Introduction to Polymers Flashcards
What are the three main classifications of polymers by structure?
Linear
Cross-linked
Branched
What are the four main classifications of polymers by molecular forces?
- Elastomers
- Fibres
- Thermoplastics
- Thermosetting polymers
What are the properties of Elastomers?
- Elastic character
- Held together by weak intermolecular forces
What are the properties of Fibres?
- High tensile strength
- Less elasticity
- Strong intermolecular forces
What are the properties of thermoplastics?
- Can be easily softened when heated with little change due to the lack of cross-linking
What are the properties of thermosetting polymers?
- Polymers which undergo permanent change upon heating
- When heated, they become highly cross-linked and the links prevent the material from moving/deforming
What are the advantages of Thermoplastic polymers?
- Recyclable
- High impact resistance
- Remoulding/reshaping capabilities
- Hard crystalline or rubbery surface options
What are the disadvantages of Thermoplastic polymers?
- More expensive than thermosets
- Lose shape and mechanical integrity if heated
What are the advantages of Thermosetting polymers?
- More resistant to high temps
- Highly flexible
- Thick to thin wall capabilities
- High levels of dimensional stability
What are the disadvantages of Thermosetting polymers?
- Cannot be recycled
- More difficult to surface finish
- Cannot be remoulded or reshaped
What is the glass transition temperature with reference to thermoplastics?
- 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
What are short term tests for?
Determining strength and modulus
What are long term tests for?
Creep (long term static loads)
Fatigue (long term fluctuating loads)
What is prototyping used for?
- Used for trial and modification
- Methods for preparing the prototype should be similar to intended full scale method
What limiations would arise from a machined prototype?
- 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
What are the two most important design considerations when dealing with plastics?
Provision of uniform wall thickness and structural integrity
Why are plastics designed with rounded corners?
To improve structural strength and make removal from moulds easier
What are the main considerations when designing with plastics?
- Minimise number of parts
- Avoid seperate fasteners
- Provide parts with integrated self-locking features
- Take advantage of flexibility (ie self locking features)
What are some common thermoplastics and their properties?
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
What are some common examples of thermosetting plastics and their properties?
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)
