While demand for recycled PET packaging sheet continues to grow, an issue converters are increasingly running into is quality. Food authorities and consumers expect the highest quality in terms of colour, purity, odour and mechanical properties, yet this is often not able to be achieved with existing equipment designed for processing virgin materials.
Investments in completely new, purpose-designed lines for processing recycled materials are not always feasible, whether for economic or space reasons. Gneuss has developed a suite of solutions that can also be retrofitted to existing systems to tackle this problem.
“The demand for PET packaging sheet, which contains a high proportion of recycled material or is even made of 100% of it, has recently increased enormously. With the know-how in PET processing and recycling as well as the customized key components, existing plants can be optimised and the processing window significantly extended,” said Gneuss.
The Bad Oeynhausen, Germany-based polymer and extrusion technology company offers, among other things, its Rotary Filtration Systems with patented filter disc technology, enabling these to operate continuously, with constant pressure and fully automatically. A major problem confronting processors are the high and fluctuating levels of contamination of the input material. With the RSFgenius Filtration Systems, screen cleaning is carried out fully automatically during the running production process via an integrated back-flushing piston system, in which only a small portion of filtered melt is regularly shot through the dirty screen via a narrow gap by means of high-pressure pulses. The quantity required for this is freely adjustable and in practice corresponds to about 0.01 to 1 % of the throughput. This means that the loss is significantly lower than with other systems available on the market, which operate with losses of up to 5%.
Next to the mechanical cleaning process by the filtration system, a thermal cleaning takes place in the extruder. Here, the processing unit must work gently in order to exert as little mechanical stress as possible on the polymer, yet achieve the thorough degassing and further decontamination of the melt. Gneuss’ Multi-Rotation System (MRS), while based on a conventional single screw extruder, is equipped with a multiple screw section, for optimum devolatilisation. The polymer melt is delivered into a large single screw drum, equipped with eight small extruder barrels, parallel to the main screw axis, containing the “satellite” screws. These are driven by a ring gear in the main barrel and rotate in the opposite direction to the main screw while they rotate around the screw axis. This disproportionately increases the surface exchange of the polymer melt. Precise control of the melt temperature is possible as the temperatures of all the surfaces in contact with the melt can be controlled. In this way, even with a simple vacuum of 25 to 30 mbar an enormous degassing and decontamination performance is ensured. The process has received EFSA approval for the processing of 100 % post-consumer PET on thermoforming sheet processing lines and a letter of no objection from the FDA for producing post-consumer recycled plastic for food-contact uses.
One customer that decided in favour of this retrofit solution is a thermoformer, located in Chile, said Gneuss. “After purchasing the filtration systems, they then opted for an extruder. The company was thus able to increase the proportion of recycled material in the PET thermoformed trays to up to 100 %,” reported Gneuss.
An online viscometer completes the processing units supplied by Gneuss, although these, too, are available separately – ‘ideal as a retrofit solution’, said Gneuss. The viscometer measures a small partial flow of the melt during ongoing production and determines melt temperature and pressure. Based on internal calculations, the online viscometer constantly provides the representative shear rate and dynamic viscosity. Depending on the values determined, the vacuum in the degassing zone of the MRS extruder is readjusted to keep the melt viscosity within the specified tolerances.