acrylic resin Flashcards
ideal properties acrylic resin
Non Toxic, Non Irritant
Be of an acceptable Colour / Translucency
Be Easy & inexpensive to manufacture
Be Easy to repair – as the acrylic is not indestructible
Has to cope with impacts
polymerisation of PMMA
Acrylic resin undergoes FREE RADICAL ADDITION POLYMERISATION. ie
The Chemical union of two molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule.
It needs C to C double bonds.
heat cured acrylics - powder
Initiator (Benzoyl Peroxide, 0.2 - 0.5%)
PMMA Particles – pre-polymerised beads
Plasticiser - allows quicker dissolving in monomer liquid eg dibutyl phthalate
Pigments – to give “natural” colour
Co-polymers - to improve mechanical properties eg ethylene glycol dimethacrylate
heat cured acrylics - 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
the technique for heat curing acrylic resin
acrylic in its dough-like form is inserted into the mould recess…taking up the shape of the patient’s dentition.
Now the acrylic needs to be CURED to form a strong solid denture base
The two halves of the vessel are clamped together. Ready to be subjected to the heating cycle required to cause polymerization.
It’s essential there is efficient polymerisation – lots of crosslinking of the MMA monomers - that will produce a high molecular weight polymer, one with good mechanical properties.
expected acrylic properties
non toxic
non irritant
cannot absorb oral fluids
acrylic mechanical properties
in terms of mechanical properties though, acrylic is poor
Acrylic resin is able to withstand impacts reasonably well – but it’s far from indestructible.
It’s fatigue strength – the ability to resist repeated application of low level stresses – eg during mastication- is reasonable too.
It resists abrasion quite well, keeping its polished finish. But over prolonged use surface layers may be removed, affecting its appearance;
and causing it to feel rough when contacted by a patient’s tongue.
If acrylic teeth are used then obviously the expansion and contraction won’t be a problem; but if porcelain teeth used then there will be a mismatch as they will expand less than the acrylic resin denture base.
One of the clear disadvantages of acrylic resin is its thermal conductivity – it’s low.
self-cured acrylics
It has a similar composition to the heat cured version
BUT here a TERTIARY AMINE in the liquid activates the initiator BENZOYL PEROXIDE – not heat
chemical activation of acrylic yields less efficient polymerisation
differences between heat and self-cured acrylic resin
Heat cured has a higher Molecular weight – making it stronger – will survive longer – more convenient for patient
But the heat curing process may cause Porosity – this is where the technician’s skills are invaluable to prevent this;
As a dentist you should examine each denture thats produced – check for flaws (ie bubble below surface)
As for the Self cured PMMA:
More monomer is uncured so there’s more risk of it being an irritant – which can have a significant impact on patient’s soft tissue;
You need to inform patient of this risk and instruct them to notify you asap if there are any signs
Although SC - fits its cast better, water absorption in mouth makes oversized
SC also has poorer colour stability (tertiary amines susceptible to oxidation)
Neither is ideal !
improved form of acrylics
One type incorporated rubber toughening agent (butadien styrene) – to stop cracks forming in the denture base when it experienced an impact.
Another type had fibres – made up of different sorts of materials eg carbon and ultra-high molecular wt polyethylene, and glass
These materials have proven to be difficult to manufacture.
improved version high impact heat cure acrylic resin
is used here in GDH.
The new ingredients give the acrylic a greater degree of ductility which helps to mitigate the effect of any micro-cracks that may be present.
And it also gives it greater flexural strength – increasing its chances of surviving for longer.
another version of PMMA
is “pour n cure” acrylic resin. This uses smaller powder particles such that it produces a fluid mix – not a dough like form.
The fluid mix is poured into the mould
however the mechanical performance is inadequate
summary acrylic resin
Heat cured acrylic resin is still the most commonly used material for dentures.
Their mechanical properties are satisfactory – though this sometimes involves workarounds such as using them in greater bulk;
And accepting that care needs to be exercised when a patient ingests hot drinks.
No serious alternative has been developed.
