Page 186 - Materials Chemistry, Second Edition

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Sustainability of Municipal Solid Waste Management
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and in developed countries by private companies. This stage is labor inten-
sive and requires no technical experience. Collection is usually made direct
to the final dumping site or via a transfer station, since these are the most
practical methods in developing countries.
The next step in the initial stage is sorting of plastics, which depends
on the manufacturing demands. It could be sorted based on color or type.
Sorting is usually carried out by women and children in developing coun-
tries because of low wages and no technical skills are required. The working
conditions at this stage are not hygienic because generally the sorting stage
falls between collection and cleaning, which leaves the workers exposed to
contaminated plastics dirt, and chemicals (Lardinos and van de Klundert,
1995).
In sorting there are basic guidelines that should be followed. Sorting
separates plastic films from rigid plastics, each undergoing a further separa-
tion based on color and type. The waste plastics are first sorted and classi-
fied into bottles, transparent plastics, rigid plastics, and flexible plastics;
within these a further separation is done based on type and color.
While sorting plastic we need to separate different kinds from each
other because while reprocessing the plastic waste we need to know which
type of plastics we are dealing with. This is because plastic types have dif-
ferent properties and reprocessing them together will produce a product with
poor properties, unexpected durability, and poor appearance. Some plastics
may even look similar and need testing to find out their type. To identify the
plastic type a chemical test such as infrared analysis could be used.
However, experience can help in this field but in case of doubt we have to
revert to testing (Lardinos and van de Klundert, 1995).
Some basic tests include:

• To distinguish between thermoplastics and thermosets, press a piece
of wire just below red heat into the material. If it penetrates, it is a
thermoplastic, if not, it is a thermoset (Vogler, 1984).
• To distinguish between the types of plastics, scratch the surface with
the fingernail to see the material’s flexibility. However, this test needs
an experienced person and is not always reliable since the material
could have been exposed to many weathering conditions and could
become brittle and rigid and cannot be scratched. A thin material
may seem flexible and thick (Vogler, 1984).
• To separate plastics from non-plastics, or to separate between plastics
types, we can use the flotation test. This test is useful to distinguish
between PP and HDPE, and between HDPE and LDPE. The test basi-
cally consists of differentiating between the different densities of the
two plastics by mixing them with water and alcohol in certain propor-
tions. It was found that in a mixture with an exact density of 0.925,
HDPE will sink and PP will float and in a mixture with a density of
0.93, HDPE will sink and LDPE will float. However, for overlapping