PMMA Flashcards
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
what do methacrylate monomers do in the liquid of heat cured acrylic?
dissolve PMMA particles
what does hydroquinone do in heat cured acrylic?
prolongs acrylic shelf life by reacting with free radicals produced by background heat
what do co-polymers allow?
cross linking with polymers to improve acrylic mechanical properties
what does the powder/liquid mix of acrylic allow?
produces dough like material which is handled easily and customised to shape, minimises polymerisation shrinkage, reduces heat reaction
why do you need efficient polymerisation?
to give high molecular weight (good mechanical properties)
what are the heating schedules for acrylic?
1 - 7hrs at 70degrees, 2hrs at 100degrees, slow cool
2 - 16hrs at 72degrees
3 - 20-20-20 (remove heat for 20mins, heat to 70degrees for 20mins, heat to 100degrees for 20mins)
what happens if the temperature of acrylic curing exceeds 100 degrees?
it turns gaseous and many voids are created which is undesirable
what do internal stresses do?
decrease strength and fatigue strength (low level forces applied repeatedly over a long time), warping, repair problems
how do internal stresses occur?
thermal expansion, cooling rate, shape and size, curing pressure, curing cycle
what happens if acrylic is undercured?
free monomer produced (irritant) and material has low molecular weight so poor mechanical properties
what happens if acrylic is fast curing?
possibly gaseous porosity
what happens if there is too much monomer present in powder/monomer ratio?
contraction porosity
what happens if there is too little monomer in powder/monomer ratio?
granularity
what does porosity do?
affects strength, appearance, rough sensation to tongue, absorbs saliva so poor hygiene
when does gaseous porosity occur?
when monomer is boiling at 100 degrees
where does gaseous porosity occur?
in bulkier parts of the denture
what is contraction porosity?
polymerisation shrinkage
what causes contraction porosity?
too much monomer, insufficient excess material, insufficient clamp pressure
where does contraction porosity occur?
anywhere dough is not sufficiently packed
in reality what is the thermal conductivity of acrylic?
low which is poor
why should you not use boiling water to clean a denture?
because the softening temperature of acrylic is 75 degrees so it would warp
