Gypsum products – Compositions, Properties and uses

8/4/2019 Gypsum products Compositions, Properties and uses

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GYPSUM PRODUCTS

Presented By :Dr. T. M. HAROON

M.D.S -1st yr.Department of Prosthodontics


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IntroductionClassification of gypsum productsManufacture of gypsum productsCompositionSetting ReactionProperties

Setting timeViscosityCompressive strengthSurface Hardness and Abrasion resistanceTensile StrengthSetting expansionReproduction of detail.


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Manipulation

Infection control

Uses

Storage of gypsum productsGypsum bonded investment

References


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Introduction

Gypsum is a naturallyoccurring mineral foundin various parts of theworld, particularly in

Germany and NovaScotia.

Chemically, the gypsum

produced for dentalapplicatios is nearly purecalcium sulfate dihydrate(CaSO4.2H2O).

Gypsum under SEM


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Different forms of gypsum havebeen in use for many centuries forconstruction purposes and formaking artifacts. For instance,

alabaster used in building of KingSolomons Temple was a form of

gypsum.

Uses in Dentistry : preparation of

study models for oral andmaxillofacial structures and asimportant auxiliary materials fordental laboratory operations that are

involved in the production of dentalprostheses

It is supplied as powders of variouscolors, in small preweighed sachetsor in large bags, sacks.


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Classification of Gypsum Products

Gypsum products

Impression Plaster Gypsum bonded investmentModel Plaster

Dental stone

dental stone, High strengthDental stone, High strength,

High expansion


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Types of Gypsum products

Five types identified by ADA specificationNo. 25 are as follows:

Type I Impression Plaster

Type II Model Plaster

Type III Dental stone / Hydrocal

Type IV Dental stone, High strength /

Improved stoneType V Dental stone, High strength, High

expansion


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Calcination of gypsum

Prolonged heating of a substance at sometemperature below its melting temperature is

known as calcination


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CaSO4.2H2O 110 1300 C CaSO4.H2O130 2000 C CaSO4

Gypsum Plaster or stone Hexagonal

(calcium sulfate (calcium sulfate anhydrite

dihydrate) hemihydrate)

200 10000 C CaSO4orthorhombic anhydrite

Thus the principal constituent of the dental plasters and

stones is the calcium sulfate hemihydrate (CaSO4 .H2O). Depending on the method of calcination,different forms of the hemihydrate ( or ) can beobtained.


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Manufacturing process of Dental plaster andstone

Depending on the method of calcination there are two forms of

hemihydrate which are chemically same but differ mainly incrystal size, surface area and degree of lattice perfection.

The form (dental plaster) consists of large, irregularly shapedorthorhombic crystal particles with capillary pores

The form consists of smaller, regularly shaped crystallineparticles in the form of rods or prisms.

The -modified hemihydrate is made by boiling gypsum in a30% aqueous solution of calcium chloride & magnesiumchloride. This process yields the smoothest , most densepowder particles and is primarily used for dies. The -hemihydrate is called artificial stone, die stone or improvedstone.


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DentalplasterGypsum is first ground to fine particles

and then heated in an open kettle orkiln at a temperature of 110 130 0 C.The process is called dry-calcination.The resulting product is hemihydratecrystals formed.

CaSO4.2H2O 110 1300 C CaSO4.H2O

Gypsum Heat Plaster(calcium sulfate (calcium sulfate

hemihydrate)dihydrate)


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Contd..

Dental stone

Gypsum is calcined under pressure in anautoclave at 1200 - 130 0C at 17 psi for 5 7 hrs.The product obtained is much stronger andharder than hemihydrate.

The resulting product is hemihydrate crystalsformed.

CaSO4.2H2O 120 1300 C CaSO4.H2O

Gypsum 17 psi stone

(calcium sulfate (calcium sulfate hemihydrate)dihydrate)


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Improved stoneGypsum is calcined by boiling it in 30%calcium chloride solution. The chlorides

are then washed away or autoclaved inpresence of sodium succinate 0.5%.

After controlled grinding the powders have

an even higher apparent density and yieldan even stronger set.

Contd..


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Dental stone, High strength, Highexpansion

This is essentially a high strength dental

stone with much lower W/P ratio & highersetting expansion.

The low water powder ratio increases thestrength and addition of suitable modifier

increases the setting expansion.

Contd..


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Synthetic stoneThis is manufactured from a by-product ofphosphoric acid manufacture.

The manufacturing process is a closelyguarded secret with its manufacturers.

Contd..


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Differences between and - hemihydrate

hemihydrate (stone) - hemihydrate (plaster)It consist of cleavage fragments It is a fibrous aggregate of fineand crystals in the form of rods crystals with capillary pores.

or prisms.

Crystals are prismatic and more Crystals are spongy, irregularregular in shape and compact. in shape and porous.

W/P Ratio: 0.3 W/P Ratio: 0.5

Low setting expansion High setting expansion

Set gypsum is stronger and Set gypsum is of lowerharder. Strength.

High abrasion resistance Low abrasion resistance.


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Setting of Gypsum Products

(CaSO4)2 .H2O + 3 H2O 2 CaSO4.2H2O +unreacted (CaSO4)2 .H2O + Heat

Hemihydrate is four times more soluble in water

than is the dihydrate near room temperature (200

C).

The products formed during calcination, allreact with water to form gypsum, but at differentrates.


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Setting reaction of gypsum productsTheories of setting

Colloidal theory

Hydration theory

Dissolution-precipitation theory based on dissolutionof plaster and instant recrystallization of gypsum,followed by interlocking of the crystals to form the set.


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Crystalline theory

This theory is more widely accepted. It was firstproposed by a French chemist Henry Louis Chatelier(1887).

In 1907, the theory received the support of a famousDutch chemist Jacobus Hendricus Vant Hoff.

The setting is explained on the basis of difference insolubility of hemihydrate and dihydrate.

The hemihydrate is four times more soluble in waterthan is the dihydrate near room temperature.


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When the hemihydrate is mixed with water, asuspension is formed that is fluid and workable.

The hemihydrate dissolves until it forms a saturatedsolution.

This saturated hemihydrate solution is supersaturatedwith dihydrate, so the dihydrate precipitates out.

As, the dihydrate precipitates, the solution is no longer

saturated with the hemihydrate, so it continues todissolve.


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Thus, the process continues, that is, dissolutionof the hemihydrate and the precipitation of thedihydrate occur as either new crystals form orfurther growth occurs on those already present.

The reaction is continuous and proceeds until nofurther dihydrate precipitates out of solution.


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The Microstructure of Set Gypsum

The set material consists of a tangled aggregate ofgypsum crystals, with length in the range of 5 to 20m. Two distinct types of microscopic porosity can beseen in the mass.

Microporosity caused by residual unreacted water.These voids are spherical and occur between clumpsof gypsum crystals.

Microporosity resulting from growth of gypsumcrystals. These voids are associated with settingexpansion and are smaller than the first type. Theyappear as angular spaces between individual crystalsin the aggregate.


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Water-Powder RatioThe ratio of the water to the powder employed for

mixing is usually expressed as water powder ratioor simply as W:P.The W/P ratio is a very important factor in decidingthe physical and chemical properties of the finalproduct.

Recommended W/P RatioImpression plaster : 0.50 to 0.75.Dental plaster : 0.45 to 0.50.

Dental stone : 0.28 to 0.30.Improved stone : 0.22 to 0.24.High strength : 0.18 to 0.22.high expansion.


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Variation in the water powder ratio has apronounced effect on the set product.

These are as follows:

setting time,Setting expansion, and

compressive strength,

Excess water prolongs the setting time.


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Properties

The important properties of gypsum products are :SETTING TIME

VISCOSITY

STRENGTH COMPRESSIVETENSILE

SURFACE HARDNESS AND ABRASIONRESISTANCE

SETTING EXPANSIONREPRODUCTION OF DETAIL.


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Mixing time is the time from the addition of the

powder to the water until mixing is completed.

Mechanical mixing time of plaster and stone is

20 -30seconds whereas for hand spatulationatleast 1min to obtain a smooth mix.


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Working time is the time available to use aworkable mix, one that maintains an evenconsistency that may be manipulated. Generally a3 minutes working time is adequate.

The freshly mixed mass is semifluid in consistencyand quite free flowing and can be poured into themould in any shape.


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SETTING TIME

The time that elapses from the beginning ofmixing until the material hardens is known assetting time.

Initial setting time - As the reaction proceeds,

however, more hemihydrate crystals react to formdihydrate crystals. The material becomes rigid(but not hard). It can be carved but not moulded.This is known as initial setting time.2

Final setting time - The time at which the materialcan be separated from the impression withoutdistortion or fracture.2


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Tests for determining Setting Time

Loss of gloss test for initial set - As reactionproceeds, some of the excess water is taken informing the dihydrate and the gloss disappears fromthe surface of plaster mix.

Initial set 9 minutes.Exothermic reaction - The temperature rise of themass may also be used for measurement of settingtime, as the setting reaction is exothermic.

Penetration tests - By using penetrometers.Types of PenetrometersVicat needleGillmore needles


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Gillmore needles:

Two typessmall and large

The small Gillmore needlehas 1/4 Ib weight anddiameter of 1/12" while thelarge Gillmore has 1 Ib wt and

diameter of 1/24".Initial Gillmoretest The mixtureis spread out and the smallerneedle is lowered on thesurface.The time at which it no longerleaves an impression is calledthe initial set.Initial set 13 minutes. Initial Final

Gilmore needles


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Vicat needle

It weighs 300 gm, and theneedle diameter is 1 mm.

The needle with a weightedplunger rod is supported andheld just in contact with themix and soon released afterloss of gloss.

The time elapsed until theneedle no longer penetratesthe bottom of the plaster is thesetting time.

The setting time obtained withthe Vicat needle is the similarto the initial Gillmore, in mostinstances.

Final Gillmore test- The Heavier Gillmore needle is


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Final Gillmoretest The Heavier Gillmore needle isused for the next stage in the setting process.The time elapsed at which the needle leavesonly a barely visible mark on the surface is

known as the final setting time.


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Ready for Use Criterion

The ready for use criterion is a subjectivemeasure of the time at which the set materialmay be safely handled in the usual manner. It isnot determined by any designated test; theability to judge readiness improves withexperience.

Technically, set material may be considered

ready to use when compressive strength isatleast 80% of that which would be attained at 1hr. Most modern products reach ready-to-usestate in approx. 30 min.


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Control of Setting TimeManufacturing process.

Mixing and spatulation (time and rate).

Water / Powder ratio.

Temperature.

Accelerators & Retarders


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Manufacturing Process

Three methods to control the setting time thathave been incorporated into the commercialproducts available is

By increasing or decreasing the solubility of thehemihydrate.

By increasing or decreasing the nuclei ofcrystallization.

By accelerating or retarding the setting time byincreasing or decreasing the rate of crystalgrowth.


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Fineness:

Finer the hemihydrate particle size, the faster theset, because hemihydrate dissolves faster and thegypsum nuclei will be more numerous andtherefore, more rapid rate of crystallization occurs.

ImpuritiesDuring manufacturing process if the calcination isincomplete and gypsum particles are present or if

they are added in the plaster or stone the settingtime will be reduced because of increase in nucleiof crystallization.


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Mixing and spatulation

Within practical limits, the longer and faster the plasteris mixed, the faster it will set because nuclei ofcrystallization are broken and well distributed withinthe mass.

Water/Powder ratio

More the water used for mixing, the fewer the nucleiper unit volume. Thus setting time will be prolonged.


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TemperatureThere is little change between 00C and 500 C but if thetemperature of the plaster water mixture exceeds 500 C, a

gradual retardation occurs.At 100C the solubility of hemihydrate and dihydrate are equalin which case no reaction can occur and the gypsum will notset.At higher temperature range (500 C - 100C) the tendency forany gypsum crystals formed are converted back to thehemihydrate form.

Modifiers (Accelerators and Retarders)

Modifiers are chemicals added in order to alter some of theproperties and make it more acceptable.If the chemical added, decreases the setting time, it is called anaccelerator, whereas if it increases the setting time, it is called aretarder.


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AcceleratorsFinely powdered gypsum (terra alba 1%):Acts by providing additional nuclei ofcrystallization.

In low concentrations, salts like sodiumsulphate (upto 3.4%), potassium sulphate (2to 3%) and sodium chloride (upto 2%), are

accelerators.They act by making the hemihydrate moresoluble.


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Retarders

In higher concentrations sodium chloride and sodiumsulphate (above 3.4%), act as retarders. In higherconcentrations the salt precipitates and decreases thnuclei of crystallization.

Acetates, borates, citrates, tartrates, and inorganicsalts like ferric sulphate, chromic sulphate, aluminiumsulphate, are retarders, which act by reducing the ratof solution of hemihydrate, or by inhibiting growth ofdihydrate crystals.

Some additives react with hemihydrate, e.g. solubletartrates and citrates precipitate calcium tartrate andcitrate, respectively.


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Borax (1-2%) is the most effectiveretarder. During setting it forms a coatingof calcium borate around the hemihydrate.

Thus the water cannot come in contact

with the hemihydrate.Colloids such as gelatin, glue, agar,coagulated blood, etc. are effectiveretarders, presumably acting by nucleipoisoning.


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VISCOSITY

Garber, Powers, Brandau2 observed morevoids in casts made from the stones withthe higher viscosities.

Impression plaster is used infrequently, butits low viscosity makes its possible to takeimpressions with a minimum of force on

the soft tissues (mucostatic technique)


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STRENGTH

The strength increases rapidly as the materialhardens after the initial setting time.

Compressive strength

The compressive strength is inversely relatedto the W/P ratio of the mix.

The more water used to make the mix, the

lower the compressive strength.


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Effect of W/P ratio on the compression strength ofgypsum products

MATERIAL W/P RATIO COMPRESSIVE(ml/g) STRENGTH (MPa)

Model plaster 0.45 12.5

0.50 11.0

0.55 9.0

Dental stone 0.27 31.0

0.30 20.5

0.50 10.5

High strength, 0.24 38.0

Dental stone 0.30 21.5

0.50 10.5

Factors Affecting Strength


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Factors Affecting Strength

The free water content (excess water) -The greaterthe amount of free water in the set stone, the less the

strength.Wet strength (green strength) is the strength obtained

when the water in excess of that required for thehydration of the hemihydrate is left in the set gypsum.

The wet strength (compressive), for dental plaster is 9MPa and for improved dental stone is 35 MPa.

Dry strength is the strength of gypsum when theexcess free water is dried. The strength is two or more

times greater than the wet strength.Excess water may be removed from gypsum cast bylow-temperature drying. But here is no strengthincrease until the last 2% of free water is removed.


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This strength increase on drying is reversible,

thus soaking a dry cast in water reduces itsstrength to the original level).

Drying at higher temperature must be carefullycontrolled. Loss of water of crystallizationoccurs rapidly at 100C or higher and causes

shrinkage and a reduction in strength.


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Effect of drying on the strength of plaster of paris

DRYINGPERIOD (hr)

COMPRESSIVE

MPa

STRENGTH

Psi

LOSS INWT(percent)

2 9.6 1400 5.1

4 11.7 1700 11.9

8 11.7 1700 17.4

16 13.0 1900 -

24 23.3 3400 18.0

48 23.3 3400 18.0

72 23.3 3400 -


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. AUJ Yap et al. Microwave drying of highstrength dental stone: Effects on dimensionalaccuracy. Op Dent; 2003 : 193-99.

studied effects on dimensional accuracy ofmicrowave drying of high strength dentalstone and concluded that microwave radiation

at 490 W for 60 seconds is acceptable fordrying Type IV high-strength dental stone.

J Prosthet Dent. 2002 May;87(5):499-502.


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y; ( )

Tensile strength of type IV dental stones dried in a microwave oven.Hersek N, Canay S, Aka K, Cifti Y.SourceFaculty of Dentistry, University of Hacettepe, Ankara, Turkey. [email protected]

STATEMENT OF PROBLEM:

It is known that drying dental stones in a microwave oven can save time, but the

strength of the material may be affected by different drying methods.PURPOSE:

This study evaluated the diametral tensile strength (DTS) of type IV gypsumproducts at different time intervals using microwave and air-drying methods.

CONCLUSION:

Within the limitations of this study, microwave oven drying had a

positive effect on the diametral tensile strength of type IV dental

stones.
http://www.ncbi.nlm.nih.gov/pubmed?term=%22Hersek%20N%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Canay%20S%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Ak%C3%A7a%20K%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Cift%C3%A7i%20Y%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Cift%C3%A7i%20Y%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Ak%C3%A7a%20K%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Canay%20S%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Hersek%20N%22%5BAuthor%5D


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Other factors affecting strength are:

W/P ratio: The more the water, the greaterwill be the porosity and less the strength.

Spatulation: Within limits, the strengthincreases with increased spatulation.

Incorporation of accelerators andretarders: Lowers both the wet and the drystrengths.


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Tensile Strength

Gypsum is a brittle material, thus weakerin tension than in compression.

The one hour tensile strength of model

plaster is approximately 2.3 MPa.When dry, the tensile strength doubles i.e.4.1 MPa after 40 hrs at 450 C.

The tensile strength of dental stone istwice than that of plaster i.e. about 8 MPa.


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HARDNESS AND ABRASION RESISTANCE

This is related to the compressive strength. Thehigher the compressive strength of the hardeningmass, the higher the surface hardness.

After the final setting occurs, the surface hardness

remains practically constant until most of the excesswater is dried, after which it increases.

The surface hardness increases at a faster rate than

the compressive strength, since the surface of thehardened mass reaches a dry state earlier than theinner portion of the mass.


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The average dry surface hardness of the type IVstones is 92 (RHN) and

Type V stone is 82.


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Mixing high strength dental stone with acommercial hardening solution containing

colloidal silica (about 30%) improves the surfacehardness of the set gypsum.2

Studies suggest that the commercial hardeningsolutiondo not improve the abrasion resistanceof high strength dental stones.


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Paul E. Harris et al. Alterations ofsurface hardness with gypsum die

hardners. J Prosthet Dent. 2004; 92: 35-8

studied alterations of surface hardness

with gypsum die hardeners(cyanoacrylates & clearcoat) andevaluated that the die hardeners did notincrease the surface hardness of thegypsum materials ( Type III & die stone)but may play role in preventing brittlefracture of the die margin.


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REPRODUCTION OF DETAIL

Gypsum die do not reproduce surface detail as wellas epoxy dies because the surface of the setgypsum is porous on a microscopic level.

Air bubbles are often formed at the interface of the

impression and gypsum cast because freshly mixedgypsum does not wet some rubber material well.

The use of vibration during the pouring of a castreduces the presence of air bubbles.

Rinsing the impression and blowing away excesswater can improve the detail recorded by thegypsum die material.


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SETTING EXPANSION

The dimensional increase that occursconcurrent with the hardening of variousmaterials, such as plaster, dental stone, die

stone and dental casting investment.

GPT 8th edition

Two types of expansion

Normal setting expansion

Hygroscopic setting expansion


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Depending on the composition of the

gypsum product, linear expansion maybe as low as 0.06% to as high as 0.5%.

Normal Setting ExpansionThe setting expansion that occurs withoutwater immersion is normal SettingExpansion.

Hygroscopic Setting Expansion

The setting expansion that occurs underwater is hygroscopic Setting Expansion


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The theory is illustrated as:

In Stage I, the initial mix is represented by the threeround particles of hemihydrate surrounded by water.

In Stage II, the reaction starts and the crystals of thedihydrate begins to form.

Under normal expansion, the water around theparticles is reduced by hydration and the particlesare drawn more closely together by surface tensionaction of the water.

Under hygroscopic expansion, the distance betweenthe particles remain same at this stage.


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In Stage III, the water around the particles are

decreased and crystals of dihydrate grow,contact each other, and the setting expansionbegins.

The particles tend to be drawn together, butcontraction is opposed by outward thrust ofthe growing crystals whereas the crystals are

not so inhibited in hygroscopic expansionbecause the water is again replenished fromthe outside.


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In Stages IV and V, the effect becomesmore marked. The crystals intermesh andentangle much sooner than those underhygroscopic conditions which grow more

freely before the intermeshing finallyprevents further expansion.


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Importance of setting expansion in dentistry,

setting expansion may be both wanted orunwanted. It is undesirable in impression plaster,dental plaster and stone as it will result in aninaccurate cast or change in the occlusal relation if

used for mounting.

Increased setting expansion is desired in case ofinvestment materials as it help to compensate the

shrinkage of the metal during casting.


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Control of setting expansion

Increased spatulation increases setting expansion.Increase in W/P ratio reduces the setting expansion.

Modifiers generally reduce the setting expansion.

Potassium sulphate 4% solution reduces the settingexpansion.

Sodium chloride and ground gypsum increases settingexpansion.


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Uses of Gypsum products

The uses of gypsum products in dentistry iswidespread and are as follows:

In the preparation of a cast for a denture.

To form moulds for the casting of dentalrestorations with molten metal.

For the preparation of study models for oral

and maxillofacial structures.


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Impression plaster was used extensivelyfor impressions of mouth

To attach cast to an articulator.

For bite registrationVarious type of plaster is used to makedies over which dental prosthesis are

constructed.Flasking of waxed teeth.

M i l ti


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Manipulation

To secure optimum properties, the water should bemeasured and the powder weighed.

The measuring and mixing technique is summarized

as follows:Measuring the water

The water is usually dispensed by volume in agraduated cylinder.


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Measuring the powder

The powder can be weighed in grams with asimple balance or scale.

Adding Powder and water

The preferred method is to add the measuredwater first followed by the gradual addition of thepreweighed powder.

Mixing


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Hand Mixing

Hand mixing is usually

done in a flexible plasticor rubber bowl with astraight stiff-bladedspatula to combine thepowder and water.

The mix should besmooth, homogeneous,workable, and free of airbubbles.

A minimum of airinclusion in the mixedproduct is desirable toprevent surface bubbles

and internal defects.


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Mixing is usually

accomplished with awiping motion against thesides of the bowl, toeliminate lumps and airbubbles.

A smooth, homogeneousmix should be obtained inapproximately I minute.

Vibration should beimmediately followedafter mixing and duringpouring of the gypsum.


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Vacuum Mixing

Mixing done mechanically under vacuumprovides a gypsum mix that is free of airbubbles and homogeneous in consistency.

AdvantageProvides a stronger and denser cast

Disadvantage

Equipment is expensive.


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Storage of gypsum products


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Storage of gypsum products

Gypsum products are sensitive to changes in therelative humidity of the atmosphere.Hemihydrate powders absorb atmosphericmoisture from humid air, and this causes gradualdeterioration.At relative humidity of 70% or higher, hydrationbegins on the surface of the particles, forming afine coating of gypsum crystals.In the early stages of deterioration these act as

nuclei of crystallization and shorten the settingtime.


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If deterioration proceeds further the mix will notset properly because insufficient hemihydrateremains to form a coherent set mass.

This is equally likely to occur in gypsum-bonded

investments, and will have the same effects.

T o avoid deterioration, all gypsum productsshould be stored in airtight waterproof

containers, in a dry laboratory area.

Care for the Cast


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Care for the Cast

If the gypsum cast has to be soaked in water it mustbe placed in a water bath in which plaster debris isallowed to remain constantly on the bottom of the

container to provide a saturated solution of calciumsulfate at all times.

This is known as 'slurry water'.

If the cast is washed in ordinary water, surface layer

may dissolve, hence slurry water is used to preservesurface details. Such a procedure also causes anegligible expansion.


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Infection control

If an impression has not been disinfected it iswise to disinfect the stone cast.

Gypsum products may be disinfected by:Immersing cast in a disinfection solution

e.g., 0.525% sodium hypochlorite.7

Spraying the cast with disinfectant.Addition of disinfectant at time of mixing to thestone.


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Einar Berg, Dr Odont, Oyunn Nielsen, Nils

Skaug. High-level microwave disinfectionof dental gypsum cast. Int J Prosthodont2005; 18: 520-525.

studies on high-level microwave disinfectionof dental gypsum cast revealed that

microwave irradiation of casts for 5 minutes at900W gives high level disinfection thatcomplies with European Standard EN 1040.

J Prosthet Dent. 1991 May;65(5):713-8.

An evaluation of dental stones after repeated exposure to spray


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An evaluation of dental stones after repeated exposure to spray

disinfectants. Part I: Abrasion and compressive strength.Stern MA, Johnson GH, Toolson LB.

SourceDepartment of Prosthodontics, University of Washington, School of Dentistry, Seattle.

AbstractThis study investigated the effect of repeated applications of spray

disinfectants on gypsum surfaces. Types III and IV gypsum products

were evaluated in combination with iodophor, acid glutaraldehyde,

phenol, and water spray. Results demonstrated greater resistance to

abrasion with increasing numbers of water or disinfectant spray

applications. Acid glutaraldehyde spray decreased the compressive

strength of type III stone by 26%, phenol increased the compressivestrength of type IV stone by 18%, and iodophor had no significant

effect on either stone relative to compressive strength.

Gypsum bonded investment
http://www.ncbi.nlm.nih.gov/pubmed?term=%22Stern%20MA%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Johnson%20GH%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Toolson%20LB%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Toolson%20LB%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Johnson%20GH%22%5BAuthor%5Dhttp://www.ncbi.nlm.nih.gov/pubmed?term=%22Stern%20MA%22%5BAuthor%5D


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Classified by ISO (1990) as:Type IThermal expansion type, for castinginlays and crowns.Type IIHygroscopic expansion type, for casting

inlays and crowns.Type III For casting complete and partial denturebases

UseGypsum bonded investment used as moldmaterials for casting dental gold alloys.

Gypsum bonded investment


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CompositionRefractory (Silica) 55 75 %

Binder ( hemihydrate) 30 -35 %Modifying agents 5 %

Function of each constituent

Refractory (Silica : cristobalite or quartz)Acts as a refractory (heat resisting) during heating.

Regulates thermal expansion.Increases setting expansion of stone.

Binder ( hemihydrate)


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It binds and holds the silica particles together.

Modifying agentsTo increase the thermal expansion of investment.ColoringReducing agents: They reduce any oxides formed on

the metal by providing a non-oxidizing atmosphere inthe mouldModifying chemicals: They regulate settingexpansion and setting time and also preventsshrinkage of gypsum when heated at temperatureabove 3000 C, e.g. boric acid and sodium chloride.Investment containing Boric acid when heated showsincrease in compressive strength ranging from +40to + 50 %.


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Rate of setting

The same physical changes occur during the setting ofgypsum plaster, when gypsum bonded investment set

Manipulation time

Investing the wax pattern or pouring the investment

cast must be complete while the mix is still fluid.Setting time

This is usually given as Gillmore or Vicat initial set.

Preferably mold heating should be delayed untilsetting expansion is complete (usually 1 2 hrs fromthe start mixing)


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Strength

Inlay casting investments have wet compressivestrength 2-6 MPa

Denture casting investments have compressive

strength 9-14 MPa when set.In both types change in the compressive strengthon heating to the recommended mold temperaturevary according to the presence of additives to

control investment thermal expansion.


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Expansion

Inlay casting investments under laboratory conditionsshow total expansion in the range of 1.5 2.5 %.

Denture casting investments gain increased strength

by having higher gypsum content, at the expense ofthe refractory component, so their total expansions aresomewhat lower in the range of about 1.3%.


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Methods for casting small restorations can beclassified into two

Hygroscopic expansion technique in which all ormost of the expansion is gained when the investment

sets. The setting expansion greatly increased byexposure to additional water. Thermal expansion ofthe investment is relatively low.

Thermal expansion technique in which both settingand thermal expansion contribute importantly to moldexpansion.


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Setting expansion

Setting expansion in air

Linear expansion of most inlay are in the rangeof 0.1 0.6 %.

Setting expansion in water (hygroscopic settingexpansion ) 0.3 2.0 %.


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Hygroscopic expansion techniqueThe mold is heated to temperature of 4800 C, at which

temperature thermal expansion is relatively low.

Investments for use in these technique are based onquartz refractory.

Under this temperature the disadvantage is thatcarbon remaining from burnout of the wax pattern isoxidized slowly.

Prolonged heating atleast 1 hr is needed in thistechnique.

Thermal expansion technique


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Thermal expansion technique

The mold is heated to about 700 0 C temperature.

A higher mold temperature should not be used orbreak down of the calcium sulfate binder can occur inpresence of carbon, thereby liberating sulphurdioxide.

The reaction which involves reduction of the calciumsulfate binder, takes place in two stages:

CaSO4 + 4C CaS + 4CO

3CaSO4 + CaS 4CO + 4 SO2

The sulphur dioxide formed in the second reactioncause sulfide formation on gold alloy casting,resulting in discoloration and embrittlement of alloy.


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Control of expansion

CompositionThe manufacturer adjusts investment expansion by

choice of refractory and binder and additives.

W/P ratio

Decreasing the W/P ratio increases both settingexpansion (in air or water) and thermal expansion.

Period of exposure to water

In Hygroscopic expansion techniques, additionalcontrol can be obtained by varying the length of timethe setting investment is exposed to an aqueousenvironment.


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Effect of time of immersion

Immersion before the initial set results in greaterexpansion.

Spatulation

Shorter the mixing time, the less is expansion.

Effect of shelf life of the investment

The older the investment, the less is thehygroscopic expansion.


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Factors affecting strengthUse of hemihydrate increases the compressivestrength (in comparison to hemihydrate)

Use of modifying agents increases the strength

References


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References

1. Phillips science of dental materials.Kenneth J. Anusavice, W.B. Saunderscompany. Eleventh edition, 2005.

2. Restorative dental materials. Robert G.

Craig & John M. Powers. Restorative DentalMaterials. 11th ed, 2002.

3. Glossary of Prosthodontics terms, 8thedition.

4. Dental materials and their selection. WilliamJ. OBrein 3rd ed, 2002.


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5. Applied dental material. McCabe

6. Einar Berg, Dr Odont, Oyunn Nielsen, NilsSkaug. High-level microwave disinfectionof dental gypsum cast. Int J Prosthodont

2005; 18: 520-525.7.Mohammed Aleem abdullah. Surface

detail, compressive strength, and

dimensional accuracy of gypsum castsafter repeated immersion in hypochlorite.J Prosthet Dent; 95: 462-8.


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8. Paul E. Harris et al. Alterations of surfacehardness with gypsum die hardners. JProsthet Dent. 2004; 92: 35-8.

9. AUJ Yap et al. Microwave drying of high

strength dental stone: Effects ondimensional accuracy. Op Dent; 2003 :193-99.


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Gypsum products – Compositions, Properties and uses

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