[PDF]recycling plastics

RECYCLING PLASTICS



Plastics - what are they and how do they behave?

Plastics are organic polymeric materials consisting of giant organic molecules. 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.



Figure 1 : Polymer structures

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 use, however,
tends to be much lower.

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.



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Linear Polymer



Branched Polymer



Cross-linked Polymer




Recycling of Plastics



Intermediate Technology Development Group



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. (This is not to say
that standards can be low - the consumer will always demand a certain level of quality).

• 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.

Plastics for recycling

Not all plastics are recyclable. There are 4 types of plastic which are commonly recycled:

• Polyethylene (PE) - both high density and low-density polyethylene.

• Polypropylene (PP)

• Polystyrene (PS)

• Polyvinyl chloride (PVC)

A common problem with recycling plastics is that plastics are often made up of more than
one kind of polymer or there may be some sort of fibre added to the plastic (a composite) to
give added strength. This can make recovery difficult.



Plastics fact file



• More than 20,000 plastic
bottles are needed to obtain 1
tonne of plastic.

• It is estimated that 100 million
tonnes of plastics are produced
each year.

• The average European throws
away 36kg of plastics each
year.

• 4% of oil consumption in
Europe is used for the
manufacture of plastic
products.

• Some plastic waste sacks are
made from 64% recycled
plastic.

• Plastics packaging totals 42%
of total consumption and very
little of this is recycled.



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Recycling of Plastics



Intermediate Technology Development Group



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).




Figure 2: Mixed waste plastic requiring sorting before it can be
recycled. ©World Resource Foundation



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. (Lardinois 1995)



Identification of different types of plastics

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. Dose the plastic burn? If so, what colour?

Fingernail test. Can a sample of the plastic be scratched with a fingernail?



Test PE PP PS PVC*



Water


Floats


Floats


Sinks


Sinks


Burning


Blue flame with


Yellow flame


Yellow, sooty


Yellow, sooty




yellow tip, melts


with blue base.


flame - drips.


smoke. Does




and drips.






not continue
to burn if
flame is
removed


Smell after


Like candle wax.


Like candle wax


Sweet


Hydrochloric


burning




- less strong
than PE




acid


Scratch


Yes


No


No


No



*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.
Source: Vogler. 1984



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.



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Recycling of Plastics



Intermediate Technology Development Group



A coding system has also been introduced in the United States 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. More information is available from the Association of Plastic
Manufacturers in Europe (APME). See useful addresses section later in this brief.

Collection

When thinking about setting up a small-scale recycling enterprise, it is advisable to first carry
out a survey to ascertain the types of plastics available for collection, the type of plastics
used by manufacturers (who will be willing to buy the reclaimed material), and the economic
viability of collection.

The method of collection can vary. The following gives some ideas;

• House to house collection of plastics and other materials (e.g. paper).

• House to house collection of plastics only (but all types of polymer).

• House to house collection of certain objects only.

• Collection at a central point e.g. market or church.

• Collection from street boys in return for payment.

• Regular collection from shops, hotels, factories, etc.

• Purchase from scavengers on the municipal dump.

• Scavenging or collecting oneself.

The method will depend upon the scale of the operation, the capital available for set-up,
transport availability, etc.

Processing of reclaimed plastic - processes and technology for small-scale recycling
enterprises

• Initial upgrading. Once the plastic has been collected, it will have to be cleaned and
sorted. The techniques used will depend on the scale of the operation and the type of
waste collected, but at the simplest level will involve hand washing and sorting of the
plastic into the required groups. More sophisticated mechanical washers and solar
drying can be used for larger operations. Sorting of plastics can be by polymer type
(thermoset or thermoplastic for example), by product (bottles, plastic sheeting, etc.), by
colour, etc.




Figure 3: Collection of Waste Materials. © World Resource Foundation



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Recycling of Plastics



Intermediate Technology Development Group



Industrial
Waste


Commercial Agricultural Municipal
Waste Waste Waste




Collection




Cleaning




Sorting




Size
Reduction




Sorted, clean
Plastic pieces




Extrusion




Strands/strings




Pelletising




Pellets


Extrusion

Pipes, tubes


Injection Blow Film
Moulding moulding moulding

Miscellaneous Bottles Bags, sheets
items



Figure 4: Flow chart of a typical waste plastic reprocessing stream in a low-income country
(TOOL 1995)



• Size reduction techniques. Size reduction is required for several reasons; to reduce
larger plastic waste to a size manageable for small machines, to make the material
denser for storage and transportation, or to produce a product which is suitable for
further processing. There are several techniques commonly used for size reduction of
plastics;

• Cutting is usually carried out for initial size reduction of large objects. It can be
carried out with scissors, shears, saw, etc.

• Shredding is suitable for smaller pieces. A typical shredder has a series of rotating
blades driven by an electric motor, some form of grid for size grading and a
collection bin. Materials are fed into the shredder via a hopper which is sited above
the blade rotor. The product of shredding is a pile of coarse irregularly shaped
plastic flakes which can then be further processed.

• Agglomeration is the process of pre-plasticising soft plastic by heating, rapid cooling
to solidify the material and finally cutting into small pieces. This is usually carried out
in a single machine. The product is coarse, irregular grain, often called crumbs.

• Further processing techniques

• Extrusion and pelletising. The process of extrusion is employed to homogenise the
reclaimed polymer and produce a material that it subsequently easy to work. The
reclaimed polymer pieces are fed into the extruder, are heated to induce plastic
behaviour and then forced through a die (see the following section on manufacturing
techniques) to form a plastic spaghetti which can then be cooled in a water bath
before being pelletised. The pelletisation process is used to reduce the 'spaghetti' to
pellets which can then be used for the manufacture of new products.



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Recycling of Plastics



Intermediate Technology Development Group



• Manufacturing techniques

• Extrusion. The extrusion process used for manufacturing new products is similar to
that outlined above for the process preceding pelletisation, except that the product is
usually in the form of a continuous 'tube' of plastic such as piping or hose. The main
components of the extrusion machine are shown in Fig. 2 below. The reclaimed
plastic is forced along the heated tube by an archimedes screw and the plastic
polymer is shaped around a die. The die is designed to give the required dimensions
to the product and can be interchanged.

• Injection moulding. The first stage of this manufacturing process is identical to that
of extrusion, but then the plastic polymer emerges through a nozzle into a split
mould. The quantity of polymer being forced out is carefully controlled, usually by
moving the screw forward in the heated barrel. A series of moulds would be used to
allow continual production while cooling takes place. See Figure 2 below. This type
of production technique is used to produce moulded products such as plates, bowls,
buckets, etc.

• Blow moulding. Again the spiral screw forces the plasticised polymer through a die.
A short piece of tube, or 'parison' is then enclosed between a split die -which is the
final shape of the product - and compressed air is used to expand the parison until it
fills the mould and achieves its required shape. This manufacturing technique is
used for manufacturing closed vessels such as bottles and other containers. See
Figure 2 below.

• Film blowing. Film blowing is a process used to manufacture such items as garbage
bags. It is a technically more complex process than the others described in this brief
and requires high quality raw material input. The process involves blowing
compressed air into a thin tube of polymer to expand it to the point where it becomes
a thin film tube. One end can then be sealed and the bag or sack is formed. Sheet
plastic can also be manufactured using a variation of the process described.



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Recycling of Plastics



Intermediate Technology Development Group





Hollow mandril forms
inner surface of
bottle neck




Eitruder and die
as above



Mould closes at second
stage to nip end of
tube shut



Compressed air
expands plastic
tube against
mould



Hopper containing plastic
in form of pellet or powder

Heating elements and cooling coils



Two part split mould







n O ^


o





o


□


3


a








W


n





E



-o o ° O a O



Nozzle




Clamp
holds
mould
shut



Spiral screw

Figure 5: Plastic manufacturing techniques; extrusion (top), injection moulding (middle)
and blow moulding (bottom).



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Recycling of Plastics



Intermediate Technology Development Group



Products from plastic

There is an almost limitless range of products that can be produced from plastic. However,
the market for recycled plastic products is limited due to the inconsistency of the raw
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