Sem 2 - Polymers Flashcards
Describe a thermoset plastic
A polymer that once having been cured by a chemical reaction will not soften or melt hen chemically treated
It has a network polymer structure and become permanently hard in formation.
Harder stronger and has more dimensional stability than thermoplastics
Describe Thermoplastics
They are semi crystalline structure which soften when heated; they can then be remolded or extruded
irreversible degradation occurs when the polymer is raised to a high temperature
Relatively soft
Describe Elastomers
Polymeric materials that may experience large and reversible elastic deformations. This is due to the linear structure with occasional cross links which provides a memory so the material can return to its original shape.
Thermoplastic elastomers are a copolymeric material which exhibits elastomeric behaviour but is thermoplastic in nature
Crystalline state of Polymers
Crystalline regions disperse in the amorphous material.
Crystalline regions are thin platelets with chain folds at faces with chain folded structure.
Polymers are rarely 100% crystalline because its difficult for all chains to be aligned
Define Isotactic
the functional groups are all located on the same side and are often semi crystalline
Define Atactic
the functional groups are alternating, so the don’t favour stacking and are in glass states
The influence of temperature and strain rate of thermoplastics
Decreasing T: - increases modulus - increase tensile strength -decreases % elongation Increasing strain rate - same effects as decreasing T
Strength limiting processes in polymers
- Brittle Fracture
- Cold Drawing
- Shear Bending
- Crazing, micro-cracks associate with local cold drawing
- Viscous flow when the secondary bonds n the polymer melt
At what point do polymers become brittle
0.75 Tg (of glass transition temperature)
Cold drawing process (Macroscopic polymer deformation)
- Two adjacent chain folded lamellae and interlamellar material
- Amorphous tie chains elongate
- Two adjacent chain folded lamellae and interlamelar amorphous material before deformation
- Increase in lamellar crystallite thickness due to bending and stretching of chains in crystalline regions
- Separation of crystalline block segments
- Orientation of block segments and tie chains with tensile axis
Factors that favour brittle fracture
- Reductions in T
- Increase in strain rate
- Sharp notch
- Increase in specimen thickness
- Modification to polymer which increases Tg
Explain crazing in thermoplastic polymers
Very localised deformation leads to microvoids with fibrillar bridges between them. If the load is sufficient the bridges will elongate and break.The craze is followed by a crack
How does an amorphous polymer behave
A glass at low T’s
A rubbery solid at T (above Tg)
A viscous liquid at higher T’s (above Tm)
Define viscoelastic creep
irreverisible process in which long molecular chains slide along each other
Describe an experiment to measure viscoelastic behaviour
Stress relaxation experiments
- specimen is strain rapidly in tension to a predetermined strain level
- stress required to maintain this strain is measured as a function of time at a constant T
- stress decreases with time
- relaxation modulus is a function of time and temperature
How do crystalline and amorphous structures from?
Through cooling:
- crystalline forms when the molecules are ordered
- amorphous go through glass transition and become a rigid solid while maintaining disordered molecular structure
Feature of melting that are found in polymers but not in ceramics or metals
- Occurs over a range of temperatures
- Melting behaviour depends on the polymer history (crystallisation T affects the thickness of the lamellae
- Impurities in the polymer and imperfections in the crystals will decrease Tm
- Tm will be elevated by increasing the rate heating
Describe Glass Transition in amorphous and semi-crystalline polymers
- Due to a reduction in the motion of large segments of the molecular chain with decreasing temperature (secondary bonds are formed)
- On cooling, glass transition is the gradual transformation from liquid to a rubbery material then a rigid solid
- Glass transition temperature is the temp at which the material changes from rubbery to rigid
How are the service limits found of polymers
They are defined by the upper limit being Tm and the lower limit being Tg of applications
Tm and Tg also influence the fabrication and processing parameters
The properties which affect Tg and how
Increase Tg:
-Increasing molecular weight
-Decrease flexibility of backbones
-Polar group (increase intermolecular interaction)
-Bulky groups (increase the energy barrier for rotation)
Decrease Tg
-Decrease crystallinity
Considerations for forming techniques
- the type of plastic (thermoplastic or thermosetting)
- the temperature at which the thermoplastic softens
- the atmospheric stability of the material being formed
- the geometry and size of the finished product
Forming techniques of thermoplastics
If amorphous it is above Tg and if semi-crystalline it is above Tm
An applied pressure must be maintained during cooling for the shape is retained
Thermoplastics can be recycled, remelted and reformed
Forming techniques of thermosets
- preparation od a linear polymer as a liquid with low molecular weight
- converted into the final hard and stiff shape during second stage in a mould.
- may occur with heating or with a catalyst
-Cross links and networks are formed
-Thermoset polymers can be removed from the mould because they are geometrically stable
They do not melt and are usable at higher temperatures
Describe Compression or transfer moulding
Forming process where the polymer is placed directly into the heated metal mould, softened by the heat and forced to conform to the shape of the mould as the mould closes
Describe Injection moulding
Most widely used technique for fabricating thermoplastics
- Very quick process
- can also be applied to thermosetting polymers but curing occurs resulting in longer cycle times
Describe extrusion
Viscous thermoplastics are put under pressure through an open ended die
used for rods n stuff
