German company Gneuss has developed the MRS extruder - a single screw extruder with a special degassing section. The polymer melt is delivered into a large single screw drum. The drum contains 8 or 10 (depending on the model size) small extruder barrels, 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 extruder barrels which are cut into the drum of the multi rotation system are approximately 30 per cent open to ensure the optimum melt transfer into the barrels and so that the evacuation can take place without restrictions. Further, precise control of the melt temperature is possible as the temperatures of all the surfaces in contact with the melt can be controlled accurately.
Due to its patented multiple screw section, Gneuss says that the MRS makes a very large area available and allows good degassing performance, even with a vacuum of only 20-40 mbar. The extrusion of undried PET bottle flakes or pellets (for example, up to moisture contents of 12,000 ppm water) is therefore possible without making any compromises. This even applies to high viscosity end products such as strapping tape.
This evacuation technology (and the multiple screw section) is based on the robust and proven single screw extruder concept. In this way, the MRS avoids the problems of alternative multiple shaft or screw designs (intermeshing) which are considerably more sensitive to mechanical damage due to their tight clearances. This last point can be decisive in the reprocessing of PET bottle flake which frequently includes coarse contamination. The company adds that further positive effects of the MRS technology are the 100 per cent dehumidification of PET, as well as the possibility of increasing the intrinsic viscosity of this material.
The extrusion drum – the heart of MRS technology
Due to the multi rotation elements, Gneuss says that a melt surface is made available which is far greater than that in conventional extruders. For example, the MRS system creates a melt surface exchange rate which is 25 times greater than a co-rotating twin screw extruder. Compared with other multi-screw systems, the MRS is characterized by its extremely compact and rugged design. The rotating satellite screws run in individual bearings and are therefore comparable with a drum containing a number of single screws.
According to Gneuss, the evacuation or degassing system is modular and can be matched to individual requirements. The position, length and design of themodules can be varied. The MRS system can be used as an element of an extruder screw or can be integrated in a polymer melt pipe transfer system.
Table 1: Extruder comparisons. * In expansion section | | Single screw extruder | Twin screw extruder | MRS |
| Surface (cm2)* | 100% | 150% | 450% |
| Surface exchange(m2/min)* | 100% | 200% | 5,000% |
| Free volume (cm3)* | 100% | 150% | 300% |
PET
Gneuss says that the performance of the MRS system has been demonstrated in several production lines. Firstly, the MRS was tried in PET applications. Several lines (sheet, non-woven, granulating and strapping lines) have now been equipped with the MRS extruder. The chemical process inside the extruder is a reversible one. Moisture in the PET shortens the molecular chains, and therefore the viscosity and mechanical properties are reduced. This chemical reaction is reversible and the point of equilibrium can be driven to one or the other side by reducing or increasing the water content.
For example, a line was established in South America for producing thermoforming film out of 100 per cent bottle flakes, totally un-dried and where the humidity is up to 10,000 ppm. In this line the polymer was analysed to find out the viscosity during processing. The intrinsic viscosity (i.V.) of the un-dried processed PET bottle flakes was analysed as well as the polymer upstream of the degassing section, directly afterwards and in the finished product.
In the first section of the extruder the viscosity of the polymer is reduced, because the water content is integrated into the chains of the PET. The more moisture is in the input, the more the decrease. In the degassing section these water molecules are extracted by vacuum (driven by diffusion). In this section the reversed chemical reaction takes place. Therefore, the MRS is able to increase the chain length, the molecular weight, the viscosity and the mechanical properties of the polymer. The viscosity build-up can be controlled by the vacuum level itself; consequently it is possible to use an online viscosity measurement to control the vacuum in order to stabilize the viscosity level by a closed control loop.
After leaving the MRS section the extruder builds up the necessary pressure for the die head or the downstream filtration system with no more hydrolysis degradation. Gneuss adds that the main conclusion is that it is possible to process bottle flakes without pre-drying and using a ‘simple’ vacuum system of roughly 25 to 30 mbar, which can be achieved by a conventional water ring pump.
Table 2: Energy savings. * Figures provided by customers | | MRS | Twin Screw | Single Screw |
| Crystallization | - | - | 90 Wh/kg |
| Drying < 50 ppm | - | - | 120 Wh/kg* |
| Pre-drying to
1000 ppm | - | 60 Wh/kg* | - |
| Extruder drive and heating | 295 Wh/kg | 230 Wh/kg* | 240 Wh/kg* |
| Vacuum | 45 Wh/kg | 90 Wh/kg* | - |
| Booster pump | | 30 Wh/kg* | - |
| Total | 350 Wh/kg | 410 Wh/kg* | 450 Wh/kg* |
Energy savings with the MRS system
In addition to the good mechanical properties of the tapes the energy consumption of a strapping line plant was measured as well. Compared to conventional single screw processing with crystallization and pre-drying down to a water content of less than 50 ppm, the specific energy consumption is much less, around 20 per cent, because of the considerable energy costs for heating the pellets by hot air during the preparation for extrusion. In comparison to twin screw processing of the flakes the energy consumption is also less, because a high vacuum of less than 5 mbar is needed with the twin screw and in addition to that the flakes normally have to be pre-dried down to around 1000 ppm, and there is a booster pump needed as well.
Gneuss concludes that the specific energy consumption of the MRS technology is 15-25 per cent less than conventional technologies, as well as offering higher flexibility and less maintenance on dryers and high vacuum systems.
Results
According to Gneuss, the new MRS extrusion system offers extremely efficient devolatilization or degassing of the polymer melt due to a high surface renewal rate. The diffusion process is significantly increased by the multiple screw system. The company adds that the MRS extrusion system (including the multi rotation section) is based on proven and rugged single screw extruder technology. The company claims that this system is therefore extremely well suited to the processing of highly contaminated polymers (for example, post-consumer waste).
Since the MRS extrusion system requires the input material to be neither pre-dried nor crystallized, the system is an economically attractive alternative to conventional technologies for processing PET. The MRS has already been proven as a reliable and practically maintenance free system for around 1½ years in a range of different applications.