Film having gas permeability

Details Film having gas permeability Film having gas permeability

1999-12-02

2002-02-19

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C428S515000, C426S118000, C426S415000, C426S419000

Reexamination Certificate

active

06348271

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a film having a good gas-permeability. More particularly, it relates to a film that can be used as a wrapping material for fresh produce such as vegetables and fruit because its properties of permeating gases such as water vapor, oxygen and ethylene suitably are required for them.
BACKGROUND ART
Recently, plastic films are widely used in food wrapping, textile wrapping, etc. In particular, as a representative of polyolefin films, polypropylene film drawn at least in a monoaxial direction (hereafter, sometimes abbreviated as drawn PP film) have been used in many fields since it has excellent transparency, gloss, and further high mechanical strength so that it is excellent in printing processability, bagging processability and filling workability thereinto.
Generally, drawn PP films have increased film densities since orientation increases the regularity of molecular orientation, so that gases such as water vapor and oxygen become more difficult to pass through. In particular, their use for wrapping fresh produce such as vegetables and fruit is undesirable since insufficient supply of oxygen to the wrapped contents, insufficient exhaustion of ethylene gas generated by the wrapped contents or the like will decrease the freshness of the wrapped contents. Further, water vapor generated by a vegetable or fruit forms water droplets on the inner surface of the film contacting the wrapped contents and the droplets drip onto the vegetable or fruit, which is a defect that might cause its discoloration or putrefaction. In other words, fresh produce such as vegetables and fruit respire by themselves after harvested and will be aged and putrefied. It is ethylene gas generated by fresh produce themselves such as vegetables and fruit that causes them to respire. As a wrapping film that can retain the freshness of the fresh produce have been studied or partly used films containing a porous structure such as activated carbon or Oya-stone. An adsorptive effect of the porous structure being utilized therein, they are intended to restrain the fresh produce from respiring by themselves by absorption of the ethylene gas, which have been generated from the fresh produce, and fixation onto the porous structure of it in the film. However, the porous structures are limited in adsorptivity. For example, once they absorb moisture and their adsorptivity reaches near saturation, then their ability of exhausting ethylene will be decreased greatly. Furthermore, in the environment where excessive moisture is present, the porous structures that have once adsorbed ethylene will release a portion of the ethylene. Therefore, it cannot be said that they have an ability to retain sufficient freshness as a wrapping film therefrom for fresh produce that are generally stored or circulated under high humidity conditions. On the other hand, it is said that fresh produce will lose the qualification as a commercial good when they lose usually 5% or more of their moisture. To maintain freshness, it is also necessary to restrain moisture from transpiring appropriately. On the contrary, excessive humidification or dew formation also causes putrefaction of the wrapped fresh produce. While wrapping films are required of high transparency in order to appeal the quality of the wrapped contents such as freshness through the film for consumer's examination, films containing a porous structure will decrease transparency due to the existence of the porous structure. At present, drawn polypropylene films circulated for wrapping fresh produce are used by blending a droplet preventing agent with the film to prevent dew formation still being unable to control the transpiration of water vapor or by being perforated by physical means during the process up to the wrapping step in order to increase aeration of water vapor, oxygen, carbon dioxide, etc.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of the above-described defects of the drawn PP films and has for its object to provide a film suitable for wrapping fresh produce and propose a film that can control gases related to the physiological activity of fresh produce and restrain the transpiration suitably. More particularly, an object of the present invention is to provide a film that is excellent in productivity and profitability in wrapping vegetables or fruit due to the fact that the step of perforation conventionally conducted can be eliminated since gases such as water vapor, oxygen, and ethylene advantageously permeate the film and the permeability can be controlled, additionally the film having transparency, gloss, and further excellent mechanical strength, particularly the film being suited for wrapping fresh produce.
To achieve the above-described objects, the present inventors have made intensive research and as a result have found that a film which is a monolayer or multilayer film having a thickness [D
1
] of 10 to 100 &mgr;m and having the following characteristics (1) to (3):
( 1) [T
H2O
] (a permeability of water vapor)=9-50 (unit: g/m
2
·24 Hr, in accordance with JIS z-0208),
(2) [T
O2
] (a permeability of oxygen gas)=600-12,500 (unit: nmol (STP)/m
2
·s·100 kPa, in accordance with JIS K-7126A)
(3) [T
ethylene
] (a permeability of ethylene gas)=600-22,500 (unit: nmol (STP)/m
2
·s·100 kPa, in accordance with JIS K-7126A) have excellent properties, thus completing the present invention.
That is, in a first aspect, the present invention provides a gas permeable film having the above-described three types of permeability in good balance.
In a second aspect, the present invention provides a film which comprises a propylene resin composition comprising a propylene polymer component and a copolymer component comprising ethylene and propylene, wherein a limiting viscosity ratio of the propylene polymer component and the copolymer component, and a weight ratio of the propylene polymer component and the copolymer component are within specified ranges respectively, wherein the film is drawn in at least one direction, and the film having a thickness [D
1
] of 10 to 100 &mgr;m and has the following characteristics (1) to (3):
(1) [T
H2O
] (a permeability of water vapor)=9-50 (unit: g/m
2
·24 Hr, in accordance with JIS Z-0208),
(2) [T
O2
] (a permeability of oxygen gas)=600-12,500 (unit: nmol (STP)/m
2
·s·100 kPa, in accordance with JIS K-7126A)
(3) [T
ethylene
] (a permeability of ethylene gas)=600-22,500 (unit: nmol (STP)/m
2
·s·100 kPa, in accordance with JIS K-7126A)
Further, in a third aspect, the present invention provides a monolayer film for wrapping fresh produce, which comprises a propylene resin composition comprising a propylene polymer component and a copolymer component comprising ethylene and propylene, wherein a content of the propylene polymer component [W
PP
] is 93 to 30% by weight and a content of the copolymer component [W
RC
] is 7 to 70% by weight based on the weight of the composition, and a limiting viscosity of the copolymer component [&eegr;]
RC
is 1.5 to 3.0 dl/g and a ratio ([&eegr;]
RC
/[&eegr;]
PP
) of the limiting viscosity of the copolymer component [&eegr;]
RC
to the limiting viscosity of the propylene polymer component [&eegr;]
PP
is in the range of 0.5 to 1.2, wherein the propylene resin composition being processed and drawn in the range from 3 to 60 times, the monolayer film having a thickness [D
1
] of 10 to 100 &mgr;m and a relationship between [D
1
] and [W
RC
] of 0.1≦W
RC
/D
1
≦7.0, and the film having the following characteristics (1) to (3):
(1) [T
H2O
] (a permeability of water vapor)=9-50 (unit: g/m
2
·24 Hr, in accordance with JIS Z-0208),
(2) [T
O2
] (a permeability of oxygen gas)=600-12,500 (unit: nmol (STP)/m
2
·s·100 kPa, in accordance with JIS K-7126A)
(3) [T
ethylene
] (a permeability o

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