Fundamentals of Polymers Flashcards
What is a Polymer?
A polymer is a compound comprised of long-chain molecules that consist of repeating units, called mers, connected end to end
What are the three basic categories of polymers
- Thermoplastics
- Thermosetting
- Elastomers
What are the Properties of Polymers compared to metals?
- lower strength
- lower hardness
- lower stiffness
- lower density
- lower temp resistance
What are useful properties of engineering polymers?
- High strength or modulus to weight ratio
- Low electrical and thermal conductivity
- Resistance to corrosion and chemicals
- Relatively low cost
- Variety of colours, transparency
- Easy processing
- Low cost
What is the polymer architecture of a Amorphous thermoplastic?
Amorphous and linear, with random molecular structure, easy to thermoform as they soften over a range of temperatures
What is the polymer architecture of a thermoset?
Amorphous and cross linked, strengthens when heated but can not be remolded or heated after initial forming
What is the polymer architecture of a Semicrystalline thermoplastic?
Have separate domains of molecules, amorphous and crystalline or highly ordered molecular structure
- has two transition temperatures
What happens to Amorphous thermoplastics at Glass transition temperature (Tg) ?
- They do not have a sharp melting point but do have a Tg
- At Tg polymer becomes leathery, can be pressed into a desired shape
- > Tg polymers behave like rubbery materials
- <Tg polymers have relatively little mobility
When can semicrystalline thermoplastics be moulded?
In there molten state at temps above there melting point
What do polymer processing techniques rely on?
Heat, pressure and time
What are polymers processed as?
Powders, doughs, granules, sheets and liquid resins
What are important parameters in polymer processing?
- tool cost
- cycle time
- material cost
- shape capability
- scrap
What are the three general phases in polymer processing?
- Heating - to soften or melt plastic
- Shaping/forming - under constraints of some kind
- Cooling - to retain shape
