PMMA Flashcards
what does PMMA stand for
poly methyl methacrylate
what is it used for
denture base most commonly
why would an acrylic denture not fit?
impression flaws
material and method not accurate
heat curing process -> temp too high or low
fracture
masticatory forces
warped
suffers wear
is acrylic dimensionally accurate?
yes
it needs to be dimensionally stable and accurate in order to fit and retain
does acrylic have high softening temp
yes, in order not to melt when consuming hot things
but ensure not to wash acrylic dentures in boiling water
is acrylic unaffected by oral fluids?
yes
does acylic have high youngs modulus
no, it has poor mechanical properties and LOW youngs modulus
which means that it is not as stiff and rigid as we would ideally want it to be
why do acrylic dentures need high elastic limit
dentures will not permanently deform easily when large stresses are applied
what should the ideal thermal expansion of acrylic be?
should match the thermal expansion of the artificial teeth so that there is no internal stresses during the cooling phase in manufacturing
what are artificial teeth made of?
acrylic resin too
is acrylic high or low density and why
acrylic is low density which is good
lighter so upper wont fall out due to gravity
does acrylic have high or low thermal conductivity?
it has low but ideally should be high so that there is transmission of thermal stimuli to the mucosa and wont burn yourself
acrylic resin has higher or lower thermal expansion than composite resin?
higher
4 properties u can infer from the stress/strain curve?
PL = point up till which stress/strain rs is linear
EL = point beyond which there is permanent deformation
gradient = rigidity = youngs modulus
y axis is any type of strength
how to compensate for poor mechanical properties in acrylic?
BULK
what reaction to form PMMA?
free radical addition polymerisation
name of the monomer in polymerisation
methacrylate (contains the C=C)
4 steps for polymerization of PMMA
- activation
- initiation
- propagation
- termination
how to do activation
- use of initiator to provide free radical
at heat > 72 deg or self curing
what is the name of the initiator
- initiator being benzoyl peroxide which is a symmetrical molecule
what is propogation
- growing polymer chain
composition of pmma
powder and liquid (that are mixed afterwards)
powder
- initiator
- PMMA particles
- plasticiser
- pigments
- copolymers
liquid
- methacrylate monomer
- inhibitor hydroquinone
- copolymers
what does a plasticiser do
so powder can dissolve quicker in monomer liquid
what do co polymers do
improve the mechincal porperties
what does the inhibitor do
hydroquinone
prolongs shelf life
stops setting rxn from starting accidentally
inhibitor reacts with free radicals produced by heat or UV light
heating schedule
7 hours at 70deg cel
2 hours at 100 deg cel
SLOW COOL to minimise internal stresses
why do we want high temperature in heat curing
more polymerisation = high molecular weight = maximises properties
what happens if temp exceeds 100 degrees when we heat cure acrylic
bubbbles
gaseous porosity
poorer mechnical properties
how to prevent internal stresses in acrylic denture?
make sure the acrylic denture base and the artifical teeth are of the same thermal expansion
can overcome this problem also by COOLING SLOWLY
how does internal stresses affect the denture
- decreases strength
- decreases fatigue strength
- fractures more easily
- warps
what happens if denture is undercured
- excess free monomer that are irritants
- low molecular weight so poorer mechanical properties (refer back to high temp -> high molecular weight -> better mechanical properties )
what happens if acrylic is over cured
gaseous porosity
ideal powder: liquid ratio?
3:1
what happens if you add too much monomer or too little monomer
too much monomer -> contraction porosity
too little monomer->
granularity
monomer is found in the liquid so if you mess up the powder: liquid ratio, might end up with wrong amount of monomer
what is contraction porosity and when does it occur
- polymerisation shrinkage that all resins experience
- occurs where dough is not sufficiently packed
problems with porosity
- affects appearance , takes on stains
- affects strength, less strong
- rough sensation to the tongue
- absorbs saliva , poor hygiene
3 reasons for contraction porosity
- too much monomer
- insufficient excess material (not enough packing into flask)
- insufficient clamp pressure
