Transparent plastic can

A transparent plastic can which comprises a transparent body formed of a laminated plastic structure, the inner surface of which is coated with a thin layer of silicon oxide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a plastic can which is used as a container 
for food and drink. 
2. Description of the Background: 
Metal cans have conventionally been used as containers for food and drink. 
Recently plastic cans, plastic bottles, and plastic cups, have realized 
increasing use in the handling and storage of food and drink items because 
of their ease of production and disposal and because of their low cost. 
The plastic containers are now available in many varied forms because they 
can be easily prepared by injection molding, blow molding, and vacuum 
forming techniques. These containers are used for the handling and storage 
of a large variety of foods and drinks including edible oils, seasonings, 
and desserts. 
In the past there has been significant activity in the development of food 
and drink handling containers, as is evident from the following list of 
patent applications: Japanese Patent Laid-open Nos. 11146/1983, 
153629/1983, and 209561/1983, and Japanese Utility Laid-open Nos. 
35315/1984 and 35333/1984. The containers described are plastic cans which 
usually have a can body made of laminated material. The cans meets 
different requirements depending on the particular application of the 
container. Plastic cans for foods and drinks are required to conform to 
food sanitation laws and must have such properties as water resistance, 
oil resistance, retortability, self-supporting, and gas barrier 
properties. 
The can body of conventional plastic cans is illustrated by the partly 
enlarged sectional view of FIG. 4. (See, for example, Japanese Patent 
Laid-open No. 214627/1984.) In this particular example, cylinder (1') 
which forms the can body, is made up of a content protecting layer which 
is the laminated film (2) and a supporting layer. The content protecting 
layer is composed of the following five layers superposed one over the 
other as shown in FIG. 4: 
A: Undrawn polypropylene layer, (70 .mu.m thick (innermost layer)) 
E: Adhesive layer (carboxylic acid-grafted polypropylene, 7 .mu.m thick) 
F: Aluminum foil (9 .mu.m thick) 
G: Undrawn polypropylene layer (30 .mu.m thick) 
One edge (4) of the content protecting layer is folded back on itself and 
the facing surfaces are bonded together. The edge (5) of the adjacent 
layer overlaps and is bonded to the folded portion. The supporting layer 
of the can is made of the following four layers which form the laminate of 
the supporting layer: 
B: Urethane adhesive layer (4.5 g/m.sup.2 (inner layer)) 
6: Wrapping sheet of undrawn polypropylene (200 .mu.m thick) (This layer 
fills the hollow portion between the ridges formed by folding and 
overlapping the edges of the content protecting layer) 
7: Intermediate plastic layer (About 600 .mu.m thick, made of a 1:1 mixture 
of polypropylene and calcium carbonate.) 
8: Top coating layer (10 to 20 .mu.m thick, made of polypropylene block 
copolymer.) 
The transparent food and drink containers which are in current use 
generally are mostly of a single-layered structure formed from a clear 
plastic such as polyester, polypropylene, and polystyrene. These 
containers, however, have poor gas barrier properties and do not preserve 
foods and drinks for long periods of time. In an attempt to improve the 
gas barrier properties of these materials, it has been proposed to form 
the container of a multi-layer structure, with a polyvinyl alcohol layer 
being interposed between two polypropylene layers. This type of container 
has the drawback that polyvinyl alcohol loses its gas barrier properties 
upon moisture absorption. Thus, it is not suitable for long-term storage 
of water containing foods and sterilized foods. 
In another embodiment of conventional plastic cans, the can body 
incorporates a layer of aluminum foil, which is necessary for gas barrier 
properties, and a layer of a polypropylene-calcium carbonate mixture. This 
type of can has an opaque body which obscures the contents of the can. 
Opaque cans are undesirable containers for those products which must 
appeal to the consumer's eye. In addition, the conventional plastic cans 
can not be heated by an electronic oven because the aluminum foil layer 
reflects the microwave radiation. A need therefore continues to exist for 
a food and drink, storage and handling container which obviates the 
above-stated difficulties of conventional containers. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a plastic 
food and drink container which has good gas barrier properties and which 
is not opaque in appearance. 
Briefly, this object and other objects of the present invention as 
hereinafter will become more readily apparent can be attained by a 
transparent plastic can which comprises a transparent body formed of a 
laminated plastic structure, the inner surface of which is coated with a 
thin layer of silicon oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The central feature of the present invention is a container, the body of 
which is formed of a transparent laminate, upon which is deposited or 
sputtered a layer of silicon oxide. The silicon oxide may be either 
silicon monoxide or silicon dioxide. The two silicon materials may be used 
individually or in combination with each other. 
The substrate onto which the silicon oxide material is coated, which forms 
the body of the container includes the likes of biaxially oriented 
polypropylene film, polyester film, biaxially oriented nylon film, 
polycarbonate film and polystyrene film. 
The deposition or sputtering of silicon oxide on the substrate may be 
accomplished by any known method. The silicon oxide layer which is formed 
should be thick enough so that the product has sufficient gas barrier 
properties. The thickness of the silicon oxide layer is usually 200 .ANG. 
and above, preferably 500 .ANG. to 2000 .ANG.. 
Normally, the use of one silicon oxide layer is sufficient to obtain a 
satisfactory product. However, two or more silicon oxide layers may be 
formed, if desired or necessary. The silicon oxide layer should be an 
internal layer rather than an outer layer in the laminate. 
The other layers of the laminated structure of the container body are 
selected with a view to the properties desired for a particular use of the 
container or can. For example, the laminate should have an innermost 
polyolefin layer when it is intended to store food or drinks in the can. 
The outer layer of conventional cans is made of a polypropylene-calcium 
carbonate mixture. In the practice of the present invention however, this 
outer layer is a transparent polypropylene layer. 
In forming a cylinder preparatory to manufacturing a completed can 
structure by joining the edges of laminated film which constitute the 
content protecting layer of the product can, one edge (4) of the laminate 
film (2) is folded back and the other edge (5) overlaps the folded 
portion. The contacting edges are then bonded, as shown in FIG. 1, so that 
this section of the laminated film does not come into contact with the 
contents of the can. The joint thus formed may be spiral or straight in 
the longitudinal direction. 
The silicon oxide layer formed by vacuum evaportion or sputtering provides 
the product container with good gas barrier properties, even when watery 
foods and sterilized foods are stored therein. The product container of 
the present invention therefore provides for effective food preservation 
for long periods of time. In addition, the silicon oxide layer is 
transparent to both visible light and microwave energy. 
Having generally described this invention, a further understanding can be 
obtained by reference to certain specific examples which are provided 
herein for purposes of illustration only and are not intended to be 
limiting unless otherwise specified. 
EXAMPLE 1 
A three-layered laminated film was produced from a 25 .mu.m thick biaxially 
oriented polypropylene film having a silicon oxide layer vacuum evaporated 
thereon to a thickness of 1000 .ANG., a 70-.mu.m thick undrawn 
polypropylene film, and a 30-.mu.m thick undrawn polypropylene film. The 
films of the laminate were bonded to each other in the order mentioned 
with a urethane adhesive in an amount of 4.5 g/m.sup.2. (The surface of 
the film bearing the silicon oxide layer was placed inside.) 
The laminated film was made into a can body (1) using the apparatus 
schematically shown in FIG. 3. The process employed to make the can body 
is as follows: 
The laminated film (2) fed from the stock roll (10) passes through the edge 
folding unit (11) that folds back one edge (4) of the laminated film (2). 
The film then passes under the mandrel (12) and reaches the pressing roll 
(13). On the other hand, the wrapping sheet (6), fed from the stock roll 
(14), is coated (on one side) with an adhesive by the adhesive applicator 
(15), followed by drying with hot-air dryer (16). The wrapping sheet (6) 
is bonded to the laminated film (2) by the pressing roll (13). The 
laminated sheet (17), thus formed is wound around the mandrel (12). The 
overlapped edges (4, 5) of the laminated film (2) are bonded together by 
heat from air heater (18). The laminated sheet (17), which is wound around 
the mandrel, is then covered with molten plastic extruded from the T-die 
(19). The molten plastic is smoothened by smoothening belt (20), which 
results in the formation of intermediate plastic layer (7). The 
intermediate plastic layer (7) is further covered with molten plastic 
applied by doctor knife (21), which results in the formation of top 
coating layer (8). The cylinder leaving the mandrel is cut to length by 
the cutter (22). 
The can body produced as mentioned above has the structure shown in FIG. 1 
(enlarged sectional view) and in FIG. 2 (longitudinal sectional view). The 
content protecting layer of the laminated film (2) is made up to 70-.mu.m 
thick, undrawn, polypropylene layer (A), urethane adhesive layer, 4.5 
gm/m.sup.2, (B), 1000 .ANG. thick silicon monoxide deposited film layer 
(C), 25-.mu.m thick biaxially oriented polypropylene layer (D), urethane 
adhesive layer, 4.5 g/m.sup.2 (B), and 30-.mu.m thick, undrawn, 
polypropylene layer (H). The laminated film (2) is spirally wound into a 
cylinder in such a manner that one edge (4) of the laminated film (2) is 
folded back and bonded and the other edge (5) overlaps the folded part, 
followed by bonding. The supporting layer (3) placed outside the content 
protecting layer has a structure similar to that of the conventional can 
body shown in FIG. 4; but the layer of polypropylene-calcium carbonate 
mixture is replaced by 100% polypropylene. 
The can body described above was provided with injection molded plastic can 
lids to complete fabrication of the can. The can was transparent and the 
contents were visible. The can had good gas barrier properties and was 
capable of being heated in an electronic oven. 
EXAMPLE 2 
A can body was produced in the same manner as described in Example 1, 
except that the biaxially oriented polypropylene film was replaced by a 
12-.mu.m thick polyethylene terephthalate film and except that a melt 
extruded polypropylene layer 36 is used in the place of a deposited sheet 
6. This apparatus for the production of the tube is basically the same as 
the apparatus which is schematically illustrated in FIG. 3, except that 
the parts pertaining to the deposited sheet 6, i.e., stock roll 14, sizing 
device 15, and hot air drier 16 shown in FIG. 3, are no longer used. In 
their place, the device illustrated in FIG. 6 is installed. In the 
apparatus of FIG. 6, a T-die 30 is positioned over a portion of a press 
roll 13 and a cooling roll 31 is positioned opposite to press roll 13 
across an intervening space three times the thickness of the laminated 
film. The laminated film 2 having one lateral edge thereof folded back by 
the edge-folding device is brought in from the right-hand side of FIG. 6. 
The folded portion is heated by a cartridge air heater 18a and then 
pressed down into fast union by the press roll 13. Via the T-die 30, the 
molten plastic 36 is extruded in a prescribed width and applied on the 
laminated film 2 as illustrated in FIG. 7. The applied molten plastic is 
cooled by cooling roll 31, rolled to an even thickness, and forwarded to 
the mandrel 12. Thereafter, the laminated film and the applied plastic are 
treated as illustrated in FIG. 3 to complete a tube for a can barrel. The 
can body produced as above has the structure shown in FIG. 5 (enlarged 
sectional view). The can body was provided with metal can lids to complete 
fabrication of the can. As in Example 1, the can was transparent and the 
contents were visible. The can had good gas barrier properties and was 
capable of being heated in a microwave oven. 
EXAMPLE 3 
The plastic cans prepared as described in Examples 1 and 2 were examined 
for their performance in a field test. For comparison, containers of 
approximately the same size, which were constructed of single layer of 
polypropylene, were also examined in the same manner. 
Sample of the cans of the present invention were filled with various types 
of cooked foods as shown in Table 1. The cans were then sterilized with 
hot water at 125.degree. C. The taste of the foods was evaluated by an 
organoleptic test immediately after sterilization and 12 months after 
preservation in a room kept at a temperature of 20.degree. to 30.degree. 
C. (Before the test, the foods were heated using a microwave oven.) The 
organoleptic test was conducted by 10 panelists. The test results are 
indicated by the point system shown below for each quality of appearance, 
flavor, taste, texture, and odor. The criteria for the point system are as 
follows: 
5.0 . . . Excellent 
4.0 . . . Good 
3.0 . . . Fair (acceptable as a commercial product) 
2.0 to 1.0 . . . Poor (no commercial value) 
0 . . . Inedible 
TABLE 1 
__________________________________________________________________________ 
Food Preservation In Plastic Cans Of The Invention And Polypropylene 
Cans 
Just after sterilization 
Cans of 
After preservation for 12 months 
Raw Sterili- 
PP cans 
Invention 
Polypropylene cans 
Cans of the invention 
Type of Food 
Materials 
zation 
Point 
Color 
Point 
Color 
Point 
Color Others 
Point 
Color 
Others 
__________________________________________________________________________ 
Short-neck 
Short-neck 
50 min 
5.0 
Good 
5.0 
Good 
1.0 
Browning 
Flavor and 
4.0 
Slightly 
Taste 
clam boiled 
clam, salt taste de- 
brown 
slightly 
in plain water teriorated; lost 
(as food material) no com- 
mercial 
value 
Bamboo Shoots 
Bamboo 65 min 
5.0 
Good 
5.0 
Good 
2.0 
Browning 
Unusual 
4.3 
Slightly 
Feeling 
boiled in 
shoot taste; poor 
brown 
slightly 
plain water feeling poor 
(as food material) 
Common Mushroom, 
50 min 
5.0 
Good 
5.0 
Good 
2.0 
Browning 
Unusual 
4.5 
Slightly 
Taste 
mushroom salt, L- taste; poor 
brown 
slightly 
(as food material) 
ascorbic acid, feeling; lost 
pH adjustor, no com- 
seasonings mercial 
value 
Mixed Soy sauce, 
40 min 
5.0 
Good 
5.0 
Good 
1.0 
Browning 
Taste lost, 
4.5 
Slightly 
Taste 
seasonings 
Sake, edible salty, no 
brown 
slightly 
for Chinese 
oil, tomato commercial lost 
Food (Mabo 
catsup, value 
Nasu) ginger root, 
seasonings, 
scallops, 
beef extract, 
salt 
Mixed Tomato, red 
45 min 
5.0 
Good 
5.0 
Good 
1.0 
Tomato 
Taste lost, 
4.3 
Tomato 
Taste 
seasonings 
beets, salt, browned 
unusual slightly 
slightly 
for Borsch 
seasonings, taste, browned 
lost 
spice, meat taste, no 
extract, commercial 
vegetable value 
extract 
Mixed Edible oil, 
50 min 
5.0 
Good 
5.0 
Good 
2.0 
Browning 
Curry taste 
4.5 
Slightly 
Taste 
seasonings 
milk, wheat lost; no brown 
slightly 
for Pork curry 
flour, sugar, commercial lost 
salt, protein value 
hydrolyzate, 
meat extract, 
Kaorina 
Soup (cream- 
Sweet corn, 
60 min 
5.0 
Good 
5.0 
Good 
1.0 
Browning 
Taste lost; 
4.5 
Slightly 
Corn 
ed corn) as 
milk, wheat unusual brown 
taste 
processed food 
flour, sugar, flavor; no slightly 
salt, edible commercial lost 
oil, season- value 
ings, protein 
hydrolyzate, 
meat extract, 
spice 
"Sankaini" 
Bamboo 70 min 
5.0 
Good 
5.0 
Good 
2.0 
Browning 
Taste lost; 
4.5 
Slightly 
Taste 
as processed 
shoot, tuna unusual brown 
slightly 
food flake, flavor; no lost 
elephant commercial 
food, value 
seasonings 
__________________________________________________________________________ 
It can be observed from Table 1 that there is no significant difference 
between the plastic cans of the present invention and the polypropylene 
cans when tested immediately after sterilization. However, after storage 
at room temperature for 12 months, the foods in the polypropylene cans 
deteriorated to such an extent that they were almost of no commercial 
value, whereas the plastic cans of the present invention preserved the 
foods almost intact. No difference was observed between the plastic cans 
of Example 1 and those of Example 2. The plastic cans of the present 
invention permit heating of the contents therein with a microwave oven 
after they are opened. 
The present invention provides for a new dimension in the use of plastic 
cans which can be easily produced at low cost and easily disposed of after 
use. The plastic can of the present invention exhibits good gas barrier 
properties, and it preserves watery foods and sterilized foods over long 
periods of time. The plastic can of the present invention is transparent 
and makes the contents of the can appealing to the consumer's eye. The can 
is also transparent to microwave energy. This feature permits the contents 
of the can to be heated by a microwave oven while the contents are in the 
can. 
Having now fully described the invention, it will be apparent to one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.