Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-02-28
2002-04-02
Eashoo, Mark (Department: 1732)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S244240, C264S171130, C264S171230, C264S177190, C264S210200, C264S288400
Reexamination Certificate
active
06364987
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing gas barrier film, in which the coating film produced has excellent, oxygen gas barrier properties.
2. Description of the Related Art
For producing oxygen gas barrier films and wrapping materials comprising them, various methods are known. Of known wrapping materials, aluminium (hereinafter referred to as Al) foil has perfect oxygen gas barrier properties, but when used by itself, it often has pin holes. Therefore, proposed is a method of laminating Al foil with other layers to give laminate films. However, since laminate films with Al foil are not transparent, the contents wrapped with them are difficult to see. Other problems with them are that, after incinerated, they give a residue of Al, and that metal detectors could not be applied to them for detecting metals in the contents wrapped with them. Another method also proposed comprises depositing aluminium, silica or the like on substrates to produce gas barrier films, which, however, is still problematic in that the films produced will easily have pin holes when folded.
For producing other oxygen gas barrier films, for example, known is a method of coating substrates with polyvinylidene chloride (hereinafter referred to as PVDC). PVDC absorbs little moisture and exhibits good gas barrier capabilities even in high-humidity conditions, and is therefore used for coating various substrates. The substrates to be coated with PVDC include films of, for example, biaxially-oriented polypropylene (hereinafter referred to as OPP), biaxially-oriented nylon (as ON), biaxially-oriented polyethylene terephthalate (as OPET), cellophane, etc. However, PVDC-laminated films are problematic in that they give hydrogen chloride gas when incinerated as wastes.
For producing still other oxygen gas barrier films, also known is a method of coating filmy substrates with completely-hydrolyzed, non-modified polyvinyl alcohol optionally followed by heating them (see Japanese Patent Laid-Open No. 32924/1994). Polyvinyl alcohol films exhibit excellent oxygen gas barrier capabilities when they absorb only a small amount of moisture. In general, however, they absorb much moisture, and their oxygen gas barrier properties are greatly degraded in high-humidity conditions having a relative humidity of around 70% or higher. Polyvinyl alcohol films are formed by coating a solution of polyvinyl alcohol having a concentration of a few % to 10% or so onto substrates through gravure coating, roll coating or the like. In that method, the barrier layer of polyvinyl alcohol formed generally has a thickness of a few microns after dried. Therefore, if the films are stretched for improving their barrier properties, the thickness of the gas barrier layer will be reduced to smaller than 1 micron. As a result, the films could not substantially enjoy the effect of orientation. This is another problem with polyvinyl alcohol films. For reducing moisture absorption by polyvinyl alcohol and for improving the orientation of polyvinyl alcohol through stretching, proposed is a method of using an ethylene-vinyl alcohol copolymer (hereinafter referred to as EVOH) which has an ethylene comonomer content of more than 20 mol % (see Japanese Patent Laid-Open No. 116259/1993). This method is still problematic in that the degree of orientation of the polymer films produced therein is still low. If the degree of ethylene modification for the polymer is increased so as to increase the orientation of the polymer films, the melting point of the polymer resin will be lowered and, as a result, the films could not undergo satisfactory heat treatment.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method for producing gas barrier film, in which the coating film produced is influenced little by moisture and has excellent, oxygen gas barrier properties.
We, the present inventors have assiduously studied various methods for producing gas barrier films which are influenced little by moisture and have excellent oxygen gas barrier properties and which can therefore be substitutes for PVDC films, and, as a result, have found a method for producing excellent gas barrier films. The method we have found comprises extruding a polyvinyl alcohol which contains not more than 19 mol % of &agr;-olefin units with not more than 4 carbon atoms onto a substrate film, then stretching it at least 3-fold and subjecting it to heat treatment at a temperature not lower than 120° C. Based on this finding, we have completed the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Polyvinyl alcohol to be used for producing the gas barrier film of the invention is described in detail.
Polyvinyl alcohol for use in the invention contains not more than 19 mol % of &agr;-olefin units with not more than 4 carbon atoms. As the &agr;-olefins, preferred are ethylene and propylene, especially ethylene.
In polyvinyl alcohol for use in the invention, the degree of modification with &agr;-olefin units with not more than 4 carbon atoms is not more than 19 mol %, preferably falls between 3 and 19 mol %, more preferably between 5 and 18 mol %. If it is larger than 19 mol %, the melting point of the polyvinyl alcohol will be low, and the polymer crystal will partly fuse while heated after coated, whereby the barrier capabilities of the polymer films produced will be poor. Further in case it is not less than 3 mol %, the polymer films could exhibit good barrier capabilities in high-humidity conditions at a relative humidity of around 70% or more, and, in addition, they could be stretched to a satisfactory degree of orientation.
The degree of hydrolysis of polyvinyl alcohol for use in the invention preferably falls between 90 and 99.99 mol %, more preferably between 97 and 99.95 mol %, even more preferably between 99 and 99.90 mol %. Polyvinyl alcohol having a degree of hydrolysis of larger than 99.99 mol % is difficult to produce on an industrial scale. On the other hand, polyvinyl alcohol having a degree of hydrolysis of smaller than 90 mol % will be disadvantageous as its films could not exhibit satisfactory gas barrier capabilities.
The degree of polymerization of polyvinyl alcohol for use in the invention is preferably not larger than 2000, more preferably not larger than 1750, even more preferably between 100 and 1500, still more preferably between 200 and 1400. Polyvinyl alcohol having a degree of polymerization of higher than 2000 is not preferred, since its viscosity is too high and its coatability is poor. Polyvinyl alcohol having a degree of polymerization of lower than 100 is also not preferred, since the strength of its films is low and pin holes are often formed in the films. The degree of polymerization of polyvinyl alcohol is measured according to JIS K6726.
Polyvinyl alcohol for use in the invention contains not more than 19 mol % of &agr;-olefin units with not more than 4 carbon atoms. Preferably, it further contains silyl group-containing units in an amount of not larger than 5 mol %. The silyl group-containing unit content of polyvinyl alcohol may be at most 5 mol %, but more preferably at most 3 mol %, even more preferably between 0.1 mol % and 1.5 mol %. If the silyl group-containing unit content is larger than 5 mol %, the stability of polyvinyl alcohol in aqueous solution will be extremely low. For introducing silyl groups into polyvinyl alcohol, preferably employed is a method of using a vinylalkoxysilane (e.g., vinyltrimethoxysilane, vinyltriethoxysilane, etc.) as the comonomer component.
Polyvinyl alcohol for use in the invention may be prepared in any known method.
It is desirable that polyvinyl alcohol for use in the invention contains sodium acetate in an amount of from 0.01 to 2% by weight. The sodium acetate content of polyvinyl alcohol preferably falls between 0.02 and 1% by weight, more preferably between 0.03 and 0.5% by weight, even more preferably between 0.03 and 0.45% by weight. If the sodium acetate content oversteps the range falling between 0.01 and 2% by weig
Shiraga Sadahiko
Takada Shigeki
Eashoo Mark
Kuraray Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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