Environmental Engineering Reference
In-Depth Information
Fig. 11.6
PET material
recycling
The mechanical recycling process discussed above is not able to provide bottle
grade PET. For this purpose additional steps have to be taken. First of all, only
material with a sufficient purity can be used and contaminated material should
be avoided. After material has been selected, the next steps are the same as for
fiber grade PET: sorting, grinding, washing, separation from other plastics. Then
a decontamination step occurs in which the flake surfaces are coated with caustic
soda (NaOH) at 150 °C. The surface including possible contaminants is etched
away. Then volatile compounds are removed at temperatures of more than 200 °C.
The flakes are extruded to pellets and a solid state polymerization is carried out
in order to increase the recyclate's molar weight which may have been reduced
by a number of causes. Firstly, during its life time, PET suffers from thermal,
mechanical, and environmental stress; secondly, during processing, temperatures
up to 300 °C are used which can cause a reduction in the molar weight; finally,
mechanical influences and the presence of water might cause shortening of the
chain length. Solid state polymerization restores the molar weight and improves
