GYSPSUM PRODUCTS

 Gypsum is naturally occurring white powder in form of CaSO4. 2H2O

 Then is converted in to hemihydrate by process called calcination.
 We use hemihydrate in our lab work to form various gypsum products by adding water And
again there it is converted to dihydrate

APPLICATIONS
In Dentistry
2. Various types of plasters are used to make moulds, casts and dies over which
dental prostheses and restorations are made .
3. To attach casts to an articulator.
4. For bite registration (e.g. to record centric jaw relation).
5. Dental investments: When plaster is mixed with silica it is known as dental
investment. They are used to form moulds into which molten metal is cast.

Model= refers to replica of several teeth and their associated soft tissues
Die= refer to replica of single tooth

CLASSIFICATION
ISO standard for dental gypsum
Type I —dental plaster , imprerssion
Type II —Dental plaster, model
Type III —Dental stone , die and model
Type IV —dental stone , hingh strength , low expansion
Type V —Dental stone, high strength, high expansion.

Requirements of dental cast materials

1. dimensional accuracy
2. mechanical properties
3. material should be fluid at a time when it is poured into impression , so that fine details can be
recorded
4. low contact angle between model and impression material , at 0 degree complete wetting takes
place
5. bio compatible
6. good color

MANUFACTURE OF GYPSUM PRODUCTS

 The process of heating gypsum for manufacturing plaster is known as calcination.
 When heated, gypsum (calcium sulphate dihydrate) loses part of its water of
crystallization and changes to calcium sulphate hemihydrate.
Alpha and Beta Hemihydrate
 Depending on the method of calcination there are two forms of hemihydrates.
 Beta hemihydrate (plaster)
• Alpha hemihydrate (stone)

Dental plaster ( plaster of paris)

 Produced by a process known as calcination
 When gypsum is heated to a temperature of 120 degree centigrade , this produces irregular ,
porous particles reffered to as beta hemihydrate .
 It is produced in an open kettle , hence we can say produced by dry calcination.
120 degree centigrade
 CaSO4.2H2O ————————————→ CaSO4.1/2H2O(beta hemihydrate)

Dental stone
 Can be produced by two methods
 If gypsum is heated to a temperature of about 125 degree centigrade under steam pressure in
an autoclave a more regular and less porous hemihydrate is formed , refereed to as alpha
hemihydrate
 if gypsum may be boiled with solution of salt such as CaCl2 , this produce the material similar
to that produced by autoclave with even less porosity ( may also be reffered as improved stone)
 calicination is called wet calcination

Note = Chemically both alpha and beta type are the same. They differ mainly in
crystal size and form.

Water-Powder Ratio
The W/P ratio is a very important factor in deciding the physical and chemical
properties of the final product.
Example ,The higher the water-powder ratio, the longer is the setting time and
weaker will be the gypsum product. Therefore water/powder ratio should be kept
as low as possible, but at the same time sufficient to produce a workable mix.

Water Requirement of a Product is Affected by?

 Shape and compactness of crystals Thus, irregular, spongy plaster particles need
more water than the denser stone.
• Particle size distribution Grinding of the powder breaks up needle like crystals.
This improves packing characteristics and reduces the water needed.

Recommended W/P Ratio

Dental plaster = 0.55 ( requires more water bcz of porous particles and dry calination)
Dental stone= 0.33 (require less water bcz of less porosity and wet calcination)
Theoretical ratio= 0.186 ( required to satisfy a reaction)
 The fluidity of Dental gypsum
 The fluidity of dental gypsum is normally measured by one of two methods in ISO standard
 For type 1 and type 2 materials a slump test is recommended
 For type 3, 4 and 5 materials , core penetration test is recommended .

Setting process
 It begins rapidly after mixing powder and water
 First stage in the process is that water becomes saturated with hemihydrate which as solubility
of about 0.8%
 Dissolved hemihydrate is then rapidly converted to di hydrate which has solubility of about 0.2%
 Crystals of dehydrate are spheruletic in nature and grow from specific sites called nuclei of
crystallization .(they may be small particles of impurity)
 Two stages can be identified during setting reaction
1) First is the time at which material develops the property of weak solid and will not flow
readily(reffered to as initial setting time)
2) Second is the final setting time as which material Is hard enough and cannot be worked
easily on it.

SETTING REACTION
 When plaster is mixed with water it takes up one and a half molecules of water, i.e. it
regains its water of crystallization and becomes calcium sulphate dihydrate.
 The reaction is exothermic and is the same for all gypsum products.
 Maximum temperature reaches during the stage when final hardening occurs
 Temperature rise is negligible at initial set
 Magnitude of temperature rise depends upon bulk of material can be great as the 30 degree
centigrade at the center of the mass of setting material.
 This temperature may be maintained for several minutes due to thermal insulating
characteristics of material.
 This marked rise in temperature can be used to good effect when flasking dentures since it
softens the wax of trial denture and enable it to remove easily from mould

SETTING TIME
 The time elapsing from the beginning of mixing until the material hardens is called setting
time.
 Mixing time is the time from the addition the powder to the water until mixing is complete.
 Initial setting time: As the reaction proceeds, however, more hemihydrate crystals react to
form dihydrate crystals. The viscosity of the mass is increased, be carved.
 Final setting time :The time at which the material can be separated from the impression
without distortion or fracture.
Measurement of Setting Time
 Exothermic reaction The temperature rise of the mass may also be used for measurement of
setting time, as the setting reaction is exothermic.
 Penetration tests By using penetrometers.(Vicat needle(diameter 1mm) and gillmore needles)
used by ISO standard for judging setting time.
Factors Affecting Setting Time
1. Manufacturing process.
2. Mixing and spatulation (time and rate).
3. Water/Powder ratio.
4. Temperature.
5. Modifiers.

1) Manufacturing Process
 If calcination is incomplete and excess gypsum (dihydrate) is left in the final product, the
resulting plaster will set faster.
 Fineness: Finer the hemihydrate particle size, the faster the set, because — hemihydrate
dissolves faster and the gypsum nuclei will be more numerous and therefore, crystallization is
faster.

2)Mixing and spatulation

 Within limits, the longer and faster the plaster is mixed,the faster it will set because nuclei of
crystallization are broken and well distributed within the mass.

3) Water/Powder ratio
 More the water used for mixing, the fewer the nuclei per unit volume. Thus setting time will be
prolonged.
4) Temperature
 On increasing from a room temperature of 20°C to the body temperature of 37°C, the rate of
reaction decreases, and the setting time is lengthened.
5) Modifiers (Accelerators and Retarders)
 If the chemical added, it decreases the setting time, it is called an accelerator, whereas if it
increases the setting time, it is called a retarder.
 Potassium sulphate is a commonly used accelerator , it increases the solubility of hemihydrate
 Borax is most widely used retarder it decreases the solubility

SETTING EXPANSION
Setting expansion is of two types
— Normal setting expansion
— Hygroscopic setting expansion.

Normal Setting Expansion

 All gypsum products show a linear expansion during setting, due to the outward thrust of the
growing crystals during setting.
 Maximum rate of expansion occurs at the time when temperature is increasing the most
 Increased setting expansion is desired in case of investment materials as it helps to compensate
the shrinkage of the metal during casting.

Control of setting expansion

• Increased spatulation increases setting expansion.

• Increase in W/P ratio reduces the setting expansion.
• Modifiers(potassium sulphate and borax both) generally reduce the setting expansion.

Note : For accuracy in dental procedures, the setting expansion has to be minimised. The manufacturers
achieve this by addition of K2SO4. This, however, reduces the setting time. To counteract this, retarders
like borax are also added (borax also reduces setting expansion).

Hygroscopic Setting Expansion

 When a gypsum product is placed under water before the initial set stage, a greater expansion
is seen. This is due to hygroscopic expansion.

Properties of set material

 Strength depends on porosity of set material and time for which the material is allowed to dry
out after setting
 W/P ratio is directly proportional to porosity
 W/P ratio is inversilty proportional to strength
 Dental stone has greater strength than plaster due to low W/P ratio
 Gypsum is very brittle material
 Attempts to improve the mechanical properties have involved the impregnation by a polymer
such as acrylic resin and use of wetting agents which enable the material tobe used a lower W/P
ratio
Advantages and disadvantages
 Gypsum model and die have advantage of being inexpensive and easy to use
 The accuracy and dimensional stability are good and are able to produce the fine details from
impression
 It has disadvantage of mechanical properties not being idea and brittle nature of gypsum leads
to fracture easily.
 Problems occasionally arise when gypsum model and die materials are used in conjugation with
alginate impression
 The surface of model may remain soft due to an apparent retarding effect which hydrocolloids
have setting on gypsum products.