Stock material or miscellaneous articles – Composite – Of polyamide
Reexamination Certificate
1999-02-12
2001-08-14
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of polyamide
C428S475800, C428S476100, C428S476300, C428S476900
Reexamination Certificate
active
06274246
ABSTRACT:
The present invention relates to a multi-layer sealable film having at least one polyamide-containing layer, optionally one or more EVOH-containing layers, as well as a sealing layer which is at least of unilayer structure, particularly for the production of thermoformed packaging units. The multi-layer film is characterised by a moisture content of at least 0.5% and at most 2.5% with respect to the total weight of the polyamide-containing layers and of the EVOH-containing layers. The multi-layer film according to the invention is distinguished by its improved thermoforming properties, particularly by its improved conversion and by the reduction of striated, irregularly shaped regions, by its very good processability on machines and by its high resistance to perforation. The present invention also comprises the use of said multi-layer films for packaging units, particularly for foodstuffs.
Foodstuffs are frequently packed on thermoforming, filling and sealing machines, which are also simply termed thermoforming machines, in hollow packaging units comprising a thermoformed hollow film and a plain outer film. After thermoforming and the introduction of the filling material into the hollow which is thus formed, the two films are sealed to each other to form a closed container by a heat-sealing operation. The mode of operation of machines such as these and the construction of films which are preferably processed on machines such as these are described, for example, in
The Wiley Encyclopedia of Packaging Technology
(Eds. M. Bakker, D. Eckroth; John Wiley & Sons, 1986) and by Nentwig (Joachim Nentwig: Kunststoff-Folien, Carl Hanser Verlag 1994, Munich).
In many applications it is of crucial importance to the introduction of the filling material that the film is capable of adapting to the hollow contours which are predetermined by the moulding tool, and that the new contour which is thus obtained is retained to the greatest possible extent even after the packaging unit has left the forming station. In contrast, if the film hardly assumes the contours of the tool at all due to an inferior thermoforming capacity, or if the contours subsequently alter, due to shrinkage for instance, it is no longer possible under some circumstances to introduce the filling material into the hollow for reasons of space. A requirement such as this arises, for example, during the packaging of smoked ham, which in many instances is cut into an approximately cubical shape with pronouncedly sharp edges and corners. This results in the necessity of providing a correspondingly sharp-edged hollow, i.e. one which matches the contours of the thermoforming tool as completely as possible. However, if the hollows are of too rounded a shape due to inadequate forming capacity of the film, the ham to be packaged cannot be placed completely in the hollow. If this problem is tackled by increasing the depth of thermoforming using the same hollow contour, the ham does in fact match the hollow, but unwanted folds can be formed in the film situated in the region of the corners of the hollow and base of the hollow, due to the poor correspondence between the contours of the ham and the hollow.
If filling materials are packed which comprise solid constituents, particularly sharp-edged constituents, a further important requirement arises, namely a high resistance to perforation of the film by segments of filling material such as these; this is also hereinafter termed perforation resistance. The packaging of fish is also subject to the requirement of a high resistance to perforation of the packaging film by segments of filling material such as these, particularly if the portion of fish to be packed may contain bones or bone fragments. A suitable method of measuring the perforation resistance is described in connection with the examples.
The design of multi-layer, flexible thermoforming films according to the prior art is explained below.
Unless stated otherwise, the convention employed here is that the abbreviations for plastics according to DIN 7728 or ISO 1043-1987 (E) are used for the description of the polymers contained in the individual layers.
For multi-layer structures, the layer sequence is given by stringing together the abbreviations for the polymers of the corresponding layers or by symbols which are explained elsewhere, separated from each other by double oblique strokes. The side of the sealing layer is always on the right when using this convention. Moreover, only part of the entire layer sequence which makes up the film may be given. In cases such as these, the side of the sealing layer is likewise always on the right, and layers which are not given, or combinations of layers, are recognisable by three dots, . . . Mixtures of different polymers are recognisable by the algebraic sign + and by the combination of the components in brackets ( ). Additional data on percentage compositions may also optionally be given here. Unless stated otherwise, the percentage compositions in such cases are always given in proportions by weight, which are expressed with respect to the total weight of the mixture. For example, the expression . . . //PA//EVOH// . . . //(PE-LD+PE-LLD)//d describes a structure with an unspecified outer layer or an externally situated layer sequence, followed by a layer which substantially consists of polyamide, followed by a layer which substantially consists of an ethylene/vinyl alcohol copolymer (EVOH), followed by an unspecified layer or layer sequence, followed by a layer which comprises a mixture of polyethylene of low density (PE-LD) and an ethylene/&agr;-olefine copolymer (PE-LLD), and with a following layer on the sealing side which is specified in more detail by d.
Melting point data is given below with respect to the value determined by DSC analysis (differential scanning calorimetry analysis) according to ASTM 3418.
Flexible, thermoformable, heat-sealable films usually possess a multi-layer structure and contain one or more layers of polyamide (PA) or mixtures comprising polyamide. PA6, i.e. polycaprolactam, is predominantly employed as the polyamide. However, other types of PA are also used, such as those given in the Table below, particularly for special applications with stringent requirements. The polyamide-containing layers impart a high mechanical stability to the film at room temperature and at customary temperatures of use. They soften on heating and thus enable the film to be thermally deformed to form a hollow.
The term “polyamide” is to be understood in its widest sense, namely to mean polymeric compounds which are linked to each other by an amide group —NH.CO— (see also: Kohan (Ed.): Nylon Plastics Handbook, Hanser Publishers, 1995, Munich). Polyamides can be characterised by the monomers which they contain. A distinction is made between polyamides which are obtained from a monomer by the condensation polymerisation of &ohgr;-aminocarboxylic acids or by the polymerisation of lactams thereof to form a polyamide 6 type, and those which are formed from at least one monomer of the diamine and dicarboxylic acid type by condensation polymerisation to produce a polyamide 66 type. A third group, hereinafter termed copolyamides, is understood to comprise those polyamides which contain lactams, diamines and dicarboxylic acids, or which contain more than one diamine and more than one dicarboxylic acid.
Examples of lactams include &egr;-caprolactam and &egr;-laurolactam. Examples of diamines include m-xylylenediamine or hexamethylenediamine. Possible dicarboxylic acids include adipic acid, sebacic acid, isophthalic acid, terephthalic acid or dodecanedioic acid.
Polyamides are characterised by numbers which give the number of C atoms in the starting material, or—when there are two components—which give the number of C atoms in the diamine (first number) and in the dicarboxylic acid (second number), or by an abbreviation of the description of the diamine or of the dicarboxylic acid (e.g. PA MXD6 from the diamine m-xylylenediamine and the dicarboxylic acid adipic acid).
Abbrevi-
&ohgr
Eggers Holger
Gasse Andreas
Kaschel Gregor
Franks James R.
Gil Joseph C.
Kiliman Leszek
Wolff Walsrode Aktiengesellschaft
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