One other method is chemical recycling, where the polymer is broken down into smaller molecules that can be easily separated from impurities.
In chemical recycling, also known as feedstock recycling, plastics are used to make raw materials for petrochemical processes. Common examples of such processes are cracking and hydrogenation.
Processes like these place restrictions on the types of materials they can handle and each method has slightly different restrictions and tolerances. Generally, however, these processes cannot handle untreated waste and sorting is necessary to remove fractions containing heavy metals, fillers and halogen compounds.
A simple way to imagine chemical recycling is that it is the reverse of the process used to create the polymer chains. Going back to the basics of Chapter 2, polymers are made up of a series of monomers joined together in a polymerisation process. Chemical recycling is often a depolymerisation or degradation process. Here, the macromolecules are broken back into smaller chemical units. Once the monomer is recovered, it can be used to make new polymers.
A second type of reaction takes the polymer back one stage further to produce feedstock that can be used to produce the monomer or other petroleum products such as waxes and paraffin. The relationship between these two routes is shown in Figure 8.1.
Chemical recycling is especially suited to polymers formed by condensation reactions such as polyethylene terephthalate (PET),
Petroleum products, e.g., paraffin, waxes recycling of polymers
Nylon and PU as the process is the reverse of the one used to create them. However, the waste must be reasonably free from other polymers or impurities.
These technologies are exploited in industry. For example, the depolymerisation of Nylon 6 has been particularly well researched and has been practised since the 1960s . In 1997, DSM Chemicals North America and Allied Signal commissioned a worldscale facility to depolymerise Nylon 6 from carpet waste . It opened in 1999 and 25% of these depolymerised monomer building blocks are used to make new nylon 6 compounds, marketed under the tradename AKULON ReCap with a particular focus towards their use in automotive applications .
The recovery of polyester materials such as PET can be achieved using a process called hydrolysis. It is the reverse of the reaction (polycondensation) used to make the polymer in the first place, in that the addition of water or water vapour to the process causes decomposition. DuPont hold a patent on one such process that converts the polyester back to carboxylic acid . Hydrolysis is also used for the breakdown of polyurethane (PU) foams. However, the PU reaction is slightly more complicated than the one for PET, as there may be other by-products. This can produce mixtures of substances that may require further separation.
A pilot plant was opened in the UK by ICI in 1998 to look at the feasibility of chemically recycling polyurethane, their method being a process called split-phase glycolysis . Like hydrolysis, the method is complicated by the presence of by-products. Commercial depolymerisation units using the glycolysis of polyurethanes operate in Germany, Austria and Denmark . The requirement for separation can be avoided in this case by further chemical reactions. The basic chemistry involved in these reactions is presented by Ehrig  for those who require further details.
One material particularly suited to depolymerisation is polymethyl methacrylate (PMMA), more commonly known as acrylic. The route for depolymerisation is shown in Figure 8.2. To show the long history of depolymerisation technology, a process to depolymerise PMMA was developed and patented in Germany as far back as 1949 . However, this method lead to the recovered material being contaminated with lead, which obviously limits application for this particular process. An alternative and more
CH3 CH3 CH3
CH2 0 OH2 C OH2 c
COOCH3 COOCH3 COOCH3
0H3 CH3 CH3
COOCH3 COOCH3 COOCH3
Figure 8.2 Depolymerisation of PMMA
environmentally friendly method has been developed using a twin-screw extruder, which heats material beyond its depolymerising temperature. The monomer is recovered as a gas and condensed into a barrel . This process is still at the laboratory stage. In 1997, ICI Acrylics and Mitsubishi Rayon announced a joint venture to develop more efficient depolymerisation technologies for PMMA . The PMMA is crushed and thermally treated at 500 °C to produce methyl methacrylate monomer, which can be used to produce new PMMA materials.
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