The setting of agar is often called gelation, a changes from sol(solid) to gel phase and vice versa. Induced by tempera temperature ture change ±
Liquefaction temperature (LT) (LT)
70 to 100C Gel convert to sol when heated up
±
Gelation temperature (GT)
37 and 50C sol transform to gel when cooled GT 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 cool to 43ºC
agar hydrocolloid (hot)
agar hydrocolloid (cold)
(sol)
(gel) 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 100C for 10 mins [ process to convert gel sol ] 2) Store in the tray in the 2 nd chamber at 65C [ no specific time ] 3)
Store in the third chamber at 46C for 3 mins
4) Take out the seat tray and cool it down at 13C for 3 mins [ process to convert sol
gel ]
ADVANTAGES
Dimensionally
Hydrophilic
accurate
-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
Indications Study model o Removable fixed partial dentures o
framework
Examples o
Jeltrate
o
Coe alginate
COMPOSITION (filler) 60%
Diatomaceous earth
Calcium sulphate (reactor) 16%
Sodium or potassium or triethanolamine alginate (active ingredients) 15% Zinc
measured quantity of water and powder are added in the rubber bowl A
proper W/P ratio as pacified by the manufacturer
2)
Mix
3)
for 45 to 60 seconds
stirring motion to wet the powder with water
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 ; Type I (fast set) : 1-2min o Type II (normal) : 2-4.5 min. o
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
Dry
has a tendency, after it sets to lose (syneresis) or absorb (imbibition) water. 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 cant be modified
Poor tear strength especially in thin sections.
Distortion
setting.
may occur if tray is shifted during
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/impres sion-materials http://www.juniordentist.com/alginateimpression-material.html http://www.dentalorg.com/manipulation-ofalginate-impression-material.html
