Gypsum Products Dental Materials

Gypsum Products Dental Materials

Write short notes on accelerators and retarders.
Or
Write briefs on accelerators and retarders.
Or
Write short notes on modifiers used in gypsum.
Answer:

Accelerators and Retarders:

Accelerators and retarders are also called as modifiers. Modifiers are chemicals added in order to alter some of the properties and make it more acceptable to the dentist.

Accelerators:

• If the chemical added increases the rate of reaction and decreases the setting time it is called an accelerator.
• In low concentrations, salts like sodium sulphate (up to 3.4%), potassium sulphate (2 to 3%), and sodium chloride (up to 2%) are accelerators. They act by making the hemihydrate more soluble.
• An accelerator changes hemihydrates to be much more soluble than dehydrate thus accelerating the chemical reaction.
• Reduction in setting time caused by accelerators depends on the amount and the rate of solubility of hemihydrates.
• Various examples of accelerators are:
  • 2% NaCl
  • 3.4% Na
  • 2SO4
  • 2% K 2SO4
  • Terra alba (ground gypsum) 1%
  • 0.1% lime
  • 1% gum Arabic
  • Potassium sodium tartrate

In above examples if NaCl and Na2SO4 when added more than the mentioned concentration, it act as retarder

Retarders:

If the chemical added, increases the setting time and decreases the rate of reaction it is called as retarder.

• Retarders act by forming an adsorbed layer on hemihydrates to reduce its solubility.
• In higher concentration sodium chloride and sodium sulfate (above 3.4%) act as retarders.
• In higher concentrations the salt precipitates and poisons the nuclei of crystallization.
• Acetates, borates, citrates, tartarates, and inorganic salts like ferric sulphate, chromic sulphate, aluminium sulphate, are retarders which act by nuclei poisoning by reducing the rate of solution of hemihydrates or by inhibiting growth of dihydrate crystals.
• Borax (1–2%) is the most effective retarder. During setting it forms a coating of calcium borate around the hemihydrates.
• Thus, water cannot come in contact with hemihydrate.
• Colloids such as gelatin, glue, agar, coagulated blood, etc. are effective retarders.
• They retard rate of setting reaction by getting adsorb on crystals or calcium sulphate dihydrate nucleation sites and interfere hydration reaction.