PMMA Flashcards
What are the ideal mechanical and thermal properties of a denture material?
- High Young’s (Elastic) Modulus
- High Proportional Limit / Elastic Limit
- Dimensionally accurate and stable in use
- High Softening Temperature (Tg)
- Unaffected by Oral Fluids
- Thermal expansion similar to artifical teeth to avoid internal stresses
- High thermal conductivity (keep patient safe)
- Low density to aid retention (gravity)
- High faigue strengeth
- High hardness
- High transverse strength
What are the ideal chemical properties of a denture base?
Non Toxic, Non Irritant
Colour / Translucency
What is transverse strength?
(flexural strength). It measures a material’s ability to withstand bending or flexing forces
How is transverse strength determined?
by subjecting the material to a three-point bending test, where a sample is supported at both ends and force is applied in the middle until it fractures.
What is free radical addition polymerisation and what type of molecule is involved?
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)
What are the steps of acrylic polymerisation?
- Activation - of initiator (benzoyl peroxide/camphorquoine/ lucerin) to provide 2 free radicals
- Initiation - free radicals break C=C bond in monomer and transfer free radicals
- Propagation - growing polymer chain
- Termination - of polymerisation
What are the components of heat cured powder acrylic? What do they do?
- Initiator (benzyol peroxide - provides free radicals)
- PMMA Particles – pre-polymerised beads
- Plasticiser - allows quicker dissolving in monomer liquid eg dibutyl phthalate
- Pigments – to give “natural” colour
- Co-polymers - to improve mechanical properties eg ethylene glycol dimethacrylate
What are the components of heat cured liquid acrylic? What does each component do?
- Methacrylate Monomer (dissolves PMMA particles polymerises)
- Inhibitor (Hydroquinone, 0.006%) (prolongs shelf life - reacts with any free radicals produced by heat, UV light)
- Co-polymers (improve mechanical properties - particularly cross-linking of polymers)
What is required to give a high molecular weight to the acrylic?
efficient polymerisation
What are the properties of acrylic?
- Non Toxic, - yes;
- Non Irritant - if no monomer released, few patients
allergic - Unaffected by Oral Fluids - water absorption, virtually insoluble in fluids taken orally
- Mechanical Properties - poor, increase in bulk to compensate
- Fatigue Strength / Impact strength
– “fairly resistant” but can be cause of failure - High Hardness / Abrasion Resistance
– retains good polish, some wear over time - Thermal Expansion = Artificial tooth
- OK if acrylic teeth used , significantly higher than porcelain teeth
- High Thermal Conductivity
– low-poor - Low Density
– low (good) but increase in bulk to overcome poor mechanical
properties offsets advantage - High Softening Temperature - 75 oC – OK for ingested hot fluids
– DON’T use boiling water for cleaning - Dimensional accurate and stable in use - OK - Linear Contraction 0.5% - Acceptable
How does the polymerisation of self curing differ to heat cured?
As heat cured except benzoyl peroxide is activated by promoter* (tertiary amine) in liquid
eg * dimethyl-para-toluidine
Why would self cured acrylics be used?
lower temperature - less thermal contraction hence better dimensional accuracy
How is self cured acrylic activated and what does this result in?
- Chemical activation - less efficient
- Hence lower molecular weight
- Hence poorer mechanical properties & Tg lower
What is a result of chemical activation?
More unreacted monomer
- which acts as plasticiser, softening denture base, reducing transverse strength
- potential tissue irritant, compromising its biocompatability
