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
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
3
Q
what is transverse strength
A
- flexural
- 3 point loading
- denture needs to be able to cope with impact
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
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
6
Q
what are the 4 stages of acrylic polymerisation
A
- activation
- initiation
- propagation
- termination
7
Q
what is activation
A
- of initiator to provide free radicals
8
Q
what is initiation
A
- free radicals break C=C bond in monomer and transfer free radical
9
Q
what is propagation
A
- growing polymer chain
10
Q
what is termination
A
- end of polymerisation
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
12
Q
what is in the liquid of heat cured acrylic
A
- methacrylate monomer
- inhibitor (hydroquinone 0.006%)
- co-polymers
13
Q
what does methacrylate monomer do
A
- dissolves PMMA particles
- polymerises
14
Q
what does the inhibitor do
A
- prolongs shelf life
- reacts with any free radicals produced by heat, UV light
15
Q
what do co-polymers do
A
- improv mechanical properties
- particularly cross-linking of polymers
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
17
Q
what is essential to have in acrylic heat curing
A
- efficient polymerisation
- gives high molecules wight polymer which gives good mechanical properties
18
Q
what is efficient polymerisation
A
- lots of cross linking of MMA molecules
- essential
19
Q
what can limit efficient polymerisation
A
- high temperature but gaseous porosity
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%
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
what is the chemical activation of self curing acrylic
- 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
what is the difference in unreacted monomer between self cured and heat cured acrylic
- chemical cured = 3 to 5% unreacted monomer
| - heat cured = 0.2% to 0.5% unreacted monomer which is much more effective
27
what is the dimensional accuracy of self cured acrylic
- 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
what is the dimensional accuracy of heat cured acrylic
- the contraction during manufacture, coupled with the expansion during use from date absorption means it is a little undersized
- under-sized
29
what is the molecular weight of heat cured
- high
| - makes it stronger
30
what are some drawbacks of self cured
- higher monomer levels = irritant
- oversized in mouth from water absorption
- poorer colour stability = tertiary amines susceptible to oxidation
31
what is the fracture rate of acrylic resin dentures
- 10% fracture within 10 years
32
how can we strengthen acrylic resin dentures
- high impact resistant materials
| - incorporate fibres
33
what does high impact resistant materials do
- incorporate rubber toughening agent
- stop crack propagation
- long term fatigue problems
34
what fibres can be incorporated into acrylic resin dentures
- carbon, ultra-high molecular weight polyethylene, glass
| - difficult processing - ongoing
35
what is 'ultra-high' heat cure
- 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
what are the properties of ultra-high heat cured
- exceptional flexural strength = increases chances of surviving for longer
- superior fracture toughness (ductility)= helps mitigate effect of any micro-cracks
37
what are pour 'n' cure resins
- 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
what are light activated denture resins
- 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
what are light activated resins used for
- mostly as customised impression tray material and repair of fractured dentures
40
what are some radiopaque polymers
- metal inserts
- inorganic salts
- co-monomers containing heavy metals
- halogen containing co-monomers or additives
41
are metal inserts good
- weaken denture
| - poor aesthetics
42
are inorganic salts good
- barium sulphate
- low concentration = not radiopaque
- high concentration = weak base
43
are co-monomers containing heavy metals good
- barium sulphate
| - poor mechanical properties
44
are halogen containing co-monomer good
- tribromophenymethacrylate
- may act as a plasticiser - soften it
- expensive
- no indication if they are good or not
45
what are some alternative polymers
- nylons
- vinyl polymers
- polycarbonates
46
what is bad about nylons
- water absorption
| - causes swelling and softening
47
what are some vinyl polymers
- polyvinyl acetate
- polyvinyl chloride
- styrene
48
what is bad about vinyl polymers
- injection moulding which is expensive
| - soften when at temperature of 60 degrees
49
what is good about polycarbonates
- can withstand high temperatures more than acrylic = 150 degrees
- good impact strength
50
what is bad about polycarbonates
- injection moulded which is expensive
| - develop internal stresses during use that can cause distortion leading to poor fit
BDS3 - dental material science
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