Without going very deep in chemistry, these are polymers. Their toughness may depend on the polymer chain and some tough chemisty.
The great question is: how are these very common materials made?
My opinin is they can be primarily made or recycled from the available (recycled).
Since they are non biodegradable, i could prefer the latter especialy at starting.
Plastics may be classified in different categories
Reasons for recycling plastics
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.
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?
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).
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House to house collection of plastics only (but all types of polymer).
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House to house collection of certain objects only.
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Collection at a central point e.g. market or church.
•
Collection from street boys in return for payment.
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Regular collection from shops, hotels, factories, etc.
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Purchase from scavengers on the municipal dump.
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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.
•
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.
- Industrial waste
- Commercial waste
- Agricultural waste
- Municipal waste
process of recycling plastics
- Collection
Extrusion
Methods used in moulding plastics
- Injection moulding
- Blow moulding
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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.
•
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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