Recycling Recycling of
Recycling Recycling of Bioplastics Production Waste In most plastics production processes there is waste coming along with the products that converters aim at. Next Generation Recyclingmaschinen GmbH (NGR) at Feldkirchen/Austria for long already is a specialist in machinery for the recycling of these materials at plastics processors and dedicated recyclers. The expertise has of course first been developed for conventional thermoplastics. Nevertheless recycling of bioplastics is getting more and more important for NGR over the past years. Production waste can be melt lumps, sprues and defective parts in injection moulding, parison waste in blow moulding or for example edge trim, cutoffs and offspec startup material in film and fibre production processes. It is obvious that the best and most economical way of treating this is avoiding it, but experience shows that still in most of the cases 2 to 10% of the production material is lost due to process reasons. As long as this cannot be avoided, it is at least essential to recover these spoilt materials and bring them back to the production process. Considering that this is state of the art for oil based plastics like polyethylene that represent a value of ca. 1 €/kg already, this can only gain importance for bioplastics that often cost 3 €/kg or even more. The easiest way of recovering these materials is to shred them to chips and refeed them to the process together with the new material, but as easy as that is, it can lead to process difficulties like: • Inconsistent feeding performance of the production extruder • Air inclusions in the melt that lead to defects of the finished products • No way to remove process materials like printing inks and similar So, in many cases it is favourable to use a recycling extruder, bringing the material back to melt and then to granules that are of the same quality as the virgin material. First of all, by this, the problem of feeding processed material to an extruder is shifted from the production line to the dedicated recycling extruder. On the other hand, there are several technologies available that are just developed to continuously feed different types of plastic material like films, fibres, lumps etc. to an extruder screw. The choice of the optimal recycling technology is mainly driven by the objective to bring the material through the process without damaging its chemical and physical properties. Some conventional plastics even have to be handled with care during processing in order to avoid material degradation. The more this is an issue for most biopolymers, some of them being processed at relatively low temperatures of 140°C or being very sensitive to oxidation due to being exposed to air when heated. A traditional way of feeding materials to a recycling extruder is the use of a cutter-compactor. In principle, this is a cylindrical hopper with a fast rotating knife disk at the bottom. By this knife disk, the material 52 bioplastics MAGAZINE [06/10] Vol. 6
Recycling is cut and agglomerated by the heat induced by the fast rotating disk and then fed to the extruder screw by centrifugal force. Opposed to that concept, NGR recycling machines feed the material to the extruder by an integrated cutter-feeder, which consists of a slow rotating cutter shaft with knives that cut against fixed knives like a scissors and a feeding zone that conveys the material to the extruder without pre-heating it. Experience shows that the cutter-compactor often comes to its limits when used for biodegradables, as the heat being put to the material before the extruder – when the material is still in contact with air – leads to degradation that damages the material. The NGR Cutter-Feeder-Extruder on the other hand has proven its ability to recycle most of the commonly used biodegradables, such as starch or PLA based products. Additional to the benefits of the cutter-feeder, screws, vacuum vent and melt filter can be tailored to the special requirements of biodegradables as for example low heatup of the material or the removing of printing inks. The graphs on the right show the molecular mass distribution of several biodegradables, characterized by gel permeation chromatography (GPC). This method has proven to be quite sensitive in monitoring slight changes in the molecular mass distribution resulting from the inevitable heat stress in any polymer processing step. Both the graphs show the molecular mass distribution measured on a product (film in one case, nonwoven in the other), compared to that measured on the pellets of the recycled product. For both examples a slight shift to the right for the recycled product (pellets), but in a level that is to be expected and acceptable for any processing step. NGR expects that with the increasing use of biodegradables, recycling of processing waste will gain further importance. So, NGR see themselves well prepared for the actual and future demands of these applications. Intensity Intensity 0,8 0,6 0,4 0,2 0- 0,2 -0,4 -0,6 -0,8 GPC for a PLA nonwoven and the recycled product thereof 5 6 7 8 9 10 11 12 13 14 -1 Elution Volumen 2 1,5 1 0,5 0 -0,5 -1 GPC for a starch based film and the recycled product thereof Pellets Nowowen 5 6 7 8 9 10 11 12 13 14 Elution Volumen Pellets Film Article contributed by: Uwe Bonten Area Sales Manager Next Generation Recyclingmaschinen GmbH Feldkirchen, Austria www.ngr.at bioplastics MAGAZINE [06/10] Vol. 6 53
