Biaxially oriented polypropylene film, the use of the film...

Details Biaxially oriented polypropylene film, the use of the film... Biaxially oriented polypropylene film, the use of the film...

1999-09-15

2002-06-25

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C264S210700, C264S290200, C428S451000, C428S461000, C428S515000, C428S516000, C428S903300, C428S910000

Reexamination Certificate

active

06410132

ABSTRACT:

The invention relates to a biaxially oriented polypropylene film composed of at least a base layer. The film has good processing performance and good optical properties and, after it has been metallized or has been coated with oxidic materials, is a very good oxygen barrier. 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 effectiveness in particular with respect to gases, good dimensional stability when heated and excellent layflat.
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 or oxidically coated, biaxially oriented polypropylene films. A highly effective barrier against water vapor is best achieved with metallized or oxidically coated, bilaxially oriented polypropylene films. There are other applications in which both a very effective water vapor barrier and an acceptable oxygen barrier are desirable.
2. Description of Related Arts
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect in metallized or oxidically coated biaxially oriented 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 Universtät München, 1995: “Barriereeigenschaften aluminiumbedampfter Kunststoffolien” [Barrier properties of aluminum-metallized plastic films]) gives detailed results of investigations on the influence of the substrate surface on barrier properties in various plastic films.
DESCRIPTION OF THE INVENTION
An object of the present invention is thus to provide a mono- or coextruded, biaxially oriented polypropylene film which also has good optical properties. After it has been metallized or has been coated with oxidic materials, the film should also, compared with the prior art, be a better oxygen barrier and be easy to produce and to process.
The gloss, at least of the surface to be vapor-coated, should be greater than 120, and the haze of the film should be less than 3.0%.
In the present case low oxygen permeation means that less than 30 cm
3
of oxygen per square meter per day should diffuse through the metallized or oxidically coated film when it is exposed to air with a pressure of 1 bar.
In terms of other properties the film should be at least as good as known packaging films of this type. It should, for example, be easy and cost-effective to produce, and process easily (i.e., not block) on conventional machinery.
The object is achieved by means of a biaxially oriented polypropylene film (OPP film) having one or more layers and with at least a base layer B, which at least 50% by weight is composed of thermoplastic polypropylene, and the film has a planar orientation delta p which is greater than 0.0138.
To achieve the desired oxygen permeation of metallized or oxidically coated films according to the invention, the planar orientation delta p of the novel film must be greater than a prescribed value. This value is given by delta p=0.0138.
Therefore, to achieve good oxygen barriers in metallized or oxidically coated OPP films, a high planar orientation delta p is required. If the planar orientation delta p of the film is smaller than the value stated above, the barrier in the sense defined here is poor. If the planar orientation delta p of the film is greater than the value stated above, the barrier in the sense defined here is good.
In a preferred embodiment of the novel film, the planar orientation delta p of the novel film is greater than 0.0140, and in a very particularly preferred embodiment it is greater than 0.0143.
In the preferred and particularly preferred embodiments the novel film is, in its metallized or oxidically coated form, a very good oxygen barrier.
Moreover, it has proven advantageous for achieving a high barrier for the refractive index n
ND
, (N
a
) in the direction of the thickness of the film to be less than a prescribed value. This value is n
ND
=0.1495.
According to the invention, the film has at least a one-layer structure. As a one-layer structure, the film comprises a layer which is a base layer B and comprises the pigments necessary for the processing of the film. However, the film may have a two-layer structure. It then comprises layers which are a base layer B and the outer layer A. In a preferred embodiment of the invention, the film has a three-layer structure and has, on one side of the layer B (=base layer), the layer A, and on the other side of the layer B, another layer C made from polypropylene. In this case at least one of the two outer layers, but preferably both outer layers, comprise(s) the pigments required for the production and processing of the film.
In principle, a variety of raw materials may be used as materials for the various layers. However, it is preferable for the production of the individual layers to be based on polypropylene raw materials.
The base layer of the novel film that has more than one layer comprises polyolefins, preferably propylene polymers, and other additives if desired, in effective amounts in each case. The base layer generally comprises at least about 50% by weight of the propylene polymers, preferably from about 75 to 100% by weight, more preferably from about 80-100%, and in particular from 90 to 100% by weight, based in each case on the base layer. The polypropylene is also preferably a thermoplastic polypropylene and can contain internal and inert particles.
The propylene polymer generally contains from 90 to 100% by weight of propylene units, preferably from 95 to 100% by weight, in particular from 98 to 100% by weight, and generally has a melting point of 120° C. or above, preferably from 150 to 170° C., and generally has a melt flow index of from 0.5 to 8 g/10 min, preferably from 2 to 5 g/10 min, at 230° C. and with a force of 21.6N (DIN 53 735). Preferred propylene polymers for the base layer are isotactic propylene homopolymers with an atactic proportion of 15% by weight or below, copolymers of ethylene and propylene with an ethylene content of 10% by weight or below, copolymers of propylene with C
4
-C
8
&agr;-olefins with an &agr;-olefin content of 10% by weight or below, and terpolymers of propylene, ethylene and butylene with an ethylene content of 10% by weight or below and with a butylene content of 15% by weight or below, and particular preference is given to isotactic propylene homopolymer. The percentages by weight given relate to the respective polymer.
A mixture of the abovementioned propylene homopolymer and/or copolymers and/or terpolymers and/or other polyolefins, in particular those made from monomers having from 2 to 6 carbon atoms, is also suitable if the mixture comprises at least 50% by weight of propylene polymer, in particular at least 75% by weight. Other polyolefins which are suitable in the polymer mixture are polyethylenes, in particular HDPE, LDPE and LLDPE, if the proportion of these polyolefin

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