Food packaging stretch film

Details Food packaging stretch film Food packaging stretch film

1995-12-26

2001-02-13

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C428S519000, C428S521000, C525S088000, C525S240000, C525S241000

Reexamination Certificate

active

06187449

ABSTRACT:

The present invention relates to a stretch film to be used for food packaging. Particularly, it relates to a stretch film made of a material containing substantially no chlorine.
Heretofore polyvinyl chloride type films have been mainly used as so-called prepackaging stretch films to wrap fruits, meats or vegetables placed on light weight trays. Such polyvinyl chloride type films have not only properties suitable for packaging, such as good packaging efficiency and neat packaging finish, but also superiority in the product quality admitted by both distributors and consumers such that they are excellent in the elastic recovery to regain the initial shape even when deformed e.g. by a finger upon wrapping, they are excellent in the bottom sealing property, and peeling of films during transportation or display of wrapped products scarcely takes place, and thus the commercial value of the wrapped products can be maintained.
However, problems such as generation of hydrogen chloride gas during incineration and elution of plasticizers contained in large amounts, have been pointed out against the polyvinyl chloride films. Accordingly, various studies have been made on materials to be substituted for the polyvinyl chloride films. Particularly, stretch films of various structures employing polyolefin type resins have been proposed. For example, stretched films having structures of e.g. an ethylene/vinyl acetate copolymer (EVA), EVA/polybutene-1/EVA and EVA/linear ethylene-&agr;-olefin copolymer/EVA have been proposed.
However, it has been difficult to satisfy all of the required properties such as the packaging efficiency, packaging finish, elastic recovery and bottom-sealing property. Further, a non-vinyl chloride type stretch film having EVA laminated on each side of a layer of a hydrogenated product of a styrene/butadiene block copolymer, has also been proposed (Japanese Examined Patent Publication No. 59622/1993), but such a film is not fully satisfactory with respect to the packaging efficiency, the packaging finish and the bottom sealing property, although it has a merit that the elastic recovery against deformation is good.
Under the circumstances, the present inventors have conducted extensive studies and as a result, have succeeded in obtaining a non-vinyl chloride type stretch film excellent in the above-mentioned various properties.
Thus, the present invention provides a food packaging stretch film made of a resin containing substantially no chlorine and having a storage modulus (E′) of from 5.0×10
8
to 5.0×10
9
dyn/cm
2
and a loss tangent (tan &dgr;) of from 0.2 to 0.8, as measured by dynamic viscoelasticity measurement at a frequency of 10 Hz at a temperature of 20° C.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The stretch film of the present invention is made of a resin which contains substantially no chlorine.
The film of the present invention has a storage modulus (E′) of from 5.0×10
8
to 5.0×10
9
dyn/cm
2
as measured by dynamic viscoelasticity measurement at a frequency of 10 Hz at a temperature of 20° C., and a loss tangent (tan &dgr;) of from 0.2 to 0.8.
Heretofore, stretch films employing various non-vinyl chloride resins have been proposed. The present inventors have found that by imparting certain specific viscoelastic properties to a stretch film, the recovery behavior of the film when the film once stretched is permitted to recover upon release of the stress, will be the optimum behavior for stretch wrapping.
Here, if E′ is less than 5.0×10
8
dyn/cm
2
, the film is so soft that the stress against deformation tends to be too small, whereby the packaging efficiency tends to be poor, and the tension of the film upon wrapping tends to be insufficient. Accordingly, such a film is not suitable as a stretch film. On the other hand, if E′ exceeds 5.0×10
9
dyn/cm
2
, the film tends to be hard and poor in stretching, whereby deformation or crushing of trays is likely to be led.
If tan &dgr; is less than 0.2, the recovery behavior against stretching of the film tends to be instantaneous, and the film tends to recover in a short period of time before it is folded back at the bottom of a tray, whereby the film may not be well stretched, and wrinkles are likely to form. Further, with respect to the heat sealing condition at the bottom, in the case of stretch wrapping, no adequate fusion by heat is usually conducted, whereby after wrapping, peeling of the bottom seal is likely to take place during the transportation or display. On the other hand, if tan &dgr; exceeds 0.0, the packaging finish will be good, but the film tends to undergo plastic deformation, and the tension against an external force of the wrapped product tends to be so weak that during transportation or display, the film at the upper side of the tray tends to be slackened, for example, when wrapped products are piled up, whereby the commercial value of the wrapped products is likely to be impaired. Further, in an automatic wrapping, the film tends to be stretched in the longitudinal direction, whereby a problem such as chucking failure is likely to occur. A particularly preferred range of tan &dgr; is from 0.30 to 0.60.
A stretch film may sometimes be used at low temperatures, and accordingly it is desired to have excellent low temperature characteristics (particularly low temperature stretchability). For this purpose, the storage modulus (E′) of the film is preferably within a range of at most 1.5×10
10
dyn/cm
2
as measured by dynamic viscoelasticity measurement at a frequency of 10 Hz at a temperature of 0° C.
Various material may be mentioned as specific examples of the material satisfying the above viscoelasticity properties. One of them is a copolymer of a vinyl aromatic compound with a conjugated diene, or a hydrogenated derivative thereof, wherein the conjugated diene block has a glass transition temperature of at least −20° C. (The glass transition temperature will hereinafter be referred to simply as Tg.)
The copolymer of a vinyl aromatic compound with a conjugated diene, or a hydrogenated derivative thereof, usually has features such that it has rubber elasticity and is flexible and less susceptible to rupture, and it has excellent transparency. Further, by properly balancing the rigid nature inherent to the vinyl aromatic compound and the elastomeric nature inherent to the conjugated diene by selecting e.g. the polymerization mode, the object of the present invention can readily be accomplished. Here, as the vinyl aromatic compound, styrene is most typical, but o-styrene, p-styrene or &agr;-methylstyrene may, for example, be also employed. As the conjugated diene, butadiene, isoprene or 1,3-pentadiene may, for example, be mentioned.
As conventional copolymers of this type, a styrene/butadiene block copolymer and a styrene/isoprene block copolymer are known, and some of them are practically used for films. However, conventional copolymers of this type which are commonly used have Tg on the side far lower than 0° C., usually around −50° C., and at normal temperatures, the loss tangent (tan &dgr;) as described hereinafter, is very small.
Suitable for the present invention may, for example, be the one having a layer containing a block copolymer wherein the weight ratio of a vinyl aromatic compound to a conjugated diene is within a range of from 40/60 to 10/90, and Tg of the conjugated diene block is at least −20° C.
Specifically, it may be the one having a block prepared by random copolymerization of e.g. styrene to a conjugated diene, or the one having a tapered block having a conjugated diene block copolymerized with e.g. styrene with a certain concentration gradient.
Particularly preferred may, for example, be the one using, as the conjugated diene block, an isoprene block having a high 3,4-bond ratio. It is known that the higher the 3,4-bond (vinyl bond) ratio of the isoprene block, the higher Tg. In this sense, it is particularly effective for th

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