heat cured acrylic resin (PMMA) Flashcards
Acrylic dentures error sources
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
ideal properties of a denture base material
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
ideal mechanical properties of a denture base material
high young’s/elastic modulus
- digit
- applying a large stress produces small strain
high proportional/elastic limit
- only large stresses will cause permanent deformation
ideal thermal properties of a denture base material
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
How is PMMA formed?
through free radical addition polymerisation of methacrylate monomer
outline the stages of acrylic polymerisation
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
What initiator is used in the polymerisation of acrylic?
benzoyl peroxide
What activation is used for polymerisation of acrylic?
heat >72 degrees
or self cured
Heat cured acrylic - powder constituents
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
heat cured acrylic - liquid constituents
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
Why is a powder-liquid mix used for polymerising heat cured acrylic?
to produce dough like mixture that can be handled/mixed easily and customised to desired shape
to reduce heat of reaction
minimise polymerisation shrinkage
Factors related to internal stresses of acrylic
thermal expansion
cooling rate
shape and size
curing pressure
curing cycle
under cured heat cured acrylic - consequences
free monomer
- irritant
low molecular weight
- poor mechanical properties
fast cured heat cured acrylic - consequences
possibly gaseous porosity
Incorrect powder/monomer ratios - consequences
too much monomer = contraction porosity
too little monomer = granularity
Porosity effects on acrylic
affects strength
affects appearance
rough sensation to tongue
absorbs saliva - poor hygiene
gaseous porosity - causes and how to avoid
caused by monomer boiling (100 degrees C)
occurs in bulkier parts
to avoid
- be careful and cure slowly
- exothermic reaction
What is contraction porosity?
polymerisation shrinkage
Contraction porosity - causes
too much monomer
insufficient excess material
insufficient clamp pressure
occurs anywhere where dough is not sufficiently packed
acrylic resin properties
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
acrylic softening temperature
75 degrees
- ok for ingested hot fluids
- don’t use boiling water for cleaning
