While addressing the challenges posed by new applications, it is important to review the steps involved during any compounding and review useful fundamentals to avoid simple problems. A compounding facility consists of some preliminary or up-stream processing, actual compounding in the twin-screw extruder followed by down-stream process to finish and package the pellets or directly produce profiles. The compounding process is one that results in the actual ‘chemical union’ of the ingredients. These steps include:
- Pre-treatment (such as drying or coating)
- Pre-mixing (such as blending in a high speed mixer)
- Storing and transporting material to silos and storage hoppers
- Feeding using volumetric or gravimetric feeding systems (includes requirements for preheating)
- Compounding (melting, mixing and metering) followed by screening (if needed) and pelletising (or in-line product formation)
- Product finishing such as classifying post-drying and packing
Steps in compounding
Pre-treatment: The temperature, pressure and moisture conditions in the extruder can depolymerise or unzip the polymer during processing. Drying is really important while processing certain polymers that are produced by polycondensation. Moisture content during melting should be as low as possible (0.005 per cent) in the case of polyesters.
Coating of fillers such as calcium carbonate or titanium-dioxide may be carried out by the supplier or in some cases at the compounding facility. These two fillers (CaCO3 and TiO2) materials and Carbon black are known for agglomerating during mixing and coating helps in greatly reducing this effect.
In case of LFRTP technology, glass roving has to be heated and coated with a coupling agent or hot melt. Wood polymer compounding used to require elaborate pre-treatment process. While processing such low bulk density material, a pre-treatment to bind the particles together was employed. Solutions are now available to avoid such preparation and handle low bulk density material directly.
Pre-mixing: Dry mixing with solid and liquid ingredients with low, medium or high intensity mixers is called blending or pre-mixing. Generally, the polymer does not melt during the process. Wetting or blinding agents such as waxes could melt during this process.
The sequence of addition of certain organic colourants is something important to avoid re-agglomeration. Since all the ingredients used in the formulation can never be fed individually either due to their vast numbers or minute quantities. Pre-mixing of selected ingredients with some quantity of resin is always a requirement. This is generally true despite use of may feed ports and a large number of gravimetric feeders. Using a pre-mix improves product consistency (percentage of ingredients) from pellet to pellet. It is important to perform mixing at a different level or in an isolated mixing station to avoid contamination with the product.
