PMMA Flashcards
State common error sources in dentures
- Production
- initial impression flawed so material + method is not 100% accurate - Curing process
- refinements at chair side required - Usage
- Pt may agree denture fits initially then overtime experience problems
What denture problems can occur for the patient over time? (4)
- Uncomfortable
- Fractures
- Becomes warped
- Surfaces suffer wear
State 3 ideal properties of a denture base
- Dimensionally accurate + stable in use
- Must fit in pt’s mouth and be retained - High softening temp (Tg)
- Must not distort during ingestion of hot fluids/cleaning - Unaffected by oral fluids
What does the elastic limit represent on the stress strain diagram?
The stress beyond which a material cannot reform to its original shape
PERMANENTLY DEFORMED (even after releasing the stress factor)
Ideal mechanical properties of a denture base (2)
- High YM
- Large stress produces small strain - High elastic limit
- Only large stresses will cause permanent deformation
Define free radical polymerisation
The 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
What are the 4 steps in acrylic polymerisation
- Activation
- Initiation
- Propagation
- termination
Describe events at ACTIVATION stage of acrylic polymerisation
Activation of initiator to produce 2 free radicals
Describe events at INITIATION stage of acrylic polymerisation
Free radicals break the C=C bond in monomer and transfer free radicals
Describe events at PROPAGATION stage of acrylic polymerisation
Growing polymer chain
Describe events at TERMINATION stage of acrylic polymerisation
Termination of polymerisation occurs
Ideal thermal properties of denture base (2)
- Thermal expansion
- Avoids internal stresses on cooling during manufacture - High thermal conductivity
- Transmission of thermal stimuli to mucosa
Name the initiator for polymerisation
Benzoyl peroxide
What makes up the heat cured acrylic POWDER (5)
- Initiator
- Benzoyl peroxide 0.2-0.5% - PMMA particles
- Pre polymerised beads - Plasticiser
- Allows quicker dissolving in monomer liquid - Pigments
- Give natural colour - Co-polymers
- To improve mechanical properties
What makes up the heat cured acrylic LIQUID (3)
- Methacrylate monomer
- Dissolves pMMA particles - Inhibitor (Hydroquinone)
- Prolongs shelf life as it reacts with any free radicals produced by heat/UV light - Co-polymers
- Improve mechanical properties, particularly cross-linking of polymers
Describe the important of the powder/liquid mix (3)
- Forms a dough like material that can be handled/mixed easily and customised to a desired shape
- Minimises polymerisation
shrinkage - Reduces heat of reaction
State the proportions for powder/liquid mix
3 to 3.5/1
What can exceptionally high temperatures lead to?
Gaseous porosity within the acrylic weakens the material
What are internal stresses of the denture base related to? (5)
- Shape and size e.g. notches
- Curing pressure
- Curing cycle
- Thermal expansion
- Cooling rate
What’s the effect of under curing the base?
- Free monomer (irritant)
- Low molecular weight
- POOR mechanical properties
What’s the effect of fast curing the base?
- Creates bubbles within the material, which increase gaseous porosity and weaken the material
What’s the effect of too much monomer?
Contraction porosity
What’s the effect of too little monomer?
Granularity
What does porosity affect?
- Strength
- Affects appearance
- Rough sensation to tongue
- Absorbs saliva - poor hygiene
What areas of the denture is porosity more present in?
Bulkier parts of the denture
Compare polymerisation shrinkage for just monomer / powder+ momomermix ? (2)
- Monomer alone = 21% (by volume)
2. Powder + Monomer mix = 7%
State the causes of contraction porosity (3)
- Too much monomer
- Insufficient excess material
- Insufficient clamp pressure
Where does contraction porosity occur?
Anywhere where the dough is NOT sufficiently packed
How do internal stresses lead to repair problems?
> Decreased strength
Decreased fatigue strength
Warping e.g. during finishing
Repair problems
Ideal properties for patient comfort and aesthetics (3)
- Low density
- Aids retention of upper denture (the heavier the material, the greater the gravity pull) - Colour/translucency
- To match natural tissues - Non toxic/ non irritant
Looking at the acrylic temp changes graph, what do temperatures over 100 lead to?
Gaseous porosity - bubbles that weaken the material
Purpose of efficient polymerisation?
Gives high molecular weight polymer
Good mechanical properties
