Course: B.Tech.

Subject: Engineering Chemistry

Unit: III(D)

• 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).

• 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,….

Ethene Ethene Ethene

Polyethene

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:

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.

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.

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

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,…..

Polyethylene

Properties: good chemical and electrical properties, strength depends on composition

Applications: bottles, garbage cans, house wares, bumpers, toys, luggage

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

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,

References1. Engineering chemistry by Jain and Jain

2. http://matse1.matse.illinois.edu/polymers/prin.html