Introduction to Polymers

Question 1

What are the three main classifications of polymers by structure?

Answer 1

Linear
Cross-linked
Branched

Question 2

What are the four main classifications of polymers by molecular forces?

Answer 2

• Elastomers
• Fibres
• Thermoplastics
• Thermosetting polymers

Question 3

What are the properties of Elastomers?

Answer 3

• Elastic character

- Held together by weak intermolecular forces

Question 4

What are the properties of Fibres?

Answer 4

• High tensile strength
• Less elasticity
• Strong intermolecular forces

Question 5

What are the properties of thermoplastics?

Answer 5

• Can be easily softened when heated with little change due to the lack of cross-linking

Question 6

What are the properties of thermosetting polymers?

Answer 6

• Polymers which undergo permanent change upon heating

- When heated, they become highly cross-linked and the links prevent the material from moving/deforming

Question 7

What are the advantages of Thermoplastic polymers?

Answer 7

• Recyclable
• High impact resistance
• Remoulding/reshaping capabilities
• Hard crystalline or rubbery surface options

Question 8

What are the disadvantages of Thermoplastic polymers?

Answer 8

• More expensive than thermosets

- Lose shape and mechanical integrity if heated

Question 9

What are the advantages of Thermosetting polymers?

Answer 9

• More resistant to high temps
• Highly flexible
• Thick to thin wall capabilities
• High levels of dimensional stability

Question 10

What are the disadvantages of Thermosetting polymers?

Answer 10

• Cannot be recycled
• More difficult to surface finish
• Cannot be remoulded or reshaped

Question 11

What is the glass transition temperature with reference to thermoplastics?

Answer 11

• It is a ductile-brittle transition temp.
• Values of Tg below room temp define elastomers
• Values of Tg above room temp define rigid, structural polymers

Question 12

What are short term tests for?

Answer 12

Determining strength and modulus

Question 13

What are long term tests for?

Answer 13

Creep (long term static loads)

Fatigue (long term fluctuating loads)

Question 14

What is prototyping used for?

Answer 14

• Used for trial and modification

- Methods for preparing the prototype should be similar to intended full scale method

Question 15

What limiations would arise from a machined prototype?

Answer 15

• Effect of weld lines cannot be studied
• Grooves by machining can sometimes greatly reduce the strength properties compared with those of an injected moulded part
• The strength and rigidity of extruded bars and sheets can be higher than those of an injected moulded part on account of their higher crystallinity

Question 16

What are the two most important design considerations when dealing with plastics?

Answer 16

Provision of uniform wall thickness and structural integrity

Question 17

Why are plastics designed with rounded corners?

Answer 17

To improve structural strength and make removal from moulds easier

Question 18

What are the main considerations when designing with plastics?

Answer 18

• Minimise number of parts
• Avoid seperate fasteners
• Provide parts with integrated self-locking features
• Take advantage of flexibility (ie self locking features)

Question 19

What are some common thermoplastics and their properties?

Answer 19

High Density Polyethylene (HDPE)
- Lightweight, strong, weather resistant, easily moulded

Polyvinyl Chloride (PVC)
 - Rigid, high resistance to chemicals, excellent mechanical strength 
eg rods, tubes, sheets

Polysyrene (PS)
- Easy to process, electrical insulator, optical clarity
easily moulded, brittle, poor UV resistance, poor impact strength

Nylon
- Low friction (good for gears etc), good strength, temperature resilience, chemical compatability, easily mouldable, high melting temp, low impact strength, affected by UV exposure

Question 20

What are some common examples of thermosetting plastics and their properties?

Answer 20

Alkyds
- Rigid, heat resistant, tough, dimensional stability
(eg automotive distributor caps, circuit breakers)

Epoxies
- Rigid, clear, very tough, chemical resistance, good adhesion properties, low curing, low shrinkage (eg adhesives, coatings)

Melamines
- Hard, opaque, tough, scratch resistant, self extinguishing (ie decrative laminates, dinnerware)