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RECYCLING OF POLYMERS
A TERM PAPER SUBMITTED IN FULFILLMENT OF
THE REQUIREMENTS FOR THE COURSE
ENVIRONMENT AND ECOLOGY
IN
MECHANICAL ENGINEERING
SUBMITTED BY SUBMITTED TO
M.GOPIKRISHNA Mr. PRAVEEN SHARMA
10907035 (Assistant Professor)
RME016A19 Dept. Of CHEMISTRY
DEPARTMENT OF MECHANICAL ENGINEERING (2009-2013)
LOVELY PROFESSIONAL UNIVERSITY
JALANDHAR– 144403

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Acknowledgement
I place on record and warmly acknowledge the continuous encouragement, invaluable supervision,
timely suggestions and inspired guidance offered by our guide Mr. PRAVEEN SHARMA, chemistry
department, in bringing this report to a successful completion.
I am grateful to Prof. Gurpreet Singh Phull, Head of the Department of Mechanical Engineering,
for permitting me to make use of the facilities available in the department to carry out the project
successfully. Last but not the least I express my sincere thanks to all of my friends who have
patiently extended all sorts of help for accomplishing this undertaking.
Finally I extend my gratefulness to one and all that are directly or indirectly involve in the
successful completion of this TERM PAPER work.
M.Gopi Krishna
(10907035)

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ABSTRACT
Plastics have become common materials of our everyday lives, and many of their properties, such
as durability, versatility and light-weight, can be a significant factor in achieving sustainable
development. However, plastic applications also contribute to the growing amounts of solid waste
generated, as plastic products are often used only once before disposal. The disposal problem is not
simply technical, but it also has social, economic and even political aspects. This is the reason why
several different methods have been explored and applied for solving the problems associated with
polymer waste handling and disposal. Plastics cause serious environmental problems. Although
they are not intrinsically dangerous, they take up a huge amount of space in landfills and they are
made from a non-renewable resource, namely fossil fuels. For these reasons it is important that,
where possible, plastics are recycled.

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CONTENTS
Description Page no.
ACKNOWLEDGEMENT………………………………………….. 2
ABSTRACT ………………………………………………………… 3
CONTENTS…………………………………………………………. 4
CHAPTER-1 INTRODUCTION………………………………….... 5
1.1 PLASTICS..................................................................................... 5
1.2 THE PROBLEM OF PLASTICS.................................................. 6
1.3 IMPACT ON THE ENVIRONMENT ......................................... 6
1.4 THE SOLUTION ........................................................................ 8
1.5 IDENTIFYING PLASTICS.............................................................. 8
1.6 WHY RECYCLE PLASTICS .................................................... 9
1.7 WHAT KIND OF PLASTIC IS RECYCLABLE.............................. 9
1.8 WHAT KIND OF PLASTIC IS NOT RECYCLABLE..................... 10
1.9 SOURCES OF WASTE PLASTIC ..................................................... 10
CHAPTER-2
2.1 Recycling Of Polymers ……………………………….. 11
2.2 Recycling Techniques …………………………….. 13-20
CHAPTER 3
3.1 Uses of Recycled Plastic....................................................................... 21
3.2 Environmental Impact............................................................................ 21
CHAPTER-4 CONCLUSION............................................................... 22
CHAPTER-5 REFERENCES................................................................ 22

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Chapter1
Introduction
1.1 Plastics – what are they and how do they behave?
 Plastics are organic polymeric materials consisting of giant organic molecules.
 There are many different types of plastics depending on their molecular make up and shape.
 Plastic materials can be formed into shapes by one of a variety of processes, such as
extrusion, moulding, casting or spinning.
 Modern plastics (or polymers) possess a number of extremely desirable characteristics; high
strength to weight ratio, excellent thermal properties, electrical insulation, resistance to
acids, alkalis and solvents, to name but a few.
 These polymers are made of a series of repeating units known as monomers.
 The structure and degree of polymerisation of a given polymer determine its characteristics.
Linear polymers (a single linear chain of monomers) and branched polymers (linear with
side chains) are thermoplastic that is they soften when heated.
 Cross-linked polymers (two or more chains joined by side chains) are thermosetting, that is,
they harden when heated.
 Thermoplastics make up 80% of the plastics produced today.
 Examples of thermoplastics include;
 high density polyethylene (HDPE) used in piping, automotive fuel tanks, bottles, toys,
 low density polyethylene (LDPE) used in plastic bags, cling film, flexible containers;
 polyethylene terephthalate (PET) used in bottles, carpets and food packaging;
 polypropylene (PP) used in food containers, battery cases, bottle crates, automotive
parts and fibres;
 polystyrene (PS) used in dairy product containers, tape cassettes, cups and plates;
 Polyvinyl chloride (PVC) used in window frames, flooring, bottles, packaging film,
cable insulation, credit cards and medical products.
 There are hundreds of types of thermoplastic polymer, and new variations are regularly
being developed.
 In developing countries the number of plastics in common uses, however, tends to be much
lower.

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 Thermosets make up the remaining 20% of plastics produced.
 They are hardened by curing and cannot be re-melted or re-moulded and are therefore
difficult to recycle.
 They are sometimes ground and used as a filler material.
 They include: polyurethane (PU) coatings, finishes, gears, diaphragms, cushions,
mattresses and car seats; epoxy – adhesives, sports equipment, electrical and automotive
equipment; phenolics – ovens, handles for cutlery, automotive parts and circuit boards (The
World Resource Foundation).
 Nowadays, the raw materials for plastics come mainly from petrochemicals, although
originally plastics were derived from cellulose, the basic material of all plant life.
1.2 The problem with plastics
 Plastics are polymers, chains of molecules produced by smaller molecules called monomers.
 There are many different types of plastics depending on their molecular make up and shape.
 To help identify the different plastics, a Plastics Identification Code is stamped on the final
product to indicate what type of resin it contains.
 The code is displayed as a number inside a triangle of chasing arrows.

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1.3 Impact on the environment
1) Greenhouse Gases:
 Fossil fuels such as natural gas, oil and coal are used in production process of
plastic, emitting dangerous greenhouse gases and toxic chemicals.
 As plastic decomposes gases are produced, particularly methane.
 Methane is 20 times stronger than carbon dioxide and represents up to 4% of
emissions from landfill2.
2) Natural Resources:
 Plastics are made from non-renewable resources that, once depleted, cannot be
replaced.
3) Persistence in the Environment:
 Most plastic is not biodegradable and will survive in the environment for hundreds
of years.
 Rather than biodegrading, plastic photo degrades, breaking down into smaller and
smaller pieces.
 Plastic is also lightweight and moisture resistant, meaning it can float easily in air
and water, and travel long distances.
4) Landfill Space:
 Australians use 1.3 million tonnes of plastic each year.
 We are great recyclers, with 46% of waste recycled each year; however, this means
that over half of our waste still ends up in landfill causing serious problems for the
environment.
5) Threat to Marine Life:
 Every year more than 6 million tonnes of rubbish is dumped into the world’s oceans.
 80% of this waste is plastic, with an estimated 46,000 pieces of plastic per square
mile of ocean.
 Plastic waste including plastic bags, food packaging, and abandoned fishing nets can
be deadly to marine life.
 Turtles, whales, and sea birds mistake rubbish for food or get entangled in it causing
painful injuries or even death.
 It is estimated that marine rubbish, mostly plastic, is killing more than a million
seabirds and 100,000 mammals every year.

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1.4 THE SOLUTION
The best way to limit the plastic waste that you create and to prevent rubbish
from going to landfill is to avoid, reduce, reuse and recycle. Plastics are increasingly
used in our everyday life, thus recycling is more important than ever to reduce
waste. Identifying the type of plastic is essential because each type of plastic is
recycled differently.
1.5 Identifying plastics:
The Plastics Identification Code is stamped on all plastic products to identify the type of
resin used. The types of plastics are as follows:-

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1.6 Why recycle plastics?
 In ‘western’ countries, plastic consumption has grown at a tremendous rate over the
past two or three decades.
 In the ‘consumer’ societies of Europe and America, scarce petroleum resources are
used for producing an enormous variety of plastics for an even wider variety of
products.
 Many of the applications are for products with a life-cycle of less than one year and
then the vast majority of these plastics are then discarded.
 In most instances reclamation of this plastic waste is simply not economically
viable.
 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).
 This means that plastic litter is often the most objectionable kind of litter and will be
visible for weeks or months, and waste will sit in landfill sites for years without
degrading.
 Although there is also a rapid growth in plastics consumption in the developing
world, plastics consumption per capita in developing countries is much lower than in
the industrialised countries.
 These plastics are, however, often produced from expensive imported raw materials.
 There is a much wider scope for recycling in developing countries due to several
factors:
 Labour costs are lower.
 In many countries there is an existing culture of reuse and recycling, with
the associated system of collection, sorting, cleaning and reuse of ‘waste’ or
used materials.
 There is often an ‘informal sector’ which is ideally suited to taking on small-
scale recycling activities. Such opportunities to earn a small income are
rarely missed by members of the urban poor.
 There are fewer laws to control the standards of recycled materials.
 Transportation costs are often lower, with hand or ox carts often being used.
 Low cost raw materials give an edge in the competitive manufacturing
world.
 Innovative use of scrap machinery often leads to low entry costs for
processing or manufacture.
 In developing countries the scope for recycling of plastics is growing as the amount of
plastic being consumed increases.
1.7 What kind of plastic is recyclable?
 Not all plastics are the same and your local council may only be able to recycle
certain types through your kerbside recycling program.
 In most areas, plastics labelled 1, 2, and 3 can be recycled, although many councils
are now extending their recycling programs to include those labelled 4 through 7.
 Contamination of recyclables is a problem because it raises the costs for collectors,
recyclers and the community.

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1.8 What is not recyclable?
 Plastic bags, bin liners, and cling wrap are not recyclable.
 These plastics can get stuck in the sorting equipment in recycling facilities causing it
to stop or break.
 Often bottle tops and lids cannot be recycled with the bottle as they may be made of
a different type of plastic.
 Polystyrene foam is generally not recyclable.
 This includes the spongy black foam trays that meat is often packaged in at
supermarkets.
 It also includes some takeaway containers and hot drink cups.
 Other items that cannot be recycled in the normal recycling bins from your council
are disposable nappies, and syringes.
1.9 Sources of waste plastics
 Industrial waste (or primary waste) can often be obtained from the large plastics processing,
manufacturing and packaging industries.
 Rejected or waste material usually has good characteristics for recycling and will be clean.
 Although the quantity of material available is sometimes small, the quantities tend to be
growing as consumption, and therefore production, increases.
 Commercial waste is often available from workshops, craftsmen, shops, supermarkets and
wholesalers.
 A lot of the plastics available from these sources will be PE, often contaminated.
 Agricultural waste can be obtained from farms and nursery gardens outside the urban areas.
 This is usually in the form of packaging (plastic containers or sheets) or construction
materials (irrigation or hosepipes).
 Municipal waste can be collected from residential areas (domestic or household waste),
streets, parks, collection depots and waste dumps.
 In Asian cities this type of waste is common and can either be collected from the streets or
can be collected from households by arrangement with the householders.

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CHAPTER 2:-
2.1 RECYCLING OF POLYMERS
 Plastics cause serious environmental problems.
 Although they are not intrinsically dangerous, they take up a huge amount of space in and
fills and they are made from a non-renewable resource, namely fossil fuels. For these
reasons it is important that, where possible, plastics are recycled.
 The recycling of plastics is carried out in a five step process.
Step 1- Plastics collection
This is done through roadside collections, special recycling bins and directly from
industries that use a lot of plastic.
Step 2 - Manual sorting
At this stage nails and stones are removed, and the plastic is sorted into three types:
PET, HDPE and 'other'.
Step 3 - Chipping
The sorted plastic is cut into small pieces ready to be melted down.
`
This stage removes contaminants such as paper labels, dirt and remnants of the
product originally contained in the plastic.
Step 5 - Pelleting
The plastic is then melted down and extruded into small pellets ready for reuse.
Some recycled plastic is then used in applications similar to those for which virgin
plastic is used. The remaining plastic is made into a variety of objects such as
drainage mats and hard board.
Step 1 - Plastics collection
Plastics for recycling come from two main sources: post consumer plastics and post
industrial plastics. Post consumer plastics are those which have already been used by people. These
are the plastics collected in plastics recycling bins and at domestic roadside collections. Post
industrial plastics, on the other hand, are rejects from industry — off cuts, damaged batches etc.
These plastics are collected either directly from the industry or collected by the local council,
squashed into bales and sold to a recycler.
Step 2 - Manual Sorting
In theory, every type of plastic can be recycled. In practise in New Zealand only codes 1
PET) and 2 (HDPE) are recycled. The incoming plastic is manually sorted into these two codes and
'other', and the three separate streams sent off to be chipped. It is particularly important that all PVC
is removed from the PET stream as the more sophisticated sorting used later on cannot differentiate
between these two types of plastic. Any rocks, nails, metal etc.that is mixed in with the plastic is
also manually removed at this stage.
Step 3 - Chipping
Each sorted stream of plastic is then sent separately to a chipper. This is a cylinder of blades
Somewhat like an old-fashioned lawnmower in a vessel with a 10 mm grill floor. The blades cut the
material until it is small enough to fall through the grill.
Step 4 - Washing
The chips are then washed to remove glue, paper labels, dirt and any remnants of the
product they once contained. Both the "other" stream and the PET stream are washed at around
90oC for at least twelve minutes, while the HDPE (which has a much lower melting point) must be
Washed below 40oC to prevent discolouration.
The wash solution consists of an alkaline detergent in water, which removes dirt and grease

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And degrades protein. The detergent used is an alkaline, cationic detergent (i.e. an alkaline solution
containing a cationic surfactant). Dishwashing detergents are usually anionic, because glass, china
etc. usually build up a negative surface charge. This means that positively charged dirt particles are
attracted to them, so an anionic detergent is needed to remove the dirt. If a cationic surfactant were
used it not only would be incapable of removing the dirt, but it would itself stick to the surface to
be cleaned, making it greasy.
However, plastics acquire a positive surface charge, meaning that a cationic detergent is needed to
clean them. Cationic surfactants are much less common than anionic ones, but they are used for
shampoos and for fabric softeners1. Surfactants are explained in more detail in the article on soaps
and detergents.
During washing the agitator in the wash tank acts as an abrasive, grinding off the glue of the
Labels and reducing any paper labels to fibres. The plastics are then separated from the glue, paper,
dirt etc. in a spinning tower in which this very fine material is forced out through small holes, while
the plastic itself remains inside. The plastics are then further rinsed and then (in the PET and HDPE
streams) separated on the basis of weight. This is done using a water cyclone which is designed to
separate out the given plastic from all the others. In the case of PET, it is heavier than all the others
and so 95% of the PET falls to the bottom while the remainder of the PET and everything else rises
to the top. Unfortunately, PVC is of about the same weight as PET and so cannot be separated in
this step. For this reason it is very important that all the PVC was removed during manual sorting.
The product at this stage can be sold for extruding, but it is only appropriate for extruding
through wide extrusion nozzles as it doesn't pack efficiently enough for narrow nozzles, hence most
of it is polluted before sale.
Step 5 - Pelleting
This is done by melting the chips and extruding them out first through a fine grill to remove
any solid dirt or metal particles that have made it through the treatment thus far and then through a
die of small holes. If the plastic was simply allowed to extrude from these holes it would come out
as spaghetti-like strings and quickly tangle together. However, it is sprayed with water as it comes
out (to prevent the plastic from sticking together) and cut off by rotating knives to give small, oval
pellets.

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2.2 Recycling Techniques:-
 Plastics have become common materials of our everyday lives, and many of their properties,
such as durability, versatility and light-weight, can be a significant factor in achieving
sustainable development.
 However, plastic applications also contribute to the growing amounts of solid waste
generated, as plastic products are often used only once before disposal.
 The disposal problem is not simply technical, but it also has social, economic and even
political aspects.
 This is the reason why several different methods have been explored and applied for solving
the problems associated with polymer waste handling and disposal.
 The alternatives of practical techniques for solid waste management are shown in Figure
 Even though external recycling is not the most profitable technique for the treatment of
plastic waste, it will have a significant role in the future.
 In spite of the application of clean technologies and waste elimination, it is not expected
that the amounts of plastic wastes will decline, thus, new recycling methods will have to be
developed.
 From the perspective of catalysis, chemical recycling of plastic wastes is the most
noteworthy of plastic waste recovery techniques.
Energy recovery
Waste incineration, or controlled burning, is typically considered as a disposal method,
because it is usually applied as a method of reducing the volume of miscellaneous
municipal waste. However, incineration of plastics can also be seen as recovery method, as

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plastics could replace the application of other oil based fuels. It can be viewed that the
plastic application is the first purpose of oil, and energy production is the secondary task.
Indeed incineration with energy reclamation is considered as a recovery method and, due to
their high energy content, plastic waste is a valuable fuel. The heat capacity of plastics and
some other materials are shown in the table
Heat capacity of plastics and some other materials.
Mechanical recycling
Plastics can also be recovered from waste via mechanical recycling. The mechanical recycling process
involves a number of operational steps: separation of plastics by resin type, washing to remove dirt and
contaminants, grinding and crushing to reduce the plastics’ particle size, extrusion by heat and
reprocessing into new plastic goods. This type of recycling is mainly restricted to thermoplastics
because thermosets cannot be remoulded by the effect of heat.
Mechanical recycling of plastics is limited by the compatibility between the different types of polymers.
Presence of a polymer dispersed in a matrix of a second polymer may dramatically change the
properties and hinder the possibilities to use it in the conventional applications. A good example of this
is the impacts of polyvinyl chloride (PVC) during polyethylene terephthalate (PET) processing. Only a
small amount of PVC in the recycled PET strongly reduces the commercial value of the latter. Another
problem with mechanical recycling is the presence in plastic waste of products made of the same resin
but with different colour, which usually impart an undesirable grey colour to the recycled plastic.
In addition, most polymers suffer certain degradation during their use due to effects of temperature,
ultraviolet radiation oxygen and ozone. Therefore, recycled polymers exhibit lower properties and
performance than the virgin polymers, and are useful only for undemanding and lesser value
applications. Recycling of plastics without prior separation by resin produces a material with
mechanical properties similar to timber. Hence, it is often used for the replacement of timber in certain
applications. A higher quality of recycled plastics is achieved when separation by resin is carried out
prior to the remoulding step. Stages and their relations in the mechanical recycling of plastics are shown
in Figure

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Feedstock recycling
Feedstock recycling of plastics, also referred to as chemical or tertiary recycling is based on the
decomposition of polymers by means of heat, chemical, or catalytic agent, to yield a variety of products
ranging from the chemical monomers to a mixtures of compounds with possible applications as a source
of chemicals or fuels. The chemical recycling processes can be classified into three main areas (Janssen
and van Santen 1999):
1. Recycling to fuels (gasoline, liquefied petroleum gas (LPG) and diesel oils)
2. Recycling to monomers
3. Recycling to industrial chemicals.
Depending on recyclable plastic types, desired composition and molecule weight of products, many
different methods of feedstock recycling can be implemented within above areas. For example,
following Figure illustrates the methods for the feedstock recycling of plastics and rubber.
Up till now. Only a small number of chemical recycling methods have been commercially realized but
the interest in more efficient processes is still growing due to the emerging need of polymer waste
recycling in the future. At the present, feedstock recycling is more limited by process economy than by
technical reasons. The factors which determine the profitability of alternative feedstock recycling
methods are the degree of separation required in raw wastes, the value of the products obtained, and the
capital investments in the processing facilities.
According to the separation steps required, the methods can be ordered as follows: gasification <
thermal treatments hydrogenation < catalytic cracking < chemical depolymerization. However, the
feedstock methods can be ordered also according to the commercial value of the products. In that case,
the order of methods will be as follows: thermal oils ≈≈ synthesis gas < hydrogenation oils ≈ catalytic
oils < monomers. It is interesting to note that the required pre-treatments and product value follow
almost reverse orders.
However, comparison of required pre-treatments and product value is not enough. Many other factors
should be included for an adequate comparison of these methods. For instance the possibility of
carrying out the treatment in existing or new facilities, minimum size of the industrial plants needed to
be profitable, required investments and plants location are such factors.

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Hydrogenation
Hydrogenation of plastics is a potential alternative for breaking down the polymer chain. Compared to
treatments in the absence of hydrogen, hydrogenation leads to the formation of highly saturated
products, avoiding the presence of olefins in the liquid fractions, which favours their use as fuels
without further treatments. Moreover, hydrogenation promotes the removal of hetero atoms, such as
chlorine (Cl), nitrogen (N) and sulphur (S), in the form of volatile compounds. However, hydrogenation
suffers several drawbacks, mainly due to the cost of hydrogen and the need to operate under high
pressure.
Even though some non-catalytic hydrogenation processes have been developed, most of the
hydrogenation processes require the presence of bi-functional catalysts to promote hydrogen addition
reactions. A typical catalyst in the hydrogenation includes transition metals, such as palladium (Pt),
nickel (Ni), molybdenum (Mo) and iron (Fe), supported on acid solids such as alumina, zeolites or
amorphous silica-alumina.
Gasification
Gasification can be considered to be a partial oxidation process of carbonaceous material leading
predominantly to a mixture of carbon monoxide (CO) and hydrogen (H
2
). It is also called synthesis gas
or syngas because of its application in a variety of chemical synthesis. Gasification has been initially
developed for coal conversion, but it has been further applied also to the processing of heavy petroleum
fractions and natural gas.
Gasification is an efficient treatment for polymeric waste because of its several advantages: it is not
necessary to separate the different polymer types, and it is possible to mix plastic wastes with other,
non-plastic solid waste before gasification. However, the profitability of a gasification process largely
depends on the value and applications of the synthesis gas. Syngas can be used for the synthesis of
various chemicals, such as methanol, ammonia or acetic acid, but it can also be burned in combustors.
However, incineration of synthesis gas cannot be really considered as a feedstock recycling of plastics,
rather it is considered as a means of energy recovery.
When oxygen or air is used as a gasification agent, the content of agent in the reaction must be kept
low, in order to avoid complete oxidation into carbon dioxide and water. Gasification can be promoted
by metal catalyst, which is typically added in aqueous solutions.
The basic reactions during gasification of carbonaceous material are shown in following scheme

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The principles of gasification of pure polymeric wastes are similar to the gasification of
carbonaceous material. However, certain details have to be taken into account when plastic and
rubber wastes are processed. For instance, the heterogeneity of the starting materials, the problem
of feeding the highly viscous melted plastics, and the possible formation of corrosive compounds
such as hydrochloric acid (HCl) and polyvinyl chloride (PVC) are some examples of details that
have to be taken into consideration.
Chemical depolymerization
During the chemical depolymerization process, the polymer is cracked to the original monomer in
the presence of different reagents. Recycled monomers are identical to those used in the preparation
of virgin polymers, consequently, plastics prepared from both fresh monomers and
depolymerization end products have similar characteristics and quality.

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Chemical depolymerization is the most established method of plastic feedstock recycling, even
though it is restricted to the recycling of condensation polymers and there are no applications of
decomposition of other polymers. The total volume of condensation polymers accounts for less than
15 % of all plastic wastes. As examples of common condensation polymers, polyesters, polyamides
and polyacetals can be mentioned. Condensation polymers are obtained by the random reaction of
two molecules, which proceeds with the liberation of a small molecule as the chain bonds are
formed. In the chemical depolymerization, the reverse reaction of polymer formation takes place
through the reaction of those small molecules with the polymeric chains. Depending on the
chemical agent used to break down the polymer, different depolymerization routes are envisaged:
for instance glycolysis, methanolysis, hydrolysis and ammonolysis. An example of
depolymerization of polyethyleneteraphtalate (PET) is shown in Figure
Thermal treatment
Thermal treatment is a collective term to describe different methods and processes developed for
breaking down polymeric materials simply by treatment at high temperature in an inert atmosphere.
They are mainly used for the feedstock recycling of addition polymers, whereas condensation
polymers are preferably depolymerised by reaction with certain agents.
Thermal decomposition of polymers can be considered as a depolymerization process in only a few
cases, given that thermal decomposition of most polymers leads to a complex mixture of products,
containing low monomer concentrations. The types and distribution of products derived form the
thermal degradation of each polymer depend on a number of factors: the polymer itself, the reaction
conditions and the type and operation mode of the reactor, for instance.
There is some confusion regarding a thermal treatment of polymers is to be described as
depolymerization, cracking, thermolysis or pyrolysis. For example, the term pyrolysis refers to the
thermal decomposition of polymeric material at high temperatures (above 600 ºC), whereas thermal
cracking refers to degradation at lower temperatures. However, in some cases, the process is not
confined to any of the above process characteristics, for instance in the case when the temperature
is continuously varied. In this situation it is difficult to assign one term to be used to describe the
process.

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Thermal degradation of plastics and rubber proceeds through a radical mechanism, which may
involve three different decomposition pathways
1. Random scission at any point in the thermal backbone leading to the formation of smaller
polymeric fragments as primary products, which in turn may be subjected to additional
random cracking reactions.
2. End-chain scission, where a small molecule and a long-chain polymeric fragment are
formed. If the small molecule released is the starting monomer, the thermal degradation
process can be considered as an actual depolymerization or unzipping process.
3. Abstraction of functional substituent’s to form small molecules. In this case, the polymer
chain may retain its length or the release of the small molecule may be accompanied by
cleavage of the polymeric chain.
In many cases, several of these pathways occur simultaneously. During the thermal
degradation of many polymers, other reactions may also occur at the same time. For
instance during the cracking reactions isomerisation, cyclization, aromatization and
recombination can also take place. Thus, an increase in the degree of branching of the
polymeric chains is usually observed, as they are reduced in length by thermal
decomposition.
Catalytic cracking and reforming
Catalytic cracking and reforming of plastic wastes are based on contact of the polymer with a
catalyst that promotes its cleavage. In fact, plastic degradation proceeds in most cases by a
combination of catalytic and thermal effects, which cannot be isolated. Besides catalytic cracking,
the use of catalysts is usual also in other earlier mentioned processes, such as gasification and
partial oxidation of plastics. However, there is no chemical agent incorporated to react directly with
the polymer during the catalytic cracking process and the products derived from the polymer
decomposition are not usually the starting monomers.
There are many advantages in catalytic cracking compared to thermal cracking. For example,
polymer molecules start to break down at lower temperatures. In consequence of the lower
temperature, the energy requirement is also lower. Further, if the rates of reactions between
catalytic and thermal cracking are compared, the catalytic process is faster than the thermal process
because of lower activation energy. Using of catalysts also improves the quality and selectivity of
products because the product distribution can be varied and controlled by the selected catalysts.
All these factors illustrate the great potential of catalytic cracking for the conversion of polymeric
wastes into valuable components. However, this method also suffers from drawbacks and problems,
which are still not completely solved. For instance with time, the catalysts are deactivated by the
decomposition of carbonaceous residues, and by poisons present in the raw waste stream such as
chlorine (Cl) and nitrogen (N) compounds. Moreover, the inorganic compounds contained in the
plastic wastes tent to remain with the catalysts, hindering their later recovery and re-use. For these
reasons, catalytic cracking is mainly applied to polyolefinic wastes of relatively high purity,

21. Page 21 of 22
requiring a number of pre-treatment steps to remove compounds that may negatively affect the
catalysts.
Other difficulties arise from the high viscosity of the molten plastic, which hinders its flow through
conventional fixed bed reactors. These problems are largely avoided when the catalytic conversion
is combined with a simple thermal treatment, aimed at reducing the viscosity of the mixture and
enabling the separation of unwanted components.
A wide variety of catalysts have been found effective in promoting the decomposition of plastics
materials: Friedel-Crafts catalysts, acidic and basic solids, bi-functional solids, etc. The most
common catalysts used in plastics cracking are acidic solids, mainly alumina, amorphous silica-
alumina and zeolites. These catalysts are typically used in petroleum processing and by
petrochemical industries. They have very different textural and acidic properties, which directly
determine their catalytic activity and product selectivity. This is an important factor, because the
initiation step of polymer catalytic degradation depends on the type of acid sites and leads to
different to cracking pathways.
CHAPTER 3
3.1 Uses of recycled plastic
 Recycled plastic can be used for anything that virgin plastic is used for except the packaging
Of food.
 In general, the pelletised plastic is sold by the recycling company to other companies for
moulding into a wide variety of products.
 Some of it is used locally and the remainder is exported to Asia and the United States. PET
is often made into fibres to make carpet and clothing, while the "other" stream is usually
used to make a wood substitute.
 Two products that are made on site by New Zealand Recycling Ltd. are:
1) A hardboard substitute made from HDPE. Most of the HDPE received is from milk
bottles, but a small proportion is made from containers that have held strong-smelling
substances such as toilet cleaners. The perfume remains in the plastic, so they are
unsuitable for normal re-use. However, NZR has recently developed a board made of
this plastic sandwiched between two layers of LDPE. These will be used industrially as
a cheap, durable and recyclable hardboard substitute.
2) FLOMAT is an American product which is used instead of scoria as a drainage material.
The mats consist of a series of fabric pockets filled with chipped plastic from the "other"
stream. Water drains down through the plastic to a drainage pipe attached to the base of
the mat. So long as well-washed plastic is used (so that the mat contents do not rot)
these are a very long-lasting and space efficient alternative to scoria draining behind
retaining walls etc.
3.2 ENVIRONMENTAL IMPLICATIONS
As stated above, plastic recycling prevents damage to the environment via excessive land filling
and use of non-renewable resources. The process is also largely environmentally safe, with the only
effluent being from the wash water. This is recycled in the plant as much as possible to minimise
water use and when it is finished with it is still sufficiently clean to be dumped into the sewers.

22. Page 22 of 22
CHAPTER 4
Conclusions
 Plastics have become common materials of our everyday lives and many of their properties
contribute to sustainable development.
 However, at the end of their useful life, plastics waste may cause a waste management
challenge.
 This problem is aggravated by the fact that plastic applications are often used only once
before disposal.
 Waste incineration, or controlled burning, is typically considered as a disposal method
because of its application for a mere reduction of the volume of waste. However,
incineration with energy recovery is considered as a recovery methods, as plastics can
replace other oil based fuels.
 The polymers in plastics can be recovered via mechanical recycling. This process involves
a number of operations including separation of plastics by resin-type, washing to remove
dirt and contaminants, grinding and crushing to reduce the plastics particle size, extrusion
by heat, and reprocessing into new plastic goods. This type of recovery is mainly restricted
to thermoplastics, because thermosets cannot be remoulded by the application of heat.
 The chemical recycling processes can be classified into recycling to fuels, monomers or
industrial chemicals. During chemical recycling processes, plastic wastes can be
remanufactured into valuable chemical feedstock by a large variety of thermal or catalytic
processes. Thermal processes are less sensitive than catalytic processes to dirt and critical
impurities, such as Cl, S, N and heavy metals, however, the end products are mostly of
lower quality and of lesser value. In the future, catalysis may offer important contributions
to the efficiency of feedstock recycling, provided that the problems of catalyst deactivation
by contaminants can be overcome in an economic viable way.
The alternative methods for feedstock recycling of plastic and rubber wastes can be summed up
into the following classes:
1. Hydrogenation
The polymer is degraded by the combined actions of heat, hydrogen and many cases
catalysts.
2. Gasification
Plastic wastes react with oxygen and/or steam to produce synthesis gas (CO and H2
).
3. Chemical depolymerization
Plastic wastes react with certain agents to yield the starting monomers.
4. Thermal cracking
Plastic wastes are decomposed by the effect of heat in an inert atmosphere.
5. Catalytic cracking and reforming
The polymer chains are broken down by the effect of catalyst, which promotes cleavage
reactions.
CHAPTER 5
References:-
http://www.lotfi.net/recycle/plastic.htm
l
http://www.bpf.co.uk/sustainability/pla
stics_recycling.aspx
www.zerowaste.sa.gov.au/About.mvc/Recycli
ngTips
www.unep.org/publications/search/pub_detail
s_s.asp?ID=4021
www.marineparks.wa.gov.au/marine-park-
protectors/marine-litterfacts.html