PETpla.net Insider 04 / 2011

REFERENCES  1 Kohlgrüber, K.: Der gleichläufige Doppelschneckenextruder - Grundlagen, Technologie, Anwendungen. Hanser, München 2007 MATERIAL / RECYCLING 12 PET planet insider Vol. 12 No. 04/11 www.petpla.net Experimental investigations and results In addition to the theoretical investigations, practical trials were conducted on an M-A-S 55 conical co- rotating twin-screw extruder. The trials were conducted at four to five different rotational speeds, using a die with an adjustable discharge opening height. The throughput for the individual meas- urements varied between about 300 and 560kg/hr. This figure clearly shows that the pressure and mass flow have a linear function of speed. Pressures achieved with this die opening varied between 90 and 110bar. The FEM program is used to cal- culate the volume flow and pressure at the end of the screw as well as the pressure along the screw. The screws have the ability to build up a pressure of 180bar. Viscosity and shear rate have a major effect on processing. The Erd- menger profile requires shear rate and viscosity to be calculated for many positions in the channel in order to findt he dissipated energy. At a higher shear rate, the amount of energy dis- sipated also increases ( ) - a desirable effect at the beginning of the extrusion process, as this pro- vides a major part of the heating and facilitates melting. On the other hand, when melt flow alone is involved, the requirement is for the lowest possi- ble temperature increase. ln the case of the conical system, the speed in the front region of the barrel is lower because of the smaller outside diame- ter and, as a result, the shear rate and dissipation decrease (Fig. 1). In this case, the energy density decreases by 50% over the length of the dis- charge zone, which can be attributed to the quadratic term for the shear rate. Therefore the MAS extruder pro- vides ‘low shear’ processing. Use for PET recycling Processing of PET bottle regrind provides the ideal processing prerequi- sites for conical twin-screw extruders. The processing unit has a large feed-in volume, with unusually high torque being transmitted because the screw diameters are so large. This mechani- cal strength leads to the high load capacity of the thrust bearings, which are designed to be correspondingly robust in response to the increasing distance between screw centre lines. As the investigations demonstrated, the equipment has not only the pro- cessing capability to build up the pres- sure but also the mechanical ability to absorb the pressure, so that a melt pump is not necessary, even when employing a 30μm melt filter. The slowly rotating extruder screws operate at high fill levels and pick up the material with little shear. A small amount of friction is introduced into the material only in the front zone of the screw, which is where only the pressure build-up occurs. This translates into what is, overall, gentle and energy-saving polymer process- ing, in the case of PET, in particular. It was also found to be possible, under appropriate conditions, to achieve the parameters needed for further processing, such as IV degradation of less than 0.02dl/g; ideal colour value b* and extremely low acetalde- hyde (AA) content. The fill level of the screw was found to be an essential parameter, one that permits very fine adjustment of the plasticising capac- ity. The stock temperature is almost inversely proportional to the fill level. This enables the achievement of stock temperature from 275°C to 295°C, without difficulty and using the same screws. The plasticizsing unit of the extruder is further characterised by two-fold venting, which (compared to conventional systems) provides twice the venting area and completely independent venting through the use of a dam element (Fig. 2). If neces- sary, a high-performance vacuum can also be connected. The throughput spectrum from 150kg/h (M-A-S 45) to 2,000kg/h (M-A-S 90) is covered by the four extruder sizes 45, 55, 75 and 90. As it is common in compounders, the screws are made in segments, in order to ensure the greatest possible flexibility. www.mas-austria.com Figure 2: Processing unit with two-fold venting