Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2001-06-26
2002-07-16
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S323000, C428S328000, C428S329000, C428S330000, C428S331000, C428S910000, C428S480000, C264S173110, C264S173120, C264S173150, C264S173160, C264S288400, C264S290200
Reexamination Certificate
active
06420019
ABSTRACT:
SUMMARY OF THE INVENTION
The invention relates to a transparent, biaxially oriented polyester film having at least three layers, with at least one base layer B, and at least two outer layers A and C, which, due to its low planar orientation &Dgr;p, is highly suitable for use as a lid film, in particular as a lid film for yoghurt pots using the mix pap system. The invention also includes the use of the film and a process for its production.
BACKGROUND OF THE INVENTION
Lids, e.g. lids for yoghurt pots, are a known application of biaxially oriented polyester films. They are used here as an alternative to aluminum films. Lids made from polyester film are currently essentially divided into the two types described in some detail below:
Firstly, lids composed solely of polyester film. In the case of lids of this type, the upper side of the film (i.e. of the lid) is printed and the underside of the film (i.e. the inner side of the lid) is provided with a surface coating which is used for adhesive bonding of the lid to the pot. To improve the shelf life of the yoghurt, both the pot and the lid have to have adequate light-protection properties. This is generally achieved by coloring the materials for the lid and for the pot with titanium dioxide or with other suitable pigments or dyes. Another important requirement placed upon the lid film is that its mechanical properties have been selected so as to prevent initiation or propagation of tearing during pull-off from the pot. There must also be no delamination of the film during pull-off from the pot. The film obtains the mechanical properties required by way of biaxial stretching and by being sufficiently thick. The mechanical properties of the lid (stiffness, puncture resistance, tear propagation resistance, etc.) improve more than proportionally as the thickness of the film increases. Polyester films of thickness from 30 to 100 &mgr;m have proven successful for yoghurt lid applications. However, the thickness of the film cannot be raised without restriction, since as thickness increases material costs rise and sealing cycle times are adversely affected, with undesirable economic consequences. The advantages of these lids are clear:
The polyester film lid has a smooth, highly glossy surface which ensures excellent color reproduction and is therefore visually highly attractive.
The film has up to 700% higher puncture resistance than conventional material, resulting in substantially greater protection of the product.
The lid is easy to open without leaving fragments of the lid on the pot in the manner observed when using conventional materials. This is a property particularly valued by the consumer.
The complete separation of pot from lid is highly advantageous for the recyclability of the two materials. The lid film is composed of thermoplastic polyester, which has excellent recyclability. The product is therefore both environmentally safe and environmentally compatible.
In addition, the lid is completely free from metal. This ensures a very high level of safety during metal detection, which is increasingly frequently employed during food production. Using this method it is therefore possible to determine with high accuracy the proportion of metallic constituents present in the contents.
Besides the abovementioned lids composed solely of polyester there are also known lids which are composed of a composite made from transparent, biaxially oriented polyester film and paper. Polyester film here is unlike the type first mentioned in being a comparatively thin biaxially oriented film, generally of thickness about 12 &mgr;m. The two materials (paper and polyester film) are laminated using a suitable adhesive and processed to give lids. The stiffness required from the lid is provided by laminating polyester film to paper to give a thick laminate. Here, the side of the polyester film located toward the paper is generally aluminum-metallized. This means that the lid still has the metallic character shown by a lid made purely from aluminum foil. The other side of the polyester film (outward-facing side of the film, facing away from the composite) is coated with a suitable adhesive and then adhesively bonded to the pot made from PP or PS. The laminate described has become known in the packaging market as the “mix pap system”.
In principle, the latter type has the same good properties as the former type. However, it is capable of substantially more cost-effective production than the former type, but it always has to be borne in mind that it is substantially more susceptible to mechanical damage than the former type since the film used is thinner. The laminate cannot have the same good mechanical properties as the polyester film of about 60 &mgr;m thickness. Particularly during pull-off of the lid from the pot, tearing can initiate and propagate in the polyester film, or there can be delamination. This makes the pull-off performance of the lid unpredictable. Satisfactory pull-off behavior is a particular reason for optimizing the mechanical properties of the polyester film, but avoiding any resultant increase in its production costs. The polyester film used in the present application should moreover be highly transparent and of high gloss, particularly so as to permit the metallic character of the metallized lid to be utilized as effectively as possible for promotional purposes. Known commercially available films do not fully meet this requirement. For example, the TERPHANE® 10.62 product data sheet from Toray Plastic Europe describes a polyester film used for this application. The specific mechanical properties required for the application are characterized by way of what is called the ISOTROPY values. The only optical property listed is haze, the value being about 3.5% according to the data sheet. Nothing is said about the gloss of the film, despite the fact that it is these two properties which substantially characterize its appearance.
The Nu Roll® 12 PXO product data sheet from Nu Roll moreover describes a polyester film used for this application. The specific mechanical properties required for the application are characterized by way of what is called the ISOTROPY values. The only optical property listed is haze, the value being about 2.3% according to the data sheet. Nothing is said about the gloss of the film, despite the fact that it is these two properties which substantially characterize its appearance.
EP-A 0 685 509 describes a transparent, biaxially oriented polyester film for laminating cans, composed of a mixture of from 70 to 40% by weight of polyethylene terephthalate and from 30 to 60% by weight of polybutylene terephthalate, the film being characterized by the following parameters
the crystallization temperature is in the range from 65 to 120° C.
the second transition temperature is above 40° C.
the planar orientation lap is in the range from 0.12 to 0.19
no white spots occur in the film once it has been laminated to the can.
Due to its composition and its resultant mechanical and optical properties, the film is unsuitable for mix pap applications.
EP-A 0 707 979 describes a transparent, biaxially oriented polyester film for thermal transfer applications and having a specific coating made from water-soluble substances based on urethane, on polyester, and on acrylates. The film is characterized by planar orientation &Dgr;p in the range from 0.145 to 0.169. The coating used and the pigment system used make the film unsuitable for mix pap applications. The film is too expensive (coating costs are too high, since the film cannot be recycled), and its haze is too high and its gloss is too low.
EP-A 0 612 790 describes a transparent, biaxially oriented polyester film for magnetic tape applications, comprising a particle system made from crosslinked polymers. The film is to be characterized by a planar orientation &Dgr;p above 0.14. All of the films listed in the examples have a planar orientation &Dgr;p of 0.17. The planar orientation &Dgr;p of 0.17 given in the examples and the pigment system used make the film unsuitable for mix pap
Hilkert Gottfried
Peiffer Herbert
Chen Vivian
Mitsubishi Polyester Film GmbH
ProPat L.L.C.
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