Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Patent
1996-05-29
1997-04-15
Nold, Charles
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
428515, 428500, 428519, 428521, B32B 2708
Patent
active
056208030
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a shrink packaging material, and more particularly to a heat shrinkable polypropylene laminate film having an intermediate layer mainly comprising a syndiotactic polypropylene and having a good packaging machine applicability and excellent propagation tear resistance, low temperature shrinkability and heat resistance.
BACKGROUND ART
Stretched films such as polyvinyl chloride films, polypropylene films and polyethylene films are hitherto known as heat shrinkable films.
Among them, heat shrinkable polypropylene films have been generally used in high speed automatic packaging, since they have a good film firmess and are excellent in packaging machine applicability. However, as compared with heat shrinkable polyvinyl chloride films, the heat shrinkable polypropylene films exhibit heat sealability and heat shrinkability only at fairly high temperatures. Moreover, the propagation tear resistance and impact strength of the film are low, so it cannot necessarily be said that the heat shrinkable polypropylene films have a good applicability to packaging of heavy weight articles and the like.
The above-mentioned heat shrinkable polypropylene films are the heat shrinkable films made of crystalline isotactic polypropylene which is an already known material, copolymers composed mainly of polypropylene such as propylene-ethylene copolymer and propylene-ethylene-butene terpolymer, and mixtures of these polymers with thermoplastic polymers capable of forming a film when mixed with these polymers.
In order to eliminate the above-mentioned defects, particularly to improve the heat sealability and the propagation tear resistance, packaging films using a syndiotactic polypropylene in the surface layers are reported in Japanese Patent Publication Kokai Nos. 5-77309, 5-200957 and 5-245992.
However, though these proposals improve the heat sealability and the propagation tear resistance, it cannot be said that the low temperature shrinkability is satisfactory. Also, since the ratio of the syndiotactic polypropylene layers to the whole layers is small, the effect on the propagation tear resistance is small. Further, because the syndiotactic polypropylene having a low melting point is used in the surface layers, the films have a defect of whitening if a package is shrinked at high temperatures.
The present invention is intended to provide a film constitution which improves the low temperature heat shrinkability and the propagation tear resistance without deteriorating the packaging machine applicability and optical property that conventional heat shrinkable films using a polypropylene resin possess, and moreover which has a wide range of heat shrinking temperature.
DISCLOSURE OF THE INVENTION
The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, they have found that a heat shrinkable polypropylene film having excellent packaging machine applicability and low temperature heat shrinkability and an improved propagation tear resistance is obtained by using a specific polypropylene resin and conducting coextrusion with setting a specific layer thickness ratio. Thus they have arrived at the present invention.
That is to say, the heat shrinkable film of the present invention is a polypropylene laminate film stretched at least two times in both the machine and transverse directions, characterized in that the surface layers comprise an isotactic polypropylene and the intermediate layer consists essentially of 50 to 100% by weight of a syndiotactic polypropylene and 0 to 50% by weight of an isotactic polypropylene, and that the thickness of the intermediate layer is not less than 30% of the total thickness of all layers, the thickness of each of the surface layers is at least 1 .mu.m, and the shrinkage of the film at 100.degree. C. is not less than 25% in both the machine and transverse directions.
The isotactic polypropylene used in the surface layers is known as a crystalline polypropylene, and includes a propylene-ethylene copolymer
REFERENCES:
patent: 4659612 (1987-04-01), Balloni et al.
patent: 5254394 (1993-10-01), Bothe et al.
patent: 5482780 (1996-01-01), Wilkie et al.
Horita Fumio
Isozaki Hideo
Morita Shuichi
Oyama Toshikatsu
Sueoka Koji
Kohjin Co., Ltd.
Nold Charles
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