Xujin Intro Flashcards
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
What is responsible for viscous and elastic properties?
Viscous - diffusion of atoms inside a material (they will not go back when force is removed - energy dissipated)
Elastic - stretching of bonds between atoms (they will go back when force is removed - energy recovered)
How does crystallinity effect the service temperature of polymers?
Amorphous - rubbery above Tg which limits its temperature range
Crystalline - Retains dimensional stability almost up to melting point so can be used at high service temps
What are some noticeable difference in crystalline and amorphous polymers?
Crystalline - sharp melting point, usually opaque, high shrinkage, solvent resistant
Amorphous - broad softening range, usually transparent, low shrinkage, solvent sensitive
What are some examples of crystalline and amorphous polymers?
Crystalline - PE, PTFE, PP, Nylon
Amorphous - PMMA, PS
How does cross linking affect how a polymer acts at Tg and Tm?
More cross linking raises Tg and increases melt viscosity. Higher cross linking = higher modulus, more brittle
What are some differences between thermoset and thermoplastics?
Thermosets - use rubbery phase for shaping, harden with cross lining by temp or catalyst, liquid goes irreversibly to solid, can withstand high T, no scrap recovery
Thermoplastics - use melt in shaping stage, harden by freezing melt, liquid-solid reversible, scrap recovery possible, can’t withstand high temps
How does molecular weight affect relaxation of orientated molecules?
Higher molecular weight = takes longer for molecules to relax
What does rapid cooling do to orientation?
Doesn’t allow enough time for complete relaxation of molecules so leaves frozen in stress in the part.
How can you remove frozen in stress?
Heating above softening temperature - high shrinkage or distortion means more frozen in stress
What are the types of copolymer?
Alternating - two monomer units alternating
Block - blocks of monomer units together
Random - random chunks of monomers
Graft - chain of same monomer with branches of a different monomer
What is a blend?
Physical mixing of two or more different polymers - usually wont mix but compatibilisers can be used such as a block polymer of the two polymers we wanted to blend
What is a compatibiliser?
A copolymer like a block one that can be used to help blend two types of polymer
What’s the difference between copolymers and blends?
Copolymerisation is a chemical modification process where two monomers are joined to make a new, different polymer
Blending is combining two or more polymers to take advantage of the properties of each one
