Hygroscopic Expansion Of Dental Investments

Hygroscopic Expansion Of Dental Investments

Write a short note on hygroscopic setting expansion.
Or
Write a brief on hygroscopic expansion.
Or
Write briefly on hygroscopic setting expansion.
Or
Write in brief on hygroscopic expansion.
Answer:

When a gypsum product is placed underwater before the initial set stage, a greater expansion is seen. This is due to hygroscopic expansion. When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely. Under dry conditions this additional water is not available and as expansion occurs the aqueous phase in the mix is reduced to a fim over the growing crystals. It is greater in magnitude than normal setting expansion.

Factors Affecting Hygroscopic Setting Expansion:

• Composition: Finer is the particle size of silica more is the hygroscopic setting expansion. High is the silica content more is the expansion.
• W/P ratio: High is the water powder ratio of the original investment water mixture less is the expansion.
• Temperature: As high is the temperature of immersion
  waterless is surface tension and the greater is the expansion.
• Effect of time of immersion: Immersion done before the initial set causes greater expansion.
• Speculation: Shorter is the mixing time, the less is the hygroscopic setting expansion.
• Effect of amount of added water: More is water added during settling period greater is the expansion.
• Effect of shelf-life of investment: Older the investment, less is the hygroscopic expansion.
• Confinement of investment: Confiement of investment by walls of the container or the wax pattern reduces hygroscopic setting expansion.