Dental Materials Quiz: Gypsum and Stone

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.