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Exam > Dental Materials Science > Flashcards

Dental Materials Science Flashcards

1
Q

stress

A

force is applied to an area of a material
stress Pa = force / unit area

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2
Q

strain

A

when stress is applied to an object it causes it to change shape
level of shape change = strain

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3
Q

proportional limit

A

highest limit where stress & strain on diagram continue linearly i.e. they are directly proportional to one another

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4
Q

elastic limit

A

greatest stress that can be applied to a material without causing permanent deformation

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5
Q

ultimate tensile strength

A

highest amount of stress that can be applied to a material without it breaking up

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6
Q

fracture stress

A

stress needed for fracture

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7
Q

young’s modulus

A

measure of how much a material will change its shape with stress i.e. a high YM means a large stress is required to cause a small strain
stress / strain = YM

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8
Q

brittle

A

where only a small permanent deformation is needed before fracture of material so distance between elastic limit & UTS will be small

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9
Q

ductile

A

large permanent deformation before there is fracture of material so distance between elastic limit & UTS will be large

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10
Q

elasticity

A

ability for a material to return to its original shape after stress is removed
high elasticity = flexible
low elasticity = rigid

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11
Q

grain

A

single crystal lattice with atoms orientated in different directions, when there is a change in direction of a crystal plane there is a grain boundary
with fast cooling, smaller grains are formed and these will have better mechanical properties

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12
Q

dislocation

A

in all crystal lattices there are imperfections which can slip & propagate through the metal to the grain boundary; as this changes the lattice it means it changes the shape of the metal, these can be impeded and will increase elastic limit, UTS, hardness & corrosion resistance
ductility / impact resistance are lowered

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13
Q

3 methods of impeding dislocations

A
  1. if grains are fine this will limit the amount of defects in crystal lattice
  2. cold working will push dislocations to grain boundary before metal is finally shaped
  3. use of alloys as they include atoms of different sizes making dislocations harder
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14
Q

cold working

A

work done on a metal at a low temp i.e. below recrystallisation temp - causes slip so dislocations will collect at grain boundaries which will allow for metal to become stronger & harder
this process will increase residual stress due to instability in crystal lattice & can lead to atoms returning over time to their original positions distorting metal shape - relieved by heat annealing

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15
Q

stress relief annealing

A

alloy is heated creating thermal vibrations allowing migration of atoms which eliminates stresses by allowing atoms to rearrange within their grains yet the structure of grains & mechanical properties remain unchanged
MUST be done below recrystallisation temp

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16
Q

recrystallisation

A

temperature by which metal / alloy forms larger equiaxed grains modifying the current structure of metal alloy & it lowers elastic limit / hardness but increases ductility
this is not wanted as it spoils benefit of cold work but may be necessary to gain correct shape
increasing cold work will decrease recrystallisation temp

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17
Q

phase

A

physically distinct homogenous structure which can have more than 1 component in this situation it is also a solution

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18
Q

solution

A

homogenous mixture of components at an atomic scale

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19
Q

solid solution

A

solution in which 2 or more metals coexist forming a common lattice structure at an atomic scale

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20
Q

3 types of solid solution

A
  1. random substitutional solid solution
  2. ordered substitutional solid solution
  3. interstitial solid solution - atoms of different sizes
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21
Q

solution hardening

A

where a solid solution forms with metal atoms of different sizes - this distorts grain structure & impedes dislocation movement to grain boundary therefore improving mechanical properties: elastic limit, UTS, hardness

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22
Q

ordered hardening

A

similar to solution hardening
ordered structure will impede dislocations

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23
Q

liquidus

A

temp at which solids start to crystallise

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24
Q

solidus

A

temp at which alloy has completely crystallised

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25
Q

coring

A

if alloy with large difference between liquidus & solidus is cooled rapidly coring will occur
this is where there are different alloy combinations throughout the grain so initial grain formation is not the same as the alloy combination because atoms are not able to diffuse throughout lattice as they could when cooled slowly
smaller grains in cored alloys = improved mechanical properties but decreased corrosion resistance
to overcome - homogenising annealing

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26
Q

homogenising annealing

A

alloy is reheated alloying atoms to diffuse causing all grain compositions to become homogenous without altering grain structure (as long as temp below recrystallisation temp)

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27
Q

eutectic alloys

A

metals which are soluble in the liquid state but insoluble in a solid
as a solid they will form 2 phases with each metal forming physically different grains
at point where liquidus & solidus coincide the crystallisation process will occur at 1 single temperature and grains form simultaneously
these will be hard but very brittle

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28
Q

partial solid solubility of AgCu

A

solid solution can only form at certain compositions
below H1 & above H2 a solid solution will form but between there will be 2 solid phases both of which have alpha & beta grains
beta grains are Cu rich & Ag low however they are in a solid solution above H2
if it is not cooled rapidly, the solid solution is not maintained as there is time allowed for atoms to diffuse and a 2nd phase to form
rapid cooling necessary to maintain beta structure at room temp even if the solid solubility does not allow it

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29
Q

hardness

A

resistance to scratching or its indentation resistance

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30
Q

compressive strength

A

maximum stress a material can sustain upon crush loading

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31
Q

tensile strength

A

maximum tensile stress a material can withstand

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32
Q

fatigue strength

A

measures failure of material by repeated applications of small loads

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33
Q

transverse strength

A

ability for material to deflect the load to be resisted in another area of the material

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34
Q

impact strength

A

ability of material to withstand large stresses applied rapidly to material

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35
Q

thermal expansion coefficient

A

how much material will expand when heated

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36
Q

thermal conductivity

A

arbitrary measure of how much heat is transmitted through material

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37
Q

flow

A

change in shape of a material with a static load e.g. amalgam

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38
Q

creep

A

after long periods of low stresses below elastic limit materials may flow causing permanent deformation i.e. causing ditched margins in amalgam

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39
Q

configuration factor

A

amount of tooth bonded tooth surfaces to unbonded in a restoration

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40
Q

adhesion

A

attraction between 2 dissimilar molecules / materials

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41
Q

cohesion

A

attraction between 2 similar molecules / materials

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42
Q

crazing

A

cracks appear on surface of material - produced by mechanical stresses / different TECs

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43
Q

viscoelastic behaviour

A

occurs in elastic impression materials
undergoes elastic deformation but when removed there is some permanent strain leading to overall permanent deformation

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44
Q

how to decrease permanent deformation in IMs

A

if load time is less i.e. impression removed with a sharp pull there is less overall permanent strain (lower deformation)

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45
Q

comp resin properties depend on (4)

A
  1. filler particle size
  2. amount of filler
  3. material used as filler
  4. coupling agents used
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46
Q

impact of filler particle size on CR

A

the more filler the less flowable CR is
large amount of filler means it will not undergo much polymerisation shrinkage making it more porous allowing bacterial ingress
lower filler content means more polymerisation shrinkage so less porous but will be difficult to apply
generally want a middle version

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47
Q

impact of particle size on CR

A

larger particles = stronger CR but will have issues with finishing & staining
smaller particles = give smooth filled structure but have inferior mechanical properties
modern resins are a hybrid of both

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48
Q

comp resin flaws

A
  • low TEC so insulates pulp but this is higher than enamel & dentine
  • slightly radiopaque
  • not anticariogenic
  • experiences high polymerisation shrinkage that can affect bond strength so must be built in increments
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49
Q

CRs made from (5)

A

resin BISGMA
glass filler particles
camphorquinone
low weight dimethacrylate
silane coupling agent

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50
Q

CR component properties

A
  1. BISGMA resin - undertakes FRAP that forms a large chain bonding glass filler particles together; exothermic reaction that may harm pulp
  2. glass filler particles - mainly silica
  3. camphorquinone - source of free radicals for FRAP, molecule is activated by blue light & causes increased strength/viscosity/RMM of resin but not all reacts (35-80% unreacted in CR)
  4. low weight dimethacrylate - used to adjust viscosity & reactivity of CR
  5. silane coupling agent - help bond between resin & glass filler as any water contaminating CR will impact this
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51
Q

how to use CR

A
  • etch with phosphoric acid 20s enamel 10s dentine
  • this removes smear layer produced from use of slow speed & allows collagen matrix to be formed so bonding agents can infiltrate
  • remove with water & dry slightly
  • overdrying dentine will cause collagen matrix collapse
  • primer HEMA (hydroxyethyl methacrylate) added to infiltrate etched enamel & collagen matrix; required as BISGMA is hydrophobic & dentine is wet so allows for intimate bond. this is blown gently over surface
  • BISGMA then added to infiltrate etched surface & increase contact angle so more contact with tooth between restoration & tooth surface, will also bond with CR & allow for greater retention
  • this is blown dry & light cured; this mix dentine & resin produced is the hybrid layer which will stop microleakage
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52
Q

why add flowable CR to cavities

A

to lower stress caused by polymerisation shrinkage of CR & on high configuration factor areas to decrease stress on these areas

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53
Q

why add flowable CR to cavities

A

to lower stress caused by polymerisation shrinkage of CR & on high configuration factor areas to decrease stress on these areas

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54
Q

when will bonding system in CR not work

A

it won’t work well for tertiary dentine so would use a RMGI in this case as this reacts with tooth surface and increases bonding to tooth

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55
Q

what will decrease strength of amalgam (3)

A
  1. undermixing
  2. too much Hg after condensing
  3. if rate of packing is too slow
56
Q

issues with amalgam

A
  • exhibits creep
  • TEC is 3x that of tooth tissue; causes issues with microleakage
  • thermal conductivity is high so could potentially damage pulp (need lining material)
  • can undergo corrosion; polishing margins & Cu enriched will help avoid this
  • poor aesthetics
  • not anticariogenic
  • can cause galvanisation if in contact with other metals i.e. opposing amalgam causing pain
57
Q

why use spherical over lathe cut particles in amalgam

A

in spherical:
- less Hg required lowering perceived toxicity
- higher tensile strength
- higher early compressive strength
- less sensitive to condensation
- easier to carve

58
Q

why polish amalgam 24hrs after placement

A

compressive strength adequate after 24hrs but before 1hr is very poor so must come back

59
Q

composition of amalgam

A

liquid - Hg
powder - Ag, Sn, Cu, Zn
Sn & Ag form Ag3Sn - the gamma phase; this is the material that reacts with the liquid Hg to form amalgam

60
Q

why use Cu enriched amalgam

A
  • higher early strength
  • less creep
  • higher corrosion resistance
  • better marginal durability
    should have at least 6% Cu
61
Q

2 different compositions of Cu enriched amalgam

A
  1. dispersion modified Cu enriched amalgam - AgCu spheres with conventional lathe cut alloy
  2. single composition Cu enriched amalgam - AgSnCu powder with lathe cut & spherical particles
62
Q

setting reaction of conventional amalgam

A

Ag3Sn + Hg -> Ag3Sn + Ag2Hg3 + Sn7Hg9
y + mercury -> unreacted y + y1 + y2

63
Q

compare different gamma phases of amalgam

A

y = good strength & corrosion resistance
y1 = good corrosion resistance
y2 = weak & poor corrosion resistance as it is the most electronegative of the 3

64
Q

uses of GIC

A

temporary restorations, core build up of a tooth, lining & luting

65
Q

key adv of GIC

A

will produce chemical bond with enamel & dentine helping prevent microleakage & transfer force to natural tooth effectively
will release F- over time
bond not as strong as acid etch for composite due to brittleness of glass ionomer, also requires clean & conditioned surface (polyacrylic acid generally accepted as best conditioner)

66
Q

issues with GIC

A

very brittle, poor tensile strength, poor wear resistance, abraded & eroded by acid over time, during gelation may partially dissolute if any materials are not protected

67
Q

composition of GIC

A

glass powder - silica SiO2, alumina Al2O3 and a CaF2 flux with Ca & Al phosphates; these are fused together & ground to fine particles
liquid phase - polyacrylic acid (ionic monomer), copolymer of acrylic & itaconic acid, tartaric acid (to control setting characteristics)

68
Q

3 stages of GIC setting

A
  1. dissolution
  2. gelation
  3. hardening
    acid base reaction which produces a salt & silica gel
69
Q

dissolution stage of GIC setting

A

acid breaks down into hydrogen ions & polyions
hydrogen ions form acid which will react with glass producing silica gel around a core of unreacted glass
Ca Al Na F ions also produced

70
Q

gelation phase of GIC setting

A

initial setting with Ca ions crosslinking the polyacrylic acid
done by chelating the carboxyl groups on polyacrylic acid molecule
this crosslinking is not ideal as the calcium can chelate with the same polyacrylic acid molecule

71
Q

hardening phase of GIC setting

A

problem with Ca crosslinking with same polyacrylic acid molecule is overcome by trivalent Al ions
process starts after 30mins and can take up to 7 days

72
Q

why RMGIC over GIC

A

added resin in RMGIC will reduce brittleness & rapid set reaction by light cure which helps protect surface during initial set before hardening reaction takes place, adhesion to tooth surface is much better, better tensile strength

73
Q

uses of porcelain

A

dentures, porcelain jacket crowns, metal ceramic crowns, bridges, veneers, inlays, onlays

74
Q

adv of porcelain

A

chemically stable
biocompatible
thermal insulator
high compressive strength

75
Q

disadv of porcelain

A

low transverse/tensile/impact strength
very brittle
very hard - so much so they may wear away natural opposing teeth
porosity is inevitable

76
Q

4 stages of production of porcelain

A
  1. moulding & compaction - to remove spaces, reducing shrinkage
  2. firing - to reduce porosity
  3. cooling - slowly to avoid cracks due to poor thermal conductivity
  4. glazing - to create smooth surface, reduce surface cracks & porosity
77
Q

porcelains fused to alloys

A

this overcomes brittleness & low tensile strength
bonding via:
1. mechanical
2. chemical - electron sharing in oxide layer during firing
3. stressed skin effect - both materials have different TECs leading to compressive forces aiding bonding process

78
Q

types of porcelain alloy & their +/-

A

high gold alloy - bonds well but YM low, melting range too low
low gold alloy - less gold which will increase melting temp & improve mechanical properties
silver palladium alloy - good mechanical properties but difficult to cast
nickel chromium alloy - highest mpt & YMs but low bond strength & high casting shrinkage

79
Q

adv of PMMA

A

non toxic
non irritant as long as no monomer released
biocompatible
not affected by oral fluids
hardness & abrasion resistance generally high

80
Q

disadv of PMMA

A

mechanical properties poor so bulk much be increased

81
Q

difference in using acrylic & porcelain teeth in pmma denture

A

TEC for acrylic teeth = same
TEC for porcelain teeth = lower so acrylic will expand more than porcelain & cause crazing

82
Q

problems with using porcelain teeth in PMMA denture (2)

A

thermal conductivity is low so heat not transferred to palate so scalding can occur
higher abrasion resistance of porcelain teeth than natural dentition could wear opposing natural teeth away

83
Q

composition of PMMA

A

acrylic powder - initiator to provide free radicals for FRAP, PMMA particles (pre polymerised beads to speed up reaction), plasticiser to allow for quicker dissolving in liquid, copolymer to increase mechanical properties & reduce crazing by crosslinking polymer chains
heat cured acrylic liquid - methacrylate monomer which will polymerase, copolymer to improve mechanical properties, inhibitor in low quantities to react with any free radicals produced by heat / UV light

84
Q

setting reaction of PMMA

A

FRAP
activation phase - initiator provides free radicals
initiation phase - free radicals break C=C in monomer producing new free radical
propagation phase - monomer will grow then terminate which is a random event

85
Q

production of PMMA denture base

A

produced in mould
liner used to stop monomer penetrating mould & stop water also
lots packed into mould & then clamped down under pressure to ensure correct proportions kept
correct amount of monomer used, sufficient excess, sufficient clamp pressure will help prevent polymerisation shrinkage
heated then cooled slowly

86
Q

problems with PMMA production

A

high temp needed but if too high (>100) causes gaseous porosity
if base undercured it can lead to free monomers being left which are an irritant & monomers may be of a low molecular weight causing poor mechanical properties

87
Q

why must PMMA base be cooled slowly

A

PMMA base will have different TECs so cool slowly to allow relief of any internal stresses that may develop as these can decrease strength & fatigue strength

88
Q

why are correct proportions necessary in PMMA

A

allows base to be handled, mixed & moulded into correct shape
also decreases heat of exothermic reaction so less gaseous porosity
minimises polymerisation shrinkage

89
Q

why use self cure acrylic

A

produced at lower temp so less thermal expansion & better dimensional accuracy

90
Q

why not to use self cure acrylic

A

chemical activation less efficient so lower molecular weight molecules produced causing reduced mechanical properties & lower softening temp, more free monomer left over so more likely to be irritant
so really not much benefit

91
Q

if ptx allergic to acrylic what denture bases can be used

A
  • nylons; high water absorption
  • vinyl polymers; low softening temp
  • polycarbonates; high internal stresses causing distortion on use
92
Q

SS denture base form

A

austenitic SS

93
Q

why are Cr and Ni added to SS denture base

A

when quenched, austenitic ss will change to martensitic SS which is not useful so Cr & Ni added to prevent this as well as improving corrosion resistance & UTS

94
Q

main issues with SS as denture base

A

inaccuracy
ss swaged into correct shape but dimensional inaccuracy due to:
- contraction of die not matching model expansion
- elastic recovery of steel
- damage to the die in the high pressure
- difficult to ensure uniform thickness throughout due to uneven pressure

95
Q

3 different types of gypsum used in dentistry

A
  1. plaster
  2. stone
  3. improved stone
96
Q

adv of gypsum

A

depends on manufacture & crystalline structure but generally;
adequate strength // dimensionally stable & accurate // cheap // good colour contrast // cam expand slightly on setting so crowns bridges dentures will be slightly big & not too tight which can be worked with

97
Q

disadv of gypsum

A

low tensile strength // poor abrasion resistance // poor surface detail // poor at wetting surface of rubber IMs so less dimensionally accurate

98
Q

factors affecting properties of gypsum

A

increasing powder = increased no of nuclei of crystallisation so increased expansion & decreased set time
increased spatulation = same as above
increasing temp = no effect on expansion
addition of potassium sulfate = will decrease expansion producing much more accurate cast

99
Q

composition of gypsum

A

different hydrated versions of calcium sulfate

100
Q

setting reaction of gypsum

A

opposite of manufacture of gypsum; conversion of hemihydrate calcium sulfate to dihydrate calcium sulfate:
(CaSO4)2.H2O + 3H2O -> 2CaSO4.2H2O

101
Q

use of investment materials

A

produce alloy castings of an object
occurs by process of lost wax technique - wax pattern is covered by investment material then burnt out & molten material poured in

102
Q

2 properties of investment material that allow lost wax technique to work

A
  1. must be porous to allow for escape of trapped gases
  2. must expand & contract at the same rate as the alloy itself
103
Q

4 different types of investment materials

A
  1. dental stone / plaster - acrylic
  2. gypsum bonded materials - gold casting
  3. phosphate bonded materials - ceramics or base metal alloys
  4. silica bonded materials - base metal alloys
104
Q

2 main components of investment material

A
  1. binder - used to form solid mass i.e. gypsum / phosphate / silica
  2. refractory - used to withstand high temperatures & provide expansion i.e. silica
105
Q

methods of expansion of investment materials

A

inversion expansion - silica undergoes this at 575 degrees in which structure expands
hygroscopic expansion - water molecules attracted between crystals via capillary forces forcing them apart; increased by increasing water temp / silica powder / low powder liquid ratio
these both help produce accurate alloy

106
Q

gypsum bonded investment materials

A

only used up to 1200 degrees as after this they become chemically unstable
only used for gold alloys therefore
mostly silica but also has hemihydrate
undergoes thermal, inversion, hygroscopic & setting expansion to produce 1.4% expansion sufficient for gold alloys
has smooth surface, easy manipulation, good porosity, adequate strength if correct powder liquid ratio used

107
Q

phosphate bonded investment materials

A

more chemically stable than gypsum
contains silica, ammonium phosphate & colloidal silica solution instead of H2O - increases strength but still undergoes hygroscopic expansion
high green strength (strength at room temp), easy to use, high strength, porous

108
Q

silica bonded investment materials

A

very chemically stable, high strength, high amounts of thermal & inversion expansion due to lots of silica
not porous so need vents & manipulation is complicated

109
Q

waxes are thermoplastic - what does this mean

A

rigid at low temps but will flow at high temps so they can only be manipulated at high temps

110
Q

2 main types of wax and their uses

A
  1. modelling wax - jaw reg; easy to mould when softened without tearing flaking or cracking, easy to carve & won’t distort large amounts at mouth temp
  2. casting wax - metallic framework for RPDs; can be burnt out in lost wax technique with no residue
111
Q

when should non elastic impression materials be used and when should they not

A

in edentulous ptx where there is mobile soft tissue
should not be used where there are large undercuts
non elastic not as accurate as elastic

112
Q

properties of impression compound

A
  • heated to allow it to flow & if fully flowable may damage oral mucosa
  • won’t produce very accurate impression
  • high TEC so likely to distort between mouth temp & room temp
  • sterilisation is difficult; need an autoclave
    however
  • non toxic & non irritant
  • setting time adequate
  • low cost & long shelf life
113
Q

composition of impression compound

A

contains
resins - keeps material hard when cold, wax - cause thermoplasticity,
stearic acid - acts as plasticiser to make more flexible,
filler - reduces stickiness & shrinkage pigments - colour

114
Q

impression paste properties

A

non elastic so can deform easily
can only be used in edentulous ptx and lips must always be covered in petroleum jelly
setting time 3-8 mins so not long
non toxic & non irritant

115
Q

composition of impression paste

A

2 pastes; one with ZnO & oil (oil used as plasticiser and to turn powder into paste) and one with eugenol, inert filler, hydrogenated resin & an accelerator which are used to produce cohesion acceleration & form a past

116
Q

properties of alginate

A

irreversible hydrocolloid
accuracy quite good, records fine detail
good setting
non toxic & non irritant
must be removed with sharp jerk removing as much permanent deformation as possible
if large undercut alginate not adequate as it has a poor tear strength instead addition silicone should be used as this is less rigid & has a higher tear strength
a larger bulk (5mm minimum) of alginate can help reduce tear strength

117
Q

why must care be taken when storing alginate

A

it undergoes:
syneresis - loss of water
imbibition - uptake of water
so must be stored with damp cotton wool after sterilisation

118
Q

composition of alginate

A

salt of alginic acid
calcium sulfate
trisodium phosphate - delays gel formation
filler particles - provide cohesion & strength
modifiers - improve surface of material flavourings

119
Q

setting reaction of alginate

A

water added to powder
2NanAlg +nCaSO4 -> nNa2SO4 + CanAlg

120
Q

what are elastomers & egs

A

much more accurate than other impression materials, better dimensional stability over time, more tear resistant, less viscoelastic & lower viscosity. Egs - polysulfides, condension, addition curing silicones & polyethers

121
Q

setting of elastomers

A

polymerisation reaction with crosslinking of polymer chain
byproducts produced could affect dimensional stability & cast compatibility of mould

122
Q

elastomer viscosity

A

high known as heavy / putty
medium known as regular / monophase
low known as light / wash
as viscosity goes from low to high the accuracy of the surface detail, thermal contraction & polymerisation all decrease but dimensional stability will increase therefore; low viscosity elastomers are used as a wash around teeth where accuracy is imperative and underneath a high viscosity material is used to keep material stable, accurate & free from thermal contraction

123
Q

polysulfide elastomers

A

base paste & catalyst paste
water = byproduct, will contract over time leading to permanent deformation so model should be poured as soon as possible
working & setting time much higher than other elastomers

124
Q

condensation silicone elastomers

A

base paste & catalyst paste
setting depends on crosslinking agent used
alcohol = byproduct so significant contraction so models must be poured asap

125
Q

addition curing silicone elastomers

A

base paste & catalyst paste
setting reaction will produce no byproducts so dimensional stability of impression will not be affected over time
working & setting time adequate
adequate tear strength
less rigid than polyethers so can be used in large undercuts
but is hydrophobic like most impression materials

126
Q

polyether elastomers

A

base past & catalyst paste
stable & do not undergo much plastic deformation
much lower working & setting time than other materials so more comfortable for ptx
tear strength adequate
only come in medium viscosity
will expand more than silicones due to heat

127
Q

2 common rpd alloys

A

CoCr
type IV gold

128
Q

type IV gold composition

A

Cu - allows solid solution to be formed in all proportions causing solution & ordered hardening, increases corrosion resistance (no coring), reduces density & mpt
Ag - causes solution hardening
Platinum - causes solution hardening, finer grain structure with better properties but can cause coring; palladium has similar properties & is cheaper but produces coarser grains so reduced mechanical properties & can cause porous casting
Zn - scavenger molecule
Ni - increases hardness & strength
Indium - causes finer grain structure

129
Q

production of type IV gold alloys

A

heat treated then quenched after casting to produce fine grains
then homogenising anneal to remove coring
cold worked then stress relief annealed to remove dislocations
heat hardened to cause precipitation hardening & improve order

130
Q

CoCr composition

A

Co & Cr used to form solid solution, they both will increase strength, hardness & rigidity
Cr - used for corrosion resistance due to passive oxide layer formed on surface
Ni increases ductility
Molybdenum - added to reduce grain size and so increase strength

131
Q

production of CoCr alloy

A

high mpt (1200-1400) so silica / phosphate bonded investment material used (not gypsum)
overheating must be avoided as this causes coarse grains
must not be cooled too fast or slow as this incorporates carbides
finishing more difficult & time consuming as it has a much higher abrasion resistance than type IV gold but this means it will wear better in mouth & polish retained more easily

132
Q

type IV gold v CoCr denture bases

A

type iv gold much more ductile
UTS similar in both
type iv gold can be manipulated much more with the same amount of stress before plastic deformation occurs
type iv gold = dense so will be heavy for ptx
CoCr = more rigid than type iv
CoCr = higher hardness so polish retained betetr
on casting type iv gold will not shrink as much as CoCr making it easier to produce

133
Q

ss dental wire properties

A

high stiffness, springiness, ductility, corrosion resistance, reasonably easy to weld together
used in ortho

134
Q

what is weld decay

A

chromium carbides will precipitate at grain boundaries causing alloy to become brittle
less chromium in centre of grains meaning it is more susceptible to corrosion; this effect minimised by using low carbon steels or stabilised ss which contain metals that preferentially form carbides not at grain boundaries

135
Q

what will EDTA do

A

remove smear layer & lubricate the canal

Exam (14 decks)

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