3 - Acrylic resin Flashcards
What are the ideal properties of acrylic resin? (11)
- dimensionally accurate
- high softening temperature
- high thermal conductivity
- unaffected by oral fluids
- non toxic
- stable in use
- low density
- radiopaque
- variable colour
- easy to repair
- inexpensive
What is transverse strength?
- flexural strength
- 3 point loading at the pivot point
- relevant In the palate aspect of denture bases (2 masticatory forces upwards, force from palate downwards)
Describe the polymerisation of PMMA.
- free radical addition polymerisation
- union of two molecules without the elimination of a small molecule
What are the steps of polymerisation?
- activation (initiator provides free radicals)
- initiation (free radicals form C=C bonds)
- propagation (growing polymer chain)
- termination
What are the components of heat cured acrylic? (5+3)
POWDER
- benzoyl peroxide (initiator)
- PMMA particles
- dibutyl phthalate (plasticiser)
- pigment
- ethylene glycol dimethacrylate (co-polymers)
LIQUID
- methacrylate monomer
- hydroquinone (inhibitor, requires UV activation)
- co-polymers
Describe the mechanical properties of heat cured acrylic. (7)
- good fatigue and impact strength
- high hardness
- thermal expansion matches acrylic teeth
- high softening temperature (75 degrees)
- low thermal conductivity
- low density
- dimensionally accurate and stable
How does the composition of self cured acrylic differ from that of heat cured acrylic?
- the same with the addition of a promoter
- tertiary amine
- eg dimethyl-para-toludine
Describe the properties of self cured acrylic.
- better dimensional accuracy to heat cured
- chemical activation is less efficient = more irritant
- lower molecular weight = poorer mechanical properties and softening temperature is lower
What can be added to acrylic resin dentures to strengthen them?
- high impact materials
- rubber toughening agent (butadiene styrene)
- fibres (carbon, glass, ultra-high molecular weight polyethylene (UHMPE))
What is an example of a radiopaque monomer?
Barium sulphate
Why is nylon a poor denture material?
Imbibition causes swelling and softening
Why are vinyl polymers poor denture materials?
- injection moulding is expensive
- low softening temperature
Why are polycarbonates poor denture materials?
- injection moulding is expensive
- high softening temperature BUT
- internal stresses cause distortion and a poor fit
