Environmental Engineering Reference
In-Depth Information
bottles are returned to the shop by costumers. This requires the establishment of a
collection system often connected with a refund system and transport back to the
bottling plant. This is most worthwhile environmentally when the distance between
refilling and sales is short, since long distances might require higher energy con-
sumption in transport than saved by reusing. Another example is the reuse of mod-
ules from broken down electronic devices, which can be used as spare parts for
identical machines. Since whole pieces with their complex structures are reused, the
loss of order (entropy) is reduced to a minimum.
Recycle
Recycling comprises many very different measures for the reduction of
waste. Mechanical recycling is described as primary recycling if the mechanical
properties of the recyclate (the recycled material) resemble those of the original
material. This can only be achieved for a few plastic materials, since even materials
with the same molecular structure differ in their molecular weight and frequency of
branches, resulting in differences in their properties. Moreover, impurities and addi-
tives have a negative impact on the recyclate. One material that satisfies the require-
ments for primary recycling is PET. About 40 % of PET consumption is related to
the production of PET bottles. After separated collection from other plastics, PET
is cleaned, ground, and treated to repair damage caused by the first use. Since PET
fractions of different origin still have comparable properties, no deterioration of
properties is observed. Today's PET recyclate can be re-used in contact with food
(food grade).
In case the recyclate cannot meet the original specifications, the process is de-
scribed as secondary recycling. Especially if plastics from different origins are
mixed, consistent properties cannot be maintained; 'down-cycling' is observed. As
an example, polyolefins such as low density PE (LDPE), high density PE (HDPE),
linear low density PE (LLDPE), and PP have very similar densities, which make
separation difficult. As a consequence, pure fractions of one material are not ob-
tained and material properties do not match requirements. These materials are often
used for lower quality applications. The structural organization of the material is
lost during grinding when primary and secondary recycling is employed, while the
molecular structure remains unchanged.
The purpose of tertiary recycling is the recovery of chemicals, which can be
reused in the chemical industry and especially in plastics production. For this
purpose, the chains of the macromolecules are split thermally or chemically and
monomers recovered. For instance, polymethyl methacrylate (PMMA) decom-
poses thermally at 450 °C forming methyl methacrylate (MMA), which can be
reused to polymerize to PMMA. The recovery of monomers from PET is more
complicated, requiring additional chemicals. In the presence of water, terephthalic
acid and ethylene glycol are obtained; in the presence of ethylene glycol bis2-
hydroxyethyl terephthalate (BHET) is formed and these compounds can be used
for the production of new PET. The demands on the material are still high. Clean,
correctly sorted materials are preferred. Therefore, chemical information remains
intact with the formation of low molecular weight compounds, even if the macro-
molecular chain is destroyed.
