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
