Environmental Engineering Reference
In-Depth Information
material properties. The pellets derived from this process can be used for applica-
tions in contact with food.
11.5.2
Chemical Recycling
Even though bottle grade PET can be obtained by mechanical recycling, the dete-
rioration of properties cannot be stopped in the long run. In general, PET recyclate
has to be mixed with new PET in order to ensure the properties required. For this
purpose, solvolytic processes are helpful for the recovery of monomeric materials
from PET.
The reaction of PET with ethylene glycol is called glycolysis (when water is
used, it is hydrolysis). Both are reverse reactions of the polymerization process,
which we discuss here. The synthesis of PET relies today on the reaction of tere-
phthalic acid with ethylene glycol in two steps. In the first step, terephthalic acid
reacts with an excess of ethylene glycol to obtain BHET. Water is released and has
to be removed from the system in order to avoid a back reaction. In the second
step, PET polymer chains are formed by the polycondensation of BHET. Ethylene
glycol that is released during this step is removed using temperatures of more than
200 °C and a vacuum. Since the chain length distribution is controlled by equilibria
between the reacting compounds, effective separation of excessive ethylene glycol
is required in order to obtain PET with the required properties.
Glycolysis is the reversed second step of PET polymerization. Before glycolysis,
the used PET is ground, washed, and foreign materials are separated as described
for the mechanical recycling of PET. The cleaned PET flakes are melted and then
added to ethylene glycol. The reaction is carried out for several hours at tempera-
tures about 200 °C. Metal salts like zinc acetate are frequently used as catalysts.
The product consisting mainly of BHET, but with higher oligomers also present, is
cooled down, precipitating the product. The solid product is then separated from the
remaining ethylene glycol, which is purified by distillation and used again. BHET
is recrystallized for purification and used as it is for the polymerization of PET
(omitting the first reaction step). One disadvantage of this process is that impurities
are not sufficient removed. Especially long heating times cause the formation of
diethylene glycol, which is then incorporated in the polymer chain, changing the
optical and mechanical properties.
The Teijin process (Fig.
11.7
) uses the BHET for the production of dimethyltere-
phthalate (DMT) by transesterification with methanol. Then DMT is used for the
production of high purity terephthalic acid that can be used for the production of
bottle grade PET.
The direct hydrolysis of PET requires high pressure water, making this process
expensive. Temperatures higher than the melting point of PET at about 250 °C and
a pressure of about 2 MPa are necessary. Such harsh conditions can be reduced by
using alkaline or acid conditions. Sodium terephthalate is obtained in NaOH solu-
tion, making it necessary to recover terephthalic acid by adding additional acid
