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
2000-05-26
2002-09-03
Michl, Paul R. (Department: 1714)
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
active
06444751
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for commercially advantageously producing vinyl alcohol polymer compositions while suppressing generation of their property unevenness, the compositions having excellent gas-barrier properties, in particular excellent gas-barrier properties under high-temperature and high-humidity conditions, and being capable of maintaining the excellent gas-barrier properties even after being repeatedly bent.
BACKGROUND ART
Materials for packaging foods and other various items are often required to have good gas-barrier properties, in particular good oxygen gas-barrier property, in order to prevent the contents from being degraded by, for example, oxidation with oxygen. Especially on packaging of foods, presence of oxygen causes microorganisms to proliferate and rot the contents. Conventional packaging materials for foods are therefore provided with a gas-barrier layer to protect the contents against penetration of oxygen or the like.
Examples of such a gas-barrier layer are metal foils and layers on which a metal or metal compound has been deposited. In general, aluminum foils and layers on which aluminum or silica has been deposited are used for this purpose. However, use of these metals causes the contents to become invisible and the packaging materials to become difficult to dispose after use.
Vinyl alcohol polymers having good gas-barrier properties, such as polyvinyl alcohol and ethylene-vinyl alcohol copolymer are sometimes used as the gas-barrier layer. These vinyl alcohol polymers have the advantages of being transparent and causing less problem on disposal after use, so that they have been applied to wider range of uses.
These vinyl alcohol polymers show good gas-barrier properties because they crystallize due to hydrogen bonding of hydroxyl groups present in their molecules. They exhibit therefore good gas-barrier properties when they are dry. However, when they have absorbed moisture such as water vapor present in the surrounding atmosphere, which loosen the hydrogen bonds, they tend to become poor in gas-barrier properties. As a result, it is difficult to provide conventional vinyl alcohol polymers, such as polyvinyl alcohol, with satisfactory gas-barrier properties under high-temperature and high-humidity conditions.
A method for decreasing the moisture-absorbing property of vinyl alcohol polymers comprises copolymerizing an olefin such as ethylene, yielding ethylene-vinyl alcohol copolymer. However, this kind of copolymerization, which decreases the content of vinyl alcohol units, tends to impair the inherent gas-barrier properties. Consequently, this method cannot provide a vinyl alcohol polymer being capable of exerting good gas-barrier properties under both dry condition and high-temperature and high-humidity conditions.
A process for preparing an organic/inorganic complex comprising an organic polymer and a metal oxide dispersed relatively finely in the polymer has been proposed. This process comprises, making use of the what is known as sol-gel process, polycondensing a metal alkoxide such as a silicon alkoxide in the presence of an organic polymer.
For example, Japanese Patent Application Laid-open Nos. 278968/1997 and 99390/1996 disclose, in order to obtain vinyl alcohol polymer-based materials possessing improved water resistance, a process which comprises polycondensing a metal alkoxide in the presence of a vinyl alcohol polymer or modified products thereof, to obtain compositions comprising the vinyl alcohol polymer and a metal oxide. This process however causes, during the polycondensation, a decrease in phase-separated state (i.e. increase in the phase-separation size). This is attributable to the high crystallinity of the vinyl alcohol polymer or modified products thereof used. In the meantime, several reports have proposed a process capable of producing compositions (organic/inorganic complexes) with improved phase-separation state (i.e. with finer phase-separation size) of polyvinyl alcohol and a metal oxide (silica). This process comprises the steps of dissolving tetramethoxysilane and polyvinyl acetate in a ratio by weight of 2:1 or 5:1, together with hydrochloric acid as a catalyst, in methanol to prepare a solution and, while evaporating low boiling substances containing the methanol from the solution, conducting both polycondensation of the tetramethoxysilane and saponification of the polyvinyl acetate in parallel with each other in the same reaction zone. See for example Polymer. Prep. Jpn. 45(5) p.907 (1996) and Appl. Organometal. Chem. 12 p.755 (1998).
According to the study made by the present inventors, it has been found that the compositions comprising a vinyl alcohol polymer and a metal oxide obtained by the above process using the vinyl alcohol polymer or its modified products as a starting material are not only insufficient in the fineness of phase-separation size but have a large unevenness in the gas-barrier properties, which vary depending on the place of the prepared composition. The compositions therefore cause a high out-of-grade ratio, when used as films for packaging foods or as gas-barrier materials such as gas-barrier layers for such films. In addition, it has also been found that the gas-barrier properties of these compositions cannot be said to have sufficient moisture resistance.
The present inventors' study has also revealed the following fact. That is, the above compositions of polyvinyl alcohol and a metal oxide (silica) obtained by the process comprising using tetramethoxysilane and polyvinyl acetate in a ratio by weight of 2:1 or 5:1 (i.e. the molar ratio between the silicon atom present in the tetramethoxysilane and the acetyl group in the polyvinyl acetate of 113:100 or 283:100) exhibit, when exposed to repeated bending, impaired gas-barrier properties, perhaps because of formation of fine gaps. The compositions are thus unusable as films for packaging foods or as gas-barrier layers for such films.
Accordingly, an object of the present invention is to provide a commercially advantageous process for producing a vinyl alcohol composition containing a metal oxide with little property unevenness, said composition being capable of maintaining its excellent gas-barrier properties at high levels even under high-temperature and high-humidity conditions and after being repeatedly bent.
DISCLOSURE OF THE INVENTION
As a result of an intensive study to solve the above problems, the present inventors found that the above object can be achieved by providing a process which comprises using a solution comprising a vinyl carboxylate polymer and a metal alkoxide in an organic solvent and, while removing low boiling substances, conducting both saponification of said vinyl carboxylate polymer and polycondensation of said metal alkoxide in parallel with each other, said solution containing a specific amount based on the vinyl carboxylate polymer of said metal alkoxide. The present invention was completed based on this finding.
The present invention thus provides a process for producing vinyl alcohol polymer compositions comprising a metal oxide and a vinyl alcohol polymer, which comprises using a solution comprising a metal alkoxide or oligomers thereof, a vinyl carboxylate polymer and an organic solvent, and, while removing low boiling substances containing the organic solvent, conducting both polycondensation of said metal alkoxide or oligomers thereof and saponification of said vinyl carboxylate polymer in parallel with each other and in the same reaction zone, said solution containing said metal alkoxide or oligomers thereof in a ratio of 0.01 to 100 moles of the metal atom contained in said metal alkoxide or oligomers thereof based on 100 moles of the acyl group originating from the vinyl carboxylate unit present in said vinyl carboxylate polymer.
BEST MODES FOR CARRYING OUT THE INVENTION
The present invention is now described in detail.
The process for producing vinyl alcohol polymer compositions according to the present invention is based on the known process (hereinafter someti
Chujo Yoshiki
Itoh Kanenori
Komiya Yukiatsu
Tamaki Ryo
Kuraray Co. Ltd.
Michl Paul R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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