Laminated sheets and deep-drawn containers made therefrom

The present invention relates to thermoforming shrinkable composite films on copolyamides and ethylene copolymers which are particularly suitable for packaging foods, such as meat and sausage products, on thermoforming systems.

Composite films made of polyamides, preferably PA-6 and polyethylene, have already been used for food packaging. However, when these composite films are used as vacuum packaging on deep-drawing systems, wrinkles appear around the packaged product. These folds not only interfere optically, but can also easily damage the packaging by being punctured by mechanical stress.

There is therefore a considerable need to provide thermoformed packaging, in particular food packaging, which has a wrinkle-free appearance.

DE-AS 2538892 already discloses a process for producing composite films made from polyethylene and from polyamide. The polyamide consists of 80-90 parts by weight of s-caprolactam and 10-20 parts by weight of AH salt. The polyethylene can be a copolymer which is predominantly composed of ethylene units.

From the BASF company publication Ultramid KR 4600 (August 1975) it is known to use the polyamide components specified therein for the production of coextruded polyamide / polyethylene composite films, which are further processed, for example, by the thermoforming process. The shrinking behavior does not meet the requirements.

The invention relates to a composite film which can be shrunk by deep drawing and which consists of<ul id="ul0001" list-style="none"><li>1) at least one layer of at least one copolyamide with laurolactam and with at least 50 wt .-% s-caprolactam units and a melting temperature in the range of 75-215 ° C, preferably 160-200 ° C, and optionally a saponified ethylene / vinyl acetate copolymer and / or ionomer resin,</li><li>2) at least one heat-sealable layer made of an ethylene copolymer of at least 50 mol% of ethylene units and</li><li>3) if appropriate, there is a conventional adhesive or adhesion promoter layer between layers 1) and 2).</li></ul>

Another object of the invention is wrinkle-free thermoformed packaging, in particular for foods such as sausage and meat, which have been produced from the composite films according to the invention.

To produce the layer 2) suitable for surface sealing, copolymers of ethylene and α, β-unsaturated monomers, such as α, β-unsaturated monocarboxylic acids with C₃-C₅which are composed of at least 50 mol% of ethylene can be used. The copolymers preferably contain 2-25 mol% of a, β-unsaturated monocarboxylic acid units, which are optionally partially or completely present as alkali metal, alkaline earth metal or Zn salt, such as Zn and / or Ca salts. The melt index of the copolymers should be in the range of 0.5-40 g / 10 min, preferably 1.5-5.0 g / 10 min, measured in accordance with DIN 53 735. Copolymers of ethylene / (meth) acrylic acid with 2-25 mol%, preferably at least 10 mol%, of (meth) acrylic acid units which are present as zinc and / or Ca salts (ionomer resins) are particularly preferably used.

Modified ethylene / vinyl acetate copolymers with 2-40% by weight, preferably with 3-10% by weight, of vinyl acetate units are also suitable for producing layer 2). However, quaternary polymers which are composed of 71 to 90% by weight of ethylene and 3 to 9% by weight of an aliphatic, ethylenically unsaturated carboxylic acid with C.₃-C₅, 1 to 20 wt .-% of an ester of an aliphatic, ethylenically unsaturated carboxylic acid with alcohols, the C_l-C₈ contain, as well as from 0.3 to 5 wt .-%, preferably 0.8 to 3 wt .-%, isobutylene, the sum of the wt .-% must be 100 in each case. These are particularly preferably ethylene-vinyl acetate copolymers which, in addition to vinyl acetate units, consist of 71-90% by weight of ethylene, 3-9% by weight of acrylic acid, 1-20% by weight of the t-butyl ester of acrylic acid and 0.8 -3% by weight of isobutylene are built up.

In particular when using the above-mentioned vinyl acetate copolymers to produce the heat seal layer, the bond should be applied by means of a conventional adhesion promoter layer 3), preferably based on the ionomer resins described above, between the carrier film made of copolyamide and the heat seal layer made of ethylene copolymers or based on the ethylene / vinyl acetate copolymer described above .

It is also possible to add the ionomer resins to the copolyamides in an amount of 5-30% by weight so as to increase the bond strength.

Gas permeability, especially for 0₂ To lower it, it is advantageous to add the amounts of 5-30% by weight to the copolyamides of ethylene vinyl acetate with 50-80% by weight vinyl acetate units, which are at least 90% saponified, and to produce the carrier film therefrom.

The composite films according to the invention should have a thickness of 90-300 μm, preferably 160-250 μm, in order to give sufficiently stable thermoformed packaging. A thickness of 40-120 μm, preferably 60-100 μm, and a thickness of 50-200 μm, preferably 100-180 μm, should be maintained for the copolyamide layer.

The composite films according to the invention are produced by customary processes, preferably by coextrusion according to the film blowing process, and are expanded 1: 2-1: 6, preferably 1: 3-1: 4, in order to ensure a volume shrinkage of at least 40% after deep-drawing the packaging . The packaging, preferably for food, the sausage or meat, is produced on the conventional thermoforming packaging machines from the composite films according to the invention.

In order to get wrinkle-free packaged products, the following treatment methods of the thermoformed packaging according to the invention are available:<ul id="ul0002" list-style="none"><li>a) The filled thermoformed packaging is evacuated in the machine and then sealed. It is then shrunk for 1-5 s at 80-100 ° C, preferably 85-95 ° C by immersion in a water bath.</li><li>b) The filled, sealed thermoformed packaging is briefly subjected to hot air treatment at 170-240 ° C, preferably at 220-240 ° C, preferably in a hot air or hot steam tunnel.</li><li>c) It is also possible to effect the shrinkage by contact heat in the thermoforming packaging machine. For this purpose, the packaging, which is already filled, is sealed using a suitable sealing tool for surface sealing - ideally at the top and bottom - in the vacuum chamber of the thermoforming machine and at the same time shrunk. The surface sealing also serves the purpose of preventing juice from escaping, which is often observed in many foods.</li></ul>

The packaging according to the invention not only has a wrinkle-free appearance, but is also distinguished by improved mechanical strength, puncture resistance and flexural strength, since the corners of the film trough thinned out by deep drawing regain considerable thickness and mechanical strength as a result of the shrinking process. This is accompanied by a reduction in the gas and water vapor permeability of the trough and thus a longer shelf life of the food.

example 1

From a copolyamide 6/12 from 98% by weight s-caprolactam and 2% by weight laurolactam and from an ethylene (meth) acrylic acid copolymer with a melt index of 5.0 g / 10 min (Surlyn⁰) a composite film is produced by coextrusion under the following conditions: temperatures in the mold 235-240 ° C, in the extruder (for layer 1) 230-245 ° C, at a speed of 50 rpm, (for layer 2) 235-240 ° C at a speed of 70 rpm; Take-off speed approx. 9 m / min, inflation ratio 1: 2. The total film tube thickness achieved here is 160₁1m.

Example 2

For a composite film consisting of a copolyamide 6/12 layer (as in Example 1) and an ethylene-vinyl acetate copolymer layer, with 8% by weight vinyl acetate units and with a melt index of 1.3 g / 10 min, the layers are made under the same conditions as in Example 1 produced by the film blowing process and the two film layers bonded to one another using a commercially available polyurethane-based adhesive.

Example 3

Layer 1) of a composite film consisting of a mixture of 10% by weight of a commercially available ionomer resin, 20% by weight of an ethylene vinyl alcohol polymer (70% by weight vinyl alcohol) and 70% by weight copolyamide 6/12 (as in Example 1) and Layer 2) consisting of an ethylene-vinyl acetate copolymer with a VAc content of 8% by weight are used as blown films under the same conditions as described in Example 1, produced and bonded together using a commercially available polyurethane-based adhesive (layer 3).<tables id="tabl0001" num="0001"><img file="EP0062822B2_D0001.tif" /></tables>