Extrusion International 4-2017

35 Extrusion International 4/2017 ary kneading pins (“teeth”) inserted through the barrel, injection of liquid additives into the polymer melt and use of screws with kneading and con- veying flight sections. The result is homogenous filler dis- tribution in the polymer matrix melt, with fillers completed “wetted” by polymer and firmly bonded by use of adhesion promoters, such as silane on aluminium hydroxide (ATH), a filler with flame retardant properties. Guntern described trials masterbatch production trials with 40wt% carbon black (CB) compounded with 57% LDPE and 3% additives. As low-density filler, CB needs to be precisely metered and homogenously mixed into the melt with optimum shear, Guntern stated. This highly filled CB masterbatch was compounded at 5wt% with 95wt% LDPE, producing a semiconductive compound with 2wt% CB content, as used for intermediate power cable sheaths, where good CB dispersion is key to formation of an effective con- ductive network in the plastic. Extruded film photographs showed better CB filler dispersion with low fil- ter pressure values (FPV) and with the Buss MX Kneader running at 150 kg/h rather than towards or at 250 kg/h. But with CB fed partly at the first screw sec- tion together with LDPE and additives, as well by later side feeding on its own, the film strips showed dispersion ben- efited more from a 60/40 ratio of CB split-feeding than with 70/30. Overall cost per litre of compounds produced from highly filled mas- terbatches is lower than with direct compounding of fillers into polymers, Guntern said, although there are theo- retical limits to high filler content, de- pending on spherical particle packing between fillers. He criticised that raw materials are purchased by weight, irrespective of pressure, temperature and bulk den- sity, while in reality, compounds are used by volume. His compounding cost comparisons therefore focussed on cost per litre. Dr Jan Diemert of Fraunhofer ICT insti- tute for chemical technology described how “extractive compounding pro- cesses” result in virgin and especially recycled compounds with low emis- sion and odour. He said materials and process parame- ters affect emissions while compound- ing and in injection moulding part production, as do the part’s final envi- ronment and e.g. decoration. Diemert addressed both total volatile organic content (VOC) and emissions causing fogging. For example, flax fi- bre reinforced PP composites typically have a high 1,000 µg/kg VOC level, within which over 200 µg/Kg fogging. Emissions can often be reduced, but in practice only usually to a limited extent, by attention to dispersion, throughput and machinery wear, as well as processing temperatures, resi- dence time, shear energy and the ex- tent of oxidation, Diemert said. Significant reduction can however be obtained by using the compounding process to reduce emissions. This is achieved by combining process stages, appropriate use of vacuum venting, use of scavenging agents, ensuring continuous extraction, extension of “dry” strand length prior to granu- lation and venting over extruded strands, Diemert suggested. He said injection of process gas such as carbon dioxide (CO 2 ) into the polymer melt at an early stage acts as a scaven- ger by extracting and stripping out of “contamination” from the melt when it is vented out further along the barrel. Diemert also spoke about scaveng- ing chemical trials in PP compounding, where nitrogen performed similarly to CO 2 (60 µg/g), water lesswell (just under 60 µg/g), but isopropanol was better (30 µg/g). CO2 scavenging with fibrous cel- lulose fibre (FCP) reinforced PP resulted in VOC falling from 78 to 22 µg/g and fogging from 360 to 125 µg/g. He said extractive compounding with scavengers is highly effective, requires low investment cost and has great po- tential for optimisation, even though it is laborious to introduce on compound- ing lines. In general, intelligently opti- mised processes result in lower emis- sions, along with significant reduction in odour perception, he observed. Although extractive compounding hardly affects established process stag- es, Diemert said caremust be taken that to avoid polymer hydrolysis or machin- ery corrosion. Scavanger material costs range from practically zero for water to 5-10 Eurocents/kg for highly concentrat- ed CO 2 or alcohol, he added. Dr Ralf Kühn from the R&D depart- ment at extrusion machinery producer Coperion described simulation in de- sign and operation of the Coperion ZSK 40 twin-screw extruder. He said material transport simulation in ex- truders has become an integral part of the design and optimisation process, as verified calculation methods can be applied to e.g. distribution and direc- tions of pressure and shear at different points in the pressure build up zones prior to and just before the die plate. Partially filled areas and simple mix- ing processes can be simulated, as can mixing element capability. Simulation of more complex processes, as well as of transport of solid material and melt- ing in the extruder feeding zones rep- resent “challenges that still have to be solved in future”, as they will also be “important tools in long-term future design processes”, Kühn stated. Among approaches adopted is mix- ing simulation based on mechanics of a falling highly viscose drop becom- ing dispersed through elongation and shear forces in a low viscosity polymer matrix, as well as a low viscosity drop in a polymer matrix of the same viscosity, and how the drop changes direction, deforms and is distributed under the pressure and shear as it is transported by distributive mixing and dispersive mixing elements. Kühn showed an example of how FEM (finite element modelling) simulation has been used to optimise tempera- ture distribution in ZSK heating and cooling elements, through consid- eration of the length of the heating process, peripheral loss of energy and distortion of sealing surfaces. SKZ-KTT GmbH www.skz.de