hydro colloid (agar & alginate)

HYDROCOLLOIDS( AGAR AND ALGINATE)

GROUP A

AGAR (REVERSIBLE) HYDROCOLLOIDS

• The setting of agar is often called gelation, a changes from sol(solid) to gel phase and vice versa.

• Induced by temperature change– Liquefaction temperature (LT)70⁰ to 100⁰CGel convert to sol when heated up

– Gelation temperature (GT) 37⁰ and 50⁰C sol transform to gel when cooledGT is critical for impression making

PROPERTIES OF AGAR

• Agar gel is converted to sol by heating in water (boiling) 100oC and become gel again by cooling it to 43.3oC. Once converted to a sol, it will remain as fluid for extended periods at 65.7oC.

• Reversible hydrocolloid – transformation of gel is reversible with application of heat.

PROPERTIES OF AGAR

• It is highly accurate when removed from mouth but may altered

Shrink when stored in air or 100% relative humidity and

Expand when stored in water as with alginate.

PROPERTIES OF AGAR

• Have tear strength of 715gm/cm and compressive strength of 8000gm/cm2( for tray materials). The syringe materials have properties about one third those of tray materials.

COMPOSITION

• Agar is an organic hydrophilic colliod (polysaccarides) extracted from certain types of seaweed.

Component Function Composition

Agar Brush-heap function

13-17%

Water Reaction medium >80%

Borax Strength 0.2-0.5%

Sulfate Accelerator 1.0-2.0%

Wax, hard Filler 0.5-1.0%

Thymol and glycerine

Bacteriacidal and plasticizer

COMPOSITION

• agar hydrocolloid (hot) agar hydrocolloid (cold)

• (sol) (gel)

• cool to 43ºC

• heat to 100ºC

Syringe and tray material

MANIPULATION

• The temperature lag between Gelation temperature and Liquefaction temperature of the agar makes it possible to use agar as a dental impression material.

• The temperature required in each of the steps are critical.

MANIPULATION By using a 3 – chamber conditioning unit :

1) First chamber - Liquify the agar at 100⁰C for 10 mins [ process to convert gel sol ]

2) Store in the tray in the 2nd chamber at 65⁰C [ no specific time ]

3) Store in the third chamber at 46⁰C for 3 mins

4) Take out the seat tray and cool it down at 13⁰C for 3 mins[ process to convert sol gel ]

ADVANTAGES

• Dimensionally accurate• Hydrophilic

-displaces moisture, blood, fluids • No custom tray or adhesives• Pleasant flavor • No mixing required

DISADVANTAGES

• Material must be prepared in advanced • Tears easily • Dimensionally unstable

-must be poured immediately-can only be used for single cast

• Difficult to disinfect

IRREVERSIBLE HYDROCOLLOID (ALGINATE)

• COMPOSITION• PROPERTIES

• MANIPULATION• ADVANTAGES AND DISADVANTAGES

ALGINATE

• Most widely used impression material

• Indicationso Study modelo Removable fixed partial dentures

framework

• Exampleso Jeltrateo Coe alginate

COMPOSITION• Diatomaceous earth (filler) 60%• Calcium sulphate (reactor) 16%• Sodium or potassium or triethanolamine alginate

(active ingredients) 15%• Zinc oxide 4%• Potassium titanium fluoride (improves gypsum

surface) 3%• Sodium phosphate (retarder) 2%• Coloring & flavoring agents traces

MANIPULATION1) A measured quantity of water and powder are

added in the rubber bowl proper W/P ratio as pacified by the manufacturer

2) Mix for 45 to 60 seconds stirring motion to wet the powder with water

3) Once its moistened, manipulate with a rapid and vigorous figure of 8 motion

4) Once a smooth mix is attained, place in the impression tray

5) Remove after 2 to 3 minutes After gelation (loss of tackiness)

PROPERTIES OF ALGINATE

• Mixing and setting times• Permanent deformation• Flexibility• Strength• Dimensional stability

Mixing and Setting Time • Alginate reacts chemically, temperature is a major

factor in the setting time. • The colder the temperature of the water the longer

it takes to set, and vice versa• Ideal water temperature is 68 degrees or room

temperature.• Mixing time : 45 to 60 seconds• Setting time : 1.75 to 2 minutes• Types ;

o Type I (fast set) : 1-2mino Type II (normal) : 2-4.5 min.

Permanent Deformation

• Commercial alginate have actual values of 96% to 98% recovery (2% to 4% permanent deformation).

• Alginate impression material is flexible but not perfectly elastic.

• Amount of permanent deformation is increased when : – the time before testing is shortened – the amount of deformation during removal is increased– the time that is held under compression is increased– thinner mixes are used.

Flexibility

• Amounts of powder and water influence the flexibility of the set alginate.

Strength

• The compressive strength - 5000 to 9000gm/cm2.• The tear strength - 380 to 700gm/cm2. • The tear strength of alginates are a function of the

rate at which the impression is deformed• Strength of alginate impression materials is increased

if thick mixed are used• The tear and compressive strengths at the time of

removal of the impression increase if the time of removal is delayed

Dimensional Stability

• Alginate has a tendency, after it sets to lose (syneresis) or absorb (imbibition) water.

• Dry conditions -loses water & shrinks• Immersed in water -imbibes moisture or

swells.

ADVANTAGES

• Inexpensive• Hydrophilic• The process is not time consuming• Easy to manipulate• Requires no special instrument• It makes a reasonably accurate impression• It allows for undercuts• It causes no great pain to the patient• Pleasant taste and odour

DISADVANTAGES

• Low dimension stability – due to syneresis and imbibitions

• Chemically set – can’t be modified• Poor tear strength especially in thin sections.• Distortion may occur if tray is shifted during

setting.

REFERENCES

• Dental Materials, Properties and Manipulation 2004 : Robert G. Craig, John M. Powers, John C.Wataha.

• Applied Dental Materials 8th edition; 2006 ; John F. McCabe, Angus W.G Walls.

• http://www.docstoc.com/docs/20044711/impression-materials

• http://www.juniordentist.com/alginate-impression-material.html

• http://www.dentalorg.com/manipulation-of-alginate-impression-material.html