DMT SBA Flashcards
Polymerisation reaction questions initiation, addition, propagation, termination
Activation
- Heat activation Apply heat- add reactants to container and heat up
- Light activated materials- light used to set/cure material
- Chemically activated polymers
- Forms radicals
Initiation
- The free radicals first react with the monomer
- W/o monomers would randomly join
- Add initiator which starts the reaction
- Initiator must be stable
Propagation
- Each time a monomer meets a radical the C-C double bond opens
- This extends Chain length- chains have different lengths
Termination
- Doesn’t mean 100% of monomers joins to become polymer (never 100% conversion rate)- residual monomers are cytotoxic
- 2 chains that are radicals will cancel each other out- residual monomer that hasn’t reacted
- Liquid to viscous so hard for monomers to meet chains to react
Chemical species that form radicals
initiator. Benzoyl peroxide & CQ - free radical initiator
Which of the following increases working time
Retarder
Time available for mixing and manipulating a material.
From commencement of mixing until the material can no longer be effectively used. SHOULD BE PLASTIC AT WT.
Composition of dental wire
Austenitic stainless steel – 18/8 stainless steel (chromium/nickel)
They have the highest corrosion resistance- however we cannot get martensite to form
% Ni/Cr in austenite and martensite steel and functions
AUSTENITE- 18% Cr and 8% Ni added to steel (Fe-C)
- Orthodontic wires
MARTENSITIC stainless steel – 12/0 stainless steel
drills, burrs
Martensite steel
12% Cr cutting instruments and probes e.g. drills
Why do light cures have filters
to protect the eye of the dentist, retinal damage from high intensity visible light. Harmful spectrum light waves
Lowering glass transition temperature
plasticiser – lowers Tg
What is order hardening
When a random solid changes to a highly ordered one i.e. to form a super lattice. This process requires diffusion of atoms
- Heat to above 450°C- below RcT temp (close however)
- Allow atoms to diffuse (slowly cool)
What is precipitation hardening
This is a heat treatment method used to increase the yield stress of malleable materials including steel. This relies on changes in solid solubility with temperature to produce fine particles of an impurity phase. This prevents movements of dislocations or defects in a crystal’s lattice
heat treatment- Can increase hardness and yield strength of partially soluble alloys
Stress strain curve pointing to different parts:
Yield stress
Proportional limit
Young’s modulus
Yield stress – point where plastic deformation begins
Proportional limit – stress which corresponds to limit of proportionality
Young’s modulus – the linear slope of the curve. The steeper the curve, the more stiff the material is
Which are initiators, activators, monomers, polymers, retarders and fillers
Initiators – Benzoyl peroxide and CQ
Activators – N,N- dimethyl-p-toluidine
Monomers – Vinyl
Retarders - delay setting (C)- delays the increase of visocity over a given period of time so there is time to mix things
Filler- PMMA beads added to MMA in dentures
•PMMA already polymerised, takes up volume- reduces shrinkage
Cold working
Mechanical shaping (forging, rolling, drawing) at low temperatures. In cold working, work is done below the RcT.
Grains go from equated to fibrous
increase in yield strength and hardness
limit to ductility- too much work/deforamation can cause fractures/device failure
Homogenization, which alloys can’t undergo this
When you heat the alloy below the RcT, to get smaller grains and this gets rid of coring.
atoms can move but If heat to temperature below Rct – atoms won’t “jump” grain boundaries
Insoluble alloys cannot undergo homogenization.
Eutectic point
when an alloy has single melting point rather than a range
