Plastics

  • To recognize common thermoplastics and thermosets.

  • Distinguish typical properties of common plastics with those of metals and ceramics; and understand the relationship between the mechanical, chemical, and physical properties of the plastics and their molecular, micro and macro structures.

  • Demonstrate deep understanding of the major manufacturing processes of plastics and analyze the advantages and disadvantages of those processes for making specific parts.

Thermoplastic Polymers:

These are the linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling.These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres. Some common thermoplastics are polythene, polystyrene, polyvinyls, etc.
A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently.
Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (Nylon).

Thermosetting Polymers:

These polymers are cross linked or heavily branched molecules,which on heating undergo extensive cross linking in moulds and again become infusible. These cannot be reused. Some common examples are bakelite, urea-formaldelyde resins, etc.
Thermoplastic polymers differ from thermosetting polymers (Bakelite, vulcanized rubber) since thermoplastics can be remelted and remolded.
Thermosetting plastics when heated, will chemically decompose, so they cannot be recycled. Yet, once a thermoset is cured it tends to be stronger than a thermoplastic.
Typically, linear polymers with minor branched structures (and flexible chains) are thermoplastics. The networked structures are thermosets.

Solved Examples

Solved Example:

54-2-01

Thermoplastic materials are those materials which:

Correct Answer: A

Solved Example:

54-2-02

The word ‘polymer’ is meant for a material made from _______________.

Correct Answer: B

Solved Example:

54-2-03

One of the characteristic properties of polymer material _____________ . (ISRO Scientist ME 2013)

Correct Answer: C

Solved Example:

54-2-04

These polymers cannot be recycled:

Solution:
Thermoset plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermosets ideal for high-heat applications such as electronics and appliances. Due to this they cannot be recycled.

Correct Answer: B

Solved Example:

54-2-05

In general, strongest polymer group is: ______________.

Correct Answer: B

Solved Example:

54-2-06

Strong covalent bonds exists between polymer chains in ______________.

Correct Answer: B