Learning Objectives:

• Understand various general characteristics of a wide variety of elastomers and fibres, ceramics and glass materials.
• To recognize common thermoplastics and thermosets.

Elastomers and Fibres:

Elastomers are rubber like solids with elastic properties. In these elastomeric polymers, the polymer chains are held together by the weakest intermolecular forces. These weak binding forces permit the polymer to be stretched. A few cross-links are introduced in between the chains, which help the polymer to retract to its original position after the force is released as in vulcanised rubber. The examples are buna-S, buna-N, neoprene, etc.

FML, CC BY-SA 2.5, via Wikimedia Commons

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.

Ceramics:

This category of materials includes things like tile, bricks, plates, glass, and toilets. Ceramics can be found in products like watches (quartz tuning forks-the time keeping devices in watches), snow skies (piezoelectric-ceramics that stress when a voltage is applied to them), automobiles (spark plugs and ceramic engine parts found in race cars), and phone lines. They can also be found on space shuttles, appliances (enamel coatings), and airplanes (nose cones). Depending on their method of formation, ceramics can be dense or lightweight. Typically, they will demonstrate excellent strength and hardness properties; however, they are often brittle in nature. Ceramics can also be formed to serve as electrically conductive materials, objects allowing electricity to pass through their mass, or insulators, materials preventing the flow of electricity. Some ceramics, like superconductors, also display magnetic properties.
Ceramics are generally made by taking mixtures of clay, earthen elements, powders, and water and shaping them into desired forms. Once the ceramic has been shaped, it is fired in a high temperature oven known as a kiln. Often, ceramics are covered in decorative, waterproof, paint-like substances known as glazes.

Silar, CC BY-SA 4.0, via Wikimedia Commons

Glass:

The mechanical properties of glasses are dual in nature:

• Below the glass transition temperature, T$_g$, glass is a rigid brittle material.
• Above T$_g$, glass behaves as a viscous liquid.

with behavior characterized by continuous deformation (at a rate inversely related to viscosity) rather than a fixed elastic strain in response to stress.

Types of Glasses:

• Annealed:

  • No residual stresses at room temperature
  • Large and sharp segment breakage.
• Tempered:
  • Surface quenched below \(T_g\)
  • Slow cooled to room temperature.
  • Surface residual compressive stress,
  • Core residual tensile stress
  • Finely fragmented and dull breakage.

• Laminated: Two ordinary layer of annealed glass with a central layer of polymer.

Dmitry Makeev, CC BY-SA 4.0, via Wikimedia Commons

Solved Example: 54-1-01

What are the major classes of engineering materials?

A. Metals, ceramics and semiconductors

B. Polymers, metals and composites

C. Metals, ceramics, polymers and semiconductors

D. Metals, ceramics, polymers, semiconductors and composites

Correct Answer: D

Solved Example: 54-1-02

One of the characteristics of polymer is:

A. High temperature stability

B. High mechanical strength

C. High elongation

D. Low hardness

Correct Answer: C

Solved Example: 54-1-03

Which of the following polymer is thermosetting polymer?

A. Terylene

B. Polystyrene

C. Bakelite

D. Neoprene

Correct Answer: C

Solved Example: 54-1-04

Which one of the following is a natural polymer?

A. Bakelite

B. Silk

C. Kevlar

D. Lexan

Correct Answer: B

Solved Example: 54-2-01

Thermoplastic materials are those materials which:

A. Do not become hard with the application of heat and pressure and no chemical change occurs

B. Are formed into shape under heat and pressure and results in a permanently hard product

C. Are flexible and can withstand considerable wear under suitable conditions

D. Are used as a friction lining for clutches and brakes

Correct Answer: A

Solved Example: 54-2-02

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

A. Single element

B. More than one element

C. Pure material

D. All of the above

Correct Answer: B

Solved Example: 54-2-03

One of the characteristic properties of polymer material _____________ .

A. High temperature stability

B. High mechanical strength

C. High elongation

D. Low hardness

Correct Answer: C

Solved Example: 54-2-04

These polymers cannot be recycled:

A. Thermoplasts

B. Thermosets

C. Elastomers

D. All polymers

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: ______________.

A. Thermoplasts

B. Thermosets

C. Elastomers

D. All polymers

Correct Answer: B

Solved Example: 54-2-06

Strong covalent bonds exists between polymer chains in ______________.

A. Thermoplasts

B. Thermosets

C. Elastomers

D. All polymers

Correct Answer: B

Solved Example: 54-3-01

Traditional ceramics comprise three main components, clay, quartz and feldspar. What is the main role of the quartz component?

A. To bind the refractory components together

B. To improve the workability of the system prior to firing

C. To lower the cost of the product

D. To control the colour of the product.

Correct Answer: C

Solved Example: 54-3-02

Traditional ceramics comprise three main components, clay, quartz and feldspar. What is the role of the feldspar component?

A. To bind the refractory components together

B. To improve the workability of the system prior to firing

C. To lower the cost of the product

D. To control the colour of the product.

Correct Answer: A

Solved Example: 54-3-03

Which of the following descriptions would not be applicable to ceramics?

A. Mixed ionic and covalent bonding

B. Brittle

C. Ductile

D. Structure comprising Metal cations with non-metallic anions

Correct Answer: C

Solved Example: 54-3-04

The word ‘ceramic’ refers to:

A. Soft material

B. Hard material

C. Burnt material

D. Dry material

Ceramics are generally made by taking mixtures of clay, earthen elements, powders, and water and shaping them into desired forms. Once the ceramic has been shaped, it is fired in a high temperature oven known as a kiln.

Correct Answer: C

Solved Example: 54-3-05

The major ingredients of traditional ceramics are:

A. Silica

B. Clay

C. Feldspar

D. All polymers

Correct Answer: D

Solved Example: 54-4-01

Most commercial glasses consist of:

A. Lime

B. Soda

C. Silica

D. All polymers

Correct Answer: D

Solved Example: 54-4-02

Common glass is also known as:

A. Potash lead glass

B. Soda-lime glass

C. Bottle glass

D. Borosilicate glass

Correct Answer: C