UNIT - I
RECYCLING & PLASTICS WASTE MANAGEMENT
What is plastics waste ?
Plastic pollution Plastic pollution is the accumulation of plastic objects and particles in the Earth's
environment that adversely affects humans, wildlife and their habitat.
RECYCLING OF PLASTICS - INTRODUCTION
Recycling of plastics started almost simultaneously with the development of processing techniques for
thermoplastics. Various technologies are involved in the recycling of post consumer plastics waste for wide
variety of plastics waste from various consumer and industrial products.
Plastics Recycling (thermoplastics) is an undaunted challenge and task as there are about 10-12 main
polymer types and thousands of different resin grades and blends available for commercial applications.
In addition to this, plastics or polymers have become increasingly multi-component through the use of
multi-layers, laminates and composites.
Furthermore, many polymers are rarely additive free. Normally they contain a large amount of additives,
formulants and modifiers such as fillers, pigments, antioxidants and flame-retardants which can further
interface with the recycling process. Another significant factor is that a single polymer (e.g. polyethylene)
is found in a wide range of melt flow indices and thermal stability.
1 Why Recycling of Plastics Waste
World wide, Plastic consumption has grown at a tremendous rate over the past two or three decades.
Many of the plastics applications are having life-cycle of less than one year and then the vast majority of
these plastics are discarded and become part of Municipal Solid Waste (MSW).
Due to chemical inertness or non-biodegradability of the plastics, it is imperative to recycle or reuse the
plastics waste by proper technique for effective plastics waste management.
In industry (the automotive industry for example) there is a growing move towards reuse and reprocessing
of plastics for economic, as well as environmental reasons, with many praiseworthy examples of
companies developing technologies and strategies for recycling of plastics. Not only is plastic made from
a non-renewable resource, but it is generally non-biodegradable (or the biodegradation process is very
slow).
Due to non-biodegradability plastic litter is often the most objectionable and will be visible for weeks or
months, and waste will remain in landfill, dump yard, etc for years without degrading. Hence, many
countries across the globe follow the proper process of reuse and recycling, with the associated system of
collection, sorting, cleaning and then recycling of waste for value addition and waste management.
2 Recovery & Recycling Concepts of Plastics
In developing countries like India the scope for recycling of plastics is enormous as the plastics
consumption has grown significantly in the recent year and reached about 5 million tones, which is
expected to touch about 12.5 Million tonnes in year 2010. In case of Plastics waste, due to its non-
biodegradability, a system of integrated waste management is essentially required to manage waste in an
environmentally and economically sustainable way.
The integrated waste management includes value addition to the waste by Recovery and Recycling
processes.
The percapita consumption of plastics in India is about 4.2 kg as against the world's average of 26 kg and
Indian plastics waste recycling industry is able to recycle about 2.5 Million tons of plastics waste per year
by mechanical recycling. In India, the plastics waste constitute only about 1-2% in MSW as compared to 8-
10% in the developed countries of the world.
However, the 60% of plastics waste in India is mechanically recycled as compared to world average of only
15-20%. Energy recovery by incineration of MSW is also widely practised in countries like Japan, Canada,
UK, USA, Germany, etc. In Japan, about 140 incinerators produce about 700 MW of power. Similarly, the
technology for mixed waste or commingled plastics waste has been developed in France, USA, Canada for
conversion of waste into value added products
.2.TYPES & SOURCES OF PLASTICS WASTE, COLLECTION AND
SEGREGATION
2.1 Types and Sources of Waste
a. Plastics in Municipal solid waste: The most important potential source of plastics waste is found in
Municipal Solid Waste (MSW), Plastics constitute between 8-10 weight % of MSW in developed countries
whereas, in India it varies between 1-2 weight %. Most of the plastics waste found in MSW are mainly
packaging materials, consisting of various grades of PE, PP, PVC, PS. PET. Engineering plastics may occur
under the form of kitchen utensils, consumer, electrical & electronic products, etc. About 95% of plastics
consumption in House-hold & packaging applications is discarded as waste.
b. Plastics waste from the agriculture sector: Agriculture primarily uses PE, PP and PVC plastics. The plastic
materials used in agriculture generally have a short to medium life-span.
Examples: Short-lived: Nursery bags, covering of greenhouses and fertilizer sacks, Medium-lived: irrigation
pipes, drums, tanks
The waste generated is estimated at 60% of the plastics consumption in Agriculture.
c. Plastics waste from the automotive sector: Currently, 20-25% of an automobile points is made of
non-metallic materials (glass, rubber, plastics, etc.). It is estimated that about 35% of plastics used in
automotive are being discarded as waste.
d. Plastics waste from the construction sector: The construction industry uses mainly PE and PVC
plastics. The construction sector typically uses plastics in applications for much longer than any other
major industry sector, making it difficult to estimate the waste generated based on consumption. 10% of
current consumption of plastics in construction becomes part of waste-stream.
e. Plastics waste from the large industry and distribution sectors: This sector is the second after
the MSW to produce large amount of plastics wastes. It includes bags, drums, containers, packaging film,
etc. It is estimated that 90% of the consumed plastics are discarded.
f. Processing Industry: This plastics wastes are generated during the plastics processing. Plastics consist
of, runners, distorted and incomplete formed products, surpluses, products rejected during quality control
and material arising during the starting-up or the shutting-down of the plant.
In the competitive environment, no industries can afford to waste these raw materials. Waste is carefully
segregated at the source, according to its nature, colour, and additives. Contamination by dust, oil or
burned (i.e. thermally decomposed) material is avoided. In general, the plastics wastes is ground and
recycled, either directly in the same or a similar production, or in a less critical application.
g. Polymer Manufacturers: Plastics wastes generated by polymer manufacturers consists of: Production
wastes, such as the deposits formed on the walls of polymerization vessel or the drivers, or sludges
separated from process waters, Off grade products; Extrusion purging and lumps; Floor sweepings:
Wastes arising during quality control and laboratory testing.
h. Hospital and Medical Waste: Plastics is widely used in different healthcare and medical applications.
The waste from hospital requires special care due to contamination and hazardous nature. Eco-friendly,
clean recycling or incineration technique is preferred for managing hospital and medical waste.
2.2 Collection & Segregation
a. Collection of Plastics Waste: In India, sorting is not carried out before disposal of the plastics waste
at source as being practised in developed countries. The plastics waste from the domestic use, consumer
and in public places are mixed waste consisting of plastic bottles, disposable cups and plates, packaging
material, milk pouches, PVC foot wear, carry bags, shopping bags, etc. The problem with household waste
are, it is dirty having food contaminants and difficult to separate by any mechanical means.
Plastics waste is collected from houses, garbage and junkyards. The plastics waste collected from these
sources by rag-pickers, and sold to scrap dealers. The scrap is segregated by rag- pickers based on quality
and the grade of the material and visual identification. The manually segregated plastics waste material is
sold to reprocessors by the scrap dealers.
After collection the wastes are separated into different categories based on the type of the resin, grade
and colour. Most of the sorting is done manually because of the low cost. In India, Sorting of plastics waste
by mechanical means or by using automatic devices are very difficult to use because of the high
contamination and mix of various items of plastics waste which are collected from the garbage. However,
in the developed countries, where waste is sorted at source itself automated equipments for cleaning
sorting & recycling are feasible option.
b. Segregation of plastics waste by simple Identification techniques: There are several
simple tests that can be used to distinguish between the common types of polymers so that they may be
separated for processing.
The water test: After adding a few drops of liquid detergent to some water put in a small piece of plastic
and see if it floats.
Burning test: Hold a piece of the plastic in a tweezers or on the back of a knife and apply a flame. Does
the plastic burn? If so, what colour, smoke and odour?
Fingernail test: Can a sample of the plastic be scratched with a fingernail?
To confirm PVC, touch the sample with a red-hot piece of copper wire and then hold the wire to the flame.
A green flame from the presence of chlorine confirms that it is PVC. To determine if a plastic is a
thermoplastic or a thermoset, take a piece of wire just below red heat and press it into the material. If the
wire penetrates the material, it is a thermoplastic; if it does not it is a thermoset. A coding system has also
been introduced in different countries across the globe to aid identification of plastics for reclamation. It
is based on the 'Recycle Triangle' with a series of numbers and letters to help with identification.
c. Segregation Techniques Based on Density:
The techniques rely on the fact that the target plastic will often have a different density from the foreign
materials, including other plastics. The target plastic stream can be separated from undesirable materials
having different density, by placing the commingled material in a medium having an appropriate density.
If a medium is chosen with a density between that of two different types of plastic having sufficiently
different densities, a separation can be effected by simply placing the plastics in a vessel containing the
medium. The material less dense than the media will float and the more dense material will sink. The most
simple density separations use sink-float tanks and various versions have been used by plastic beverage
bottle recyclers for years, often using only water.
Most rigid plastics from durable goods are denser than water, so the density of the medium used in the
tank must be increased to greater than that of water by adding a modifier to the water or using
a different liquid to create "heavy media". Salt-water solutions using sodium or calcium chloride can reach
approximately 1.2 specific gravity, which is usually sufficient for most separations. Higher density salt or
other solutions are also possible and used frequently by other industries. The major drawbacks to using
heavy media include economic and environmental considerations associated with lost heavy media and
residual media contamination on the recovered plastic. Adequate rinsing can address the residual media
concern for many plastics.
Hydrocyclones are often used to enhance the effectiveness of density separations from both a throughput
and purity standpoint, but their operation is less well understood. They can provide a greater driving force
(centrifugal verses gravity) to the separation, enhance material wetability and increase throughput.
d. Segregation by Selective Dissolution: The selective dissolution process is based on
physical principles without destruction of the polymer molecules to feedstock compounds. The process
was developed specifically for sorting of commingled plastics by dissolving and precipitating at different
level of solubility. The Rensselaer technology has developed a pilot-scale selective dissolution system.
In this process the unsorted (or commingled) plastics are shredded, washed, and then selectively dissolved
in a common solvent. Since each type of plastic dissolves at a different temperature, it is possible to
dissolve and remove them from the mixture one type at a time.
In a typical process run on pilot system, unsorted plastic pieces shredded into 3/4-inch pieces were used.
They get washed to remove impurities, but any remaining contaminants won't cause problems and will be
removed later in the process. After drying, the pieces are fed into a mixing vessel along with a xylene
solvent at room temperature (15°C). At this temperature, polystyrene dissolves upon contact with the
solvent and forms a solution of about 6% PS by weight. The polystyrene solution is drained from the mixing
vessel through filters to a holding tank, leaving behind the other undissolved plastics. The process is
repeated to flush out any remaining polystyrene.
In the next round, mix the remaining commingled plastics in xylene at 75°C. This dissolves the traces of
LDPE. The LDPE solution is drained and the cycle gets repeated with increasing xylene
temperatures for HDPE and polypropylene. When only PVC and PET are left in the mix, xylene/
cyclohexanone solvent would be added. The PVC dissolves out first at 120°C and, finally, the PET dissolves
at 180°C. The polymer solutions are stored in six holding tanks from which each is moved to a flash
devolatilization vessel and a devolatilizing extruder where the polymer is separated from the solvent. All
solvent is reused in the process and any impurities are collected for use as fuel, as are any low molecular
weight vapours released by the polymers, such as hexane and hexene. The pure polymer is made into
pellets and packaged for use.
Selective dissolution also works with the plastics in durable goods such as carpets, consumer electronics,
and automobile components. Reclaiming these plastics has been complicated in the past. The plastics used
in these applications are often combined with something else like a metal part in such a way that one
cannot mechanically separate or rework them. The point is, we can tailor the process to fit each situation.
In the case of many durable goods, the value of the reclaimed plastics is much higher than that of PET or
other packaging.
Figure Demonstration of selective dissolution)
The Advantages and Disadvantages NIR-based sorting systems are:
Advantages
➢ It is possible to use conventional fibre-optics allowing easy and inexpensive remote sampling. •
Portable and robust units are available.
➢ NIR penetrates deeper into the polymer than Mid InfraRed Spectroscopy (MIR).
➢ Measurements can be performed in reflectance without sample contact.
➢ Instruments have no moving parts and can operate in vibration-prone environments.
Disadvantages
➢ Nature of peaks is not always clear since they are due to overtones and not fundamental peaks.
➢ Carbon black strongly absorbs and scatters at NIR frequencies making it difficult to probe dark
plastics.
