DMT Flashcards
Elastic, plastic and viscoelastic, explain using models and graphs what these mean
Elastic
Model = spring
When load applied - deforms quickly
If load held it stays deformed and when removed return to original dimensions
Plastic
Model = dashpot
When load applied deforms quickly
If load is held it stays deformed and when removed it stays deformed
Viscoelasticity
Model = dashpot and spring (in series = Maxwell model) (in parallel = Voigt model)
Maxwell = only some deformation returns
Voigt = all deformation returns but slowly
What effect does adding plasticiser to a polymer have on Tg?
Reduces Tg
Acts as a lubricant between chains so they cannot get as close together
Residual monger can act as a plasticiser
Force?
Equation?
Results from an outside agency acting upon a body to change its momentum
load x acceleration
Define stress
Internal forces set up inside a body to oppose and externally applied force
Define strain
The amount of deformation that occurs due to an applied stress
Define tensile strength
Maximum stress a material can withstand before breaking
Define stiffness
The mount of resistance against deformation
Define ductility
How much a material can be pulled
Define proportional limit
The point where stress is not proportional to strain (switch from elastic to plastic)
Define Young’s modulus
Measures stiffness - defining relationship between stress and strain
Define resilience
How much energy something can take before it deforms
Define toughness
How much energy something can take before it breaks
Define malleability
How much something can be compressed
Define yield strength
Point on Young’s modulus graph (stress and strain) where plastic deformation occurs
3 methods to shape metals and alloys
Casting
Working
Amalgamation
Soluble vs insoluble alloys
Soluble - atoms form into planes
Insoluble - atoms don’t form into planes
Working time
Time from start of mix until the material can no longer be effectively used (should be in the mouth by this point)
At RT
Setting time
Time from start of mix until the material has minimum properties for function
At suitable temp e.g. mouth
What is quenching?
Rapid cooling of a solid in a cooler liquid
Effect of quenching?
Increases speed of cooling - atoms closer together - more grains per volume, more grain boundary per unit of volume
More grain boundaries = higher yield strength (atoms cannot move as much)
What is glass transition temperature?
The temperature at which the material switches from being rubbery to glassy
What type of material does glass transition temperature affect?
Thermoplastic polymers
Thermoplastic
Type of polymer that hardens on cooling and softens on heating
Thermosetting
Type of polymer that hardens on setting and does not soften on heating
All cross-linked polymers are thermosets
Gaseous porosity
Increased temperature leads to vaporised monomer causing weakness and failure
Where does porosity occur?
Thickest part of the denture (most volume)
Contraction porosity
Polymers are denser than monomers so polymerisation causes shrinkage = contraction porosity = weakness and failure
Interstitial alloy
When the atoms forming the alloy are very different in size the smaller element sits int he interstitial gaps between the larger ones = solid solution
The type of molecule is the molecule forming free radicals?
An initiator molecule - weak bond e.g. BPO
Through what step does an initiator molecule become a free radical?
Activation (heat or light)
What increases working time?
Retarder by delaying setting
How to reduce polymerisation contraction?
Use larger monomers or add a filler that will not polymerise
Composition of orthodontic wire
18/8 austentitic stainless steel
18% Cr 8% Ni added to steel
What is steel?
An alloy of carbon and iron (carbon much smaller = so in interstitial sites)
Why is steel so useful?
Number of phases with different properties
Austentite = stable at useful temperatures
Ferrite
Cementite = ionic solution (hard but brittle)
Pearlite = ferrite and cementite (depends on concentration of carbon)
How does C concentration affect mechanical properties of perlite?
Increase C = increased hardness, yield strength, ductility
Purpose of light cures
Protect the eyes of the dentist because high intensity visible light can cause retinal damage
What increase Tg?
Increased polymerisation (lower residual monomer) Change monomer = C=C bond is more rigid than Si-o
What decreases Tg?
Plasticiser
Large pendant groups
Order hardening
Heat treatment on a solid to increase hardens and yield strength by making the atoms form ordered solid solution
Precipitation hardening
Heat treatment on a solid to increase yield strength and hardens in partially soluble alloys
Must be below RcT
Eutectic point
Point at which homogenous mixture of substances has a single melting point
Why does steel have no more than 2% carbon?
2% carbon is its solubility limit
unstable above this
Rheology?
Study of deformation and flow of material, measurement of how viscosity changes
Newtonian
Pseudoplastic
Dilatant
Viscosity does not change as shear rate increases/decreases
Viscosity decreases as shear rate increases (ketchup)
Viscosity increases as shear rate increases (bullet proof vest)
Thixotropy
No flow until sufficient pressure is applied
