This document discusses polymers and plastics. It defines polymers as long-chain molecules formed by linking repeating molecular units (monomers) through polymerization. The earliest synthetic polymer was Bakelite, developed in 1906. Modern plastics started in the 1920s using raw materials from coal and oil. Polymers can be thermoplastics, which soften when heated and harden when cooled, allowing reshaping, or thermoset plastics, which permanently cure into shape upon heating. Common polymers include polyethylene, polycarbonate, and nylon, each with different properties and applications ranging from bottles to gears. Additives are added to polymers to modify properties like strength, color, and weather resistance.

1. Course: B.Tech.
Subject: Engineering Chemistry
Unit: III(D)

2. • The earliest synthetic polymer was developed in 1906,
called Bakelite.
• The development of modern plastics started in 1920s
using raw material extracted from coal and petroleum
products (Ethylene). Ethylene is called a building block.
• Polymers are long-chain molecules and are formed by
polymerization process, linking and cross linking a
particular building block (monomer, a unit cell).

3. • When Additives are added then they are known as
Plastics.
• The term polymer means many units repeated many
times in a chainlike structure.
• Most monomers are organic materials, atoms are
joined in covalent bonds (electron-sharing) with other
atoms such as oxygen, nitrogen, hydrogen, sulfur,
chlorine,….

4. Ethene Ethene Ethene
Polyethene

5. Thermoplastics
As the temperature is raised above the melting point, the
secondary bonds weaken, making it easier to form the plastic
into any desired shape.
 When polymer is cooled, it returns to its original strength
and hardness.
The process is reversible.
 Polymers that show this behavior are known as
thermoplastics.
Plastics can be classified into two major types:

6. Thermosetting Plastics (thermosets)
Thermosetting plastics are cured into permanent shape.
Cannot be re-melted to the flowable state that existed before
curing, continued heating for a long time leads to degradation or
decomposition.
This curing (cross-linked) reaction is irreversible.
Thermosets generally have better mechanical, thermal and
chemical properties.
They also have better electrical resistance and dimensional
stability than do thermoplastics.

7. Bonding –
 monomers are linked together by covalent bonds,
forming a polymer chain (primary bonds).
 The polymer chains are held together by secondary
bonds.
The strength of polymers comes in part from the length
of polymer chains.
The longer the chain, the stronger the polymer.
More energy is needed to overcome the secondary
bonds.

8. Linear
polymers
A sequential structure
resulting in thermoplastics
like nylon, acrylic,
polyethylene.
Branched
polymers
Side branch chains are attached
to the main chain which interferes
with the relative movement of the
molecular chains.
This results in an increase in
strength, deformation resistance
and stress cracking resistance.
2

9. Additives are added to polymers in order to obtain or improve
certain properties such as strength, stiffness, color, resistance to
weather and flammability.
Ultraviolet radiation (sunlight) and oxygen cause polymers to
become stiff and brittle, they weaken and break the primary
bonds.
Fillers such as fine saw dust, silica flour, calcium carbide are
added to reduce the cost and to increase harness, strength,
toughness, dimensional stability,…..

10. Polyethylene
Properties: good chemical and electrical
properties, strength depends on composition
Applications: bottles, garbage cans, house wares,
bumpers, toys, luggage

11. Polycarbonates
Properties: very versatile and has dimensional stability,
good mechanical and electrical properties, high
resistance to impact and chemicals
Applications: optical lenses, food processing equipments

12. Nylon
Properties: good mechanical and abrasion resistance
property, self-lubricating, resistant to most chemicals but it
absorbs water, increase in dimension is undesirable
Applications: mechanical components; gears, bearings,
zippers, surgical equipments,

13. References
1. Engineering chemistry by Jain and Jain
2. http://matse1.matse.illinois.edu/polymers/prin.html