ADC Dental Materials Science 2 — Questions and Answers
Question 1: Irreversible hydrocolloid (alginate) impression material is classified as:
- A thermoplastic, non-elastic material
- An elastic material that sets via irreversible chemical reaction and cannot be reused (Correct answer)
- An elastomeric addition-polymerization material
- A rigid non-elastic impression material
Correct answer: An elastic material that sets via irreversible chemical reaction and cannot be reused
Alginate is an elastic hydrocolloid that undergoes an irreversible chemical crosslinking reaction, so it cannot be remelted or reused unlike reversible hydrocolloid.
Question 2: Addition silicone (polyvinyl siloxane) impression materials have superior dimensional stability compared to condensation silicones because:
- They contain more filler particles
- Their addition polymerization produces no volatile by-products (Correct answer)
- They absorb water from the oral environment
- They have a much lower viscosity
Correct answer: Their addition polymerization produces no volatile by-products
Addition silicones polymerize without releasing any by-products, so there is no volume loss after setting, resulting in excellent long-term dimensional accuracy.
Question 3: Type III dental stone (improved stone) differs from Type II dental plaster in that it has:
- A higher water-to-powder ratio and lower compressive strength
- A lower water-to-powder ratio and higher compressive strength (Correct answer)
- Greater setting expansion and more porosity
- A faster setting time due to added accelerators
Correct answer: A lower water-to-powder ratio and higher compressive strength
Type III stone (hemihydrate crystals are more regular in shape) requires less mixing water, producing a denser, stronger set product with compressive strength ~35 MPa compared to ~12 MPa for plaster.
Question 4: The solidus temperature of a casting alloy is defined as the temperature at which:
- The alloy first begins to melt on heating
- The alloy is completely solidified on cooling (Correct answer)
- The alloy should be poured into the mold
- The alloy undergoes annealing
Correct answer: The alloy is completely solidified on cooling
The solidus temperature is the highest temperature at which the alloy is entirely in the solid state; below this temperature, no liquid phase exists.
Question 5: For a porcelain-fused-to-metal (PFM) restoration to resist porcelain fracture during firing and function, the metal alloy must:
- Have a coefficient of thermal expansion (CTE) close to that of the veneering porcelain (Correct answer)
- Have a much higher melting point than the porcelain
- Contain at least 75% gold by weight
- Have low hardness so porcelain can indent it slightly
Correct answer: Have a coefficient of thermal expansion (CTE) close to that of the veneering porcelain
Compatible CTEs (metal slightly higher than porcelain) place the porcelain in slight compression on cooling, which glass ceramics tolerate well; large CTE mismatches cause tensile stresses that crack the porcelain.
Question 6: Condensation silicone impression materials are less dimensionally stable than addition silicones primarily because:
- They have higher viscosity and trap more air
- They release ethanol as a by-product of the condensation polymerization reaction (Correct answer)
- They absorb oral fluids during setting
- They require longer intraoral setting time
Correct answer: They release ethanol as a by-product of the condensation polymerization reaction
Condensation silicones polymerize via a reaction that releases low-molecular-weight ethanol, which evaporates from the set impression over time, causing shrinkage.
Question 7: The recommended casting temperature for most dental gold alloys is:
- Equal to the solidus temperature of the alloy
- Equal to the liquidus temperature of the alloy
- Approximately 100–150°C above the liquidus temperature (Correct answer)
- Approximately 50°C below the liquidus temperature
Correct answer: Approximately 100–150°C above the liquidus temperature
Casting at 100–150°C above the liquidus provides enough superheat to ensure the alloy remains fully fluid, fills all mold details, and allows complete compensation for heat loss during casting without premature solidification.
ADC Dental Materials Science 2
Irreversible hydrocolloid (alginate) impression material is classified as: