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
Uncured monomer - Chemical Vs Heat Cure?
Chemical cure :–
3 to 5% unreacted monomer (risk of dimensional instability)
Heat cure :–
0.2 to 0.5% unreacted monomer
How is the dimensional accuracy of self cure compared to heat cure?
- Fits original cast better than heat cured BUT
- Water absorption gives expansion
- Hence :
Self cured over-sized
Heat cured under-sized – better tolerated
Heat Cured vs Self Cured?
- HC -higher molecular weight – stronger
- HC – curing process may cause porosity
- SC - higher monomer levels - irritant
- SC - fits cast better but water absorption in mouth makes oversized
- SC - poorer colour stability (tertiary amines susceptible to oxidation)
- SC - other properties - as HC
What % of acrylic resin dentures fracture within 3 years?
10%
How were the acrylic resin dentures strengthened?
what were the attempts?
high impact resistant materials – incorporate rubber toughening agent (butadien styrene) – stop crack propagation – long term fatigue problems
incorporate fibres (carbon, UHMPE – ultra-high molecular wt polyethylene, glass)
difficult processing -ongoing
What is ultra hi?
high impact acrylic resin
- exceptional flexural strength
- superior fracture toughness (ductility)
- a slight bending aspect which keeps the material from being
brittle and subject to cracking and/or breaking.
What are pour n cure resins?
- similar to SC
- smaller powder particles
- fluid mix pour into mould
- good fitting but poor mechanical properties
What are light activated denture resins?
How are they used and when?
- urethane dimethacrylate matrix plus acrylic copolymers
- microfine silica fillers - small amounts to control rheology
- photoinitiator systems
- adapted to cast
- cured in light chamber
- used mostly as customised impression tray material & for repair of fractured dentures
How have polymers been made radioaque and what are the disadvantages of each technique?
metal inserts - weaken, poor aesthetics
inorganic salts (eg barium sulphate)
- low conc - not radiopaque
- high conc - weak base
comonomers containing heavy metals eg barium sulphate
- poor mechanical properties
halogen containing comonomers or additives eg tribromophenylmethacrylate
may act as plasticiser
expensive
? promising
What are the options to be used as a polymer if there is an allergy to acrylic?
nylon
vinyl polyers
polycarbonates
What happens to nylon?
water absorption
it swells
What is the softening temperature of vinyl polymers?
Tg = 60 c
softening in use
How can polycarbonates and vinyl polymers made?
injection moulding
What is the softening temperature of polycarbonates?
150 c
What are the advantages and disadvantages of polycarbonates?
high temperature causes internal stresses which causes distortion in use
* good impact strength
