Week 2 - polymers

Question 1

what are polymers?

Answer 1

A molecule of high relative
molecular mass, the structure of
which essentially comprises the
multiple repetition of units derived,
actually or conceptually, from
molecules of low relative molecular
mass

Question 2

What is the limitin oxygen index?

Answer 2

the minimum amount of oxygen to support combustion within the material. The smaller the LOI, the more flammable.
LOI>26+ are self extinguishing/ fire retardent, 20<LOI<26 are kind of retardent and LOI<20 will always burn in air.
air is 20.95%

Question 3

How can the limiting oxygen index be changed or enhanced in materials?

Answer 3

Fillers and Fibres
These can improve the resistance of a polymer by increasing the LOI. They
may char forming a protective layer on the component. Examples include glass fibres, mineral fillers.
Flame retardants
Additions added to the polymer that release an inert gas which forms a barrier to oxygen (air) and inhibits the combustion.

Question 4

What are the main reasons issues arise to make polymers fail?

Answer 4

• Material issues - phase separation, poor selection, differential expansion, delamination, adhesive debonding, blistering, contamination (foreign body, self), excess recycle with virgin polymer.
• Manufacturing - Poor mixing, process defects (weld lines, porosity, wall thickness, overheat etc.), assembly defects (thermal welds, adhesive failure etc.)
• Design - Stress concentrators, stress
  raisers, aesthetics
• Installation - Strain from dissimilar materials, over-tightening, loose fitting, wrong dimensions
• In-service conditions

Question 5

What are mechanical and other (5) reasons there can be failure in polymers? and give examples of each.

Answer 5

The mechanical weight average can control the properties of the polymer.
Mechanical:
* Fracture (Brittle or ductile)
* Creep
* Fatigue
* Impact
* Wear
* Yielding, crazing
* Distortion
* Environmental Stress Cracking
* Plasticiser bleeding
* Swelling
Thermal
* Degradation, depolymerisation
* Dimensional Instability
* Shrinkage
* Fire, Slow combustion
* Thermal fatigue
Chemical
* Oxidation, Ozone attack
* Attack by Chlorine (Chlorinolysis)
* Hydrolysis (water, acid, alkali)
* Stress Corrosion Cracking
* Other chemicals
Optical
* UV light
* Ionising radiation
* Photo-tendering (Fading)
Electrical
* Arcing
* Electrostatic build-up

Question 6

When is degradation an advantage?

Answer 6

When we want to compost the materials this is a good thing. Also in stents that can insert into stents and the expand in the right place etc.