Environmental Engineering Reference
In-Depth Information
materials (oil, coal, natural gas) and refinement/processing that yields
monomers (2) polymerization of this monomer into a plastic resin that is
made available as resin pellets or powder, (3) intimate mixing of the plastic
resin with fillers and additives, and (4) forming (or shaping) this mix to
processing are combined into a single box as both operations are often
carried out by a single company.
4.2.1.1 Resin Manufacture
Plasticresinismanufacturedprimarilyfromcrudeoilornaturalgas(though
other fossil fuels and biomass might also be used for the purpose). Refining
crude oil yields chemicals that are converted into monomers, the basic
building blocks of polymers. These are reacted or polymerized in the
presence of appropriate catalysts to yield plastic resin. A more detailed
discussion of resin manufacture follows later in this chapter. The end
product of this step will be the virgin plastic resin in the form of prils or
powder.
4.2.1.2 Compounding
Compounding is the intimate mixing of plastic resin in the form of pellets
or powder with a variety of additives and fillers. The resin is melted and
the melt is mixed with the additives to ensure their even dispersion in the
plastic matrix. Given the range of available additives, thousands of different
formulations or “compounds” of plastic are theoretically possible.
Compounders are specialists in designing and mixing resin formulations
into pellets (or dry blended into powders) for use by the processors. Heavy
equipment such as compounding extruders, roll mills, and internal mixers
are used to carry out this operation.
Though not as popular in the 1980s, the custom compounder segment in
the US plastics industry is still viable. But compounding is increasingly
undertaken by resin manufacturers or more commonly by plastic
processors. This often results in cost savings (and even environmental
benefits) as the resin does not have to be transported. Pre-made
formulations or “compounds” are commercially available for processors to
use. Some of these are available as “concentrates” or masterbatches where
the additives and fillers are incorporated at levels that are much higher
than that typically used in the final product. The processer generally mixes
an appropriate ration of the masterbatch with the virgin plastic pellets,
