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HYDROCOLLOIDS

- Joel Koshy Joseph



TOPICS TAKEN:

Colloidal solution

Gelation

Syneresis

Imbibition Composition of Agar and alginates



For recording details of edentulous areas rigid impressionmaterials are used.

But for the areas of teeth where sever undercuts are

present such rigid impression materials can’t be used. Here such materials must be used which could record the

details and at the same time not fracture or distort formits original shape.

So ELASTIC IMPRESSION materials are used.



TYPES OF ELASTIC IMPRESSION MATERIALS

Hydrocolloids

Elastomeric materials



Colloids are often classified as the fourth state of matter,the colloidal state.

Such solutions contains two phases:

1. Dispersion medium

2. Dispersed particles



Our colloidal solution is somewhere in between theextremes of the very small molecules in solutionand the very large particles in the suspension.



In the colloid the particles in the dispersed phaseconsist of molecules held together either by primarybonds or secondary bonds.

The sizes of the colloid particles ranges from 1 to 200nm.



Colloids in its liquid like consistency is called sol.

Whereas its semi-solid form is called gel.

in gel state the dispersed phase agglomerates to form

a brush heap structure.The dispersion medium is held in the interstices

between the fibrils by capillary attraction oradhesion.



The hydrocolloids used are:

Agar (reversible hydrocolloid)

Alginate(irreversible hydrocolloid)



Sol

Gelation

Gel

Solation



In agar, the secondary bonds hold the fibrilstogether, such bonds breaks at elevatedtemperatures. We get the sol form again from thegel. So called reversible hydrocolloid.

Whereas in alginates, strong primary bonds hold upthe colloids or the particles. Such bonds doesn’t

break even at elevated temperatures. Thus calledirreversible hydrocolloid.



The process of Gelation is different for Agar andalginate.

For Agar, the process is temperature dependent.

whereas its independent for alginate.

GELATION



GELATION

IT’S A PROCESS OF CONVERSION OF SOL TO A GEL

Gelation process for Agar:

Agar initially is available in the form of a gel.

This gel is converted to a Sol state by heating

it.(Liquefaction). 70 ºC to 100 ºC.When desired consistency is obtained, with bearable

temperature, the agar is poured onto the tray andloaded in the mouth. (Tempering) 37 ºC to 50 ºC

The whole mixture is now set by allowing it tocool.(Gelation)



The colloids agglomerates with each other by formingsecondary bonds like vanderwal bonds.

Such bonds has very weak force of attraction, whichbroken up at elevated temperatures.



GELATION PROCESS IN ALGINATE

Here a chemical reaction takes place.

The colloidal particles are present as the solalginates.

These include sodium and potassium alginates.These alginates react with calcium sulphate and form

calcium alginate which is the gel.



Reaction:

K2nAlg +n CaSO4 n K2SO4 + CanAlg

The above reaction is very fast.

With such rate of reaction, the operator would not getsufficient time to manipulate the impression.

Thus a retarder is used to decrease the rate ofreaction. And therefore we get an increased

working time.The retarder used is sodium phosphate.



Reaction:

2Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4 .ºC

When sodium phosphate is added, this being more

reactive than the Sol Alginate reacts with thecalcium sulphate and form calcium phosphate andsodium sulphate.

When all sodium phosphate has been used up thenthe remaining calcium sulphate reacts with the solAlginate forming the gel that is cal alginate.



SYNERESIS

Like all the impression compounds hydrocolloidsalso face dimensional instability.

this is because of the properties like Syneresis.

The gel may loose water by evaporation from itssurface or by exuding fluids on to the surface by theprocess known as Syneresis.

The gel shrinks as the result of evaporation andSyneresis.



IMBIBITIONS

Such alterations in the dimensions can be rectifiedby the process like imbibition.

If a gel is place in the water, it absorbs water .

Thus the altered dimensions can be rectified.



COMPOSITION:

Agar Hydrocolloid –

Agar- its an organic hydrophilic compound- 8-15%

Water – serves as the reaction medium- >80%

Borax – used for strength, retarder

Potassium sulphate- accelerator

Wax, diatomaceous earth, clay, rubber and inertpowders- used to control the strength viscosity andrigidity.

Thymol and glycerine- bactericidal agent andplasticizers

Pigments and flavors- for the convenience of the patient



They are available in two forms:

Syringe material - low viscosity

Tray material- with comparatively higher viscosity



Alginate –

Chiefly consists of soluble alginates in the form ofpowder. - 15%

Soluble alginate – sodium, potassium, ortriethanolamine alginate

Water – reaction medium

CaSO4 – Reactor - 16%

ZnO – filler particles - 4%

Potassium titanium Fluoride – accelerator- 3%

Diatomaceous Earth – filler particles - 60%

Sodium Phosphate – Retarder -2%



REFERENCE

Phillips’



THANK YOU

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