PMMA

Question 1

what does PMMA stand for

Answer 1

poly methyl methacrylate

Question 2

what is it used for

Answer 2

denture base most commonly

Question 3

why would an acrylic denture not fit?

Answer 3

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

Question 4

is acrylic dimensionally accurate?

Answer 4

yes

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

Question 5

does acrylic have high softening temp

Answer 5

yes, in order not to melt when consuming hot things

but ensure not to wash acrylic dentures in boiling water

Question 6

is acrylic unaffected by oral fluids?

Answer 6

yes

Question 7

does acylic have high youngs modulus

Answer 7

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

Question 8

why do acrylic dentures need high elastic limit

Answer 8

dentures will not permanently deform easily when large stresses are applied

Question 9

what should the ideal thermal expansion of acrylic be?

Answer 9

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

Question 10

what are artificial teeth made of?

Answer 10

acrylic resin too

Question 11

is acrylic high or low density and why

Answer 11

acrylic is low density which is good

lighter so upper wont fall out due to gravity

Question 12

does acrylic have high or low thermal conductivity?

Answer 12

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

Question 13

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

Answer 13

higher

Question 13

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

Answer 13

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

Question 13

how to compensate for poor mechanical properties in acrylic?

Answer 13

BULK

Question 13

what reaction to form PMMA?

Answer 13

free radical addition polymerisation

Question 14

name of the monomer in polymerisation

Answer 14

methacrylate (contains the C=C)

Question 15

4 steps for polymerization of PMMA

Answer 15

1. activation
2. initiation
3. propagation
4. termination

Question 16

how to do activation

Answer 16

• use of initiator to provide free radical

at heat > 72 deg or self curing

Question 17

what is the name of the initiator

Answer 17

• initiator being benzoyl peroxide which is a symmetrical molecule

Question 18

what is propogation

Answer 18

• growing polymer chain

Question 19

composition of pmma

Answer 19

powder and liquid (that are mixed afterwards)

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

liquid
- methacrylate monomer
- inhibitor hydroquinone
- copolymers

Question 20

what does a plasticiser do

Answer 20

so powder can dissolve quicker in monomer liquid

Question 21

what do co polymers do

Answer 21

improve the mechincal porperties

Question 22

what does the inhibitor do

Answer 22

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

Question 23

heating schedule

Answer 23

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

Question 24

why do we want high temperature in heat curing

Answer 24

more polymerisation = high molecular weight = maximises properties

Question 25

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

Answer 25

bubbbles gaseous porosity poorer mechnical properties

Question 26

how to prevent internal stresses in acrylic denture?

Answer 26

make sure the acrylic denture base and the artifical teeth are of the same thermal expansion can overcome this problem also by COOLING SLOWLY

Question 27

how does internal stresses affect the denture

Answer 27

- decreases strength - decreases fatigue strength - fractures more easily - warps

Question 28

what happens if denture is undercured

Answer 28

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

Question 29

what happens if acrylic is over cured

Answer 29

 gaseous porosity

Question 30

ideal powder: liquid ratio?

Answer 30

3:1

Question 31

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

Answer 31

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

Question 32

what is contraction porosity and when does it occur

Answer 32

- polymerisation shrinkage that all resins experience - occurs where dough is not sufficiently packed

Question 33

problems with porosity

Answer 33

- affects appearance , takes on stains - affects strength, less strong - rough sensation to the tongue - absorbs saliva , poor hygiene

Question 34

3 reasons for contraction porosity

Answer 34

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