PMMA

Question 1

what are the error sources in production of acrylic resin dentures?

Answer 1

initial impression flawed
-material/method inaccurate
curing process
imperfect fit
-chairside refinements

Question 2

what are the error sources of dentures in usage?

Answer 2

fractures
uncomfortable
warping
surface wear
fits for short period

Question 3

what are the basic requirements of a denture base material?

Answer 3

replaces function of natural teeth
goes into mouth
seen by other people

Question 4

what are the ideal properties?

Answer 4

dimensionally accurate and stable in use
high softening temperature
-hot foods/cleaning
unaffected by oral fluids
thermal expansion=to artificial tooth
high thermal conductivity
low density
non toxic/irritant

Question 5

why is a high proportional/elastic limit important?

Answer 5

only large stresses will cause permanent deformation

Question 6

what is a free radical addition polymerisation?

Answer 6

chemical union of 2 molecules either the same/dif to form a larger molecule w/o the elimination of a smaller molecule
-involves molecules w/ C=C

Question 7

what is the polymerisation molecule in PMMA?

Answer 7

methacrylate monomer

Question 8

what are the stages of acrylic polymerisaion?

Answer 8

activation
-of initiatory to provide free radicals
initiation
-free radicals break C+C bone in monomer and transfer free radicals
propagation
-growing polymer chain
termination

Question 9

what is the initiator?

Answer 9

benzoyl peroxide

Question 10

what is the activation?

Answer 10

heat >72 celcius

or self cured

Question 11

describe acrylic propagation?

Answer 11

electrically charged radical reacts with monomer
charge transferred to monomer
this reacts with another monomer and charge transferred to monomer end of molecule
grows in size, weight and strength

Question 12

what are the components of the powder?

Answer 12

initiator
PMMA particles
plasticiser
-quicker dissolving in monomer liquid
pigments
-natural colour
co-polymers
-improve mechanical properties

Question 13

what are the components of the liquid?

Answer 13

methacrylate monomer
-dissolved PMMA particles
inhibitor
-prolongs shelf life
-reacts with any free radicals produced by heat/UV light
co-polymers
-improve mechanical properties
-particularly cross-linking of polymers

Question 14

why is powder/liquid mix used?

Answer 14

to produce dough-like material that can be handled/mixed easily and customised to desired shape
reduce heat of reaction
minimise polymerisation shrinkage

Question 15

why is efficient polymerisation needed?

Answer 15

to give high molecular weight polymer

i.e. good mechanical properties

Question 16

what are the 3 heating schedules?

Answer 16

7 hrs 70 C, 2 hrs 100C
72*C 16hrs
20-20-20 reverse curing

Question 17

what is cooling used for?

Answer 17

mould material & acrylic

• dif thermal expansion coefficients
• internal stresses
• cool slowly

Question 18

what are possible internal stresses?

Answer 18

low strength
low fatigue strength
warping
repair problems

Question 19

what are internal stresses related to?

Answer 19

thermal expansion
cooling rate
shape and size
curing pressure
curing cycle

Question 20

what happens if acrylic is under cured?

Answer 20

free monomer
-irritant
low molecular weight
-poor mechanical properties

Question 21

what happens if acrylic is fast cured?

Answer 21

gaseous porosity

Question 22

what happens if the powder/monomer ratio is incorrect?

Answer 22

too much monomer
-contraction porosity
too little monomer
-granularity

Question 23

what does porosity affect?

Answer 23

strength
appearance
rough sensation to tongue
absorbs saliva
-poor hygiene

Question 24

what are the properties of acrylic?

Answer 24

non toxic
non irritant if no monomer release, can be allergy
unaffected by oral fluids
good colour/translucency
thermal expansion=acrylic artificial tooth, not porcelain
low thermal conductivity :(
low density
poor mechanical properties
reasonably high softening temp
ok dimensionally accurate & stable

Question 25

what is contraction porosity?

Answer 25

polymerisation shrinkage
monomer=21%
powder+monomer=7%
causes:
-too much monomer
-insufficient excess material
-insufficient clamp pressure