Gypsum Products Dental Materials Question And Answers

Gypsum Products

 

Question 1. Write a short note on the impression plaster.
Answer:

Impression plaster is a gypsum product. It is also known as type I or dental plaster or impression plaster. Impression plaster was one of the earliest impression materials in dentistry

Composition of impression plaster:

Read And Learn More: Dental Materials Question And Answers

Manufacture and Setting Reaction:

The powder form of plaster is manufactured by calcination in which gypsum is subjected to high temperatures to remove water content from it. This leads to conversion of calcium sulphate dehydrate to calcium sulphate hemihydrates in which other modifiers are added.

• CaSO₄. 1/2 H₂O + 3H₂O CaSO₄.2H₂O + 3900 Cal/gmol

Manipulation of impression plaster:

The premeasured powder is sifted into a flxible bowl which consists of a premeasured quantity of water and the material is mixed with a straight stainless steel spatula till a smooth mix is obtained. As the water/powder ratio is slightly high, it is not possible to load a thin consistency mix onto a stock tray so a closely adapting custom tray made of either acrylic resin or shellac base plate is used.

Once the impression is made to pour a cast, a separating medium such as varnish, lacquer, or detergent soap are needed since both the cast material and the impression material used are gypsum products. After about 30 to 60 minutes, the cast should be removed from the impression material by immersing the assembly into hot or warm water.

Advantages of impression plaster:

• Its hydrophilic nature and thin consistency allows it to flow and record fine details with least pressure.
• Dimensional changes are very minimum as compared to hydrocolloids.
• It is economical as the manipulation of material does not require any costly equipment.

Disadvantages of impression plaster:

• The taste is unpleasant.
• Due to the water sorption nature, it produces an increased dry sensation in the mouth of the patient.
• Gag reflux can cause increase fluidity during impression-making.
• It cannot be used in undercuts because of brittleness.
• Material can irritate the underlying soft tissues due to heat evolved during the settling process of dental plaster.
• At times separation from the cast is difficult if separating medium is not applied.

Uses of impression plaster:

• To record completely edentulous arches and regions.
• Used in dental soldering procedures for assembling and recording the relationship of crowns and politics.
• As bite registration material

Question 2. Classify gypsum products. Describe the difference between alpha and beta hemihydrates and their uses.
Or
Classify gypsum products.
Or
Write the difference between α and β hemihydrates.
Or
Classify gypsum products used in dentistry and Differentiate between dental plaster and dental stone.
Answer:

Classification of Gypsum Products

ISO 6873:2013:

• Type 1: Dental plaster for impressions
• Type 2: Dental plaster
  • Class 1: For mounting
  • Class 2: For models
• Type 3: Dental stone for models
• Type 4: Dental stone (high strength, low expansion)
  for dies
• Type 5: Dental stone (high strength, high expansion)
  for dies.

According to ANSI/ADA specification no. 25:

Difference between Alpha Hemihydrate and Beta Hemihydrate:

Uses of Alpha Hemihydrate:

• Uses of alpha hemihydrate:
  • For preparing the master cast.
  • For making mould.
• Uses of beta hemihydrates:
  • For making study cast and models.
  • For making moulds, for curing dentures
  • For mounting casts on articulator.

Question 3. Write short notes on accelerators and retarders.
Or
Write briefs on accelerators and retarders.
Or
Write short notes on modifiers used in gypsum.
Answer:

Accelerators and Retarders:

Accelerators and retarders are also called as modifiers. Modifiers are chemicals added in order to alter some of the properties and make it more acceptable to the dentist.

Accelerators:

• If the chemical added increases the rate of reaction and decreases the setting time it is called an accelerator.
• In low concentrations, salts like sodium sulphate (up to 3.4%), potassium sulphate (2 to 3%), and sodium chloride (up to 2%) are accelerators. They act by making the hemihydrate more soluble.
• An accelerator changes hemihydrates to be much more soluble than dehydrate thus accelerating the chemical reaction.
• Reduction in setting time caused by accelerators depends on the amount and the rate of solubility of hemihydrates.
• Various examples of accelerators are:
  • 2% NaCl
  • 3.4% Na
  • 2SO4
  • 2% K 2SO4
  • Terra alba (ground gypsum) 1%
  • 0.1% lime
  • 1% gum Arabic
  • Potassium sodium tartrate

In above examples if NaCl and Na2SO4 when added more than the mentioned concentration, it act as retarder

Retarders:

If the chemical added, increases the setting time and decreases the rate of reaction it is called as retarder.

• Retarders act by forming an adsorbed layer on hemihydrates to reduce its solubility.
• In higher concentration sodium chloride and sodium sulfate (above 3.4%) act as retarders.
• In higher concentrations the salt precipitates and poisons the nuclei of crystallization.
• Acetates, borates, citrates, tartarates, and inorganic salts like ferric sulphate, chromic sulphate, aluminium sulphate, are retarders which act by nuclei poisoning by reducing the rate of solution of hemihydrates or by inhibiting growth of dihydrate crystals.
• Borax (1–2%) is the most effective retarder. During setting it forms a coating of calcium borate around the hemihydrates.
• Thus, water cannot come in contact with hemihydrate.
• Colloids such as gelatin, glue, agar, coagulated blood, etc. are effective retarders.
• They retard rate of setting reaction by getting adsorb on crystals or calcium sulphate dihydrate nucleation sites and interfere hydration reaction.

Question 4. Write a short note on setting of dental plaster.
Answer:

The setting of Dental Plaster:

When 1 g mol of calcium sulfate hemihydrates reacts with 1.5 g mol of water, the hemihydrates revert to dihydrate forming 1 g mol of calcium sulfate dihydrate by hydration, with liberation of 3900 calories of heat.

Here the heat which is evolved in reaction is equal to the heat originally used in calcinations.

• (CaSO₄)₂.H₂O  +  3H₂O → 2CaSO₄.2H₂O  +  unreacted (CaSO₄)₂.½ H₂O  +  Heat
• Hemihydrate  +  Water → Dihydrate  +  unreacted hemihydrates  +  3900 cal/g mol

Three theories of settng have been put forward, i.e.

• Colloidal theory
• Hydration theory
• Dissolution-precipitation theory.

Out of these theories most acceptable theory is dissolution precipitation theory which is:

Dissolution-Precipitation Theory (Crystalline Theory): 

This theory is widely accepted. According to the theory, the plaster dissolves and reacts to form gypsum crystals which interlock to form the set solid. The setting reaction is explained on the basis of difference in solubility of hemihydrates and dihydrate.

Hemihydrate is four times more soluble than dihydrate. When hemihydrate is mixed in water it forms a flid workable suspension. Hemihydrate dissolves until it forms a saturated solution. Some dihydrate is formed due to the reaction. The solubility of dihydrate is much less than hemihydrate, the saturated hemihydrate is supersaturated with respect to the dihydrate. All supersaturated solutions are unstable.

So the dihydrate crystals precipitate out. As the dihydrate precipitates out, the solution is no longer saturated with hemihydrate and so it continues to dissolve.The process continues until further dihydrate precipitates out of the solution. Initially, there is little reaction and thus little or no rise in temperature.

This time is referred to as the induction period. As the reaction proceeds gypsum is formed in the form of needle-like clusters called spherulites. Continued growth and intermeshing of crystals of gypsum leads to thickening and hardening of the mass into a strong solid structure.

Summary of Dissolution-Precipitation Theory:

Question 5. Define the initial and final setting time of gypsum products. State their importance.
Or
Enumerate the factor that controls the setting time of gypsum hemihydrates.
Or 
Answer:

Setting time: The time elapsing from the beginning of mixing till the material hardens is called as “setting time”.

Initial Setting Time: The time elapsing from the mixing till the material becomes rigid (but not hard) it can be carved but not moulded, this is known as “initial setting time”. It is of 5 to 7 minutes.

Final Setting Time:  The time at which the material can be separated from the impression without distortion or fracture. It is 30 to 45 minutes.

Factors Controlling Setting Time of Gypsum Hemihydrates:

Factors controlling setting time are:

• Manufacturing process: Finer is the particle size of hemihydrate, the faster it sets.
• Mixing and speculation: As faster and longer the plaster is mixed, faster it will set because nuclei of crystallization are broken and well distributed within the mass.
• Water/Powder ratio: As more water is used for mixing,
  few are the nuclei per unit volume and so the setting time increases.
• Temperature: As room temperature is increased to the
  body temperature, the rate of reaction increases slightly, and setting time decreases.
• Accelerator: As chemical added decreases the setting time is known as accelerator. Sodium chloride and potassium sulphate act as accelerators in hemihydrates.
• Retarders: They form a layer on hemihydrates to reduce solubility. Acetates, borates, citrates tartrates, and salt like ferric sulphate, chromic sulphate, and aluminum sulphate are the retarders that act by nuclei poisoning and reducing the rate of solution of hemihydrates.

Question 6. Describe setting time in detail in relation to gypsum products used in dentistry. What are the different methods of changing the setting time?
Answer:

Setting time: The time elapsing from the beginning of mixing till the material hardens is called as “setting time”.

Setting time is divided into two parts, i.e.

• Initial Setting Time: The time elapsing from the mixing till the material becomes rigid (but not hard) it can be carved but not moulded, this is known as “initial setting time”. It is of 5 to 7 minutes.
• Final Setting Time: The time at which the material can be separated from the impression without distortion or fracture. It is 30 to 45 minutes. For methods of changing setting time refer to the heading factors controlling the setting time of

Question 7. Enumerate the gypsum products used in dentistry.
Or
Tabulate their difference for physical properties, composition, and uses.
Or
Write about different types of gypsum products
Answer:

Gypsum is a mineral mined in various parts of the world.

Gypsum is also an industrial by-product.

• Gypsum was first found in mines around the city of Paris, so it is also called the “plaster of Paris”.
• The mineral gypsum CaSO4, 2H2O is usually white to yellowish-white in color and is found as a compact mass.
• Products of gypsum are used extensively in dentistry.

Enumeration of Gypsum Products Used in Dentistry

According to ADA Specification /ANSI specification:

Tabulation of Difference for physical properties, composition, and uses is given in the table:

Difference Between Gypsum Products :

Question 8. Explain the setting expansions of investment materials. Describe them in details.
Answer:

Setting Expansion of Investment Materials:

Normal Setting Expansion:

• A mixture of silica and dental stone results in setting expansion which is more than when the gypsum product is used alone.
• The silica particles probably interfere with the intermeshing of the crystals as they form. Thus, the thrust of the crystals is outward during growth.
• ADA specification No. 2 for type I investments permits maximum setting expansion in air of 0.5%

Hygroscopic Setting Expansion:

• When gypsum products are allowed to set in contact with water, the amount of expansion exhibit is much greater than the normal setting expansion.
• The increased amount of expansion is because the water helps the outward growth of crystals.
• This expansion is known as hygroscopic setting expansion.
• Investment should be immersed in water before initial set is complete.
• ADA specifications no. 2 for type II investments require minimum of 1.2% and maximum of 2.2% expansion.

Thermal Expansion:

• In case of gypsum investment thermal expansion is achieved by placing the mould in furnace at a temperature below than 700°C, if temperature is more than 700°C, the investment is breakdown and gases are released which contaminate gold alloy.
• The thermal expansion of gypsum-bonded investment is directly related to the amount and type of silica.
• A considerable amount of quart or allotropic form of silica is necessary to counterbalance the contraction in the gypsum during heating.

When investment is heated:

• Gypsum contracts between 200 to 400°C and a slight expansion takes place between 400°C to 700°C and a large contraction occur after this.
• If considerable amount of allotropes of silica present in the investment this contraction can be eliminated and change in expansion.
• Because when heated, quart or crystabolite changes its crystalline form this occurs at a transition temperature.
• Quart when heated, inverts from “low” form known, as alpha quart to a “high” form known as beta quartz at a temperature of 370°C.
• Crystabolite similarly when heated inverts from ‘low’ or alpha crystabolite to high or beta crystabolite form at temperature between 200°C and 270°C.
• The density changes (decreases) as alpha form changes to beta form with a resulting increase in volume and a rapid increase in linear expansion.

Question 9. Write in detail about hygroscopic expansion.
Answer:

Hygroscopic Setting Expansion:

When a gypsum product is placed underwater before the initial set stage, a greater expansion is seen. This is due to hygroscopic expansion. When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely. Under dry conditions, this additional water is not available and as expansion occurs the aqueous phase in the mix is reduced to a film over the growing crystals. It is greater in magnitude than normal setting expansion.

Importance: It is used to expand some gypsum-bonded investments.

Factors Affecting Hygroscopic Setting Expansion:

• Composition: Finer is the particle size of silica more is the hygroscopic setting expansion. High is the silica content more is the expansion.
• W/P ratio: High is the water-powder ratio of the original investment mixture, and less is the expansion.
• Temperature: As high is the temperature of immersion waterless is surface tension and greater is the expansion.
• Effect of time of immersion: Immersion done before the initial set causes greater expansion.
• Speculation: Shorter is the mixing time, the less is hygroscopic setting expansion.
• Effect of amount of added water: More is water added during settling period greater is the expansion.
• Effect of shelf-life of investment: Older the investment, less is the hygroscopic expansion.
• Confinement of investment: Confinement of investment by walls of the container or the wax pattern reduces hygroscopic setting expansion.

Question 10. Describe its properties, water/powder ratio and setting expansion.
Or
Give the composition and properties of dental plaster.
Answer:

Gypsum is a mineral mined in various parts of the world. Gypsum is also an industrial by-product. Gypsum was fist found in mines around the city of Paris, so it is also called the “Plaster of Paris”.

• The mineral gypsum CaSO₄. 2H₂O is usually white to yellowish-white in color and is found as a compact mass.
• Products of gypsum are used extensively in dentistry.

Properties  of Gypsum Products:

The following are the properties of gypsum materials:

1. Strength
2. Tensile strength
3. Compressive strength
4. Dimensional stability
5. Hardness and abrasion resistance
6. Porosity
7. Reproduction of surface details

1. Strength:

• Strength of the set calcium sulfate dihydrate is dependent on the water/powder ratio, i.e. W/P ratio.
• The lesser is the W/P ratio, the greater is the strength
• W/P ratio is dependent on the type of hemihydrate particle.
• Type II gypsum (model plaster) has a higher W/P ratio than type IV gypsum (high-strength stone) and therefore less strength, because the hemihydrate particles in the plaster are more porous and irregular, and thus require more water for wetting than the dental stone.

Strength depends on the following factors, i.e.

• The material used whether calcined or autoclaved, and the additives used
• W/P ratio
• Vacuum mixing

The strength of gypsum increases rapidly as the material get harden after the initial settng time; moreover, free water content of set product affects the strength.

So this is the reason the two strength property of gypsum is reported as:

• Wet strength: It is the strength when water in excess of that required for hydration of the hemihydrate is left in the specimen. It is also known as green strength.
• Dry strength: It is the strength obtained when the excess of water has been driven of by drying. Dry strength may be two or more times higher than wet strength.

Removal of approximately 8.8% excess water in a cast will double the strength and hardness. The reason is the excess water adds mobility to the dihydrate crystals, so fier dihydrate crystals reinforce the larger dihydrate crystals as the water evaporates and thus strengthen the mass.

The drying process normally takes a week at normal room temperature and humidity which may be enhanced by increasing the temperature. But, this should be done with caution because as the cast is heated above 60°C, the water of hydration will also be driven of and the cast will be weakened.

2. Tensile Strength:

• Wet tensile strength of plaster is very low, i.e. 2 MPa.
• This is because of the porosity and brittle nature of the material. So, the teeth and margins of the model can be easily damaged if handled roughly.
• Dental stone has double the tensile strength as compared to plaster and is therefore preferred for the production of die material.
• Tensile strength is less than the compressive strength.
• Materials which are mixed with a high W/P ratio have lower tensile strength as compared to those mixed with a low W/ P ratio.

3. Compressive Strength:

• It is the mechanical property used for assessing the strength of gypsum products.
• Compressive strength values for dental plaster, dental stone, and die stone which are 12 MPa, 30 MPa, and 38 MPa, respectively.
• Mostly the compressive strength is affected by W/P ratio used. Reduction in the amount of water required to produce an acceptable mix results in significant improvement in compressive strength.
• Presence of voids decreases the compressive strength; so use of a vibrator is recommended for pouring the impression.
• To obtain a quick dry compressive strength and dry surface hardness, molds, casts, or dies are dried in an oven which dehydrates the gypsum leading in a reduction of strength.

4. Dimensional Stability:

• As material gets set, there is little or no significant dimensional change at room temperature.
• Set material should not be stored at high temperatures, i.e. between 90°C and 110°C.
• If the material is stored, the water of crystallization is removed and the dihydrate is converted back to hemihydrate which leads to the contraction of cast.
• As compared to a dental stone contraction of plaster is more and it also loses its strength.
• These contractions may occur during storage in air above room temperature.
• So, it is not safe to store or heat a stone cast in air at a temperature higher than 55°C.

5. Hardness and Abrasion Resistance:

• Hardness is related to compressive strength.
• The higher the compressive strength of the hardening mass, the higher the surface hardness.
• After the final setting occurs, the surface hardness remains practically constant until most of the excess water is dried, after which it increases.
• The surface hardness increases at a faster rate than the compressive strength since the surface of the hardened mass reaches a dry state earlier than the inner portion of the mass.
• Commercial hardening solutions are available to increase the surface hardness of stone.
• However, surface hardness and abrasion resistance are not always related, for example, epoxy resin is more abrasion resistant than die stone, even though die stone is harder of the two.
• Mixing high-strength dental stone with commercial hardening solutions containing colloidal silica improves the surface hardness of set gypsum.
• The use of die hardening solution does not necessarily improve the abrasive resistance as other factors too affect the abrasive resistance.
• The use of disinfectant chemicals on gypsum dies can cause surface erosion and affect the surface hardness.

6. Porosity:

• Increase in the W/P ratio causes a more porous set material.
• Plaster is more porous as compared to dental stone or die stone.
• This is due to the increase in W/P ratio used for the manipulation of plaster.

7. Reproduction of Surface Details:

• Types I and II gypsum products, i.e. impression plaster and model plaster reproduce a groove of 75 µm in width, while type III, IV, and V, i.e.
• Dental stone, die stone, and dental stone—high strength and high expansion reproduce a groove of 50 µm in width.
• Gypsum dies are not able to reproduce surface details, so epoxy or electroformed dies reproduce the surface details much better.
• Air bubbles are formed at the interface of impression and gypsum cast because freshly mixed gypsum does not wet some elastomeric impressions well.
• Incorporation of non–ionic surfactants improves the wetting of the impression.
• Use of vibration during the pouring of the cast reduces air bubbles.
• If the impression gets contaminated in which the gypsum die is poured by saliva or blood also affects the detail of reproduction.
• Rinsing the impression and blowing away the excess water improve the details recorded.

8. Water/Powder Ratio (W/P Ratio):

• Ratio of the required quantity of water for 100 mg of powder ,is known as the water/powder ratio.
• W/P ratio is a very important factor in deciding the physical and chemical properties of final product. It has also a pronounced effect on the setting time.
• So more is the water in the mix longer is the settling time?
• The strength of the gypsum is inversely proportional to W/P ratio. So W/P ratio should be kept low as possible but at the same time sufficient to produce a workable mix.
• Too low a water/powder ratio will produce a very thick consistency that may not flow in the finer detail of impression.
• The W/P ratio varies with the commercial brands of dental plaster or dental stone.

Various recommended W/P ratios of various gypsum products are:

Setting Expansion of Gypsum Products:

It is measured by using an extensometer. Setting expansion is of two types, i.e.

1. Normal settng expansion
2. Hygroscopic settng expansion.

• Normal Setting Expansion: All gypsum products show a linear expansion during setting due to the outward thrust of the growing crystals.
  • Crystals growing from the nuclei not only intermesh but also intercept each other during growth.
  • In dentistry, setting expansion may be both desirable and undesirable depending on the use.
    It is undesirable in impression plaster, dental plaster, and stone as it will result in an inaccurate cast or change in the occlusal relation if used for mounting.
  • Increased setting expansion is desired in the case of investment materials as it helps to compensate for the shrinkage of the metal during casting.
• Control of Settng Expansion:
  • Mechanical mixing reduces setting expansion when compared to hand mixed stone.
  • An increase in W/P ratio reduces the setting expansion.
  • Modifiers generally reduce the settng expansion.
  • Potassium sulphate 4% solution reduces setting expansion from 0.5 to 0.06%.
  • Sodium chloride and borax also decreases 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.
  • When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely.
  • Under dry conditions, this additional water is not available and as expansion occurs the aqueous phase in the mix is reduced to a fim over the growing crystals.
  • It is greater in magnitude than normal setting expansion.

Question 11. Write a short note on WET strength.
Answer:

Wet strength is the strength property of gypsum.

• It is also known as green strength.
• Wet strength is the strength obtained when the water in excess of that required for hydration of hemihydrates is left in the specimen.
• Wet strength is lower as compared to dry strength.
• The wet strength (1 hour compressive strength) for model plaster is 12.5 MPa, for dental stone is 31 MPa and for die stone is 45 MPa.

Question 12. What are gypsum products?
Or
Write about manufacturing and theories of setting of gypsum products.
Answer:

Most commonly used materials that produce positive replication of oral structures are known as gypsum products. Gypsum products are derived from mineral gypsum which is chemically known as calcium sulfate dihydrate. When powdered gypsum get heated by calcinations to remove the part of its moisture content, the gypsum product a fie powder of calcium sulphate hemihydrate is formed.  So dehydration of gypsum leads to the production of gypsum products which ranges from hydrous to anhydrous form of calcium sulfate.

Manufacturing of Gypsum Products:

The process of heating gypsum for the manufacture of plaster is known as calcinations. Mined gypsum is ground and heated. When heated, gypsum (calcium sulphate dihydrate) looses part of its water of crystallization and changes to calcium sulphate hemihydrate. On further heating the remaining water of crystallization is lost. First, hexagonal anhydrite (soluble anhydrite) is formed. Later, orthorhombic anhydrite (insoluble anhydrite) is formed.

Manufacture of Dental Plaster:

Gypsum is ground and heated in an open letter or kiln at a temperature of 110 to 130° C. The process is called dry calcination. β Type of crystals are formed. Microscopically, firous aggregate of fie crystals with capillary pores. They are then ground to breakup the needle-like crystals. This improves packing.

Manufacture of Dental Stone:

Gypsum is calcined under steam pressure in an autoclave at 120 to 130°C at 17 lbs/sq inch for 5 to 7 hours. Thus, the product obtained is much stronger and harder than b hemihydrate.

Manufacture of High Strength (α modified) Stone:

The gypsum is calcined by boiling it in 30% calcium chloride solution. The chlorides are then washed away or autoclaved in presence of sodium succinate 0.5%. These particles are the densest of all three types. After controlled grinding these powders have an even higher apparent density and yield a stronger set. Microscopically it is cuboidal in shape.

Theories of Setting:

Three theories have been put forward, i.e.

1. Colloidal theory
2. Hydration theory
3. Dissolution—precipitation theory.

• Colloidal Theory: This theory was given by Mahaelis in 1893. The theory proposes that when mixed with water, plaster enters into a colloidal state through a sol-gel mechanism. In the sol state, hemihydrate combines with water to form dihydrate. As the water is consumed the mass turns to a ‘solid gel’.
• Hydration Theory: The hydration theory suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set solid mass.
• Dissolution-Precipitation Theory (Crystalline Theory):

This theory is widely accepted.

• According to the theory, the plasterdissolves and reacts to form gypsum crystals which interlock to form the set solid.
• The settling reaction is explained on the basis of difference in solubility of hemihydrates and dihydrate.
• Hemihydrate is four times more soluble than dihydrate. When hemihydrate is mixed in water it forms a flid workable suspension.
• Hemihydrate dissolves until it forms a saturated solution. Some dihydrate is formed due to the reaction.
• The solubility of dihydrate is much less than hemihydrate, the saturated hemihydrate is supersaturated with respect to the dihydrate.
• All supersaturated solutions are unstable. So the dihydrate crystals precipitate out.
• As the dihydrate precipitates out, the solution is no longer saturated with hemihydrate and so it continues to dissolve.
• The process continues until further dihydrate precipitates out of the solution.
• Initially, there is little reaction and thus little or no rise in temperature. This time is referred to as the induction period.
• As the reaction proceeds gypsum is formed in the form of needle-like clusters called spherulites.
• Continued growth and intermeshing of crystals of gypsum leads to thickening and hardening of the mass into a strong solid structure.

Question 13. Write a short note on setting expansion.
Answer:

Setting expansion is of two types:

1. Normal settng expansion
2. Hygroscopic setting expansion.

1. Normal Setting Expansion:

All gypsum products show a linear expansion during setting, due to the outward thrust of the growing crystals. Crystals growing from the nuclei not only intermesh but also intercepteach other during growth.

In dentistry, setting expansion may be both desirable and undesirable depending on the use. It is undesirable in impression plaster, dental plaster, and stone as it will result in an inaccurate cast or change in the occlusal relation if used for mounting. Increased setting expansion is desired in the case of investment materials as it helps to compensate the shrinkage of the metal during casting.

• Control of Setting Expansion:
  • Mechanical mixing reduces setting expansion when compared to hand-mixed stone.
  • Increase in W/P ratio reduces the setting expansion.
  • Modifirs generally reduce the setting expansion.
  • Potassium sulphate 4% solution reduces setting expansion from 0.5 to 0.06%.
  • Sodium chloride and borax also decreases setting expansion.

2. Hygroscopic Setting Expansion:

When a gypsum product is placed underwater before the initial set stage, a greater expansion is seen. This is due to hygroscopic expansion. When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely.

Under dry conditions, this additional water is not available and as expansion occurs the aqueous phase in the mix is reduced to a fim over the growing crystals. It is greater in magnitude than normal setting expansion.

Question 14. Write a short note on hygroscopic setting expansion.
Or
Write a brief on hygroscopic expansion.
Or
Write briefly on hygroscopic setting expansion.
Or
Write in brief on hygroscopic expansion.
Answer:

When a gypsum product is placed underwater before the initial set stage, a greater expansion is seen. This is due to hygroscopic expansion. When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely. Under dry conditions this additional water is not available and as expansion occurs the aqueous phase in the mix is reduced to a fim over the growing crystals. It is greater in magnitude than normal setting expansion.

Factors Affecting Hygroscopic Setting Expansion:

• Composition: Finer is the particle size of silica more is the hygroscopic setting expansion. High is the silica content more is the expansion.
• W/P ratio: High is the water powder ratio of the original investment water mixture less is the expansion.
• Temperature: As high is the temperature of immersion
  waterless is surface tension and the greater is the expansion.
• Effect of time of immersion: Immersion done before the initial set causes greater expansion.
• Speculation: Shorter is the mixing time, the less is the hygroscopic setting expansion.
• Effect of amount of added water: More is water added during settling period greater is the expansion.
• Effect of shelf-life of investment: Older the investment, less is the hygroscopic expansion.
• Confinement of investment: Confiement of investment by walls of the container or the wax pattern reduces hygroscopic setting expansion.

Question 15. Write a short note on the die stone.
Answer:

Die Stone: Die stone is of two types, i.e.

•  Type IV: Die stone with high strength low expansion
• Type V: Die stone with low strength and high expansion.

Properties of Die Stone:

Manipulation of Die Stone:

The thick mix should be prepared and vibrated in a rubber base impression. The base is poured in a dental stone or dental plaster. Die stone should be left for 24 hours to gain maximum hardness and the cast should be separated 1 hour after pouring.

Uses of Die Stone:

• Type IV: Die material for fabrication of wax patterns for dental cast metal restorations including intracoronal and radicular.
• Type V: Die material for fabrication of wax patterns for dental cast metal restorations where high expansion is needed to compensate for the alloy shrinkage such as base metal alloy.

Advantages of Die Stone:

• They have the ability to reproduce fie detail and sharp margins.
• They are compatible with all impression materials.
• Their dimensional accuracy is considered to be accurate.
• They are relatively inexpensive.
• They are easy to manipulate.

Disadvantages of Die Stone:

• Mechanical properties are not ideal.
• They have susceptibility for abrasion during the carving of wax patterns.

Question 16. Write short notes on α and β hemihydrates.
Answer:

Alpha Hemihydrate: It is also known as a type III gypsum product or dental stone or hydrocal.

Manufacture of Alpha Hemihydrate:

Here gypsum is calcined under steam pressure in an autoclave at 120°C to 130°C at 17 lbs/sq inch for 5 to 7 hours. So the product obtained is much stronger and harder. Microscopically there are cleaved fragments and crystals inform of rods and prisms.

Composition of Alpha Hemihydrate:

A stone with a setting time established by the addition of proper quantities of both accelerator and retarder is known as a balanced stone.

Properties of Alpha Hemihydrate:

Advantages of Alpha Hemihydrate:

• It is inexpensive and is easy to manipulate.
• Dimensional stability and accuracy is good.
• It has the ability to reproduce fie details and sharp margins of impressions.

 Disadvantages of Alpha Hemihydrate:e:

• Its mechanical properties are not ideal.
• Fractures of teeth from stone casts can occur with careless handling.
• It has low abrasion resistance.

Uses of Alpha Hemihydrate:

• It is used to pour the workable casts in the fabrication of complete dentures.
• It is used to process the denture base because the stone has adequate strength.

Beta Hemihydrates: It is also known as dental plaster or model plaster or laboratory plaster or mounting plaster.

Manufacture of Beta Hemihydrates:

Gypsum is ground and heated in an open letter or klin at a temperature of 110 to 130°C. The process is known as dry calculations. Here β types of crystals are formed. Microscopically, fibrous aggregate of fie crystals with capillary pores are present. They are then ground to break up the needle-like crystals. This improves packing.

Composition of Beta Hemihydrates:

 Properties of Beta Hemihydrates:

Advantages of Beta Hemihydrates:

• It is very easy to trim the models.
• It is least expensive of all the gypsum materials.

Uses of Beta Hemihydrates:

It is used for pouring the cast of oral structures when high strength and high abrasion resistance is not required, and dimensional accuracy is not typical.

• It is used for filing a flsk in denture construction.
• It is used for mounting stone models in articulators.

Manipulation of Alpha Hemihydrates or Beta Hemihydrates:

Dental stone or plaster is mixed in flxible rubber or plastic bowl with stif bladed plaster spatula. Take proper W/P ratio in rubber bowel and add the pre-weighted quantity of powder. Spatulate with stirring motion for 45 to 60 seconds. Tap the bowel or keep it on vibrator to avoid air bubbles and immediately pour it in the impression by inversion or boxing method.

Question 17. What is the mechanism of action of cold mould seal?
Answer:

When cold mold seal is applied to dry plaster surface sodium alginate reacts with calcium sulphate to form insoluble skin of calcium alginate on the plaster surface which in turn blocks the porosities on the surface of plaster. Thus, any other material that comes in contact with plaster will not adhere to it.

Question 18. Add in detail about the setting reaction of dental plaster and add a note on the uses of different gypsum products.
Or
Write in detail the setting reaction of calcium sulphate dihydrate.
Answer:

Setting Reaction of Dental Plaster:

When 1 g mol of calcium sulfate hemihydrates reacts with 1.5 g mol of water, the hemihydrates reverts to dihydrate forming 1 g mol of calcium sulfate dihydrate by hydration, with liberation of 3900 calories of heat. Here the heat which is evolved in the reaction is equal to the heat originally used in calculations.

• (CaSO₄)₂.H₂O + 3H₂O→ 2CaSO₄.2H₂O + unreacted (CaSO₄)₂.½ H₂O + heat
• Hemihydrate + Water →  Dihydrate + unreacted hemihydrates + 3900 cal/g mol

Uses of Different Gypsum Products:

Question 19. Classify gypsum products. Describe the setting reaction and microscopic structure of the set material.

Answer:

Setting Reaction of Gypsum Products:

When 1 gram mol of calcium sulfate hemihydrates reacts with 1.5 gram mol of water, the hemihydrates reverts to dihydrate forming 1 gram mol of calcium sulfate dihydrate by hydration, with liberation of 3900 calories of heat. Here the heat which is evolved in the reaction is equal to the heat originally used in calcinations.

• (CaSO₄)₂.H₂O + 3H₂O →2CaSO₄.2H₂O + unreacted (CaSO₄)2.½H₂O + Heat
• Hemihydrate + Water →  Dihydrate + unreacted hemihydrates + 3900 cal/g mol

Microscopic Structure of Set Material:

• Set material composed of an entangled aggregate of gypsum crystals having a length of 5 to 10 µm. Two different types of microscopic porosities are seen in the mass.
• Microporosity caused by residual unreacted water. These voids are spherical and occur between clumps of gypsum crystals.
• Microporosity results from the growth of gypsum crystals.
• These voids are associated with setting expansion and are smaller than the first type.
• They appear as angular spaces between individual crystals in aggregate.

Question 20. Compare and contrast dental plaster, dental stone and die stone.
Answer:

Question 21. ADA specification, types, uses, manipulation, and setting process of gypsum products.
Answer:

Uses of Gypsum Products:

• As study models for the oral and maxillofacial structures.
• As cast and die material.
• As impression material.
• As investing material in flaking procedures for acrylic complete and partial denture processing.
• As investment material for the casting of metallic restorations.

Manipulation of Gypsum Products

• ProportioningFor attaining maximum strength water powder/ratio should be optimum.
• Recommended water/powder ratio for various gypsum products is:
  • Impression plaster: 0.50–0.75
  • Dental plaster: 0.45–0.50
  • Dental stone: 0.28–0.30
  • Die stone, Type 4: 0.22–0.24
  • Die stone, Type 5: 0.18–0.22.
• Instrument Used: It is a flexible rubber bowl and stiff bladed spatula.

Hand Mixing Procedure:

Water is taken first. The powder is then dispensed as per the proper water/powder ratio. The powder is sifted in water in rubber bowl. Now it is allowed to setting for 30 seconds and also to minimize air entrapment, stirring of the mix is done vigorously.

Inside of the bowl is wiped by a spatula so that there is wetting of powder and a breakdown of lumps. Mixing is continued until a smooth creamy mix is obtained. Spatulation is completed in 45 to 60 seconds.Vibration of mix is done either on the vibrator or by repeated tapping against a bench. Now the gypsum product is poured in the impression taking care not to entrap the air.

Mechanical Mixing:

It is done under the vacuum and this provides strong and dense casts. A uniform homogeneous creamy mix with the less incora portion of air is obtained by using an automated vacuum mixer. A good mix should have a glossy surface with smooth creamy consistency. The incorporation of air voids in the mix is reduced by use of vacuum mixers. The weighed powder is incorporated in 15 seconds by hand speculation in a special container, followed by 20 to 30 seconds of mechanical mixing of high frequency and low amplitude under a vacuum