Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1998-12-01
2001-08-07
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S220000, C428S476300, C428S475800, C428S476100, C428S476900
Reexamination Certificate
active
06270882
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multi-layer heat sealable film which consists of a strength-imparting sequence of layers, a heat sealable layer, optionally together with a bonding layer located between them. The strength-imparting sequence of layers may itself comprise one or more plies and contains a layer of a linear copolymer of an alternating structure prepared from carbon monoxide and at least one ethylenically unsaturated olefin, which layer may optionally be coextruded with layers containing polyamide. The heat sealable layer may also comprise multiple plies. Since the multi-layer film according to the invention retains elevated stiffness and dimensional stability even when exposed to heat and moisture, it is distinguished by outstanding machine processing characteristics in conventional packaging machinery. The film is additionally distinguished by very high flex crack resistance. The invention also concerns the use of the stated film on machinery for the production of tubular bags sealed on all sides and on machinery for the production of thermoformed tray packages and the use of the film for packaging foodstuffs. The film of the invention is halogen-free.
BACKGROUND OF THE INVENTION
The majority of packaging for foodstuffs, snacks and similar products is produced on form/fill/seal bagging machines. The mode of operation of such machines and the structure of films preferably processed on such machines is described, for example, in
The Wiley Encyclopedia of Packaging Technology
(editors M. Bakker, D. Eckroth; John Wiley & Sons, 1986) and in
Nentwig
(Joachim Nentwig, Kunststoff-Folien, Carl Hanser Verlag 1994, Munich).
Form/fill/seal bagging machines form a tube from a continuously fed film, introduce he contents therein and, once the film has subsequently been heat sealed on all sides around the contents, separate from the fed end of the tube a bag-shaped container, formed from a continuous section of film and filled with the product. The stated container is referred to below as a tubular bag. For reasons of economy, elevated machine running speeds are advantageous for the performance of these operations. Particular requirements are consequently placed upon the flexible packaging film used.
Moreover, in many cases the contents requires the least possible exchange of the atmosphere present in the package with the ambient air around the package. Thus, for example, in the case of oxygen-sensitive contents, the interior of the package may be provided with an oxygen-depleted atmosphere. The penetration of water vapour may, however, also be disadvantageous in the case of moisture-sensitive goods. In order to maintain the desired atmospheric conditions the packaging film must accordingly, on the one hand provide an elevated diffusion barrier to such unwanted gases while, on the other, have no macroscopic leaks, such as pores, which allow convective air flow into or out of the package.
One essential precondition for elevated packaging speeds on form/fill/seal bagging machines is elevated stiffness of the film used. The modulus of elasticity to DIN EN ISO 527 may be used as a suitable measure of the stiffness of a film. Elevated stiffness is required in order to draw it through the machinery with the least possible deformation despite the elevated forces applied thereto. Thus, for example, deformation of the film in the area to be heat sealed results in irregularities in the sealed seams. Apart from impairing the appearance of the film, this may, under certain circumstances, also result in leakage, for example if creases are included in the heat seal and consequently in a failure of the function of the package formed from the film.
Particular requirements with regard to the thermal softening behaviour of the film apply to so-called horizontal form/fill/seal bagging machines, which heat seal the film in the machine direction by means of a heated, driven pair of rotating rollers, through which the areas of the film to be sealed are passed. Before reaching the heat sealing rollers, the film is appropriately preheated. In the zone between preheating and heat sealing, the film is in a thermally softened state and thus has a particular tendency to be introduced irregularly into the heat sealing rollers, so giving rise to irregular seams. Optimum processability of the film entails a heat sealable layer which melts at low temperatures combined with a supporting layer which is as stiff and dimensionally stable as possible at elevated temperatures.
If the stated machines are shut down temporarily, the sections of film already unwound from the feed roll remain in extended contact with the ambient atmosphere. In many cases, especially if the prevailing climatic conditions are hot and moist, certain films may absorb moisture from the ambient air and not only soften but also, under certain circumstances, curl due to the non-uniform increase in volume of the inner and outer layers, or they may shrink or expand due to moisture absorption or undergo an accompanying structural change, such as post-crystallisation. These changes may have such a serious effect on the functioning of the packaging machine that it is impossible subsequently to process the exposed areas of film after such shut-downs. Film curling may be qualitatively assessed using the measurement method specified in relation to the characterisation of the Examples according to the invention.
For transport purposes the bags produced in the manner described above are conventionally consolidated in a transport package containing a large number of packages. A typical example of such a transport package is a carton made from paperboard. Placing the packages in the transport package and transport itself expose the packaging film to elevated stresses.
This frequently results in the formation of creases and folds in the packages. Vibration during introduction into the transport package and during transport repeatedly exposes the film to forces which are transferred by mechanical impact against the inside of the transport package or, within the transport package, against adjacent packages. Such contact results in particular stress in the area of a fold. This may thus result, after a certain number of such impacts, in local failure of the film at the fold. This gives rise to a pore which, by allowing air exchange and consequent spoilage of the contents, may result in failure of the package. The resistance of a film to such stress will be referred to below as flex crack resistance, which may, for example, be quantified by the measurement method specified in relation to the characterisation of the Examples according to the invention.
For the prior art film structures described below, it is generally the case that, at an identical film thickness, increased stiffness of the film due to appropriate selection of one of the stated materials, or, with an identical material, increased film thickness, both result in reduced flex crack resistance thereof.
For these reasons, the stated packages are conventionally produced using multi-layer films having a layer or sequence of layers which ensures film stability and is conventionally located on the outer side of the film, here denoted the support film, followed by a layer, the primary function of which is to provide adequate adhesion to the single or multi-layer sequence of layers located on the inner side, here denoted in brief as the heat sealable layer.
The thickest possible heat sealable layer is advantageous in order to ensure that sealing of the package is effective around creases too. A thick support film also contributes towards elevated mechanical stability of the film. On the other hand, on economic grounds (firstly due to material costs and secondly due to the required elevated machine running speeds), the films should be as thin as possible. An excessively thick support film furthermore has a particular tendency to fail by flex cracking. Depending upon package size and contents, favourable thickness ratios for flexible films in tubular bag films are around 15 to 25
Brandt Rainer
Eggers Holger
Lund Klaus
Sperlich Bernd
Franks James R.
Gil Joseph C.
Kelly Cynthia H.
Shewareged B.
Wolff Walsrode AG
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