Compounding machinery for Rigid PVC

During the first half of the nineteenth century the creation of PVC from vinyl chloride (VC) was first described. Large-scale PVC production started in 1928 in the USA and 1930 in Germany. By the time the Second World War ended, it was already the most-produced plastic. With a chlorine content of 56.7% molar mass, PVC is a welcome co-product in chlorine chemistry. Due to their low content of hydrocarbon-based components, PVC materials have a comparatively favourable energy balance and CO2 footprint.

Rigid PVC (PVC-U) is produced in a compounding machinery by hot/cold mixing in the powder phase. For all subsequent processes requiring pellets, such as injection moulding, this is followed by compounding and pelletizing on a Buss Kneader. This two-stage compounding process likewise applies in case of high fillers content or specific quality demands. For simpler formulations, premixing in the powder phase may be sufficient.

Compounding technology for rigid PVC - application in syringes as an example

Yellowish granules from a rigid PVC compounding process

Compounding technology for rigid PVC - application in white and red plastic tubes as an example

Rigid PVC compounding technology by BUSS – application in white plastic frames and bars as an example (PVC-U).

Compounding technology for rigid PVC (PVC-U) - application in white pipes as an example

A PVC window frame from rigid PVC demonstrates the use of PVC-U produced in a compounding machinery

Typical applications

Thanks to its good mechanical, electrical and optical properties, as well as good chemical resistance, rigid PVC is very useful for many applications. It is used in plant and machinery applications for pressure pipes, couplings and fittings, blowers and fans, air ducting, valves, pumps, or in the chemical industry for tanks and liners made of rigid PVC (PVC-U). In the building industry rigid PVC is used e.g. for wastewater pipelines, roof gutters and downpipes, gas pipes, drainage pipes, window frames, facade elements, ventilation shafts and anti-glare screens. In electrical technology, rigid PVC is used for insulating conduits, transparent distribution-box covers, housings and cable conduits. Examples of rigid PVC applications in the packaging industry include diffusion-resistant bottles for oils and other liquids.

Compounding requirements for rigid PVC

Compounding requirements can be summed up as follows. The rigid PVC dryblend powder, which also contains other constituents like stabilizers, additives, fillers, reinforcement materials and flame retardants, must be intensively mixed in the compounding machinery. The dispersive and distributive mixing is critical, and all in compliance with well defined temperature limits.

The Buss Kneader can make the most of its strengths profile at uniform, moderate and if necessary, adjustable shear rates. Compounding and pressurizing are separated and optimized in a two-stage system. Low specific energies with the most intensive mixing processes, volumetric scale-up procedures, and maximum availability are possible thanks to wide operation windows. It is these advantages that underline the technological and market importance gained in rigid PVC compounding technology since the beginning of mass production in the mid twentieth century.

Typical plant layout for compounding rigid PVC

Typical plant layout for a rigid PVC (PVC-U) compounding machinery

BUSS compounding machinery for rigid PVC offers the following specific benefits

Uniform, moderate shear rates
Uniformly moderate shear rates allow controlled mixing in the compounding machinery at lower temperatures while imparting only the required shear for the task at hand. The narrow shear rate distribution compared to alternative systems ensures uniform shear histories for every individual particle. This results in high quality compounding with reduced energy input.
Precise temperature control
The BUSS Kneader permits precise temperature control owing to controlled energy input and uniform, moderate shear rates, along with precise temperature monitoring by thermocouples mounted at relevant positions along the barrel in hollow-bored kneading pins surrounded by polymer.
Intensive distributive mixing
The BUSS Kneader achieves intensive distributive mixing because the combined rotation and axial motion of the Kneader screw generates extensional flow, a large number of shear interfaces, and cross channel mixing.
High filler loadings
Filler loadings up to 90% are possible with BUSS compounding machinery technology owing to partitioning into two or three feeder openings, the use of feed-in vessels such as side-feed screws, separate gravimetric feeding of filler, removal of trapped air by back venting, and excellent conveying efficiency. The moderate shear rates allow excellent handling of the highest viscosities at such high loadings.
Low process temperatures
The separate execution of compounding in the BUSS Kneader and pressure build-up in the discharge unit allows mixing at low pressure and temperature. The requirements of each individual process section are addressed with ingenious screw designs for optimized temperature profiles.

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