PMMA
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how do errors arise in acrylic
initial impression flawed, curing process, may not achieve perfect fit, fractures, uncomfortable, warped, surface suffers wear, fits patient for a short period
what are the ideal properties for acrylic dentures?
replaces function of natural teeth, goes into patients mouth, is seen by other people, dimensionally accurate and stable, high softening temperature, unaffected by oral fluids
on the stress strain diagram at what point is the material elastic?
on the straight slope
on the stress strain diagram at what point is the material inelastic?
at the point where the slope changes to a curve
what is young’s modulus?
stress/strain
for acrylic what do you want the elastic modulus to be?
high so the material is rigid (applying large stress produces small strain)
for acrylic what do you want the proportional limit/elastic limit to be?
high so only large stresses will cause permanent deformation
what do you want the thermal expansion to be?
equal to the thermal expansion of the artificial tooth to avoid internal stresses on cooling during manufacture
what do you want the thermal conductivity to be?
high to allow transmission of thermal stimuli to mucosa
what do you want the density to be?
low to aid retention of the upper denture
what is 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
what are the stages of acrylic polymerisation?
activation, initiation, propagation, termination
what happens in activation?
initiator is activated to provide free radicals
what happens in initiation?
free radicals break C=C bonds in monomer and transfer free radicals
what happens in propagation?
growing polymer chain
what happens in termination?
polymerisation is terminated
what is the initiator of polymerisation?
benzoyl peroxide
what does the initiator need to be activated?
heat >72 degrees and self cured
what are the powder components of heat cured acrylic?
initiator, PMMA particles, plasticiser, pigments, co-polymers
what are the PMMA particles in the acrylic powder?
pre-polymerised beads
what does the plasticiser in acrylic powder allow?
quicker dissolving in monomer liquid
what do pigments in heat cured acrylic do?
give natural colour
what do co-polymers do in powder of heat cured acrylic?
improve mechanical properties
what are the components of the liquid in heat cured acrylic?
methacrylate monomer, hydroquinone, co-polymers