Polymers Flashcards
What is a polymer?
Extremely long chains, can be linear or branched
Describe linear chains
Just have primary and secondary bonds
Describe cross linked chains
Covalent bonds inside and in between the chain, makes the structure more rigid
Describe semicrystalline
Chain of polymers folded in an orderly pattern
Describe amorphous
Chain of polymers that are folded randomly
What mechanical property does high amorphous provide?
High toughness
What mechanical property does crystallinity provide?
High tensile strength
What are tie molecules?
They start in the crystalline region, pass through amorphous region and rejoin the next crystalline region, (they hold the structure together)
What is Tg?
Glass transition temp, characteristic of amorphous materials
What is Tm?
Melting temp, characteristic of crystalline materials
What happens below and above Tg?
Below polymer is brittle, above polymer is ductile
What do plasticisers do?
Change the Tg and therefore change the mechanical properties at different temps
What are thermoplastics?
A material that can be moulded and shaped when heated (can be recycled)
What are thermosets?
Once formed, does not become mouldable or or soften when reheated
What are elastomers?
Any material that can be stretched multiple times its original length without breaking
Describe the structure of thermoplastics
Can be amorphous or semi-crystalline, between linear chains are Van der Waals or linear bonds
Describe the structure of thermosets
Mainly amorphous, covalent bonds between lightly cross-linked chains
Describe the structure of elastomers
Mainly amorphous, covalent and Van der Waals bonds between fully cross-linked chains
Describe the process of extrusion
Hopper feeds pellets into a turning screw which passes them through a heater, the molten plastic is then forced through a die to form extrudate
Describe the process of injection moulding
Hopper feed pellets into turning screw which passes through a heater, the molten plastic is then injected into the molten mould
Describe the process of blow moulding
Same as extrusion, molten plastic then forms a parison which gets expanded into the shape of the mould
Describe the process of film blowing
Same as extrusion, molten plastic forms air bubble which is then rolled into sheets
Describe the process of melt spinning
Same as extrusion, molten plastic falls die forming thin fibres
