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Elastic Impression Materials :Elastic Impression Materials :

HydrocolloidHydrocolloidss

What are colloids ?What are colloids ?

Dissolving solid (solute) into Liquids (solvent) could result in 1 of 3 states

1. Solution2. Suspension

3. Colloid

If the solvent of such colloid is the water, the material is named

hydrocolloid

HydrocolloidsHydrocolloids

Hydrocolloids are usually exist either in Sol (Viscous liquid) or Gel (Jelly-like) form.

The sol can change into gel through:

1. Physical (Reversible) reaction

Sol gel

2. Chemical (Irreversible) reaction

Sol gel

Hydrocolloid impression Hydrocolloid impression materialsmaterials

2 types of hydrocolloid can be used for making impressions:

1. Agar (reversible hydrocolloid)2. Alginate (irreversible hydrocolloid)

To make an impression using hydrocolloid materials,

The material should be introduced into patient’s mouth in their sol state (To give sufficient flow and

register the details) and left inside to reach the gel state (Setting) before removal.

1. Agar Hydrocolloid1. Agar Hydrocolloid impression materialimpression material

The material is elastic and usually used in special type of stock trays to make 2ry impressions for

fixed restoration (inlay, onlay, crown and bridge).

Dental applications of agar Dental applications of agar hydrocolloidhydrocolloid

1. 1. Making 2ry impression for inlay, only crown and bridge work

2. Making agar-alginate impression (Laminate impression technique)

3. Laboratory duplication of stone casts

4. Help in making acrylic dentures using the fluid resin technique

Presentation formsPresentation forms

CompositionComposition

1. 1. Agar…. 12-15% 2. Water…. 85%3. Borax…. (strengthening agent) 4. K2So4 …. (to overcome the retarding effect of

borax on gypsum cast material)5. Thymol…. (Antifungal) 6. Coloring agent

Usually the consistency of the tray material is higher than that of the syringe material as a

result of higher agar content

Setting reactionSetting reaction The material changes from Sol to Gel via physical

reaction (Reversible hydrocolloid )

Heating at 70-100oCSol Gel

Cooling at 37-45oC

Factors affecting the gelation time:1. Sol Temperature 2. Cooling temperature

3. Agar content 4. Thickness of the material

5. Surface area

Mode of material’s settingMode of material’s setting

Tray Tissue

Water cooling system tray

Mode of agar setting

Agar impression material

Oral tissues

Circulating water 13-18oC

Boiling room)100oC(

Syringe and tray materials left for 10-15

min.

Storage room)60-65oC(

Syringe and tray materials

stored up to 24 hrs.

Tempering room )45oC(

Tray material is only tempered for

2 min.

Material’s ConditioningMaterial’s Conditioning

PropertiesProperties

1. Biological propertiesProper tempering of the agar tray material

is recommended to avoid burning of the oral soft tissue

Reusing of the agar impression material is not advisable because of the possibility of cross-infection

PropertiesProperties2. Interfacial properties The material is is hydrophilic, easily flow over wet

tissues registering fine details.

No separating medium is required at the time of impression pouring

Setting of polymeric die material is affected by the water content of the agar

Gypsum model material should be mixed with hardening solution; otherwise a cast with chalky easily abraded surface will result.

PropertiesProperties3. Mechanical properties Agar has low tear strength as a result of the

higher water content , however jerk (Rapid) removal of the set impression will help in overcoming this drawback

Agar material shows a lower degree of permanent deformation than that of alginate due to the absence of filler particles within the composition

PropertiesProperties4. Chemical properties Agar material sets via physical

reaction can be reused

A degree of shrinkage is expected on material’s gelation (due to cooling)

3 other reasons are responsible for impressions’ dimensional changes

PropertiesProperties1. Imbibition:

Agar material can absorb (Gain) water if present in wet conditions expansion

2. Synersis:

Agar material can exudates (Loss) water if present in humid conditions shrinkage

3. Dehydration:

Agar material can evaporate (Loss) water if present in dry conditions Shrinkage


5. Practicability

Making agar impression is complex and sensitive procedure

A special equipments (conditioner & water-cooling system trays) are required

Needs circulating cold water (18oC) to help the gelation

Lengthy setting time could reach 8 min

Jerk removal of the impression and immediate pouring are both recommended

Storage of impressions is risky and needs certain precautions


2. Alginate Hydrocolloid 2. Alginate Hydrocolloid impression materialimpression material

Alginate impression material is elastic and usually used in stock trays to make primary

impressions.

Other applications:

1. Final impression for complete denture

2. Agar-Alginate impression technique

3. As wash material over 1ry compound impression

Presentation formsPresentation forms

A. Powder to be mixed with water- Regular & Fast set - Dust free- Chromatic

B. 2 paste system (Siliconized alginate)

Powder compositionPowder composition1. Sodium or potassium alginate (The principal

ingredient)

2. Calcium sulfate (Reactor)

3. Na-phosphate (Retarder)

4. Diatomaceous earth (Filler)

5. Potassium sulfate (Provides proper setting of gypsum poured against the alginate impression)

6. Color indicator in chromatic alginates

7. Glycol in dust-free alginate coating the powder particles

Setting reactionSetting reaction Chemical reaction named gelation

Calcium sulfate Sodium phosphate

Calcium phosphate + Sodium sulfate (1)

Calcium sulfate Sodium alginate

Insoluble calcium alginate (2)

The speed of the reaction is affected by;

1. L/P ratio 2. Temperature

3. Mixing rate 4. powder particle size

5. Additives

Mode of material’s settingMode of material’s setting

Tissue TrayStock tray

Mode of alginate setting

Alginate impression material

Oral tissues

Making the material readyMaking the material ready


1. Biological propertiesAlginate is biologically safe (Has no bad

effect on the patient health)

Inhalation of dust at the time of manipulation could precipitate bronchial asthma in allergic patients. Accordingly, dust-free alginate is advisable.

PropertiesProperties2. Interfacial properties Alginate impression material is hydrophilic

The material is able to register the complete denture foundation. But, not used to make final crown and bridge impressions

No separating medium is required at the time of impression pouring

Setting of polymeric die material is affected by the water content of the alginate

Gypsum model material should be mixed with hardening solution, otherwise a cast with chalky easily abraded surface will result.


PropertiesProperties3. Mechanical properties Alginate has higher tear strength than that of

agar as a result of its filler content

Jerk removal of the set impression is recommended to minimize the rate of both impression tearing and permanent deformation

PropertiesProperties4. Chemical properties Aliginate material sets via chemical reaction

(gelation) single use only

Several factors such as temperature, liquid/powder ratio, particle size, mixing rate and the amount of retarder could affect the rate of the reaction and the setting time.

Imbibition, synersis, and dehydration are also noticed with the material

PropertiesProperties5. Practicability Making alginate impression is simple and less

sensitive procedure

The powder container should be shacked very well before dispensing the powder ratio to help in redistributing the ingredients.

The container should be hermetically sealed all the time to avoid moisture contamination and deterioration of material’s powder

Regular perforated or rim-lock stock trays are recommended for making alginate impression.

Both the regular set and the fast set alginates are available

Jerk removal of the impression and immediate pouring are both required

Storage of impressions is risky and needs certain precautions


Agar-Alginate ImpressionAgar-Alginate Impression(Laminate impression)(Laminate impression)

Purpose: to overcome the complexity and high cost of making agar impressions.

How: register the fine details of the prepared teeth using syringe type agar hydrocolloid and record the adjacent teeth using alginate

The agar sets by the effect of alginate mixing water and both materials cohere depending on their colloidal nature

Alginate overallimpression

Agar impressionmaterial

Stock tray

Oral tissues

Prepared tooth

A. Dimensional stability:

Due to the ability of hydrocolloids to gain or loss water (imbibitions, synersis and dehydration). A considerable degree of Dim. Changes is expected unless;

1. Immediate pouring of the impression in stone2. Storage of impressions in 2% potassium sulfate or

100% humidity

100% humidity could be achieved by wrapping the impression with wet paper towel and both are stored in sealed plastic bag.

Care and handling of hydrocolloid Care and handling of hydrocolloid impressionsimpressions

Thank you