What is compounding

As Compound (engl. For Composite, composition) refers to plastics to which additional fillers, reinforcing materials or other additives have been added. The compounding means that at least two substances are firmly bonded to one another. The aim of compounding is to modify the properties of the plastics for a specific application. For example, plastics for components in outdoor areas are made UV-resistant and less sensitive to hydrolysis by adding so-called stabilizers. The process of making a compound is called compounding. In the case of thermoplastic compounding, it takes place with the aid of twin-screw extruders. If two polymers are compounded, e.g. PA with ABS, this creates a so-called polyblend. From a terminological point of view, such a blend is a special compound, but a compound is not necessarily a blend.

Compounding goals

The goals of compounding are varied and depend on the desired properties of the later component. Here, however, compromises often have to be made in the sum total of the requirements, since, as is so often the case when solving technical problems, individual requirements are diametrically opposed. However, the following are the most important goals of compounding:

  • Change in the mechanical properties of the base polymer. In this case, mechanical parameters such as tensile strength, elongation at break (see also tensile test) and impact strength are set by adding reinforcing materials and fillers, as well as modifying impact strength.
  • Color settings. The color required by the customer is set by adding pigments or so-called masterbatches. This often results in initial conflicting goals, as certain color settings sometimes have a very clear influence on the mechanical properties.
  • Flame retardant. The addition of flame retardants can prevent plastics, which are otherwise easily flammable by nature, from igniting.
  • Addition of stabilizers and stabilizer systems. The main reasons for stabilization are:
    • Temperature-initiated chain degradation during processing. This can result from excessive shear of the material or from too long dwell times in the processing machines. It is prevented by a simple stabilization designed for a short-term load.
    • Temperature-initiated chain degradation in the application. Plastic parts that are exposed to high temperatures during use, e.g. in the engine compartment of a motor vehicle, must be specially stabilized against this load.
    • Improvement of weather resistance: Plastic parts in outdoor areas are exposed to severe damage from oxidation and hydrolysis. These can be compensated to a certain extent by using special stabilizers. Depending on the base polymer and stabilization, these effects can be delayed for different lengths of time.
  • Addition of processing aids. This group of substances essentially improves the processing of the polymers. In this way, demolding in the injection molding process is simplified, for example, by using mold release agents. This group of additives is less relevant for the end application.

Used aggregates

Reinforcement fabrics

  • Glass fiber: Short ("KGF") or long glass fiber ("LGF") are the most frequently added reinforcing materials. Compared to e.g. carbon fibers, they are significantly cheaper.
  • Carbon fibers: The lightest, but also the most expensive fiber for reinforcements.
  • Wollastonite: Wollastonite is a borderline case between reinforcement and filling. Due to its rod-shaped crystal structure, a reinforcing effect can be achieved by adding it.


Other additives

  • Heat stabilizer
  • lubricant
  • Antiblocking agent
  • Antistatic agents
  • UV stabilizer
  • Laser markers
  • Nucleating agent (crystallization accelerator)
  • Flame retardant
  • Dyes
  • Impact modifier
  • Teflon

Common compounds

  • all TPE (thermoplastic elastomers)
  • colored materials
  • PP with 20% talc
  • PP with 40% chalk
  • PP with 30% glass fiber (KFG or LGF)
  • PA 6 or 66 with 30% glass fiber (KGF or LGF)
  • ABS with 16% glass fiber (KGF)
  • PC with 20% glass fiber
  • ABS, PC, PP flame retardant

Other uses of the term

  • In bearing technology, adding solid lubricants lowers the coefficient of friction and the wear of the materials.
  • In the field of sports, a "compound" bow is a piece of sports equipment that uses rollers and cables to reduce the pulling weight to be held when fully extended.
  • The compound brake is a special brake construction in vehicle construction.

Category: Plastics Processing