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.

MODIFIED ALGINATES:

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.

DISINFECTION

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)

ELASTOMERS

Polysulfide

Condensation polymerizing silicone

Addition polymerizing silicone

Polyether

Each type is further divided into four viscosity classes

Light body

Medium / regular body

Heavy body

Putty

GENERAL PROPERTIES

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

Dimensional stability

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.

Curing shrinkage

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

POLYSULFIDES

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

Light bodied

Medium bodied