The plastic material suppliers often carry out compounding. They will receive a quantity of virgin base polymer, such as polypropylene, from a polymer manufacturer and create a number of different formulations and grades for sales to their customers, who will require materials that can meet the specific needs of their end use applications. For example, simply by adding pigments, they can create a range of colours. In some cases this process might be continuous: instead of pelletising, the result may be an extruded product such as pipe.
In considering the thermal history of plastics, most material will already have passed through some kind of a processing cycle before it even goes to the final manufacturer to make saleable production components. Examples of possible supply chains are shown in Figure 6.5.
When considering recycling of these materials, a similar supply chain may operate. Recyclate materials may be utilised by either the polymer supplier or the compound supplier to produce grades containing all recyclate, a mixture of recyclate and additives, or a mixture of recyclate and virgin material. Additives for recyclates will be discussed in greater detail in Chapter 7. A possible supply chain for recyclates is shown in Figure 6.6.
Processors may also perform compounding themselves, to customise materials for their own specific requirements. This may
include the addition of colourants, regrind materials or other processing aids as required. Often additives and colorants are purchased as masterbatches.
Masterbatches are concentrated mixtures of colours and/or additives incorporated into suitable carrier materials. This enables easy incorporation of the selected masterbatch formulation into the production process. Often, masterbatches are used to improve the downtime when changing colour and materials, which improves production efficiency. It is more expensive to buy a masterbatch than an additive. However, the advantages, especially in cases where numerous colour changes are required, can quickly repay the cost. For example, carbon black pigment is very fine and incredibly difficult to remove from feed hoppers and mixers (and the production workers!), whereas a black masterbatch containing perhaps 20% black pigment in a carrier material such as low-density polyethylene (LDPE) can be added to the mixture at 1-5%, leaving the hoppers and mixers easy to clean, ready for the next blend.
Whether an additive is used in concentrated masterbatch form or in its standard form will vary depending on the requirements of particular producers. Masterbatches are routinely used for blow moulding and film blowing in commodity plastics such as polypropylene (PP), LDPE and high-density polyethylene (HDPE).
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