Natural Fiber Composites (NFC) Compounding

For thousands of years natural fibers were mankind’s primary material. Houses and ships were built of wood, while flax and hemp were used for making ropes and utilitarian fabrics such as sails and corn sacks. During the industrial revolution and in modern times, new applications have been developed with the use of chemical agents for bonding natural fibers with natural fiber composites compounding into strong components for use in various industries.

Typical applications

The specific properties of natural fibers have always made them a desirable material. They possess good mechanical characteristics such as such as tensile and impact strength, elongation and elasticity modulus, paired with low weight. These are based on renewable raw materials with excellent CO2 and energy balances, which offer a competitive price. Altogether, these materials bring significant growth in NFC applications and production volume accordingly. Among their main application fields are the automotive and aircraft industries.

Compounding requirements

Natural fiber composites are made with a thermoplastic or a thermosetting polymer matrix. The thermoplastics can be PE, PP, PS, PVC or PLA, while the thermosetting polymers are mainly acrylate, epoxide and phenol resins. The most important natural fibers used are flax, hemp, jute, kenaf, sisal, abaca and cotton. Other frequently used materials include wood fiber, sawdust and rice husks. In many cases, optimal bonding of the matrix and fibers is ensured by using adhesion promoters.

Indispensable for natural fiber composites compounding is perfect dispersive and distributive mixing at low product temperatures, in order to process the additives and typically high content of 30-70% natural fibers according to product specifications without thermally damaging the fibers. Also required are multiple feed points and if necessary the injection of liquid adhesion promoters at predetermined positions.

The BUSS Kneader’s specific capabilities really come to the fore in these applications. Due to the operating principle of natural fiber composites compounding, the enormous number of mixing cycles at moderate and uniform shear rates enables unsurpassed mixing efficiency and product quality at maximum throughput rates. As a side effect of the moderate shear rates, wear and tear is significantly lower than with other kinds of screw extruders. The process chamber is modularly designed by in-house experts according to specific applications.

Synthetic wood terrace planks being produced with raw mass chunks from natural fiber composites compounding systems by BUSS.

The BUSS Kneader’s two-stage system deliberately separates the compounding and pressure build-up stages so that each stage can be independently optimized. The hinged housing of the BUSS Kneader within the natural fiber composites compounding and the retractable casing of the discharge extruder ensure fast and easy access for high system availability. Together with broadly based BUSS process expertise, the modular design and therefore adaptable arrangement of the entire compounding line make the BUSS Kneader first choice for compounding natural fiber composites.

Typical plant layout for natural fiber composites compounding systems

Typical plant layout for natural fiber composites compounding systems.

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.

  • Limited damage to polymer, fiber and high-structured fillers
    The BUSS Kneader’s uniform and moderate shear rates result in controlled shear and low temperature profiles. This guarantees better mechanical and electrical properties, improved flow characteristics, and lower consumption of expensive formulation components.

  • 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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  • Laboratory Kneader MX 30