13hydrocolloids-140604162732-phpapp02.pdf

Elastic Impression Materials :

Hydrocolloids

What are colloids ?  1. 2. 3.

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Dissolving solid (solute) into Liquids (solvent) could result in 1 of 3 states Solution Suspension Colloid If the solvent of such colloid is the water, the material is named hydrocolloid

Hydrocolloids 

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 materials  1. 2.

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2 types of hydrocolloid can be used for making impressions: Agar (reversible hydrocolloid) 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 Hydrocolloid impression 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 hydrocolloid 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 forms

Composition 1. 2. 3. 4.

Agar…. 12-15% Water…. 85% Borax…. (strengthening agent) 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 reaction 

The material changes from Sol to Gel via physical reaction (Reversible hydrocolloid ) Heating at 70-100oC Sol 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 setting Tray Circulating water 13-18oC

Tissue Water cooling system tray

Agar impression material

Oral tissues

Mode of agar setting

Material’s Conditioning

Boiling room (100oC)

Storage room Tempering room (45oC) (60-65oC)

Syringe and Tray material is Syringe and only tempered for tray materials tray materials 2 min. stored up to 24 left for 10-15 hrs. min.

Properties 1. Biological properties  Proper 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

Properties 2. Interfacial properties 

The material is is hydrophilic, easily flow over wet tissues registering fine details.

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No separating medium is required at the time of impression pouring

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

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

Properties 3. 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

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

Properties 4. Chemical properties  Agar material sets via physical reaction  can be reused 

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

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3 other reasons are responsible for impressions’ dimensional changes

Properties 1. 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

Properties 5. Practicability 

Making agar impression is complex and sensitive procedure

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A special equipments (conditioner & watercooling system trays) are required

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

Properties 

Lengthy setting time could reach 8 min

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Jerk removal of the impression and immediate pouring are both recommended

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Storage of impressions is risky and needs certain precautions

2. Alginate Hydrocolloid impression 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 forms A. -

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

B. 2 paste system (Siliconized alginate)

Powder composition 1. 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 reaction Chemical reaction named gelation Calcium sulfate  Sodium phosphate  Calcium phosphate  + Sodium sulfate   (1)

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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 setting Tissue

Tray Stock tray

Alginate impression material

Mode of alginate setting Oral tissues

Making the material ready

Properties 1. Biological properties  Alginate 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.

Properties 2. Interfacial properties 

Alginate impression material is hydrophilic

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

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No separating medium is required at the time of impression pouring

Properties 

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

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

Properties 3. 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

Properties 4. 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.

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

Properties 5. 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.

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

Properties 

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

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Both the regular set and the fast set alginates are available

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Jerk removal of the impression and immediate pouring are both required

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Storage of impressions is risky and needs certain precautions

Agar-Alginate Impression (Laminate impression)

Agar impression material Stock tray Alginate overall impression Oral tissues Prepared tooth 

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

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

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

Care and handling of hydrocolloid impressions 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. 2.

Immediate pouring of the impression in stone 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.

Thank you