PMMA Flashcards
what are the denture errors often based on
production
usage
how does production result in an unfitting denture
o The initial impression is flawed as there is no material nor method that is 100% accurate
o The curing process has severe potential error sources
o You may not achieve the perfect fit at first attempt and sometimes refinements may be required at chairside
how does usage result in an unfitting denture
o Fits the patient only for a short period and they may feel overtime that it doesn’t fit them
o Fractures during use or by accident
o Uncomfortable
o Becomes warped
o Surface suffers wear, roughens and becomes uncomfortable
what are the ideal properties of a denture base material
dimensionally accurate and stable in use high softening temperature unaffected by oral fluids overtime high young modulus high proportional limit/elastic limit thermal expansion = artificial teeth high thermal conductivity low density natural colour non toxic/irrititant
why is it important that the material is dimensionally accurate and stable in use
o Must fit patients mouth AND be retained
why is it important that the material has a high softening temperature
so it does not distort during ingestion of hot fluids or during cleansing
why is it important that there is a high young modulus
o Want it to be rigid so that large stresses produce small strain so that it doesn’t change shape
why is it important that there is a high proportional limit/elastic limit
o Only large stresses will cause permanent deformation
why do we want thermal expansion = artificial teeth
we want to avoid internal stresses on cooling during manufacture
why do we want high thermal conductivity
o We want transmission of thermal stimuli to mucosa to avoid scalding of back of the throat or oesophagus
why do we want a low density
o To aid retention of upper denture as the heavier the material the greater the gravity pull
how is an acrylic resin denture made
made by the process of free radical addition polymerization to form poly methyl methacrylate.
what is the monomer
methyl methacrylate.
what is free radical addition poylermisation
the chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule. It involves molecules with C=C bonds.
what bond are we looking to break
the double bond
what are the stages of acrylic polymerization
- Activation of the initiator to provide free radicals
- Initiation of the free radicals to break the C=C bond in the monomer and transfer free radical
- Propagation of the growing polymer chain
- Termination of polymerization
what is the initiator
is benzoyl peroxide – C6H5COO-OOCH5C6
how can activation occur
can be through heat (>72 degrees celcius) or through self cure.
what is the difference between self cure and heat cure
The chemistry of the self cure resin is identical to that of the heat cured resins except that the cure is initiated by a tertiary amine rather than heat.
what does activation provide
C6H5COO*
what are the 2 components of PMMA
powder
liquid
brought together in deep curing process
what does the powder consist of
initiator PMMA particles plasticizer pigments co-polymers
what are the PMMA particles
pre-poylmerised beads
o PMMA that has been produced before, ground into beads that is put into the powder itself which will react with the liquid
what is the plasticizer
o Will allow for quicker dissolving in monomer liquid e.g dibutyl phthalate
o The improved dissolution of the monomer liquid speeds up the process
what do the pigments do
give natural color
what are the co-poylmers
improve mechanical properties e,g ethylene glycol dimethacrylate
what does the liquid consist of
methacrylate monomer
inhibitor
copolymer
what does methacrylate monomer do
dissolves PMMA particles (polymerises)
what is the inhibitor
(hydroquinone, 0.006%)
what does the inhibitor do
o Prolongs shelf life – reacts with any free radicals produced by heat, UV light. this prevents any free radicals that are produced inadvertently
what do the co-polymers do
o Improve mechanical properties – particularly cross linking of polymers
what is the reason that there is a powder/liquid mix
- To produce a dough like material that can be handled/mixed easily and customized to the desired shape.
- It also reduces the heat of the reaction
- Minimizes polymerization shrinkage
what are the portions of powder to liquid by volume
3-3.5/1
what are the proportions of powder to liquid by weight
2.5/1
what are the mixing stages
sandy (tacky)
dough (packing)
what is the heat cure technique
A flask containing the teeth set in plaster is placed with the dough and then closed under pressure such that the excess dough is squeezed out.
In addition, by adapting the dough to the model and trimming off any excess, cold-cure varieties of the acrylics are easily manipulated (when in the doughy stage) to produce special trays.
how is polymerization shrinkage reduced
the beads have already been polymerized
why should the container be sealed at all times and in a dark glass bottle
The monomer is extremely volatile and highly flammable so the container must be kept sealed at all times and must be kept away from naked flames. The container is a dark glass bottle which extends the shelf life of the monomer by avoiding spontaneous polymerisation from the action of light.
what are examples of heating schedules
- 7 hours to 70 degrees celcius + 2 hours to 100 degrees + slow cool
- 72 degrees for at least 16 hours
- 20-20-20 reverse curing
what is 20-20-20 reverser curing
o Place flask in boiling H2O
o Remove heat for 20 minutes
o Heat to 70 degrees for 20 minutes
o Then heat to 100 degrees for 20 minutes
why should the temperature not go above 100 degrees
gaseous porosities will be produced
why do internal stresses occur
The mould material and acrylic have different thermal expansion coefficients and so there is the possibility that internal stresses will be made inside the acrylic so to minimize that you have to cool the flask gradually to allow relief.
what can internal stresses cause
- Reduced strength
- Fatigue strength
- Warping e.g during finishing
- Repair problems
what is internal stresses related to
- Shape and size e.g notches
- Curing pressure
- Curing cycle
- Thermal expansion
- Cooling rate
what happens if it is under cured
then there is free monomer present which is an irritant and it will have a low molecular weight which will result in poor mechanical properties
what happens if it is cured too fast
then there is risk of possible gaseous porosity
what happens if there is monomer
If there is too much monomer then this results in contraction porosity. This is associated with poylmerisation shrinkage.
what happens if there is too little monomer
there will be granularity on the surface of the material
how does porosity effect the denture base material
- Affects strength
- Affects appearance
o Roughness attracts plaque - Rough sensation to tongue
- Absorbs saliva resulting in poor hygiene
describe gaseous porosity
On polymerization, there is an exothermic reaction and this could cause the temperature of the resin to rise above 100 degrees Celsius which is just above its boiling temperature. If this temperature is exceeded before the poylermisation process is completed, gaseous monomer will be formed – which is the cause of the gaseous porosity
how should gaseous porosity be avoided
. The occurrence of gaseous porosity can be avoided by allowing the temperature to be raised in a slow and controlled fashion.
where does gaseous porosity occur
It occurs in the bulkier areas of acrylic
what are the causes of contraction porosity
- Too much monomer
- Insufficient excess material
- Insufficient clamp pressure
where can contraction porosity occur
anywhere where dough is not sufficiently pakced.
is acrylic non toxic
yes
is acrylic non irritant
provided that no monomer is released but some patients may be allergic
is acrylic affected by oral fluids
no
is thermal expansion the same as the artificial tooth
yes if tooth is also acrylic
no if porcelain
what is the thermal conductivity of acrylic
low
making it poor
how are the mechanical properties of acrylic
poor
so we incase in bulk to compensate
what is the density of acrylic
low
but increase in bulk offsets advantage
what is the softening temperature of acrylic
high - 75 degrees
okay for ingesting hot fluids
is acrylic dimensionally accurate and stable in use
it has a linear contraction of 0.5% and this is acceptable
what are the advantages of acrylic
- Excellent aesthetics
- Easy and cheap to process
- Low density
what are the disdavtnages of acrylic
- Has barely adequate strength characteristics
- Susceptible to distortion
- Has a low thermal conductivity
- Radiolucent
