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Issue 06/2017

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  • Bioplastics
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Basics Blown Film

Basics Blown Film Extrusion By Michael Thielen Blown Film Extrusion is an established process which is used to manufacture a wide range of thin commodity and specialized plastic films mainly for the packaging industry, but also for other sectors, such as for example mulch films for agricultural applications. Also known as Film Blowing Process, this extrusion process generally comprises the extrusion of a molten thermoplastic tube and its constant inflation to several times its initial diameter. This forms a thin, tubular product which may be used directly, or indirectly by slitting it to create a flat film. Materials used In the process of Blown Film Extrusion, the common resins that are used are polyethylenes (LDPE, HDPE and LLDPE). However, various other materials, including many biobased and biodegradable plastics, can also be used in this process, as a blend with other resins or even as single layers or in multi-layer film structures. In few instances of multilayer production, when the individual materials are not able to gel together, a multi-layer film might delaminate. This can happen if polyethylene or polypropylene is combined with other thermoplastics. Hence, to overcome this issue, various tiny layers of special adhesive resins are used purposefully in between. These tiny layers are called tie layers. Process of blown film extrusion The extrusion and subsequent tube-forming of the plastic melt is done via an annular slit die, generally vertically in the upward direction, for the formation of a thin walled melt tube. The introduction of air takes place through a hole in the die’s center for blowing up the tube just like a balloon. The cooling of the hot film is done by a high-speed air ring that blows onto it. This air ring is mounted above the die. Then the following procedures take place: The tube of the film continues its movement upwards (constantly cooling) until it is squeezed by two opposing flat surfaces (collapsing frame) to collapse the tube before it enters the primary nip rolls at the top of the tower structure. The now collapsed tube is transported on idler rolls down the tower by the secondary tension-controlled nip. On higher output lines, an exchange of air inside the bubble is necessary. This is called IBC (Internal Bubble Cooling). Finally, the collapsed tube is kept as it is or is slit into two individual sheets or webs. Then the film is wound onto cores to make film roll stock. The film can also be sent to an in-line sealing machine to make bags. This process can also be carried out off-line at a later stage. Depending on where the inflated melt-film starts to solidify (so-called frost line) a short neck process or – if the frost line lies rather high – a long neck process are being distinguished. As an example, PE-LD is usually run on in a short neck process, whereas PE-HD is preferably run on long neck equipment. Advantages of blown film extrusion • In a single operation, flat as well as gusseted tubing can be formed. • Regulation of film thickness and width with the control of air volume in the bubble • Capability of biaxial orientation, which allows uniformity in all the mechanical properties • Very high productivity • Allows combination of different materials as well as properties Applications of blown film extrusion In this extrusion process, the blown film is used either in tube form (for plastic sacks and bags) or a sheet can be used by slitting the tube. Typical applications of the Blown Film Extrusion or Film Blowing includes following: industry packaging • shrink film • stretch film • bag film • container liners consumer packaging • packaging film for frozen products • shrink film for transport packaging • food wrap film • packaging bags • form, fill and seal packaging film laminating film • laminating of aluminium or paper used for packaging milk, coffee, and similar products agricultural film • Mulch film • greenhouse film • crop forcing film • silage film • silage stretch film films for packaging medical products [1, 2] Blown film extrusion of bioplastics Most bioplastics can be run on conventional blown film extrusion equipment. However, due to their different flow behavior – just like changing any other polymer – most bioplastics need a new and exact adjustment of the extrusion dies, output speeds, temperatures etc. for the respective operating points. 48 bioplastics MAGAZINE [06/17] Vol. 12

Basics As an example, some particularities of BASF’s ecovio ® blown film types (F and FS - grades) shall be mentioned here: The ecovio blown film grades can be run on conventional blown film extrusion lines in thicknesses ranging from usually 8 up to 250 μm (depending in the product type). All existing downstream equipment (winding-, printing-, cutting- and welding/sealing, bag-making equipment etc) can be used. Conventional spiral mandrel die heads of the latest generation can be used to process ecovio F and FS. The range of usable die gaps is rather wide. Existing metallocene‐ dies with gap widths of 1.2-1.8 mm can be used as well as die gaps as narrow as 0.8 mm. Ususally ecovio is being processed in a normal- (or short)-neck process (PE-LD). To a limited extent, it can also be run on existing PE-HD long neck equipment. The neck-length however, should be significantly shorter as for running PE-HD. Usual blow up ratios for ecovio are in the range of 1:2.5 up to 1:4 (e.g. mulch films). The ecovio F and FS grades are pre-compounded with a certain amount (1~2 %) of slip- and antiblock masterbatches to adapt sliding properties, avoid fold formation and reduce blocking of the film. However, additional adding of such additives may be advised. [3] Sources: [1] [2] [3] BASF brochure “ecovio® Biologically degradable solutions for extrusion applications” The 3-layer plant of the machine manufacturer Hosokawa Alpine. The picture shows the plant in the Augsburg test centre (Photo courtes Hosokawa-Alpine) Nip Rolls Collapsing Frame Stabilizing Cage Air Ring Winder Extruder Die bioplastics MAGAZINE [06/17] Vol. 12 49

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