Page 187 - Materials Chemistry, Second Edition

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Sustainable Industrial Design and Waste Management
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densities of plastics like PP and LDPE, the fingernail test and visual
inspection can be more reliable (Vogler, 1984).
In order to distinguish between PS and PVC, another flotation test
is carried out using pure water and salt. The existence of salt in water
forces the PVC and dirt to sink and the PS to float. However, the
amount of salts need not be measured; it is achieved from experience
based on trials (Vogler, 1984).
• A burning test is used to differentiate between the different types of
plastics based on the color and smell of the flame produced. The test
is carried out as follows: cut a 5 cm length of a strip of plastic mate-
rial, tapered to a point at one end and 1 cm wide at the other. Hold
the sample over a stove and light the tapered end. However, the per-
son carrying out this test should be careful not to be too close to the
sample under test and not to inhale the smoke as it might contain
hazardous substances (Vogler, 1984).

The techniques used in testing plastics differ between the developed and
developing countries. In the developing countries, the technology and expert-
ise are not available in the informal sector. Whereas in the industrialized
countries mechanical separation techniques are used. Instrumental analyti-
cal methods are becoming more available like infrared spectroscopy and
thermal analysis (Lardinos and van de Klundert, 1995).
Some of the techniques developed by industrialized countries concern-
ing the separation of plastics include a method of separating the plastic
packaging materials using velocity – this plays an important role when iden-
tifying about six packaging plastics. In another technique velocity plays a
minor role and is used to distinguish between about 30 different plastics,
mainly those made from engineering plastics (Wogrolly et al., 1995). Other
methods include separating the plastics based on density, surface structure,
ferromagnetism, conductivity, color, etc. Since the purity of the finished
product depends on the sorting accuracy, we need to make a distinction
between sorting due to density, flotation, electrostatic sorting, thermal sep-
aration, and sorting of plastics by the hand picking method discussed earlier
(Wogrolly et al., 1995).
Other separation methods include the bottle sorting mechanisms,
which can be either manually as described earlier, semi-automatic or auto-
matic (Wogrolly et al., 1995). However, the problem of separation sometimes
becomes more difficult when mixed plastics are involved. Therefore, the
Society of Plastics Industry in the United States has developed a coding sys-
tem shown in Figure 5.12 using symbols and numbers for the types of plas-
tic used. This system has also been introduced in Europe and most countries
worldwide. This coding system requires updating since a number of plastic
types are included under the number “7”.
The next step in the initial stage is the cleaning, which usually con-
sists of washing and drying the plastics. It is important to wash the plastic