PMMA Flashcards
What are the ideal properties of PMMA
- dimensionally accurate and stable in use
- high softening temp (must not distort when drinking hot drinks/ when cleaning)
- unaffected by oral fluids over time
- high young’s modulus (rigid)
- high elastic limit (only large stresses will cause permanent deformation)
- thermal expansion similar to artificial tooth
- high thermal conductivity (to transmit thermal stimuli to mucosa)
low density - colour matches natural tissues
- non toxic, non irritant
what does PMMA stand for
Polymethylmethacrylate
what does elastic limit mean and do we want PMMA to have a low or high one
the stress which beyond which the material will become deformed because when you release that force the material will not restore
a high one (so even on high stresses it won’t deform)
what reaction does PMMA undergo
free radical addition polymerisation
define free radical addition polymerisation
Chemical union of TWO molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule
why do we need free radical addition polymerisation
Looking to break down double bond (C=C) so it links to other molecules and the material becomes rigid. We want cross linking to happen
Happens in stages
what undergoes free radical addition polymerisation
methacrylate monomer
what are the 4 stages in acrylic polymerisation (define them)
- Activation - of inhibitor to provide free radicals
- Initiation - free radicals break C=C bond in monomer and transfer free radical
- Propagation - growing polymer chain
- Termination - of polymerisation
what is used as an initiator
benzoyl peroxide
how is it activated
- heat >72oC
- self cured
what are the free radicals which are produced
2 radicals:
C6H5COO* (?)
(i think these are produced by the breaking down of benzoyl peroxide)
what happens following free radical addition polymerisation
propagation
keeps it’s electrical charge, so propagates, original molecule just keeps getting longer
What are the two components and how are they brought together
Powder and liquid
heat-curing
What is inside the powder
- initiator (Benzoyl peroxide)
- PMMA particles (pre-polymerised beads)
- plasticiser - allows quicker dissolving in monomer liquid
- pigments - gives natural colour
- co-polymers - to improve mechanical properties
what is inside the liquid
- methacrylate monomer- dissolves PMMA particles
- inhibitor (hydroquinone) - prolongs shelf life, reacts with any free radicals produced by heat to prevent them, UV light
- co-polymers - improve mechanical properties, particularly cross-linking of polymers
why do we have the mix of powder and liquid
- to produce dough-like material that can be handled/mixed easily and customised to desired shape
- reduce heat of reaction
- minimise polymerisation shrinkage
what are the mixing stages for the powder and liquid mix
sandy - tacky
dough - packing
how is the mixing carried out with the powder and liquid
- brush applied giving separation
- pack dough into investment material
- important to allow for excess material
- take ‘flasks’ and clamp together
- controlled heating schedules
why is it important to have efficient polymerisation
to give high molecular weight polymer i.e. good mechanical properties
why is it important that heating is controlled
because the wrong temperatures could result in porosities which aren’t acceptable
what temp does the majority of the heating happen at
70oC
what temp should be stayed clear of during heating of acrylic
100oC
why does the flask need to cool down gradually following curing
because of the possibility of internal stresses
what would internal stresses (if not cooled gradually) during cooling result in
- decreased strength
- decreased fatigue strength
- warping e.g. during finishing
- repair problems
what would internal stresses affect
- shape and size e.g. notches
- curing pressure
- curing cycle
- thermal expansion
- cooling rate
what could under curing result in
- free monomer (irritation)
- low molecular weight (poor mechanical properties)
what could fast curing result in
possibly gaseous porosity
what could incorrect powder/monomer ratios result in
too much monomer:
- contraction porosity
too little monomer:
- granularity
what can porosities affect
- strength
- appearance
- rough sensation to tongue
- absorbs saliva (poor hygiene)
where do gaseous porosities often occur
in bulkier parts
how do we avoid gaseous porosities
cure slowly
- be careful of exothermic reaction
- avoid temps above 100oC
what is the affect of adding the powder to the monomer on polymerisation shrinkage
- monomer alone = 21%
- powder and monomer mix = 7%
greatly reduces
what are the causes of contraction porosities
- polymerisation shrinkage
- too much monomer
- insufficient excess material
- insufficient clamp pressure
- occurs anywhere where dough is not sufficiently packed
is acrylic non-toxic
yes
is acrylic non-irritant
- provided no monomer released
- some patients allergic
how affected is acrylic to oral fluids
- virtually insoluble in fluids taken orally
how are the thermal expansion properties of acrylic
we want:
thermal expansion = artificial tooth
actually:
OK, if acrylic teeth used, significantly higher than porcelain teeth
how are the thermal conductivity properties of acrylic
we want:
high
actually:
low (poor)
how are the mechanical properties of acrylic
(YM, PL, EL)
poor, increase in bulk to compensate
how are the colour/transleucent properties of acrylic
good
how are the densitiy properties of acrylic
- low (good)
- but, need to increase in bulk to overcome poor mechanical properties offsets this advantage
how are the softening temperature properties of acrylic
we want:
high
actually:
- 75oC
- ok for ingested hot fluids
- don’t use boiling water for cleaning
is acrylic dimensionally accurate and stable in use?
OK
- linear contraction 0.5% (acceptable)
what is acrylic resin most commonally used for
denture base material
Identify the incorrect statement regarding PMMA:
A. Acrylic polymerisation involves activation, initiation, propagation and termination
B. Efficient polymerisation leads to high molecular weight polymers and good mechanical properties
C. Too much monomer leads to contraction porosity
D. Self Cure acrylics cure at a lower temperature, meaning less thermal contraction and better dimensional accuracy
E. Self cure acrylics have a more efficient activation, so the polymers are of greater molecular weight and there are better mechanical properties
E. Self cure acrylics have a more efficient activation, so the polymers are of greater molecular weight and there are better mechanical properties
