Dental Gypsum Bonded Investment Composition

Dental Gypsum Bonded Investment Composition

Describe the composition and properties of gypsum-bonded investment material.
Answer:

Composition of Gypsum-bonded Investment Material:

“What Is Dental Gypsum Bonded Investment”

Properties of Gypsum-bonded Investment Materials:

• Thermal Behavior of Gypsum: When gypsum is heated to a high temperature, it shrinks and fractures. At 700°C, it shows slight expansion and then a great amount of contraction. The shrinkage is due to the decomposition and the release of sulfur dioxide. It contaminates the casting with the sulfides of silver and copper. So the gypsum-bonded investments should not be heated above 700°C.
• Thermal Behavior of Silica: When heated, quart or cristobalite changes its crystalline form. This occurs at a transition temperature, characteristic of the particular form of silica.
  • Quart when heated, inverts from a ’low’ form known as alpha-quart to a high form called as beta-quart at a temperature of 375°C.
  • Cristobalite similarly when heated, inverts from low or alpha-cristobalite to high or beta-cristobalite at a temperature between 200°C and 270°C.

The beta forms are stable only above the transition temperature. It changes back to the low or alpha form that occurs upon cooling in each case. The density changes (decreases) as alpha-form changes to beta-form with a resulting increase in volume and a rapid increase in linear expansion.

Expansion  of Gypsum-bonded Investment Material:

• Normal Setting Expansion: As water get mixed with investment powder, water surrounds the calcium sulfate crystals and converts them into calcium α-dihydrate. Setting expansion enlarges the mold partially for casting shrinkage of gold alloys.
  • The effectiveness of setting expansion depends on the thermal expansion of the wax pattern.
    Expansion in the wax pattern is due to exothermic heat which is released from the investment while setting. As the soft is the wax and thin is the wax pattern, more is the expansion since the investment can move soft and thin wax more easily.
  • ADA specifications No. 2 for type I investment permit maximum setting expansion in the air of 0.5%. Modern investments show a setting expansion of 0.4%.
• Hygroscopic Setting Expansion: Hygroscopic setting expansion is a continuation of normal setting expansion when it is allowed to set in the presence of water. Water has to be incorporated before the setting of the investment takes place.
  • This water replaces the water of hydration and prevents the confinement of crystal growth.
    So the crystals continue to grow outward in the presence of water which causes more expansion.
  • Water here is provided to gypsum crystals from a wet liner which is placed over the inner surface of the casting ring or from the exposed surface of the investment which is immersed in water.
  • Hygroscopic setting expansion is 5 to 6 times that of normal setting expansion. ADA specification N o. 2 for type II investments requires a minimum of 1.2% and a maximum of 2.2% expansion.

“Importance Of Gypsum Bonded Investment In Dentistry”

• Thermal Expansion:
  • In the case of gypsum investment, thermal expansion is achieved by placing the mold in the furnace at a temperature below 700°C (If the temperature is more than 700°C, the investment is a breakdown, and gases are released which contaminate the 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: The gypsum contracts between 200 to 400°C and a slight expansion takes place between 400°C to 700°C and a large contraction occurs after this.
  • If a considerable amount of allotropes of silica are present in the investment this contraction can be eliminated and change in expansion.
• Because when heated, quart or cristobalite changes its crystalline form this occurs at a transition temperature
  • Quart when heated, inverts from a “low” form known as alpha quart to a “high” form known as beta-quart at a temperature of 370°C.
  • Crystabolite similarly when heated inverts from ‘low’ or alpha crystalline to high or beta-crystalline form at temperatures 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.
  • The permissible minimum and maximum thermal expansion for gypsum-bonded investments are 0 to 0.5% at 500°C and 1 to 1.6% at 700°C temperatures.’

“Risk Factors For Defects In Gypsum Bonded Investment”

Strength:  According to ISO 15912:2006, the compressive strength for investments should not be less than 2 MPa when tested 2 hours after setting.

The porosity: More the gypsum crystals present in the set investment, the less is its porosity. Less is the hemihydrate content and the greater the amount of gauging water, the more is its porosity. A mixture of coarse and fine particles exhibits less porosity than an investment composed of uniform particle size.

“Early Signs Of Problems With Gypsum Bonded Investment”

Fineness: A fine particle size is preferable to a coarse one. The faster the investment, the smaller will be the surface irregularities on the casting.

Storage: Investments should be stored in airtight and moisture-proof containers. Purchase in small quantities.