Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Producing multilayer work or article
Reexamination Certificate
2001-01-17
2002-12-24
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Producing multilayer work or article
C264S321000, C264S554000
Reexamination Certificate
active
06497838
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hollow foamed polyolefin-based resin container having a hollow portion in at least its side wall part that can be used as containers for retorted food products and other containers, and to a process for producing the same.
2. Description of the Related Art
Although polystyrene foam has hitherto been used as containers for food products such as retorted food products, noodles in cup and soup in cup, the use of polypropylene-based foam has been studied in recent years. These containers for food products are required to have, for example, improved heat retaining property and heat insulation property that enable the containers to be touched by hands even when their contents are heated to high temperatures.
Containers for food products composed of polystyrene foam or polyolefin-based resin foam are generally produced by vacuum- or pressure-forming foamed resin sheets. Containers made of foamed polyolefin-based resin are disclosed in Japanese Patent Kokai Publication Nos. 9-226851 and 11-34251.
All the above-mentioned known foamed polyolefin-based resin containers, however, have only one resin layer constituting the containers and do not satisfy the requirement about heat retaining property, heat insulation property and the like enough. Neither process for producing a hollow foamed resin container nor process for producing a plurality of hollow foamed resin containers at the same time through a series of steps is disclosed in these publications.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a hollow foamed polyolefin-based resin container having a hollow portion in at least its side wall part that can fully meet requests for heat retaining property, heat insulation property and the like even when it is used as containers for retorted food products and the other containers, and to provide a process for producing the same.
The hollow foamed resin container of the present invention is characterized by being a hollow polyolefin-based resin container comprising an inner layer and an outer layer, at least one of the inner and outer layers being formed of a foamed polyolefin-based resin sheet, wherein at least a side wall part of the container has a hollow portion between the inner and outer layers.
The above-mentioned hollow foamed polyolefin-based resin container may be produced by the vacuum-forming using a pair of mold members fittable each other including a male mold member having a configuration of the inner surface of the container and a female mold member having a configuration of the outer surface of the container and accompanying a forming processing in which a (plane) resin sheet for forming the inner layer is made to firmly touch the outer surface of the male mold member and a (plane) resin sheet for forming the outer layer is made to firmly touch the inner surface of the female mold member.
The hollow portion that is formed in the side wall part of a container may be one formed throughout the entire side wall part or one partly formed in the side part. Moreover, a structure having many hollow portions partitioned by ribs may be also possible. A hollow portion may be formed in the bottom part of a container.
In the hollow foamed polyolefin-based resin container of the present invention, it is preferable that the resin forming the foamed layer has an expansion ratio of from 1.5 to 8 times and a thickness of not greater than 6 mm and a draw ratio [(the height of the container)/(the minimum dimension of an opening of the container)] of the container of from 0.6 to 1.5. Such a container was difficult to be produced by the conventional techniques, particularly by vacuum- or pressure-forming accompanying forming processing when foamed polyolefin-based resin is used.
The minimum dimension of an opening of a container means, for example, the diameter of the opening in the case where the configuration of the opening of the container is a circle, the dimension of each side of the opening in the case where the opening is a square, the dimension of a shorter side in the case where the opening is a rectangle, and a distance between the opposite sides in the case where the opening is a regular hexagon.
It is desirable that the foamed polyolefin-based resin sheet to be used in the hollow foamed polyolefin-based resin container of the present invention has a non-foamed polyolefin-based resin layer having a thickness of from 5 to 100 &mgr;m on one side of the foamed layer. Furthermore, it is more desirable that the foamed polyolefin-based resin sheet to be used in the hollow foamed polyolefin-based resin container of the present invention has a non-foamed polyolefin-based resin layer having a thickness of from 5 to 100 &mgr;m on each side of the foamed layer.
Moreover, it is desirable that the foamed polyolefin-based resin sheet to be used in the present invention is a sheet in which at least two layers are laminated wherein each of the layers is composed of a three-layer structure of non-foamed polyolefin-based resin layer (non-foamed layer/foamed polyolefin-based resin layer (foamed layer)
on-foamed polyolefin-based resin layer (non-foamed layer). In particular, it is more desirable that the foamed polyolefin-based resin sheet to be used in the present invention is a sheet in which at least two layers of the aforementioned three-layer structured foamed polyolefin-based resin sheets are laminated, namely having at least five-layer structure of non-foamed layer/foamed layer
on-foamed layer/foamed layer
on-foamed layer.
The use of the foamed polyolefin-based resin sheet in which a non-foamed polyolefin-based resin layer is laminated improves the vacuum-formability of a hollow foamed polyolefin-based resin container. Particularly, the deeply drawing property is improved and a foamed polyolefin-based resin container having a draw ratio of not less than 0.6, which has heretofore been said difficult to be obtained, can be produced easily in a low proportion defective.
The non-foamed layer in the foamed polyolefin-based resin sheet is desirably formed of a polyolefin-based resin having a long-chain branch.
By designing the foamed polyolefin-based resin sheet to be such a laminated foam using the polyolefin-based resin having a long-chain branch as a non-foamed layer, the strength of the original resin sheet as a raw material in a heated and softened state is enhanced and the vacuum- or pressure-formability of hollow foamed polyolefin-based resin container is improved.
It is desirable that the polyolefin-based resin has a branching index [A] satisfying 0.20≦[A]≦0.98.
The polyolefin-based resin which has a branching index [A] satisfying 0.20≦[A]≦0.98 has especially high strength in its molten state among polyolefin-based resins. Providing such a non-foamed resin layer as a surface layer may give an effect of preventing the generation of unevenness caused by cells formed in the internal foamed layer by a foaming agent or caused by breakage of cells. This may result in a multi-layer foamed polyolefin-based resin sheet having high surface smoothness and superior secondary processabilities such as vacuum-formability. This also improves the vacuum- or pressure-formability of hollow containers, particularly the formability in multiple-piece production.
In the case where the branching index of the polyolefin-based resin having a long-chain branch is out of the aforementioned range, a melt viscosity decreases and the formability of hollow containers also deteriorates.
The branching index expresses the degree of long-chain branching and is a value defined by the following formula:
Branching index [
A]=[&eegr;]Br/[&eegr;]Lin
In the formula, [&eegr;]Br is the intrinsic viscosity of a branched polyolefin and [&eegr;]Lin is the intrinsic viscosity of a semicrystalline linear polyolefin, which is mainly isotactic, having a weight average molecular weight substantially equal to t
Kitayama Takeo
Usui Nobuhiro
Fitch Even Tabin & Flannery
Kuhns Allan R.
Sumitomo Chemical Company Limited
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