PMMA Flashcards
What are the ideal features of a denture
it replaces the function of natural teeth
goes into the px mouth and fits properly
has good aesthetics as its seen by there other people
good value
What are ideal mechanical properties of a denture material
- Dimensionally accurate and stable in use
- High softening temperature (Tg)
- Unaffected by oral fluids
- Thermal expansion (not expand or contract too much)
- Low density
- High thermal conductivity
- Radiopaque
- Non toxic/irritant
- Colour/translucency is suitable
- Easy and inexpensive to manufacture
- Easy to repair
What are the ideal properties of a denture material
• High YM • High proportional limit • High transverse strength • High fatigue strength • High impact strength High hardness/abrasion resistance
What is transverse strength
- Flexural strength
* How well does the denture cope with stresses that cause deflection
What is the pivot point of denture
• The pivot point is the palate, forces are being applied on either side and the denture may fracture at the pivot point and this is the worst case scenario for acrylic resin
What is the insidious failure of a denture
where the acrylic resin withstands the impact created and microcracks appear in the subsurface so it may fail later
What is free radical addition polymerization
- 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 is the monomer
methacrylate monomer
What are the stages of polymerization
○ Activation - of initiator to provide free radicals
○ Initiation - free radicals break C=C bond in monomer and transfer free radicals
○ Propagation - growing polymer chain
○ Termination - of polymerisation
What is the composition of the powder in heat cured acrylic
initiator PMMA particles plasticizer pigments copolymer
What is the ignitor
benzoyl peroxide 0.2-0.5%
What is PMMA particles
prepolymerised beads
What is plasticizer for
allows for quicker dissolving in monomer liquid e.g dibutyl phthalate
What is the pigment for
to give ‘natural’ colour
What is the copolymer for
to improve mechanical properties e.g ethylene glycol dimethacrylate
What is the composition of the liquid component
○ Methacrylate monomer - dissolves PMMA particles (polymerises)
○ Inhibitor
○ Co-polymers - improve mechanical properties (particularly cross-linking of polymers)
What is the inhibitor
(hydroquinone, 0.006%)
What is the technique for heat cured acrylic
- A vessel containing the mould material and artificial teeth is used
- Dough form acrylic is packed in
- Mould is pressed and takes up the shape of the patient’s dentition
- The acrylic needs to be cured to form a strong solid denture base
- It is subjected to a heat cycle to cause polymerisation
Why is effect polymerization important
• Efficient polymerisation is needed to give a high molecular weight polymer i.e good mechanical properties hence a high temperature is required but gaseous porosity limits
Is acrylic non toxic
yes
Is acrylic non irritant
if no monomer released, only few patients are allergic
Is acrylic unaffected by oral fluids
water absorption is possible which can cause distortion but acrylic resin is fortunately insoluble in fluids taken orally
How are the mechanical properties of acrylic
poor but we increase in bulk to compensate
How is the fatigue/impact strength of acrylic
fairly resistant but can be a cause of failure
What is the high hardness/abrasion resistance
retains good polish but there is some wear overtime which can feel rough to the patients tongue
How is the thermal expansion of acrylic
equivalent to that of artificial tooth - okay if acrylic teeth are used but it is significantly higher than porcelain teeth
How is the thermal conductivity of acrylic
low/poor
What is the density of acrylic
low which is good but the increase in bulk to overcome the poor mechanical properties offsets this advantage
How is the softening temperature of acrylic
okay for ingested hot fluids but don’t use boiling water for cleaning
Is acrylic dimensionally stable
okay, linear contraction 0.5% - acceptable
How is the expansion of acrylic
- Manufacture: 0.5% linear contraction
* Usage - 0.4% expansion
What is the composition fo self cure
○ The same as heat cured except benzoyl peroxide is activated by promoter (tertiary amine e.g dimethyl-para-toluidine) in liquid
Why self cure
• Lower temperature since not required to initiate polymerisation so less thermal contraction so should give a better dimensional accuracy
What are the disadvantages of self cure
- Chemical activation is less efficient so there is a lower molecular weight and poorer mechanical properties as well as the Tg being lower
- There is more unreacted monomer which acts as a plasticiser, softening the denture base and reducing its transverse strength
- Its also a potential tissue irritant, compromising its biocompatibility
- Chemical cure leaves 3-5% unreacted monomer compared to 0.2-0.5% in heat cure
What is the issue of dimensional accuracy for self cure
- Fits the original cast better than heat cured but water absorption gives expansion
- Self cured is oversized and heat cured is undersized but undersized is better tolerated
Compare self cure with heat cure
- HC - higher molecular weight so it’s stronger
- HC - curing process may cause porosity
- SC - higher monomer levels which can cause irritation and have a significant impact on the patient’s tissue
- SC - fits cast better but water absorption in mouth makes it oversized
- SC - poorer colour stability as the tertiary amines are susceptible to oxidation
- Neither are ideal
Why were variations of acrylic made
• Acrylic resin and dentures have poor strength and toughness with 10% fracturing within 3 years
What are the attempt to strengthen acrylic
○ High impact resistance materials which was done by incorporating rubber toughening agent (butadien styrene) to stop crack propagation and long term fatigue problems
○ Incorporate fibres (carbon, UHMPE - ultra high molecular weight polyethylene, glass)
§ Difficult processing and manufacturing
Describe ultra hi (used in GDH)
- High impact heat cure acrylic resin that exudes quality and gives the technician confidence
- It has been formulated with flexural strength and superior fracture toughness (ductility)
- The two features together give it a slight bending aspect which keep the material from being brittle and subject to cracking and breaking
Describe pour’n’cure resins
- Similar to self cure
- Smaller powder particles to produce a fluid mix not a dough like form
- The fluid mix is poured into a mould
- A good fitting denture can be produced but the mechanical properties are inadequate
What are light activated denture resins
- Urethane dimethacrylate matrix plus acrylic copolymers
- Microfine silica fillers - small amounts to control rheology
- Photo initiator system
- Adapted to cast
- Cured in a light chamber
- Used mostly as customised impression tray material and for repair of fractured dentures
why should a denture material be radiopaque
so if any fragments broke off and there is a risk of it being swallowed then a radiograph can confirm
What are alternative materials that have been developed for radiopacity
metal inserts
inorganic solts
comonomer containing heavy metails
halogens containing comonomer or additives
Why are metal inserts an issue
they weaken the material and result in poor aesthetics
Why are inorganic salts not ideal
§ In low concentrations it is not radiopaque
§ In high concentrations it is a weak base
Why are comonomer containing heavy metals not ideal
○ Comonomers containing heavy metals e.g barium sulphate which results in poor mechanical properties
Why are halogen containing comonomer or additives not ideal
§ May act as plasticiser
§ Expensive
§ Promising
For those with a proven allergy to acrylic what can be tried instead
nylons
vinyl polymers
polycarbonates
What is the issue with nylons
§ Water absorption leading to swelling and softening
What is the issue with vinyl polymers
§ E.g polyvinyl acetate, polyvinylchloride, styrene
§ Injection moulding (expensive)
§ Softening temperature of 60 degrees celcius
What are the issue with polycarbonates
§ Injection moulded
§ Softening temperature of 150 degrees
§ They develop internal stresses during use and can result in distortion and poor fit
