heat cured acrylic resin (PMMA)

Question 1

Acrylic dentures error sources

Answer 1

production
- flawed initial impression
- curing process - potential error sources
- may not achieve perfect fit at first attempt
usage
- fractures
- uncomfortable
- surface suffers wear
- fits patient for only a short period

Question 2

ideal properties of a denture base material

Answer 2

dimensionally accurate and stable in use
- must fit in patient’s mouth and be retained
high softening temperature
- must not distort during ingesting of hot fluids or during cleaning
unaffected by oral fluids over time
non toxic/non irritant
low density
- aid retention of upper denture (heavier = greater gravity pull)
colour/translucency

Question 3

ideal mechanical properties of a denture base material

Answer 3

high young’s/elastic modulus
- digit
- applying a large stress produces small strain
high proportional/elastic limit
- only large stresses will cause permanent deformation

Question 4

ideal thermal properties of a denture base material

Answer 4

thermal expansion equal to artificial tooth
- avoids internal stresses on cooling during manufacture
high thermal conductivity
- transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus

Question 5

How is PMMA formed?

Answer 5

through free radical addition polymerisation of methacrylate monomer

Question 6

outline the stages of acrylic polymerisation

Answer 6

activation of initiator to provide free radicals
initiation
- free radicals bread C=C bond iii monomer and transfer free radicals
propagation - growing polymer chain
termination - of polymerisation

Question 7

What initiator is used in the polymerisation of acrylic?

Answer 7

benzoyl peroxide

Question 8

What activation is used for polymerisation of acrylic?

Answer 8

heat >72 degrees
or self cured

Question 9

Heat cured acrylic - powder constituents

Answer 9

initiator
- 0.2-0.5% Benzoyl peroxide
PMMA particles
- pre - polymerised beads
plasticiser
- allows quicker dissolving in monomer liquid
pigments
- to give natural colour
co-polymers
- to improve mechanical properties

Question 10

heat cured acrylic - liquid constituents

Answer 10

methacrylate monomer
- dissolves PMMA particles - polymerises
inhibitor - hydroquinone 0.006%
- prolongs shelf light - reacts with any free radicals produced by heat or UV light
co-polymers
- improves mechanical properties - particularly cross linking of polymers

Question 11

Why is a powder-liquid mix used for polymerising heat cured acrylic?

Answer 11

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

Question 12

Factors related to internal stresses of acrylic

Answer 12

thermal expansion
cooling rate
shape and size
curing pressure
curing cycle

Question 13

under cured heat cured acrylic - consequences

Answer 13

free monomer
- irritant
low molecular weight
- poor mechanical properties

Question 14

fast cured heat cured acrylic - consequences

Answer 14

possibly gaseous porosity

Question 15

Incorrect powder/monomer ratios - consequences

Answer 15

too much monomer = contraction porosity
too little monomer = granularity

Question 16

Porosity effects on acrylic

Answer 16

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

Question 17

gaseous porosity - causes and how to avoid

Answer 17

caused by monomer boiling (100 degrees C)
occurs in bulkier parts
to avoid
- be careful and cure slowly
- exothermic reaction

Question 18

What is contraction porosity?

Answer 18

polymerisation shrinkage

Question 19

Contraction porosity - causes

Answer 19

too much monomer
insufficient excess material
insufficient clamp pressure
occurs anywhere where dough is not sufficiently packed

Question 20

acrylic resin properties

Answer 20

non toxic
non irritant
- if no monomer released
- some patients may be allergic
unaffected by oral fluids
good colour/translucency
thermal expansion = artificial tooth
- only if acrylic teeth used - significantly higher than porcelain teeth
low thermal conductivity
- poor
poor mechanical properties
- increase in bulk to compensate
low density
- needed in bulk to overcome poor mechanical properties
- this offsets advantage
dimensionally accurate and stable in use
- linear contraction 0.5% - acceptable

Question 21

acrylic softening temperature

Answer 21

75 degrees
- ok for ingested hot fluids
- don’t use boiling water for cleaning