DMT practicals Flashcards
What do impression materials require ?
a large working time- can be mixed, loaded and set into the mouth
the time taken from the start of mixing to the material no longer being usable at room temperature
When a material is first set in the mouth which type of deformation should it undergo ?
plastic- retains the shape of the impression
What is the state of deformation during setting of impression materials ?
viscoelastic
What is the state of deformation when impression materials are set ?
elastic- cant be permenantly deformed
What type of deformation should occur in the working time ?
plastic
What type of deformation should happen in the setting time ?
elastic
If the recovery is 0 the material is ?
elastic
Acrylic and resin materials undergo setting by ??
polymerisation
What is polymerisation ?
the process by which small molecules are joined covaleltnty into large molecular chains
What are the dimensional changes that happen in polymerisation ?
monomers are usually liquid/gas and this leads to solid formation in polymerisation- increase in viscosity and contraction occurs
What can dimensional changes be measured with ?
dialtometer
How do denture base materials initiate polymerisation ?
when acrylic beads are added and they absorb water
What are the stages of form that denture base materials go through when polymerising ?
sandy stage
stringy
douhy
What is mixed with MMA to intitate polymerisation of denture base materials ?
PMMA mixed with MMA
PMMA is polymer
How does the dilatometer work ?
when the polymer is at dough stage
but into water bath that has a liquid callbrated pipette attached
rises in liquid due to volume changes in PMMA
What are gypsum materials used for ?
impression materials
How does gypsum set ?
not by polymerisation
What are gysum products made of ?
calcium sulphate hemihydrate
plaster and stone
What are dental plaster and stone used for ?
record of intra oral structures
How is dental plaster and stone provided and how is it set ?
provided as powder- mix with water
set by exo reaction
What happens when dental plaster mixes with water ?
forms calcium sulphate dihydrate
this is less soluble
precipitates out in solution
precipitation occurs until no further dihydrate to precipitate out
How can the setting of dental gypsum be measured ?
extensometer
How can dentures be subjected to rapidly applied stress ?
dropping the denture
What does the charpey impact tester do ?
provide a rapidly applied force with sufficient magnitude
What does the charpey impact tester consist of ?
weighted pendulum which drops down
What happens when the pendulum hits a specimen ?
the pendulum should fracture
the pendulum looses energy and it max acsent i sless as a result
What can be used to measure impact strength ?
difference between specimen height and no specimen height
Why can brittle materials fracture under their stress ?
they have notches which concentrate stress
If a notch has a large effect on impact strength it is ?
notch sensitive
What 2 properties are needed to determine if a material is notch sensitive ?
Ep
Ei
What is Ei ?
energy of initiation
energy of failure of unnotched specimens
What is Ep ?
energy of propagation
energy needed to fail notched specimens
What is the relationship between Ep and Ei in a notch senstitive material ?
Ei > Ep
Impact strength is also known as ?
toughness
How do we usually measure diametral tensile strength ?
dumb bell shaped specimens
apply tensile force
Why can we use conventional methods to measure tensile strenght in brittle materials ?
they would shatter
How do we measure diamteral tensile strength in materials ?
apply a compressive froce
this sets up tensile forces within the specimen that are equal and opposite direction and perpendicular to the force application
What happens as we apply more compressional stress ?
the internal forces will increase
material breaks down the diameter
Which types of materials can only have their diamteral tesnile strength measured ?
rigid materials
brittle materials would shatter
How do we calculate stress ?
force/cross sectional area
What is adhesion ?
attachment of one surface to another as a result of molecular attraction or interlocking with the surfaces
What are the 3 types of adhesion ?
chemical
physical
complex
What is chemical bonding ?
molecular attraction
What is physical bonding ?
interlocking of surfaces of adhesive
What is complex bonding ?
combination of physical and chemical
What are the materials used in white fillings ?
glass ionomers
composites
What are composite materials ?
resin based
filled with glass filler
Do composites bind directly to the tooth ?
no
thye need an adhesive
How do you apply a composite ?
acid to remove mineral content
dentine is porous
resin is applied and uses chemical process to infiltrate porosities
composite is applied to resin
How does composite bind ?
physical process
resin uses chemical process
What are the layers in a composite filling ?
dentine
resin
composite
How do GIC set ?
acid base reaction
Does GIC need an adhesive ?
no
GIC binds to dentine withour need for adhesive
How does GIC adhere to dentine ?
chemical process
Does composite or GIC have a higher shear bond strenght ?
composite- as it has an adhesive
What are the 3 modes of failure ?
adhesive
cohesive mixed
What is an adhesive mode of failure ?
smooth surface
failure of interface of resin and composite
What is a cohesive mode of failure ?
rough surface
either some composite left behind
or some dentine removed
What is a mixed mode of failure ?
smooth and rough
mix of adhesice and cohesive
What do tooth coloured fillings consist of ?
glass filler
incorporated into glass matrix
Do tooth colured fillngs adhere directly ?
no they rely on an adhesive
How do composites set ?
by polymerisation
How can we measure the heat released in polymerisation ?
calorimeter
How can polymerisation be initiated ?
be mixed with an initiator
inititated by light
How does light polymerisation work ?
Require exposure to a visible light source
What is the wavelength of visible light ?
380 to 700 nm
Why are chemically activated polymers harder to use than light activated ?
chemcially activated have shorter working time
light activated materials have infinite working time
What is the disadvantage of using a curing light ?
it can only set composites to the depth the light can reach
as a result have to do composite fillings in layers
What are the factors affecting cure ?
distance
angle
depth
What is the relationship between the diameter of the cure tip and light intensity ?
the smaller the diameter of the light tip the higher the light intensity
What happens if composite is not fully polymerised ?
releases monomer that leads to insenstitvity
What is the relationship between energy of light polymerisation and energy of light only ?
energy of light polymerisation- energy of light= energy chemical polymerisation
