Multi rotation system extruder

PET Bottle-to-Bottle recycling is, from both an environmental and economic viewpoint, an extremely interesting process. An increasing number of processors manufacture new bottles from post-consumer PET bottles which are in most cases suitable for food contact containers. The basic prerequisites are however a high product quality and suitability for food contact. Gneuss Kunststofftechnik GmbH (Bad Oeynhausen, Germany) has the ideal concept via its Multi Rotation System (MRS) extruder, Melt Filtration System and Online Viscometer. This system has unrestricted FDA approval for PET reprocessed on this system.

Although in recent years, several different processes for PET bottle-to-bottle recycling have become established, the high quality requirements on the final product mean that with the higher contamination levels of post-consumer bottles, closed-loop recycling is difficult. The efficient decontamination performance of the MRS extruder offers new possibilities. This system has not only convinced the FDA, but a growing number of processors throughout the world are taking advantage of this technology.

Preparation of the used bottles
Before the bottles can be processed, foreign materials must be separated. These are often sorted by hand. Then, the bottles are shredded and washed in a hot washing process whereby dirt, adhesives and labels are removed. In a floatation bath, lighter polymers (predominantly the PP caps) are separated by bulk density (sink/swim). A centrifugal dryer removes the surface water; metals are sorted out of the product flow. Separation of PVC and sorting by colour is also possible. The flakes are now ready for processing. This usually consists of an extruder with melt filtration system and a pelletiser. In order to achieve the required viscosity for injection moulding the pellets to pre-forms for bottles, the dried pellets go through a solid state polymerisation (SSP). This last process can be continuous (for larger throughput rates) or discontinuous (for smaller throughput rates).

PET pre- drying: a thing of the past!
When PET is processed at high temperatures and in the presence of moisture, hydrolysis takes place: the polymer chains break down and the viscosity decreases. It is therefore usual practice to dry the materials for up to 8 hours and at temperatures of up to 200°C before processing the material in an extruder. The MRS extruder from Gneuss enables PET processors to completely dispense with this extremely energy and maintenance intensive equipment (to say nothing of the space requirements and the lack of flexibility when material has to be kept in a drier for so long). With the MRS, the molecule chains are rebuilt as moisture is removed from the PET in the melt phase. This is thanks to the extremely large surface area and surface area exchange rate under vacuum in the MRS section. This extremely efficient surface area is extremely advantageous with regard to removing toxic volatiles.

The MRS extruder is based on a single screw extruder, with a special devolatilising section. In this section, the polymer flows through an enlarged section of single screw: a rotating drum. This drum contains eight grooves with satellite screws. These screws are driven by a ring gear. They rotate around the main screw in the opposite direction to the main screw. This increases the effect of the polymer melt surface area exchange disproportionately. The grooves in the drum are approximately 30% open so that the volatiles can expand out of the melt freely. The temperature spectrum of tAdidas