3.2. Mechanical recycling
Mechanical recycling is reprocessing of the used plastics to
form new similar products. This is a type of primary and secondary
recycling of plastic where the homogeneous waste plastics are
converted into products with nearly same or less performance
level than the original product. Efforts were made by the polymer
technologists in the 1970s to recover materials from waste plastics
suitable for second use but practical experience has shown that
reprocessing of mixed contaminated plastics produces polymer
polyblends that are inferior mechanically and lacking in durability
(which is explained due to peroxidation) compared with those
produced from virgin polymers [20]. Although at first sight
mechanical recycling of plastic wastes appears to be a ‘green’
operation, the re-processing operation is not cost effective as it
needs high energy for cleaning, sorting, transportation and
processing in addition to the additives used to provide a
serviceable product [21]. Again, materials recycling of household
waste plastics is particularly difficult when they are contaminated
with biological residues or, as is usually the case, when they are a
mixture of different kinds of plastics. Technology is being
introduced to sort plastics automatically, using various techniques
such as X-ray fluorescence, infrared and near infrared spectroscopy,
electrostatics and flotation. However the economic viability
and practicability of such process in industrial application is not
apparent [27]. Entrepreneurial effort has gone into the development
of special processing equipment to convert mixed plastics
wastes to wood or concrete substitutes in the manufacture of fence
Fig. 2. Different routes for plastic waste management.
236 A.K. Panda et al. / Renewable and Sustainable Energy Reviews 14 (2010) 233–248
Author's personal copy
posts, benches, boat docks, etc., but there are serious doubts about
the ecological benefits of doing this. Some limited success has been
achieved with mixed plastics wastes in the manufacture of
plastics-based underground chambers by increasing wall dimensions
to match the load-bearing strength of concrete. In this
application, there is no significant long term deterioration due to
exposure to the weather but this procedure could never utilise
more than a small fraction of the mixed polymer wastes available
[22]. Considerable academic interest has centered round the use of
‘compatibilizers’ (more correctly, solid phase dispersants to
upgrade the mechanical performance of mixed plastics polyblends
but in general this is an expensive and energy-intensive procedure
which cannot be justified for domestic mixed plastics wastes
[20,21]. In this way, it is apparent that mechanical recycling,
although employed widely, is not a suitable method when the
quality of secondary produce and ecological aspects are considered.