Coex film
A coex film ( coextruded film) is a versatile plastic film that tries to combine several properties of different film materials, for example barrier or welding properties. It is a composite film made up of several layers. It is technically possible to combine up to six layers of film. For example, a waterproof and opaque foil can be connected to create a security bag. Another advantage is the extreme tear resistance of the film. In practice, it can only be opened or torn open by destroying it, which makes manipulation of the packaged goods immediately visible.
Renewable raw materials are now increasingly being included in the material mix.
Manufacturing
Coex films are produced both in the blown process to form tubular films and as flat films with a biaxial stretching process. At least two extruders are used to liquefy plastic granulate to form polymer melts and filter them using a filter made of sintered metal . In the blow molding process, the two melt streams are combined in a multilayer ring nozzle to form a common film. After exiting the nozzle, the material is inflated and stretched through an air channel in the middle of the nozzle to form a bubble (dome). The resulting polymer tube solidifies in the process. The amount of air, the amount of granulate and the take-off speed determine the film thickness.
The production of coextruded flat films is more complicated. The polymer melts are brought together either via a multilayer nozzle or via what is known as adapter technology. In the latter, the melt flows to be combined are brought together in front of the nozzle via an adapter. One makes use of the properties of a laminar flow . The two (or more) melt flows are brought together to form a common melt flow using a specially designed adapter. This common melt flow is processed into a pre-film via a so-called coat hanger nozzle. This still liquid pre-film is sprayed onto a casting roller to cool down. A further process step, namely pinning (also: application), is necessary for air-free pressing of the still liquid film onto the casting roll. In the case of polyolefin films, an air nozzle is used for this purpose, which presses the film against the casting roller with a fine but strong jet of air.
In the case of polyester films ( polyethylene terephthalate , PET) an electrically charged wire (direct current) with a high voltage of up to 10 kV is used. The opposite pole is the earthed casting roller. During this process, the dipoles (-OH) in the polyester press the liquid polyester melt against the casting roller. The aim is to avoid an air layer between the polymer film and the roller. This would hinder the heat transfer.
The resulting pre-film is then stretched lengthways and crossways. The result is biaxially stretched films (BO films). The stretching usually takes place in two stages. In the first stage, the pre-film is passed over several heated rollers. In the case of PET, the temperature must be above the glass transition temperature. The film, which has now been warmed up, is stretched in the following part by faster-running rollers. Such a film is stretched three to four times its original length and then cooled again. A so-called clip chain is used for the second stretch. This holds the film at the edges of the film, heats it up again and extends it in width. The stretch factor is similar to the longitudinal stretch between three and four.
This stretched film is fixed in a final process step. This takes place in the clamped chain at PET at approx. 220 ° C. The film is then cooled back to room temperature and the thick film edges on which the film was clamped in the clip chain are cut off. It is necessary to cool down between the individual process steps in order to avoid unnecessary crystallization of PET, which would prevent the following process steps. The stretching process gives the film its final properties such as tear strength , elongation or shrinkage . A film can be produced from several granulates with unrelated materials by means of coextrusion.
use
A typical example of a simple coex film is film for potting soil - black on one side and white on the other. The ingredients of the potting soil are protected from harmful UV light by the black coating on the inside. The white side is used for printing. Beverage packaging, plastic bottles, for example for ketchup, or bags and bags as well as bags for vegetables and fruit are made from coex film.
High-quality coextruded films are produced exclusively as flat films through biaxial spreading. Examples of this are the carrier films for audio and video tapes, which were previously used in the corresponding cassettes. These had a smooth surface on one side and a rough surface on the other. The rough surface was achieved by adding fine pigment particles to the polymer. Such surfaces are important so that the foils can be wound easily. They allow the film layers to be shifted against each other. Two smooth surfaces practically stick together (glass plate effect). Another example is X-ray films. These are foils that are practically crystal clear and still easy to wrap. For this purpose, an unpigmented core layer is coextruded on both sides with a very thin, slightly pigmented outer layer. It is a three-layer film. The cloudiness or transparency is hardly influenced by the thin outer layers. Further examples of coextruded films are sealable films. To do this, the main layer of the polymer (e.g. PET) is combined with a copolyester with a lower melting point. One step in the production of such films is fixing: the film is clamped in the clip chain and heated; During this procedure, the copolyester layer melts again and remains on the film as an amorphous sealing layer. Without melting the copolyester layer in the fixation, it would remain crystalline and would therefore not be sealable.
A special variant of coextruded films are multilayer films, i.e. films that consist of many very thin individual layers. To produce them, two melt streams are brought together using adapter technology to form a common melt stream. This two-layer melt stream is then passed through a static mixer, e.g. B. directed the Kenics mixer. With each mixing element, the number of melt layers doubles. Depending on the polymers used, this results in shimmering colored foils that are used for decorative purposes.
In the automotive industry, the coex technology is used to manufacture plastic fuel tanks (KKB) by combining an inner, diffusion-proof layer with an outer, mechanically stressable layer. Several functional layers can be combined depending on the manufacturer and intended use. The state of the art is up to six layers (multi-layer).
Individual evidence
- ↑ TransPack-Krumbach KG: Technical term coex film in the packaging material glossary. In: www.transpack-krumbach.de. January 24, 2020, accessed January 27, 2020 .