The alginate sets by chemical reaction, where as agar gels by means of contact with cool alginate. Since agar is in contact with teeth maximum detail can be obtained.
Traditionally alginate is supplied in two component system, powder and water. But other forms are also available.
In the form of a solution containing water and a reactor of plaster of Paris
As a two paste system one containing solution and other calcium reactor. These materials contain silicone and are supplied in tray viscosity and syringe viscosity.
Rinse the impression
Place in zip lock bag, pour in disinfectant to cover all surfaces
Seal for 10 minutes
Remove and rinse prior to pouring stone
Chlorhexidine has also been incorporated to limit cross infection (Hydrogum + CHX- Zhermack)
Condensation polymerizing silicone
Addition polymerizing silicone
Each type is further divided into four viscosity classes
Medium / regular body
Excellent reproduction of surface details. The low viscosity is able to record fine details
They are hydrophobic. So oral tissues in the area of impression should be dry.
Co-efficient of thermal expansion of elastomers is high. So thermal contraction of set material occurs when transferred to room temperature
Tear strength is excellent. So impressions can be removed from thin areas.
The elastomeric materials can be copper/silver plated
Shelf life of elastomers is good
For visco-elastic material slow elastic recovery may control for sometime after withdrawal of the impression, producing dimensional change. In this case the dimensional change results in more accurate impression.
But many impression materials contain volatile substances either as primary components or as by products of setting reaction. Loss of such volatile materials results in shrinkage of impression material resulting in decrease in accuracy. So for majority of materials, accuracy can be maintained by pouring the cast soon after the removal of impression.
Loss of byproducts
Polyether being hydrophilic absorbs water and loses soluble plasticizers causing change in dimension
Thermal contraction when transferred from mouth to room temperature
Incomplete elastic recovery after deformation during removal
Amount of filler
Uniform thickness of material gives more accurate impression
Time of pouring of impression. Impression should be powered after elastic recovery and before dimensional change
Also called as Mercaptan or Thiokol
MODE OF SUPPLY
It is supplied in tubes as base and accelerate in paste form
Available in three viscosities