Polycondensation polymer polyethylene terephthalate reacts to heat, mechanical stress and moisture by breaking down the polymer chain length and thus decreasing the viscosity. Systems which counteract this process are therefore essential for the production of regranulate. Twin-screw or other multi-shaft extruders, which ensure gentle processing, are commonly used for processing PET residual materials. They are also equipped with degassing devices for removing moisture and volatile constituents. After being processed in the extruder, the manufactured regranulate then often undergoes post-condensation in a solid-state polycondensation (SSP) reactor, so that high IV values can be attained.
At the plastics trade fair, the companies presented their continuously operating reactors as an alternative to familiar methods.
The JUMP solution offered by Gneuss enables viscosity to be increased by between 0.2 and 0.3 dl/g when used in combination with the specially developed Multi Rotation System (MRS) extruder. The machinery manufacturer highlights the compact design of the JUMP as a particular advantage, as it only requires a small floorprint and stands at just 2.5 metres tall. The inner workings of the reactor are constructed such that they are easily accessible and therefore easy to clean. The process takes places as follows: The PET melt, removed of most of the volatile constituents by a simple 25 mbar vacuum in the MRS, reaches the JUMP and passes through several slowly rotating, mixing elements, forming a polymer film whose surface is constantly being renewed. The reactor is under a vacuum which ensures that glycol and other volatile constituents are safely removed. Surface exchange and also dwell times can be varied widely, so that dwell times of between 5 and 100 minutes can be achieved. According to Gneuss, one of the advantages of the end product is that the lack of oxygen intake during processing prevents to the greatest possible extent the yellowing of the materials.
The so-called liquid-state polymerisation (LSP) process from NGR utilizes a pre-treated PET melt, which in this case comes from a shredder-feeder-extruder combination. As an example, the company explains the processing phase in the P:REACT reactor for residual materials from the fibre industry, which have an initial IV value of 0.56 dl/g and, in addition to a moisture content of 3,600 ppm, may also contain up to 15% spin oils: After the first processing stage in the shredder-feeder combination, which already includes one degassing process, the IV value increases to 0.66 dl/g ± 0.015 dl/g and the spin oils have been safely removed. Then, the temperature in the reactor is maintained at a consistent level and a second degassing process takes place. The IV value, which is measured online, can be adjusted during the dwell time. The end product after granulation is an rPET granulate with an IV value of 0.7 g/dl. The builder of recycling systems views the low energy consumption of 0.12 kwh/kg including granulation and the simplicity of the process to be particular advantages of the system. Furthermore, the IV value fluctuations, which are well-known in discontinuous processes, are significantly smaller. Thanks to FDA approval, rPET granules manufactured in this way could even be reused in contact with food products.
Better known from plant engineering for their specially developed large polyester polycondensation systems, Aquafil Engineering now offers EverPET, a complete system for processing recyclable materials. This system also consists, for example, of a cutter compactor, an extruder, the EverPET IV Lift continuous reactor and an optional granulation unit. As, according to Aquafil Engineering, it does not always make sense to granulate the processed melt, the melt could also be put directly back into the production process, for example, for fibre manufacturing. Among the advantages of their complete system with reactor, Aquafil Engineering lists operating principles which which are gentler on materials than conventional extrusion systems, along with reduced yellowing and gelling.