Laminated three-layer film

Details Laminated three-layer film Laminated three-layer film

1993-04-16

1994-12-27

Buffalow, Edith

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

428349, 428516, 428910, B32B 2700

Patent

active

053764374

DESCRIPTION:

BRIEF SUMMARY
INDUSTRIAL UTILIZATION FIELD

The present invention relates to a laminated three-layer film and more particularly to a heat-sealable film superior in sealability under a reduced pressure.


PRIOR ART

In conventional laminate films superior in heat-sealability, in the case where a biaxially oriented film (A) of polypropylene is used as a base layer, a heat-sealing layer (C) is laminated to only one side of the base layer (A) to provide a (A)/(C) laminate film, or the heat-sealing layer (C) is laminated to both sides of the base layer (A) to provide a (C)/(A)/(C) laminate film, or different heat-sealing layers (C) and (C') are laminated to both sides of the base layer (A) to provide a (C)/(A)/(C') laminate film. In all of these cases, the base layer and the heat-sealing layer(s) are in contact with each other. (See Japanese Patent Laid Open No. JP53-28685A and Patent Publication No. JP59-26470B.)


OBJECT OF THE INVENTION

Such conventional laminate films superior in heat-sealability have been disadvantageous in that when the atmospheric pressure changes from a normal pressure to a reduced pressure, for example when the films are exposed to the air of a high ground during transport over a mountain, there occurs cleavage at the interface between the base layer (A) and the heat-sealing layer (C) or (C'), which is apt to cause bursting of the bags formed by the films.
It is the object of the present invention to solve such a problem of the prior art and provide a laminated three-layer film superior in heat-sealability and difficult to burst even under a reduced pressure.


SUMMARY OF THE INVENTION

The laminated three-layer film according to the present invention is characterized in that a base layer (A) which is a biaxially oriented film formed of a crystalline polypropylene, a cushion layer (B) which is a film of an olefin polymer lower in melting point than the base layer (A), and a heat-sealing layer (C) which is an olefin polymer film, are laminated together in the order of (A)/(B)/(C), that the three layers are in the following ranges in terms of the degree of surface orientation, and that when the cushion layer (B) and the heat-sealing layer (C) are of the same composition, the degree of surface orientation of the cushion layer (B) is smaller than that of the heat-sealing layer (C):


PREFERRED EMBODIMENTS OF THE INVENTION

In the base layer (A) of a biaxially oriented film formed of a crystalline polypropylene in the present invention, the polypropylene is preferably a homopolymer (hereinafter referred to simply as "PP"), but it may be a copolymer of propylene and a small amount of another .alpha.-olefin. In view of the rigidity and stretchability required for the base layer, it is preferable for the polypropylene to have a melting point not lower than 155.degree. C. and an intrinsic viscosity [.eta.] of 1.5 to 2.5 dl/g. The biaxial orientation of the base layer is obtained by a conventional biaxial orienting method. It is optional which of simultaneous biaxial stretching and sequential biaxial stretching is to be adopted, provided it is necessary that the degree of surface orientation P.sub.A should be in the range of 2.0 to 15.0, preferably 2.5 to 13.0.
The surface orientation degrees (P.sub.A, P.sub.B, P.sub.C) are determined at absorbance ratios d.sub.841 /d.sub.890 and d.sub.841 /d.sub.810 according to Laser Raman microscopy. An in-plane difference in the degree of surface orientation, .DELTA.P.sub.A (.vertline.P.sub.TD -P.sub.MD .vertline.), which is also determined by the same Laser Raman microscopy, is not specially limited if only the degree of surface orientation is within the foregoing range. It is well known that the closer to zero, the better the length-breadth balance of film. But in the case of a film obtained by sequential biaxial stretching, a suitable .DELTA.P.sub.A value is in the range of 0 to 13, preferably 6 to 12. If P.sub.A is smaller than 2.0, preferably smaller than 2.5, the resulting laminate film will be low in rigidity and difficult to effect packaging, and if it exceeds 15

REFERENCES:
patent: 4726999 (1988-02-01), Kohyana et al.
patent: 4740421 (1988-04-01), Suzuki et al.
patent: 4786533 (1988-11-01), Crass et al.
patent: 4921749 (1990-05-01), Bossaert

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