Gypsum 2010
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Transcript of Gypsum 2010
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Ed. by Harsini,
drg 2010
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Dental Applications :
Gypsum products are used mainly formaking positive reproductions or
replicas of oral structures. Casts, dies, or models
Other uses: impression, mounting,
part of investment
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Artikulator mounting
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Desirable properties(1) Accuracy(2) Dimensional stability(3) Ability to reproduce fine detail
(4) Strength and resistance toabrasion(5) Compatibility with the impression
material(6) Color(7) Biological safety
(8) Ease of use
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Manufacture
Chemical and Physical
properties
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For dental purposes in form of
Calcium sulfate dihydrate(CaSO4.2H2O)
Gypsum is a common name of calcium sulfate dihydrate.
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Dental gypsum products
Primary constituent = Calcium sulfate
hemihydrate,CaSO4.H2O
From a process called CalcinationCalcination: heating process
performed by the manufacturer to
create the hemihydrate powder formof the product
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CaSO42H2O CaSO4H2O +100-130oC
Calcination
Dehydration
H2O
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This process is reversible.
CaSO4H2O+ H2o CaSO42H2O+
Rehydration
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Types of Dental Gypsum
Plaster
Stone
High-Strength or Improved Stone
Other types of gypsum: impression
plaster, mounting plaster,..etc
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Plaster
Manufacturedby grindingthe
gypsumrock toa fine powder
andthenheatingthe powder in
anopencontainer eliminate
water &shatter the crystal
Porous, irregular powder particles
Sometimes is referredtoas -
hemihydrate or Type II stone.
Weakest andleast expensive
Usually white incolor. Used
mainly where strengthis not a
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Made fromgypsumby carefully
controlledcalcinationunder steam
pressure inaclosedcontainer.
Powder particle ismore regular,
more uniforminshape, andlessporous
Is oftenreferredtoas -
hemihydrate, Type III stone, or
Hydrocal.
Various colors. Usedmainly for
makingcasts for diagnostic
purposes andcasts usedfor
complete andpartial denture
Stone
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High-Strengthor ImprovedStone
Made fromgypsumbycalciningthe gypsum
inacalciumchloride solution.
Very dense powder particlein cuboidal
shape witha reduce surface area
Strongest andmost expensive
Oftenreferredtoas Type IVstone,
die stone, densite, andmodifiedhemihydrate
Usedmainly for makingcasts or dies
for inlay andcrownfabrication
Anewly developedhigh-strength stone
withahigher compressive strength
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WorkingWorkingCast withCast withRemovableRemovableDiesDies
RemovableRemovableDie withDie with
Waxed InlayWaxed Inlay
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Forms of hemihydrate
-hemihydrate
(plaster of Paris)
Sponginess,
porous,
irregular
-hemihydrate
(dental stone)
Well-formedcrystals,
regular, more dense
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Setting Reaction
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Somethingtoremember!
All gypsumproducts set by the same reaction.
CaSO 4 H 2O+1H 2O CaSO 4 2H 2O+Heat
After calcinationprocess, some calciumsulfate
dihydrate remaininthe powder.
Bothhemi- anddihydrate are water soluble but
the hemihydrate is four times more soluble.
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Hemihydrate + water suspension
(fluidandworkable)
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Dihydrate
(supersaturated)
Dihydrate precipitate out in
formof needle-like crystals.
(some calciumsulfate dihydrate
remaining
inthe powder are
nuclei for precipitation.)
Solnnolonger sat. withhemihydrate
dissolution&precipitationor
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Formation of needle like crystals Mass
thickens
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Water/Powder ratio
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Theoretically, if 100 gof any hemihydrate were
combinedwith19 ml of water, all the hemihydrate wouldbe convertedtothe dihydrate.
(19 ml = sufficient water for 100 ghemihydrate toreact)
However inpractice, this amount of water will
not produce a mass that canbe manipulated
andpouredintoanimpression. (toodry and
crumbly mix) Therefore, inpractice, excess
water must be addedwhenone is mixingto
produce a workable mass .
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Various gypsumproducts require different amount of
water tomix.
- hemihydraterequireslesswater than hemihydrate hemihydrate=>Veryporous, irregular, donot pack together verywell
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Dependsonthe physical characteristics of the powder
particles
Plaster (Type II) 45-50 ml/100g(0.45-0.50)
Stone (Type III) 28-30 ml/100g(0.28-0.30)
Improvedstone(Type IV) 19-24 ml/100g(0.19-0.24)
different consistencies of aworkable mix
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The excess water does not react but is simply trappedin the mass whenit sets.
porosities or voids inthe mass
Set plaster has the lowest
density (most porous) because
it hadthe most excess water
andthe most voids inthe mass.
Set high-strengthstone has the
highest density.
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The water/powder ratiohas a direct effect onthe
properties of eachgypsumproduct andmust be
controlledfor optimumresults.
Higher W:Pratio(Thinner mix)
Longer settingtime
Weaker product (more free water more porositieswhenset)
Lower hardness
Reduceexpansion(inthesametypeof gypsum)
Lower W:Pratio(Thicker mix)
Broader crystals stronger andmoresolidmass
Shorter settingtime
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Setting Time
Types of settingtime
Factors tocontrol settingtime
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TIMETIME
GYPSUMGYPSUMSetting StagesSetting Stages
MixingMixingIntervalInterval
WorkingWorkingIntervalInterval
SettingSettingIntervalInterval
MixingMixing
TimeTime00:000:00:000:00 1:001:00
WorkingWorkingTimeTime
0707SettingSettingTimeTime
11:0011:00
Final Set =Final Set =Setting TimeSetting Time
Initial Set =Initial Set =Working TimeWorking Time
LOSS OF GLOSSLOSS OF GLOSS
LargeLargeGilmoreGilmoreNeedleNeedle
SmallSmallGilmoreGilmoreNeedleNeedle
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Workingor Initial Setting
Time
Represents the available
time for manipulating
Indicates partial progress
of thesettingreaction
Loss of gloss
Vicat needle does not
completely penetrates.
8to16 minutes
Final SettingTime
Represents the timeuntil the
settingmass becomes rigidand
canbe separatedfromthe mold
Indicates the major completion
of the hydrationreaction
Failure of penetrationof a
fingernail or knife.
Dissipationof
the heat of reaction
30 to45 minutesStart mixing Material starts toset. Material sets.
Mixingtime
Workingtime/Initial settingtime(8-16min)
Final settingtime
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Control of the SettingTime
Controlledby the manufacturers particular
formulation
3 methods
Control solubility of the hemihydrate
Control number of nuclei for crystallization
Control rate of crystal growth
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Accelerators
Increase the solubility of the hemihydrate
without increasingthe solubility of the dihydrate
inorganic salts
SodiumChloride upto2%conc., Potassiumsulfate >2%
conc., Sodiumsulfate upto3.4%conc.
Powderedgypsum(CaSO4.H2O) or slurry of
groundgypsumcasts
Increase nuclei of crystallization
Increase spatulation(time andspeed)
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GYPSUMGYPSUMSetting MechanismSetting Mechanism
Dissolution of hemihydrateDissolution of hemihydrate
Precipitation of dihydratePrecipitation of dihydrate
Crystal expansion and interlockingCrystal expansion and interlocking
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Control of SettingExpansion
Expansioncanbe advantage or disadvantage. In
general, minimal settingexpansionis desirable.
accurate dimensional reproduction
Less W:Pratioand/or Longer mixingtime (within
practical limit) expansion (inthe same type of stone) Higher W:P
ratio
fewer nuclei/unit volume
space betweennuclei less growthinteractionof the dihydrate
crystals andless outwardthrust
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t
What will happenif
the settingprocess
occurs under water?
Expansionbecause additional
crystal growthis permitted
Hygroscopic SettingExpansion
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Hygroscopic SettingExpansionStageI: initial mix
StageII: crystalsof dihydrateare
formed
Water aroundtheparticlesis
reducedbythehydrationandthe
particlesaredrawnmoreclosely
together bythesurfacetension
actionof thewater
hydratedwater isreplaced
StageIII: crystalsgrow
Water isdecreased, particles
aredrawntogether
water isreplaced, crystalsgrow freely
StageIVandV
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Reproductionof Detail
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Reproductionof detail of animpression
depends onthe compatibility and
interactions betweenthe model material
andthe impressionmaterial.
The ability of the material toflowintoand
register finer detail andfiner lines is used
torate its detail of reproduction.
Factors: Compatibility + Wettability
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Tensile Strength
Important whenseparatingdies froman
impression.
Fromdiametral tensile tests, ingeneral:
1 hr strengthis 1/2 the dry strength.
Plasters tensile strengthis 1/2 that of die
stone.
Tensile strengths are usually 1/5 the
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MixingThe preferredmethodof
mixingis toaddthe measured
water first, followedby gradual
additionof the powder.
The use of anautomatic
vibrator, of highfrequency and lowamplitude, is helpful.
The guessworkof repeatedly
addingwater andpowder to
achieve theproper consistency
must be avoided
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Pouringthe Model
Themixedgypsumis placedinto
theimpressioninincrements,
withuse of vibrationtoenhance
theflowof the material intothe
impression.
Allowthematerial toflowacross
theimpression, withavoidance
of entrapment of air.
Alternatively, the teethandsoft tissues may be
pouredinstone or die stone andallowedtoset, and
the base canbe made fromplaster later.
The impressionshouldnot be removedfromthe
gypsumfor 45 to60 minutes toallowthe final set to
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MANIPULATIONMANIPULATIONof Gypsum Productsof Gypsum Products
Proportion P and LProportion P and L
Transfer to impressionTransfer to impression
Bulk PBulk P
Microstone
Pre-packaged PPre-packaged P
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Desirable Properties
Types of gypsum: Plaster, Stone, Improved stone
SettingReaction
W:PRatio
SettingTime
SettingExpansion, Hygroscopic Expansion Reproductionof Detail
StrengthandHardness
Summary