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Recovery and Recycling of Particulate Fillers from Polymer Composites
There are two aspects to this: recovery and recycling of the filler itself, or recovery and recycling of the filled composite.
Where the filled polymer itself can be recycled, then this is preferable to separating it from the polymer first. This applies mainly to thermoplastic composites, where nearly all filler recycling is carried out in this way. This type of recycling is discussed in section “Recycling in the Form of a Filled Polymer.”
The situation is different for cross-linked polymers (thermosets and elastomers) which are not so readily reprocessable, and this is where efforts on recovery and recycling of fillers themselves are concentrated today. These efforts are largely focussed on carbon black and, to a much lesser extent, on calcium carbonate.
Recovery of Carbon Black
Carbon blacks are one of the main particulate fillers in use and also relatively expensive. This leads to both pressures to recycle and an economic incentive to do so. Elastomer applications consume about 90% of all carbon black production with tires making up over 70% of this sector. Most of the attention has thus been on recycling of carbon black used in elastomer applications, especially tires. This has been assisted in some regions (notably the European Union) by the banning of tires from going to landfill. There is also an advantage here from the fact that tires and some other elastomer wastes are already collected and segregated. Because of the large quantities used, recovery and recycling of carbon blacks is receiving much attention, with end of life tires (ELTs) providing a large and readily available source, with high levels of carbon black. The cross-linked nature of rubber makes direct recycling difficult due to cross-linking (but not impossible, see section “Carbon Black Recycling in Elastomer Compounds”) and so recovery and recycling of the filler itself is receiving a great deal of attention.
Removal of the rubber from ELTs using pyrolysis is the most investigated process, and a number of commercial pyrolysis processes have been developed. These processes involve heating in the absence of air and can be used to break down organic matter into volatile products, allowing the fillers to be recovered for reuse. They are seen by some as the best way of closed loop recycling for tires. The following description is based on tire pyrolysis, but can be applied with slight modification to other sources of filled elastomers.
Pyrolysis is usually applied to tires after the steel and textiles have been removed by shredding. The steel can be recycled. Carefully controlled heating is then used to convert the rubber into gaseous and liquid fractions which are swept out of the reactor, leaving behind an ash. Some processes use added catalysts to help the breakdown processes (usually some form of active clay). The ash is mainly reinforcing filler (carbon black and/or precipitated silica) together with carbonaceous residues from the breakdown of the rubber and other organic content and various minor inorganics such as zinc oxide from the cure system together with catalyst residues where these have been used.
The energy for the pyrolysis process can be obtained by burning some of the gaseous fraction. The rest of the pyrolysis oil can be used for a variety of purposes, including energy generation.
Currently the ash, which is referred to as pyrolysis carbon black (pCB), is the most valuable product; and the economics of the operation depend on maximizing income from this source
Table 3 Typical properties of commercial pyrolysis carbon blacks (From Norris and Bennett 2014)
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