denture base polymers: which is the most widely used denture base material? what are the two types of it?
- polymethyl methacrylate (PMMA)
- heat cured and self cured
heat cured PMMA is made out of?
constituents of each component?
- power & liquid
- powder:
1 polymethylmethacrylate beads
2 initiator: benzoyl peroxide
3 pigments, opacifiers - liquids:
1 methyl methacrylate
2 inhibitor (hydroquinone)
3 cross linking agent (ethylene glycol dimathacrylate
heat curing of PMMA: steps
- mixing of powder and liquid to form what?
- how does the process of curing work?
- what is the activator of the process?
- what happens when temp is raised to 60deg?
- what happens when the process has started?
- forms dough, which is packed in the mould
- free radical addition polymerisation
- the application of heat
- raising of temperature causes breakdown of benzoyl peroxide to form free radicals
- it generates heat and the reaction continues
what kind of reaction occurs on curing? this has consequences on?
what does the cross linking agent help to do?
- exothermic reaction, it has consequences on how the material is cured
- improves the mechanical properties of the cured material
powder-liquid ratio for PMMA?
what happens if there is too much powder?
- 2.5 : 1
- too much powder -> the particles will not be wetted sufficiently, which may result in porosity
what is the contraction shrinkage for:
- monomer only cured?
- powder-liquid?
- 21%
- 5-6%
PMMA - temperature for curing? and for how long? what happens after?
- 7 hrs at 70deg, 3 hours at 100deg
- cooling to room temperature before coming out of mould
properties of an ideal denture base material
- allow attachment of teeth
- good appearance
- ability to achieve a good polish, characterise with stains
- comfortable in oral function
- not too bulky
- absence of taste from material itself
- thermal conductivity: effective insulator a disadvantage
- coverage of palate: will it affect taste?
- biocompatible with soft and hard tissues of denture bearing area
- should not cause sensitivity reactions in oral mucosa
- not affected by extremes of heat - glass transition temp.
why should patients never clean dentures by placing it in boiling water? what should be done?
glass transition temperature is 105deg - this means that the denture base may distort when placed in boiling water
put dentures in water below 65 degrees
other properties of ideal denture base?
- generally good mechanical properties, but some limitations
- dimensionally stable in oral function
- ideally rigid - high value of modulus of elasticity
- elastic limit: need high value to avoid permanent deformation in function
- high impact strength: acrylic poor
- poor flexural strength
- high abrasion resistance: denture cleaners
- inert in mouth: not affected by oral fluids, microorganisms, food materials
- easy to clean
- easily modified if broken or if new teeth added in
- accuracy in construction
why is a correct liquid/power mix needed?
reducing polymerization shrinkage to 5-6%
how does the injection moulding technique minimize shrinkage?
it compensates for shrinkage by allowing additional material from the reservoir into the mould
how to improve impact strength?
- elastomers: may help prevent crack propagation by creating an internal shock absorber mechanism
- metal strengtheners: striking plates, full palates
- inserts - carbon fibres or silicone
how does gaseous porosity happen? how to prevent?
- monomer boils at 100.3 deg
- exothermic reaction on curing
- need to ensure that heat does not go beyond 100deg until most of monomer has cured
crazing:
- what is it?
- how to avoid?
- where on the denture can it occur?
- crack that forms on the surface of the denture which may weaken it
- patient should keep denture in water when not using
- around porcelain teeth in denture base, or around metal inserts (striking plates)
self cured resin: can be used for?
can be used to do chairside modifications to the denture, does not need to go to lab. e.g. modification of immediate dentures
components of self-cured PMMA? what are the components of each?
powder:
- polymethyl methacrylate beads
- initiator: benzoyl peroxie
- pigments, opacifiers
liquid:
- methyl methacrylate
- activator: dimethyl-p-toludine (tertiary amine)
- inhibitor (hydroquinone)
- cross linking agent (ethylene glycol dimethacrylate)
self cured PMMA:
how does curing process start?
heat required?
needs what to cure most effectively?
- activator reacts with benzoyl peroxide to produce free radicals -> start curing
- no
- needs pressure
how do the polymer beads in self cured resin differ from those in the heat cured resin?
how is this a disadvantage to self cured resin?
- size of polymer beads smaller than in heat cured resin to allow dissolution in monomer
- this results in lower glass transition temperature around 75-80deg, meaning that the denture is more susceptible to warpage during use
disadvantages of self cured resin?
- no heat used -> less efficient curing, significant amount of remaining uncured monomer
