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Gentle and cost efficient processing of highly filled PVC compounds

Coperion’s two-stage compounding system Kombiplast

PVC has a greater variety of formulation options than virtually any other thermoplastic polymer. Slight changes in additives, plasticizer and filler composition strongly influence the properties of the compound. However, the common denominator for all PVC formulations is that they need gentle processing in order to avoid excessive plastification or even degradation. Naturally, besides the necessity to create high quality products the main focus in PVC processing is on economic efficiency.

With its Kombiplast Coperion offers a very flexible two-stage system for PVC processing meeting both high quality products manufactured at high economic efficiency. In order to pelletize the PVC it has to pass a die plate. As the comparably high backpressure in the compounder simultaneously creates a high product temperature, Coperion’s Kombiplast decouples the two process steps of plastification/ homogenization and pressure build up before the die plate. The Kombiplast system consists of a co-rotating twin screw extruder ZSK for compounding and a single screw extruder (ES-A) in which the pressure build up before the die plate is realized very gently. The strands are cut into pellets by Coperion’s eccentric hot face pelletizer (EGR). The new design of the knife rotor allows for an almost complete elimination of fines during the cutting process.

The process section of the ZSK extruder has a modular design consisting of individual barrels and screw elements aligned on screw shafts. Hence, the set-up of the machine can be easily adapted to the requirements of the products. With an optimized configuration of the barrels and especially the screws a large variety of formulations can be run without any changes in the set-up.

No feed limitation for highly filled PVC compounds
For formulations containing large amounts of filler (> 100 pph) such as CaCO3 or Al(OH)3 the mixing process becomes more and more inefficient due to a large volume of the mixer, longer mixing times and higher amount of plasticizer to overcome absorption by the filler. At the same time the required size of the compounder increases in order to avoid feed limitation.

To compensate these disadvantages Coperion equips the Kombiplast system with a ZSK extruder in Mv PLUS design. This twin screw extruder model has an increased screw volume due to larger barrel bore (Do) and reduced inner screw diameter (Di). The result is a larger screw channel depth of the twin screws that allows for improved feed intake, reduced shear input, lower product temperatures and lower product stress. The quality of the shear sensitive PVC products increases obviously.

In addition Coperion separates the feeding of the filler from dryblend feeding. As the bulk properties of PVC dryblend such as flow ability and bulk density vary a lot depending on the formulation and the mixing process , the dryblend is fed to the process section not through a top hopper but by high accuracy Coperion K-Tron feeding equipment in the side feeder ZS-B. The Coperion K-Tron feeders ensure the right consistency of the formulation at any time and the ZS-B itself is a short co-rotating twin screw extruder. The feed hopper is stirred and the barrels are cooled in order to avoid sticking of the dryblend.

While the dryblend is fed by a ZS-B side feeder into the process section of the ZSK Mv PLUS and is plastified, the filler is added downstream by a second ZS-B side feeder. Excess air brought in by the filler is removed first atmospherically in the ZSK Mv PLUS and in a second step in the transfer chute between the twin screw extruder and the single screw. Both, the ZSK Mv PLUS as well as the side feeders ZS-B are operated starve fed. That means that their screw speed can be adapted in a wide range – and therefore economic - without being feed limited.

Feed Enhancement Technology for improved feed intake
In case of poorly flowing dryblend or fillers the side feeders can be equipped by Coperion’s patented Feed Enhancement Technology (FET): a vacuum is applied in the feed zone to a wall section which is porous and permeable to gas. The solids conveying is improved by two effects in this area: First, the bulk density is increased by removing air, second the friction is changed in the area of the FET insert by the filter cake.

As a result, the throughput of products being feed limited previously can be further increased, in many cases 20-30% or even more. Alternatively, the screw speed of the compounder can be decreased while keeping the throughput at the same level. In any case, the specific energy consumption SEI and therefore the shear stress on the product and energy costs can be further decreased.

Typical set-up for the production of highly filled PVC compounds