THERMOPLASTICS Flashcards
Examples of addition polymers?
Polyolefins (PE, PP), PVC and PS
What are thermoplastic materials?
Chemical compounds based on carbon atoms, often complex compounds
Examples of condensation polymers?
PA, PET, PC
Homopolymers?
Polymer formed from a single monomer:
-A-A-A-A-A-
Copolymers?
Two types of monomers joined in a single polymer chain:
-A-B-A-B-A-B-
Block copolymers?
When the two monomers cluster together and form blocks of repeating units inside the polymer chain:
-A-A-A-B-B-B-A-A-A-B-B-B-
Terpolymers?
Polymer from three monomers
Polymer blends?
Class of material analogous to metal alloys, at least two polymers are blended together to create a new material with different physical properties.
General properties of amorphous materials?
- BRITTLE (apart from PC or by the addition of rubber e.g. PVC and ABS)
- MORE STABLE (low creep) under sustained load
- EASIER TO MANUFACTURE at temps above Tg
- TRANSPARENT to light
- LESS RESISTANT TO SOLVENTS
General properties of semi-crystalline materials?
- LESS BRITTLE (except in low temps)
- Wide range of strength and stiffness
- Suitable for fibre reinforcement
- LESS STABLE (more creep) under sustained load
- More difficult to manufacture (increased molecular weight due to crystal structure, so more viscous)
- Less transparent to light
- More resistant to solvents
What is the equivalent to metal grain structure in plastics?
SPHERULITE
What does creep do to stiffness?
Creep decreases stiffness
What is impact resistance dependent on?
Temperature (more so than metals) and dry or wet conditions
How is heat deflection temperature tested and what does it test?
Place sample in hot oil and measure deflection. Tests ability to resist elevated temperature
What is underwriters laboratory (UL) thermal index?
Test for various plastics : at what temp will strength be halved?
How do plastic coefficient of thermal expansions compare to that for metals?
Much larger than for metals, expand significantly under elevated temperature (dimensional accuracy is a concern)
What is Environmental Stress Cracking (ESC)?
Plastics degrade in same way as metals - (SCC) oxidation between grain boundaries weakens the metal - occurs in SPHERULITE structure in plastics.
How can you judge ESC?
water absorption (lower the better), permeability (lower the better), solvent resistance (higher the better, semicrystalline), ultraviolet radiation degradation (low) and strength (high)
What is volume resistivity?
Ratio of DC voltage per unit thickness to amount of current per unit area passing through the material :
Conductive > 4
Insulative 12-20
Static dissipative 4-11 ( a measure for antistatic or electrostatic discharge - A SAFETY PROPERTY!!)
What is electrostatic discharge (ESD)?
A sudden and momentary electric current that flows when excess electric charge stored on an electrically insulated object finds a path to an object at a different potential
What are the food and drug administration (FDA) and United States pharmacopeia (USP) classes?
VII (best, lowest biological risk), VI, V, IV (worst, highest biological risk)
What is toxicology?
The biological effects cause by plastics. There are seven classes based on animals testing (either by extraction, implantation or both). Mice and rabbits are mostly used for a specific period of time. The animal’s response is observed.
What are flammability UL ratings?
HB (High burning), V2, V1, V0 (lowest flame spread)
What is the limiting oxygen index?
The minimum oxygen content that has to be present in order to sustain a fire (e.g. PVC requires 50% oxygen rating to continue burning) - the higher the better.
What are processing temps related to?
Tg and Tm
What is recrystallisation?
Very important to semi-crystalline materials, cool the plastic slowly and keep at it’s recrystallisation temp so that crystals can form themselves - if this isn’t done right the plastic will be very amorphous. The higher the recrystallisation temp the worse.
How does drying time effect processing properties?
The longer the drying time the lower the economy (hours/degC)
How to judge fluidity of melt?
Melt index - starts with 10 (PE) to 0 - higher the better. Can also judge by Tm
Are semicrystalline materials generally easy to bond?
No
What are the recycling symbol numbers and what do they mean for recyclability?
1 - PET 2 - HDPE 3 - PVC 4 - LDPE 5 - PP 6 - PS 7 - others The lower the recycling number the better.
What is the CO2 footprint?
The mass of carbon dioxide resulting directly from the production of a kg of the material. This includes emission due to: processing, the manufacture of feedstock, and the transport associated with the primary production process.
What are reinforcements?
Typically high strength, high modulus, fibrous materials that form a good bond with the polymer.
Why is short fibre used?
To improve strength, stiffness and dimensional and thermal stability
How much reinforcement can be used?
Up to 50% by weight! but the higher the content of fibre the more difficult the moulding process.
What type of plastic can be reinforced?
Semicrystalline
What is the rile of mixtures used to calculate the resultant composite properties?
Pc = 0.4PfVf + Pp(1-Vf)
Pc is composite property
Pf is fibre property
Vf is volume fraction of fibre
Pp is plastic property
How may the volume fraction of the fibre Vf be calculated?
Vf = (m(f)/rho(f))/((m(f)/rho(f)) + (m(p)/rho(p))
