Environmental Engineering Reference
In-Depth Information
materials since separation of small inorganic particles from highly viscous solvoly-
sis derived solutions by filtration or centrifugation causes problems. However, hy-
drolysis in steam atmosphere at more than 400 °C offers the opportunity of decom-
posing PET without incineration. Gaseous terephthalic acid can be decarboxylated
over a CaO catalyst and benzene is obtained as a basic material for the chemical
industry.
11.6
PVC Recycling
Recycling of PVC faces various problems caused by its low thermal stability and
the high content of additives. PVC degradation starts around 100 °C with the elimi-
nation of HCl, making the presence of stabilizers essential. Also other additives
such as plasticizers and impact modifiers are present, making material recycling
difficult. However, flame retardants are barely used, since the high chlorine content
of 56 % (wt.) for pure PVC assures sufficient flame resistance.
Chemical recycling is hindered by the high chlorine content as well. The easy
elimination of HCl does not allow monomer recovery. Although most of the chlo-
rine is released easily, chlorine content in the oil remains unacceptably high during
conversion into fuel requiring an additional dechlorination step prior to liquefaction
and gasification.
11.6.1
Material Recycling
The Vinyloop process allows the recovery of PVC from PVC-containing waste
streams. PVC materials are often composites containing metals or fibers. For the
separation of these materials, the PVC waste is first ground and then dissolved in
methylethylketone (2-butanone). Metals, cotton and PET fibers, as well as other
contaminants are not dissolved and removed by centrifugation. Then steam is
introduced into the PVC solution and methylethylketone is vaporized. PVC re-
mains in the aqueous slurry and is separated from the liquid phase by centrifu-
gation. Additives which are soluble in methylethylketone, but insoluble in water
are precipitated with PVC and remain in the recyclate. This applies mainly to
the plasticizer. The properties of the recyclate can be modified by the addition of
additional additives.
The properties of recycled PVC can be upgraded by the chemical modification
of the polymer. The chlorine atom in PVC has a high reactivity that can be used for
introducing new functional groups into the polymer by nucleophilic substitution. In
this way, the substitution of chlorine by isothiocyanate (-N = C = S) has antibacte-
rial effects on the polymer. Long chain groups such as dodecathiolate (-S-C
12
H
25
)
could replace plasticizer like phenolates, which pose a threat to human health and
environment when leaching from PVC.
