PMMA Flashcards
What are the ideal mechanical and thermal properties of a denture material?
- High Young’s (Elastic) Modulus – rigid (stiff)
– ie applying a large stress produces small strain - High Proportional Limit / Elastic Limit
– only large stresses will cause permanent deformation - Dimensionally accurate and stable in use
- High Softening Temperature (Tg) (not distort during ingesting of hot fluids)
- 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)
What material is used for denture bases?
Acrylic / Polymethylmethacrylate (PMMA)
What is free radical addition polymerisation and what type of molecule does it involve?
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 two methods of intiator activation in PMMA?
heat >72 c
self cured
What is the initiator molecule that provides the free radicals?
benzoyl peroxide
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 the proportion of powder to liquid?
3/3.5 to 1
to produce a dough-like material that can be easily handled
What does heat curing need in order to provide material with good mechanical properties?
must be done in a way that causes efficient polymerisation – as this will yield a high molecular weight polymer – and that means good mechanical properties, such as mechanical strength.
What are the two heat curing cycles of the water bath? A vs B
Flask of ACRYLIC RESIN placed in water bath:-
(a) at 70oC for 7hrs.100oC for 2hrs. Slow Cool
(b) at 100 oC for several hours
Why does the monomer not exceed 100 degrees in cycle A?
GASEOUS POROSITY is avoided (or minimised)
Why does the monomer exceed 100 degrees in cycle B?
due to the exothermic polymerisation reaction it undergoes after about 2hrs– this cycle would produce material with many gaps.
Why should cooling be done slowly?
If cooled too quickly this could lead to internal stresses within the acrylic resin – which would weaken it.
cooling should be done slowly to minimise internal stresses.
What is fatigue strength?
where low level stresses applied repeatedly over a long time, cause fracturing