Composite metal-coated polyester films with barrier properties

Details Composite metal-coated polyester films with barrier properties Composite metal-coated polyester films with barrier properties

2000-03-16

2002-08-20

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Composite

Of metal

C428S215000, C428S480000, C428S910000, C427S404000, C427S407100, C427S412500

Reexamination Certificate

active

06436544

ABSTRACT:

The present invention is concerned with metallized composite films based on polyester and having good barrier properties against gases.
Polyester films, in particular polyethylene terephthalate, are largely used for packaging, owing to their numerous advantages, such as their mechanical properties, their transparency, their lack of toxicity, and because they have no taste or odour.
However, their properties as barriers for gases can limit their use in applications requiring a high level of protection for the packaged items, against the action of outside gases, in particular against the action of the oxygen of air, or conversely, can not maintain a constant gas composition within the packaging.
In order to overcome this drawback, it was proposed in the GB patent 1 126 952 to deposit on a polymer film a solution of a polyvinyl alcohol, in order to create a layer of polyvinyl alcohol, the latter having good gas-barrier properties. In order to enable a good adhesion of the polyvinyl alcohol layer on films of acetate cellulose, polycarbonate or polyethylene terephthalate, an intermediate layer of a polyurethane adhesive is provided between the film base and the polyvinyl alcohol layer.
Patent EP-A-0 254 468 also describes a composite film comprising a film base of a synthetic thermoplastic polymer, such as a polyamide, a polyethylene, a polypropylene or a polyester, having two coatings on the same face of the film base; the first coating being adjacent to the film base and being comprised of a urethane primer applied in a solvent, which enables, when dry, a dispersion in an aqueous solution of a polyvinyl alcohol to wet the primer coating, the second coating being placed on the dried surface of the first coating and including a material based on a polyvinyl alcohol acting as a barrier to gases.
The present invention is concerned with metallized polyester films having good barrier properties to gases, owing to a coating including a polyvinyl alcohol placed directly on the surface of the polyester film without the presence of an intermediate adhesive layer, while providing good adhesion properties between the polyester film base and the coating.
More precisely, it is concerned with a metallized composite film stretched biaxially, based on a polyester and having improved barrier properties to gases, characterised in that it includes a polyester film base of 5 &mgr;m to 50 &mgr;m, coated on one of its two faces with a layer including a polyvinyl alcohol which has an average degree of polymerization in number equal to or in excess of 350 and a water-dispersible copolyester with sulphonyloxy residues, the layer including the polyvinyl alcohol and the water-dispersible copolyester being in turn coated with a metal layer, said composite film exhibiting a permeability to oxygen measured at 23° C. under a relative humidity of 50%, equal to or lesser than 0.4 cm
3
/m
2
/24 h.
The polyester forming the film base can be selected from those polyesters which are normally used for obtaining semi-crystalline bi-oriented films. These are film-forming linear polyesters, which crystallise when oriented and which are obtained in the usual manner from one or several aromatic dicarboxylic acids or their derivatives (for example, esters of lower aliphatic alcohols or halogenides) and from one or several aliphatic glycols. As examples of aromatic acids, one can mention phthalic acid, terephthalic acid, isophthalic acid, naphthalene 2,5-dicarboxylic acid and naphthalene 2,6-dicarboxylic acid. These acids can be associated to a minor amount of one or several aliphatic or cycloaliphatic dicarboxylic acids, such as adipic acid, azelaic acid and hexahydroterephthalic acid. As non limiting examples of aliphatic diols, one can mention ethylene glycol, 1,3-propanediol and 1,4-butanediol. These diols can be associated with a minor amount of one or several aliphatic diols with a higher carbon content (neopentylglycol for example) or cycloaliphatic diols (cyclohexanedimethanol). Preferably, the film-forming crystallisable polyesters are polyterephthalates or polynaphthalene dicarboxylates of alkylene diols and, in particular polyterephthalate of ethylene glycol (PET), of 1,4-butanediol or copolyesters including at least 80% in moles ethylene glycol terephthalate residues. Advantageously, the polyester is a polyterephthalate of ethylene glycol having an intrinsic viscosity measured at 25° C. in ortho-chlorophenol comprised between 0.6 and 0.75 dl/g.
The polyester forming the film base must be selected in such a manner that its temperature at the beginning of the melting be higher than the temperature at which the bi-stretched film comprising the layer of the polyvinyl alcohol and of the water-dispersible copolyester is heated during its preparation.
When the thickness of the film including the polyvinyl alcohol and a water-soluble copolyester with sulphonyloxy residues is lesser than 0.6 &mgr;m, the average roughness Rz of the film base (such as defined in standard DIN 4768) is lesser than or equal to 0.40 &mgr;m on the face of the film carrying the layer of the polyvinyl alcohol and of the water-dispersible copolyester and this face exhibits, on the average, not more than 20 peaks having a height equal to or in excess of 1 micrometre and not more than 150 peaks having a height comprised between 0.4 and 1 micrometre, per square millimetre.
The peak height distribution mentioned above for defining the surface topography of the polyester film base, can be determined, in a known manner, and in particular through visual inspection with an interferential microscope which makes it possible to count the number of interference rings for a light having a known wavelength. The interferometers most frequently used are the NOMARSKI, the MIRAU and the MICHELSON interferometers.
Concerning the industrial implementation of these processes for the preparation of films according to the invention, in which the speed of the machine in which the films are processed is generally in excess of 100 meters per minute, it is particularly preferable that the face of the film base carrying the layer of polyvinyl alcohol and of the water-dispersible copolyester have no more than an average of 20 peaks having a height equal to or in excess of 1 micrometre and no more than 100 peaks having a height comprised between 0.4 and 1 micrometre, per square millimetre.
The polyvinyl alcohol entering into the composition of the layer of the polyvinyl alcohol and of the water-dispersible copolyester can be any polyvinyl alcohol. In practice, the polyvinyl alcohols generally available commercially have a degree of hydrolysis of about 73% to more than 99% (which means that they have a content of from about 73% to more than 99% of vinyl alcohol residues in their formula). Except when stated otherwise, this does not exclude the use of polyvinyl alcohols with a degree of hydrolysis lesser than 73%, when available. In the present description the terms <<degree of hydrolysis>>, <<degree of saponification>> and <<content in vinyl alcohol residues>> are interchangeable.
The water-dispersible copolyesters with sulphonyloxy residues combined with the polyvinyl alcohol in the coating are copolyesters derived from at least one aromatic dicarboxylic acid and from at least one aliphatic diol, and carrying a plurality of sulphonyloxy groups of the general formula (I):
—(—SO
3
—)
n
M  (I)
in which n is equal to 1 or 2, M is a hydrogen atom, an alkali metal, an earth-alkali metal, a cationic ammonium or a cationic quaternary ammonium.
The term <<water-dispersible copolyester>> designates in the present application copolyesters which are soluble in water or copolyesters forming stable homogeneous dispersions.
The water-dispersible copolyesters carrying sulphonyloxy residues are already known, in particular from patent FR 1 602 002 and from patent EP-A-0 540 374, to which one can refer for more detailed information on their preparation and their composition. They are obtained through the polycondensation of one or of sev

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