Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
1998-05-14
2002-08-06
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S336000, C428S430000, C428S458000, C428S480000, C428S910000
Reexamination Certificate
active
06428882
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a biaxially oriented polyester film having a base layer of at least 80% by weight based on the total weight of the base layer of at least one thermoplastic polyester, and having a cover layer functioning as a barrier layer. The invention furthermore relates to the use of the film and to a process for its production.
2) Prior Art
In many food and drink packaging applications, there is demand for a high barrier effect with respect to gases, water vapor and flavors (this having the same significance as low transmission or low permeability). A well-known process for producing packaging of this type consists in high-vacuum aluminum metallizing of the plastics films used for this purpose. Another well-known process consists in coating the films with ceramic materials, such as SiO
x
, AlO
x
or MgO
x
. The barrier effect with respect to the above-mentioned substances depends essentially on the type of polymers in the film and the quality of the barrier layers applied. For example, metallized, biaxially oriented polyester films have a very high barrier effect with respect to gases, such as oxygen, and to flavors. Metallized, biaxially oriented polypropylene films are in turn very effective barriers with respect to water vapor.
The good barrier properties of metallized or ceramically coated films mean that they are used in particular for packaging foodstuffs and other consumable items, for which long storage or transport times create the risk that the packaged foodstuffs become spoilt or rancid or lose flavor if there is an inadequate barrier; examples are coffee, snacks containing fats (nuts, potato chips, etc.) and drinks containing carbon dioxide (in pouches). In addition, coated polyester films are suitable for heat-insulation of technical devices. For example, polyester films metallized with aluminum can be used to improve insulation in refrigerators. The insulation produced with these films works on the vacuum-flask principle, the insulation essentially consisting of two laminates (more than one layer of aluminum-metallized polyester films) into which, for example, sheets of foam are welded. The sheets of foam, welded in under reduced pressure, serve essentially as distance pieces between the two laminates. For the required reduced pressure to be maintained, the laminates must exhibit a very high oxygen barrier.
If polyester films to which a metallized aluminum layer has been applied are to be used as packaging material, then they are generally a constituent of a composite film (laminate) having more than one layer. Bags produced therefrom may be filled, for example, on a vertical tubular bag forming, filling and sealing machine (vffs). The bags are sealed on their inward side (i.e. on the side facing their contents), the sealable layer consisting generally of polyethylene or polypropylene. The composite film here typically has the following structure: polyester layer/aluminum layer/adhesive layer/sealable layer. If the laminate thickness is from about 50 to 150 &mgr;m, the thickness of the metal layer is only from 20 to 50 nm. Even a very thin aluminum layer is therefore sufficient to achieve adequate light protection and very good barrier properties.
The oxygen barrier or the oxygen transmission is generally measured not on the laminate or the packaging itself, but on the metallized or ceramically coated polyester film. To ensure good quality of the foodstuffs or other consumable items even after relatively long storage times, the oxygen transmission (identical with permeability) of the metallized film may not be greater than 2 cm
3
/m
2
bar d, (cubic centimeters, per square meter, bar, day) but in particular not greater than 1 cm
3
/m
2
bar d. In the future, the demands of the packaging industry will head toward still higher barriers with attempts to achieve permeability values of significantly less than 1.0 cm
3
/m
2
bar d.
There have been many reports of the relationship between oxygen barrier and aluminum-metallized film (substrate). A detailed review of the relevant prior art can be found, for example, in the dissertation by H. Utz (Munich Technical University, 1995: “Barriereeigenschaften aluminiumbedampfter Kunststoffolien [Barrier properties of aluminum-metallized plastics films]”).
There has so far not been sufficient investigation into the detailed basis for the barrier effect of the metallized film. Clearly, the substrate surface and the type of substrate polymer and its morphology are variables with significant influence. It is generally assumed that smooth surfaces result in better barrier properties (cf. Utz, page 38 ff). In this connection, von Weiss (cf. “Thin Solids Films” 204 (1991), p. 203-216) has shown that titanium dioxide particles introduced into a coating in different concentrations after aluminum-metallizing result in higher oxygen transmissions as TiO
2
proportion rises. The results of the studies by Utz are said to show that there is no direct relationship between the surface roughness of the PET film and the oxygen barrier.
It is also known that the oxygen barrier can be improved by selecting particular polymers for the film serving as substrate (G. Schricker: Metallisierte Kunststoffolien für höherwertige Verpackungen [Metallized plastics films for high-quality packaging], in ICI 5th International Metallising Symposium 1986, Cannes). Polyesters, for example, are particularly suitable, specifically those made from ethylene glycol and terephthalic acid or from ethylene glycol, terephthalic acid and naphthalene-2,6-dicarboxylic acid. Besides these, polyamides, ethylene-vinyl alcohol copolymers (EVOH) and polyvinylidene chloride may also be used with advantage. For example, U.S. Pat. No. 5,506,014 describes a copolyester made from (a) from 45 to 85 mol % of terephthalic acid, (b) from 10 to 40 mol % of naphthalenedicarboxylic acid and (c) from 5 to 15 mol % of a dicarboxylic acid having from 2 to 8 carbon atoms and (d) ethylene glycol; (the molar percentages are based on the total proportion of dicarboxylic acids). This polyester is claimed to have better barrier properties with respect to gases. It is used, inter alia, for producing bottles or containers, and also films of various thicknesses. A disadvantage of the raw materials mentioned is that they are significantly more expensive than polyethylene terephthalate (PET) or are unsuitable and/or not permitted by the authorities for use in food and drink packaging.
There are also known coextruded polyester films having a base layer of polyethylene terephthalate, of a polyester made from ethylene glycol, terephthalic acid and naphthalene-2,6-dicarboxylic acid and/or of a polyester made from ethylene glycol and napthalene-2,6-dicarboxylic acid, and having (at least) one cover layer of copolymers made from polyethylene terephthalate and polyethylene-2,6-naphthalate or from polyethylene-2,6-naphthalate homopolymers. At least one of the cover layers contains pigments. It is also known that such films can be coated or metallized.
EP-A 0 602 964 describes a polyester film for magnetic recording media (in particular magnetic tapes) which has more than one layer. It comprises one layer which is composed primarily of a polyester made from ethylene glycol and naphthalene-2,6-dicarboxylic acid, and at least one other layer which consists essentially of a copolyester built up from ethylene glycol units, diethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The film may be coated on one or both sides.
It is an object of the present invention to provide a coated, biaxially oriented polyester film which is distinguished by a high oxygen barrier; (less than 0.3 cm
3
of oxygen per square meter and per day diffusion through the film when it is exposed to air at a pressure of 1 bar). In its other properties, the film should be at least equivalent to the known packaging films of this type. In addition, it should be simple and cost-effective to produce. A further object was to improve the gloss of the coat
Bennett Cynthia
Hilkert Gottfried
Peiffer Herbert
Roth Werner
Mitsubishi Polyester Film GmbH
Thibodeau Paul
Zacharia Ramsey
LandOfFree
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