PMMA - polymethylmethacrylate

Question 1

why may dentures not fit the first time?

Answer 1

production
* initial impression is not accurate
* curing process of acrylic
* may need chairside refinements
usage
* fractures - handling and masticatory forces
* uncomfortable
* becomes warped
* surfaces suffers wear
* fits patient only for a short period

Question 2

what are the ideal properties of PMMA?

6

Answer 2

• dimensionally accurate and stable in use (fit and retention)
• high softening temperature (Tg - doesn’t distort when cleaning/drinking hot liquids)
• unaffected by oral fluids over time
• low density to aid retention
• colour/translucency to match natural tissues
• non-toxic and non-irritant

Question 3

what are the ideal mechanical properties of PMMA?

Answer 3

• high young’s (elastic) modulus - rigid (large stress produces small strain)
• high proportional limit / elastic limit - only large stresses will cause permanent damage

Question 4

what are the ideal thermal properties of PMMA?

Answer 4

• thermal expansion should match the thermal expansion of the artificial tooth
• avoid internal stresses on cooling during manufacture
• high thermal conductivity to mucosa to avoid scalding of throat and mucosa

Question 5

describe how PMMA is produced and the steps

Answer 5

free radical additional polymerisation
* activation - heat >72degres or self cured to give free radicals
* initiation - free radicals break C=C bond in monomer and transfer free radicals
* propagation - growth of polymer chain
* termination - end of polymerisation

Question 6

what are the components of heat cured acrylic?

Answer 6

powder
* initiator - benzoyl peroxide
* PMMA particles
* plasticiser
* pigments
* co-polymers
liquids
* methacrylate monomer
* inhibitor - hydroquinone
* co-polymers

Question 7

why would you use powder/liquid PMMA?

Answer 7

• produces a dough like material than can be handled/mixed easily and customised
• reduced heat of reaction
• minimise polymerisation shrinkage

Question 8

what are the three heating schedules for PMMA?

Answer 8

1. 7 hours to 70 degrees + 2 hours to 100 degrees + slow cool
2. 72 degrees for at least 16 hours
3. 20-20-20 reverse curing (place flask in boiling water, remove heat for 20mins, heat to 70degrees for 20mins, heat to 100degrees for 20mins)

polymerisation exotherm at 2 hours

Question 9

how shoudl acrylic be cooled?

Answer 9

• should be cooled slowly to allow relief
• if cooled too quickly, can lead to internal stresses in acrylic
• mould material and PMMA have different thermal expansion coefficients so contract at different rates when cooling

Question 10

what can happen if PMMA is cooled too quickly? (internal stress)

Answer 10

• decreased compressive and tensile strength
• decreased fatigue strength (low level repeated stresses over a long time can cause fracturing)
• warping e.g. during finishing
• minor repairs later on are problematic

Question 11

what factors are related to internal stresses?

Answer 11

• thermal expansion
• cooling rate
• shape and size e.g. notches
• curing pressure
• curing cycle

Question 12

what are some problems with curing?

Answer 12

• under cured - free monomers are irritants and low molecular weight = poor mechanical properties
• fast curing - possibel gaseous porosity

Question 13

what are some problems with incorrect powder/monomer ratios?

Answer 13

• **too much **monomer = contraction porosity
• too little monomer = granularity

Question 14

how does porosity affect PMMA?

Answer 14

• decreases strength
• affects appearance
• rough sensation to tongue
• absorbs saliva - poor hygiene

Question 15

how does gaseous porosity form in PMMA? how would you prevent this?

Answer 15

• develops during the heat curing stage
• monomer boiling at 100degrees and produced bubbles to the PMMA surface especially in bulkier parts
• cure slowly to prevent (curing is exothermic)

Question 16

what are the causes of contraction porosity?

Answer 16

• too much monomer
• insufficient excess material
• insufficient clamp pressure
• where PMMA is not sufficiently packed

Question 17

what are the properties of PMMA?

Answer 17

• non-toxic :)
• non-irritant :)
• unaffected by oral fluids :)
• good colour/translucency :)
• thermal expansion = artificial tooth :/
• low thermal conductivity :(
• poor mechanical properties :(
• low density but increased bulk to improve mechanical properties offsets this :/
• high softening temperature (75degrees) :)
• dimensionally accurate and stable in use :)

mechanical properties