1. Dr .Omar S.M.J. Ali PhD Orthodontic
119
ماده سنيه / ثاني اسنان كركوك
د.عمر
23/11/2017
Dental Materials
Dr .Omar S.M.J. Ali
PhD Orthodontic

2. Gypsum Material
A number of gypsum products are used in dentistry as adjuncts to dental operation.

3. Types of Gypsum 1. Type I: Impression plaster.
2. Type II: Dental plaster.
3. Type III: Dental stone (medium strength stone).
4. Type IV: Improved stone (high strength stone) (die stone).
5. Type V: high strength/high expansion stone.   

5. Application Of Gypsum
1-Impression plaster. 2- Mounting the casts to the articulation. 3- Form casts and dies. 4- Used as a mold for processing dental polymers. 5- Used for bite registration (record centric jaw relation).

6. Properties of ideal model material (gypsum products):
Dimensional stability, no expansion or contraction during or after setting.
High compressive strength to withstand the force applied on it.
Hardness, soft material can be easily scratched.
Reproduce the fine details
Produce smooth surface.
Reasonable setting time
Compatible with the impression material.
Can be disinfected without damaging the surface.

7. Chemical and physical nature of G.M.
Most gypsum products are obtained from natural gypsum rock. Because gypsum is the dihydrate form of calcium sulfate, on heating, it loses 1.5 g mol of its 2 g mol of H2O and is converted to calcium sulfate hemihydrate (CaSO4. 0.5H2O). When calcium sulfate hemihydrate is mixed with water, the reverse reaction takes place, and the calcium sulfate hemihydrate is converted back to calcium sulfate dihydrate.

8. Plasters
produced when the gypsum mineral is heated in an open kettle at a temperature of about 110° to 120°C (dry calcination). The hemihydrate produced is called β-calcium sulfate hemihydrate. Such a powder is known to have a somewhat irregular shape and is porous in nature. These plasters are used in formulating model and lab plasters.

9. Stones :
are produced when the gypsum is dehydrated under pressure and in the presence of water vapor at about 125°C (wet calcination), the product is called hydrocal. The powder particles of this product are more uniform in shape and denser than the particles of plaster. Calcium sulfate hemihydrate produced in this manner is designated as α-calcium sulfate hemihydrate. Hydrocal is used in making low- to moderate-strength dental stones.

10. High-strength stones
are produced when the gypsum rock is boiling in a 30% calcium chloride solution, after which the chloride is washed away with hot water (100°C), the product is called densite, and the material is ground to the desired fineness. This variety is made by gypsum The calcium sulfate hemihydrate in the presence of 100°C water does not react to form calcium sulfate dihydrate because at this temperature their solubilities are the same.

12. 1-Potassium sulfate, and terra alba (set calcium sulfate dihydrate) are effective accelerators.
2- Sodium chloride in small amounts shortens the setting reaction but increases the setting expansion of the gypsum mass. 
3- Sodium citrate is a dependable retarder.
4- A mixture of calcium oxide (0.1%) and gum arabic (1%) reduces the amount of water necessary to mix gypsum products, resulting in improved properties.

13. 5-The first effect of increasing temperature is a change in the relative solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate, which alters the rate of the reaction. As the temperature increases, the solubility ratios decrease, until 100°C is reached and the ratio becomes one. As the ratio of the solubilities becomes lower, the reaction is slowed, and the setting time is increased.

14. 6- The second effect is the change in ion mobility with temperature
6- The second effect is the change in ion mobility with temperature. In general, as the temperature increases, the mobility of the calcium and sulfate ions increases, which tends to increase the rate of the reaction and shorten the setting time.
Practically, the effects of these two phenomena are superimposed, and the total effect is observed.

15. Effect of Humidity
Plaster can easily absorb water vapor from a humid atmosphere to form calcium sulfate dihydrate. The presence of small amounts of calcium sulfate dihydrate on the surface of the hemihydrate powder provides additional nuclei for crystallization. Increased contamination by moisture produces sufficient dihydrate on the hemihydrate powder to retard the solution of the hemihydrate. Experience has shown that the common overall effect of contamination of gypsum products with moisture from the air during storage is a lengthening of the setting time.

16. Effect of Colloidal System and PH
Colloidal systems such as agar and alginate retard the setting of gypsum products. Accelerators such as potassium sulfate are added to improve the surface quality of the set CaSO4 .2H20 against agar or alginate.
Liquids with low pH, such as saliva, retard the setting reaction. Liquids with high pH accelerate setting.

17. Powered/Water Ratio
The operator also can change the setting time of model plaster to a certain extent by changing the water/powder (W/P) ratio. The W/P ratio has a pronounced effect on the setting time. The more water in the mix of model; (plaster, dental stone, or high-strength dental stone); the longer the setting time.

18. Compressive Strength
When set, gypsum products show relatively high values of compressive strength. The compressive strength is inversely related to the W/P ratio of the mix. The more water used to make the mix, the lower the compressive strength. Model plaster has the greatest quantity of excess water, whereas high-strength dental stone contains the least excess water. The set model plaster is more porous than set dental stone, causing the apparent density of model plaster to be lower.

19. Surface roughness and Abrasion resistance
After most excess water is evaporated from the surface, the hardness will increase. Attempts have been made to increase the hardness of gypsum products by impregnating the set gypsum with epoxy or methyl methacrylate monomer that is allowed to polymerize.

20. Tensile strength
The tensile strength of model plaster and dental stone is important in structures in which bending tends to occur because of lateral force applications, such as the removal of casts from flexible impressions. Because of the brittle nature of gypsum materials, the teeth on the cast may fracture rather than bend.

21. Manipulation
When any of the gypsum products is mixed with water, it should be spatulated properly to obtain a smooth mix. Water is dispensed into a mixing bowl of an appropriate size and design. The powder is added and allowed to settle into the water for about 30 seconds. This technique minimizes the amount of air incorporated into the mix.

22. The spatulation can be continued either by:
1- Hand using a spatula.
2- Hand-mechanical spatulator.
3- Power-driven mechanical spatulator.