Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1999-10-13
2002-03-05
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C264S209300, C264S209400, C264S209500, C428S520000, C428S910000
Reexamination Certificate
active
06352785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field to the Invention
The present invention relates to shrinkable films considering the environment which are suitably used for wrapping foods or the like and have good oxygen barrier property, processability, transparency and strechability, especially.
2. Description of the Related Art
Shrink packaging, that uses stretched films (heat shrinkable films) which are widely adopted in industrial packaging such as food packaging, pharmaceutical drug packaging and industrial parts packaging, is performed by, for example, charging an object into a bag formed of a stretched film, removing air in the bag if demanded, sealing the opening of the bag, and heating the stretched film to shrink it.
Packaging and delivering some types of foods such as cheese, pork, smoked or processed meat and, especially, fresh blood-red meat require to keep qualities of these products at good conditions for a long time until the products are gotten by consumers. Especially, in the case of the fresh blood-red meat, since oxygen accelerates degradation (decomposition) of the meat caused by microorganisms, it is important to securely protect the meat from oxygen for a long time.
In order to prevent oxygen to pass, wrapping having a multilayer structure comprising a resin sheet or film with good oxygen barrier property are often employed. An oxygen barrier resin material is required to have process ability which means good ease to handle it during forming, transparency at a time of forming, performance of having good strechability and shrinkability after a stretched film is formed, or the like as well as oxygen barrier property.
Oxygen barrier resin materials employed for conventional shrinkable films have some problems. Namely, for example, they have an insufficient oxygen barrier property. Alternatively, even if they have a good oxygen barrier property, they require to be processed at a high temperature because of their high melting points, and are easy to cause heat degradation. Cost for maintaining equipment for processing at high temperature, therefore, becomes expensive.
Shrinkable films used as wrappers are required that the wrapped objects are easy to be seen. When objects wrapped with such shrinkable films are sold as commercial products, consumers can buy them after examining the actual products and can directly confirm degradation or defects of the products. In addition, sellers can show appearance of the objects wrapped to buyers. Therefore, transparency of wrapping is important.
On the other hand, films used for shrink packaging is demanded to have a property to shrink well by heating. Additionally, lowering a temperature at which a film can be shrunk by heating is demanded from the viewpoint of shortening a wrapping cycle. In order that a stretched film is shrunk by heating, the stretched film must be heated at a temperature higher than the stretching temperature in its manufacture. It is, therefore, thought that a heat shrinking temperature of a stretched film depends on a stretching temperature in the manufacture of the stretched film.
A shrinkable film is usually manufactured by stretching a film at a temperature higher than room temperature to orient molecules followed by cooling it to stop motion of the molecules, and when wrapping is carried out, it is shrunk by reheating to relieve the orientation of the molecules. Stretching and shrinking behavior of films varies depending on the types of the polymers used. However, what is required of wrapping films, especially those with the oxygen barrier property, is to have characteristics that the films can be stretched at a relatively moderate temperature and subsequently can be shrunk at a moderate temperature.
Additionally, the more a stretching degree of a film becomes, the more its shrinking rate becomes. Hence the more a stretching rate of a specific film becomes, the more its shrinkability in the whole temperature range, especially at a low temperature, becomes.
Especially in order to make wrapping foods and containers more efficient, the speed of a wrapping line must be increased. In addition, a shrinkable film capable of exhibiting a high shrinking rate at a lower temperature is demanded in order to prevent wrapped products to degrade due to a high temperature.
A shrinkable film is often required to have, for example, heat sealability, gas barrier property and tearing strength as well as the aforementioned characteristics. When one intends to satisfy these characteristics required, there is a problem that materials which can be used are restricted and a heat shrinking rate is limited. On this account, development of shrinkable films using specified materials and layer compositions which can shrink well at a low temperature might be needed.
By the way, since a saponified ethylene/vinyl ester copolymer is superior to other resins in transparency and a gas barrier property, especially the oxygen barrier property, films formed thereof have been widely used in wrapping applications. Since the saponified ethylene/vinyl ester copolymer (especially, a saponified ethylene/vinyl acetate copolymer) has high rigidity and poor stretchability (especially, stretchability at a low temperature,) a temperature at which the saponified ethylene/vinyl ester copolymer can be stretched is high. When the saponified ethylene/vinyl ester copolymer is stretched at a low temperature, there will cause cracking, uneven stretching, blushing, etc., and stretched films with insufficient performance are adversely obtained. For this reason, improvement in stretchability of the saponified ethylene/vinyl ester copolymer is demanded.
For example, Japanese Laid-Open Patent Publication Nos. 53(1978)-88067 and 59(1984)-20345 disclose improvement in stretchability of a saponified ethylene-vinyl ester copolymer by adding a variety of plasticizers. The saponified ethylene-vinyl ester copolymers obtained by such conventional methods, however, do not have enough stretchability.
Although nylon, polyvinylidene chloride, polyvinyl alcohol, liquid crystal polymers and the like have been hitherto employed as gas barrier materials, there have been known that nylon and polyvinylidene chloride are inferior to the saponified ethylene/vinyl ester copolymer in the gas barrier property, and on the other hand, polyvinyl alcohol and liquid crystal polymers are inferior to the saponified ethylene/vinyl ester copolymer in stretchability. These gas barrier materials do not have a satisfactory balance between the gas barrier property and the stretchability.
The present inventors have made an intensive investigation to develop a shrinkable film with good oxygen barrier property, processability, transparency and stretchability, and as a result, have accomplished the present invention.
SUMMARY OF THE INVENTION
The present invention provides;
1. a shrinkable film characterized by being a laminate film having at least one layer formed of a gas barrier resin composition satisfying the following expressions (1) and (2) and by having an average shrinking rate in its longitudinal and transverse directions after immersion in a 120° C. oil bath for ten seconds of 45% or more:
ln(
OTR/
22.5)+0.0698(
T
im
−157)<−0.06 (1)
T
2
−T
1
≦20 (2)
(where in the expression (1), OTR and T
im
are, respectively, an oxygen transmission rate (cc/m
2
·day·atm) of a 1-&mgr;m thick portion of a film formed of the gas barrier resin composition at 23° C. and a relative humidity of 0% and a melting point of the gas barrier resin composition, and in the expression (2), T
1
is a haze (%) of a 30-&mgr;m thick film of the gas barrier resin composition left at 23° C. and a relative humidity of 48% for 48 hours and T
2
is a haze (%) of a 30-&mgr;m thick film of the gas barrier resin composition left at 40° C. and a relative humidity of 90% for 24 hours.)
2. The shrinkable film according to the above 1 wherein its average heat shrinking rate in the longitudinal and transverse directions after immersion in an 80° C. oil bath for 10 seconds is 20% or more.
Hanada Satoshi
Kuroda Ryuma
Sakaya Taiichi
Nakarani D. S.
Sumitomo Chemical Company Ltd.
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