Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1999-04-21
2002-04-23
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S336000, C428S339000, C428S457000, C428S458000, C428S469000, C428S480000, C428S910000, C264S288400, C264S290200
Reexamination Certificate
active
06376042
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
Biaxially oriented polyester films are used in packaging and in industry primarily where there is a need for their advantageous properties, i.e. good optical properties, high mechanical strengths, good barrier effect in particular against gases, good dimensional stability when heated and excellent layflat.
In food packaging applications, packaging technology requires a high barrier effect against gases, steam 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 metalizing of the plastic films used for this purpose. Other well-known processes include coating the films with oxidic materials (e.g. SiO
x
or Al
x
O
y
) or water glass. Essentially, the coatings used are transparent.
The barrier effect against the substances mentioned above depends essentially on the type of the polymers in the film and the quality of the barrier layers applied. Thus, a very high barrier effect against gases, such as oxygen and flavors, is achieved in metalized, biaxially oriented polyester films. A barrier effect against steam is achieved in metalized, biaxially oriented polypropylene films.
The good barrier properties of metalized or oxidically 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 foods lose flavor and become spoiled rancid if there is an inadequate barrier. Examples of such foodstuffs and consumable items are coffee, snacks containing fats (nuts, potato chips, etc.), and drinks containing carbon dioxide (in pouches).
If polyester films metalized with an aluminum layer or having applied oxidic layers are used as packaging material, they are generally a constituent of a multilayer composite film (laminate). Bags produced therefrom can be filled, for example, on a vertical tubular bag forming, filling and sealing machine. The bags are heat-sealed on their inward side (i.e. on the side facing the contents), the heat-sealable layer consisting, for example, of polyethylene. The composite film here typically has the following structure: polyester layer/aluminum or oxide layer/adhesive layer/heat-sealable layer. The thickness of the metal or oxide layer is only from 10 to 80 nm. Even this very thin functional layer is sufficiently effective to achieve adequate protection from light and very good barrier properties.
The gas or oxygen barrier or the oxygen transmission is generally measured not on the laminate or the packaging itself, but on the metalized polyester film. To ensure good quality of the foods or other consumable items even after relatively long storage times, the oxygen transmission (identical to permeability) of the metalized film may not be greater than 2 cm
3
/(m
2
.bar.d), but in particular not greater than 1.5 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 less than 1.0 cm
3
/(m
2
.bar.d) for metalized films.
DESCRIPTION OF THE RELATED ARTS
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect of metalized or oxidically coated biaxially oriented polyester films nor of how this may be decisively improved. Variables which are clearly important are the substrate surface, and the substrate polymer and its morphology.
Weiss et al., in “Thin Solids Films” 204 (1991), p. 203-216, studied the influence of the roughness of a substrate layer on permeability. For this study, polyester films were coated with lacquer which contained various concentrations of titanium dioxide particles. In the experiments described, the concentration of titanium dioxide particles in the coating varied from 2 to 20% by weight. Using this method, the roughness R
a
of the coated substrate surface could be varied from 43 nm (uncoated and coated film, without titanium dioxide) to 124 nm. In his experiments, increasing roughness (increasing proportion of TiO
2
) of the coated surface resulted in markedly higher oxygen transmissions after metalizing with aluminum. However, the largest step increase in oxygen transmission was seen when the coated film (0% by weight) was compared with the uncoated film, although the surface roughness of the substrate surface was the same in both cases. Merely coating the film gave a deterioration in the barrier from about 0.43 cm
3
/(m
2
.d.bar) (plain film) to about 19 cm
3
/(m
2
.d.bar) (coated film). A further uncertainty concerning the transferability of this work to commercial products is created by the fact that the aluminum layer was applied using a laboratory evaporator. When compared with an industrial metalizer, this method achieves essentially low permeability values, and the influence of the substrate surface on the barrier properties cannot be clearly seen.
Other detailed results of studies on the influence of the substrate surface of polyester films on their barrier properties can be found in the dissertation by H. Utz (Technische Universität München 1995: “Barriereeigenschaften Aluminiumbedampfter Kunststoffolien” [Barrier Properties of Aluminum-Metalized Plastic Films]).
EP-A-0 490 665 A1 describes a single-layer biaxially oriented polyester film for magnetic recording tape; the film contains
a) from 0.05 to 1.0% by weight of &ohgr;-alumina having an average particle diameter in the range from 0.02 to 0.3 &mgr;m, and
b) from 0.01 to 1.5% by weight of inert particles of a type other than &ohgr;-alumina and having an average particle diameter in the range from 0.1 to 1.5 &mgr;m, these particles being larger than the &ohgr;-alumina particles.
The surface of this film is formed by a large number of elevations/protrusions which are described by the relationship
−11.4×+4<log
y<−
10.0×+5 where
y>
30, ×>0.05 &mgr;m.
In this equation, ×(&mgr;m) is a height above a standard level and y is the number of elevations (number/mm
2
) if the elevations are sectioned at a height of ×. The distribution of the elevations is measured with a standard apparatus for measuring roughness. This text gives no information concerning improvement of the barrier properties, the gloss or the haze.
The prior art also discloses films which have different roughnesses on their two surfaces (dual surface). These films are suitable in particular for magnetic recording media and essentially have different topographies (e.g. surface A smooth, surface B rough). These texts generally provide means of improving the processing properties of the film but not its barrier properties.
EP-B-0 088 635 describes a coextruded biaxially oriented polyester film having at least two layers, of which a layer A consists of thermoplastic resin and a layer B comprises thermoplastic resin and fine particles. The roughness R
a
of the outer surface of the layer A in the film is less than 5 nm and the outer surface of the layer B is either
a surface having a roughness R
a
of from 5 to 40 nm and a large number of depressions and a large number of protrusions which are arranged in a particular arrangement or
a surface which has protrusions formed on a level area and which is covered by a layer C, which consists of a lubricant and has a roughness R
a
of from 5 to 40 nm.
A disadvantage of film surface A is that it blocks, both with itself and with certain other surfaces (e.g. rubber rolls). The film cannot be processed cost-effectively; in particular during metalizing in vacuo, the film tends to tear because of its high tendency to block, and this can cause great cost problems. The film is unsuitable for the purposes of the object to be achieved.
It is also known that the gas or oxygen barrier can be improved by selecting particular polymers for the film serving as substrate (Schrikker, G.: Metallisierte Kunststoffolien für Höbherwertige Verpackungen [Metalized Plastic films for High
Davis Richard Lee
Mahl Hans
Peiffer Herbert
Chen Vivian
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Mitsubishi Polyester Film GmbH
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