Xujin Intro Flashcards
(37 cards)
What is and isn’t included in polymer processing?
Conversion of raw material into a finished part, including mixing and compounding
Can’t include polymerisation/chemical reactions
Which shear viscosity equations do we use for Newtonian and pseudo plastic?
Newtonian - normal model (shear stress/shear strain rate)
Pseudo plastic - power law model
What equation is used for elongational or tensile viscosity?
Troutonian model: lambda = tensile stress/tensile strain rate
What does viscoelastic mean?
Polymers are predominantly viscous (permanent deformation) but also are partially elastic (recoverable deformation)
What are Tg and Tm?
Tg is glass transition temp. - above this the polymer is elastic
Tm is melt temp. - above this the polymer is viscous/fluid flow
What are some process that use the melt state?
Extrusion, injection, blow, and rotational moulding
What are some examples of process that use heat softening rather than melting?
Vacuum forming, stretch blow moulding, film and fibres
What processes use thermosets?
Moulding - injection, transfer, compression, and reaction injection moulding
Lamination - glass, carbon fibres in liquid thermoset resins
Use as adhesives, surface coatings, and paints
Why are elastomers rubbery?
They are above Tg at ambient temp so in the elastic region.
What processes use elastomers?
Compounding - incorporating additives, mastication (intensive shear mixing process to reduce molecular weight)
Shaping - extrusion, injection, transfer, and compression moulding
What are the different types of polymerisation?
Addition (chain) polymerisation - opening a bond to increase the length of the chain
Condensation (step) polymerisation - reaction of two ends with the elimination of something like water (Alcohol + Acid = Polyester)
Polyaddition - reaction between two ends with no formation of volatiles
Ring-open
Living
What degree of polymerisation should most chains be to be useful?
At least 500 monomer units long
What are commercial polymer DoP’s?
10^3 - 10^5
What is the most important property of polymers?
High molecular weight - affects resistance to stress, fatigue, toughness, creep, flexural and shear modes, and cracking
What is Number Average Molecular Weight?
Mn = sum of Ni x Mi / sum of Ni
Mi = each molecular weight
Ni = number of molecules of that molecular weight present
So basically the average molecular weight (weighted by how many there are at each molecular weight)
What is Weight Average Molecular Weight?
Me = sum of Ni x Mi^2 / sum of Ni x Mi
What is Molecular Weight Distribution of polymers?
Polymers have different molecular weights. The distribution of chain length is called molecular weight distribution (MWD)
What is the polydispersity index (PDI)?
PDI is the breath of the molecular weight distribution (ratio of weight average molecular weight to number average molecular weight)
PDI = Mw/Mn
How does polymerisation type affect MWD?
Monomers will be 1
Chain polymers are 1.05-3.5
Condensation polymers are 3-20
What motion allows a polymer to be above its glass transition temperature?
Long range segmental motion
What is free volume?
Space in a solid or liquid that’s not occupied by polymer molecules
What happens to free volume as temp is reduced?
Free volume reduces until eventually there is not enough for molecular movement - this is Tg, where polymer glass is ‘frozen’
How does perfect and imperfect crystallisation alter the volume as temp is increased?
In perfect, there is linear increase in volume with temperature until the melting point, when volume increases by a large mount
In imperfect, the volume will start to increase by a large amount before reaching the melting point.
How does the modulus change with temp comparing crystalline and amorphous?
Crystalline stays a tough solid, then loses most rigidity at melt temp where it becomes a viscous flow
Amorphous lowers in modulus a lot at the Tg where it becomes rubbery then continues decreasing to a viscous flow after Tm
