DAVIDENKO, Alexandr Vladimirovich (per. Ostrovskogo, 48-5 Rostov-na-Donu, 8, 34401, RU)
RYZENKO, Sergei Petrovich (ul. Iliicha 44A-3, Rostov-na-Donu, 9, 34402, RU)
BORODAEV, Sergei Vasilievich (ul. 2-ya Koltsevaya 94, Rostov-na-Donu, 4, 34400, RU)
DAVIDENKO, Alexandr Vladimirovich (per. Ostrovskogo, 48-5 Rostov-na-Donu, 8, 34401, RU)
CLAIMS
1. A synthetic tubular casing for food products, characterized in that it comprises a coextruded outer layer having a relief fibrous-netlike structure.
2. The synthetic casing according to claim 1, characterized in that a relief fibrous- netlike structure is formed by alternating of thicker and thinner regions, wherein the maximum thickness of the layer in the thicker region is 80 μm, the minimum in the thinner region - 0 μm.
3. The synthetic casing according to claim 2, characterized in that the thick region has linear dimensions from 0.1 mm to 250 mm in a longitudinal direction and from 0.1 mm to 15.0 mm in a lateral direction, more preferably from 0.1 mm to 100.0 mm in the longitudinal direction and from 0.1 mm to 3.0 mm in the lateral.
4. The synthetic casing according to claim 2, characterized in that the thin region has linear dimensions from 0.1 mm to 100.0 mm in a longitudinal direction and from 0.1 mm to 30.0 mm in a lateral direction, more preferably from 0.1 mm to 40.0 mm in the longitudinal direction and from 0.1 mm to 15.0 mm in the lateral.
5. The synthetic casing according to claim 1, characterized in that the coextruded outer layer is made from a polymer or a mixture of polymers, each of which has a melt flow index from 0.2 to 15 g/ 10 min, measured according to ASTM D 1238 A.
6. The synthetic casing according to claim 1, characterized in that it comprises at least 2 layers.
7. The synthetic casing according to claim 1, characterized in that it is made straight, bent or helical.
8. The synthetic casing according to claim 1, characterized in that it is made monoaxially or biaxially oriented. 9. The synthetic casing according to claim 1 , characterized in that it is cut into flat films (strips).
10. The synthetic casing according to claim 1, characterized in that it is obtained by the blown-bubble extrusion method or by the process of biaxial orientation stretching. |
MULTILAYER TUBULAR FOOD CASING
WITH VARIABLE OUTER LAYER THICKNESS THE OUTER LAYER HAVING RELDEF FIBROUS-NETLIKE STRUCTURE
Field of Invention The instant invention relates to a multilayer coextruded tubular food casing in particular for sausages and processed cheeses, with an outer layer that has variable thickness and relief fϊbrous-netlike structure, which imparts a pleasing appearance to the casing.
Background of Invention
In the last decades, artificial sausage casings on the basis of synthetic polymers, mainly polyamide, have become widespread. Such casings have a number of advantages as compared with earlier used natural, artificial protein and cellulose casings. Synthetic sausage casings are distinguished by high mechanical strength, by better barrier properties in respect to moisture and oxygen, which increases the possible storage life of sausage products, by impermeability in respect to pathogenic microorganisms. Furthermore, they are produced with the aid of simple and highly productive extrusion technologies, which are not accompanied by the formation of toxic liquid wastes as, for example, in the production of a cellulose casing.
However, casings on the base of synthetic polymers in respect to their appearance significantly differ from the natural or artificial protein casings which consumers are used to. For them, a smooth, glossy finish with an unnatural shine is typical, while natural casings, as a rule, are distinguished by a matt surface, and artificial protein - by a fibrous structure. Usually, additional attractiveness is imparted to synthetic casings by means of a printed image applied onto their surface. However, such a printed image usually covers only a part of the surface of the casing, since it is a serious technical problem to coat a casing completely by a printed image, this being related to the difficulty or impossibility of applying a decorative layer onto the edges of a folded casing, to the permanent pollution of the printing drum, etc.
Another method of changing the appearance of synthetic casings is to introduce excipients into the polymer film material.
In DD patent No. 247830, published 22 July 1987, a sausage casing is claimed that is suitable for both uncooked smoked and hard smoked and also for cooked sausage products. The sausage casing in accordance with the invention is made of polyurethane with starch as the excipient. The thermoplastic composition consists of 60 - 90 wt.%, preferably 70 - 95 wt.% of polyurethane, and 2 - 40 wt.%, preferably 5 - 30 wt.% of starch. The claimed sausage casing may also be prepared from a polyurethane-cellulose composition with the
introduction of starch. Starch disperses well in polyurethane components or a polyurethane melt, and therefore the mixture does not have inclusions. The gas and water vapor permeability of the casing depends on the content of starch. In accordance with the invention an extrudable, non-sticky film is obtained that has a natural appearance and high gas and vapor permeability. Drawbacks of the claimed films include their large thickness about 0.1 mm, which results in an increase of the cost of sausage casings.
A sausage casing of polyurethane elastomer, the properties of which are similar to the properties of natural intestine casing, is claimed in patent DD No. 257384, published 15 June 1988. The polyurethane composition comprises 70 -98 wt.%, preferably 80 - 95 wt.% of polyurethane, 2 - 30 wt.%, preferably 5 - 20 wt.% of microcrystalline cellulose. The claimed casing when filled does not form folds and readily allows air, smoke and water vapor to pass through it, has a good capability for shrinkage, and an outer appearance similar to the properties of natural intestine, is physiologically harmless. The sausage casing is suitable for uncooked smoked and hard smoked and also for cooked sausage products. However, in the presented examples, the thickness of the film is relatively large and is 70 - 150 μm, and furthermore, there is a need for preliminary compounding of the thermoplastic mixture, which also increases the cost of the final product.
In application WO 03/073861 Al, published 12 September 2003, a food casing is described that is permeable to water vapor and smoke, that is especially suitable for uncooked smoked sausage of the salami type. It is made of a thermoplastic mixture, which comprises at least one aliphatic polyamide and/or copolyamide, one inorganic and/or organic excipient and one aliphatic and/or aromatic copolyamide having glycolic and/or polyglycolic units. Carbohydrates, which may consist of natural polysaccharide and/or a derivative thereof and also branched and crosslinked polysaccharides and derivatives thereof, are used as the excipient. Synthetic thermally stable fibers or powders on the base of polymers may be used. Inorganic excipients or reinforcing materials, such as glass fiber, glass thread, mineral cotton thread, talc, clay, etc. are also suitable. Organic excipients provide for increased permeability of the casing in respect to gas and water vapor. By varying the size of the excipient grains, its type and weight fraction, it is possible to control the roughness of the casing surface, as a result of which the casing acquires a silky, matt appearance, looking like parchment and making it look like a natural intestine.
A similar solution is disclosed in WO 03/073862 Al, published 12 September 2003, for a single-layer or multilayer casing with a matt, rough surface, giving it the appearance of a natural casing. This casing is the one most similar to the claimed casing. For its preparation a thermoplastic mixture composition as in application WO 03/073861 is used. In the
preferable variant, the thermoplastic mixture consists of at least one (co)polyamide and at least one organic and/or inorganic excipient. The fraction of the (co)polyamide in that case is 50 - 99 wt.%, preferably 65 - 98 wt.%, while the total fraction of the organic and/or inorganic excipient is 1 - 50 wt.%, especially preferable is 2 - 35 wt.% of the total weight of the thermoplastic mixture. The inventors claim vegetative powders and fibers, synthetic thermally stable fibers or powder on the base of fluoropolymers, polysulfons, polyestersulfons, polyesterketones, polyphenylsulfides, polyaramides, polyimides, aromatic polyesters, polyquinoxalines, polyquinolines, polybenzimidazoles, as well as carbonic fibers as organic excipients. Fibers or spherical particles of glass-like materials are suitable inorganic excipients. In an especially preferable variant of embodiment of the invention, the length of the organic fibers or grain size of such substances is 15 — 500 μm, the length of inorganic fibers is 1 — 250 μm. The introduced excipients create a rough microcontour of the casing surface. The method of producing such films provides for two stages. At first granulated material is prepared from a mixture of organic material and an excipient, then granulated material is processed into a tubular film by the blown-bubble extrusion method or by a method of biaxial orientation stretching. Multilayer casings are prepared by the method of coextrusion with aid of a head for coextrusion of multilayer films, wherein the layer containing the excipient forms the outer layer. Such a casing is used as a sausage casing, especially for cooked sausages, but its surface is only slightly rough. Furthermore, such films do not have a sufficiently high tensile strength along/across and with an increase in the amount of excipient the casing acquires the capability of tearing like paper.
At the same time, such polyamide casings, like other known casings with an outer layer of polyamide, are likely to disrupt in the processes of being filled, being subjected to thermal treatment and cooling. Brief Description of the Invention
The object of the instant invention is to create a casing, which is distinguished by its outer appearance from known polymer casings, which does not have the shine typical for them, and which is pleasant to the touch. A synthetic tubular casing for food products according to the invention is characterized in that it additionally comprises a coextruded outer layer having a relief fibrous-netlike structure.
Furthermore, it should have a reduced tendency to tearing when its surface is disturbed.
Polymer sausage casings are traditionally made from polyamide. In multilayer sausage casings the outer layer more often that not is a biaxially-oriented polyamide, which is a fragile material and in the case of even insignificant damage is easily torn in the direction of
the axis of orientation. Furthermore, this layer should be sufficiently thin, otherwise the casing will not have the required degree of elasticity. Damage to the casing may take place at different stages of its production and use upon insufficiently precise adjustment of the equipment, for example, during the applying of printed image, shirring, stuffing with sausage meat, clipping. Wherein, if obvious disruptions in the operation of the equipment, resulting in damage to all the layers of the casing, are as a rule obvious and easily fixed, minor damage affecting only the outer layer is not always detected, which results in breaks of the casing during thermal processing of the sausage and, accordingly, to losses of the raw meat material at the meat processing plants. Furthermore, if a sausage during the selling thereof is cut into separate pieces, damage to its surface quite often occurs and this results in a crack appearing along the length of the casing and loss of good appearance.
One of the known methods used for protection of the outer polyamide layer against damage is the application (lamination, coextrusion) of a polyolefϊn layer onto its surface. Wherein improvement is really achieved in respect to resistance against punctures and tearing. However, in that case too, the casing has a smooth shiny surface, and furthermore a polyolefin layer has reduced engagement with clips, as a result of which technological problems arise upon fulfillment of the clipping, filling and cooking of the sausage. The clips may be displaced during the filling of the casing and also in the process of heat treatment or cooling, resulting in deviation of the form of a separate sausage from the cylindrical form (cylindrical sausage is transformed into pear-shape body) or damage to the casing or a clip may slip off the sausage.
Therefore, one more object of the invention is improvement of the fixation of the clips during the clipping on the surface of the casing, which is especially suitable for the production of cooked sausage products. Detailed Description of the Invention
As a result of research carried out by the inventors of the instant invention, it was unexpectedly found that the objects of the invention may be achieved by creating a synthetic tubular casing for food products, which additionally comprises a coextruded outer layer having a relief fibrous-netlike structure. Wherein the relief fibrous-netlike structure is formed by alternating thinner and thicker regions, wherein the maximum thickness of the layer in the thicker region is 80 μm, in the thinner region this layer may be thinned up to zero.
The thick region has linear dimensions from 0.1 mm to 250 mm in a longitudinal direction and from 0.1 mm to 15.0 mm in a lateral direction, more preferably from 0.1 mm to 100.0 mm in the longitudinal direction and from 0.1 mm to 3.0 mm in the lateral.
The thin region has linear dimensions from 0.1 mm to 100.0 mm in a longitudinal direction and from 0.1 mm to 30.0 mm in a lateral direction, more preferably from 0.1 mm to 40.0 mm in the longitudinal direction and from 0.1 mm to 15.0 mm in the lateral.
The outer layer of the casing may be made from a polymer or a mixture of polymers, each of which has a melt flow index from 0.2 to 15 g/ 10 min, measured according to ASTM D 1238 A.
The casing comprises at least 2 layers.
It may be made straight, bent or helical, monoaxially or biaxially oriented. Furthermore, it may be cut along one or two edges with the formation of one or two strips of polymer film accordingly.
Wherein, any layer or layers of the casing may additionally comprise a pigment and/or dye and/or matting additives.
Furthermore, the outer layer may comprise antiblocking additives and/or gliding additives and/or matting additives. And the outer layer may be colored in a color that is in contrast to the color of the preceding colored layer.
Furthermore, the synthetic tubular casing according to the invention may be obtained by the blown -bubble extrusion method or by the process of biaxial orientation stretching.
Thus, the result of the instant research was the creation of a tubular casing for food products in which there is no shine inherent in synthetic casings and which has a relief fibrous-netlike structure. The relief fibrous-netlike structure of the casing is visually perceived as a net applied onto the surface of the casing, wherein the length of the net cells may vary within different limits: from fine-cellular to large-cellular and even remind one of a fiber, positioned parallel to the longitudinal direction of the casing. The absence of a shine on the surface of the casing and the presence of a relief fibrous-netlike structure distinguish it immediately from the number of prior art synthetic casings and make it pleasing for the consumer. Furthermore, the casing is pleasant by feel.
The creation of an outer layer having a relief fibrous-netlike structure is possible for all polymer sausage casings having a different structure, from one and more layers to their sensible number and combination.
The number of polymer layers, their sequence and the selection of materials for production of the layers is determined on the whole by the functional purpose of the layer.
The layers intended for lengthy storage of the food products are made in the form of a multilayer structure in which layers alternate, providing barrier properties in respect to water vapors, for example, polyolefin, and layers of polymers, providing barrier properties of the
casing in respect to oxygen, in particular, polyamides, copolymers of ethylene and vinyl alcohol, polyurethanes, etc. In order to produce single-layer and multilayer casings for food products, which in the process of curing pass through the stages of smoking or microbiological maturing, thermoplastic materials are selected, which provide a sufficiently high gas and vapor permeability of the casing and also permeability in respect to the smoke components of the smoke.
The thermoplastic materials used within the frame of the instant invention for the production of the outer layer and subsequent layers should satisfy a number of requirements, in particular: • they should be technologically compatible in the process of coextrusion, i.e., should have intersecting temperature processing ranges, wherein the use of technological additives, plasticizers, etc. is provided for controlling (adjustment) of the temperature ranges;
• they should have a value of the melt flow index (further — MFI) of the layer within the range of from 1 to 4 g/10 min at the extrusion temperature; plasticizers, gliding additives, nucleating agents, etc. may be used to correct the melt flow index of the melt.
The lower limit of the viscosity of the melt is related to the stability of the technological process of stretching the polymer material in the foπn of a film. The use of thermoplastic materials having a MFI less than 1 g/10 min results in local infringements of a continuous layer and, as a consequence thereof, in frequent breaks of the tubular casing. The upper limit of viscosity is due to the irregular thickness of the film layer. The use of thermoplastic materials having a MFI higher than 4 g/10 min results in irregularity of the thickness of the tubular casing by more than 15%. In the case where a mixture of polymers is used as the thermoplastic material, the necessary viscosity of the melt is achieved by use of polymers with a MFI in the range from 0.2 to 15 g/10 min, measured according to ASTM D 1238 A. At the same time in respect to the inhomogeneity of the outer layer thickness, such strict restrictions in respect to the continuity and irregularity of the thickness do not exist, and therefore not only a mixture of polymers may be used for its production, but also any polymer having a melt flow index from 0.2 to 15 g/10 min measured according to ASTM D 1238 A. All of the conventionally used polymer materials used for the production of synthetic sausage casings relate to such thermoplastic materials.
The thermoplastic material may be selected from the group consisting of polyamides, thermoplastic polyurethanes, block copolymer comprising polyester blocks and polyamides blocks, olefin polymer materials, (co)polyesters, thermoplastic elastomers on the base of styrene copolymers, thermoplastic cellulose esters.
A polyamide should be understood to mean an aliphatic polyamide and/or aliphatic copolyamide and/or partially aromatic copolyamide and/or a polymer mixture, the composition of which includes at least one of these compounds. The aliphatic polyamide and/or copolyamide is selected from the group comprising polyamide 6, polyamide 66, polyamide 610, polyamide 12, copolyamide 6.66, copolyamide 6.12. The producers and trade names of the aforesaid polyamides are well known to persons skilled in the art.
Partially aromatic copolyamide should be understood to mean copolyamides comprising in the polymer chain at least 25% aromatic units, for example polyamide 61.6T, polyamide MXD6. Polyamide 61.6, polyamide 6I.6T.66, polyamide 6T.66 and others also relate to such copolyamides.
Olefin polymer material should be understood to mean homopolymers of ethylene and/or propylene, copolymers of ethylene and/or propylene, ionomers. Copolymers of ethylene and/or propylene, additionally comprising monomer residues of α-olefϊns with the number of carbon atoms from 4 to 10, and/or unsaturated carboxylic acids and/or cyclic anhydrides of unsaturated dicarboxylic acids and/or esters of vinyl alcohol also relate to olefin polymer material.
Thermoplastic esters of cellulose, used as the thermoplastic material, comprise ester residues of at least one of the acids selected from the group consisting of acetic, propionic, butyric acids. The outer layer having a relief fibrous-netlike structure may be extruded together with the polyamide layer additionally comprising at least one water-soluble polymer selected from homopolymers and/or copolymers of vinylpyrrolidone, N-vinylalkylamide, alkyl oxazoline, alkyl glycol, vinyl alcohol, acrylamide, alkylenoxide, (met-)acrylic acid, maleic acid and/or cellulose ester, wherein its fraction is 5 — 40 wt.%. The outer layer may be coextruded with the polyamide layer, additionally comprising starch and/or dextrine and/or starch esters. A casing produced in such a manner, acquiring the advantages in accordance with the purpose of the invention, does not lose the permeability to smoke substances.
The synthetic tubular food casing with variable outer layer thickness may comprise an additional adhesive layer positioned between the outer and the polymer layer below it, if these layers do not have mutual adhesion. The adhesive material for polyamide and olefin polymer material is selected to be an olefin polymer material modified by at least one grafted compound selected from the group consisting of unsaturated carboxylic acid, glycidal ester of unsaturated carboxylic acid, a cyclic anhydride of dicarboxylic acid. Adhesive layers may also be between other layers of casings under condition that there is no mutual adhesion between them. At the same time, adhesives may be included directly in the composition of
one of the adjacent (touching) layers. Which adhesives should be used in the combination of some or other layers for achievement of adhesion between them is completely obvious to one skilled in the art.
In accordance with the purposes and objects of the invention, it is completely obvious that the claimed casing may comprise two or more layers. What is important is that the outer layer should have a relief fibrous-netlike structure. Depending on the purpose of the casing for the production of one or another kind of sausage (cooked sausages of different diameters, including linked sausage and also cooked-smoked and fresh-smoked sausages), processed cheeses, the synthetic casing construction is selected. As a rule, it is sensible to use a casing consisting of from 2 to 7 layers, but the number of casings may be more. Layers, for example, on the base of polyolefins and/or copolymers of ethylene and vinyl alcohol (saponified copolymers of ethylene and vinyl acetate) and/or (co)polyamides and/or polyesters and/or polyurethanes with different sequence of their positions may be the additional layers. Wherein, the layers may also comprise mixtures of polymers relating both to any of the aforesaid classes and to different classes. Thin adhesive layers, the thickness of which is mainly 1 — 5 μm, may be located between the layers.
The presented list of materials only confirms the possibility of their use within the frame of the instant invention, but does not limit the use of other thermoplastic materials satisfying the aforesaid criteria and is not comprehensive. In accordance with the objects of the invention, one of the methods of creating the claimed synthetic casing for food products is the extruded method of producing casings with subsequent orientation. However, it differs from the known methods of producing multilayer casings in that it provides for the introduction of at least one foaming agent into the thermoplastic material of the outer layer, with the subsequent coextrusion of at least two layers of thermoplastic materials through the extrusion head with an annular die into a multilayer primary tube, cooling the original tube with subsequent heating, blowing and oriented stretching into a secondary tube, thermofixation of the secondary tube and winding. Then the casing may be subjected to applying of a printed image thereon, shirring, transforming into ring-like or spiral shape, etc. The foaming agent in the present invention should be understood as a substance, which is gaseous at the temperature that melted polymeric composition (extrudate) has while it leaves the extruder die and at atmospheric pressure or a substance decomposing within the extruder with the release of a substance which is gaseous under the same conditions. (It is obviously that the foaming agent should be solid or liquid at the room temperature and atmospheric pressure). In the latter case the foaming agent and the extrusion conditions
should be selected in such a manner that decomposition would begin in any zone of the extruder except the first (compression zone), since otherwise loss of the gaseous substance is possible. Organic and inorganic substances, decomposing upon heating, in particular, sodium hydrocarbonate, potassium hydrocarbonate, zinc carbonate, ammonium carbamate, urea, azodicarbonamide, citric acid, 5-phenyltetrazole, and also substances comprising a residual amount of moisture or comprising water in the form of crystalline hydrates may be used as the foaming agents. In order to create a casing with an outer layer having a relief fibrous- netlike structure, it is preferable to introduce a foaming agent in an amount of 0.1 — 5.0 wt.%, more preferably 0.3 - 2.0 wt.%, into the thermoplastic material of that layer. An abrupt expansion of the gas takes place upon outlet from the head, resulting in foaming of the thermoplastic material with the formation of open cells, separated by the partitions of polymer material. For the purposes of the instant invention, it is preferable to use foaming agents in the form of a concentrate in a polymer carrier. The polymer carrier may be a polyolefin, polyamide or other thermoplastic material, used in the production of the indicated outer layer, wherein the polymer excipient may either coincide or differ from a concretely used thermoplastic material of the outer layer. An effect on the character of distribution of the heterogeneity of the outer layer is provided by the concretely used concentrate of the foaming agent, also by its concentration in the thermoplastic material, and also by orientation of the casing (monoaxial or biaxial) and the coefficient of stretching. The maximum thickness of that layer in the region of thickening is 80 μm, the minimum — 0 μm, that is the layer in some places disappears completely. As different from the closest prior art, the embodiment and use of such an inhomogeneous by thickness layer as a single-layer casing is not possible. The use of such a method is illustrated by examples 3, 5 - 7. Another method of creating the claimed synthetic tubular casing is the method of coextrusion of layers of a polymer film with the use of a multilayer head. A distinction of the method in this case is the use of dies rotating in opposite directions to create the outer layer. The threads of thermoplastic material flowing from the apertures of the dies intersect and adhere each to other and to the surface of foregoing layer, forming different structures (different textures) of the outer layer, from netlike to fibrous. The materials of the outer and lower-lying layers may have an identical nature, for example, polyamide/polyamide or polyethylene/polyethylene, which ensures good interlayer adhesion. Realization of this method is presented in example 1. In the case where materials of different nature are used, for example, a polyolefϊn/polyamide pair, up to 50 wt.% of an adhesive material is additionally introduced into the polyolefin layer in order to provide interlayer adhesion. The coextruded outer layer, made in accordance with the instant method,
has the appearance of a network, the thickness of the fibers and the dimensions of the cells of which are determined by the technological parameters of the extrusion process, such as the productivity of the extruder, the angular velocity of rotation of the die, the ratio of the longitudinal and transversal stretching. The thickness of the fiber of the grid coating does not exceed 80 μm.
One more method of creating a casing with a rough fibrous- netlike structure of the outer layer is the coextrusion method of producing a casing, a distinction of which consists in carrying out hot stamping of the primary tube in the process of coextrusion processing of the thermoplastic materials in a multilayer casing with subsequent oriented stretching. The primary tube is heated to a temperature of 70 0 C and passed through a pair of heated metal shafts having an engraved image on the surface thereof corresponding to the image of the inhomogeneous in respect to thickness outer layer, after which it blows up into the secondary tube. The method of heating the primary tube is determined by the material of the outer layer. For example, for polyamides and polyesters heating with IR radiation, a hot bath or heating with hot vapor to a glass-transition temperature is used; for olefin polymers, heating with IR radiation or heating with hot vapor to the Vicat temperature is used. The temperature of the shafts carrying out the stamping is also established depending on the material of the outer layer.
Furthermore, the claimed casing may be prepared with the introduction into the thermoplastic material of the outer layer of up to 25 wt.% of an additional polymer unlimitedly mixing in a melt with the starting thermoplastic material, but with limited compatibility therewith in a solid state and forming fiber (filament) with the main content of the additional polymer in the outer layer. As a result of the die stretching, the region of inhomogeneous composition is oriented along the tube. During the selection of the additional polymer, an essential characteristic is the melting temperature. In order to form lengthy fibrous regions it is necessary that the melting temperature of the thermoplastic material and of the additional polymer would differ by at least 40 0 C.
Since oriented stretching is carried out while heating the primary tube to the softening temperature (of polyolefins) or the glass-transition temperature (polyamides) of the main material, this determines one more criterion for the selection of the additional polymer. It is necessary that the softening temperature or the glass-transition temperature of the additional polymer differs from the oriented stretching temperature by at least 1 O 0 C. If this condition is fulfilled the orientation stretching leads to layer structure wherein the regions with a more content of basic material are transformed into a smooth areas of film and the regions with a more content of additional material are transformed into salient "fibers". For
an outer layer on the basis of polyamide PA 66, an acceptable additional material is polyamide PA 12; for an outer layer on the basis of polyethylene an acceptable additional material is polypropylene.
Use of this method is illustrated by example No. 9. However, persons skilled in the art may propose other methods of forming a layer that is inhomogeneous in respect to thickness, which does not in any manner affect the essence of the technical effect achieved upon use of the instant invention, since it is due to the presence of a fibrous-network structure in the outer layer.
Casings produced in accordance with the instant invention have enhanced mechanical properties. Such integrity defects of the polymer layer as cracks, punctures, scratches do not spread by themselves and do not extend beyond the cell of a net of an outer layer with variable thickness. Artificial casings from natural raw material (for example, Belkozin, Cutisin) have, as is different from synthetic sausage casings based on polyamide, less strength in the case of stretching loads. In some cases, in order to provide strength to such artificial casings during stretching loads, they are glued or melted with the tissue of the grid. It is obvious that a similar effect is also observed in this case.
A physical value characterizing the mechanical stability of a film against breakage upon damage to the surface is the tear resistance determined, for example, according to GOST 26128-84. Corresponding data are presented in the examples and in the table. Furthermore, in the case of embodiment of the outer layer from thermoplastic polyolefin having the structure of the outer layer in accordance with the invention, slipping down of the clips from the sausage is less often observed as compared with the same kind of casing, but having a solid polyolefin layer.
The following method was used to assess the strength of engagement of the clips to the casing:
• The preliminarily prepared sections of the casing are clipped together with the loop.
• A control mark of its initial position is applied on the casing below the clips by a marker.
• The loop of the clipped section of the casing is put on a hook of a device for testing samples, which is a platform, in the center of which a rod with a hook at the end is secured perpendicular to the plane of the platform. The weight of the device is selected depending on the caliber of the casing being tested: 3 kg with a caliber of the casing of 35 - 65 mm and 5 kg - with a caliber from 66 mm and more.
• The casing is slowly raised at the clipped end in such a manner that the platform would be at a height of about 10 cm from the floor and is held hanging for 5 seconds. The
device is lowered, the sample removed, it is given consideration and the displacement of the clips is measured.
The results of the tests are considered to be positive if the form of the clips does not change and displacement of the clips or their slipping off from the sample does not occur. During tests according to this method of 100 samples of a casing produced in accordance with example 3, the clips are displaced and slip off from 4 samples, while during a test of the same number of samples of a casing with a continuous outer layer (example 4, comparative), the clips slip off 17 samples.
The caliber of the casing in accordance with the invention may vary over a wide range: from caliber 19 to 290.
It is obvious to one skilled in the art in this field that a casing may be easily made bent, annular or spiral by using the known methods.
The casing in accordance with the invention, depending on the number of layers included therein, is suitable for producing cooked sausages of any caliber diameter, including links of different diameter, having both limited effective life and lengthy storage time, and also for sausages and processed cheeses having the taste and smell of smoked products.
Brief description of the drawings, explaining the essence of the invention:
Figs. 1 — 3, 5 show photographs of the outer appearance of casings prepared in accordance with examples 5 — 7, 9. Fig. 4 shows an optical microphotography of a cross-section cut of a casing prepared in accordance with example 6 (a negative image).
Possible embodiments of the instant invention are presented in the following examples, which are given by way of illustration and not by way of limitation.
The following conditional designations of the materials are used in the examples of embodiment of the instant invention (the brands of the materials and the producer are presented in the parentheses):
• PEi - metallocene high density polyethylene (Finacene ER 2278, Atofina)
• PE 2 - high pressure polyethylene (HPPE, brand 15813-020, OOO Kazanorg- synthesis) • SC - superconcentrate of chalk (Aksoy 106123, Aksoy plastik)
• SP, - concentrate of foaming agent (Basko T0027, OOO PNF "Bars-2")
• SP 2 - concentrate of foaming agent (Corducell ET 9520, NEMETZ Additive Plastic GMBH)
• Adg - HPPE, grafted with maleic anhydride (Bynel 623, Du Pont) • PA, - polyamide 6 (Ultramid B4, BASF)
• PA 2 - polyamide 6.66 (Ultramid C35, BASF)
• PA 3 - partially aromatic polyamide 61.6T (Selar 3426, Du Pont)
• PA 4 - polyamide MXD 6 (MX-Nylon 6007, Mitsubishi Gas Chemical Company, Inc.) • PA 5 - polyamide 12 (Grilamid L 25, EMS)
• SW - superconcentrate of white pigment (Schulman 8160, A. Schulman)
• SR - superconcentrate of red pigment in HPPE (Schulman 5330, A. Schulman)
• SG - superconcentrate of goldish pigment in PA 6 (Sicopas 30-1305, BASF)
Example 1 A mixture comprising 80% PA 1 , 14% PA 2 , 4% PA 3 and 2% SG is loaded into an extruder for the production of an outer layer. A mixture comprising 80% PAj, 14% PA 2 , 4% PA 3 and 2% SR is loaded into an extruder for producing a polymer layer adjacent to the outer layer. The melts are passed into. extruder heads. A continuous tube is extruded through an annular die, while a layer with variable thickness is extruded through rotating dies. The prepared primary tube is cooled to a temperature of 4O 0 C, folded into a flat tape then heated to a temperature of 70 0 C and subjected to biaxial orientation by simultaneous blowing and longitudinal stretching. As a result of the orientation, a secondary tube is obtained, which is subjected to thermofixation for 15 seconds at a temperature of 150 - 165°C with the simultaneous shrinkage of the tube by 20% in a longitudinal direction and 10% across. As a result a finished tube is obtained, which is wound into a roll. The casing has a tear strength of 0.39 kgf/mm.
Example 2 (comparative)
A single-layer polyamide casing is produced from a mixture of 80% PAi, 14% PA 2 , 4% PA 3 and 2% SR with the use of a single-layer head with an annular die in accordance with example 1.
The tear resistance of the casing is 0.19 kgf/mm.
Example 3
The following are used for the production of a triple-layer casing: a mixture of 59.3% PE 1 , 30% SW 5 10% SC and 0.7% SP, - for the outer layer; a mixture of 80% PA 1 , 14% PA 2 , 4% PA 3 and 2% SR for another polymer layer and resin Bynel 623 (Du Pont) between them as a third layer.
The compositions are loaded into extruders and extruded through a multilayer head with an annular die. The primary tube is cooled, folded into a flat strip, then heated to a temperature of 70 0 C and subjected to biaxial orientation by simultaneous blowing and
longitudinal stretching. The secondary tube is subjected to thermofixation for 15 seconds at a temperature of 145 - 150°C with the simultaneous shrinkage of the tube by 20% along and 10% across. As a result a finished tube is obtained which is wound into a roll. The tear strength of the casing is 0.62 kgf/mm. Example 4 (comparative * )
A triple layer casing with a homogeneous by thickness, smooth, continuous outer layer is produced by the cόextrusion method in accordance with example 3, using a mixture of 60% PE 1 , 30% SW and 10% SC for production of an outer layer, a mixture of 80% PA 1 ,
14% PA 2 , 4% PA 3 and 2% SR for the other polymer layer and an adhesive Bynel 623 (Du Pont) between them.
Upon testing 100 samples of the casing produced in accordance with example 3, the clips were displaced on or slipped off 4 samples, and upon testing the same number of samples of a casing produced in accordance with example 4, the clips were displaced on or slipped off 17 samples. Example 5 — 7.
The compositions presented in the table were loaded into seven extruders attached to a seven-layer head. A multilayer primary tube with a diameter of 26 mm is formed through an extrusion head. The primary tube is folded into a flat strip, then heated to a temperature of 7O 0 C and subjected to biaxial orientation by simultaneous blowing and longitudinal stretching. As a result of the orientation a secondary tube is obtained, which is subjected to thermofixation for 15 seconds at a temperature of 145 - 150 0 C, and simultaneous relaxation (shrinkage) of the tube by 20% along and 10% across. As a result a third (finished) tube with a diameter of 80 mm is obtained, which is wound into a roll.
Photographs of the outer view of casings prepared in accordance with examples 5 - 7 are presented respectively in Figs. 1 - 3. The relief fibrous-netlike structure of the outer layer, obtained as a result of using the described technology, is clearly seen in the photographs.An optical microphotograph (negative image) of the cross section of the casing having a relief fibrous-netlike structure and produced in accordance with example 6 is shown in Fig. 4. The rough character of the outer layer of the casing, occurring as a result of a burst of bubbles, is clearly seen on that photograph.
Example 8 (comparative).
The composition presented in the table is loaded into five extruders attached to a five- layer head and a five-layer casing is obtained in accordance with the technology described in examples 5 - 7 with an outer layer that is homogeneous in respect to thickness, is smooth and
continuous.
Example 9.
A five-layer casing with an inhomogeneous in respect to thickness outer layer, having a rough fibrous-grid structure, is obtained in accordance with example 8. The compositions for production of the layers are presented in the table.
A photograph of the outer view of the casing is presented on Fig. 5. The rough structure of the outer layer, formed by lengthy fibers of polymer material, is clearly seen on the photograph.
It follows from the table that the casings obtained in accordance with the instant invention are characterized by significantly better values of tear strength.
A casing with a relief fibrous-netlike structure in respect to outer appearance clearly stands out from the known polymer casings. It does not have the shine that is inherent in traditional synthetic casings; the alternating thin and thick regions make it pleasant to feel, and the unusual pattern of the texture make it recognizable and attractive for a consumer.
TABLE
* The average thickness of the layers is presented, calculated on the basis of the productivity of the extruders and the specific weight of the layer material.
** Conditional designations of the materials - see the text.
*** The resistance to tearing was determined in accordance with GOST 21628-84 (the sample for tearing of type I, direction of tear — across the machine.
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