Rigid PVC or PVC-U (Un-plasticized) 2018-06-21T10:24:03+00:00

Rigid PVC or PVC-U (Un-plasticized)

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 is produced 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 process likewise applies in case of high fillers content or specific quality demands. For simpler formulations, premixing in the powder phase may be sufficient.

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. 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

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. 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 leadership gained in rigid PVC compounding since the beginning of mass production in the mid twentieth century.

Typical plant layout for rigid PVC

BUSS compounding systems offer the following specific benefits

  • Uniform, moderate shear rates
    Uniformly moderate shear rates allow controlled mixing at low-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 has been well known for precise temperature control since more than 6 decades. This is achieved via controlled energy input due to uniform, moderate shear rates and precise temperature monitoring by thermocouples mounted in pins along the barrel.

  • 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 technology thanks to 2 or 3 feed openings, separate gravimetric feeding of filler, removal of trapped air by back venting, and excellent conveying efficiency. The moderate shear rates allow perfect handling of the highest viscosities at such high loadings.

  • Low process temperatures
    The separation of mixing 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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