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BDS2 CDS > PMMA > Flashcards

PMMA Flashcards

1
Q

what does PMMA stand for

A

poly methyl methacrylate

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2
Q

what is it used for

A

denture base most commonly

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3
Q

why would an acrylic denture not fit?

A

 impression flaws
 material and method not accurate
 heat curing process -> temp too high or low
 fracture
 masticatory forces
 warped
 suffers wear

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4
Q

is acrylic dimensionally accurate?

A

yes

it needs to be dimensionally stable and accurate in order to fit and retain

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5
Q

does acrylic have high softening temp

A

yes, in order not to melt when consuming hot things

but ensure not to wash acrylic dentures in boiling water

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6
Q

is acrylic unaffected by oral fluids?

A

yes

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7
Q

does acylic have high youngs modulus

A

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

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8
Q

why do acrylic dentures need high elastic limit

A

dentures will not permanently deform easily when large stresses are applied

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9
Q

what should the ideal thermal expansion of acrylic be?

A

should match the thermal expansion of the artificial teeth so that there is no internal stresses during the cooling phase in manufacturing

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10
Q

what are artificial teeth made of?

A

acrylic resin too

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11
Q

is acrylic high or low density and why

A

acrylic is low density which is good

lighter so upper wont fall out due to gravity

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12
Q

does acrylic have high or low thermal conductivity?

A

it has low but ideally should be high so that there is transmission of thermal stimuli to the mucosa and wont burn yourself

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13
Q

acrylic resin has higher or lower thermal expansion than composite resin?

A

higher

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13
Q

4 properties u can infer from the stress/strain curve?

A

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

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13
Q

how to compensate for poor mechanical properties in acrylic?

A

BULK

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13
Q

what reaction to form PMMA?

A

free radical addition polymerisation

14
Q

name of the monomer in polymerisation

A

methacrylate (contains the C=C)

15
Q

4 steps for polymerization of PMMA

A
  1. activation
  2. initiation
  3. propagation
  4. termination
16
Q

how to do activation

A
  • use of initiator to provide free radical

at heat > 72 deg or self curing

17
Q

what is the name of the initiator

A
  • initiator being benzoyl peroxide which is a symmetrical molecule
18
Q

what is propogation

A
  • growing polymer chain
19
Q

composition of pmma

A

powder and liquid (that are mixed afterwards)

powder
- initiator
- PMMA particles
- plasticiser
- pigments
- copolymers

liquid
- methacrylate monomer
- inhibitor hydroquinone
- copolymers

20
Q

what does a plasticiser do

A

so powder can dissolve quicker in monomer liquid

21
Q

what do co polymers do

A

improve the mechincal porperties

22
Q

what does the inhibitor do

A

hydroquinone
 prolongs shelf life
 stops setting rxn from starting accidentally
 inhibitor reacts with free radicals produced by heat or UV light

23
Q

heating schedule

A

7 hours at 70deg cel
2 hours at 100 deg cel
SLOW COOL to minimise internal stresses

24
Q

why do we want high temperature in heat curing

A

more polymerisation = high molecular weight = maximises properties

25
Q

what happens if temp exceeds 100 degrees when we heat cure acrylic

A

bubbbles
gaseous porosity
poorer mechnical properties

26
Q

how to prevent internal stresses in acrylic denture?

A

make sure the acrylic denture base and the artifical teeth are of the same thermal expansion

can overcome this problem also by COOLING SLOWLY

27
Q

how does internal stresses affect the denture

A
  • decreases strength
  • decreases fatigue strength
  • fractures more easily
  • warps
28
Q

what happens if denture is undercured

A
  • excess free monomer that are irritants
  • low molecular weight so poorer mechanical properties (refer back to high temp -> high molecular weight -> better mechanical properties )
29
Q

what happens if acrylic is over cured

A

 gaseous porosity

30
Q

ideal powder: liquid ratio?

A

3:1

31
Q

what happens if you add too much monomer or too little monomer

A

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

32
Q

what is contraction porosity and when does it occur

A
  • polymerisation shrinkage that all resins experience
  • occurs where dough is not sufficiently packed
33
Q

problems with porosity

A
  • affects appearance , takes on stains
  • affects strength, less strong
  • rough sensation to the tongue
  • absorbs saliva , poor hygiene
34
Q

3 reasons for contraction porosity

A
  • too much monomer
  • insufficient excess material (not enough packing into flask)
  • insufficient clamp pressure

BDS2 CDS (15 decks)

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