A PACKING MATERIAL FOR ASEPTIC PACKAGES

Abstract: The invention relates to a packing material in the form of a web intended for aseptic packages. The pack­ ing material consists of a laminate, the side of the pack­ ing material web which is intended to form the inside of the packages consisting of a thermoplastic layer, prefer­ ably polyethylene. This polyethylene layer has a sterile surface and is covered in turn by a thin plastic film [6] which completely covers the said polyethylene layer and maintains its sterility. On the conversion of the packing material to packages, the thin plastic film is separated from the surface-sterile polyethylene layer, whereupon the remaining packing material is formed to a tube [28] which is filled with sterile contents so as to form closed packing units [30].

Description: The present invention relates to a method of producing a packing material for aseptic packages of the type which is manufactured in that a web of packing material is formed to a tube by joining together the longitudinal edges of the web, whereupon the tube formed is filled with the intended contents and divided up into individual packages or packing containers through repeated flattening and sealing of the tube along narrow zones located across the tube, wherein the packing material is provided along the whole surface which is intended to form the inside of the packages with a bacteria-tight thin plastic coating according to the preamble of claim 1. The invention, moreover, relates to the method for the manufacture of such packing material. It is known that so-called aseptic packages of the non-returnable type can be manufactured by the filling of sterile contents into sterilized packing containers, the filling process having to be carried out, of course, under aseptic conditions. One example of such a packing method is the aseptic packing system marketed by Tetra Pak International AB which consists in that a web of a packing material, consisting of a laminate of paper and plastics and frequently also aluminium foil, is formed to a tube with interior plastic coating (usually polyethylene) by joining together the longitudinal edges of the web in the above-mentioned manner. The inside of the packing material then has to be sterilized, so that the sterile contents should keep their sterility in the package, and this is done by heating of the inner plastic layer of the tube with the help of a heater introduced into the tube or by means of a chemical process; whereby the plastic inside is put into contact with a sterilizing agent, preferably hydrogen peroxide. The most common process is, however, that a combination of chemical and thermal sterilization is used, which consists in that the web is brought into contact first with hydrogen peroxide by being dipped into a bath, and that the tube formed is subsequently heated by means of a heater introduced into the tube, so that on the one hand the hydrogen peroxide decomposes and vanishes, and on the other hand the inside of the tube is fully sterilized. In order to achieve and to maintain full sterility of the packing material web, the tube forming process must take place inside a closed sterile chamber, wherein an aseptic atmosphere is maintained under a slight pressure and the sterilization process must be accurately monitored so as to ensure in a reliable manner the complete sterilization of the inside of the material web. The plastic inside of the packing material is actually sterile when the plastic coating is applied with the help of an extrusion process, since the plastic in the coating operation has a temperature of approx. 200°C, that is a temperature which substantially exceeds the temperature at which bacteria and microorganisms can stay alive. However, immediately after cooling, the packing material produced comes into contact with air contaminated by bacteria, so that the plastic coating, sterile at the moment of manufacture, is infected, which means that the plastic surface of the packing material, which is intended to form the inside of the packing container produced, has to be sterilized when the packaging takes place. This sterilization of the packing material web with the help of thermal and/or chemical agents can be avoided, though, if the plastic layer of the packing material web is provided with a thin protective coating of a non-porous, bacteria-tight plastics which has such good adhesion to the plastic layer of the packing material web, that its sterility is maintained, but which at the same time does not actually fuse together with the plastic layer of the packing material, but can be pulled off the same when the packaging takes place, thus exposing the sterile plastic coating of the packing material. A packing material as mentioned in the opening part of claim 1 is known (FR―A―2 366 932). The thermoplastic layer forming the inside surface of the packages is coated particularly by the same material. The separation thereof is carried out by high temperatures or by interposing an additional separation sheet which creates some problems. The objective of the invention consists in good separating possibility of the coating without problems with regard to producing and use of the packing material itself and with regard to recontamination of the inside-forming surface of the packing material. The invention is claimed in claim 1. Further embodiments are claimed in sub-claims and described below, particularly by the enclosed schematic drawing, wherein Fig. 1 shows a greatly enlarged cross-section of a packing material produced in accordance with the invention, Fig. 2 shows a side elevation of a packing machine wherein the packing material is used, and Fig. 3 shows a side elevation of a second packing machine wherein the packing material is used. Fig. 4 shows schematically a side elevation of an arrangement for the manufacture of the packing material in accordance with the invention. The laminate material shown in cross-section in fig. 1 consists of a relatively rigid base layer 1 of paper or cardboard, one side of which is covered by a thermoplastic coating 2, preferably polyethylene, which coating 2 is intended to constitute the outside layer of the package and to protect the base layer 1 against moisture, oil etc. which would rapidly penetrate into the fibrous base layer and impair its rigidity, if the base layer were unprotected. The laminate comprises furthermore a gas-tight barrier layer 4 of metal foil, preferably aluminium foil, which layer is laminated to the base layer 1 by means of an intermediary thin thermoplastic layer 5, which preferably is constituted of polyethylene. The barrier layer 4 is not always present in packing material of the type referred to here, but since it constitutes an excellent gas barrier, it is in most cases advantageous to incorporate an aluminium foil layer in the laminate. The plastic layer 5 constitutes only a binder between the metal foil layer 4 and the base layer 1, and can therefore be very thin. On top of the metal foil layer 4 is placed a thicker thermoplastic layer 3 of polyethylene, which plastic layer is intended to form the inside of the package which is to be produced from the packing material. The plastic layer 3 is thicker than the plastic layer 2 because it is intended, in addition to forming a liquid barrier, also to function as a sealing layer, that is to say after folding, assembling and overlapping of the packing material, parts of the sealing layer 3, after heating and simultaneous pressing together, shall be fused together with other parts of the sealing layer 3 of the laminate so as to form tight sealing joints of high sealing strength. Onto the sealing layer 3, according to the invention, a further plastic layer, namely the coating 6 is applied which is very thin (corresponding to a gram-weight of between 5 and 10 g/m²). The coating 6 is constituted of thermoplastics of a higher melting point than the plastic material in the layer 3, namely by polypropylene. Owing to the differences in melting temperature it is difficult to obtain any surface fusion between the polyethylene layer 3 and the polypropylene coating 6, so that the polyethylene layer 3 will not attach itself to the polypropylene coating 6 with any substantial adhesive power, and it will be possible to separate the coating 6 from the layer 3 simply by pulling off the layer 6. The boundary layer 7 between the layers 3 and 6 is sterile because the polypropylene layer 6 on application by means of extrusion had a temperature exceeding 150°C, and the surface of the thermoplastic layer 3 has also been heated to this temperature during the application operation. The boundary layer 7 retains its sterility as long as the thin polypropylene coating 6 remains in position on the surface of the plastic layer 3, and at the moment when the plastic coating 6 is removed, the surface layer of the plastic layer 3 thus continues to be completely sterile. Thus, it is important to apply the polypropylene coating 6 onto the plastic layer 3 of polyethylene under such conditions that the boundary layer 7 and the two assembled plastic surfaces are heated so much that complete sterility is obtained, whilst on the other hand the adhesion between these plastic layers is so low that the two layers can easily be separated from each other. The adhesive powers must be so great, therefore, that the outer polypropylene coating 6 will remain in position and form a barrier during the transport, storage and handling of the packing material until the packing material is introduced into the packing machine, when the coating 6 will be pulled off so as to expose the sterile surface layer of the polyethylene layer 3. The packing laminate which is shown in fig. 1 can be produced e.g. with the help of an arrangement of the type which is shown schematically in fig. 4. The arrangement comprises two extruders 12, 14 by means of which a polyethylene layer 3 is applied to the material web 11 in a first extrusion operation, whilst a thin plastic layer, being a polypropylene coating 6, is applied to the polyethylene layer 3 in a second extrusion operation. The material web 11, which is assumed to consist of a base layer of a fibrous material, e.g. paper, whose outside is coated with a plastic material, e.g. polyethylene, and whose inside comprises, for example, a layer of aluminium foil, applied with the help of a thin laminating layer of polyethylene. In fig. 4 the aluminium foil layer of the material web 11 is facing upwards during the passage between the first pair of laminating and cooling cylinders 13, and with the help of the extruder 12 a polyethylene film 3 is extruded into the nip between the cylinders 13, the extruded polyethylene layer 3 on the one hand being cooled with the help of the pair of cylinders 13 so as to stabilize, and on the other hand being fixed to the aluminium foil layer of the material web 11. Since the polyethylene coating layer 3 applied is of a temperature after cooling which is less than the sterilization temperature, it is necessary in certain cases to prevent bacteria-contaminated air from coming into contact with the polyethylene layer 3 coating. This can be achieved in the manner which is shown in the figure in that a hoodlike device 40, which is of such a width that it extends over the whole material web 11, is arranged between the laminating and cooling cylinders 13, 15. Into the said hood 40 is blown sterile air, which may be obtained for example by heating or by sterile-filtering, so that a certain pressure is maintained in the hood 40 in order to prevent further bacteria-contaminated air from entering the hood. The thin polypropylene coating 6, is applied with the help of the extruder 14°. Owing to polypropylene having poor adhesive strength on polyethylene, no surface fusion will take place between the polyethylene layer 3 and the polypropylene coating 6, and it is possible for the rest, by means of the cylinder pressure of the laminating cylinders 15, to control to a certain extent the adhesion between the thin polypropylene film applied and the polyethylene layer 3 so as to obtain a degree of adhesion which is so low that the thin polypropylene coating 6 can be readily removed, but that it nevertheless adheres to such a degree that it does not detach itself during normal transport and handling of the packing material, thus making it possible for bacteria to be introduced to the polyethylene layer 3. The finished laminate web 17 is wound, in the manner described ealier, onto a magazine roll, not shown here. As mentioned above it is assumed that the temperature of the polypropylene coating 6, when it is brought into contact with the polyethylene coating 3 applied earlier, should be so high that any microorganisms and bacteria present on the plastic layer 3 would be completely destroyed. Experience shows that the temperature of the polypropylene coating 6 must exceed 150°C (preferably 200°C). In order to obtain the desired limited adhesion between the plastic layers 3 and the coating 6 and at the same time to ensure that the plastic layer 3 along its whole surface is also heated to a temperature of approx. 150°C for a time sufficiently long for all microorganisms and bacteria to be destroyed, the compression pressure as well as the temperature of the compression and cooling rollers 15 are controlled so that the cooling is not forced and that the compression pressure produces the appropriate degree of adhesion between the plastic layers. This means in practice that the cooling effect is kept very low and that the cooling rollers in actual fact are often heated to approx. 70-100°C, and that the compression pressure is kept as low as possible whilst constant monitoring ensures that the plastic layer 3 and the coating 6 are pressed against each other to establish mutual contact along the whole width of the packing material. Fig. 2 shows schematically a sketch of an automatic packing machine of the type which operates with a plane packing material web which is converted to a tube. In fig. 2 a magazine roll with packing material of the type which is shown in fig. 1 is designated by numeral 21 and the packing material web is designated 17. Furthermore an aseptic chamber, which in principle is closed, is designated 26 and a sealing device for the flattening and sealing of the tube 28 formed from the packing material web 17 is designated 29. The manufacture of the packages in principle proceeds so that the packing material web 17 is rolled off the magazine roll 21 and is passed over an upper guide roller 22, fixed in the frame of the packing machine, whereupon the packing material web 17 is passed vertically downwards towards the aseptic chamber 26 of the packing machine. At the inlet 31 to the aseptic chamber 26 the packing material web 17 passes between two rollers or cylinders 23, when the outer thin plastic film 6 is pulled off and is guided around one of the rollers 23 to be collected on a magazine roll 24. The remaining part of the packing material web, whose inner plastic layer 3 has now been exposed, is passed through the opening 31 into the aseptic chamber 26. The aseptic chamber 26 is sterilized before the start of the production with the help of superheated steam and/or a chemical sterilizing agent, whereupon the sterile atmosphere inside the sterile chamber 26 is maintained in that sterile-filtered air is blown in and in that a slight pressure is constantly maintained in the sterile chamber 26 so that further, bacteria-contaminated, air cannot enter into the chamber. The exposed sterile inner plastic layer 3 of the packing material, afters its introduction into the sterile chamber 26 through the opening 31, is thus prevented from coming into contact with bacteria-contaminated air, so that the sterile surface of the inner plastic layer 3 is preserved. To prevent the outside of the packing material from introducing bacteria into the sterile chamber which in some manner might come into contact with the sterile surface of the packing material, the outside of the packing material web in certain cases must be washed or cleaned of accumulations of bacteria. In the sterile chamber 26 the packing material web is formed to a tube 28 by means of forming devices, not shown here, and after the longitudinal edges of the packing material web have been joined together the tube is filled with the intended sterile contents which are introduced into the tube by means of the filler pipe 27 introduced into the sterile chamber 26. The formed and filled tube 28 is passed out of the sterile chamber 26 through the opening 25, whereupon the tube is sealed off with the help of the sealing device 29 in narrow sealing zones at right angles to the longitudinal axis, so as to form separate packing containers 30. The said packing containers 30 may be subjected to further shaping processes, e.g. so as to acquire parallelepipedic shape, or else it is possible, by arranging the sealing device 29 in a manner known in itself, to shape the tube to tetrahedral packages. The individual packages are separated from the tube by means of cutting through the sealing zones formed. A second realization of a packing machine is shown schematically in fig. 3 wherein the packing material web 17, which is of the type described earlier with a thin protective film of the polypropylene coating 6 covering the sterile surface of the inner plastic layer 3, is rolled off the magazine roll 32 and is passed over an upper guide roller 33. In this realization of the machine the packing material web 17 is also passed vertically downwards from the guide roller 33 at the same time as the packing material web 17 is formed to a tube 28. However, instead of the thin polypropylene coating 6 being pulled off the packing material web 17 whilst the same is plane, as in the procedure described earlier, the thin polypropylene coating 6 in the machine according to fig. 4 is pulled off only in conjunction with the actual tube formation of the packing material web 17, when e.g. the packing material web 17 may be shaped around a mandrel-like device 34 which at the same time serves as a guide surface for the pulling off of the thin polypropylene coating 6, which in the manner described earlier is then wound and collected on a magazine roll 24. By exposing the inside plastic layer 3 of the packing material web 17 only in conjunction with the tube formation, and by the said guide surface 34 for the pulling off of the thin protective film of the polypropylene coating 6 covering, at least in part, the mouth of the tube 28 formed, it should be possible to maintain a sterile atmosphere inside the tube 28 without a risk of the exposed sterile surface of the layer 3 coming into contact with bacteria-contaminated air. If extra safety against exposure of the packing material web 17 to the effect of bacteria is required, the area around the mouth of the tube 28 and the said guide surface 34 for the pulling off of the thin protective polypropylene coating 6 may be surrounded by a screenlike arrangement 35 and a slight pressure of sterile-filtered air may be maintained in the tube 28. In the same manner as before, the tube 28 is filled with sterile contents through the filler pipe 27, whereupon the sealing off of the tube to individual packing containers 30 takes place with the help of the sealing elements 29. It is a great advantage of the packing material in accordance with the invention that the inside layer of the packing material web does not have to be sterilized with the help of chemical or thermal sterilizing agents before or in conjunction with tube formation. This is of special importance where chemical sterilizing agents are concerned, since it is complicated and expensive to remove all residues of the sterilizing agent used before the packing material web is brought into contact with the contents. It is another great advantage that the aseptic chamber 26 can be made considerably much simpler for the reason that all devices and means for the removal of chemical sterilizing agent residues in general are placed in the aseptic chamber. The disadvantages associated with the invention, namely that the thin polypropylene coating 6 first has to be placed onto the packing material and then removed so as to form waste material, will in many cases be outweighed by the advantages of having a simpler aseptic system in the packing machine. Since the plastic coating 6 is constituted of pure polypropylene it can be melted and used again.

Claims: 1. Method of producing a packing material for aseptic packages of the type which is manufactured in that a web of packing material is formed to a tube (28) by joining together the longitudinal edges of the web, whereupon the tube formed is filled with the intended contents and divided up into individual packages (30) or packing containers through repeated flattening and sealing of the tube along narrow zones located across the tube, wherein the packing material (17) is provided along the whole surface which is intended to form the inside of the packages with a bacteria-tight thin plastic coating (6), wherein said inside forming surface of the packing material as well as the surface of said coating connected to said inside forming surface of the packing material are sterile and wherein both said thin plastic coating and the material forming said inside forming surface of the packing material are different thermoplastic materials and can be re-separated from each other, characterized in that the combination of the following layers of said packing material is used: a) the melting temperature of the thin thermoplastic coating (6) is higher than the melting temperature of the material of a the thermoplastic layer (3) forming the inside of the packages (30) consists of polyethylene, b) the thin thermoplastic coating (6) consists of polypropylene with a thickness of between 5 and 10 g/m² and in that the thermoplastic layer (3) and the polypropylene coating (6) are extruded by separate extruders (13, 14) in two separate but successive extruding operations, the surface or contact zone (7) of the thermoplastic layer (3) extruded is protected by sterile gas until the polypropylene coating (6) has been applied onto the contact zone (7) by means of a hood (40) between said successive extruding operations, and said thermoplastic coating (6) of polypropylene is applied to the thermoplastic layer (3) at a temperature sufficient to heat the contact zone (7) of the thermoplastic layer (3) facing said coating (6) to a temperature exceeding 150°C. 2. Method as claimed in claim 1, characterized in that a relatively rigid base layer (1) of paper or cardboard being covered at least on one side by a thin layer (5) of polyethylene is used and that a metal foil is interposed between said thin layer (5). 3. Method as claimed in claim 2, characterized in that the thermoplastic layer (3) is coated by the polypropylene coating (6) being at a temperature of about 200°C, and that said thermoplastic layer (3) and said coating (6) are compressed by cooling rollers (15) being at a temperature of between 70 and 100° C.

Inventors: PUPP, HERWIG
Applicants: AB TETRA PAK
Application year: 1981
Main classification: B65D65
IPC codes: B65B55, B65D65