Elastic

Aqueous Hydrocolloids

Non-aqueous Elastomers

Polysulfide

Silicones

Polyether

Condensation

Addition

Agar (reversible)

Alginate (irreversible)

Solution: homogenous mixture: one phase system.

Suspension: hetrogenous mixture: two phase system. Colloids: hetrogenous, particle size of the

particles are smaller than in suspension: two phase system.

Dispersed phase. Dispersion medium water Hydrocolloid

• Sol state: viscous liquid.

• Gel state: jelly like

sol

fibrils in a network pattern.

® Brush heap structure.

• Sol

reversible hydrocolloid

Or by: chemical reaction:

irreversible hydrocolloid.

Agglomeration of molecules

reduction in temperature

gel

Gel

Reversible hydrocolloids: Agar-Agar

Liquefaction temperature: 71°C-100°C.

Gelation temperature 37°C-43°C.

• Hysteresis: temperature lag or different liquefaction and gelation

temperature

Sol gel: physical change.

UsesComposition: Agar 12.5% by wt.

Borax Potassium sulphate Alkyl benzoate Water 85.5% by wt.

Syringes: low viscosity – high flow.Tubes: high viscosity – heavy body: low flow.

Agar hydrocolloidAgar (8-15%)

Polysaccharide from seaweed

Provides matrix phase which forms fibrils in the “gel” stage.

Water (>80%)Reaction medium,

controls flow properties

Boraxto strengthen the

gelretarder for the

setting of gypsum

Potassium sulfateto accelerate the

setting of gypsum

Fillersto control the

strength, viscosity, and rigidity

Alkyl benzoateanti-fungal

Pigments and flavors

agar hydrocolloid (hot) agar hydrocolloid (cold)

( sol)

cool to 43 C

heat to 100 C (gel)

Setting Reaction

9

Clinical application The use of agar requires special

equipment consisting of hydrocolloid conditioner (has 3 water bath chambers) and water -cooled tray connected to a rubber hose that delivers water.

Agar is supplied in two viscosities, thick and thin (depending on the amount of agar).

10

Preparing the material for impression:Gel heated to 71-100°C, becomes liquid

(sol)Tray and agar syringe are liquefied in 8-12

minutes in boiling water in 1st chamber2nd chamber is used to store the agar at 60-

66°C.3rd camber kept at 45-47°C to temper agar

before use to prevent burns.Tray is loaded and water hose attached and

tray is seated on teeth.

Manipulation

• Heating and Conditioning Bath

Gelation: Use of water cooling system tray with means of retention

(perforations).

Avoid iced H2O: concentration of stresses.

Avoid exerting pressure on the tray or

moving it.

Removal impression: one single stroke or

sudden pull.

reduce permanent deformation and

tearing of impression

Dimensional changes:Loss of water by Evaporation Synersis: Exuding of fluid at surface

of the gel. fibril cross linking continuescontracts with time exudes water

Imbibition:If the gel takes water expansion or swelling.

Stresses: Pressure on tray during gelation. Use of iced H2O during initial stages

of gelation rapid cooling internal stresses.

Shrinkag

e

Syneresis and Imbibition

15

H2O

H2O

16

Properties Flows well and adapts readily to hard

and soft tissue contoursAccurate reproduction (hydrophilic

nature), however, the impression need to be poured immediately.

Need to be stored in 100% humidity for short period of time if not poured immediately

Distortion is more likely to occur if impression not poured within an hour.

17

Tear strength is not high.Material suffers from loss or gain of water:

Syneresis: a process by which the gel contracts and some of the liquid is squeezed out, forming an exudate on the surface.

Evaporation: loss of water which causes the material to shrink, and impression is distorted.

Imbibition: uptake of water, this will swell and distort the impression.