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2012

Composition :

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Gypsum is a mineral naturally found in nature

Brown (cokelat) Grey White

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Dental applications:* Impression materials* Constructing casts and dies* Making replicas

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Die for dental ceramic

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Fixed Partial DentureBridge

Resin acrylic Prothesa Frame prothesa

Ideal properties of die materials* Dimensional accuracy. * Ease of use. * Accurate reproduction of fine detail. * Inert. * Good strength. * Inexpensive. * Abrasion resistant. * Good color contrast

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Properties and behaviour

Chemical properties: (exothermic reaction)

Ca sulfate dihydrate heat water loss ground Ca sulfate hemihydrate

The production of various types of gypsum is basically the same:If the heating process occurs in atmospheric pressure at

115°C, the powder particles are porous and irregular (β hemihydrate, plaster).

if heating process is under pressure (Autoclave) powder particles are regular and less porous ( α hemihydrate, or dental stone).

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A further increase in pressure and refining of the powder by grinding results in denser stone, high-strength or die stone

When stone is mixed with silica: dental investment, , a material that can withstand high heat and stress produced when molten metal is forced into molds to form indirect restorations

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Gypsum products

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Plaster of Paris Dental stone

Die stone

Physical properties

TypeType W/PW/P PorosityPorosity Compressive Compressive strengthstrength

Abrasion Abrasion resistanceresistance

Setting Setting expansionexpansion

plasterplaster 0.450.45 HighHigh 8.8 MPa8.8 MPa LowLow HighHigh

StoneStone 0.300.30 moderatemoderate 20.6 MPa20.6 MPa ModerateModerate ModerateModerate

High strength High strength stonestone

0.230.23 LowLow 34.3 MPa34.3 MPa HighHigh LowLow

High strengthHigh strength

High-expansion High-expansion stonestone

0.200.20 LowLow 48.0 MPa48.0 MPa High High High High

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Physical properties

Strength and hardness: Affected byPorosityShape of particles (large irregular particles

don’t condense well leading to decreased density).

Amount of water mixed with the powder. Higher porosity requires more water to be mixed to produce a cast which will be weaker.

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Dimensional accuracy:The higher the setting expansion, the lower

the accuracy. Setting expansion results from the growth of

crystals as they join.

Solubility: Set gypsum is not highly solubleThe greater the porosity the greater the

solubility.

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Reproduction of detail:Greater porosity decreases surface detail

production

The impression material should be compatible with the gypsum to improve detail reproduction. The best compatibility is between silicon and polyethers and gypsum

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PROPERTIESPROPERTIESof Gypsum Productsof Gypsum Products

PLASTERPLASTER STONESTONE DIE STONEDIE STONE

Setting Time:Setting Time: 11 min11 min 7 min7 min 6 min6 min

Setting Expansion:Setting Expansion: 0.20%0.20% 0.10%0.10% 0.05%0.05%Min. Crushing Strength:Min. Crushing Strength: 1,600 psi1,600 psi 3,000 psi3,000 psi 5,500 psi5,500 psiPorosity:Porosity: 35%35% 20% 20% 10%10%

Typical Water Content:Typical Water Content:> > Reaction WaterReaction Water 18 cc18 cc 18 cc18 cc 18 cc18 cc> Extra Water> Extra Water 32 cc32 cc 12 cc12 cc 6 cc6 cc

TOTAL Water =TOTAL Water = 50 cc50 cc 30 cc30 cc 24 cc24 ccPowder =Powder = 100 gm100 gm 100 gm100 gm 100 gm100 gmW/P Ratio =W/P Ratio = 0.500.50 0.300.30 0.240.24

Classification of gypsum Impression plaster

(Type I) *seldom used*

Model plaster (Type II): used forDiagnostic castArticulation of

stone castArt portion of

working castFlasking procedure

for acrylic dentures (cast end)

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Impression (negative replica), poured into gypsum to make a cast (positive replica

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Plaster

Plaster

Flask

Dental stone (Type III): used for makingFull or partial denture

modelsOrthodontic modelsFlasking procedure for

acrylic dentures (teeth end)

It requires less water,

stronger than plaster

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Dental stone casts

Dental stone, high strength (Type IV):Die stone used in

fabricating wax patterns of cast restorations (crown & bridge)

ImplantsPrecision attachments

workOften colored pink or greenAlmost 2 times stronger

than type III stone

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Die stone used in the fabrication of cast inlay / crown / restoration

High strength, high expansion dental stone (Type V)

Colored blue or greenMost costly of all gypsum

materialsLowest W:P ratio, higher

compressive strength

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Manipulation

Selection: based on the desired properties and dental application. e.g.:For a diagnostic cast, plaster can be used. For a working cast, strength is required and

accuracy, dental stone is the gypsum of choice

Working models for cast restorations require die stone

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Manipulation

Proportioning (W:P ratio)Golden rule: Manufacturer instructions

should always be followed.Variations in W:P ratio affect the set

materials’ properties such as strength and accuracy. ○ Too much water?○ Too little water?

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Manipulation

Mixing:Manual: Rubber

bowl and spatula. Powder is sifted into water to ensure good wetting and avoid air bubbles

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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

Mechanical (Mechanical (also used with vacuum)

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Manipulation

Initial setting time (working time)Working time start after mixing for 1 minuteInitial setting time: time elapsed from the

start of mix until loss of gloss (8-10 minutes)6-10 minutes of working time are available

to pour the gypsum.

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Final setting time: is reached when the materials can be safely handled, the gypsum is cool (exothermic reaction is over).

Setting expansion: Results from crystal growth during setting. Can be decreased by the addition of potassium sulfate, sodium chloride, borax.

Hygroscopic expansion. If gypsum soaked during setting, water fills pores and increases volume

It’s recommended to separate the cast from impression after 1 hour.

Strength increases 2-3 times after 24 hours

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Tests for initial setting, and final setting times Loss of gloss test for initial setting time:

loss of gloss occurs as water is taken up by gypsum to form the dihydrate. The materials does not have measurable compressive strength.

Initial Gillmore test for initial set: needles are used to indent the material until no indentation can be seen = initial setting time.

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Vicat penetrometer (A) for setting time: used for the next stage of setting (refer to figure) after loss of gloss, the plunger rod is released onto the mix. Time elapsed until the rod no longer penetrates is the setting time.

Gillmore test (B) for final setting time: a heavier Gillmore needle to determine final setting time.

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B

A

B

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TIMETIME

GYPSUMGYPSUMSetting StagesSetting Stages

MixingMixingIntervalInterval

WorkingWorkingIntervalInterval

SettingSettingIntervalInterval

MixingMixingTimeTime

00:000:00:000:00 1:001:00WorkingWorking

TimeTime0707

SettingSettingTimeTime

11:0011:00

Final Set =Final Set =Setting TimeSetting Time

Initial Set =Initial Set =Working TimeWorking Time

LOSS OF GLOSSLOSS OF GLOSS

LargeLargeGilmoreGilmore

NeedleNeedle

SmallSmallGilmoreGilmoreNeedleNeedle

Ready for use criteria

Technically, the material is considered ready when compressive strength reaches 80% of the strength attained after 1 hour.

Most products are ready to use in 30 minutes.

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Clinical tip: before separating cast from impression, ensure that no part of the tray is connected to the gypsum

Clinical tip: if alginate impression dried before cast separation, soak in water for 15 minutes.

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How to control setting time

1. Changing water : powder ratio

Increasing water Increasing water Decreasing waterDecreasing water

Retarded settingRetarded setting Shorter setting timeShorter setting time

Weaker model or castWeaker model or cast Mix difficult to manipulateMix difficult to manipulate

Inaccurate modelInaccurate model Bubbles inclusion in mixBubbles inclusion in mix

---------------- Inaccurate modelInaccurate model

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2. Spatulation: rapid and prolonged spatulation accelerates setting and also increases setting expansion.

3. Temperature: increasing water temperature to a certain level will accelerate setting.

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4. Accelerators and retarders: Manufacturers add accelerators or retarders to

gypsum. These chemicals increase or decrease gypsum solubility respectively which will alter setting time accordingly.

Clinicians can add accelerators such as potassium sulfate or set gypsum (slurry water), they act as sites for crystallization.

Setting reaction retarders: blood, saliva, alginate. If left on impression, can affect surface details of impression. Impression surface need to be properly rinsed before being poured.

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Pouring of the cast: the cast is composed of two parts which are prepared separately

The anatomical part (hard and soft tissue), impression poured using a vibrator

Art portion or base, which is important to aid in handling and articulating the casts. Can be poured in different ways:

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1. Double-pour method

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1 2

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2. Single step

Both anatomical and art portions of the cast are prepared at the same time. This method requires skill and accurate timing. Difficulty encountered:If mix is too runny?If mix started to initially set?

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Storage and clean up

Gypsum should be stored in airtight dry area. Prolonged exposure to moisture can retard setting due to decreased solubility of powder.

Relevant equipment should be kept clean to avoid unwanted acceleration of setting by set gypsum.

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Infection control

Casts should have set for 24 hours before being disinfected if necessary.

Spray rather than immerse Disinfectants commonly used:

Sodium hypochloriteiodophors Chlorine dioxide

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Other types of Die stone

Metal plated die stone: silver or copper plated to increase abrasion resistance

Epoxy die stone: resin and a hardener are used to increase abrasion resistance.

Resin reinforced die stone: resin is incorporated into the gypsum material to increase abrasion resistance.

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Investment materials

Used to form metal casting through the lost wax technique

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References Dental materials, clinical applications for

dental assistants and dental hygienists. Chapter 12

Phillips’ science of dental materials. Chapter10

Download from Google

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