Plastic is a synthetic material made from a wide range of
organic polymers such as polyethylene, PVC, nylon, etc., that
can be moulded into shape while soft, and then set into a
rigid or slightly elastic form.
Composition of plastic
Polymers + Carbon + O2 +
Organic Polymers may include :
CHA LK, STA RCH, IVORY DUST, WOOD F LOO R , ZINC
Classification of plastic
There are many ways of classifying plastics. They can be
classified considering various aspects, as according to their:
1. Behaviour with respect to heating,
2. Structure, and
3. Physical and mechanical properties.
Classification of plastic
respect to heating Structure Physical and
Themoplastic & thermosetting
Basic difference between Thermoplastics and Thermosetting plastics
Thermoplastics variety softens
by heat and hardens when
cooled down. It can be used by
remolding as many times as
Thermosetting plastics can not
be reused .This variety requires
a great pressure and
momentary heat during
molding which hardens on
The thermo-plastic or heat non-convertible group is the general term
applied to the plastics which becomes soft when heated and hard when
Thermoplastic materials can be cooled and heated several times.
They can be recycled.
When thermoplastics are heated, they melt to a liquid. They also freeze
to a glassy state when cooled enough.
Thermoplastic can be moulded into any shape.
It may melt before passing to a gaseous state.
Allow plastic deformation when it is heated.
They are brittle and glossy.
They are soluble in certain solvents.
Swell in the presence of certain solvents.
Good resistance to creep.
Thermoplastic materials have many features. Some products made from
thermoplastic materials are used for electronic applications. They protect
against electrostatic discharge and radio frequency interference.
High pressure polyethylene as applied to rigid material covered with electrical
machines, tubes, etc...
Low pressure polyethylene elastic material used for insulation of electrical cables,
Polystyrene applied for electrical insulation, handles of tools...
Polyamide used for making ropes, belts, etc...
PVC or polyvinyl chloride for the manufacture of insulation materials, pipes,
Examples and applications of thermoplastic plastic materials
The thermo-setting or heat convertible group is the general term applied
to the plastics which become rigid when moulded at suitable pressure and
This type of plastic passes originally through thermo-plastic stage. When
they are heated in temperature range of 127 ͦC to 177 ͦC, they set
permanently and further application of heat does not alter their form or
But at the temperature of about 343 ͦC, the charring occurs. This charring
is a peculiar characteristic of the organic substances.
These are soluble in alcohol and certain organic solvents, when
they are in thermo-plastic stage. This property is utilized for making
paints and varnishes from these plastics.
These are durable, strong and hard.
They are available in a variety of beautiful colours.
They are mainly used in engineering application of plastics.
Properties: good dimensional stability, excellent mechanical and electrical
properties, good resistance to heat and chemicals
Applications: electrical components requiring strength, tools and dies,
fiber reinforced epoxies are used in structural components, tanks,
pressure vessels, rocket motor casing
Properties: good dimensional stability, rigid, high resistance to heat, water,
electricity, and chemicals
Applications: laminated panels, handles, knobs, electrical components;
Polyesters (thermosetting, reinforced with glass fibers)
Properties: good mechanical, electrical, and chemical properties, good
resistance to heat and chemicals
Applications: boats, luggage, swimming pools, automotive bodies, chairs
Properties: excellent electrical properties over a wide rang of temperature
and humidity, good heat and chemical properties
Applications: electrical components requiring strength at high temp.,
waterproof materials, heat seals
Natural vs. synthetic polymers
NATURAL POLYMERS SYNTHETIC POLYMER
(Chemical composition do not
change on heating)
(Irreversible chemical process)
Remouldable Polymers Non-remouldable Polymers
These are brittle, glossy,
They possess matt effect,
elasticity, and are flexible
Polypropylene, Polystyrene, PVC
Eg: Rubber, Nylon, Glass Fibre,
This variety of plastic contains carbon chain i.e. the plastics of this group
are composed only of carbon atoms and they exhibit homogeneous
This variety of plastic is composed of the chain containing carbon and
oxygen, the nitrogen and other elements and they exhibit
Based on physical and chemical properties
These plastics have a high modulus of elasticity and they retain
their shape under exterior stresses applied at normal or
moderately increased temperatures.
These plastics have a medium modulus of elasticity and the
elongation under pressure completely disappears, when pressure
These plastics have a low modulus of elasticity and the
elongation under pressure disappears slowly, when pressure is
Soft plastics are available in a large range of colours, sizes and
USES: Used in making children’s toys eg: rattles etc., fishing baits.
These plastics are soft and elastic materials with a low modulus of
elasticity. They deform considerably under load at room
temperature and return to their original shape, when the load is
released. The extensions can range upto ten times their original
Biodegradable plastic decomposes in the natural environment. It is
produced from biopolymers called polyhydroxyalkanoate (PHA). This
material is completely biodegradable.
Biodegradation of plastics can be achieved by enabling
microorganisms in the environment to metabolize the molecular
structure of plastic films to produce an inert humus like material that is
harmful to environment.
The use of bio-active compounds compounded with swelling agents
ensures that, when combined with heat and moisture, they expand the
plastic’s molecular structure and allow the bio-active compounds to
metabolise and neutralize the plastic.
Advantages & disadvantages
Under proper conditions biodegradable plastics can degrade to the
point where microorganisms can metabolize them.
This reduces the problems with litter and reduces harmful effects on
However degradation of biodegradable plastic occurs very slowly.
Proper composting methods are required to degrade the plastic,
which may actually contribute to carbon dioxide emissions.
Some of the moulding components are:
These components are added to assist and accelerate the hardening
For instance, the ester acts as catalyst for urea formaldehyde.
They are used for quick and complete polymerization.
The fillers are inert materials and they impart strength, hardness and
other properties to the plastic.
The choice of filler should be carefully made.
It should be confirmed that the addition of a filler does not have
detrimental effect on other properties of plastic.
The filler may be used in the following forms-
These compounds are added to increase the hardness of resin.
For instance, the hexamethylene tetramine acts as hardeners for
The lubricants are applied on the surface of moulds so that the
articles of plastic do not stick to the moulds.
The application of lubricants on surface of moulds allows easy
removal of articles of plastic from the moulds.
The commonly used lubricants are graphite, parafine, wax, etc.
The addition of dyes and pigments helps in two ways, namely, they act as fillers
an they impart desired colour to the plastic.
They should be durable and adequately fast to light.
The commonly used pigments are zinc oxide, barytes, etc.
The selection of pigments should be done in such a way that their addition does
not alter or affect the other properties of plastic.
The plasticizers are the organic compounds which are oily in nature and low
They are used to separate the polymer chain by a greater distance to make the
These components are added to improve the plasticity and to impart softness to
They gives flexibility to the material and act like a lubricant.
The addition of plasticizers facilitates the moulding process of plastic articles.
The commonly used plasticizers in plastic are camphor, triacetin, tributyl
The properties of plasticizers in plastic should not exceed 10% otherwise
strength of plastic will decrease.
These components are added to dissolve the plasticizers.
For instance the alcohol is added in cellulose nitrate plastics to dissolve
Following are the process involved in the fabrication of articles of
This method of lubrication of articles of plastic is more or less the same
as the one employed in the glass industry.
A lump of plastic material is taken and by blowing, it is converted into
hollow plastic articles such as jars, bottles, toys, etc.
In this process the plastic material is allowed to pass between the
The process is used to prepare plain flat sheets of plastics.
The roller may be provided with artistic designs which will appear on
the finished product.
This process is similar in principle to that of metal casting.
The resin is heated and when it is in plastic form, it is poured into the
The curing of articles is then done either with or without the
application of heat.
During curing the low pressure may be applied if necessary.
This process is used to prepare plastics of beautiful colours and it is
most suitable for cellulose plastics.
In this process, the thermosetting resin are just applied on sheets of
paper, asbestos, cloth, wood, glass, fibre, etc. and they are subjected to
heavy pressure by allowing them to pass through rollers to form plastic
The thickness of sheets varies from 0.12 mm to 15 mm
Due to the pleasing finished surface, they are used for ornamental and
This is the most commonly adopted process for the fabrication of plastic
The general process consist in placing the raw materials in a mould and then
The moulding can be done by various methods:
NOTE: The choice of moulding method will depend on the article to be prepared.
APPEARANCE: Some plastics are completely transparent in appearance. With
the addition of suitable pigments, the plastics can be made to have appearance
of variety of attractive, opaque, stable and translucent colours.
CHEMICAL RESISTANCE: The plastics offer great resistance to moisture,
chemicals and solvents. The degree of chemical resistance depends on the
chemical composition of plastics. Many plastics are found to possess excellent
corrosion resistance. Hence they are used to convey chemicals.
DIMENSIONAL STABILITY: This property of plastic favours quite satisfactory with
that of other common engineering materials.
DUCTILITY: The plastics possess excellent electric insulating property. They are
far superior to ordinary electric insulators.
MAINTENANCE: It is easy to maintain plastic surfaces. They do not require any
protective coat of paints.
MELTING POINT: Most of the plastics have low melting point and the melting point
of some plastics is only about 50 ͦC. They cannot therefore be used in positions having
high temperatures or to convey boiling water.
OPTICAL PROPERTY: Several types of plastics are transparent and translucent.
RECYCLING: The most environmentally aware people condemn the use of plastics
for the amount of pollution caused by them in disposal. The plastics used for soft-
drink bottles, milk and juice bottles, bread bags, syrup bottles, coffee cups, plastic
utensils, etc. can be conveniently recycled into carpets, detergent bottles, drainage
pipes, fencing, handrails, grocery bags, car battery cases, pencil holders, benches,
picnic tables, roadside posts, etc.
SOUND ABSORPTION: The acoustical boards are prepared by impregnating fibre-
glass with phenolic resins. This material has absorption coefficient of about 0.67.
STRENGTH: An ideal section of plastics for structural member has yet not been
designed. The plastics are reasonably strong. The strength of plastics may be increased
by reinforcing with various fibrous materials.
Following considerations are responsible to discourage the use of plastics
as the structural material:
1. The plastics are costly.
2. The plastics are subject to creep under constant heavy loads.
3. The behaviour of plastics is very sensitive to the changes in temperature.
4. The stiffness of plastics is very poor
THERMAL PROPERTY: The thermal conductivity of plastics is low and it can be
compared with that of wood. The foamed or expanded plastics are among the leading
WEATHER RESISTANCE: Only limited varieties of plastics can be exposed to weather.
The certain plastics are seriously affected by ultraviolet light in the presence of
sunlight. The resistance to sunlight of such plastics can be improved by incorporating
fillers and pigments which absorb or reflect the ultraviolet light at the surface. Thus
the interior of plastics is protected.
WEIGHT: The plastics, whether thermo-plastic or thermo-setting, have low specific
gravity, the average being 1.30 to 1.40. The light weight of plastics reduces the
transport costs and facilitates fixing.
Add to your knowledge
The Indian Petrochemical Corporation Ltd. (IPCL), near Baroda, has put a
commercial brand of plastics, known as the ‘Koylene’. It is the lightest of all
commercial known plastics and it is available in a wide range of grades. It is tailor-
made to suit various applications such as automotive and scooter parts, box
strappings, industrial woven fabrics, ball pen refills, drinking straws, etc. This
material possesses rigidity, good gloss, ability to withstand temperatures upto
100 ͦC and easy processibility.
Uses of plastic
SECTION USED FOR JOINING PVC SHEET
ADVANTAGES of plastic
Low electrical and thermal conductivity, insulator
Easily formed into complex shapes, can be formed, casted
Wide choice of appearance, colors and transparencies
Ken Youssefi Mechanical Engineering 54
DISADVANTAGES of plastic
Low useful temperature range (up to 600
Less dimensional stability over period of time (creep effect)
Aging effect, hardens and become brittle over time
Sensitive to environment, moisture and chemicals
Uses of plastic
There are more than 10000 different kinds of plastics available in the market
and their performance abilities span those of every other known material from
soft rubber to steel.
The typical uses of plastics in building are summarized as follows:
Bath and sink units
Cistern ball floats
Corrugated and plain sheets
Decorative laminates and mouldings
Films for water proofing, damp
proofing and concrete curing
Foams for thermal insulation
Overhead water tanks
Paints and varnishes
Pipes to carry cold water
Water resistance adhesives
GE, POLYGAL, GOODSALES,
Rates difference as per texture is Rs 15 to 20
PER SQ FT.
IT IS TRANSPORTED IN THE FORM OF ROLLS
USES OF POLYCARBONATE SHEETS
SIZE - 3X100 sq ft ,4X100 sq ft (FOR 1 mm)
7X20 sq ft, 7X40 sq ft (FOR 2 TO 12 mm)
THICKNESS - 1,2,4 ,6 ,8 ,10,12 mm
THICKNESS RATES PER SQ. FT.
1 MM RS. 57
2 MM RS. 65
4 MM RS. 75
6 MM RS. 95
8 MM RS. 135
10 MM RS. 140
12 MM RS. 145