Section B Flashcards
Polymers are often produced using heterogeneous production methods. Name major safety issue that can result in the loss of control over this process that is identified with the manufacture of chain polymers using homogeneous manufacturing routes?
Tromsdorff-Norish effect or gel point or auto acceleration
Explain the influencing factors which may result in this onset of this loss of control of a homogeneous polymerisation
- the reaction is exothermic
- conversion of monomer to polymer/ molecular weight increases during reaction
- this leads to increased viscosity
- thus mass and heat transfer is severely reduced
- Thus bulk reaction medium temperature increases
- thus increasing rate of reaction so it cannot be controlled
Explain how adopting heterogeneous polymerisation methods mitigate this safety risk
- The reaction is conducted by forming droplets called micelles
- the micelles are dispersed in a continuous phase
- the continuous phase does not participate in the reaction
- It acts as a cooling medium only
- conversion of monomer to polymer during the reaction occurs in these droplets
- Thus mass transfer is improved as there is no increased viscosity within the bulk of the reactors
-Heat transfer is improved because micelles have very high surface to volume ratios
Heterogenous polymerisation processes
- Dispersion
-Suspension - Emulsion
Rank the three heterogeneous polymerisation methods to manufacture polystyrene from least safe to most
- Least: Suspension
- Intermediate: Emulsion
- Most: Disperion
Explain the reasoning behind the rank
- All methods rely on successfully breaking up a large monomer into smaller droplets
• If this doesn’t happen, auto-acceleration can occur in the large volume of monomer
• Because emulsion polymerization uses a surfactant, the physical chemistry of the system -forms micelles, thus the level of mixing is not as critical as it is with suspension polymerization.
• however suspension completely relies on shear forces to create the droplets, and the interfacial aid in this system only stabilizes the particles as they form. Thus, emulsion is safer than suspension.
• In dispersion polymerization, the monomer is miscible with the continuous phase, so there is much less chance of monomer domain issues due to poor diffusion of the monomer in the system at the outset. Again, the interfacial aid in this system only stabilizes the particles as they form.
• In emulsion polymerization, the monomer is not miscible with the continuous phase. Therefore, dispersion is safer than emulsion.
In dynamic equilibrium, increasing more pressure will…
Move the position of equilibrium to the side with less number of molecules to reduce pressure
Does a catalyst influence the position of equilibrium
No- it speeds both forward and reverse reactions
Step chain growth
- polymer growth by reaction between monomers
- low- moderate DP
- monomers are consumed quickly but MW increases slowly
- no termination
- no initiation
- polymerisation rate decreases as functional groups are consumed
Chain growth polymerisation
- polymer growth by successive additions do monomers to the end of growing chain
- high DP
- monomers are consumed slowly MW increases quickly
- polymerisation rates increases as initiator is generated
Gibbs free energy
G= moles * chemical potential ( reactants + products)
At equilibrium
Cu + Du = au + bu. ( abcd are coefficient)
Ksp solubility constant
A large KSP means forward reaction is favoured eg CaF2 KSP= Ca2+ + 2F-
Equilibrium constant
How far a chemical reaction will go
Adding inert gas for dynamic equilibrium
- Total pressure of system increases but no Keq change
- but if volume is allowed to increase position of equilibrium will move to the side with more moles of gas
Van Hoff equation
DlnKeq/ dT. =. Delta H/ RT^2
Van Hoff Plot for graph
LnKeq. =. -Delta H / RT. + delta S/R
Delta S/ R is y intercept
-delta H/R is gradient
Negative slope is endothermic
Le chatelier’s principle
If a system is disturbed the position of equilibrium will change to contract the effect of the disturbance
Equilibrium constant equation
Kc= [C]^c * [D]^d / [A]^a * [B]^b
Tromsdorff effect
- conversion of monomer increases, MW increases, viscosity increases , HMT is reduced, reaction temperature increases, rate of propagation increases
How to prevent Tromsdorff effect
- add inhibitor to kill radicals
- add crash volume of solvent/ monomer to reduce viscosity
- drop reaction contents out of bottom of reactor
Acrylic sheet casting
- initiator in a polymer resin is poured between the moulding sheets ( resin prevents leaking from cell and increases viscosity )
- resin allowed to pymerizw under controlled heating
- sheet allowed to cool in mould before removing it
Cell casting
- two sheets of glass kept apart by rubber polymer and pour resin into it
- affords high surface area to volume ratio
Batch process
Film casting
- a the in sheet of solution or low viscosity resin is applied to a heat roller
- solvent is evaporated way
Continuous
Deborah number
- describes viscoelastic behaviour of a material , tells you if you can process material
- bigger De the more solid the material, small De is more fluid
- De= deformation time*mean velocity flow / pipe length
Emulsion
- surfactant is added to water to form micelles and droplets of water migrate into micelle start swelling them
- initiator is put into water and breaks down to form radical and it now wants to be with oil - it goes into the monomer and starts reacting with the monomer in the micelle
- reaction continues micelle still swelling and monomer is still transferring
- we now have polymer in monomer
Emulsion
In emulsion polymerization, a surfactant forms micelles in water, stabilizing droplets of monomer. An initiator breaks down into radicals, which enter the micelles and start polymerizing the monomer. As the reaction continues, the micelles swell with growing polymer particles, and the polymerization progresses within the monomer droplets.
Emulsion advantages
- continuous water phase is a good conductor of heat
- droplets give large surface area
- viscosity remains close to water
Emulsion advantages
- continuous water phase is a good conductor of heat
- droplets give large surface area
- viscosity remains close to water
Suspension
- suspension polymerization involves dispersing water-insoluble monomers in water, typically as droplets
- initiator is dissolved in the monomer, and when activated, it forms free radicals
- stabilisers used to prevent the polymer particles from agglomerating. These stabilizers keep the particles suspended in water.
- it avoids issues such as micelle formation, which can occur in emulsion polymerization.
Dispersion
- a homogeneous mixture of the monomer and (dispersant)
- When polymer reach a certain molecular weight, the polymer becomes insoluble in the solvent and starts to precipitate out of the solution.
- dispersant, preventing the polymer particles from aggregating or “crashing out” of the solution. Key to maintain dispersion
- the polymer’s precipitation starts forming droplets or micelles, where the growing polymer particles are contained within. The stabilizer ensures the polymer particles remain dispersed.
- Without stabilizer, the polymer would precipitate uncontrollably.
