PMMA Flashcards
why is manufacturing a denture challenging
it is a complex 3D customised object
how do you measure the success of a denture
based on patients feedback - they will either say it fits or it doesn’t fit
what are the error sources in the production of dentures
○ Initial impression is flawed
§ Material and method not 100% accurate
§ If these aren’t perfect then the denture cannot be perfect
○ Curing process
§ Several potential error sources
○ May not achieve perfect fit at first attempt
§ Refinements at the chairside may be required
[Can make minor adjustments but the overall fit has to be close enough to the desired fit for this to work]
what are the error sources in the usage of dentures
○ Fits patient only for short period
○ Fractures
§ During use
§ Accident
○ Uncomfortable
○ Becomes warped
§ The way the denture is handled
○ Surface suffers wear
§ Gets worn away or the surface becomes rough and becomes uncomfortable for the patient
list ideal properties of dentures (not including mechanical or thermal properties)
- Unaffected by oral fluids
- Replaces function of natural teeth
- Is seen by other people
- Dimensionally accurate and stable in use
- Must fit patient’s mouth and be retained
- Low density - aids retention of upper denture as the heavier the material, the greater the gravity pull
- Colour / translucency matching natural tissues
- Non toxic, non irritant
list ideal mechanical properties of dentures
○ High young’s (elastic) modulus
§ Rigid (stiff)
§ Large stress produces small strain
○ High proportional limit / elastic limit - Only large stresses will cause permanent deformation
list ideal thermal properties of dentures
• High softening temperature (Tg)
○ Must not distort during ingesting of hot fluids or during cleaning ie won’t change with high temperatures
• Thermal expansion
○ Must be the same as the artificial tooth
§ Avoid internal stresses on cooling during manufacture
• High thermal conductivity
○ Transmission of thermal stimuli to mucosa to avoid scalding the back of throat or oesophagus
what are the available materials for making a denture
acrylic / polymethylmethacrylate (PMMA)
explain free radical addition polymerisation
- Chemical union of 2 molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
- Involves molecules with (C=C bonds)
explain the free radical addition polymerisation of methacrylate monomer
double bond is what we are looking to break down so it links to other molecules = so material becomes rigid, heavy molecule
want cross linking happen
explain acrylic polymerisation
• Activation
○ Of initiator to provide free radicals
• Initiation
○ Free radicals break C=C bond in monomer and transfer free radical
• Propagation
○ Growing polymer chain
• Termination
○ Of polymerisation
○ Polymerisation stage just stops
what is the initiator in acrylic polymerisation
Benzoyl peroxide
(NB symmetrical)
C6H5COO-OOCH5C6
what activates acrylic polymerisation
A) Heat > 72°C
- Different heat curing cycles
B) Self cured
- Not in this lecture
what does activation of polymerisation give
○ Activation gives 2 free radicals R*
§ C6H5COO*
§ Breaking down benzoyl peroxide so it is split in the middle
§ Electrical charge
what happens in propagation stage in acrylic polymerisation
R* + M --> R-M* R-M* + M --> R-M-M* R-M-M* + M --> R-M-M-M* …. And so on Initial monomer grows and cross links with other molecules
what is found in the powder of heat cured acrylic
○ Initiator
§ Benzoyl peroxide (0.2-0.5%)
○ PMMA Particles
§ Pre-polymerised beads
§ Speeds things up
□ Introduce into the powder PMMA that has been produced before and then it is ground into little beads which then react with the liquid
○ Plasticiser
§ Allows quicker dissolving in monomer liquid
§ Eg dibutyl phthalate
§ Speeds up the process
○ Pigments
§ To give “natural” colour
○ Co-polymers
§ To improve mechanical properties
§ Eg ethylene glycol dimethacrylate
what is found in the liquid of heat cured acrylic
○ Methacrylate monomer
§ Dissolves PMMA particles - polymerises
○ Inhibitor (hydroquinone, 0.006%)
§ Prolongs shelf life - reacts with any free radicals produced by heat, UV light
§ Prevents any free radicals that might be produced inadvertently as the material is being stored somewhere
§ Could be triggered by heat or light
§ Stops it from reacting before it gets to the powder
○ Co-polymers
§ Improve mechanical properties - particularly cross-linking of polymers
why are the powder and liquid mixed in a heat cured acrylic
○ To produce 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 will shrink by 21%
§ Mix it with powder and it will shrink by 7%
§ Substantial amount of shrinkage during heat curing process
what are the proportions of powder to liquid in heat cured acrylic
○ 3 to 3.5 / 1
§ P / L by volume
§ Follow instructions for ratios
○ 2.5 / 1
§ By weight
what are the mixing stages of heat cured acrylic
Sandy - tacky
Dough - packing
how is the shape of the acrylic denture acquired
- Metal vessel filled with mould material that is the shape of the patient’s mouth
- Individual teeth placed within the mould material
- Brush allows separation later in the process
- Pack dough like material in - important to pack in
- Allow excess material besides
- Take flasks and clamp them together
- Apply an appropriate pressure
why do you need efficient polymerisation in acrylics heat curing
• Need efficient polymerisation to give high molecular weight polymer ie good mechanical properties
= Better the polymerisation then the better the material you are going to get
Hence high temperature but gaseous porosity limits
○ Ideally want a high temperature
○ But if you have the wrong heat curing cycle / temperature then this will cause porosity
what are examples of heating schedules for heat cured acrylic
1) 7 hours to 70°C + 2 hours to 100°C + slow cool
2) 72°C for at least 16 hours
- Simplest
3) 20-20-20 reverse curing
- Place flask in boiling H2O
- Remove heat for 20 minutes
- Heat to 70°C for 20 minutes
- Then heart to 100°C for 20 minutes
Don’t be too concerned about the details of these
what happens when the temperature is raised about 100 degrees C in heat cured acrylic
If temperature peaks above 100°C then gases are produced which lead to porosity which leads to a weak material
Raise temp to 70°C - exothermic reaction of the polymerisation will raise it to 100°C
Then it is dropped and 70°C is maintained
Stay clear of exceeding 100°C within the acrylic
how should heat cured acrylic be cooled
Mould material and acrylic have different thermal expansion coefficients
this can create Internal stresses within the acrylic
§ Cool slowly - allow relief
Cool flasks down gradually over many hours
what is the problem with internal stresses being present in the acrylic resin
• Decrease strength
• Decrease fatigue strength
• More prone to warping at a later stage
○ Eg during finishing
• Repair problems later on
○ If the acrylic resin needs repaired (eg there is a crack on the surface) then the technician cannot fix it as easily - Will lead to fracture
○ Won’t find out at time of production or fitting but this problem will be revealed during the later stage
• Internal stresses are related to ○ Shape and size eg notches ○ Curing pressure ○ Curing cycle ○ Thermal expansion ○ Cooling rate [Complexity of the whole process]
what is the problem if the acrylic is under cured
○ Free monomer (irritant)
§ Monomer that is not cured that can leak out of the denture base
§ If patient complains of irritant then it is likely to be this problem
○ Low molecular weight
§ Poor mechanical properties
§ Easily fractured
what is the problem with fast curing the acrylic
○ Possible gaseous porosity
§ Creates bubbles within the material
§ Weakens the material
what are the problems with incorrect powder / monomer ratios
• Too much monomer
○ Contraction porosity
• Too little monomer
○ Granularity
what problems does porosity cause in acrylic
• Affects strength • Affects appearance ○ Certain roughness to the surface ○ Roughness attracts plaque - doesn’t look attractive • Rough sensation to tongue ○ Uncomfortable for the patient • Absorbs saliva - poor hygiene
where and how does gaseous porosity occur
- Monomer boiling (100°C) produces bubbles
- Occurs in bulkier parts
- Be careful NB exothermic reaction
Hence cure slowly
what is contraction porosity caused by
too much monomer
what is the polymerisation shrinkage in acrylic
○ Monomer alone = 21% (by volume)
§ Too much monomer is going to cause more shrinkage than you would expect
○ Powder + monomer mix = 7%
what causes polymerisation shrinkage in acrylic
○ Too much monomer ○ Insufficient excess material § Not enough dough packed in ○ Insufficient clamp pressure § Need pressure to force the flasks together
• Occurs anywhere where dough is not sufficiently packed
list acrylic properties (not including thermal and mechanical properties)
• Non toxic, Non irritant
○ Provided no monomer released - may cause problems at a later stage
○ some patients allergic to the monomers
• Unaffected by oral fluids
○virtually insoluble in fluids taken orally
• good, natural colour / translucency
• Low density
○ Low is good but the need to increase in bulk to overcome poor mechanical properties offsets this advantage so don’t usually get the benefit of this property
• Dimensionally accurate and stable in use
○ Linear contraction = 0.5% and this is acceptable
what are the thermal properties of acrylic
• Thermal expansion same as artificial tooth
○ Ok with acrylic teeth
○ Porcelain teeth cause a problem as they have different thermal expansions
• High thermal conductivity
○ Low - poor, doesn’t perform well
• High softening temperature
○ 75°C = ok for ingested hot fluids (tea / coffee no problem)
○ Don’t use boiling water for cleaning as it will cause the denture to warp and not fit
what are the mechanical properties of acrylic
• Mechanical properties (ie YM and PL & EL)
○ Poor, increase in bulk to compensate
○ Bulk it up to 1.5mm thickness for the acrylic resin denture to make up for the weaker mechanical properties
what is the most commonly used denture base material
acrylic resin (heat cured)
