Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1999-07-30
2001-08-28
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S035200, C428S035300, C428S461000, C428S519000, C428S910000, C426S106000, C426S126000, C426S127000, C264S173150, C264S173190, C156S244240
Reexamination Certificate
active
06280833
ABSTRACT:
The invention relates to a biaxially oriented polypropylene film which has good optical properties and which, after it has been metallized or has been coated with oxidic materials, has a high oxygen barrier value, and whose structure has at least one base layer B and at least one layer A applied to this base layer, where this layer A has a specified number of elevations of specified height and specified diameter. The invention further relates to the use of the film and to a process for its production.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Biaxially oriented polypropylene films are used in packaging and in industrial sectors, especially where there is a need for their advantageous properties, i.e. good optical properties, high mechanical strengths, good barrier action, in particular with respect to gases, good dimensional stability when heated and excellent layflat.
For most applications it is moreover desirable, e.g. for reasons of promotional effectiveness, to improve the optical properties of the polypropylene film, in particular its gloss and haze, while retaining its good processability. It is likewise desirable to improve the barrier properties of polypropylene films, for example to gain new applications.
2. Description of Related Arts
The prior art describes how the optical properties, in particular the gloss and haze, of biaxially oriented polypropylene film can be improved.
DE-A 43 06 155 describes a non-sealable, transparent, oriented polyolefin film having more than one layer, with a base layer and with at least one non-sealable outer layer made from polypropylene, where the outer layer comprises silicon particles which have had an organic aftertreatment and have an average particle diameter of from 2 to 6 &mgr;m.
The haze and processing properties of the film are improved, but the text has no teaching, concerning improvement of the barrier properties of the film. Nor does it give any kind of indication as to how the topography of a film of this type should be adjusted for simultaneous improvement of gloss and oxygen barrier.
In packaging applications for food and drink, high barrier effectiveness against gases, water vapor and flavors is generally demanded (this having the same significance as low permeation). A well known process for producing packaging of this type consists in high-vacuum aluminum metallizing of the plastic films used for this purpose. Other well known processes consist in coating the films with oxidic materials (e.g. SiO
x
or Al
x
O
y
) or with water glass. Essentially, the coatings used are transparent.
The effectiveness of the barrier 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, very high barrier effectiveness against gases, such as oxygen and flavors, is achieved in metallized, biaxially oriented polyester films. An effective barrier against water vapor is best achieved in metallized, biaxially oriented polypropylene films. There are other applications in which both a very effective water vapor barrier and an acceptable oxygen barrier are desirable.
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect in metallized or oxidically coated polypropylene films nor of how this may be decisively improved. Variables which are clearly important are the substrate surface, the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties.
The dissertation by H. Utz (Technische Universität München, 1995: “Barriereeigenschaften Aluminiumbedampfter Kunststoffolien” [Barrier Properties of Aluminum-Metallized Plastic Films]) gives detailed results of investigations on the influence of substrate surface on barrier properties in various plastic films.
DESCRIPTION OF THE INVENTION
It was an object of the present invention to provide a coextruded, biaxially oriented polypropylene film which has very good optical properties, i.e. reduced haze and in particular high gloss, at least on one surface of the film (called film surface A below). A surface A of this type would be, for example, particularly advantageous for printing or metallizing. The high gloss of the film is transferred to the print or to the metallic layer and thus gives the packaging a particularly lustrous appearance.
After metallization or after coating with oxidic materials on surface A the film should also have an oxygen barrier value better than in the prior art, and it should be easy to produce and to process. In summary, the object is to provide a film with the following combination of features:
high gloss, in particular on film surface A
low haze and
low oxygen permeation of the film after metallization or after coating with oxidic materials on film surface A.
The gloss of film surface A should be greater than 120 and the haze of the film should be less than 2.0.
Low oxygen permeation means in the present case that less than 30 cm
3
of oxygen per square meter and per day should diffuse through the metallized film when it is exposed to air at a pressure of 1 bar.
The other properties of the film should be at least equivalent to those of known packaging films of this type. It should, for example, be simple and cost-effective to produce and be easy to process on conventional machinery (i.e. the films should not block, for example).
The object is achieved by means of a coextruded, biaxially oriented polypropylene film having a base layer B, at least 80% by weight of which is composed of thermoplastic polypropylene, and having one or more other layers, where at least one outward-facing layer A has a number of elevations/protrusions N per mm
2
of film surface area which is related to their respective heights h and diameters d by the following equations
log N/mm
2
<A
h
−B
h
*log h/&mgr;m, 0.05 &mgr;m<h<1.0 &mgr;m (1)
A
h
=1.4; B
h
=2.5
log N/mm
2
<A
d
−B
d
*log d/&mgr;m, 0.2 &mgr;m<d<10.0 &mgr;m (2)
A
d
=3.4; B
d
=2.4
For the purposes of the present invention, elevations/protrusions are conical elevations/protrusions which project out of the flat film surface. To achieve high oxygen barrier values in metallized or oxidically coated films according to an object of the invention, the number of elevations/protrusions N per mm
2
of film surface area A must be less than a particular numerical value, as given by equations (1) and (2). This numerical value is uniquely given by the right-hand sides of equations (1) and (2) as a function of the height h and the diameter d of the elevations/protrusions.
Biaxially oriented polypropylene films described by the above equation have a relatively small number of elevations/protrusions in the layer A to be metallized or oxidically coated. In the range h<0.5 &mgr;m, particularly in the range h<0.4 &mgr;m and very particularly in the range h<0.3 &mgr;m, the number of elevations/protrusions is markedly lower than known from the prior art.
If the number N of elevations per unit area on the film layer A to be metallized or oxidically coated is greater than the right-hand sides of equations (1) or (2), this implies higher oxygen permeation, which is undesirable with regard to the object of the present invention. In this case, furthermore, the gloss of the film surface (in the unmetallized or uncoated state) is no longer as high, and this too is undesirable with regard to the object of the present invention.
In a preferred embodiment of the novel film the constant A
h
of the above-mentioned equation (1) has the value 1.18, and in a particularly preferred embodiment it has the value 1.0. In a likewise preferred embodiment of the novel film the constant Bh in the above-mentioned equation (1) has the value 2.2, and in a particularly preferred embodiment it has the value 2.1.
In a preferred embodiment of the novel film the constant A
d
in the above-mentioned equation (2) has the value 3.0, and in a particularly preferred embodiment i
Hilkert Gottfried
Peiffer Herbert
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Mitsubishi Polyester Film GmbH
Tarazano D. Lawrence
Thibodeau Paul
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