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EXECUTIVE SUMMARY Stretch wrap2
EXECUTIVE SUMMARY Recycling Post-Consumer stretch Wrap Film Literature Review • Significant quantities of stretch wrap film is used to stabilise palletised products throughout the materials handling industry. The estimate of usage in Australia is 16,000 tonnes are produced locally and 1,500 tonnes is imported as roll stock. Further film enters Australia as wrap around goods being imported • Stretch wrap film is usually Linear Low Density Polyethylene (LLDPE) that has been compounded with a tackifier, usually 5% Polyisobutylene, (PIB) to ensure the film clings to itself when wrapped around goods. It is usually a thin film (20 microns) that is stretched to at least 300% in wrapping pallets or boxes making it even thinner ie less than 10 micron. • The unpigmented nature of the film makes it attractive to recycle however recycling is inhibited by the sticky surface that collects contamination and the tackifiers that make the unwrapped film difficult to handle and shred to size due to the rubbery nature of the material. Labels, other films and strapping often present difficult sorting challenges. • Recycling of stretch wrap film has been developed by various companies in the USA such as Dow and Mobil where the film has been recycled into hand stretch wrap film, garbage bags and injection or blow moulded containers. Rubbermaid and Tucker Housewares produce waste bins from recycled PE blends. Mobil developed plastic lumber made from 50/50 stretch film and sawdust for outdoor applications. • In Australia stretch wrap film is not recycled in large volumes however it is used as a component of plastic pallets by Vicfam and is used as a component in “builders film” which is used as a heavy gauge moisture barrier in building construction. The market price of waste stretch wrap film is usually low eg $100-150 per tonne and the finished recycled LDPE resins sell for approx $750 per tonne. • Attempts to reuse stretch wrap film back into stretch film have not been successful when the film has come from post-consumer sources due to contamination. Recompounding of industrial film scrap is conducted by use of batch wise agglomerators that can densify the thin film into a popcorn-like particles that can be handled by conventional extruders and moulders. This process is slow and requires a skilled operator to control the process making the economics of recycling unattractive. Modern equipment has been devised by companies such as Sorema that operate on a continuous basis and remove the majority of contaminants, however processing rates a slower than for heavier gauge films. Factory Scale Trials of Recycling of Stretch Wrap Film Visy have installed a modern recycling plant for the washing and extrusion of Polyethylenes, including LLDPE, LDPE and HDPE in the form of film and bottles. The plant consists of the following: 1. Presorting area for inspection of infeed film. 2. Infeed conveyor that delivers the material to a Shredder. 3. A Shredder to reduce the film to small, free flowing pieces approx 100 mm square to facilitate removal of contamination 4. An inspection conveyor where contaminants can be removed. 5. A Wet Grinder that further reduces size to 10mm square and simultaneously washes off dirt and pulps paper labels. 6. A washing tank to further clean the film 7. A separation tank to remove heavy contaminants and plastics more dense than water such as PET, Polystyrene and PVC. 8. Centrifuges to separate the film from water. 9. Drying loops to dry the film . 10. An extruder that feeds the film via an agglomerator chamber to the feed throat of the extruder. 11. A pelletiser that cuts molten strands of LDPE and converts them into cylindrical pellets 3mm in diameter and length. By conducting four formal trials of processing stretch wrap film as well as many on-line observations when stretch wrap film was present in high concentrations, the following conclusions were reached: 1. Stretch wrap film is very difficult to shred due to its highly elastic nature. The film would often wrap itself around shafts causing throughput problems. 2. The film sourced from recyclers was frequently contaminated with PP strapping, multilayer LDPE/Nylon film, packaging tape and paper labels. 3. The grinding of film to smaller particles required a sharp set of blades to function on the stretchy film. The screen size had to be reduced from 22mm to 16mm and consequently the output of the grinders was reduced from 1500 kgs/hr (when running HDPE bottles) to a maximum of 300 kgs/hr due to the low bulk density of the film. 4. The fine gauge film ( 10 micron) was extremely difficult to wash, rinse and dry using the plant at Visy which was designed to run film at a rate of 800 kgs/hr. Drying was particularly problematic with up to 15% moisture being left with the film after the maximum drying conditions were applied. An additional drying loop was purchased to improve the drying effect however the moisture was reduced to 7% which still causes extrusion problems. 5. The extrusion of the film was made difficult by the light bulk density of the film (approx 300 kgs/m3) and the residual moisture in the film. Both of these factors inhibit the feeding of the film into the extruder limiting the output of the extruder from 1300 kgs/ hr on HDPE bottle flake to 200 kgs/hr on the film. The low bulk density makes it difficult to fill the flights of the screw at the feed section of the extruder and the moisture in the film creates steam and causes a back pressure in the melt which inhibits the positive feeding of the incoming material. 6. The energy intensity of the agglomerator was increased by the use of elevated knives on the rotor of the agglomerator. This increased the output from 200 kgs/hr to 220 kgs/hr which was still a long way from the capacity of the extruder. 7. The product of the trials was used in builders film as a 10% blend along with 10% HDPE and with 80%of LDPE. This product was tested and found to meet the strength and impact requirements of building film. 8. The use of higher levels than 10% of stretch wrap film caused problems in “blocking” ie the adhesion of two layers of film together as well as weakening the film due to the effects of higher levels of the tackifier (PIB). The tackifiers’s effects can be countered by the use of antiblocking additives however these are considered too expensive for general use at the levels required in stretch wrap blends. 9. The overall conclusion reached in processing Stretch Wrap film in Sorema recycling equipment was that the output was reduced to 25% of the capacity of running film in the equipment used at Visy Plastics. The extrusion equipment was reduced to 17 % of its maximum capacity. Both of these effects illustrate that stretch wrap film requires equipment that can handle the problems of low bulk density and can process the film in the dry state. Such equipment is available from Sorema and Erema in the form of agglomerators that work by semi-melting the film and forming rough granules that can be processed at higher extrusion speeds. Laboratory Scale Trials of recycling Stretch wrap film into stretch wrap film. Trials were conducted at RMIT’s Polymer Technology Centre on small scale extrusion equipment as well as a plant size Reifenhauser film line. Investigations were conducted to evaluate if recycled stretch wrap film resins produced by Visy Plastics could be blended with virgin resins to produce useable stretch wrap films. The following conclusions were reached from this study. 1. Residual film flake from the Visy washing system still had traces of caustic soda from the washing steps due to the high surface area associated with the film. 2. The Melt Flow Index of the recycled film was 0.76 grams per 10 mins which is less than the common stretch wrap film resins (0.9)indicating that some crosslinking was present due to the recycling process. 3. The level of gels in 100% recycled film was very high limiting the production of blown film to blends with virgin otherwise pinholes and other defects would be present. The level of gels was measured to be between 0.5% to 1.5%. 4. The use of antioxidant masterbatches did not reduce the incidence of gels in the film. 5. Reasonable film properties and appearance was achieved when at least 50 % of the blend was made from virgin resin. 6. Additional PIB was needed to restore the tack need Film made from virgin resin could be stretched at a wrap tension of 10 kg of force up to an extension of 600% with a maximum stress of 31 kgf. 7. Film made from 50/50 virgin/recycle plus 5% PIB could not be stretched at 10kgf of wrap tension due to failure at the numerous gels present. Stretching at 5kgf tension achieved an extension of 200% and at 3kgf extension of 300% was achieved. 8. Film made from 100% recycled film could not be stretched even at 1kgf tension due to the failure of the film at the numerous gels present. 9. The conclusion of the laboratory studies into making stretch wrap film from recycled stretch wrap is that it is possible to make a grade of film that is suited for Hand Wrap applications where low tensions and low stretch are used. To achieve this all of the problems of processing and purification need to be overcome. The hand wrap film will use typically double the amount of film to secure a load to a pallet due to the lower forces applied and the less stretch used on the film. This decreases the economics of using recycled film even further when compared to virgin film. The final cost of the recycled film was estimated to be $4/kg against $2.20/kg for virgin film. This emphasises the difficulty in developing the market in stretch film from recycled film. CONLUSIONS AND RECOMENDATIONS • Stretch wrap film demonstrates that it is problematic to recycle from a technical viewpoint due its tendency to collect contamination, its thin gauge, and the presence of the tackifier, PIB • The opportunity to recycle Stretch Wrap film into Hand Stretch Wrap film is limited due to the low rate of reprocessing on recycling equipment and the subsequent high cost of the finished product. • The presence of gels needs to be overcome to allow a useable film to be manufactured. Technological innovation to multilayer films that do not use PIB but use metallocene catalysed very low density LLDPE as a tacky surface may provide an improvement in the recycling of these films. Coordination with the major producers of the films may accelerate the investigation and application of this approach. • The markets for Stretch Wrap films are currently as blends with other polyethylenes in applications such as builders film and pallets where it can be used as a 10- 15% component. The penetration of stretch films in these markets is not quantified and potentially the products could absorb larger quantities. Further applications such as these need to be developed in order to increase the end markets for this ubiquitous and rapidly growing packaging medium. • An investigation of equipment dedicated to the processing of stretch wrap film into pellets may allow the processing economics to be reviewed and improved beyond those revealed by this study. • A major deficiency of the current recycling strategy for these films is the lack of information supplied to users on how to deal with used films and the need to keep the films uncontaminated as much as possible. • The development of plastic labels from LLDPE or HDPE would greatly assist in minimising contamination. • The ideal scenario for recycling Stretch Film would be a closed loop service where used film would be collected as new film is delivered. This would capture much of the film since most users also have material that will have been used in previous packaging. The film could then be processed by the small number of companies that generate the 17,500 tonnes of film roll stock. Alternately the producers could designate specific recyclers to carry out this task on their behalf.
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