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
