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For the dewatering of high-moisture wet ABS (Acrylonitrile Butadiene Styrene) rubber powder Coperion offers a high quality and very gentle dewatering solution using the ZSK twin screw extruder. The material is dried mechanically without strong compaction and low shear impact from 30~40 wt% to a final moisture content of 8~15 wt%, depending on the raw material properties.
The ZSK twin screw extruder is designed with a feed intake zone, two compression sections, two side dewatering sections with side dewatering units and a pressureless discharge.
The process starts by feeding ABS rubber powder with moisture content up to 40 wt% into the first barrel of the ZSK extruder. This section of the twin screws is designed to be partly filled only, which leads to low product pressure in a range of 0-3 bar. The pressure is increased slightly over the next conveying area up to 5 bar. The first compression step is located directly after the first side dewatering units with a product pressure range of 5-10 bar. With a special screw elements configuration the water is pressed out gently while keeping the pressure on the rubber as low as possible to keep the energy input and compaction of the raw material low.
The water that is squeezed out can escape safely through the counter rotating side dewatering units, while the rubber is kept safely inside the process. The moisture level after the first dewatering step is reduced to around 20–25 wt%.
After this first compression zone again a partly filled screw section is added. In this section the product pressure is reduced to 0-5 bar, which leads to relaxation and homogenization of the rubber powder. This section is followed by the second compression zone with two additional side dewatering units. The product pressure is increased again to around 5-10 bar and the moisture is pressed out of the rubber powder to the targeted final moisture content.
After this section the product temperature is around 100°C and the final moisture content of the product is between 8~15 wt%, depending on the material properties of the rubber powder. After this second dewatering section of the extruder again a section for mixing and homogenization is added. In this section no further pressure is applied (0-2 bar) on the rubber which leads to additional relaxation and stable product quality. The discharge of the extruder is open, so there is no further compaction of the rubber powder.
During the full process the rubber powder is squeezed very gently, and the drying of the rubber is carried out very softly, which makes sure that the physical properties of the rubber are not affected and strongly compacted rubber is prevented. These softly dried rubber particles are beneficial to the following final ABS compounding step, which leads to the highest process stability and best quality ABS polymer.
With our process parameters like screw speed, the extruder can be adjusted to the given raw material properties and the moisture content of the final product can be adjusted easily.
Since the year 2000 more than 30 Coperion production lines are using this technology successfully.