Stock material or miscellaneous articles – Hollow or container type article – Flexible food casing
Reexamination Certificate
1999-06-03
2001-10-16
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Flexible food casing
C428S035300, C428S035400, C428S036700, C428S349000, C428S474400, C428S475800, C428S476100, C428S518000, C260S001000, C524S503000
Reexamination Certificate
active
06303199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a coating agent for a film, a process for coating the coating agent on film, and the laminate film produced thereby. The laminate film can be used as a wrapping film, for example, for wrapping food.
2. Description of the Related Art
Various types of oxygen gas barrier films, and wrapping materials containing oxygen gas barrier films, are known. Aluminum (Al) foil has perfect oxygen gas barrier properties, but when used by itself, it often has pin holes. Therefore, Al foil is used as the interlayer of laminate films. However, since laminate films containing an Al foil layer are not transparent, it is difficult to see the contents of packages made with such films. Furthermore, packages made from films containing an Al foil layer cannot be used in applications in which it is desirable to use metal detectors to identify metal contamination of the contents. Finally, films containing an Al foil layer generate Al residues after incineration.
Other known oxygen gas barrier films are polyvinylidene chloride (PVDC) films and PVDC-coated films. PVDC absorbs little moisture and exhibits good gas barrier properties 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 (OPP), biaxially-oriented nylon (ON), biaxially-oriented polyethylene terephthalate (OPET), and cellophane, etc. However, PVDC-laminated films are problematic in that they generate hydrogen chloride gas when incinerated as waste.
Completely hydrolyzed, non-modified polyvinyl alcohol (PVA) films are also known as oxygen gas barrier films. PVA films exhibit excellent oxygen gas barrier properties under low-humidity conditions. In general, however, they absorb moisture, and their oxygen gas barrier properties are greatly degraded in high-humidity conditions in which the relative humidity is around 70% or higher. Another problem with PVA films is that their adhesion to OPP or OPET substrate films is poor. Some proposals have been made heretofore for attenuating the moisture absorption of PVA. For example, an ethylene-vinyl alcohol copolymer (EVOH) having an ethylene comonomer content of at least 20 mol % can be used, or both surfaces of PVA films can be coated with PDVC. However, EVOH coating solutions must contain organic solvents, and the organic solvents used are detrimental to the working environment. Alternatively, PVDC-coated films are also problematic in that they generate hydrogen chloride gas when incinerated.
The following techniques are known, relating to the present invention.
(1) JP-A 4-331246 discloses gas barrier materials prepared by coating moisture-proof plastics (e.g., polyethylene, polypropylene) with a gas barrier substance capable of dissolving in water or solvents (e.g., polyvinyl alcohols having a degree of hydrolysis of at least 95 mol %, ethylene-vinyl alcohol copolymer having an ethylene content of from 28 to 51 mol %).
(2) JP-A 6-32924 discloses oxygen barrier films prepared by coating biaxially-oriented polypropylene films or biaxially-oriented polyester films with an aqueous solution of a polyvinyl alcohol having a degree of polymerization of from 300 to 1000 and a degree of hydrolysis of from 95 to 99.5 mol %, thereby forming thereon a coating layer having a thickness of from 1.7 to 4 &mgr;m.
(3) U.S. Pat. No. 4,927,689 discloses gas barrier structures prepared by coating a thermoplastic resin substrate with a first coating layer of a urethane primer of from 0.3 to 3.0 g/m
2
and a second coating layer of a polyvinyl alcohol of at most about 2.0 g/m
2
.
(4) The present applicant's own JP-A 8-188624 discloses self-sustaining films (preferred thickness: from 10 to 100 &mgr;m) of a modified polyvinyl alcohol that contains from 1 to 20 mol % of &agr;-olefin units with up to 4 carbon atoms and has a degree of polymerization of at least 700.
(5) The present applicant's own JP-A 9-241999 discloses a coating agent for paper that comprises a modified polyvinyl alcohol containing from 3 to 15 mol % of ethylene units.
SUMMARY OF THE INVENTION
We have found that a water-soluble polyvinyl alcohol containing from 3 to 19 mol % of &agr;-olefin units, each &agr;-olefin unit containing up to 4 carbon atoms can be coated onto a film, forming a laminate having excellent oxygen gas barrier properties. Specifically, laminates comprising a layer of a water-soluble polyvinyl alcohol that contains from 3 to 19 mol % of &agr;-olefin units, each &agr;-olefin unit containing up to 4 carbon atoms, and a layer of a polyolefin, a polyester or a polyamide, have excellent oxygen gas barrier properties.
We have also found a method for producing the laminate, which comprises applying an substantially aqueous solution of a water-soluble polyvinyl alcohol that contains from 3 to 19 mol % of &agr;-olefin units, each &agr;-olefin unit containing up to 4 carbon atoms, to the surface of a polyolefin, a polyester or a polyamide film, wherein the solution contains at most 50% by weight of a lower aliphatic alcohol. Further, the coated film can be dried at a drying speed that falls between 2 and 2000 g/m
2
·min.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polyvinyl alcohol (PVA) used in the invention is a water-soluble PVA containing from 3 to 19 mol % of &agr;-olefin units, each &agr;-olefin unit containing up to 4 carbon atoms.
By water-soluble PVA, we mean that the PVA dissolves in an aqueous solution containing at most 50% by weight of a lower aliphatic alcohol. Ethylene and propylene are the preferred &agr;-olefins units.
The content of &agr;-olefin units in the water-soluble PVA can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or 19 mol %, inclusive of all values and subranges therebetween. Preferably, the &agr;-olefin content of the water-soluble PVA is between 5 and 16 mol %, more preferably between 7 and 14 mol %. If the &agr;-olefin content is smaller than 3 mol %, PVA films may not exhibit good barrier properties in high-humidity conditions. If the &agr;-olefin content is more than 19 mol %, the water solubility of PVA is too low, making it difficult to coat the PVA from aqueous solution.
The degree of hydrolysis of the water-soluble PVA of this invention may be 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 99.99 mol %, inclusive of all values and subranges therebetween. Preferably the degree of hydrolysis falls between 90 and 99.99 mol %, more preferably between 97 and 99.95 mol %, even more preferably between 99 and 99.90 mol %. PVA having a degree of hydrolysis of larger than 99.99 mol % is difficult to produce on an industrial scale. Furthermore, a thin solid film tends to form on the top of aqueous solutions of PVA in which the degree of hydrolysis is greater than 99.99 mol %. Films formed from PVA having a degree of hydrolysis less than 90 mol % do not exhibit satisfactory oxygen gas barrier properties.
The degree of polymerization of the PVA may be 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300,1400,1500, 1600, 1700, 1800, 1900, or 2000, inclusive of all values and subranges therebetween. The degree of polymerization is preferably up to 2000, more preferably up to 1000, even more preferably up to 600, still more preferably between 100 and 400. The viscosity of aqueous solutions of PVA having a degree of polymerization of higher than 2000 is too high, giving the solution poor coating properties. If the degree of polymerization is lower than 100, the strength of the resulting PVA coating is low and pin holes are often formed. The degree of polymerization of PVA is measured according to JIS K6726.
The PVA may further contain 0.1, 1, 2, 3, 4, or 5 mol %, inclusive of all values and subranges therebetween, of silyl group-containing units. Preferably, the PVA contains up to 5 mol %, more preferably up to 3 mol %, even more preferably between 0.1 mol % and 1.5 mol % silyl group-containing units. If the silyl group-containing unit content is larger than 5 mol
Shiraga Sadahiko
Takada Shigeki
Kiliman Leszek
Kuraray Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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