Learning Unit 4

Question 1

Define Thermoplastics

Answer 1

• Thermoplastics have the simplest molecular structure with chemically independent macromolecules.
• By heating, they are softened or melted, then shaped, formed, welded, and solidified when cooled.
• Multiple cycles of heating and cooling can be repeated without severe damage, allowing reprocessing and recycling.

Question 2

List 4 advantages of Thermoplastics

Answer 2

• The softening or melting by heating allows welding and thermoforming.
• The pocessing cycles are very short because of the absence of the chemical reaction of cross- linking.
  Processing is easier to monitor because there is only a physical transformation.
• Thermoplastics do not release gases or water vapor if they are correctly dried before
  pocessing.
• The wastes are partially reusable as virgin matter because of the reversibility of the physical softening or melting.

Question 3

What are 3 disadvantages of thermoplastics.

Answer 3

• When the temperature rises, it decreases due to the absence of chemical links between macromolecules.
• For the same reason, the creep and relaxation behaviors are not as good as the thermosets. During a fire, fusibility favors dripping and annihilates final residual physical cohesion.
• There are few materials workable in the liquid state.

Question 4

Define Thermosetting plastic?

Answer 4

• Therosets before hardening like thermoplastics are independent macromolecules.
• But in their final state, after hardening, they have a 3D structure obtained by chemical cross linking produced after or during the processing.
• The links created between the chains of the thermosets limit their mobility and possibilities of relative displacement and bring certain advantages and disadvantages.

Question 5

Using plastics in general have certain advantages in an economic or costing context. Name three (3) of these advantages.

Answer 5

• Design freedom: realization of all shape and size parts unfeasible with metals or wood.
• The possibility of selective reinforcement in the direction of the stresses by selecting particular composites or by part
  drawing.
• The reduction of design and production setup times.
• The opportunities of repairing the composites permit the recovery of expensive parts after damage.