Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Patent
1994-12-21
1997-06-03
Thibodeau, Paul J.
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
428 349, 428 352, 428515, 428516, B32B 2732, B32B 702, B65B 5302
Patent
active
056352867
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a heat shrinkable polyethylene laminate film, and more particularly to a heat shrinkable laminate film made of specific ethylene copolymers, having an excellent applicability to packaging machines.
BACKGROUND ART
Stretched films of polyvinyl chloride, polypropylene, polyethylene and the like are hitherto known as heat shrinkable films. Among them, heat shrinkable polyethylene films have been put to practical use from the viewpoints of possession of heat sealability and low cost. Particularly, in recent years, a heat shrinkable polyethylene film using a linear low density copolymer of ethylene and an .alpha.-olefin (hereinafter referred to as "linear low density polyethylene") attracts attention in that the impact resistance and the heat seal strength are excellent, and its utilization in various fields is expected.
The present inventors previously proposed a heat shrinkable film composed mainly of a specific ethylene/.alpha.-olefin copolymer (Japanese Patent Publication Kokai No. 62-201229). By practicing this proposed method, it became possible to obtain a film having a less unevenness in thickness and having a better low temperature shrinkability than films obtained by inflation. However, in case of application to automatic packaging machines (such as pillow type packaging machine and automatic packaging machine using centerfold films), the proposed film encounters a problem that a heat seal failure (non-sealing in part) which has not been generated, is generated because the packaging speed of packaging machines is markedly increased in recent years. When packaging is made by automatic packaging machines, it is general to conduct the heat sealing by bead sealing using a heat knife. The sealing failure phenomenon means that a sealed portion is peeled off in a shrinking step, a film is not well cut off with causing stringiness at the sealed portion when a large tension is applied to the film (for example, when bulky goods are packaged), pinholes are generated at the sealed portion, or in an extreme case a film is not sealed at all.
The sealing failure as mentioned above also occurs when a molten resin adheres to the edge of a heat knife or to a heat knife pedestal.
SUMMARY OF THE INVENTION
As a result of making further investigation thoroughly in order to eliminate the above-mentioned sealing failure, the present inventors have confirmed the following matters.
That is to say, as the time until a film is shrinked in a shrinking tunnel is shortened with increase of packaging speed, and when solidification of a molten resin does not proceed within such a short time, a part of the molten resin is drawn apart to produce pinholes, or in an extreme case to cause a complete peeling of the sealed portion.
Also, when a film lacks firmness (when the modulus of tensile elasticity is small), the film is easy to crease at the time of running and, therefore, folding of the film increases at the portion to be sealed by bead sealing, resulting in increase of pinhole formation.
Also, when running property of a film in a packaging machine is insufficient because the film is poor in slipping property, or when articles to be packaged are bulky, the bead seal portion is subject to a larger tension, and a part of the seal portion is drawn apart prior to solidification of the molten resin, thus increasing formation of pinholes.
Further, when the viscosity of the molten resin is low, the molten resin is easy to adhere to the edge of a heat knife and a pedestal for the heat knife, thus pinholes are formed at the sealed portion, or the film is not cut, or in an extreme case the film is not sealed at all.
In order to eliminate the above-mentioned problems and to provide a heat shrinkable film having a good applicability to packaging machines, the present inventors have made a study about a lamination constitution of films and various kinds of raw material resins. As a result, the present inventors have found that a heat shrinkable film having a good packaging machine applicabili
REFERENCES:
patent: 4985197 (1991-01-01), Isozaki et al.
patent: 5306549 (1994-04-01), Isozaki et al.
Koyabu Shigeyoshi
Morita Syuuichi
Moriyama Tamio
Tateiwa Masaaki
Kohjin Co., Ltd.
Tarazano D. Lawrence
Thibodeau Paul J.
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