Multilayer biaxially oriented polyester film, and the use...

Details Multilayer biaxially oriented polyester film, and the use... Multilayer biaxially oriented polyester film, and the use...

1998-09-22

2001-09-18

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Continuous and nonuniform or irregular surface on layer or...

C428S336000, C428S339000, C428S458000, C428S480000, C428S910000, C264S172190

Reexamination Certificate

active

06291053

ABSTRACT:

The invention relates to a biaxially oriented polyester film which has at least three layers and has better optical properties than films of the prior art together with very good processing performance, and, after it is metalized or coated with oxidic materials, is a good barrier to oxygen, and which is built up on each side from at least one base layer B and outer layers A and C applied to this base layer, Where these outer layers have a defined number of elevations of a defined height and diameter. The invention further relates to the use of the film and to a process for the production thereof.
BACKGROUND OF THE INVENTION
1. 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.
For most applications it is also desirable, e.g. for reasons of effective presentation, to improve the optical properties of the polyester films, in particular the gloss and the film haze. With this, the other performance characteristics of polyester films, in particular its good processability and its good barrier properties, should be retained at least or likewise improved.
2. Description of the Related Art
The prior art demonstrates how the optical properties, in particular the gloss and the haze, of biaxially oriented polyester films can be improved.
EP0 514 129 describes a transparent multilayer film which comprises a substrate of a primary layer of polymer material which, at least on one of its surfaces, has a secondary layer of polymer material which has certain concentrations and certain size distributions of glass beads and silica particles. The secondary layer can be arranged on one or on both sides of the primary layer substrate. The haze and processing properties are improved with the film, but no improvement in the gloss and the barrier properties of the film is provided in the text. There is also no indication of any kind in the text as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
EP 0 604 057 describes a transparent multilayer film which comprises a substrate of a primary layer of polymer material which is essentially free from fillers and which, at least on one of its surfaces, has a secondary layer of polymer material which contains-, as filler, in a concentration of from 100 to 1000 ppm, silicone resin having an average particle diameter of from 1.5 to 12.5 &mgr;m. A disadvantage of the silicone particles is that they are comparatively expensive and do not provide an acceptable solution for the packaging market. In addition, films which are provided with pigments of this type tend to telescope more easily during reeling. In this text there is likewise no indication of any type as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
In many foodstuff packaging applications, there is demand for 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. Other well known processes consist in 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 luxury foods, for which long storage or transport times create the risk that the packaged foodstuffs become spoilt, 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).
If polyester films metalized with an aluminum layer or having an applied oxidic layer 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 generally of polyethylene or polypropylene. The composite film here typically has the following structure: polyester layer/aluminum or oxide layer/adhesive layer/heat-sealable layer. If the laminate thickness is from about 50 to 150 &mgr;m, 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 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 foodstuffs or luxury foods even after relatively long storage times, the oxygen transmission (identical with 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 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 for metalized or oxidically coated films.
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect of metalized or oxidically coated polyester films nor of how this may be decisively improved. Variables which are clearly important are the area of the substrate and the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties.
In this connection, Weiss et al., in “Thin Solids Films” 204 (1991), p. 203-216, studied the influence of the surface roughness of a substrate layer on the permeability. For this, polyester films were coated with lacquer which contained various concentrations of titanium dioxide particles. In the experiments described, the concentrations of titanium dioxide particles in the lacquer varied from 2 to 20% by weight. Using this method, the surface roughness R
a
of the coated substrate surface could be varied from 43 nm (unlacquered and lacquered film, without titanium dioxide) to 124 nm. In his experiments, increasing roughness (increasing proportion of TiO
2
) of the lacquered surface resulted in markedly higher oxygen transmissions after metalizing with aluminum. However, the largest step increase in oxygen transmission was seen when the lacquered film (0% by weight) was compared with the unlacquered film, although the surface roughness of the substrate was the same in both cases. The lacquering alone of 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 (lacquered 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 seen clearly.
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 Kunststoffo

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