PMMA Flashcards
what is the measure of success of a denture fit
patient feedback
- natural appearance
3 error sources in denture production
initial impression is flawed – material and method not 100% accurate
curing process - several potential error sources
may not achieve perfect fit at first attempt
REFINEMENTS at the chairside required
- Little adjustments is feasible
initial impression is flawed – material and method not 100% accurate
curing process - several potential error sources
may not achieve perfect fit at first attempt
REFINEMENTS at the chairside required
Little adjustments is feasible
5 error sources in denture usage
fits patient only for short period
- with time and wear fit fails
fractures
uncomfortable
becomes warped – implications
on shape
surface suffers wear
13 ideal properties of denture base material
replaces function of natural teeth
goes into patient’s mouth
needs to be aesthetically pleasing
Dimensionally accurate and stable in use
- must fit patient’s mouth AND be retained
High Softening Temperature (Tg)
- must not distort during ingesting of hot fluids, or during cleaning
Unaffected by Oral Fluids
over time
High Young’s (Elastic) Modulus
rigid (stiff) – large stress produces small strain
High Proportional Limit / Elastic Limit
- only large stresses will cause permanent deformation
Thermal Expansion
= Artificial tooth
- Match denture base and artificial teeth
- avoid internal stresses on cooling during manufacture (more on internal stresses in Metals & Alloys)
take into account internal stresses when manufacturing
High Thermal Conductivity
transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus from hot food/drink
Low Density
- aid retention of upper denture
the heavier the material, greater the gravity pull
Colour / Translucency
to match natural tissues
Non Toxic, Non Irritant
what is the free radical addition polyermisation reaction which occurs in PMMA
Chemical union of TWO molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule
INVOLVES molecules with (C=C bonds)
- Break C=C
- Become a larger, longer molecule so more rigid
what reaction occurs in PMMA on setting
free radical addition polyermisation reaction
4 stages in acrylic polyemrsation
activation
initiation
propagation
termination
what occurs in activation in acrylic polymerisation
of initator to provide free radicals
what occurs in initiation of acrylic polymerisation
free radicals break C=C bond in monomer and transfer free radical
what occurs in propagation acrylic polymerisation reaction
growing polymer chain
what occurs in termination of acrylic polymerisation reaction
final molecule made
what is the initiator in acrylic polymerisation
benzoyl peroxide
C6H5COO-OOCH5C6
symmetrical
what are the 2 types of acrylic addition polymerisation activations
heat above 72 degrees C
or self cured
what does activation of the initiation addition polymerisation reaction achieve
two free radicals C6H5COO*
electrically charged
react with another molecule/monomer
- grows and cross links with other methacrylate monomer
monomer in PMMA
methacrylate
2 components of heat cured accylic
powder
liquid
5 parts of powder in heat cured acrylic
Initiator
- (Benzoyl Peroxide, 0.2 - 0.5%)
PMMA Particles
- pre-polymerised beads
- produced before, ground into beads, mixed into powder, reacting with liquid
Plasticiser
- allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
- improves dissolution of monomer liquid
speed up process
Pigments
- to give “natural” colour
Co-polymers
- to improve mechanical properties e.g. ethylene glycol dimethacrylate
- PMMA monomer cannot do on its own
what is the role of the PMMA particles/beads in the powder of PMMA?
pre-polymerised beads
- produced before, ground into beads, mixed into powder, reacting with liquid
what is the role of the plasticiser in the PMMA powder
- allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
- improves dissolution of monomer liquid
speed up process
what is the role of the co-polymers in the PMMA powder
improve mechanical properties e.g. ethylene glycol dimethacrylate
- PMMA monomer cannot do on its own
3 components of the liquid in PMMA
Methacrylate Monomer
- dissolves PMMA particles – polymerises
Inhibitor (Hydroquinone, 0.006%)
- prolongs shelf life
reacts with any free radicals produced by heat, UV light
- Prevent any free radicals made inadvertently when material stored somewhere
Co-polymers
- improve mechanical properties - particularly cross-linking of polymers
what is the role of the inhibitor in liquid part of PMMA?
- prolongs shelf life
reacts with any free radicals produced by heat, UV light - Prevent any free radicals made inadvertently when material stored somewhere
what is the role of the co polymers in the liquid part of PMMA?
improve mechanical properties - particularly cross-linking of polymers
what is the role of the methacrylate monomer in the liquid part of PMMA?
dissolves PMMA particles – polymerises
what is the result of mixing the liquid and powder components of PMMA?
dough-like material that can be handled/mixed easily and customised to desired shape
reduce heat of reaction
minimise polymerisation shrinkage
- monomer on its own shrink by 21% but with liquid only 7%
- substantial shrinkage factored into heat curing process
2 stages of PMMA mixing
sandy - tacky
dough - packing
how to use PMMA clamp flask
Filed with investment/mould material
- Shaped to pt mouth
- Individual teeth placed to mould material
Brush applied to palate area
Dough packed into investment materials
- Allow excess material at the sides
Clamp together
- Apply sufficient pressure
why is sufficient heat curing required for PMMA
Need efficient polymerisation to give high molecular weight polymer
i.e. good mechanical properties
better the polymerisation the better the material
use a high temperature ideally to make efficient
- but the wrong temperatures can cause porosities in the acrylic
hence high temperature but gaseous porosity limits
different heating schedules depending on the material used
- Stay clear of 100 o C in acrylic to avoid weakness
how should PMMA be cooled
Cool slowly in flask over many hours to allow relief and internal stresses
what can internal stresses in acrylic lead to
Decreased strength
Decreased fatigue
Warping e.g. during finishing
Repair problems
what are some causes internal stresses in acrylic
Shape and size e.g. Notches
Curing pressure
Curing cycle
Thermal expansion
Cooling rate
2 problems if PMMA undercured
Free monomer leak out of denure base
- Pt irritant
Low molecular weight
- Poor mechanical properties
problem if PMMA cured too fast
possible gaseous porosity
if too much monomer in PMMA
see contraction porosity
if too little monomer in PMMA
see granularity of surface material
4 effects of porosity
Affects strength
Affects appearance
- Roughness - Plaque trap
Rough sensation to tongue
Absorbs saliva - poor hygiene
gaseous porosity caused by
monomer boiling
occurs in bulkier parts
- inspect bulky areas for subsurface imperfections
what type of reaction is the free radical addition polymerisation of acrylic
exothermic
so cure slowly to avoid gaseous poroisty
what are 3 causes contraction porosity
excess monomer causing polymerisation shrinkage
insufficient excess material
insufficient clamp pressure
what are the softening temperatures of acrylic like
high
- OK for ingested hot fluids
DON’T use boiling water for cleaning
Will warp acrylic denture so no longer fits
