PMMA Flashcards
What are the ideal properties of a material being used for a denture base?
High Young's Modulus High elastic limit High transverse strength High fatigue strength High impact strength Low density High hardness/abrasion resistance High thermal conductivity Radiopaque Non toxic/Non irritant Good aesthetics Cost effective to produce Easy to repair
What is the softening temperature of PMMA?
Moderate - 75 degrees C
Must not distort during ingestion of hot fluids or during cleaning
DON’T use boiling water for cleaning
What does PMMA stand for?
polymethyl methacrylate
What are the 4 stages in acrylic polymerisation?
Activation
Initiation
Propagation
Termination
What is the initiator in PMMA polymerisation?
Benzoyl peroxide
What are the components of PMMA?
POWDER: (PPPIC) initiator (benzoyl peroxide 0.2-0.5%) PMMA particles plasticiser (e.g. dibutyl phthalate) pigments co-polymers (e.g. ethylene glycol dimethacrylate) LIQUID: (MIC) methacrylate monomer inhibitor (hydroquinone 0.006%) co-polymer
What are the methods of curing PMMA?
Self cure
Light cure
Heat cure
What problems are caused by internal stresses?
Decreased strength
Decreased fatigue strength
Warping during finishing
Repair problems
What problems can occur during curing?
Undercured - free monomer (irritant), low molecular weight (poor mechanical properties)
Fast curing - possible gaseous porosity
What problems are caused by porosity?
Affects strength and appearance
Rough sensation to tongue
Absorbs saliva - poor hygiene
If gaseous porosity occurred due to fast curing, where on the denture base would it be seen?
Bulkier areas e.g. either side of the palate
What are the properties of acrylic?
Non toxic, non irritant
Unaffected by oral fluids - virtually insoluble
Mechanical properties poor - increase bulk to compensate
Fatigue/impact strength - fairly resistant
High hardness/abrasion resistance - some wear over time
Thermal expansion = to artificial tooth - if acrylic, if porcelain then it is higher
Thermal conductivity poor
Low density - but bulk must be increase to compensate for poor mechanical properties, so this is cancelled out
High softening temperature - 75 degrees C
Dimensionally accurate & stable in use
Not radiopaque
Good colour/translucency
Easy/inexpensive to manufacture
Relatively easy to repair
What are the disadvantages of self cure PMMA?
Chemical activation is less efficient
Results in lower molecular weight and hence poorer mechanical properties. The softening temperature is also lower.
3-5% unreacted monomer.
Likely to be oversized as water absorption gives expansion.
What are the pros & cons of heat cure PMMA?
Need efficient polymerisation to give high molecular weight polymer.
Fast curing could result in gaseous porosity.
Cooling could result in internal stresses.
0.2 to 0.5% unreacted monomer.
Likely to be undersized - better tolerated.
What has been added to PMMA in attempt to strengthen it?
High impact resistant materials (e.g. rubber toughening agent) - stop crack propagation and long term fatigue problems Incorporate fibres (e.g. carbon, UHMPE, glass)
