Elastomers

Elastomers

1. ELASTOMERS

2. Elastomers refers to a group of rubber polymers
which are either chemically or physically
crosslinked , easily stretched and rapidly recover
to their original dimension when the applied
stress is released.
Chemically there are four kinds of elastomers
Polysulfide
Condensation silicon
Polyether
Addition silicon

3. POLYSULFIDES

4. COMPOSTION
BASE PASTE
Polysulfied polymer
Filler ----Lithophone or titanium dioxide
Plasticizer---Dibutyl phthalate
Accelerator –sulfur 5%
CATALYST PASTE
Lead dioxide ---initiator
Plasticizer -----same as those added in base paste
Retarder ---Oleic or stearic acid

5. Chemistry Thebasicingredientofthepolymerpasteisa
polyfunctionalmercaptanorpolysulfidepolymer.
Thislinearpolymercontainsapproximately1
mol%ofbranchestoprovideenoughpendant
mercaptangroupsaschaincross-linkingsites.This
polymerisusuallycross-linkedwithanoxidizing
agentsuchasleaddioxide.Itistheleaddioxide
thatgivespolysulfideitscharacteristicbrown
color.Duringthecondensationreactionofthe
leaddioxidewiththeSHgroupsofthepolysulfide
polymer,twophenomenaoccur:(1)chain-
lengtheningpolymerizationfromthereaction
withtheterminal-SHgroups,and(2)cross-
linkingfromthereactionwiththependant-SH
group

6. Because the pendant groups compose only a small
percentage of the available-SH groups, initially, the
polymerization reactions result in chain lengthening,
which causes the viscosity to increase. The subsequent
cross-linking reactions tie the chains together, forming
a three-dimensional network that confers ,elastic
properties to the material .

7. MANIPULATION
Withtheproperlengthsofthetwopastessqueezedontoa
mixingpadorglassslab,thecatalystpasteisfirstcollected
onastainlesssteelspatulaandthendistributedoverthe
base,andthemixtureisspreadoutoverthemixingpad.
Themassisthenscrapedupwiththespatulabladeand
againsmoothedout.Theprocessiscontinueduntilthe
mixedpasteisofuniformcolor,withnostreaksofthebase
orcatalystappearinginthemixture.Ifthemixtureisnot
homogeneous,curingwillnotbeuniform,andadistorted
impressionwillresult.

8. Medium-bodyandheavy-bodypolysulfideimpression
pastesareextremelyviscousandsticky.Consequently,
theyaredifficulttomix.However,ifsufficientforceis
appliedandspatulationisperformedrapidly,the
materialwillseemthinnerandeasiertohandle,This
phenomenonisknownaspseudoplasticity.

9. WorkingandSettingTimes
.Workingandsettingtimesforthevariouskindsof
elastomericmaterials,asmeasuredbyan
oscillatingrheometer,Anincreaseintemperature
acceleratesthecuringrateofalltheseelastomeric
impressionmaterialsandthusdecreasesboth
settingandworkingtimes.Coolingisapractical
methodofincreasingtheworkingtimeofmost
polysulfideimpressionmaterials.Thiscanbe
accomplishedbystoringthematerialsatalow
roomtemperatureorbymixingonachilled,dry
glassslab.Then,whentheimpressionmaterialis
carriedtothemouth,settingtimeisdecreasedby
thehigheroraltemperature.oleicacidisalsoan
effectiveretarderfortheleaddioxide-cured
polysulfidematerials.Conversely,addingadropof
wateracceleratesthecuringrate.

10. Thecuringrateofsome,butnotall,ofthe
polysulfideimpressionmaterialsissensitiveto
alterationsinthebase:acceleratorratio.However,
mechanicalpropertiescanbeadverselyaffected
whenmarkedchangesinthebase:accelerator
ratiooccur.Alteringthebase:acceleratorratioto
changetheworkingorsettingtimeisnot
economical,becauseaportionofthepasteisnot
used.Moreover,becausetheacceleratorpaste
containsaretarderaswellasareactor,increasing
thebase:acceleratorratiomaynotproducea
predictablechangeinthepolymerizationrate.

11. RHEOLOGY.
Polysulfideranksasoneoftheleaststiffofthe
elastomericimpressionmaterials.Thisflexibility
allowsthesetmaterialtoreleasefromundercut
areasandberemovedfromthemouthwitha
minimumofstress.Despitethelackofstiffness,
theunsetmaterialhasahighlevelofviscosity.This
thickconsistencyoftheuncuredmaterialhelps
displaceanyunwantedfluidpresentwhileseating
theimpression.Also,theexcessmaterialextruded
fromthetraydoesnotfloweasilybecauseofthe
highviscosity,reducingthepotentialdiscomfortto
thepatientduringseatingofatray.

12. Tear Energy. Polysulfides have the highest resistance to tearing.
DIMENSIONAL STABILITY.
The stone cast should be poured up immediately
because the impression is the most accurate
immediately after removing it from the mouth.
The following are sources of dimensional change:
1. During setting most polymers contract slightly
during cross-linking, because tied together, the
chains occupy less volume, and there is a reduction
in bond length.
2. After setting, the by-product (water) of the
condensation reaction is lost, which causes
shrinkage.

13. 3. Although polysulfide impression materials are water
repellent, the materials can absorb fluids if exposed to
water, a disinfectant, or a high-humidity environment
over time.
4. After setting, there is incomplete recovery of
deformation because of the viscoelastic properties.

14. HANDLING TECHNIQUE .
A polysulfide impression of a number of teeth can
be obtained at one time with the use of syringe and
tray materials. To provide a less viscous material
for use with the syringe, most manufacturers have
provided impression materials with at least two
consistencies: one for use with the tray and a
thinner one for use with the syringe. The syringe
type may also have longer working and setting
times. In addition, the syringe material contains
less filler particles, so it has greater polymerization
shrinkage and more thermal contraction. For these
reasons, it is not advisable to use the syringe
material alone.

15. The method of using both the syringe and the tray
materials is often referred to as the multiple mix
technique because two separate mixtures are
required with two separate mixing pads and
spatulas. With the elastomeric impression
materials, the mix thickens as it begins to set.
Although the consistency may be acceptable for
the tray material, it may be too thick to be
extruded from the syringe. Thus, it is advisable to
mix the tray material first and fill the tray with a
uniform thickness of material and set it aside,
while a second person begins mixing and filling
the syringe. The material is injected from the filled
syringe within and around the prepared teeth. The
filled tray is then carried to place and seated over
the syringe material.

16. The two materials should cure together. Timing the
procedure is critical because if the tray or the syringe
material becomes too set, they may not cohere. If
either material has progressed past its working time
when the two are brought together, the bond between
them will not be strong. Polysulfide has a long
working time, which can be beneficial for the student
who is just learning to make an impression,

17. DISINFECTIONof polysulfides is my 10 mins
immersion in 10% solution of hypochlorite .
SHELF LIFE.. To maximize the shelf life, always
keep the tubes tightly closed when they are not in
use. Storage in a cool environment is also
advisable. ADA Specification No. 19 requires that
after storage of the base and accelerator for 7 days
at 60 ±2°C (140 ±3.6°F), the material must still
meet the test for permanent deformation. If the
shelf life is exceeded, the components of the pastes
may separate. Poor elastic behavior exhibited as
high "set" values (lowrecovery) is another
indication that the material has exceeded its shelf
life

18. ADVANTAGES
Long working time
Proven accuracy
High tear resistance
Less hydrophobic
Inexpensive to use Long
shelf life
DISADVANTAGES
Requires a custom tray
Must be poured in stone
immediately
Potential for significant
distortion
Odor offends patients
Messy and stains cloths
Advantages and Disadvantages of
Polysulfide Materials

19. CONDENSATION SILICONE
IMPRESSION MATERIAL

20. CHEMISTRY
The polymer consists of an -w-hydroxy-terminated
polydimethyl siloxane The condensation
polymerization of this material involves a reaction
with trifunctional and tetrafunctional alkyl
silicates, commonly tetraethyl orthosilicate, in the
presence of stannous octoate These reactions can
take place at ambient temperatures; thus, the
materials are often called roomtemperature
vulcanization (RTV) silicones. The average RTV
polymer chain consists of about 1000 units. The
formation of the elastomer occurs through a cross-
linking between terminal groups of the silicone
polymers and the alkyl silicate to form a three-
dimensional network

21. .Ethyl alcohol is a by-product of the condensation
setting reaction. Its subsequent evaporation
probably accounts for much of the contraction
that takes place in a set silicone rubber.
Improvements in the chemistry of the cross-
linking reactions have resulted in addition
reaction silicones that are discussed in a
subsequent section. The advantages of the newer
silicones resulted in a significant drop in the use of
the condensation silicones. However, the
information in this section on these original
silicone impression materials is useful since it sets
the stage for the discussion of the addition

22. COMPOSTION
It is supplied as base paste and a low viscosity liquid or
catalyst paste . Because the catalyst paste is liquid
colloidal silica or micro sized metal oxide is added as
filler to form a paste

23. Manipulation.
Condensation silicones are supplied as a base paste
and a liquid catalyst or reactor. A length of the
base is dispensed from the tube onto a graduated
mixing pad. Then, one drop of the liquid catalyst is
added for each unit length of base. These materials
are somewhat difficult to mix because of the
disparity in the viscosity of the two components.
However, the color difference of the two
components provides a visual clue as to how
completely the two are blended; for example, a
homogeneous mix has a uniform color.

24. The putty material is supplied as a very thick paste
and a liquid accelerator. Because the putty is so
viscous, it is packaged in a jar rather than a tube
and dispensed by volume using a scoop. The
manufacturer's directions indicate the number of
drops of accelerator needed for each scoop. Again,
producing a wellmixed material is not easy when
putty and an oily liquid are mixed, so some
manufacturers have formulated a two-paste putty
system. With either system, the best mixing
technique is to knead the material with the fingers.
Wearing gloves adds another complication,.
because some latex gloves contain a sulfur
component that inhibits the setting Of the putty.

25. Working and setting time
Temperature has a significant influence on the rate of
curing for condensation silicone impression materials.
Chilling the material or mixing on a cool slab slows the
reaction rate. Altering the base: catalyst ratio is
another effective and practical method of changing
the curing rate of these impression materials. When
the base: catalyst ratio is modified, it is wise to test the
setting time for the new ratio before it is used for a
patient.

26. ELASTICITY.
Condensation silicone impression materials are more
ideally elastic than polysulfides. They exhibit minimal
permanent deformation and recover rapidly when
strained. Like polysulfides, these materials are not very
stiff, which means it is not difficult to remove them
from undercuts without distortion.

27. RHEOLOGY.
The viscoelastic characteristics of' these materials
suggest that they can respond elastically (snap
back like springs) or as viscous liquids that easily
sustain permanent deformation (never return to
the exact same spot, like dashpots). The material is
more likely to respond as an elastic if it is strained
rapidly; hence, impressions must be removed
quickly so that the deformation is elastic and
recoverable. Prolonging the strain by removing the
impression slowly increases the chance for the
permanent deformation to occur because the poly-
mer chains respond in a viscous manner.

28. TEAR ENERGY.
The tear resistance is low for condensation silicone
impression materials. Although they do not tear as
easily as alginates or agar hydrocolloids, they must be
handled carefully to avoid ruining a margin of a crown
preparation when it is torn.

29. DIMENSIONAL STABILITY.
The excessive polymerization shrinkage of the
condensation silicones requires a modification of
the impression-making technique to produce
accurate impressions. Rather than the custom
tray-multiple mix technique recommended for
polysulfides, a putty-wash technique is used for
condensation silicones. This technique is able to
compensate for the poor dimensional stability of
these materials. The amount of linear contraction
is two to four times greater than for the other
impression materials In addition to the large
setting shrinkage, the dimensional instability is
also caused by the loss of the volatile reaction
product, ethyl alcohol.

30. Polysulfides undergo condensation polymerization,
but the reaction product, water, is a relatively small
molecule; thus, this loss has a much smaller effect
on the material than the loss of the larger alcohol
molecule resulting from the silicone condensation
reaction. Because thepolymerization reaction
continues even after the material is "clinically set,"
the polymerization shrinkage continues. Once the
impression is removed from the mouth, the
evaporation of the reaction by-product occurs
continuously. Thus, the most accurate model is
obtained by pouring up the impression with a
gypsum stone slurry immediately.

31. BIOCOMPATIBILITY
Silicon is one of the most biologically inert materials
SHELF LIFE.
The alkyl silicates are slightly unstable, particularly if
they are mixed with a tin compound to form a single
catalyst liquid. Thus, a limited shelf life may result
because of oxidation of the tin component ,within the
catalyst. Shelflife failure may also occur as a result of
degradation of the base or cross-linkage of the base
during storage.

32. HANDLING TECHNIQUE .
A putty-wash technique is frequently used for making
condensation silicone impressions. First, the thick putty
material is placed in a stock tray, and a preliminary
impression is made. This results in what is essentially an
intraoral custom-made tray formed by the putty. Space for
the light-bodied "wash" material is provided either by
cutting away some of the "tray" putty or by using a thin
polyethylene sheet as a spacer between the putty and the
prepared teeth. After the spacer is removed, a mix of the
thin-consistency wash material is placed into the putty
impression, and then the putty wash is seated in the mouth
to make the final impression. This method is referred to as
the two-stage putty-wash technique or the reline techniq~e.
For reliable reproduction of sharp angles in cavity
preparations, it is often necessary to use a syringe and
inject the wash material within and onto the preparations.

33. An alternative to the two-step procedure is to syringe the
wash into place, then seat the unset putty over the
light-bodied material. One difficulty with combining
the wash and putty steps is that the higher viscosity
material may displace the more fluid wash material. If
this occurs, critical areas of the preparation may be
reproduced in putty rather than in the light-bodied
material, but the putty is too thick to replicate the
required detail

34. ADVANTAGES AND DISADVANTAGES OF
CONDENSATION SILICONES COMPARED
WITH POLYSULFIDE MATERIALS
ADVANTAGES
Adequate working and
setting time
Pleasant odor and no
staining
Adequate tear strength
Better elastic
properties on removal
Less distortion on
removal
DISADVANTAGES
Adequate accuracy if
poured immediately
Poor dimensional
stability
Potential for significant
distortion
Putty-wash method is
technique sensitive
Slightly more expensive
Poor to adequate shelf

35. DISINFECTION OF THE IMPRESSION .
Condensation silicone impressions can be immersed in
most of the commercial antimicrobial solutions for a
short period (less than an hour) and not experience
any adverse effects.

36. ADDITION REACTION
SILICONE
(VINYL POLYSILOXANE)

37. Chemistry
Theadditionsiliconesarefrequentlycalled
polyvinylsiloxaneorvinylpolysiloxaneimpression
materials.Incontrastwiththecondensationsilicones,
theadditionreactionpolymeristerminatedwithvinyl
groupsandiscrosslinkedwithhydridegroupsactivated
byaplatinumsaltcatalyst.Therearenoreactionby-
productsaslongasthecorrectproportionsof
vinylsiliconeandhydridesiliconearemaintainedand
therearenoimpurities.Iftheproportionsareoutof
balanceorimpuritiesarepresent,thensidereactionwill
producehydrogengaswhichresultsinpinpointvoidsin
thestonecast.

38. Composition
Base paste Catalyst paste
Vinyl silicon Divinyl polydimethyl siloxane
Polymethyl hydrogensiloxane Other siloxane prepolymer
Other siloxane prepolymer Platinium salt activator
Hybrid silicone
Retarder
Sulfur contamination from natural latex gloves inhibits
the setting of the addition silicone impression material.

39. Manipulation
Thelight-bodyandmedium-bodyvinylpolysiloxanesare
suppliedastwopastes,theputtyissuppliedastwojars
ofhigh-viscositybaseandcatalystmaterial.The
similarityofthepasteconsistenciesandtheshear-
thinningbehavioroftheseimpressionmaterialsmadethe
vinylpolysiloxanessuitableforandautomaticdispensing
andmixingdevice.Theadvantageisthatisgreater
uniformityinproportioningandinmixing,lessairis
incorporateintothemix,andthemixingtimeisreduced.
Also,therearefewerpossibilitiesforcontaminationof
thematerial.
Themixedimpressionmaterialisejecteddirectlyintothe
adhesive-coatedtrayorontothepreparedteethifthe
syringetipisinplace.

40. WorkingandSettingTime.
Incontrastwiththecondensationsilicones,the
curingrateoftheadditionsiliconesappearstobe
evenmoresensitivetoambienttemperaturethan
arethepolysulfides.Workingandsettingtimecan
beextended(upto100%)bytheadditionofa
retardersassuppliedbytherespective
manufacturerandbycoolingthemixingslab.The
additionsiliconecanalsoberefrigeratedbefore
use.Thiscoolinghaslittleeffectonviscosity.
Retardersareimpracticalwiththeautomatic
mixingdevices.

41. Elasticity
Distortiononremovalfromundercutsisvirtually
nonexistent,becausethesematerialsexhibitthelowest
strain-incompressionvalues(permanentdistortion).The
excellentelasticpropertiespresentaprobleminthatthe
heavy-bodyputtymaterialbeginstobuildupelastic
responsewhilestillintheworkingstage.Ifthematerial
iscompressedelasticallyduringtheseatingofthe
impression,thendistortioncanoccurwhenthematerial
elasticallyrebounds.Formostofthesematerials,the
stiffnessisproportionaltotheconsistencyofthe
materials;forexample,puttiesarestiff,butlight-bodied
materialsarequiteflexible.

42. Rheology.
Asoneofthemostpsuedoplasticimpressionmaterials,
theeffectofincreasedstrainrateontheunsetmaterialis
quitepronouncedforvinylpolysiloxane.Thislarge
discrepancybetweentheflowpropertiesofthematerial
understrongforce,suchasduringsyringing,andlight
force,suchasduringseatingthetray,hasallowed
manufacturerstomarkettheone-stepmaterial.Onemix
ofthismediumviscositymaterialcanbeusedfor
capturingthefinedetailandrecordingthebulkofthe
oralstructures.Whensyringed,itflowsreadilyandyet
holdsitsshapewhenplacedinanimpressiontray.With
these"single-phase"materialsintendedforbothinjection
andtrayuse,theprimaryconsiderationisviscosity.

43. TearEnergy
Theresistancetotearingisadequate,similartothatof
thecondensationsilicones.Thematerialsarehighly
viscoelastic,sousingarapidstrainrateproducesan
elasticresponseandthevinylpolysiloxaneareless
likelytotear.

44. Dimensionalstability
Vinylpolysiloxaneimpressionmaterialsarethemost
dimensionallystableofalltheexistingmaterials.No
volatilereactionby-productisreleasedtocausethe
materialtoshrink.(Notethatthehydrogengasisnota
truereactionby-product.)Theclinicallysetmaterialis
closetobeingcompletelycured,sothereislittle
residualpolymerizationtocontributetothedimensional
change.Theprimarydimensionalchangecomesfrom
thermalshrinkageasthematerialcoolsfrommouth
temperaturetoroomtemperature.
Theimpressiondoesnothavetobepouredinstone
immediatelyacastcanbepouredbetween24hoursand
1week.

45. Biocompatibility.
Vinylpolysiloxanematerialsarehighlybiocompatible.
HandlingoftheTray
Thepopularityofastocktrayratherthanacustomtray
maybeassociatedwiththeincreaseduseoftheputty
relinetechnique.ThePrimaryputtyimpressionactually
servesasacustomtrayforthewashorrelinematerial.
Ingeneral,thetrayadhesivesforpolysulfide,polyether,
andcondensationsiliconesarequitesatisfactory.
Certainputty-typesiliconesrequiremechanical
retentioninthetray,becausetheadhesiveisineffective.

46. Handling techniqueThe addition silicone materials, the putty-wash system can, and frequently does, produce a grossly inaccurate impression. This occurs if the, putty impression is held under pressure. Problems can occur because of the pressure applied to the setting putty when the simultaneous technique is used, and they can also occur with the set putty that is used in the two-stage technique. Removing the impression from the mouth releases the pressure; the putty recovers but the recovery is excessive since the elastic precompression is recovered in addition to elastic deformation recovery from undercut areas. The distortion that is produced with the stiff, compressible putty results in shorter and narrower dies. In addition to excessive pressure, some of the distortion in putty wash impressions may be attributable to inadequate spacing for the wash material.
Handlingtechnique
Theadditionsiliconematerials,theputty-washsystemcan,
andfrequentlydoes,produceagrosslyinaccurate
impression.Thisoccursifthe,puttyimpressionisheld
underpressure.Problemscanoccurbecauseofthepressure
appliedtothesettingputtywhenthesimultaneoustechnique
isused,andtheycanalsooccurwiththesetputtythatis
usedinthetwo-stagetechnique

47. .Removingtheimpressionfromthemouthreleasesthe
pressure;theputtyrecoversbuttherecoveryisexcessive
sincetheelasticprecompressionisrecoveredinaddition
toelasticdeformationrecoveryfromundercutareas.The
distortionthatisproducedwiththestiff,compressible
puttyresultsinshorterandnarrowerdies.Inadditionto
excessivepressure,someofthedistortioninputtywash
impressionsmaybeattributabletoinadequatespacingfor
thewashmaterial.

48. Theadditionsiliconesaresuppliedinawidevariety
ofviscosities,includingthesingle-phaseor
monophaseviscosity.Eventhe"less"dimensionally
stablemonophasematerialsareconsiderablymore
accurateanddimensionallystablethanthe
polysulfidesorthecondensationsilicones

49. Disinfection
Vinylimpressionmaterialsareeasilydisinfectedby
immersingin10%hypochloriteor2%glutaraldehyde
solutions.Usually,a10-toI5-minuteimmersionis
sufficient.

50. PreparationofStoneCastsandDies.
Thehydrophobiccharacteristicsofthesevinyl
polysiloxaneimpressionmaterialsmakeitdifficultto
wetthesurface,soitisdifficulttopourabubble-free
stonecastfromanadditionsiliconeimpression
material.Toavoidthissurfactantspray,adilute
solutionofsoaporcommerciallyavailablewetting
agentsmaybepreferred.
Theimpressionisalsocompatiblewithepoxyresins
usedformoreaccuratecastanddies.Thesereplica
materialscanbepouredrepeatedlyinthesame
impression,ascanalsobedonewithgypsumproducts.

51. Advantages and Disadvantages of Addition Silicones
Compared with Polysulfide Materials
ADVANTAGES DISADVANTAGES
Shorter setting time Hydrogen gas evolution in
some materials
Easy to mix-automatic
mixing devices
need careful handling and a
very dry field
Adequate tear strength more expensive, especially
with automatic mixing device
Extremely high accuracy
Undetectable distortion on
removal
Dimensionally stable even
after 1 week

52. ShelfLife
Manufacturersclaimthatthevinylpolysiloxaneshavea2-
yearshelflife,whichisconsiderablylongerthanthatof
thecondensationsilicone.Thesiliconematerialtubesor
containersmustremaintightlyclosedbecauseairhastens
theirdeterioration.Theshelflifecanbeprolongedby
storingthematerialinacool,dryenvironment.

53. POLYETHER IMPRESSION MATERIAL

54. Chemistry.
Thistypeofelastomericimpressionmaterialwas
introducedinGermanyinthelate1960s.Itisa
polyether-basedpolymerthatiscuredbythereaction
betweenaziridineringswhichareattheendof
branchedpolyethermolecules.Themainchainis
probablyacopolymerofethyleneoxideand
tetrahydrofuran.Cross-linking,andthussetting,is
broughtaboutbyanaromaticsulfonateesterofthe
type,whereRisanalkylgroup.Thisproducescross
linkingbycationicpolymerizationviatheamineend
groups.
Thismaterialwasthefirstelastomerstobedeveloped
primarilytofunctionasanimpressionmaterial

55. Composition.
Thepolyetherrubbersaresuppliedastwo
pastes.Thebasecontainsthepolyether
polymer,colloidalsilicaasfiller,anda
plasticizersuchasaglycoetherorphthalate.
Theacceleratorpastecontainsthealkyl–
aromaticsulfonateinadditiontothe
aforementionedfillerandplasticizer.

56. Manipulation
Originally,polyetherswereonlysuppliedinone
viscosity.Thepseudoplasticityofthematerials
allowedonemixtobeusedforbothsyringeand
traymaterial.Subsequently,manufacturers
suppliedanadditionalpastethatcouldbeusedto
produceathinnermix.
Thecomponentmaterialsrequireda
reformulationtoadaptthematerialtousewith
theautomaticmixingdevices.Polyetherwas
changedsothatitcouldbesuppliedinseveral
viscosities.

57. WorkingandSettingTime
Thecuringrateofthepolyethersislesssensitiveto
temperaturechangethanthatoftheaddition
siliconessomemodificationinbase:accelerator
ratiocanbeusedtoextendworkingtime.Useofa
thinneralsoextendsworkingtimewithonlya
slightincreaseinsettingtime.

58. Elasticity
Thepolyethershavealwaysbeenconsideredthe
stiffestoftheimpressionmaterials,excludingthe
highviscosityputties.Itisdifficulttoremovefrom
undercutareasbecauseofthehighmodulusof
elasticity.Thenewformulationofregularor
mediumbodymaterialsisactuallylessstiffthan
theonestephydrophilicpolysiloxaneimpression
material.Polyethersareslightlylesselasticthan
thevinylpolysiloxane.

59. Rheology
Thepsuedoplasticcharacteristicsallowedthe
originalsingle-viscositymaterialstobeusedas
bothsyringeandtraymaterials.Thethinner
componentwasusedmostlytoreducethestiffness
ofthesetmaterial.

60. TearEnergy
Tearresistanceisbetterthanthatofthe
condensationsiliconimpressionionmaterial.
However,polyetherismoreprone,totearingthan
polysulfide.

61. Dimensionalstability
Thestiffnessofthematerialmeansthattheforce
neededtoremovetheimpressionisgreaterfor
polyetherimpressionsthanfortheothertypeof
materials.Yet,therecoveryisnearlycomplete
becauseoftheexcellentelasticpropertyofthe
polyether.Thus,polyetherimpressioncanbe
pouredimmediately,afterseveralhours,orafter
severaldaysandtheresultingcastwillhavethe
sameaccuracy.Thepolyetherwillexhibittheleast
amountofdistortionfromtheloadsimposedonthe
setmaterial.Onepropertythathasanegativeeffect
onthematerialistheabsorptionofwaterorfluids
andthesimultaneousleachingofthewater-soluble
plasticizer.Thus,thestoredimpressionsmustbe
keptinadry,coolenvironmenttomaintaintheir
accuracy.

62. Biocompatibility.
Originally,therewassomeconcernabout
hypersensitivitytothepolyethercatalystsystem..
However,recentstudiesindicatenocytotoxiceffects
associatedwiththeiminecatalyst.Perhapsthemost
likelyelastomersinducedproblemforthepatient
arisesfrompiecesoftheimpressionmaterialbeing
leftinthesulcus.Theirritationcanrangefromminor
tosevere.

63. Handlingthetray
Regardlessofwhattypeoftrayisused,itis
importanttouseanadequateamountofadhesive
sothatthestressduringremovaldoesnotdislodge
thematerialfromthetray.

64. Handling Technique.
Mostclinicianscontinuetousethemonophaseor
singleviscositymaterial.Withasingle-viscosity
materialonlyonemixismadeandpartofthe
materialisplacedinthetrayandanotherportionis
putinthesyringeforinjectionintothecavity
preparation.Thefastsettingofthismaterial
requiresacliniciantoworkrapidly.

65. Disinfection
2 percent glutaraldehyde is a satisfactory
solution for most elastomers

66. Advantages and Disadvantages of Polyethers Compared
with Polysulfides
Advantages Disadvantages
Fast working and setting
times
Adequate accuracy if
poured immediately
Adequate tear strength Poor dimensional stability
Less hydrophobic-better
wetting
Clean but tastes bad
Less distortion on
removal
Stiffness requires
blocking undercuts
Long shelf life Slightly more expensive

67. Handling of the Stone cast
Pouringthestonecastinapolyetherimpressionis
mucheasierthaninonemadefromsiliconematerial.
Itisnotagoodideatoshortenthesettingtimeofthe
stonebytryingtoremovethecastearlythiswillresult
inaweakstonecastwhichmayfractureduring
removal.

68. Shelf life of polyether
Storinginacool,dryenvironmentprolongsthe
shelflife.However,chilledpolyetherimpression
materialbecomesrigidandcannotbemixed.Thus
itisnecessarytoallowthematerialtoreachroom
temperaturebeforeusing.

69. REFERENCES
Kenneth J,. Anusavice-PhilipsScience of Dental materials-
10
th
edition
Robert G Craig-Restorative dental Materials-11
th
Edition
William J. o brien-Dental matereials and their selection-
3
rd
edition
Herbet T Shillingburg-Fundamentals of Fixed
Prosthodontics-3
rd
edition
Notes on Dental Materials, E. C Combe-6
th
edition