Polymers Flashcards
Polyemers: What are they?
Long molecular chains, often covalently bonded
- good insulators of electricity, heat: yet low stability, weak,
Monomers: get heated, covalant bonds break, reform with others to create a POLYME
Homopolymer vs copolymer and examples
Homopolymer (made of cust one monomer)
e.g. polyethelene, PVC, polystyrene)
Copolymer (made of two or more monomores in a singular chain (e.g ABS)
Addition Polymeristtion:
A simple polymerisation, linking monomers with inclusion of all parts of the structure: no waste
Heat and pressure are suitable catalysts
Condensation Polymerisation
Two or more dissimilar monomers react together: make polymer with a byproduct (usually water) e.g. nylon (dicarboxylic acid and a diamine)
Thermosets
Undergo a CHEMICAL change with heat. (non-resversible)
Have network structure with covalent bonds along and between chains
Thermoset types
Epoxy resins, silicone, polyurethane, polyester resins
Thernoset vs thermoplastic properties
Thermosets:
- More rigid, chemical resistance, heat resistance, structrual integrity
Thermoplastics:
- recyclanble, flexible
Vulcanising
Minimising sliding of chains that distort natural rudder items under tensile loads
- controlled breaking of some ‘spare’ multiple bonds, formation of covalent bonds
Sulphur is introduces as vulcanising agent: requires heat and pressure
Crystabllinity
In amorphous: loosely packed chains, distroted: allows light rays to pass through (can be transparent e.g ABS, acrylic, PVC_
Crystaline are highly organised,
Thermoplastics
Soften when heated, can be repeatedly melted down and reformed.
Long covalently bonded chain structures with weak secondary bonds (Van der Waal): can be made flexible and transparent)
CONSEQUENCES Of crystallisation
Destroys clarity: more opaque
Greater shrinkage,
More rigidity, stronger, higher creep + fatigue resistance
Less ductile (tighter packing of molecules restricting movement)
More difficult to bond with adhesives and solvents BUT higher chemical and stress cracking resistance
Greater resistance to wear, good for structural applications.
Elastomers
Thermosoftening polymers still with multiple bonds in their structure after polymerisation e.g. rubbers
Vulcanising Properties
No longer able to completely soften under heating: loose elasticitym but increase strength and rigitidy
Thermosets
Undergoes a permanant chemical change with heat
Have a network structure with covalent bonds along and between chains
Thermoset vs Thermoplastics (properties)
More rigid, better chemical, heat resistance and structural integrity
vs.
Thermoplastics:
- more ductile, recyclable, but weaker, transparent
Elastomers in transport (3 types)
Fillers e.g. carbon black, silica: added to natural rubber tyres to increase resistance to abrasion and tearing
NEOPRENE: synthetic rubber, e.g. flexible hoses in hydraulic brake systems, fuel systems (crosslinked with oxides of zinc or magnesium) = resistant to oils and solvent
BUTYL Rubber: synthetic, no double bonds: long life (bike tyres, tubes): airtight.
Elastomers properties:
Highly flexible, wear resistant
Crystalline polymers vs amorphous
Completely transparent, loosely packed disordered: TRANSPARENT e.g. ABS, acrylic, PVC
Highly organised, aligned, closely packed: light cannot pass thru (opaque)
E.g. polypropylene, polyethylene
Crystallisation properties
Opaque, more shrinkage (tightly packed structure)
Rigid, stronger, more fatigue and corrosion/cracking resistance: structural applications
Less ductile, harder to bond
Vulcanising:
Minimises distortion under tensile loads
Breaks spare multiple bonds, makes covalent bonds
Introduce sulphur
Modification of polymer properties
Increase length of chain or large group of atoms
Make material crystalline
Cross-linking
Blenndong
Orientation of molecules
Copolymerisation
Prodicomg branches
Introducing additives
Polymer textiles
Polymeric materials with fibre: properties non-typical of textiles e.g. ‘shade cloth’ to protect from UV, filtration systems, seatbelts, train seat covers
Fibre manufacturing Processes:
Coagulating, stretching, washing, applying finish, drying, then cutting.
Textiles examples
Polyester
Nylon
Aramid
Olefins
PTFE (Teflon)
Polyester properties and uses
- strong, resilient, hydrophobic
- helium airships, some tyres, car parts e.g. fan belts, radiator hoses
Nylon properties and uses
Dry lubricant
- being replaced by PTFE
- resistant to acids, bases, oils
Olefids
Polyethylene or Polypropylene fibres shaped in sheets
- waterproof, used for collapsible shelters and buildings
PTFE (Teflon)
Fire resistant, stop water vapour
- filters in engines
Aramid
- nomex and Keval
- strengthened by a bacjbone of benzene rings
- strong BUT only low temperature uses
- used in aircraft, bulletproofing
Polymer Manufacturing types
Blow molding
Extrusion
Thermoforming
Calendaring
Rotational molding
Injection molding
Blow moulding
- thermoplastics
1. polymer tube lowered into mold, air forces tube to shape of mold
used for making containers e.g. oil containers
Extrusion
- Polymer granules melted, forced through die
- good for thermosoftening ploymers
- polymer tubing, bike cable coatings
Thermoforming
Thermoplastic containers
- thermoplastic sheets placed over dies, make required shape
- can be done with matching dies, vacuum or air pressure
Calendering
Thermoplastic poured into cavity between rollers, plastic squeezed through
- can be embossed with patterns
- Tiles, films, curtains
Rotational molding
Polymer poured into mold, centrifugal force makes hollow article
Silicones properties, uses
- Non-chemically reactive
- Low thermal and electrical conductivity
- Can repel water (watertight steels)
- Electronics (low thermal and electrical conductivity properties)
- Aviaton (sealants)
- lubricants
THERMOSET
Injection molding
- Polymer granules placed into hopper
- released into heated chamber where reciprocating screw forces metal closer to die (even heating due to screw)
- Forced into die, cooled then released
- good for mass manufacturing, cheap
- poor finish: sprue and split line evident
- used for small thermoplastic moldings for cars and bijes
Polyurethane Foam
THERMOSET
- resistant to water, oil, grease
- good specific strength
- adhesive when forming
- Thermal insulative
- Used for insulation e.g. roofs, wine tanks, refrigeration
Polyethylene
low melting temperature, tough, flexible, insulator, low density has branched chains reducing crystallinity, high density is linear
coating on outer of bike gear and brake cables
Polycarbonate
High strength (resistant to impact and fracture), tough materials (can be optically transparent)
- Easily manufactured.
Lightweight, good alternative to glass
Ecofriendly processing and recyclability
Good electrical properties, UV radiation protection
3D printing, food containers, medical applications, car bumpers and headlight lenses
Polypropylene
Harder then polyethelene, low density and hight heat resistance
-Packaging, automotives
Polyvinyl chloride (PVC)
- Cheap
- Strong
- Weather and chemical resistant
- Strong tensile strength
- Electrical insulation
- Pipes, flooring, car interiors and seat coverigs, wire insulation
Polyamide (nylon)
- Chemical, oil, thermal resistant
- Flexible
- High wear and abrasion resistance
- Low coefficient of friction
- Lightweight
- Water aborbant
Automotive parts, industrial vlaves, insulation for railway sleepers
Acrylic (perspex)
- High rigidity, weldability
- Insulating
- Impact, abrasion and strain resistance
- Good surface aspect
- High dimensional stability
- Dashboard components, wheel covers, helmets, pipes
Vulcanised Rubber
- Elasticity
- Weather, ozone, heat and chemical resistance
- Abrasion, oil and aging resistance
- Hoses, insulation, vibration dampers, tyres
