Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-02-22
2001-07-03
Gorr, Rachel (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S084000, C264S165000, C264SDIG007
Reexamination Certificate
active
06255388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a shrink film which contains essential components of rubber-modified styrene resins consisting of a gum polymer, a styrene-based monomer, and an alkylester(meth)acrylate, and a method of making the same. The shrink film thus obtained exhibits excellent shrinking properties suitable in the shrink packaging and labeling fields such as a lower shrink starting temperature and a larger shrinkage rate, and an improved dimensional change in the shrinking direction at room temperature (natural shrinkage). The shrink film further exhibits high glossiness, and high transparency, and possesses desirable mechanical properties and an excellent applicability to the packaging machines due to its high rigidity, and the desirable property of preventing whitening from occurring at bent portions.
2. Background Art
Conventionally, shrink films produced by polyvinyl chlorides are widely used as films for the shrink packaging due to their shrinkage properties, mechanical strength, and transparency. However, the material for shrink films has changed to styrene-butadiene copolymers, because of environmental problems due to the generation of hydrochloride gas caused by incineration of polyvinyl chlorides.
Recently, stimulated by the development of PET bottles, various shrink films provided with improved shrinkage at lower temperatures and improved mechanical properties have been developed by a method of mixing a styrene-based resin with a styrene-butadiene block copolymer, or by a method of controlling the styrene content in the styrene-butadiene copolymer or of controlling the degree of polymerization of the styrene-butadiene copolymer.
For example, JP-A-2-14186 and JP-A-2-55218 disclose shrink films in which a particular low molecular polystyrene is incorporated in a block copolymer composed of vinyl aromatic hydrocarbon and conjugated dienes. However, any shrink films composed of those block copolymers are liable to undergo natural shrinkage, and are also liable to be inferior in transparency of the film after forming the film.
Therefore, conventionally, JP-A-8-34861 disclosed a new type of styrene-based shrink film made by drawing a resin obtained by graft-polymerization of a gum polymer, a styrene-based monomer, and alkylester (meth)acrylate such that the gum polymer occupies the resin at an ratio of 1 to 20% by weight. The thus obtained resin film exhibits good properties in transparency, glossiness, and in mechanical strength, and the film exhibits good shrinkage properties such as shrinkage at low temperature and a property to prevent natural shrinkage.
However, the shrink film disclosed by JP-A-8-34861 has a drawback in that it is still likely to undergo natural shrinkage, although it is superior in mechanical strength or in rigidity, as well as in surface glossiness.
Therefore, it is the object of the present invention to provide a shrink film and to provide a method of making the same which is superior in the surface glossiness and in mechanical properties such as the mechanical strength and rigidity as well as the property of preventing natural shrinkage.
This invention was completed by the authors after a series of investigations by discovering that an excellent shrink film can be made by the use of an essential component of a grafted copolymer obtained by the graft-polymerization of styrene monomers and alkylester(meth)acrylate to a specific amount of a gum polymer, and by successive processing of a film formation by rolling, and thermal setting for adjusting the film orientation. The thus obtained shrink film possesses desirable properties such as high glossiness, high rigidity, and a property of preventing natural shrinkage.
That is, the present invention relates to a shrink film constructed by a resin composition in which the essential component is composed of a Resin A which is obtained by graft polymerization of a gum polymer (a-1), a styrene-based monomer (a-2), and alkylester(meth)acrylate (a-3), such that the gum polymer comprises 1 to 20% by weight of the Resin A; and the orientation of the film based on ASTM D1504 is controlled to be within a range of 1.5 to 5.0 MPa. Furthermore, the present invention relates also to a method of making a shrink film comprising the steps of: extruding the melted and kneaded resin composition in which the essential component is composed of a Resin A which is obtained by graft polymerization of a gum polymer (a-1), a styrene-based monomer (a-2), and alkylester(meth)acrylate (a-3), such that the gum polymer comprises 1 to 20% by weight of the Resin A; drawing the extruded resin into at least one direction at a temperature range from (Tg+5° C.) to (Tg−15° C.), where Tg is the glass-transition temperature measured by JIS K-7198; exposing the thus drawn film at an temperature range from (Tg+50° C.) to (Tg−10° C.) for 2 to 80 seconds.
The resin composition A of the present invention is a resin which is obtained by graft polymerization of a gum polymer (a-1), a styrene monomer (a-2), and alkylester (meth)acrylate (a-3), such that the gum polymer comprises 1 to 20% by weight of the Resin A. If the content of the gum polymer (a-1) is less than 1% by weight, it will be difficult for the resin composition to be extended into a film, and the impact resistance of the shrink film is degraded. In contrast, if the content exceeds 20% by weight, surface irregularities of the film caused by the gum particles increase with increased amounts of the gum polymer, reducing the transparency and the glossiness of the film, and the rigidity of the film is also degraded.
Although the gum polymer is not limited to a specific material, the polymer may include rubber having rubber-like properties at room temperature or thermoplastic elastomers. Practically, the gum polymer used in the present invention includes block-copolymer of styrene-based monomers represented by a styrene-butadiene rubber and diene-based monomers, butadiene rubber (BR), natural rubber (NR), and isoprene rubber (IR).
Among these polymers, from the point of view of obtaining a good balance of properties in transparency and impact resistance, and improved property resistance to whitening by bending, it is preferable to use block copolymers of styrene-based monomers and diene-based monomers, and butadiene rubber (BR), and it is more preferable to use block copolymers of the styrene-based monomers and diene-based monomers.
There is no limitation in selecting block copolymers of styrene-based monomers and diene-based monomers, but it is preferable to use copolymers with a content of the styrene unit is in the range of 30 to 55% by weight, because the properties such as the transparency and the impact resistance will be improved. That is, when the content of the styrene unit is 39% or more than 30% by weight, various properties of the resultant shrink film such as the capability to prevent natural shrinkage, the transparency, and the glossiness are remarkably improved, and the rigidity of the resultant film is improved so as to be more suitable for a packaging machine. In contrast, when the content of the styrene unit is 55% or less than 55% by weight, the impact resistant strength of the resultant film becomes high, and whitening by bending can be further reduced.
The average diameter of gum particles of the gum polymer (a-1) is not limited. However, it is preferable to use a gum polymer having an average diameter of the particles within a range of 0.05 to 0.8 &mgr;m, and more preferably within a range of 0.1 to 0.4 &mgr;m, because the resultant film will be provided with the better impact resistance, better transparency, better glossiness, and better capability of preventing whitening by bending.
Examples of the styrene-based monomers which are used for constructing the copolymers of the styrene-based monomers and the diene-based monomers include styrene, &agr;-methylstyrene, m-methylstyrene, p-methylstyrene, ethylstyrene, isobutylstyrene, tert-butylstyrene, (o-, m-, p-)bromostyrene, (o-, m-, p-)chlorostyren
Arai Kosuke
Murata Yoshiaki
Sawada Hidetsugu
Yamada Hirofumi
Armstrong Westerman Hattori McLeland & Naughton LLP
Dainippon Ink and Chemicals Inc.
Gorr Rachel
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