Multi-layered tubular packaging casing.

The invention relates to a multilayer, tubular packaging casing for pasty filling, in particular to an artificial sausage casing, based on polyamide.

Casings based on polyamide are increasingly used for packaging food, in particular as artificial sausage casings. For this purpose, a casing material is required for different types of sausage, which has a low water vapor permeability, so that the weight loss when storing the sausage due to water release of the sausage mass remains as small as possible. The sausage then shows a wrinkle-free, tight-fitting casing even after prolonged storage; there are no gaps between the sausage mass and the inside of the casing, which lead to jelly sales of the sausage mass. For this purpose it is known to combine the polyamide layer with a polymer layer which has a lower water vapor permeability than the polyamide layer. For other foods, however, the oxygen permeability of these composite films is still too high. Sausage mass of the liver sausage type discolors due to the addition of oxygen, so that these casings are not suitable for these types of sausage. To reduce oxygen permeability, an additional oxygen barrier layer is therefore required in the polyamide film.

EP-A-0 305 959 describes a multilayer polyamide film which is provided, for example, in the form of a heat-sealable bag for packaging meat and poultry. It consists of a barrier layer for oxygen made of an amorphous partially aromatic copolyamide (PA 61 / 6T), which consists of units of hexamethylenediamine, terephthalic acid and isophthalic acid. This copolyamide layer forms the core layer of the film. It is surrounded on both sides by polyolefin layers, which form a water vapor barrier, the inner polyolefin layer being intended to prevent the escape of moisture from the packaged food into the core layer made of copolyamide. However, polyolefin layers are disadvantageous on the inside of sausage casings because they lead to inadequate adhesion between the sausage mass and the sausage casing inner wall and promote jelly sales.

EP-A-0 127 296 (= US-A-4,606,922) had already attempted to solve this problem. It was proposed there to provide an ionomer for the inside of a multilayer polyamide film and to treat the inside with ionizing radiation. DE-A-38 16 942 (= US-A-4,888,223), in the case of a multilayer polyamide shell, to avoid the formation of wrinkles, teaches treating the inner layer of polyolefin resin with corona discharge and, if necessary, sprinkling it with starch powder. These proposals thus have the disadvantage that an additional pretreatment of the inside of the casing is required to prevent wrinkles and jelly deposits between the sausage mass and the inside of the casing.

The present invention is therefore based on the object of specifying a tubular packaging casing based on polyamide which, after being filled, adheres to the coated filling material, in particular sausage mass. The casing should be able to be used without problems as an artificial sausage casing, in particular for cooked and cooked sausages. It should have a reduced permeability compared to water vapor and atmospheric oxygen. Sausages with this coating should not show any noticeable loss of moisture even after a storage period of 4 to 6 weeks, which is noticeable in a decrease in the weight of the sausage and a wrinkled appearance, nor should jelly be removed from the sausage mass between the casing wall and the sausage mass drop. The envelope is also intended for the oxygen-tight packaging of air-sensitive goods, such as Sausage masses of the liver sausage type may be suitable so that they do not change during the storage period, in particular not discolour in an unsightly manner.

This object is achieved by the packaging envelope specified in claim 1. The dependent claims indicate expedient developments of the shell.

The casing shows a wall thickness that is comparatively thin for multi-layer packaging casings made of polyamide with good barrier properties. It is preferably at most about 60, in particular at most 45 micrometers. The lower limit is around 30 micrometers. The hose caliber is adapted to the intended use. When using the casing for cooked and cooked sausages, the tube caliber is generally 30 to 160, in particular 40 to 135 mm.

The packaging casing consists of at least three layers, namely an outer, a middle and an inner layer, the outer and the inner layer made of polyamides and the middle layer consisting of a polymer mixture with polyolefin.

The outer layer usually has the greatest thickness compared to the other two layers. The thickness is generally 14 to 48 µm with a total film thickness of 30 to 60 µm. This layer can therefore generally be regarded as the actual carrier layer of the multilayer casing. It is usually made up of a single layer. It shows a comparatively high water absorption capacity.

The outer layer absorbs water when the sausage is scalded or boiled, which is released again when the sausage is cooled and stored. As a result, the outer layer shrinks when it dries, so that the sausage casing fits tightly and wrinkle-free on the cooled sausage. Since the casing is practically waterproof and impermeable to water vapor through the middle layer, the outer layer cannot absorb moisture from the sausage mass.

The outer layer consists essentially of an aliphatic polyamide, aliphatic copolyamide or a polymer mixture of at least one of these compounds.

The aliphatic polyamide is a homopolycondensate of aliphatic primary diamines, in particular with 4 to 8 C atoms, and aliphatic dicarboxylic acids, in particular with 4 to 10 C atoms, or a homopolymer of omega-aminocarboxylic acids with 6 to 12 C atoms or their lactams. The aliphatic copolyamide contains the same units and is, for example a polymer based on one or different aliphatic diamines and one or different aliphatic dicarboxylic acids and / or one or different omega-aminocarboxylic acids or their lactams. Examples of suitable dicarboxylic acids are adipic acid, azelaic acid, sebacic acid and dodecanedicarboxylic acid, suitable diamines are tetra-, penta-, hexa- and octa-methylenediamine, and hexamethylenediamine is particularly preferred. An example of omega-aminocarboxylic acids is 11-aminoundecanoic acid, examples of lactams are epsilon-caprolactam and omega-laurolactam. Particularly preferred aliphatic polyamides are polycaprolactam (PA 6) and polyhexamethylene adipinamide (PA 66) or mixtures of these polyamides. The aliphatic copolyamide contains various of the units mentioned. A preferred aliphatic copolyamide consists of caprolactam, hexamethylenediamine and adipic acid units (PA 6/66).

The outer layer of the tubular packaging casing has a higher water absorption capacity than the middle layer and in a preferred embodiment also as the inner layer, so that it can be well watered as a sausage casing before the filling process and shows the required suppleness. The middle layer with the reduced water absorption capacity, on the other hand, enhances the barrier properties of the inner layer. PA 6 and PA 66, which are also mixed with other water-absorbent polymers, including polyester, are therefore particularly suitable for the outer layer. The addition of polyester facilitates the biaxial stretching of the tube, that is, the surprising stretching force is surprisingly reduced, and the overall strength of the sheath is also increased.

As is known, polyesters are condensation products of diols and aromatic dicarboxylic acids, in particular terephthalic acid and, if appropriate, additionally isophthalic acid. To a small extent, the polyester can be modified with aliphatic dicarboxylic acids, such as adipic acid. The diols are in particular aliphatic compounds of the formula HO- (CH₂)_n-OH (n = 2-8), such as ethylene glycol, 1,4-butylene glycol, 1,3-propylene glycol or hexamethylene glycol, and alicyclic compounds such as 1,4-cyclohexanedimethanol. Polybutylene terephthalate is very particularly preferably used in the polymer mixture. The proportion of the polyester is usually 5 to 15% by weight, based on the polymer mixture of the outer layer of the packaging envelope.

It is advisable in individual cases to modify the outer polyamide layer by adding other polymers compatible with the polyamide or with the polyamide mixture up to a proportion of 25% by weight. Suitable additional polymers are, for example, polyolefins, ie Homopolymers or copolymers of alpha-olefins with 2 to 8 carbon atoms, especially ethylene, propylene and butylene, preferably polyethylene, especially LDPE and LLDPE, as well as modified polyolefins with functional groups, such as those used in the middle layer as a suitable adhesion-promoting component , for example polyolefins with carboxyl radicals, that is to say groups consisting of acids, esters, anhydrides and salts of carboxylic acids. The polyolefin and the modified polyolefin improve the adhesion of the outer layer to the middle layer and increase the smoothness of the casing. However, they are generally only present at a maximum of 3 to 10% by weight in the outer layer in order not to significantly impair the water absorption capacity and the strength of the outer layer. The outer layer may contain customary additives, for example matting agents, in order to reduce the gloss. Furthermore, the outer layer shows good color adhesion and can be printed over the entire surface without any problems.

The middle layer of the packaging sleeve shows practically no water vapor permeability and also does not absorb any water. For this purpose, the middle layer should have a minimum thickness of 2 µm. In general, a thickness of 5 to 25 µm is sufficient to achieve the water vapor impermeability required for boiled and cooked sausage casings.

The middle layer consists of one or more polyolefinic compounds and an agent which improves the adhesion between the two polyamide layers and the intermediate polyolefin layer. This agent is either largely homogeneously distributed over the entire cross section of the middle layer or is at least in the interfaces with the adjacent polyamide layers. In the first case, the middle layer is made up of a single layer, in the second case the middle layer preferably consists of a polyolefin core and in each case a layer of polyolefin / adhesion promoter or only adhesion promoter on the two surfaces of the polyolefin core, so that in the first case there is an overall three-layer structure, in the second case a total of 5 layers results in the shell wall. In view of the desired total tube wall thickness of less than 60 μm and the small tube caliber, the three-layer structure is preferred in order to avoid difficulties in the construction of suitable coextrusion nozzles. In the preferred three-layer embodiment, the adhesion-promoting component is not used as a separate layer between the middle and the outer or provided between the middle and the inner layer, rather this connection is incorporated into the middle layer. As a result, the binding force between the middle polyolefin layer and the adjacent polyamide layers is increased, so that when the three layers are coextruded, a firm bond is formed which, even when exposed to hot water, does not experience a layer separation when the sausage is scalded.

The proportion of this adhesion-promoting component is generally 5 to 50, in particular 10 to 35,% by weight, based on the polymer mixture present in the middle layer. It should be noted, however, that the water vapor permeability of the middle layer generally increases somewhat due to the functional groups of the adhesion promoter, which is why this addition should be as small as possible. In general, 20 to 25% by weight, based on the polymer mixture of the middle layer, is therefore sufficient.

The polyolefin of the middle layer is usually a homopolymer of ethylene or propylene or a copolymer of linear alpha olefins with 2 to 8 carbon atoms or a mixture of these polymers. Suitable copolymers are C₂/ C₃- and C₃/ C₄Copolymers and C₂/ C₃/ C₄-Terpolymers, eg a mixture of C₃/ C₄Copolymer and C₂/ C₃/ C₄Terpolymer.

The adhesion-promoting component is an agent as is known for producing coextruded composites from polar and non-polar plastics such as polyamide and polyolefin. It usually consists of a polyolefin resin modified with functional groups, which is composed of ethylene and / or propylene units, optionally also further linear alpha-olefins having 3 to 10 carbon atoms. The olefinic chain ensures compatibility with the polyolefin of the middle layer and the extrudability of the adhesion-promoting component. The functional groups are in particular units of vinyl acetate, vinyl alcohol, ethylenically unsaturated carboxylic acids such as acrylic acid and methacrylic acid and their esters and salts, in particular Na and Zn salts, and also also ethylenically unsaturated carboxylic acid anhydride groups. They lead to sufficient wettability of the polyamide and polyolefin melt during coextrusion, so that a firmly adhering bond is created.

Suitable adhesion promoters consist in particular of graft polymers, copolymers or terpolymers with ethylene or propylene units with at least one comonomer from the group comprising (meth) acrylic acid, (meth) acrylic acid ester (esters of n-alkanols with 1 to 6 carbon atoms, for. As butyl acrylate), vinyl acetate and / or maleic anhydride. The proportion of units with functional groups is usually 3 to 12% by weight, based on the weight of the adhesion-promoting polymer. The modified polyolefin resins also include rubber modified polyethylene.

If the casing is to be colored, it is expedient to incorporate the dye or the color pigments into the middle layer so that they do not come into direct contact with the filling material. This measure has the advantage that the stretching process is not disturbed by the nucleating effect of color pigments. Furthermore, it has been shown that color pigments can be dispersed particularly easily and uniformly in the middle layer.

In one embodiment, the inner layer consists of aliphatic polyamide, aliphatic copolyamide or a polymer mixture of at least one of these compounds. Further polymers, for example polyolefins, polyesters or ionomers, can also be present in the inner polyamide layer up to a proportion of 15% by weight. This layer thus consists of a polymer or a polymer mixture, as has already been described above for the outer layer. However, the aliphatic polyamide of the inner layer preferably shows a lower water absorption capacity than the aliphatic polyamide of the outer layer. For the inner layer, therefore, aliphatic polyamides with relatively long methylene chains such as. B. PA 11 or PA 12 or copolyamides used with these units. The adhesion of the inner layer to the sausage mass can be improved by adding ionomers to the aliphatic polyamide. The layer of aliphatic polyamide or copolyamide prevents direct contact between the polyolefin and the sausage mass. However, the oxygen barrier property of this inner layer is comparatively low. If the casing is to have a reduced oxygen permeability, the inner layer consists of a partially aromatic polyamide and / or partially aromatic copolyamide. The partially aromatic polyamide or copolyamide consists of aliphatic and aromatic units.

In a first embodiment of the partially aromatic polyamide, the diamine units predominantly or exclusively form the aromatic units. Examples of these units are xylylenediamine and phenylenediamine units. The dicarboxylic acid units of this embodiment, on the other hand, are predominantly or exclusively aliphatic and usually contain 4 to 10 carbon atoms. Preferred aliphatic dicarboxylic acid units are sebacic acid and azelaic acid, especially adipic acid.

In a second embodiment of the partially aromatic polyamide, the diamine units predominantly or exclusively form the aliphatic units, while the dicarboxylic acid units consist predominantly or completely of residues of aromatic dicarboxylic acids, in particular isophthalic acid and terephthalic acid. The aliphatic diamine units usually consist of 4 to 8 carbon atoms, preferably of caprolactam and / or hexamethylene diamine units. The preferred partially aromatic copolyamide of the second embodiment consists of caprolactam and / or hexamethylenediamine units and terephthalic acid and / or isophthalic acid units, in particular PA 61 / 6T.

The first embodiment of the partially aromatic polyamide or copolyamide optionally contains up to 5 mol% of aliphatic diamine units and up to 5 mol% of aromatic dicarboxylic acid units. Likewise, the second embodiment of the partially aromatic polyamide or copolyamide can contain up to 5 mol% of aromatic diamine units and up to 5 mol% of aliphatic dicarboxylic acid units.

However, it has been shown that inner layers made of pure partially aromatic polyamide and / or pure partially aromatic copolyamide are relatively stiff and hard and lead to the fact that the inner sides of the flattened casing lying one on top of the other tend to stick together. Due to the action of moist contents there is also the danger that this material will become brittle.

It is therefore particularly advantageous when the casing is used as a sausage casing if the inner layer additionally contains aliphatic polyamides and / or aliphatic copolyamides. Particularly when the partially aromatic polyamide or copolyamide consists of the second embodiment mentioned, the partially aromatic polyamide is present in a mixture with a saturated linear aliphatic polyamide and / or a saturated linear aliphatic copolyamide. A shell with an inner layer made of this polyamide mixture is also particularly biaxially stretchable.

This additional aliphatic polyamide in the polymer mixture of the inner layer consists of the reaction product of an aliphatic dicarboxylic acid, preferably with 4 to 10 C atoms, with aliphatic primary diamines, preferably with 4 to 8 C atoms. Examples of suitable dicarboxylic acids are adipic acid, azelaic acid, sebacic acid and dodecanedicarboxylic acid, suitable diamines are tetra-, penta-

, Hexa- and Octa-methylenediamine, hexamethylenediamine is particularly preferred. Furthermore, the additional aliphatic polyamide of the polyamide mixture can also be composed of units of omega-aminocarboxylic acids having 6 to 12 carbon atoms or their lactams, for example of 11-aminoundecanoic acid, epsilon-caprolactam or omega-laurolactam. The aliphatic copolyamides in the mixture with the partially aromatic polyamide or Copolyamides contain various of the units of the aliphatic polyamide mentioned. A preferred aliphatic copolyamide consists of caprolactam, hexamethylenediamine and adipic acid units. PA 6, PA 66, PA 6/66, PA 11 and PA 12 or mixtures of these polymers are particularly preferred as an additional component in addition to the partially aromatic polyamide and / or copolyamide in the inner layer.

The proportion of the partially aromatic polyamide and / or copolyamide in the inner layer is preferably 5 to 85, in particular 10 to 40% by weight, based on the polymer mixture of partially aromatic and aliphatic polyamides and copolyamides. In this area of ​​the mixing ratio, a smooth, yet oxygen-impermeable inner layer is achieved. On the other hand, as the proportion of the partially aromatic polyamide / copolyamide increases, the adhesion of the sausage mass to the inner wall of the casing increases, so that the casing properties can also be varied in this regard by increasing the proportion of the partially aromatic polyamide / copolyamide.

The tubular casing is preferably produced by coextrusion of the polymers forming the individual layers through an annular die, for example using a system as described in EP-A-0 305 874. Due to the biaxial stretching, the shell is oriented and consequently shows excellent resistance to deformation and elastic elasticity. The stretching occurs at 70 to 95_°C in an aspect ratio of 2.4 to 3.8, in both the longitudinal and transverse directions. The area stretch ratio is 7 to 12. To improve the dimensional stability, partial or complete heat setting may be carried out after stretching. This is usually done at 120 to 160_° C. As a result of heat setting, the film shows up to about 90 when exposed to heat _° C only a relatively small shrinkage of less than 20, in particular less than 15% in the longitudinal and transverse directions.

The packaging casing can be easily processed into sausage casings. The gathering process can surprisingly be carried out without the addition of antiblocking agents. It is sufficient if only the usual shirring aids such as paraffin oil and water are used. The pleats are completely removed after the skin has been removed and filled and are no longer recognizable from the finished sausage.

The casing can be filled with sausage mass plump and wrinkle-free, without the filling pressure causing undesired bulging of the casing or delamination of the layers. Even with heavy sausages with a caliber of more than 100 mm and a length of more than 60 cm, the core temperature of the subsequent cooking process is 80_°C no formation of long tips, no pear-shaped deformations and no "bag-in-bag" formation observed. The sausages obtained are cylindrical and have a uniform cross section. Due to the high material strength and the excellent elastic resilience, there is no costly cooling of the sausages in cold water. A short, intermittent cooling shower with a subsequent cooling phase in air is sufficient.

Because of the increased water vapor barrier, a cooked sausage with the casing according to the invention can be stored for at least 8 weeks without serious weight loss and without wrinkling. In the case of veal sausage fillings, the graying of the sausage material occurs much later compared to stretch-oriented PA 6 casings (of the same film thickness), which can be attributed to an improved oxygen barrier.

Sausages with the casing according to the invention can be cut smoothly and without tearing. No layer separation occurs when the shell is peeled off in a spiral.

The invention is illustrated by the following examples. Percentages are in percent by weight.

example 1

Using conventional coextrusion technology, a 3-layer pipe is built up using 3 single-screw extruders and a downstream coextrusion die:<tables id="tabl0001" num="0001"><img file="EP0467039B1_D0001.tif" /></tables>molded with a diameter of 23 mm and a total wall thickness of 0.35 mm. The individual layer thicknesses are:<tables id="tabl0002" num="0002"><img file="EP0467039B1_D0002.tif" /></tables>

This tube is brought to stretching temperature (about 80 ° C.) within 2 seconds by means of IR radiation.

By applying a pressure of about 1 bar inside the heated tube, it is stretched biaxially by an area factor of 8.6 and heat-set, laid flat and wound up in a second bubble.

The thickness of the resulting film is 40 µm. A determination of the individual layer thicknesses gives:<ul id="ul0001" list-style="none"><li>25th around outer layer</li><li>10th around middle class</li><li>5 around inner layer.</li></ul>

Example 2

A 3-layer pipe is made analogously to Example 1 with a structure of:<tables id="tabl0003" num="0003"><img file="EP0467039B1_D0003.tif" /></tables>produced and stretched to form a hose and heat-set as in Example 1. The layer thickness distribution corresponds to Example 1.

As comparative examples<ul id="ul0002" list-style="none"><li>a) a single-layer tube made of an oriented polyamide / polybutylene terephthalate blend (oPAxPBT blend) according to DE-A-34 36 682,</li><li>b) an unoriented single-layer blow hose made of polyamide-12 (PA-12) and</li><li>c) a multi-layer cover with the structure of polyamide adhesion promoter polyethylene (PA / HV / PE) according to DE-A-38 16942 manufactured.</li></ul>

Table 1 contains mechanical characteristics and the barrier properties of these casings. Table 2 shows the application comparison based on meat sausage or liver sausage fillings. The advantageous properties of the casings according to the invention were particularly evident in the application technology assessment.<tables id="tabl0004" num="0004"><img file="EP0467039B1_D0004.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0467039B1_D0005.tif" /></tables>