Dental Materials Quiz: Gypsum and Stone

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Questions and Answers

What is the ratio of powder to water when mixing dentist (improved stone)?

100gm to 22ml (correct)

100gm to 30ml

200gm to 30ml

100gm to 50ml

What is the nature of the reaction when calcium sulfate hemihydrate is mixed with water?

Endothermic and prolonged

Exothermic and slow

Endothermic and rapid

Exothermic and rapid (correct)

During the setting process, what happens to the solubility of calcium sulfate dihydrate?

Remains very low (correct)

Variates depending on temperature

Becomes highly soluble

Increases significantly

What occurs first during the mechanism of setting when hemihydrate is mixed with water?

The hemihydrate dissolves in water Signup and view all the answers

Which factor is NOT associated with the setting reaction of gypsum products?

Transformation into gas Signup and view all the answers

What defines the initial setting time of gypsum products?

Time until the Gillmore needle does not penetrate Signup and view all the answers

What is the primary purpose of a model in dental procedures?

To construct an appliance like a full or partial denture Signup and view all the answers

What happens to the material during the initial set?

It can still be carved but not shaped Signup and view all the answers

What is the main difference between plaster and the dentist (improved stone) in terms of porosity?

Dentist stone has less porosity Signup and view all the answers

Which of the following is NOT an ideal requirement for model and die materials?

Require extensive processing for use Signup and view all the answers

What is gypsum chemically known as?

Calcium sulfate dihydrate Signup and view all the answers

Which of the following properties differs among the three products of calcium sulfate hemihydrate?

Particle size and shape Signup and view all the answers

What type of gypsum product is produced by heating calcium sulfate dihydrate in an autoclave?

α-hemihydrate Signup and view all the answers

At what temperature does retardation occur, and no setting takes place?

100ºC Signup and view all the answers

What is the typical water-to-powder ratio for dental plaster?

50ml of water to 100gm of powder Signup and view all the answers

Which factor increases the rate of dissolution of hemihydrate?

Grinding of the hemihydrate Signup and view all the answers

Which gypsum product consists of large irregular porous particles?

Dental plaster Signup and view all the answers

What property is crucial for a die when constructing crowns and bridges?

High strength and dimensional stability Signup and view all the answers

What effect does borax have on the setting of gypsum products?

Slows down dissolution Signup and view all the answers

How much less should the volume of the final dihydrate be compared to the total volume of hemihydrate plus water?

7% Signup and view all the answers

What can reduce the linear expansion of gypsum products during setting?

Adding salts that regulate crystal shape Signup and view all the answers

What is hygroscopic expansion in the context of gypsum products?

Expansion due to water immersion during setting Signup and view all the answers

Which of the following will not affect the setting time of gypsum products?

Rate of temperature increase Signup and view all the answers

What is the main reason for the linear expansion of gypsum products during setting?

The outward thrust of growing crystals Signup and view all the answers

What visual indication signals the initial setting time of a gypsum mass?

The surface gloss is lost Signup and view all the answers

Which tool is used to measure the final setting time of gypsum?

Gillmore needle with 1 pound weight Signup and view all the answers

How does increasing the water-to-powder ratio affect the setting time of gypsum?

It prolongs the setting time Signup and view all the answers

What is a primary factor that an operator controls that affects setting time?

Water temperature Signup and view all the answers

At what temperature does the longest setting time for gypsum occur?

0ºC Signup and view all the answers

According to the material, how does an increase in mixing time affect setting time?

It accelerates the setting time Signup and view all the answers

What is the weight of the Vicat needle used to measure the initial setting time?

300 gm Signup and view all the answers

Which of the following factors is least likely to affect the setting time of gypsum materials?

Color of the mixing bowl Signup and view all the answers

What is the primary purpose of using gypsum bonded investment materials?

To compensate for casting shrinkage Signup and view all the answers

Which factors primarily influence the strength of gypsum products?

Type of product, dryness of set material, and water/powder ratio Signup and view all the answers

How does dry strength compare to wet strength in gypsum products?

Dry strength is typically double the wet strength Signup and view all the answers

What is one of the consequences of mixing gypsum with excessive water?

It may result in a product as weak as plaster Signup and view all the answers

What is the recommended technique for minimizing air incorporation into the gypsum mix?

Sifting powder onto water and letting it sit Signup and view all the answers

Which method can significantly enhance the properties of dental stones?

Impregnation with a polymer or adding a wetting agent Signup and view all the answers

What is the significance of maintaining a correct water/powder ratio in gypsum product formulation?

It ensures an optimal strength of the set material Signup and view all the answers

What happens to gypsum products if they are stored in open containers?

They can react with atmospheric moisture, forming dihydrate Signup and view all the answers

Signup and view all the answers

Study Notes

Model and Die Materials

A model or cast is a replica of hard or soft oral tissues. It's poured from an impression and used to construct appliances like full or partial dentures.
A die is a model of a single tooth, primarily used in constructing crowns and bridges.

Ideal Requirements for Model Materials

High strength to prevent accidental breakage and resist scratching during wax carving.
Ability to produce detailed impressions.
Little dimensional change upon setting, maintaining stability.
Compatibility with impression materials to avoid interaction between impression and model surfaces.
Good color contrast with the impression material.
Affordability and ease of use.

Gypsum Products (Chemistry)

Gypsum is a naturally occurring calcium sulfate dihydrate (CaSO₄·2H₂O).
Heating gypsum to a high temperature removes water, converting it to calcium sulfate hemihydrate (CaSO₄·½H₂O). Gypsum production yields three different products with varied properties.
Variation in the manufacture of these products produces variations in particle size, shape, powder-water ratio and physical/mechanical properties. Impurities and manufacturing procedures vary the final products.

Gypsum Products (Types)

Plaster

Heated calcium sulfate dihydrate in an open vessel converts to calcined or beta-hemihydrate, producing large, irregular, porous particles.
Requires high water mixing ratio for dental applications.
Typical mixing ratio is 50 mL water to 100 g powder.

Dental Stone

Heated calcium sulfate dihydrate (in an autoclave) produces alpha-hemihydrate or hydrocal; smaller, more regular, and less porous particles result from this procedure.
Lower water-powder mixing ratio is needed compared to plaster. Typical mixing ratio is 30 mL water to 100 g powder.

Dentist (Improved Stone)

Heating calcium sulfate dihydrate in the presence of calcium or magnesium chloride results in hemihydrate particles that are less porous and more regular than plaster/dental stone.
100g powder to 22mL water is the usual mixing ratio

Setting Process (b)

Mixing calcium sulfate hemihydrate with water converts it back to calcium sulfate dihydrate, an exothermic reaction (it releases heat). This is expressed chemically as CaSO4·½H2O + 1½H2O → CaSO4·2H2O + Heat.

Mechanism of Setting (Crystalline Theory)

Some calcium sulfate hemihydrate dissolves in water.
Dissolved hemihydrate reacts with water to form calcium sulfate dihydrate.
Calcium sulfate dihydrate has very low solubility (0.2%), meaning it's stable.
Stable calcium sulfate dihydrate crystals precipitate out of solution. Subsequent hemihydrate dissolves, completing the conversion.

Factors Observed During Setting Reaction

Initial set: material becomes rigid but not hard; carving possible, but not shaping.
Final set: material becomes hard and strong.
Heat released: exothermic reaction results in heat output.
Dimensional change: Setting expansion occurs.

Properties of Gypsum Products

Setting time: time required for the reaction to complete.
Expansion: change in volume during hardening.
Strength: resistance to deformation or fracture (wet/dry strength).

Setting Time (1)

Initial setting time (working time): the time from mixing until the Gillmore needle (1¼ pound weight, 1/12 inch diameter) no longer penetrates the surface.
Final setting time: the time from mixing until the Gillmore needle (1 pound weight, 1/24 inch diameter) no longer penetrates the surface.

Measuring Setting Time

Loss of surface gloss: indicates the initial setting time when excess water is absorbed.
Penetration (Vicat needle): measures the initial setting time using a specialized needle and measuring the depth the device penetrates into the gypsum.

Factors Affecting Setting Time (a)

Water-powder ratio: higher ratio lengthens the setting time because there are fewer nuclei of crystallization.
Mixing time and rate: increase in mixing time/rate accelerates setting, potentially increasing nuclei of crystallization.
Water temperature: temperature of the mix and the environment affect setting time. The longest setting time is at 0°C; the fastest possible setting occurs at 50°C. Beyond 80°C, time is constant; above 100°C, dihydrate and hemihydrate dissolve and no setting occurs.

Factors Affecting Setting Time (b)

Fineness of the powder; increase in surface area accelerates setting.
Impurities in the powder: accelerate setting (impurities, i.e., impurities, act as nuclei); and some salts retard the setting process.

Expansion (2)

Setting expansion: theoretical final dihydrate volume is 7% less than the hemihydrate (initial) and water volume; actual linear expansion during hardening occurs. Expansion is caused by outward thrust of growing crystals.
Hygroscopic expansion: immersion in water during initial setting results in double the normal expansion.

Strength (3)

Strength depends on product type, dryness of the set material, and the water-powder ratio.
Gypsum products are brittle, exhibiting high compressive strength but low tensile strength.
Wet strength: strength when excess water is present.
Dry strength: strength after excess water evaporates (typically double that of wet strength); fully reached after 7 days.

Manipulation

Storage: store in closed containers to prevent moisture reaction and accelerated setting.
Correct water-powder ratio is essential. Mixing with too much water creates a weak set material.
Avoid air incorporation during mixing; properly sift powder onto water and mix thoroughly with a spatula.
Use vibration to eliminate air bubbles, helping the mix flow smoothly.
Mechanical mixers (under vacuum): produce superior gypsum.

Recent Developments

Fracture from models can occur due to poor mechanical properties.
Strategies are to:
- Impregnate gypsum with polymers (e.g., polyester, polystyrene, acrylics, epoxy).
- Add wetting agents (e.g., lignosulfonates) to modify water-powder ratio for harder, stronger, and denser gypsum.

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Description

Test your knowledge on dental materials, specifically focusing on gypsum products and their setting reactions. This quiz covers essential concepts related to improved stone, calcium sulfate reactions, and model requirements in dental procedures.