Separating Medium

Definition

Separating media are those substances which is used to separate two surfaces from each other.

Separating media help to separate

A.            Plaster surface to acrylic surface

B.            Plaster surface to plaster surface

C.           Plaster surface to metal surface

D.           Plaster surface to impression material surface

Classification

1. Oily and greasy substances

a)Vaseline

b)Liquid paraffin

c) Grease in crude form

d)Mineral and vegetable oil

These substances when applied, absorbs onto the plaster surface and thus aids in separation

Disadvantage

§  They weaken the plaster surface

§  Cannot be uniformly and evenly applied on the plaster surface

§  Details of impression cannot be copied precisely due to loss of surface details

Uses

Used when great accuracy is not required eg.

— Flasking of denture

— Articulating the model

— Separation of model from base during repair works

2. Solution separating media

a)Spirituous solution of sandarac and shellac

b)Acetone solution celluoid’s

c) Volatile solution of different waxes(eg. Paraffin wax)

d)6% solution of sodium silicate

e)Solution of sodium alginate(cold mould seal or alginate mould seal)

3. semi- solid separating media:

a)Soap solution

b)Dilute water glass solution

These substances cause physical effect and chemically combine with the surface layer of the plaster and somewhat alter the surface

There is little or no dimensional change if these substances are applied on the plaster surface

Mechanism of action

Soap solution is readily absorbed into the plaster surface

Oleates in the soaps are broken down into hydroxide and free oils

When absorbed, oils acts as separating media

Disadvantage

Weakens the plaster

For this reason soap solutions are most commonly used for making models from impression plaster

Solid separating media:

The substance which have no chemical and physical action on plaster surface

a)Tin foil

b)Wet cellophane paper

Tin foil

•     Tin foil is used in 0.001mm thickness

•     It is adapted and thoroughly burnished on the plaster surface

•     Care should be taken so that it does not fold up or wrinkle

•     Used basically during packing of base material(acrylic resin) to prevent the liquid (monomer) from getting absorbed in plaster

•     Tin foil reduces water absorption of acrylic denture base materials during curing

Cellophane paper

•     Should be thoroughly wet in water before spreading it over and adapting on the surface of plaster

Vaseline

It is one of the oily and greasy substances which is commonly used in dentistry for different purposes.

•     To separate tissue surface to impression material surface during impression taking

•     To separate the plaster to plaster surface during flasking and articulating the model

•     To separate plaster surface to acrylic surface during packing

•     To separate acrylic resin from tooth and tissue surfaces

Solution separating media

Preparation

1 ounce of finely powered borax

1 ounce of distilled water

1 ounce of separation solution like sandarac and shellac with some colouring agent(aniline dye)

Spirituous solution of sandarac and shellac are dissolved in equal parts of distilled water and forms a saturated solution . Colouring agents are added to distinguish from white plaster surface

6% solution of sodium silicate

This separating media is only effective on hot plaster surface

Hence known as separating media of hot surfaces

If applied on cold surfaces it will

•     Add up and form lumps on surfaces

•     Sticky to the cold surfaces

They have to be applied in two layers

Sodium alginate/ cold mould seal

## the most commonly used separating media

Composition

1. Na- alginate

2. Distilled water

3. Colouring agent (pinkish)

Uses

1. Separate plaster surface to acrylic surface

2. Separate plaster surface from metal surface

3. Separating model (plaster) from soluble plaster(impression plaster)

Na-alginate+CaSO4                 Ca-alginate + Na2SO4        

When sodium alginate is aplied on the plaster surface the sodium salt come in contact with the CaSO4  of plaster[(CaSO4).2H2O] forming Ca- alginate , which is precipitates on the plaster surface into thin impermeable layer and separate the two surfaces

Application

This is applied in 2 coatings

The first coating is applied and allowed to dry

The second coating is applied on the first coating

Application is done by camel hair brush or cotton

Applied in only one direction

Disadvantage

If used in hot surface it will heap up

They do not lump up on the cold plaster surface that’s why they are called cold mould seal