Gypsum products

Gypsum Products

Dr. Md. Arifur RahmanAssistant Professor & Head of the Dental Unit

North East Medical [email protected]

1Dr. Md. Arifur Rahman, NEMC

CAST / MODEL: A replica of several teeth and their associated soft tissues or an edentulous arch.

DIE: Replica of a single prepared tooth.Ordinary/ MODEL PLASTER: Weakest and most porous

form of gypsum product used in dentistry.Hard Plaster/ DENTAL STONE: A stronger and less

porous form of gypsum product used in dentistry.DIE STONE: The densest form of gypsum product used in

dentistry.DIAGNOSTIC CASTS: The casts generally used for patient

education, treatment planning or tracking the progress of treatment. (e.g. orthodontic models)

2

Important Terms

Dr. Md. Arifur Rahman, NEMC

WORKING CAST: The cast that is strong enough to resist the stresses of fabricating an indirect restoration or prosthesis.

POURING: The process of vibrating the flowable gypsum product into the impression.

HYGROSCOPIC SETTING EXPANSION: The amount of setting expansion that occurs when the gypsum bonded casting investment is immersed in water.

NORMAL SETTING EXPANSION: The amount of setting expansion that occurs when a gypsum bonded casting investment is allowed to set in air.

3

Important Terms (cont.)


4

Introduction

Gypsum- A mineral mined from the Earth. Occurs most widely in a massive form known as rock-gypsum.

It is the dihydrate form of calcium sulfate, usually appears white to milky yellowish and is found in compact mass in nature.

It has commercial importance as a source of Plaster of Paris.


5

Production of Calcium Sulfate Hemihydrate

Plaster and stone products are produced by calcining calcium sulfate dihydrate, or gypsum.

CaSO4. 2H2O 1100-1300 C CaSO4. 1/2H2O gypsum plaster or stone (calcium sulfate dihydrate) ( calcium sulfate

hemihydrate)


6

Types of gypsum or gypsum products Type 1- Plaster, impression Type 2- Plaster, model Type 3- Dental stones Type 4- Dental stone high strength Type 5- Dental stone , High strength / high

expansion (Identical chemical formulae- CaSo4. 1/2H2O)


7

Dental Uses of gypsum products

Dental applications: Impression

materials Constructing casts

and dies As an investment

material


8

Requirements of model and die materials1. Dimensional accuracy. 2. Adequate mechanical properties. 3. Should ideally be fluid at the time it is poured into the

impression.4. Set material should be sufficiently strong to resist

accidental fracture.5. Set material must be hard enough to resist the abrasion

during carving of wax pattern.6. Should be compatible with all the materials it comes in

contact.7. Should have good color contrast with various waxes.


9

Gypsum (calcium sulfate) is mined, ground to a powder and heated (calcined)

• Calcination drives off water and changes the original dihydrate form (2 water molecules) to a hemihydrate form( ½ of one water molecule)

Making Plaster or Stone


10

Making Gypsum Products (cont.) Gypsum product stays in its hemihydrate form until water is added back • When water is added, hemihydrate returns to a dihydrate• The new dihydrate forms as a series of interlocking

crystals (Crystallization) As the crystals form, they give off exothermic heat, which equals the amount of heat added during calcination


11

Gypsum inhemihydrateform

Water is added tohemihydrate tochange it back todihydrate form

Amount of exothermicheat given off duringCrystallization = amountadded during calcination


12

Dental Plaster

• Calcination occurs without pressure, plaster is formed.

• Plaster is called a beta hemihydrate (Type II)• Plaster is white• Plaster is used for study models and working

models on which splints and orthodontic appliances are made


13

Dental Stone If calcination occurs under steam pressure,

an alpha hemihydrate is formed (Type III Stone)

stone is usually yellow in colour. Stone is harder, stronger and more

expensive than plaster. It is used for products that require strength

such as Removable Prosthodontics.


14

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

Die stone can be any color except white (plaster) or yellow (regular stone)

Restorations are made on a die (a working replica of a single tooth)

Die stone


15

Stone is mixed with silica dental investment

Dental investment is a material that can withstand high heat and stress produced when molten metal is forced into molds to form indirect restorations

Dental Investment


16

Gypsum products – Microscopic view

Plaster of Paris Dental stone

Die stoneDr. Md. Arifur Rahman, NEMC

17

Physical propertiesType W/P Porosity Compressiv

e strengthAbrasion resistance

Setting expansion

Plaster, impression

0.40- 0.70

High 8.8 MPa Low High

Stone 0.30 moderate 20.6 MPa Moderate Moderate

High strength stone

0.22 Low 34.3 MPa High Low

High strengthHigh-expansion stone

0.20 Low 48.0 MPa High High


18

Physical properties (cont.)

Strength and hardness: Affected by Porosity Shape 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.


19

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 soluble The greater the porosity the greater the

solubility.


20

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


21

Classification of gypsum Impression plaster (Type I)

*seldom used* Model plaster (Type II): used

for Diagnostic cast Articulation of stone cast Art portion of working cast Flasking procedure for

acrylic dentures (cast end)The mix produces a weak cast compared to dental stone. It’s available is fast and regular sets. Impression (negative replica), poured

into gypsum to make a cast (positive replica


22

Plaster

Plaster

Flask


23

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

models Orthodontic models Flasking procedure for

acrylic dentures (teeth end)

It requires less water,stronger than plaster

Dental stone casts


24

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

fabricating wax patterns of cast restorations (crown &bridge)

Implants Precision attachments work Often colored pink or green Almost 2 times stronger than

type III stoneDie stone used in the fabrication of cast crown restoration


25

High strength, high expansion dental stone (Type V) Colored blue or green Most costly of all gypsum materials Lowest W:P ratio, higher compressive strength The need for higher expansion (0.1%) was to

compensate for the solidification shrinkage of some alloys used as base metals used for dental casting


26

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

required, dental stone is the gypsum product of choice

Working models for cast restorations require die stone.

For casting procedures dental investment is required.

Manipulation


27

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?

( increased water powder ratio)

Increases setting time Increased porosity Decreases strength

Too little water?( decreased water powder

ratio)

Decreases setting time decreased porosity high strength

.


28

Manipulation (cont.)

Mixing: 1.Manual &

2.Mechanical

Manual: Rubber bowl and spatula.

Powder is sifted into water to ensure good wetting and avoid clumps, and avoid air bubbles


29

Mechanical


30

Setting reactions: Theories 1.Colloidal theory

2.Hydration theory

3. Dissolution – Precipitation theory


31

Colloidal theory

Plaster enters into the colloidal state through a sol-gel mechanism.

In sol state hemihydrate particles are hydrated to form dihydrate.

As the measured amount of water is consumed the mass coverts to solid gel.

CaSO4. 1/2 + H2O CaSO4. 2H2O plaster or stone gypsum ( calcium sulfate hemihydrate) (calcium sulfate dihydrate)


32

Hydration theory

Rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.


33

Dissolution – precipitation theory Based on dissolution of plaster and instant

recrystallization of gypsum, followed by interlocking of the crystals to form the solid gel.

( most widely accepted mechanism)


34


Initial setting time-working time Working time is measured from the start of mixing

to the point where the consistency is no longer acceptable for the product’s intended purpose.

6-10 min.

Initial setting time: time elapsed from the start of mix until loss of gloss (8-10 minutes)



Final setting time: It is the elapsed time at which the Gillmore needle leaves only a barely perceptible mark on the surface.

It is reached when the materials can be safely handled, the gypsum is cool (exothermic reaction is over).


36

Manipulation (cont.) 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


37

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.


38

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.

BB



40

Properties of gypsum productstype w/p

ratioSetting time

Min.2 hr setting expansion(1%)Min. Max.

1-hr compressive strength

MpaI.Plaster, impression

0.40-0.75

4+,- 1 0.00 0.15 4

II.Plaster, model

0.45-0.50

12+,- 4 0.00 0.30 9

III.Dental stone 0.28-0.30

12+,- 4 0.00 0.20 20.7

IV. Dental stone, high strength

0.22-0.24

12+,- 4 0.00 0.10 34.5

V. Dental stone, high strength, high expansion

0.18-0.22

12+,- 4 0.10 0.30 48.3


41

Ready for use criteria The ability to judge readiness of gypsum to

be handled improves with experience.

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.


42

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.


43

How to control setting time1. Changing water : powder ratio

Increasing water Decreasing water

Retarded setting Shorter setting time

Weaker model or cast Mix difficult to manipulate

Inaccurate model Bubbles inclusion in mix

-------- Inaccurate model


44

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.


45

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.


46

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:


47

1. Double-pour method

1 2

3


48

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?


49

3. Boxing method A strip of wax is fitted around the impression then gypsum is

poured. The wax border should extend at least 0.5 inch above the highest point of the impression.


50

Storage and clean up Gypsum should be stored in air tight 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.


51

Infection control Casts should have set for 24 hours before

being disinfected if necessary. Spray rather than immerse. Disinfectants commonly used:

Sodium hypochlorite iodophors Chlorine dioxide


53

Investment materials

Used to form metal casting through the lost wax technique


54

References

Craig’s Restorative Dental Materials 13th edition by Ronald L. Sakaguchi and John M. Powers.

Dental Materials and Their Selection - 3rd edition by William J. O'Brien

Philip’s science of dental materials, 11th edition by Kenneth J. Anusavice.


Health & Medicine

Gypsum products