Biaxially oriented coextruded multilayer foil.

p0001The invention relates to a biaxially oriented, co-extruded multilayer film with a polypropylene layer as a base layer.

p0002For packaging purposes in the past were mostly cellophane films, such as cellophane^R, Used but nowadays predominantly etc. through further development of the prior art by plastic films of polyethylene, polyvinyl chloride, polypropylene. Plastics have been replaced.

p0003Particular importance is attached to the biaxially oriented polypropylene films obtained, since these can be produced by modifying the formulation of the layered structure by an appropriate choice of parameters in the co-extrusion of multiple layers and the selection of different coatings such as acrylate, PVDC for almost all packaging requirements.

p0004To use these films come with a few exceptions, such as a packaging material for cigarette, in printed form.

p0005It is known that polypropylene is nonpolar and can be polypropylene films, due to their low surface tension, not print almost. For improving the wettability and adhesion when printing and laminating polypropylene films are usually subjected to a corona discharge, thereby increasing the surface tension of about 30 mN / m to values ​​of 36-42 mN / m and the films are printable.

p0006The effect of this pre-treatment of the film by corona discharge decays with time and the surface tension reaches almost regardless of their initial value of the order of about less than / equal to 35 mN / m. Once this order has been reached, the film is almost not again printable. In other words, this means that polypropylene films remain printable only a limited time in spite of the pretreatment by means of a corona discharge.

p0007In addition to this disadvantage of only short-term printability of polypropylene films by far the more serious disadvantage of corona discharge is that there is a drastic deterioration in the odor quality of the film, wherein this degradation depends on the intensity of the corona discharge. So the sensory almost indifferent polypropylene film is deteriorated by corona discharge in their odor quality such that it no longer is suitable for many applications, where the medium can come into direct contact with the film.

p0008The object of the invention is, etc. for food packaging, filling material. to provide a polypropylene film which can be printed over a long period of time without causing a deterioration in the odor quality of the film.

p0009This object is achieved in that outer layers are applied to the polypropylene film on both sides, of which at least one outer layer has a surface tension has greater / equal to 36 mN / m, without pretreatment by corona discharge is printable and sealable.

p0010In the invention, the printable topcoat is a copolyester in the form of polycondensates of terephthalic acid and ethylene glycol or butanediol. Furthermore, it is possible that both top layers are copolyesters in the form of polycondensates of terephthalic acid, isophthalic acid and ethylene glycol or butanediol. It may be a polyolefinic sealing raw material of C₂-, C₂C₃-, C₂C₄-, C₃C₄-, C₂C₃C₄- raw materials or mixtures of these raw materials and the one cover layer a copolyester and the other top layer. The base layer is suitably made of a pure polypropylene homopolymer, equipped with additives such as antistatic agents, lubricants, stabilizers, nucleating agents, neutralizing agents and / or antiblocking agents.

p0011To reduce the stretching temperature of the biaxially oriented, co-extruded multi-layer polypropylene film of the base layer 10 are usually - 30 wt .-% of hydrocarbon resin, based on the weight of the base layer, was added.

p0012The total thickness of the multilayer sheet is generally from 15 to 60 microns, especially 20 to 40 microns, wherein the thicknesses of the surface layers 0.2 to 2 .mu.m, be in particular from 0.5 to 1.5 microns. The surface tension of the film is moving in the range of 36 to 42 mN / m.

p0013The multi-layer composite sheet according to the invention, in particular, used as sealing printable and odorless packaging film.

p0014As has already been mentioned above, the copolyester of the outer layers are thermoplastic polycondensation of two or more aliphatic or aromatic polybasic carboxylic acids and dihydric alcohols.

p0015They may as phosphorus compounds are prepared either by direct esterification of the carboxylic acids and alcohols as well as by the transesterification process, using the customary catalysts such as Ca, Li, Zn or Mn salts and stabilizers.

p0016Examples of copolyesters are polycondensates of terephthalic acid and ethylene glycol or butanediol. Instead of the isophthalic acid and adipic acid, or sebacic acid Acaleinsäure can be used as components.

p0017The polybasic carboxylic acids can contain for improving the adhesion of coatings or functional groups.

p0018Examples of such carboxylic acids are the p-hydroxy benzoic acid, 5-sodium sulfoisophthalate or Trimelittsäure.

p0019The combination of the various polyesters can also take the form of a block condensation.

p0020The ratio of the acids used may be in the range of 90:10 - 10:90 mol% can be varied. The melting point of the polycondensate and the crystallisation behavior can thereby be set according to the requirements.

p0021Mixtures of various copolyester can be used instead of a Kopolyesters. Further, the copolyester may be used in the form of blends with other polymers.

p0022Examples of the polymers which can be blended into the copolyester, polyolefin homo- and copolymers such as polyethylene, polypropylene, poly-4-methylpentene, ethylene-vinyl acetate copolymers or their saponified products, ionomers, polyamides, etc. The type and amount of the added polymers is subject to the requirements imposed on the film.

p0023The Kopolyesterschichten may contain for optimization of the slip and slip properties inert organic or inorganic particles, waxes or siloxanes, as well as further customary additives such as antistatics antioxidants, among others.

p0024The merging of the polypropylene film and the surface layers of copolyester by means of coextrusion, which called Koextrusionsmehrschichtdüsen be used which allow the setting and succession adjusting the flow rates of the individual layers of the multilayer film with high accuracy. From the use of the adapter technology in the production of the multilayer film is apart in general, since the coordination of the flow rates of the various raw materials is very difficult.

p0025The co-extrusion produced on or coated on both sides and in succession in the longitudinal direction at a temperature of 120 - 130 ° C and in the transverse direction at a temperature of 160 - film stretched 170 ° C will last heat set. This heat treatment is carried out in a conventional manner, namely at a temperature of preferably 150-160 ° C over a period of about 0.5 to 10 sec Through the use of sealable Kopolyestern switched as a result of one or both sides heat-sealable and printable films. receive. The sealing layers may also contain known additives, in particular, contain dimethyl polysiloxane.

p0026In another embodiment of the invention, the film has a three-layer structure with both sides different layers. While there is a Kopolyesterschicht on the print side as the top layer, the opposite surface of the base layer is coated with a polyolefin sealing raw material. For this C₂-, C₂C₃-, C₂C₄-, C₃C₄-, C₂C₃C₄- raw materials and mixtures of these raw materials, it is understood C₂ = polyethylene, C₂C₃ = a random copolymer of ethylene and propylene units, C₃C₄ = a random copolymer of propylene and butene units C₂C₄ = a random copolymer of ethylene and butene units C₂C₃C₄ = a random Perpolymer of ethylene, propylene and butene units. The recipes for these raw materials are described for example in German Offenlegungsschriften 32 47 998 34 44 158 and 35 09 384th

p0027Because of the opposite polypropylene different crystallization of the copolyester used, it is expedient to carry out the biaxial stretching of the multilayer film in a simultaneous frame, in which a contact with hot rollers is avoided. If the biaxial stretching as a result of stretching in the manner that is first in the longitudinal direction and then in the transverse direction, or first in the transverse direction and then stretched in the longitudinal direction, wherein the longitudinal stretching is carried out by means of rolls having different circumferential speeds and the transverse stretching in the tenter frame, need to reduce the stretching temperatures of polypropylene base layer 10-30% hydrocarbon resins are added. It is essential that the two dilations are performed in succession and at pre-given temperatures and draw ratios. The coextrusion of the base layer forming polypropylene layer and the outer layers forming copolyester or polyolefinic sealing raw materials by means of a coextrusion die, cooling the coextruded film to its solidification, reheating the film to the stretching temperature and the longitudinal and transverse stretching are carried out in a conventional manner (see. German Auslegeschrift 28 14 311). The cooling of the multilayer film to solidify and their reheating for stretch temperature is carried out with the aid of one or more rolls which are maintained at an appropriate temperature. The longitudinal stretching of the film is carried out by means of two corresponding to the desired stretching ratio running at different speeds roller pairs and transverse stretching with the aid of an appropriate tenter frame. It is expedient to cool the film after the first stretching and bring on the way to second stretching back to the required stretching temperature.

p0028In the following three examples of the multilayer film of polypropylene and sealable outer layers, and two comparative examples are described.

example 1

p0029It was coextruded a base layer of polypropylene applied to both sides sealable outer layers of copolyester through a flat. The thickness of the layers in each case is 1 .mu.m, thickness of the multilayer film 25 microns, the surface tension of 42 mN / m and the sealing temperature 135 ° C. The resulting film is very easy to print for a long time and odorless without pretreatment by corona discharge.

example 2

p0030The base layer of polypropylene on one side with a layer of copolyester having a thickness of 1 .mu.m and on the other side with a layer of sealable C₂C₃ raw material was co-extruded through a flat die. The multilayer film obtained is also readily printable and odorless without pretreatment. The sealing temperature is on the side of the cover layer from the C₂C₃ commodity around 120 ° C.

example 3

p0031There is a base layer of polypropylene on one side with a covering layer of copolyester and on the other side with a top layer of a sealable copolymers of propylene and ethylene or butene (1) units and / or a terpolymer of ethylene, propylene and butene (1) units coextruded. The sealable outer layer may have additional 5-15 wt .-% of a low molecular weight, compatible with the olefin resin composition of the sealable outer layer resin and / or of 5 - 50 wt .-% of a polypropylene homo-polymer, based on the sealable outer layer, be added. Further details of the sealable outer layer are described in DE-OS 34 44 158th

p0032The multilayer film obtained is printable and odorless, its sealing temperature is 100 ° C.

Comparative Example 1

p0033The base layer is a polypropylene layer, while the two outer layers are sealable C₂C₃ raw material layers. This coextruded multilayer film is subjected to a corona discharge to give a good printability of the multilayer film results in the fresh state, but is accompanied by a strong odor nuisance.

p0034Printability can be a function of the particular storage conditions of the film to within a period of six to eight months.

Comparative Example 2

p0035It is the same multi-layer composite film is used as in Comparative Example 1, but not pretreated by a corona discharge. The film is odorless, but not printable.