Question

a. Interstitial atoms belong to line defects, yes or no?

b. What is the influence of crystallinity increase on polymer modulus?

c. Why are polymers lighter than metals and ceramics? Two reasons.

d. How can glass fiber be made (name 2 methods/procedures)?

f. What are the three limitations of ferrous alloys, generally?

g. Why poly crystals are stronger than single crystals?

h. Can polymers be purely crystalline? Yes/No

i. What are the three functions of screen pack in polymer extruding?

Answer 1

Part (a): NO

Statement: Interstitial atoms belong to line defects

Explanation

All crystals possess some defects such as:

• point defects
  • In this defect, atoms may be missing or irregular spacing
  • examples : interstitial impurity atom, self interstitial atom etc
• linear defects
  • In this defect, a group of atoms are in irregular position. These defects are also called dislocations.
• planar defects
  • These are the interface between homogeneous regions.

Correct statement: Interstitial atoms belong to point defects.

(Interstitial atoms can be used to pin dislocation or line defects)

Part (b): Influence of crystallinity increase on polymer modulus

Explanation

• As the crystallinity increases, both modulus and yield point increases.
• Increasing crystallinity led to the following properties
  • increased stiffness
  • increased softening and melting point
  • increased tensile strength
  • Elongation at failure decreases
• Crystallinity increases (due to chain scission); there is an increase in young’s modulus of polymers during degradation.
• Conventional crystallization in polymers led to polymers with high modulus of elasticity and yield strength.
• In unoriented polymers, increasing Crystallinity may decrease strength.

Part (c): polymers are lighter than metals and ceramics

Explanation

• Molecules in metals are closely packed due to metallic bonding and in ceramics mostly due to ionic bonding whereas molecules in polymers are long and branched and are not closely packed.
• Polymers generally possess large molecular weight, lower modulus of elasticity and tensile strength
• Polymers have irregularity in their structure and have decreased density as compared to metals. Thus polymers are lighter.

Part (d): How is glass fibers made?

Explanation

There are mainly two types of glass manufacturing process:

• Direct melt process
• Marble remelt process

In both processes, solid raw materials are mixed and melted in a furnace.

In Marble remelt process, molten materials are sheared, rolled, cooled and packaged and then remelted in a fiber manufacturing unit.

In Direct melt process, molten material is passed to bushing (important part for manufacture of fiber).