Phosphate-Bonded Investment Materials: Composition, Properties, and Applications in Dentistry

Dental Materials And Their Selection

An investment is defined as a ceramic material that is suitable for forming a mold into which molten metal or alloy is cast. Phosphate-bonded investments are the most commonly used investments in the casting of palladium-based alloys and base metal alloys.

Types of phosphate-bonded investment

Based on application they are classified into two types, i.e.

• Type I: For inlays, crowns, and other field restorations
• Type II: For partial dentures and other removable restorations

Composition of phosphate-bonded investment:

Setting Reaction of phosphate-bonded investment

At room temperature, ammonium diacid phosphate reacts with magnesium oxide to give the investment green strength or room temperature strength.

NH₄ + H₂PO₄  +  MgO +  5H₂O→ NH₄Mg PO₄ + H₂O

Manipulation of phosphate-bonded investment:

• For manipulation, the power liquid ratio should be 16 to 23 mL/100  g. The powder is mixed with a measurable amount of liquid using a bowl and spatula. Hand mixing is done for 20 seconds and mechanical mixing is done for 90 seconds.
• Working time should be 8 to 9 minutes. Hand mixing or mechanical mixing under a vacuum can be done. The mixed material is vibrated into the casting ring or agar mold (RPD frameworks). The bench set for material should be for 30 to 45 minutes depending on the particular investment.

Properties of phosphate-bonded investment

• Expansion: Expansion of mold compensates for casting shrinkage. Phosphate investments have their expansion from three sources:
  • Wax pattern expansion: Heat during setting allows for proper expansion of the wax pattern.
  • Setting expansion: It is 0.7 to 1%.
  • Thermal expansion: It ranges from 1 to 1.5%.
• Strength: Its wet strength is from 4 to 10 MPa. According to the studies, there is no correlation between the wet and dry strength of phosphate-bonded investments.
• Thermal Reaction: Phosphate investments undergo thermochemical reactions when they are heated to high temperatures.
• Flow: When the phosphate investment is mixed it appears to have low flow. This material flows readily and covers the pattern when it is poured in mold under vibration. So use of a vibrator is recommended. Reducing agents should be used on wax patterns to improve wetting.
• Surface Smoothness: Early phosphate investments produce rough castings when compared to gypsum-based investments. Current phosphate investments have improved surface smoothness comparable to gypsum-bonded investments.