Compounding Sytems for Bioplastics

Bioplastics have been known for a long time. Their industrial production started in 1869, first based on cellulose, and then on casein in large quantities as artificial horn in the early twentieth century. With the technical breakthrough soon afterwards to petroleum-based plastics, with production at much lower cost, bioplastics were no longer produced. It was not until decades later, in the 1980s, that rising crude oil prices and gradually changing ecological awareness led to interesting new developments in the field of bioplastics. And thus also to new compounding systems for production.

The terms bioplastics or biopolymers are still not used consistently. However, they mostly include a large number of different plastics that fulfil at least one of two criteria:

  • Bioplastics comprise at least a proportion of renewable (vegetable) raw materials. Almost all bioplastics are bio-based.
  • Bioplastics are biodegradable, i.e. they can be degraded by naturally occurring microorganisms into water and CO2 with a small proportion of biomass. Bioplastics from fossil raw materials can also be biodegradable.

Bioplastics are defined by these two properties either on their own, or together: bioplastics are bio-based, biodegradable, or both. Conventional plastics do not meet any of these criteria. Biogenic raw materials or naturally occurring biological macromolecules such as proteins, as well as natural fibre-filled or reinforced materials (see also separate article) are therefore not included.

Typical applications

The main applications of bioplastics are packaging, consumer durables, and technical parts in the transport and construction industries.

Compounding requirements for bioplastics

To ensure the desired properties, bioplastics compounding systems demand excellent dispersive and distributive mixing at moderate shear rates and low product temperatures.

The careful incorporation of fibres, fillers and additives often requires multiple metering of dry ingredient streams and possibly also the injection of liquid additives at defined positions in the compounding machinery.

The BUSS Kneader’s specific capabilities are outstandingly suitable for such applications. Thanks to its operating principle, the enormous number of mixing cycles at moderate and adjustable shear rates enables the highest possible mixing efficiency over short processing lengths with narrow residence time spectra. The BUSS Kneader’s two-stage system systematically separates the compounding and pressure build-up stages so that each of them can be independently optimized.

The split barrel of the BUSS Kneader and the retractable casing of the discharge extruder ensure fast and easy access to the processing components for high system availability.

Together with broadly based BUSS process expertise in compounding, the modular design and therefore adaptable arrangement of the entire compounding line, make the BUSS Kneader is first choice for compounding technical bioplastics.


Typical plant layout for compounding bioplastics

Typical plant layout for a bioplastics compounding machinery

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.

  • Degassing of volatiles
    Volatiles are typically removed by a vacuum degassing opening at the end of the barrel or optionally in the discharge unit. Continual highlevel compound surface renewal is achieved with the large number of mixing cycles, striations and foldings created by the BUSS Kneader within the compounding machinery, thus enabling entrapped air or volatiles to be removed completely.
  • Precise temperature control
    The BUSS Kneader within the compounding system 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.

  • High filler loadings
    Filler loadings up to 90% are possible with BUSS compounding 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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  • Laboratory Kneader MX 30