to fall within the range mentioned above are preferable from the viewpoint of flexural rigidity and easy to produce.
For achieving sufficient heat insulating ability, the foamed layer is preferred to have a thickness of not less than about 0.3 mm. In view of heat insulation, the thicker the layer is, the more it is favorable.
Although the thickness of the non-foamed layer is not particularly restricted provided that its surface smoothness, in other words, external appearance is good, it is preferably about 1 &mgr;m or thicker, more preferably about 10 &mgr;m or thicker, with about 50 &mgr;m or thicker much more preferred. The maximum limit of the thickness of the non-foamed layer is suitably set according to the intended lightweight and moldability.
In the foamed polypropylene resin container for microwave cooking of the present invention, exemplified as the polypropylene resin forming the foamed layer are a homopolymer of propylene and a propylene copolymer containing about 50 mol % or more of the propylene unit. In the propylene copolymer, preferred examples of the component copolymerizable with propylene include ethylene and &agr;-olefins having 4 to 10 carbon atoms. Included in &agr;-olefins having 4 to 10 carbon atoms are 1-butene, 4-methylpentene-1, 1-hexene, and 1-octene. As for the content of the monomer unit other than propylene, in the case of ethylene, it is preferably about 10% by weight or less. In the case of an &agr;-olefin having 4 to 10 carbon atoms, it is preferably about 30% by weight or less.
Of polypropylene resins, for their capability of forming layers with a highly homogeneous cellular structure, (a) long chain branched polypropylene resins and (b) polypr
Hanada Satoshi
Kuroda Ryuma
Nodono Mitsunori
Sumitomo Chemical Company Limited
Van Quang T.
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