Figure 1 The Gneuss MRS (Multi-Rotation System) extruder achieves extremely efficient devolatilisation of the polymer melt and allows for the production of high quality PET sheet for thermoforming from bottle flake without any pre-drying

MRS Technology

The design of the MRS extruder is based on the proven and rugged singlescrew extruder, in this case with a special devolatilising section.
The polymer melt is delivered into a large single-screw drum. The drum contains 8 or 10 small extruder barrels (depending on the model size), parallel to the main screw axis. Installed in these small extruder barrels are the ‘satellite’ screws, which are driven by a ring gear in the main barrel. The satellite screws 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.
The barrels cut into the drum of the multi-rotation system are approximately 30% open to ensure the optimum melt
transfer into the barrels and so that the evacuation can take place without restrictions.
Thanks to this patented design, the melt drum with its extremely large surface area and extremely high surface area exchange rate offers unmatched devolatilising performance. And this at a vacuum of 30 mbar, a level of vacuum which can be achieved by means of a conventional, simple water ring pump. In comparison to conventional single-screw extruders, the melt surface area in the MRS is a factor 4,5 higher and three times higher than twin-screw extruders. The perhaps more important surface area renewal is a factor of respectively 50 and 25 higher.

Figure 2 Performance comparison

PET Concept

The first MRS extruders were supplied for PET processing without predrying, for which the system was originally conceived (sheet, fibres, strapping tape, pelletising).
The hydrolytic degradation process in PET is reversible. Moisture in the PET shortens the molecule chain length which results in reduced mechanical
properties and correspondingly in a reduced quality level of the finished product.
In the devolatilising section of the extruder, the water molecules which were present in or on the PET are removed or extracted by the vacuum (diffusion process). The melt viscosity can therefore be controlled by varying the vacuum level. Consequently, it is possible to keep the viscosity constant even when the moisture level varies by regulating the vacuum in response to viscosity changes, monitored by an Online Viscometer.
After leaving the devolatilising section, the polymer passes into a conventional single-screw extruder section, where sufficient pressure for the downstream process is created (filtration etc) without any further hydrolysis taking place.

Thermoforming sheet production

The importance of sheet for thermoforming made from PET in the packaging industry is growing rapidly. This is on the one hand due to its excellent mechanical properties and on the other due to the relatively simple processing and the low material costs, particularly if post-consumer ‘bottle flake’
is used. Thanks to new processes, it is possible to transform this ‘waste’ to the point where a sheet can be manufactured, which can be used in direct contact with food products.
With the correct processing technologies, sheet with excellent thermoforming properties can be made thinner, saving weight and therefore material. Further, the temperature stability of PET packaging is significantly better than that of other polymers so that food can be heated in the container (such as in a microwave oven).
An American manufacturer of sheet for thermoforming and thermoformed containers has – with the help of the MRS extruder concept – successfully dispensed with the expensive and energy intensive processing step of crystallising and drying the material prior to extrusion.
This company operates an extrusion line with a throughput of 1000 kg/h of PET. Most of the sheet produced is subsequently processed to containers for packaging fruit.

The MRS extruder plasticises and devolatises the material, the ‘RSFgenius’ filters the material (at 56-75 microns), the Online Viscometer measures the viscosity immediately upstream of the die and controls the vacuum pump at the devolatilising section of the extruder in order to maintain the viscosity within a narrow range. The quality of the sheet and the sheet thickness can hence be maintained at a consistently high level.

Figure 3 A 1000 kg/h Gneuss thermoforming sheet line in the USA

A smaller line of this kind is in operation in Brazil, with an MRS 110 extruder. This line is also used for the production of thermoforming sheet for packaging fruit, however with a somewhat smaller throughput rate of 600 kg/h. A mixture of reground skeletal waste, reground bottle flake and virgin material is processed, with no pre-drying and with extremely simple vacuum technology, without gas purification and without the corresponding high maintenance requirement.
In comparison with lines which operate with pre-drying, the MRS concept means that no crystallising or pre-drying is required. It was therefore possible to reduce the space requirement by 25% and the energy requirement by 15-25%, not to mention the reduced maintenance and the fact that the long time required for drying and crystallising the material in advance of processing considerably restricts the flexibility of the whole operation (for example material colour or grade changes).
In the production of PET sheet using post-consumer bottle flake, the devolatilising effect of the MRS extruder is not only useful for removing moisture: it is also useful for decontaminating the melt so that the process can be provided with an FDA ‘letter of no objection’ from the American Food & Drug Administration (FDA).

Advantages and energy savings

The values for energy consumption are the actual values obtained from Gneuss customers. Compared with a conventional single-screw extruder whereby the residual moisture level is reduced to less than 50 ppm, the specific energy requirement with the MRS extruder is some 20% lower as the higher energy costs for heating the material with hot air prior to extrusion is no longer necessary.
Processing with a twin-screw extruder would likewise avoid the need to completely pre-dry the material (although typically, the material needs to be pre-dried to approximately 1000ppm) much of the energy savings of this system over the conventional single-screw with crystallising and pre-drying is cancelled out due to the higher vacuum (<5 mbar) required and the necessity of an additional screen-changer for coarse filtration or even a melt pump due to the restrictions on back pressure of the twin-screw extruder.
The specific energy consumption of the MRS technology is therefore 15-25% less than for conventional technologies. An additional advantage is that the less extreme vacuum means far less maintenance.
The MRS system offers an economically extremely attractive alternative to the established technologies for PET processing.

Gneuss, based in Germany but active worldwide, is represented in Southern Africa by Onyx SA, contact Jimmy Schutte: phone 011 828 8167 or visit www.onyxsa.co.za or www.gneuss.com