CA2043187C

CA2043187C - Multilayered tubular packaging casing

Info

Publication number: CA2043187C
Application number: CA002043187A
Authority: CA
Inventors: Karl Stenger; Marliese Saal
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1990-05-26
Filing date: 1991-05-24
Publication date: 2001-07-24
Anticipated expiration: 2011-05-24
Also published as: HUT61503A; EP0467039B1; ATE126126T1; EP0467039B2; ES2077110T5; EP0467039A2; HU911753D0; DK0467039T4; DE9007334U1; JPH0716943A; US5185189A; FI105902B; FI912503A; CA2043187A1; DK0467039T3; EP0467039A3; FI912503A0; DE4017046A1; ES2077110T3; HU219218B

Abstract

A multilayered tubular packaging casing for pasty matter, in particular synthetic sausage casing, on a polyamide basis, is comprised of an outer layer based on aliphatic polyamide, aliphatic copolyamide or a polymer blend comprising at least one of these compounds; a central layer which is impermeable to water vapour and comprises a polyolefin and an adhesion-promoting component; and an inner layer which preferably is impermeable to oxygen and is based tin aliphatic and/or partially aromatic poly-amides and/or aliphatic and/or partially aromatic copolyamides.
The casing can be stuffed with sausage meat in a tight and crease-free manner, without bulging or delamination of the individual layers. Due to improved water vapour barrier properties, scalded sausages surrounded by the casings can be stored at least eight weeks without substantial weight loss or crease formation. As a result of the improved oxygen barrier properties, discolouration of liverwurst-type stuffings is observed later than for casings previously known.

Description

~,~, ~,J .~, e~ . , CI 3 MULT:CLAY'ERE17 TUBULAR PACKACTNG 'CASTNG
The present invention relates to a multilayered tubular packaging casing for pasty materials, in particular to a synthetic sausage casing, based on polyamide.
Casings based on a polyamide have gained an increasing importance as foodstuff packaging materials, in particular as synthetic sausage casings. Quite a number of sausage types require casing materials exhibiting low permeability to water vapour in order to make sure that the weight loss of the sausage as a result of water losses during storage is kept as small as possible. Only then will the sausage casing surround the sausage meat in a t3.ght, crease-free manner, even after relatively long storing periods, and no hollow spaces where jelly pockets can form will be observed between the inner wall of the casing and the sausage meat. For this purpose, it is known to combine the polyamide layer with a polymer layer whose permeability to water vapour is lower than that of the polyamide layer. There are, however, types of food where even the reduced permeability of these composite films is too high. For example, sausage meat of the liverwurst type discolours when it is exposed to atmospheric oxygen, so that casings of the type described above are not suited for 'them. To reduce the permeability to oxygen, the poly-amide film additionally requires an oxygen barrier layer.

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zo~31-lm3 -z-In EP-A-0,305,959, a multilayered polyamide film is disclosed, which may, for example, be used in the form of a heat-sealable bag for packaging meat and poultry.
The casing comprises a barrier layer towards oxygen, of an amorphous, partially aromatic copolyamide (PA 6I/6T) comprised of units of hexamethylene diamine, tereph-thalic acid and isophthalic acid. This copolyamide layer constitutes the core layer of the film. It is surrounded on both sides by polyolefin layers which form a barrier towards water vapourP whereby the a.nner poly-amide layer has the purpose of preventing moisture from penetrating from the packaged food into the copolyamide core layer. It is, however not advantageous to use polyolefin layers as inner layers of sausage casings, for they have an inadequate adhesion towards the sausage meat and thus favor the formation of jelly pockets.
EP-A-0,127,296 (=US-A- 4,606,922) has tried to overcome this problem. It proposes to provide a ionomer for the inner surface of a multilayered polyamide film and to treat the inner surface with ionizing radiation.
In accordance with DE-A-38 16 942 (=US-A-4,888,223) the formation of creases on a multilayered polyamide casing can be avoided by subjecting the inner layer comprising a polyolefin resin to a corona discharge treatment and optionally also to dust it with starch powder. The pro-posed solutions have the disadvantage that the inner surface must be subjected to an additional pretreatment in order to prevent the formation of creases and jelly pockets between the inner casing surface and the sausage composition.

It was therefore an object of the instant invention to propose a tubular packaging casing based on polyamide, which after being stuffed adheres firmly to its con-tents, in particular sausage meat. It should be pos-sible to employ the casing without any problems as a synthetic sausage casing, in particular for cooked or scalded sausages. The casing should have a reduced per-meability to steam and atmospheric oxygen. Even after having been stored for 4 to 6 weeks, sausages provided with the casing of the invention should neither have a noticeably reduced moisture content, which would manifest itself in a reduced weight of the sausage and a creased appearance, nor should there be observed the formation of jelly pockets between the inner casing wall and the sausage meat as a result of detachment of the casing from the sausage meat. The casing should also be suited for the air-tight packaging of goods which are sensitive to air, such as sausage meat of the liverwurst type, so that these goods remain unchanged during storage and in particular do not discolor undesirably.
Compared to other multilayered polyamide casings with good barrier properties, the casing of this invention has a low wall thickness, which preferably is not more than about 60 um, particularly preferably not more than about 45 um. The lower limit for the wall thickness is about 30 um. The diameter of the tubing is adapted to the intended use. If the casing is used for cooked or scalded sausages, its diameter will generally be 30 to 160 mm, in particular 40 to 135 mm.

~~.~r L~~~ ~~ 6~ ~:~ { i) d ~a Sa l.:~ r.,J ~~ <) d The packaging casing is comprised of at least three layers, i.e., an outer, a central and an inner layer, the outer and the inner layers being comprised of poly-amides and the central layer being comprised of a poly-mer blend including a polyolefin.
The outer layer usually is thicker than the two other layers. In the case of a total thickness of the film of 30 to 60 um, the outer layer generally has a thickness of 14 to 48 Um. Therefore, this layer can be considered as the real carrier layer of the multilayered casing.
It has a comparably high water absorbing capacity.
During the scalding or boiling of the sausages, the outer layer absorbs water, which during the subsequent cooling and storing of the sausages is set free again.
This causes the outer layer to shrink during drying, so that the casing surrounds the cooled-down sausage tightly and without creases. Since due to the central layer the casing is substantially impermeable to water and water vapour, the outer layer cannot absorb any moisture from the sausage meat.
The outer layer is essentially comprised of an alipha-tic polyamide, aliphatic copolyamide or of a polymer blend containing at least one of these compounds.
The aliphatic polyamide is a homopolycondensate com-prised of aliphatic primary diamines having in particu-lar 4 to 8 carbon atoms, and aliphatic dicarboxylic acids having, in particular, 4 to 10 carbon atoms, or is a homopolymer of omega-aminocarboxylic acids having 6 to 12 carbon atoms or of the lactams thereof. The aliphatic copolyamide comprises the same units and is, for example, a polymer based on one or various alipha-f,~ ~J ~~ r.~ .~ ~J

tic diamines and one or various aliphatic dicarboxylic acids and/or one or various omega-aminocarboxylic acids or the lactams thereof. Examples of suitable dicar-boxylic acids include adipic acid, azelaic acid, sebacic acid and dodecane dicarboxylic acid; suitable diamines include tetra-, yenta-, hexa-, and octamethylene diamine, with hexamethylene diamine being particularly preferred. 11-Amino-undecanoic acid is an example of omega-aminocarboxylic acids, and epsilon-caprolactam and omegalauric lactam are examples of lactams. Polycaprolactam (PA 6) and polyhexamethylene adipinamide (PA 66) or blends of these polyamides are particularly preferred aliphatic polyamides. The aliphatic copolyamide is comprised of several of the units enumerated above. A preferred aliphatic copolyamide is comprised of units of caprolactam, hexamethylene diamine and adipic acid (PA 6/66).
The water absorbing capacity of the outer layer is higher than that of the central layer and preferably also than that of the inner layer, so that it can be soaked with water prior to the stuffing process without problem and possesses the required smoothness and flexibility. The barrier properties of the inner layer are improved by the central layer exhibiting reduced water absorbing properties. Therefore, PA 6 and PA 66 are particularly well suited for the outer layer, it being possible for these 'two polyamides to be present in admixture with other water-absorbing polymers, including polyesters. By the addition of polyester the biaxial stretching of the tubing is facilitated, i.e., the stretching force required is reduced unexpectedly, and the overall strength of the casing is increased.

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f..,, ,~ ~= t:.~ .~. ~9 a As is known, polyesters are condensation products of diols and aromatic dicarboxylic acids, in particular terephthalic acid and optionally also isophthalic acid.
The polyester may be modified, to a minor extent, with aliphatic dicarboxylic acids, such as adipic acid. The diols are in particular aliphatic compounds of the formula HO-(CH2)n-OH (n denoting 2 to 8), such as ethy-lene glycol, 1,4-butylene glycol, 1,3-propylene glycol or hexamethylene glycol, and alicyclic compounds, such as 1,4-cyclohexane dimethanol. Polybutylene terephtha-late is particularly preferably employed in the poly-mer blend. The polyester is usually present in an amount of 5 to 15 °o by weight, relative to the polymer blend of the outer layer of the packaging casing.
In individual cases it is expedient to modify the outer polyamide layer by adding up to 25 °/ by weight of further polymers which are compatible with the poly-amide or polyamide blend. Suitable additional poly-mers include, for example, polyolefins, i.e., homo- or copolymers of alpha-olefins having 2 to 8 carbon atoms, in particular ethylene, propylene and butylene, pre-ferably polyethylene, in particular LDPE and LLDPE, and also modified polyolefins carrying functional groups, of the types employed as adhesion-promoting components for the central layer, such as polyolefins with carb-oxyl groups, i.e., groups comprising acids, esters, anhydrides and salts of carboxylic acids. By the addi-tion of the polyolefin and of the modified polyolefin, the adhesion between the outer layer and the central layer and the smoothness of the casing are improved.
However, in general, their amount in the outer layer does not exceed 3 to 10 % by weight, so as not to im-pair the water-absorbing properties and the strength of the outer layer substantially. Optionally, the outer ~:~ f'j /~ ~ .,r~ ~'t 5"'I
w2! ~..r :a 'C1y .:a. ~ '3 _ 7 _ layer furthermore contains customary additives, such as matting agents to reduce the gloss of the casing. The outer layer readily accepts ink and can be provided with prints without problem.
The central layer of the packaging casing is substanti-ally impermeable to water vapour and does not absorb water.
Lt should have a thickness of at least 2 um. In general, a thickness of between 5 to 25 um is sufficient to ensure that the casing possesses the high degree of impermeability to water vapour that is required for the production of scalded and cooked sausages.
The central layer is comprised of one or several poly-olefinic compounds and an agent by which the adhesion between the two polyamide layers and the intermediate polyolefin layer is improved. This agent may be homo-geneously distributed throughout the cross-section of the central layer or it may be present on the surface only, i.e., in the areas immediately adjacent to the adjoining polyamide layers. In the first case, the central layer is comprised of one single coat, and in -the latter case the central layer is preferably comprised of a polyolefin core coat, which on both surfaces is provided with a coat comprising a poly-olefin/adhesion promoter mixture or an adhesion promoter alone. This means that in the first case the casing wall comprises a total of three layers, whereas in the latter case it comprises a total of five layers.
In view of the desirable low total thickness of the tubing wall of less than 60 um and the possible low tubing diameter, preference is given to the three-layered build-up of the casing, in order to avoid difficulties with regard to the construction of suitable coextrusion dies. In the preferred three-~~ 'S ~ ~~ i ;~.~
-layered embodiment, the adhesion-promoting component is not applied between the central and the outer or the central and the inner layer, respectively, as an addi-tional coat, but it is incorporated into the central layer. Thereby, the adhesive bond between the central polyolefin layer and the adjoining outer and inner polyamide layers is strengthened, so that a firm composite film results from coextrusion, and even when 'the sausages are treated with hot water no layer separation is observed.
In general, the adhesion-promoting component is present in an amount of 5 to 50 % by weight, in particular of to 35 °/ by weight, relative to the polymer blend of the central layer. It should be noted, however, that in general the permeability to steam of the central layer is increased by the functional groups of the adhesion promoter, so that the added amount of adhesion promoter should be kept as low as possible. Usually, 20 to 25 °/
by weight of adhesion promoter, relative to the polymer blend of the central layer, are sufficient.
In general, the polyolefin of the central layer is a homopolymer of ethylene or propylene or a copolymer of linear alpha-olefins having 2 to 8 carbon atoms or a blend of several of these polymers. Suitable copolymers include C2/C3 copolymers and C3/C4 copolymers and also C2/C3/C~ terpolymers. A mixture comprising C3/C~
copolymer and C2/C3/C4 terpolymer is an exemplary suitable blend.

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The adhesion-promoting component is an agent known from the production of coextruded composite films from polar and non-polar plastics, such as polyamide and polyole-fin. Usually, it is a polyolefin resin modified with functional groups, the resin being comprised of units of ethylene and/or propylene and optionally also other linear alpha-olefins having 3 to 10 carbon atoms. The olefinic chain guarantees the compatibility with the polyolefin of the central layer and the extrudability of the adhesion-promoting component. The functional groups in particular are units of vinyl acetate, vinyl alcohol, ethylenically unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, and the esters and salts thereof, in particular the Na- and Zn-salts, and furthermore ethylenically unsaturated carboxylic acid anhydride groups. They impart an adequate wettability to the polyamide and polyolefin melts during coextrusion, so that a firm bond results.
Suitable adhesion promoters include, in particular, graft polymers, copolymers or terpolymers comprising ethylene or propylene units, with at least one comono-mer selected from the group including (meth)acrylic acid, (meth)acrylates (i.e.,esters of n-alkanols having 1 to 6 carbon atoms, such as butyl acrylate), ethylene vinyl acetate and/or malefic anhydride. The units carry-ing functional groups are usually present in an amount of 3 to 12 °/ by weight, relative to the weight of the adhesion-pramoting polymer. The modified polyolefin resins also include rubber-modified polyethylene.

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If the casing is to be dyed, it is expedient to work the dystuff or color pigments into the central layer, so that a direct contact with the contents of the casing is avoided. This measure provides the advantage that the stretching process is not impeded by the nucleating effect of color pigments. It has also been stated that color pigments can be dispersed particular-ly uniformly and easily when the pigments are incorpo-rated into the central layer.
In one embodiment, the inner layer is comprised of an aliphatic polyamide, aliphatic copolyamide or of a polymer blend incorporating at least one of t!~ese compounds. Further polymers, such as polyolefins, poly-esters or ionomers may be present in the inner poly-amide layer in an amount of up to 15 % by weight. This means that the inner layer is comprised of a polymer or polymer blend of the type described above for the outer layer. However, the aliphatic polyamide of the inner layer preferably has a lower water absorbing capacity than the aliphatic polyamide employed for the outer layer. Therefore, aliphatic polyamides having relatively long methylene chains, such as PA-11 or PA-12 or copolyamides comprising these units are advantageously used for the inner layer. The adhesion between the sausage composition and the inner layer can be improved by adding ionomers to the aliphatic polyamide. The layer of aliphatic polyamide or copolyamide protects the sausage meat from direct contact with the polyolefin. The oxygen barrier proper-ties of the inner layer are, however, comparatively low. If the casing is to possess a reduced permeability 6ij !?~~ !l ~,~1 ~~ '~, f:~ y ~_; ~ ...~. ZJ a to oxygen, the inner layer is comprised of a partially aromatic polyamide and/or a partially aromatic copoly-amide. The partially aromatic polyamide or copolyamide is composed of aliphatic and aromatic units.
In a first embodiment of the partially aromatic poly-amide, the aromatic units are predominantly or exclu-sively represented by the diamine units. Examples of these units include xylylene diamine and phenylene diamine units. The dicarboxylic acid units of this embodiment are exclusively or predominantly aliphatic units and as a rule they contain 4 to 10 carbon atoms.
Preferred aliphatic dicarboxylic acid units include sebacic acid and azelaic acid, and in particular adipic acid.
In a second embodiment of the partially aromatic poly-amide, the aliphatic units are predominantly or exclu-sively represented by the diamine units, whereas 'the dicarboxylic acid units are predominantly or exclusive-ly represented by radicals of aromatic dicarboxylic acids, in particular isophthalic acid and terephthalic acid. The aliphatic diamine units in general have 4 to 8 carbon atoms, and preferably are caprolactam and/or hexamethylene diamine units. Preferably, the partially aromatic copolyamide of the second embodiment comprises units of caprolactam and/or hexamethylene diamine and units of terephthalic acid and/or isophthalic acid, and in particular it is PA 6I/6T.
In the first embodiment, the partially aromatic poly-amide may contain up to 5 mol % of aliphatic diamine units and up to 5 mol % of aromatic dicarboxylic acid ~9 a d ~ "~ .,~ a'7 r,i ~, '~ j._ ;:~ _ (> o units. Likewise, in the second embodiment, the parti-ally aromatic polyamide or copolyamide may contain up to 5 mol °~ of aromatic diamine units and up to 5 mol %
of aliphatic dicarboxylic acid units.
It has, however, been found that inner layers exclu-sively comprised of partially aromatic polyamide and/or partially aromatic copolyamide are relatively unflexi-ble and brittle and that the mutually adjoining inner surfaces of the laid-flat casing tend to stick together. As a result of the action of moisture contained in the packaged goods there is, moreover, the danger of embrittlement of this kind of material.
Tn those cases where the casing is to be employed as a sausage casing, it is therefore especially advantageous additionally to incorporate aliphatic polyamides and/or aliphatic copolyamides into the inner layer. In parti-cular in those cases where the partially aromatic poly-amide or copolyamide responds to the above described second embodiment, the partially aromatic polyamide is present in a blend with a saturated linear aliphatic polyamide and/or a saturated linear aliphatic copolyamide. Moreover, biaxial stretching can be particularly well performed with a tubing having an inner layer comprised of such a polyamide blend.
The additional aliphatic polyamide contained in the polymer blend of the inner layer is the product obtained from reacting an aliphatic dicarboxylic acid which preferably has 4 to 10 carbon atoms with alipha-tic primary diamines which preferably have 4 to 8 carbon atoms. Examples of suited dicarboxylic acids include adipic acid, azelaic acid, sebacic acid and dodecanedicarboxylic acid, examples of suited diamines include tetra-, yenta-, hexa- and octamethylene diamine, hexamethylene diamine being employed parti-cularly preferably. Furthermore, the additional alipha-tic polyamide of the polyamide blend can also com-prise units of omega-aminocarboxylic acids having 6 to 12 carbon atoms or the lactams thereof, such as units of 11-aminoundecanoic acid, epsilon-caprolactam, or omega-lauric lactam. The aliphatic copolyamides in the blend with the partially aromatic polyamide or copoly-amide contain various of the above-described units of aliphatic polyamides. A preferred aliphatic copolyamide is comprised of units of caprolactam, hexamethylene di-amine and adipic acid. Particular preference is given to PA 6, PA 66, PA 6/66, PA 11 and PA 12 or blends of these polymers, as further components of the inner layer in addition to the partially aromatic polyamide and/or copolyamide.
The proportion of the partially aromatic polyamide and/or copolyamide contained in the inner layer preferably is 5 to 85 % by weight, particulary 10 to 40 °/ by weight, relative to the polymer blend comprised of partially aromatic and aliphatic polyamides and/or copolyamides. In this range of the mixing ratio, a flexible inner layer is obtained, which nevertheless is impermeable to oxygen. By increasing the content of partially aromatic palyamide/copolyamide the adhesion between the sausage meat and the inner casing wall can likewise be increased, so that the properties of the casing can be varied in this respect, -too, by increas-ing the amount of partially aromatic polyamide/
copalyamide.

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The tubular casing is preferably produced by coextru-sion of the polyrners forming the individual layers through an annular die, for example, by means of a device as described in EP-A-0,305,874. The casing is oriented by biaxial stretching and consequently exhibits excellent, resistance to deformation and good resilience properties. Stretching is performed at temperatures between 70 and 95 °C in a stretch ratio of 2.4 to 3.1, both in the longitudinal direction and in 'the transverse direction. The surface area stretch ratio is 7 to 12. To improve its dimensional stability, the stretched casing can be partially or completely heat-set. This thermal treatment is usually carried out at temperatures of between 120 °C and 160 °C. As a result of heat-setting, the shrink of the film at temperatures of up to about 90 °C is relatively low, i.e., a shrink of Less than 20 %, in particular of less than 15 %> is measured both in the longitudinal direction and in the transverse direction.
The packaging casing can be processed into sausage casings without problem. It was surprising to state that shirring can even be performed without the addi-tion of anti-blocking agents. It is sufficient to use the conventional shirring aids, such as paraffin oil and water. No shirring folds remain after 'the deshir-ring and stuffing of the casings.
The casing can be stuffed with sausage meat in a tight and crease-free manner, without undesirable bulging or delamination of the individual Layers taking place.
Even on heavy sausages having a diameter of more 'than 100 mm and a length of rnore than 60 cm, no formation of Long tags nor of pear-shaped deformations nor bag-in-bag formation is observed during the subsequent heating ao~3Z-m~3 of the sausages to core temperatures of about 80 °C.
The resulting sausages are cylindrical and have a uniform diameter over their entire length. Due to the good mechanical strength and resilience properties of the material, no expensive cooling of the sausages in cold water is required. It is sufficient to subject the sausages to short, intermittent rinsing with cold water and then to let them cool down in the ambient air.
Due to the improved barrier properties of the casings towards water vapour, scalded sausages surrounded by the casings of this invention can be stored for at least eight weeks without substantial weight loss or crease formation. As a result of the improved oxygen barrier properties, discoloration of liverwurst-type stuffings is observed markedly later than in the case of stretch-oriented PA 6 casings having the same wall thickness as the casings according to this invention. Sausages pro-duced with the casings of this invention can be cut easily and without tearing. When the casing is peeled off helically, no layer separation occurs.
The invention will be explained in greated detail by means of the Examples which follow. In the Examples, percentages denote percent by weight.
Example 1 A three-layered tubing having the following build-up is produced by means of a customary coextrusion method, using three single-screw extruders followed by an annular coextrusion die:

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Outer layer: PA 6 (Grilon F 47) Central layer: Dry blend comprising 80 % of HDPE
(Lupolen 2441 D); and 20 °/ of adhesion promoter based on a LLDPE/methacrylic acid copolymer (Plexar OH 002) *
Inner layer: PA 6 (Grilon F 47) The extruded tubing has a diameter of 23 mm and a total wall thickness of 350 um. The individual layers have the following thicknesses:
Outer layer: 220 um Central layer: 90 um Inner layer: 40um By means of IR radiation the tubing is heated to the .
required stretching temperature (about 80°C) within two seconds. By applying a pressure of about 1 bar to the interior of the heated tubing, the tubing is stretched by a surface area factor of 8.6, subsequently heat-set in the inflated state, laid flat and wound up.
The resulting film has a total thickness of 40 um. The individual layers have the following thicknesses:
Outer layer: 25 um Central layer: 10 um Inner layer: 5 um Example 2 A three-layered 'tubing having the following layer build-up is extruded, stretch-oriented and heat-set as described in Example 1:
Trade-mark ~~ 'u . r~ .'i~ C~ .1 Outer layer: PA 6 (Grilon F 47) Central layer: Dry blend comprised of 80% of HDPE
(Lupolen 2441 D); and 20 % of adhesion promoter based on a LLDPE/methacrylic acid copolymer (Plexar OH 00~) Inner layer: Dry blend comprised of 70 °/ of PA 6 (Grilon F 47); and 30 °/ of PA 6I/6T
(Grivory G ?_1*) The thicknesses of the individual layers correspond to those of Example 1.
The following comparative tubings were produced:
a) a monolayered tubing of an oriented polyamide/poly-butylene terephthalate blend (oPA/PBT blend) accord-ing to DE-A-34 36 682, b) a non-oriented monolayered blown tubing of poly-amide 12 (noPA 12), and c) a multilayered casing having the build-up polyamide/
adhesion promoter/polyethylene (PA/HU/PE) according to DE-A-38 16 942.
The mechanical data and barrier properties of these casings are compiled in Table 1 below. Table 2 shows the properties of the casings when being stuffed with sausage meat of the liverwurst type and of the slightly smoked ham sausage type. In this cornparsion, the advan-tages offered by the casings according to this inven-tion become particularly apparent.
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Claims

1. A multilayered tubular packaging casing for pasty matter, the casing comprising:

an outer layer comprising a polymer selected from the group consisting of an aliphatic polyamide, an aliphatic copolyamide and a polymer blend comprising at least one aliphatic polyamide or aliphatic copolyamide;

a central layer having a thickness of at least about 2 µm, said layer comprising polyolefin and an adhesion-promoting component; and an inner layer comprising a polymer selected from the group consisting of an aliphatic polyamide, a partially aromatic polyamide, an aliphatic copolyamide and a partially aromatic copolyamide.

2. A casing as claimed in claim 1, wherein the outer layer comprises a polymer selected from the group consisting of a polyamide, a copolyamide and a polymer blend comprising at least one polyamide or copolyamide, wherein the polyamide or copolyamide comprises i) aliphatic dicarboxylic acid units and aliphatic diamine units or ii) units of omega-aminocarboxylic acids having 6 to 12 carbon atoms or the lactams thereof.

3. A casing as claimed in claim 2, wherein the aliphatic dicarboxylic acid units have 4 to 10 carbon atoms and the aliphatic diamine units have 4 to 8 carbon atoms.

4. A casing as claimed in claim 3, wherein the aliphatic dicarboxylic acid units are adipic acid units, the aliphatic diamine units are hexamethylene diamine units and the units of omega-aminocarboxylic acids having 6 to 12 carbon atoms or the lactams thereof are selected from the group consisting of 11-amino-undecanoic acid, omega-lauric lactam and caprolactam.

5. A casing as claimed in claim 1, wherein the outer layer comprises a blend comprising polyamide or copolyamide or both polyamide and copolyamide and up to 25 % by weight, relative to the polymer blend, of a further polymer, an ionomer or a polyester.

6. A casing as claimed in claim 5, wherein the further polymer is a polyolefin.

7. A casing as claimed in claim 6, wherein the polyolefin is polyethylene.

8. A casing as claimed in claim 5, wherein the polyester is polybutylene terephthalate.

9. A casing as claimed in any one of claims 1 to 8, wherein the polyolefin of the central layer comprises a homopolymer of ethylene or propylene, a copolymer of linear alpha-olefins having 2 to 8 carbon atoms or a mixture thereof.

10. A casing as claimed in claim 9, wherein the polyolefin comprises copolymers comprising units of ethylene/propylene, ethylene/propylene/butylene or propylene/butylene, or a blend thereof.

11. A casing as claimed in any one of claims 1 to 8 and 10, wherein the central layer comprises a blend of the polyolefin and to 50 % by weight of the adhesion-promoting component.

12. A casing as claimed in claim 11, wherein the central layer comprises 10 to 35 % by weight of the adhesion-promoting component.

13. A casing as claimed in any one of claims 1 to 8, 10 and 12, wherein the adhesion promoting component of the central layer is based on polyolefin resin modified by functional groups.

14. A casing as claimed in any one of claims 1 to 8, 10 and 12, wherein the inner layer comprises one or several polymers selected from the group consisting of a partially aromatic polyamide, a partially aromatic copolyamide, a linear aliphatic polyamide and an aliphatic copolyamide, wherein the partially aromatic polyamide or partially aromatic copolyamide comprises aliphatic diamine units and aromatic dicarboxylic acid units.

15. A casing as claimed in claim 13, wherein the aliphatic diamine units have 4 to 8 carbon atoms.

16. A casing as claimed in claim 14, wherein the aliphatic diamine units are caprolactam and/or hexamethylene diamine units.

17. A casing as claimed in claim 13, wherein the aromatic dicarboxylic acid units are isophthalic or terephthalic acid units or both isophthalic and terephthalic acid units.

18. A casing as claimed in any one of claims 1 to 8, 10, 12 and 15 to 17, wherein the inner layer comprises one or several polymers selected from the group consisting of a partially aromatic polyamide, a partially aromatic copolyamide, a linear aliphatic polyamide and an aliphatic copolyamide, wherein the partially aromatic polyamide or partially aromatic copolyamide comprises aromatic diamine units and aliphatic dicarboxylic acid units.

19. A casing as claimed in claim 17, wherein the aromatic diamine units are xylylene diamine or phenylene diamine units or both xylylene and phenylene diamine units.

20. A casing as claimed in claim 17, wherein the aliphatic dicarboxylic acid units have 4 to 10 carbon atoms.

21. A casing as claimed in claim 19, wherein the aliphatic dicarboxylic acid units are adipic acid units.

22. A casing as claimed in any one of claims 1 to 8, 10, 12, 15 to 17 and 19 to 21, wherein the total amount of the partially aromatic polyamide or copolyamide is 5 to 85 % by weight relative to the blend of partially aromatic and aliphatic polyamides and copolyamides contained in the inner layer.

23. A casing as claimed in any one of claims 1 to 8, 10, 12, 15 to 17 and 19 to 21, wherein the linear aliphatic polyamide or copolyamide comprised in the inner layer is one of the polymers listed in claim 2.

24. A casing as claimed in any one of claims 1 to 8, 10, 12, 15 to 17 and 19 to 21, which comprises a three-layered coextruded and biaxially stretch-oriented tubular film which may be also heat-set.

25. A casing as claimed in any one of claims 1 to 8, 10, 12, 15 to 17 and 19 to 21, wherein the wall thickness is 30 to 60 µm, the thickness of the outer layer amounting to 45 to 80 %, the thickness of the central layer amounting to 5 to 40 %, and the thickness of the inner layer amounting to 2 to 35 %, of the total thickness.

26. A casing as claimed in claim 25, wherein the wall thickness is 40 to 50 µm.