PMMA Flashcards
what is transverse strength
3 point loading (flexural). acrylic is being flexed and pivot point is palate so when forces either side are applied this can potentially cause fracture at pivot point
what is free radical addition polymerisation
chemical union of 2 molecules either the same or different to form a larger molecule without the elimination of a smaller molecule. involves molecules with C=C double bonds
what is the polymerisation process of acrylic resin
activation - of initiator to provide free radicals
initiation - free radicals break C=C in monomer and transfer free radical
propagation - growing polymer chain
termination - of polymerisation
heat cured acrylic - powder
- initiator = benzoyl peroxide 0.2-0.5%
- PMMA particles = pre-polymerised beads
- plasticiser = allows quick dissolving in monomer liquid e.g. dibutyl phthalate
- pigments = to give natural colour
- co-polymers = to improve mechanical e.g. ethylene glycol dimethacrylate
heat cured acrylic - liquid
- methacrylate monomer = dissolves PMMA particles; polymerises
- inhibitor = hydroquinone 0.006%; prolongs shelf life reacts with any free radicals produced by heat, UV light
- co-polymers = improve mechanical properties, particularly cross linking of polymers
what do you need when heat curing acrylic
need efficient polymerisation i.e. lots of crosslinking of MMA monomers to produce a high molecular weight polymer with good mechanical properties hence high temp but this has gaseous porosity limits
acrylic properties (4)
- non toxic
- non irritant - if no monomer released, few patients are allergic
- unaffected by oral fluids - water absorption, virtually insoluble in fluids taken orally
- mechanical properties - poor so increase bulk to compensate
PMMA mechanical properties
impact strength - reasonable
high hardness / abrasion resistance
TEC - ok if acrylic teeth used
thermal conductivity - low
density - low which is good but increase in bulk to overcome poor mechanical properties
high softening temp
dimensional accuracy & stable in use
what is impact strength
ability to resist repeated application of low level stress e.g. mastication on denture base
difference in self cure and heat cure acrylic
self cure has similar composition to heat cure except benzoyl peroxide is activated by a promoter (a tertiary amine) e.g. dimethyl-para-toluidine, not heat
benefit of self cure as opposed to heat cure
polymerisation stage requires no heating stage so little thermal contraction would be experienced leading to a better fitting and more dimensionally accurate denture
issues with self cure acrylic
chemical activation yields less efficient polymerisation hence a lower molecular weight so poorer mechanical properties and Tg
also leaves unreacted monomer which acts as a plasticiser softening denture base and reducing transverse strength; this can potentially be an irritant thus compromising biocompatibility
self cure vs heat cure
HC - higher molecular weight; stronger, curing process may cause porosity
SC - higher monomer levels; may be irritant, fits cast better but water absorption in mouth makes it over sized, poorer colour stability; tertiary amines more susceptible to oxidation, other properties same as HC
improved acrylic as an alternative
it incorporates high impact resistance materials but has long term fatigue problems or difficult processing to incorporate fibres
ultra-hi as an alternative
high impact heat cure acrylic resin which has better flexural strength + ductility which helps mitigate effect of microcracks - used in GDHS
