Polyolefin film for stretch packaging

Details Polyolefin film for stretch packaging Polyolefin film for stretch packaging

2000-11-20

2003-04-01

Tarazano, D. Lawrence (Department: 1773)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C428S516000, C428S523000, C525S240000, C525S241000, C526S348000, C526S348100

Reexamination Certificate

active

06541123

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a stretch film useful for food packaging. Particularly, it relates to a stretch packaging film made of a material containing no chlorine.
BACKGROUND ART
Heretofore, polyvinyl chloride type films have been mainly used as stretch packaging films for so-called prepackaging to wrap fruits, meats or prepared food placed on lightweight 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 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 type films. Accordingly, various studies have been made on materials to be substituted for the polyvinyl chloride type films. Particularly, stretch packaging films of various structures employing polyolefin type resins have been proposed. For example, stretch packaging 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.
In recent years, studies have been actively made on stretch films of a three layer structure with two kinds, consisting of front and rear side layers of ethylene-vinyl acetate copolymer and an interlayer containing a polypropylene type resin as the main component, from reasons such as good surface property and transparency, moderate heat resistance, flexibility for materials design, economical efficiency, etc., as a stretch packaging film.
However, stretch films containing a polypropylene type resin as the main component, which have been proposed, are not fully satisfactory with respect to comprehensive evaluations in the market including principal properties suitable for packaging (automatic machine or manual packaging) such as packaging efficiency, packaging finish, elastic recovery and bottom sealing property, and economical efficiency.
The present inventors have proposed in JP-A-9-154479 a food packaging stretch film, which contains a relatively low crystalline propylene type polymer and petroleum resins, and has specific viscoelastic properties.
The film containing a propylene type polymer and petroleum resins as disclosed in the above-mentioned JP-A-9-154479 has relatively good properties such as packaging efficiency, packaging finish, elastic recovery and bottom sealing property, but is required to contain a relatively large amount (30 wt %) of petroleum resins in order to obtain desired viscoelastic properties. Accordingly, the strength of the film tends to change with time, or the petroleum resins tend to bleed out on the surface, thus causing blocking of the films when the films are rolled up, such being problematic.
DISCLOSURE OF THE INVENTION
The present inventors have conducted extensive studies and as a result, have succeeded to obtain a non-PVC type stretch film which is excellent in not only properties such as packaging efficiency, packaging finish, elastic recovery and bottom sealing property, but also stability of the film with time and economical efficiency, by using a specific soft polypropylene type resin, or by controlling viscoelastic properties. It is an object of the present invention to provide a polyolefin type stretch packaging film (hereinafter sometimes referred to as a stretch film or simply as a film), which has at least one layer containing the following component (A), and which has a storage modulus (E′) of from 5.0×10
8
dyn/cm
2
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.:
(A) a soft polypropylene type resin which satisfies the following conditions (1) to (3), which has a sum of a mesopentad percentage and a racemopentad percentage (mmmm+rrrr) of from 30 to 70% as obtained from
13
C-NMR spectrum, in which crystalline block and non-crystalline block portions co-exist in the molecular chain, and of which the stereoregularity is controlled:
(1) the glass transition temperature is at least −15° C. as measured by a differential scanning calorimeter by raising the temperature at a heating rate of 10° C./min;
(2) the quantity of heat of crystallization is from 10 to 60 J/g as measured by a differential scanning calorimeter by raising the temperature to 200° C. after the crystal fusion at a heating rate of 10° C./min, holding the resin at 200° C. for 5 minutes, and lowering the temperature to room temperature at a cooling rate of 10° C./min; and
(3) the melt flow rate (MFR) is from 0.4 to 40 g/10 min (JIS K 7210, 230° C., 2.16 kg load).
With the stretch film of the present invention, cutting and transportation of the film and wrapping can be carried out without any problem when the film is used for packaging by e.g. an automatic packaging machine, and the bottom sealing property is excellent. Further, it is possible to obtain a wrapped product with the film tightly stretched. Thus, it has excellent properties which have not been seen with conventional non-chlorine type stretch films. Further, it is excellent also in stability with time and economical efficiency.
BEST MODE FOR CARRYING OUT THE INVENTION
Now, the present invention will be explained in detail.
The stretch film of the present invention has at least one layer containing-the following component (A) and has specific viscoelastic properties as the entire film:
(A) a soft polypropylene type resin which satisfies the following conditions (1) to (3), which has a sum of a mesopentad percentage and a racemopentad percentage (mmmm+rrrr) of from 30 to 70% as obtained from
13
C-NMR spectrum, in which crystalline block and non-crystalline block portions co-exist in the molecular chain, and of which the stereoregularity is controlled:
(1) the glass transition temperature is at least −15° C. as measured by a differential scanning calorimeter by raising the temperature at a heating rate of 10C./min;
(2) the quantity of heat of crystallization is from 10 to 60 J/g as measured by a differential scanning calorimeter by raising the temperature to 200° C. after the crystal fusion at a heating rate of 10° C./min, holding the resin at 200° C. for 5 minutes, and lowering the temperature to room temperature at a cooling rate of 10° C./min; and
(3) the melt flow rate (MFR) is from 0.4 to 40 g/10 min (JIS K 7210, 230° C., 2.16 kg load).
Here, the soft polypropylene type resin as the above-mentioned component (A) in which crystalline block and non-crystalline block portions co-exist in the molecular chain, usually has such properties that it has rubber elasticity, is flexible and is less likely to be broken, and has a good transparency. Further, it is suitable to achieve the object of the present invention by balancing the proportion of the crystalline block portion of an isotactic structure and a syndiotactic structure showing stiffness with the non-crystalline block portion of an atactic structure showing elastomeric properties.
The co-existance of crystalline block and non-crystalline block portions in the molecular chain means that the crystalline block and the non-crystalline block portions are present randomly and/or blockwise. The chain length of each block portion may be optional, but the distribution of the block portions is preferably random from the viewpoint of transparency, appearance and the like in the present invention.
The polypropylene type resin is usually high crystalline and has high strength, and has a relatively high melting point and good heat resistance among polyole

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