PMMA

Question 1

how do errors arise in acrylic

Answer 1

initial impression flawed, curing process, may not achieve perfect fit, fractures, uncomfortable, warped, surface suffers wear, fits patient for a short period

Question 2

what are the ideal properties for acrylic dentures?

Answer 2

replaces function of natural teeth, goes into patients mouth, is seen by other people, dimensionally accurate and stable, high softening temperature, unaffected by oral fluids

Question 3

on the stress strain diagram at what point is the material elastic?

Answer 3

on the straight slope

Question 4

on the stress strain diagram at what point is the material inelastic?

Answer 4

at the point where the slope changes to a curve

Question 5

what is young’s modulus?

Answer 5

stress/strain

Question 6

for acrylic what do you want the elastic modulus to be?

Answer 6

high so the material is rigid (applying large stress produces small strain)

Question 7

for acrylic what do you want the proportional limit/elastic limit to be?

Answer 7

high so only large stresses will cause permanent deformation

Question 8

what do you want the thermal expansion to be?

Answer 8

equal to the thermal expansion of the artificial tooth to avoid internal stresses on cooling during manufacture

Question 9

what do you want the thermal conductivity to be?

Answer 9

high to allow transmission of thermal stimuli to mucosa

Question 10

what do you want the density to be?

Answer 10

low to aid retention of the upper denture

Question 11

what is free radical addition polymerisation?

Answer 11

chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule

Question 12

what are the stages of acrylic polymerisation?

Answer 12

activation, initiation, propagation, termination

Question 13

what happens in activation?

Answer 13

initiator is activated to provide free radicals

Question 14

what happens in initiation?

Answer 14

free radicals break C=C bonds in monomer and transfer free radicals

Question 15

what happens in propagation?

Answer 15

growing polymer chain

Question 16

what happens in termination?

Answer 16

polymerisation is terminated

Question 17

what is the initiator of polymerisation?

Answer 17

benzoyl peroxide

Question 18

what does the initiator need to be activated?

Answer 18

heat >72 degrees and self cured

Question 19

what are the powder components of heat cured acrylic?

Answer 19

initiator, PMMA particles, plasticiser, pigments, co-polymers

Question 20

what are the PMMA particles in the acrylic powder?

Answer 20

pre-polymerised beads

Question 21

what does the plasticiser in acrylic powder allow?

Answer 21

quicker dissolving in monomer liquid

Question 22

what do pigments in heat cured acrylic do?

Answer 22

give natural colour

Question 23

what do co-polymers do in powder of heat cured acrylic?

Answer 23

improve mechanical properties

Question 24

what are the components of the liquid in heat cured acrylic?

Answer 24

methacrylate monomer, hydroquinone, co-polymers

Question 25

what do methacrylate monomers do in the liquid of heat cured acrylic?

Answer 25

dissolve PMMA particles

Question 26

what does hydroquinone do in heat cured acrylic?

Answer 26

prolongs acrylic shelf life by reacting with free radicals produced by background heat

Question 27

what do co-polymers allow?

Answer 27

cross linking with polymers to improve acrylic mechanical properties

Question 28

what does the powder/liquid mix of acrylic allow?

Answer 28

produces dough like material which is handled easily and customised to shape, minimises polymerisation shrinkage, reduces heat reaction

Question 29

why do you need efficient polymerisation?

Answer 29

to give high molecular weight (good mechanical properties)

Question 30

what are the heating schedules for acrylic?

Answer 30

1 - 7hrs at 70degrees, 2hrs at 100degrees, slow cool
2 - 16hrs at 72degrees
3 - 20-20-20 (remove heat for 20mins, heat to 70degrees for 20mins, heat to 100degrees for 20mins)

Question 31

what happens if the temperature of acrylic curing exceeds 100 degrees?

Answer 31

it turns gaseous and many voids are created which is undesirable

Question 32

what do internal stresses do?

Answer 32

decrease strength and fatigue strength (low level forces applied repeatedly over a long time), warping, repair problems

Question 33

how do internal stresses occur?

Answer 33

thermal expansion, cooling rate, shape and size, curing pressure, curing cycle

Question 34

what happens if acrylic is undercured?

Answer 34

free monomer produced (irritant) and material has low molecular weight so poor mechanical properties

Question 35

what happens if acrylic is fast curing?

Answer 35

possibly gaseous porosity

Question 36

what happens if there is too much monomer present in powder/monomer ratio?

Answer 36

contraction porosity

Question 37

what happens if there is too little monomer in powder/monomer ratio?

Answer 37

granularity

Question 38

what does porosity do?

Answer 38

affects strength, appearance, rough sensation to tongue, absorbs saliva so poor hygiene

Question 39

when does gaseous porosity occur?

Answer 39

when monomer is boiling at 100 degrees

Question 40

where does gaseous porosity occur?

Answer 40

in bulkier parts of the denture

Question 41

what is contraction porosity?

Answer 41

polymerisation shrinkage

Question 42

what causes contraction porosity?

Answer 42

too much monomer, insufficient excess material, insufficient clamp pressure

Question 43

where does contraction porosity occur?

Answer 43

anywhere dough is not sufficiently packed

Question 44

in reality what is the thermal conductivity of acrylic?

Answer 44

low which is poor

Question 45

why should you not use boiling water to clean a denture?

Answer 45

because the softening temperature of acrylic is 75 degrees so it would warp