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BDS3 - dental material science > denture base materials > Flashcards

denture base materials Flashcards

1
Q

what are the ideal properties of a denture base material

A
  • replaces function of natural teeth
  • appropriate aesthetics
  • dimensionally accurate and stable in use
  • high softening temperature
  • unaffected by oral fluids
  • no thermal expansion
  • low density
  • high thermal conductivity
  • radiopaque
  • non-toxic, non-irritant
  • accepted colour/translucency
  • easy and inexpensive to manufacture
  • easy to repair
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2
Q

what are the ideal mechanical properties

A
  • high Young’s modulus
  • high proportional limit
  • high transverse strength
  • high fatigue strength
  • high impact strength
  • high hardness/abrasion resistance
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3
Q

what is transverse strength

A
  • flexural
  • 3 point loading
  • denture needs to be able to cope with impact
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4
Q

what is the pivot point of the denture

A
  • the palatal

- forces applied on either side, potentially causing fracture at the pivot point

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5
Q

what is free radical addition polymerisation

A
  • chemical union of 2 molecules either the same or different to form a larger molecule WITHOUT the elimination of smaller molecules
  • involves molecules with C=C bonds
  • acrylic resin undergoes this
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6
Q

what are the 4 stages of acrylic polymerisation

A
  • activation
  • initiation
  • propagation
  • termination
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7
Q

what is activation

A
  • of initiator to provide free radicals
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8
Q

what is initiation

A
  • free radicals break C=C bond in monomer and transfer free radical
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9
Q

what is propagation

A
  • growing polymer chain
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10
Q

what is termination

A
  • end of polymerisation
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11
Q

what is the powder in heat cured acrylic made up of

A
  • initiator -> benzoyl peroxide 0.2-0.5%
  • PMMA particles -> pre-polymerised beads
  • plasticiser ->allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
  • pigments -> to give ‘natural’ colour
  • co-polymers -> to improve mechanical properties e.g. ethylene glycol dimethacrylate
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12
Q

what is in the liquid of heat cured acrylic

A
  • methacrylate monomer
  • inhibitor (hydroquinone 0.006%)
  • co-polymers
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13
Q

what does methacrylate monomer do

A
  • dissolves PMMA particles

- polymerises

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14
Q

what does the inhibitor do

A
  • prolongs shelf life

- reacts with any free radicals produced by heat, UV light

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15
Q

what do co-polymers do

A
  • improv mechanical properties

- particularly cross-linking of polymers

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16
Q

what is the acrylic vessel

A
  • contains mould material
  • artificial teeth are placed ready for acrylic resin
  • acrylic resin in dough-like form is inserted into the mould - takes up patients dentition
  • 2 halves of the vessel are clamped together ready to be subjected to the heating cycle required to cause polymerisation
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17
Q

what is essential to have in acrylic heat curing

A
  • efficient polymerisation

- gives high molecules wight polymer which gives good mechanical properties

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18
Q

what is efficient polymerisation

A
  • lots of cross linking of MMA molecules

- essential

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19
Q

what can limit efficient polymerisation

A
  • high temperature but gaseous porosity
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20
Q

what are the properties of acrylic

A
  • non-toxic
  • non-irritant
  • unaffected by oral fluids
  • poor mechanical properties
  • fairly good fatigue strength/impact strength
  • high hardness and abrasion resistance - retains good polish but can wear over time
  • thermal expansion = artificial tooth if acrylic used, if porcelain then acrylic much higher
  • low thermal conductivity
  • low density = good as it is lightweight and comfortable for patient
  • softening temperature = 75 degrees, good for oral fluids but not for cleaning in boiling water
  • dimensionally accurate and stable in use = contraction 0.5%
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21
Q

what is the dimensional change in heat cured acrylic

A
  • 0.5% contraction from manufacturing

- 0.4% expansion from usage = through water absorption, almost makes up for contraction

22
Q

what is the composition and polymerisation of self cured acrylic

A
  • similar to heat cured

- except benzoyl peroxide is activated by a promoter in the liquid

23
Q

what could the promoter be in self curing acrylic

A
  • tertiary amine

- dimethyl-para-toluidine

24
Q

why do we use self cured acrylic

A
  • lower temperature
  • which means less thermal contraction
  • hence better dimensional accuracy
25
Q

what is the chemical activation of self curing acrylic

A
  • less efficient polymerisation
  • gives lower molecular weight
  • means more unreacted monomer
    = means denture is softer and has reduced transverse strength
    = denture therefore more vulnerable to failure
    = more chance of monomer leaching out and causing problems
26
Q

what is the difference in unreacted monomer between self cured and heat cured acrylic

A
  • chemical cured = 3 to 5% unreacted monomer

- heat cured = 0.2% to 0.5% unreacted monomer which is much more effective

27
Q

what is the dimensional accuracy of self cured acrylic

A
  • undergoes less thermal contraction during polymerisation so initially gives a better fit
  • BUT as it expands during use due to water absorption, it ends up over-sized and will fall out
  • over-sized
28
Q

what is the dimensional accuracy of heat cured acrylic

A
  • the contraction during manufacture, coupled with the expansion during use from date absorption means it is a little undersized
  • under-sized
29
Q

what is the molecular weight of heat cured

A
  • high

- makes it stronger

30
Q

what are some drawbacks of self cured

A
  • higher monomer levels = irritant
  • oversized in mouth from water absorption
  • poorer colour stability = tertiary amines susceptible to oxidation
31
Q

what is the fracture rate of acrylic resin dentures

A
  • 10% fracture within 10 years
32
Q

how can we strengthen acrylic resin dentures

A
  • high impact resistant materials

- incorporate fibres

33
Q

what does high impact resistant materials do

A
  • incorporate rubber toughening agent
  • stop crack propagation
  • long term fatigue problems
34
Q

what fibres can be incorporated into acrylic resin dentures

A
  • carbon, ultra-high molecular weight polyethylene, glass

- difficult processing - ongoing

35
Q

what is ‘ultra-high’ heat cure

A
  • a high impact heat cure acrylic resin that exudes quality and gives technician confidence
  • a slight bonding aspect which keeps the material from being brittle and subject to cracking and/or breaking
36
Q

what are the properties of ultra-high heat cured

A
  • exceptional flexural strength = increases chances of surviving for longer
  • superior fracture toughness (ductility)= helps mitigate effect of any micro-cracks
37
Q

what are pour ‘n’ cure resins

A
  • similar to self cure
  • smaller powder particles
  • fluid mix to pour into mould = not dough-like
  • perfectly serviceable dentures can be produced
  • good fitting but poor mechanical properties
38
Q

what are light activated denture resins

A
  • urethane dimethacrylate matrix plus acrylic co-polymers
  • cured in light chamber = limited depth of cure
  • has microfine silica fillers
  • has photo initiator systems
  • adapted to cast
39
Q

what are light activated resins used for

A
  • mostly as customised impression tray material and repair of fractured dentures
40
Q

what are some radiopaque polymers

A
  • metal inserts
  • inorganic salts
  • co-monomers containing heavy metals
  • halogen containing co-monomers or additives
41
Q

are metal inserts good

A
  • weaken denture

- poor aesthetics

42
Q

are inorganic salts good

A
  • barium sulphate
  • low concentration = not radiopaque
  • high concentration = weak base
43
Q

are co-monomers containing heavy metals good

A
  • barium sulphate

- poor mechanical properties

44
Q

are halogen containing co-monomer good

A
  • tribromophenymethacrylate
  • may act as a plasticiser - soften it
  • expensive
  • no indication if they are good or not
45
Q

what are some alternative polymers

A
  • nylons
  • vinyl polymers
  • polycarbonates
46
Q

what is bad about nylons

A
  • water absorption

- causes swelling and softening

47
Q

what are some vinyl polymers

A
  • polyvinyl acetate
  • polyvinyl chloride
  • styrene
48
Q

what is bad about vinyl polymers

A
  • injection moulding which is expensive

- soften when at temperature of 60 degrees

49
Q

what is good about polycarbonates

A
  • can withstand high temperatures more than acrylic = 150 degrees
  • good impact strength
50
Q

what is bad about polycarbonates

A
  • injection moulded which is expensive

- develop internal stresses during use that can cause distortion leading to poor fit

BDS3 - dental material science (9 decks)

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