GYPSUM PRODUCT

• Obtained from natural gypsum rock

• That is heated to temp. sufficiently high its converted to :• Gypsum dihydrate (up to 130oC)

• Calcium sulfate hemihydrate (up to 200oC)

• Calcium sulfate (anhydrite)

- On heating CaSO4.2H2O converted to CaSO4.½H2O- The reaction is reversible and exothermic (dihydrate hemihydrate dihydrate)- Depending on the nature raw materials of dehydration process (plaster, hydrocal, densite) plaster : fluffy, porous, less dense hydrocal : higher density, more crystalline (dental stone) densite : the most dense

Gypsum are used in dental application :

• Impression materials• Models and dies - models is replica of the fitting surface of the oral cavity - die is replica of individual teeth• Moulds - • Refractory investment

GYPSUMProduct Use in Dentistry

GYPSUM = calcium sulfate =naturally occurs as dihydrate

Heat removes water and converts dihydrate to hemihydrate.

DIRECT versus INDIRECT PROCEDURES

Dental Amalgam Inlays, Onlays, Crowns, and BridgesDental Composites Partial and Full Dentures

Temporary Appliances

[Gypsum Powder] + [H2O] [Gypsum] + [Heat]Calcium Sulfate Hemihydrate Calcium Sulfate Dihydrate

Various Gypsum Product

• Type I : impression plaster• Type II : dental plaster• Type III : dental stone• Type IV : improved dental stone / die stone• Type V : dental stone, high strength & expansion

MANUFACTUREof Dental Gypsum

PLASTER STONE DIE STONE

Chemical Name: -calcium sulfate -calcium sulfate -calcium sulfateFormula: CaSO4-(1/2)H2O CaSO4-(1/2)H2O CaSO4-(1/2)H2O

Powder Shape: Irregular Uniform UniformDensity: Porous Moderately Dense DenseProduction Steps: Heat to 115C Heat to 125C Heat to 100C

in air with steam pressure in CaCl2 sol’n

Dental Products: Plaster, Stone, Improved Stone,Impression Plaster Investment Die Stone

Common Names: [Plaster of Paris] [Hydrocal] [Densite]

GYPSUM PRODUCTS:

• Plaster• Stone, Cast Stone• Improved Stone, Die

Stone

• Specialty Stones• Investment Materials

EdentulousCast

OrthodonticModel

WorkingCast

WorkingCast withRemovableDies

RemovableDie withWaxed Inlay

GYPSUMSetting Reaction

[Gypsum Powder] + [H2O] [Gypsum] + [Heat]

[CaSO4-(1/2)H2O] + [(3/2)H2O] [CaSO4-(2)H2O] + [Heat]

Calcium Sulfate Hemihydrate Calcium Sulfate Dihydrate

Accelerators

Retarders

GYPSUMSetting Mechanism

Precipitation of dihydrate

Slurry waterCrystal expansion and interlocking

Dissolution of hemihydrate

TIME

GYPSUMSetting Stages

MixingInterval

WorkingInterval

SettingInterval

MixingTime

00:00:00 00:01:00Working

Time00:07:00

SettingTime

00:11:00

Final Set = Setting TimeInitial Set = Working Time

LOSS OF GLOSS

LargeGilmoreNeedle

SmallGilmoreNeedle

Gilmore needle:Initial setting

(Φ:1/12 inc; 1/4 pound)Final setting

(Φ:1/24 inc; 1 pound)

http://www.whipmix.com/2gypsum/microsto.html

PROPERTIESof Gypsum Products

PLASTER STONE DIE STONE

Setting Time :11 min 7 min 6 min

Setting Expansion : 0.20% 0.10% 0.05%Min. Crushing Strength:1,600 3,000 5,500 psiPorosity : 35% 20% 10%

Typical Water Content

> Reaction Water 18 cc 18 cc 18 cc> Extra Water 32 cc 12 cc 6 cc

TOTAL Water = 50 cc 30 cc 24 ccPowder = 100 gm 100 gm 100 gmW/P Ratio = 0.50 0.30 0.24

MANIPULATIONof Gypsum Products

Proportion P and L

Transfer to impression

Bulk P

Microstone

Pre-packaged P

Hand methods

Mechanically methods

Measurement & Combination of P/L

- Powder is measured on a scale by mass not by volume, but water is

measured by a small graduated cylinder

GypsumRatio

(ml in 100 g powder)

Model plasterDental stoneHigh strength dental stone

37 -50 28 – 3219 - 24

Hand methods

Longer spatulation times will tend to reduced the setting times and increase the setting expansion

Formulating model and lab plaster

- Plaster are produced by heated in an open kettle (1100-1200 C)

- ß-Calcium sulfate hemihydrate - Particle: irregular shape & porous

• Low – to Moderate Dental stone - Hydrocal are produced by dehydrated under pressure & in the presence of water vapour at 1250 C - Particle : more uniform in shape & more dense - α-Calcium sulfate hemihydrate

• Densite• Formulating as : - high-strength/low-expansion or - high-strength/high-expansion dental stone• Produced from densite made by boiling in a 30% calcium chloride solution

Hemihydrate at 100o C in the presence water doesn’t react to form dihydrate

Reaction: CaSO4.2H2O CaSO4.½H2O + 1½ H2O

Formulated with chemical that aim to:

• Modify their handling characteristics & properties• Accelerator: - Potassium sulfate K2SO4

- Terra alba (set calcium dihydrate)• Shortens the setting reaction - Sodium chloride NaCl• Retarder - Sodium citrate - Borax ( and accelerator)• Reduce the amount of water - a mixture calcium oxyde (0,1%) + gum arabic (1%)

• Volumetric contraction 7%• Effect spatulation : increased amount

spatulation causes more nuclei centers to be formed

• Effect temperature: increased temperature, the mobility Ca & sulfate ion increased, tends to increased the rate reaction & shorten setting time

• The additives in gypsum product that are used to control the degree of expansion :

• Plaster is 0.2 – 0.3 vol%• Stone & dies is0.05 – 0.10%

EFFECT of MANIPULATION

Factor Working time

Viscosity Strength

Increase W/P ratio increase decrease decreaseDecrease W/P ratio decrease increase increaseIncrease rate of spatulation decrease increase no effectIncrease temp. of water decrease increase no effectDecrease temp. of water increase decrease no effect

PROPERTIES

• Setting time: the time required for reaction to be complete and which the material can be separated from impression without distortion or fracture

• Surface hardness & abrasion resistance - High compressive strength correspond to high

surface hardness. Surface hardness increases at a faster than compressive strength.

- Abrasion resistance increased with resin impregnating

• Effect of humidity: absorb water vapor from humid atmosphere to form calcium sulfate dihydrate

- Effect colloidal systems & pH: - Colloidal systems (agar & alginate) retard the setting of gypsum product with interfering in the hydration reaction

• Tensile strength: determined with diametral test bending tends occur because of lateral force application ( on removal cast)

• Reproduction of detail: can reproduce - a groove 75 µm in width (dental plaster), - 50 µm in width (dental stone)• Setting expansion; - Dental plaster 0,2 – 0,3% - Dental stone 0,15 – 0,25%

Type Setting time Setting expansion

Compressive strength

Reproduction detail

Impression plaster

2,5 – 5,0minute

0,0 – 0,15% 4,0 – 8,0MPa

75 ± 8

Model plaster 20%minute

0,0 – 0,30% 9,0 MPa

75 ± 8

Dental stone 20%minute

0,0 – 0,20% 20,0MPa

50 ± 8

HS-LE dental stone

HS-HE dental stone

20%minute

20%minute

0,0 – 0,15%

0,16 – 0,30%

35,0MPa

35,0MPa

50 ± 8

PREVENTION forward CROSS INFECTION

Compatibility with impression :

• Agar hydrocolloid impression are more compati-ble than alginates impression

• The impression should be washed of saliva and any trace retarded the setting of gypsum

• After the impression is rinsed with water and disinfected

• The agar hydrocolloid impression must be stored in humidor from syneresis before pouring up the gypsum model

Disinfectant methods for impression

Materials Methods RecommendedAlginate

PolysulfideSiliconePolyether

ZnOE pasteImpression compound

Immersion(<10 minute )ImmersionImmersionImmersion (<10 minute )ImmersionImmersion

- Chlorine , iodophors

- Chlorine , iodophors, - Glutaraldehide, phenolic-Chlorine , iodophors, glutaraldehide, phenolic- Chlorine , iodophors- Glutaraldehide , iodophors chlorine , iodophors

Preventive for cross-infection

Disinfectant for stone cast:

• Spray disinfectant• Hypochlorites

• Iodophors

• Patient with known cases of infection(Being treated overnight gas sterilization)

INFECTION CONTROLProcedures for Gypsum Products

Infection Control Approaches (Strategies):• Additives in “impression material”• Immersion of “impression material” surfaces• Additives in “gypsum products”

Impression Tray

ImpressionMaterial

Dental Arch

GYPSUM BONDED INVESTMENT

Three types of gypsum bonded :

Type 1 Thermal expansion type : for casting inlay and crown

Type 2 Hygroscopis expansion type : for casting inlay and crown

Type 3 For casting complete and partial dentures

• According to ADA Spec. no. 2 for gold alloy• Type based on use and expansion used type I : inlay, (thermal) type II : inlay, (hygroscopic) type III: full & partial denture, (thermal)• Composition: - alpha calcium sulfate hemihydrate (60-65%) a. it bind and hold the silica particle together b. permits pouring of the mix into the mould c. It impart strength to the mould d. contributes to mould expansion

- Silica : quartz or crystobalite (30-35%) a. acts as refractory b. regulated thermal expansion c. increases setting expansion of stone d. eliminates contraction of gypsum and changes it to an expansion - Chemical modifiers (5%) a. coloring matter b. reducing agent c. modifying chemical: regulate setting expansion. Setting time, prevent shrinkage of gypsum

GYPSUM-BONDED INVESTMENT

Composition are :• Calcium sulphate α-hemihydrate ,silica (Sio2) , powdered

graphite , copper and various modifiers to control setting time

• α-hemihydrate of gypsum (25 – 45%) - causes greater strength - as a binders - provide rigidity - shrinkage after dehydration 200-4000C - slightly expansion and large contraction at 400-700oC caused decomposition of investment - product with containing carbon max. temp: 6500C

• Quartz or crystobalite (allotropic forms of Si) - provide a refractory and to regulate the thermal expansion - silica is responsible for producing the expansion - quartz undergoes inversion at 575oC from low form (α-quartz) to high form (β-quartz)•

Requirements of an investment material

• Mould must expand to compensate for the alloy shrinkage

• The powder should have a fine particle size to give a smooth surface to the casting

• The manipulation should be easy and have suitable setting time

• The set material should be porous enough to permit air in the mould cavity to escape

• At higher temp. must not decompose to give off gases that may corrode the surface

• Must have adequate strength at room temp. to permit handling, enough strength at higher temp. to withstand the impact force of the molten metal

• Powder shall be composed of calcined gypsum and silica with or without addition other agent

• Shall be uniform and free of foreign material and set cakes lumps

• Mixed with water in proper ratio applied to the dental wax pattern

• Heated in the usual manner without contaminated /resistance of substrate

• Will be found satisfactory for use in casting dental gold alloy restorations

• Fineness of the powder: 85% (number 200 sieve), 95% (number 100 sieve), 100% (number 30 sieve),

• Heated in the usual manner without contaminated /resistance of substrate

• Will be found satisfactory for use in casting dental gold alloy restorations

• Fineness of the powder: 85% (number 200 sieve), 95% (number 100 sieve), 100% (number 30 sieve),

• Casting temp. should not be critical• The material should be economical

• Setting time : should be not less than 5 – 25 min and shall not vary more than 20% from the manufacturer’s values

• Compressive strength: shall be not less 24.6 kg/cm2 (type I & II), not less than 49.2 kg/cm2 (type III)

• Thermal expansion: - shall not at any temp. with range 200 – 700 for type I & III - shall not at any temp. with range 200 – 500 for type II - shall not shrinkage/decrease in length of more than 0.15% from the max. length at any temp.

• After casting it should break away readily from the surface of metal and should not react chemically with it

• Surface defect of alloy: - shall not contaminated the surface of the alloy into it - shall not cause: - pitting, - fins - roughness surface - voids in the alloy

Factors Controlling Setting Timeinvestment

• Manufacturing process• Mixing time and rate• Water/powder ratio

• Temperature• Accelerator and retarders

Strength is affected by:

• Use alpha-hemihydrate increases compressive strength

• Use of chemical modifiers increases the strength• More water (during mixing) to reduces the strength• Heating to 700oC greatest reduction in strength when

containing sodium chloride• After has cooled to room temp. is strength decreases

(because of fine cracks)

Investing the wax pattern

• The wax pattern should be cleaned of any debris, grease or oils

• Is used a commercial wax pattern cleaner, or a diluted synthetic detergent

• The thin film of cleanser left on the pattern reduces the surface tension and permits better wetting the investment

• Care and caution to minimize air entrapment• Mix the investment with mechanical mixing under vacuum

(removes air bubbles , evacuates harmful gasses produced by the chemical reaction of the high heat investment)

Investing

• Jenis : - Gypsum bonded investment - Phospate bonded investment 700 – 1300 C - Ethyl silicate bonded investment• Terdiri dari : binder, refractory• Heating : 500 C hygroscopis 700 C thermal expansion

• Investment thickness : 6 mm the end mold cavity and end of invested ring ( gypsum bonded investment)(3 – 4 mm of the top of phosphate bonded investment )

• Placing the casting ring liner short (3.25 mm) of the end of the ring (thickness > 1 mm)

- asbestos (cause carcinogenic) - aluminosilicate ceramic liner - cellulose paper liner

6 mm 3.25 mm

A. Crucible formerB. SprueC. Formed cavityD. InvestmentE. LinerF. Casting ring

Hygroscopic low –heat technique

• Obtains the compensation expansion : - the 370C waterbath expands the wax pattern - the warm water entering the investment mold from the top adds some hygroscopic expansion - thermal expansion at 5000C

• Added expansion may be obtained by: - increasing the waterbath temperature to 400C - Using two layer of liner - increasing burnout temperature to range 600 –

6500C

Wassalamu’alaikum wr wb