This document discusses hydrocolloids, which are gelatin particles suspended in water. There are two main types used for dental impressions: agar and alginate. Agar is a polysaccharide extracted from seaweed that is widely used for cast duplication. It can be reused but has low tear strength. Alginate is derived from seaweed and sets via a chemical reaction between soluble alginate and calcium sulfate to form insoluble calcium alginate. Alginate makes accurate impressions but cannot be reused or corrected. Both materials are inexpensive and easy to use but have limitations such as dimensional instability and low strength.

1. HYDROCOLLOIDS
JIBI SARA VARGHESE
1ST YEAR POST GRADUATE

2. HYDROCOLLOID are lyosol basically
consists of gelatin particles suspended in
water and water is the dispersion medium .
Solution – uniformly dispersed solute and
solvent e.g. salt in water – exist as a single
phase
Suspension – visible insoluble particles in
solute e.g. dust in water
Emulsion – visible insoluble liquid in liquid
e.g. oil in water

3. Hydrocolloid impression
materials
2 types of hydrocolloid can be used for
making impressions:
 1. Agar (reversible hydrocolloid)
 2. Alginate (irreversible hydrocolloid)

4. AGAR
-1925 – Alphous Poller – Negacol.
-Modified & introduced in dental profession as Dentacol in
1928.
-Agar is an organic hydrophilic colloid (polysaccharide)
extracted from certain types of seaweed( kelp plant).
-It is a sulphuric ester of a linear polymer of galactose

5. USES OF AGAR:
1. Widely used at present for cast
duplication .
2. For full mouth impression without deep
undercuts .
3. As tissue conditioner.
4. Was used for crown & bridge before the
advent of elastomers.
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6. Composition
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

7. TYPES OF AGAR
1. Heavy bodied – in poly tubes or flex skins
2. Medium bodied
3. Regular bodied
4. Light bodied – in syringes

8. MANIPULATION
The following steps are recommended ;
-Heat in water at 100° C (212° F) for 8 - 10
minutes.
-Store in water at 65° C (149° F) for 8-10 hours
-Place in a tray at 65° C (149° F)
-Temper in 46° C (115° F) water for 2 minute
before taking the impression to increase the
viscosity and pressure applied to the syringe
material, also making comfortable for the patient.
The solution is taken directly from then storage
compartment, is first ejected at the base of the
preparation and then the remainder of the
prepared tooth is covered.

9. In the meantime, tray material is tempered, water soaked
outer layer is soaked with a dry guaze before placing to
ensure good union between tray and syringe material. The
tray material is positioned with passive pressure.
ADA specifications no.11 specifies that gelation
temperature must not be less than 37°C (98.6°F)
or more than 45°C (113°F).
Gelation is accomplished by circulating
Cool water with a time not less than
5 minutes.
After gelation, impression is withdrawn in
One piece, parallel to line of withdrawal
of prepared tooth.

10. - Excess is trimmed and impression is stored in 2 %
potassium sulfate, controls water volume and has better
surface integrity in stone for 5 to 20 mins.
-The potassium sulfate solution gives a harder surface to
the stone die material.
- The potassium sulfate should not be washed out of the
impressions before the dies are poured.
- The impression must be blown carefully; the impression
should not be dehydrated, but an excessive amount of
water or moisture should not be left in the impression.
- Impression must be poured immediately after retrieval,
and disinfected with iodophor, bleach or glutaraldehyde.

12. BOILING /LIQUIFACTION SECTION
• 1 0 mins in boiling water (100 c)
• Every time material is reliquified, 3 mins should
be added. this is because it is more difficult to
break down the agar brush heap structure after a
previous use.
STORAGE SECTION
• 65 – 68ºC Temp. is ideal. It can be Stored in the
sol condition till needed.
TEMPERING SECTION
46ºC for 2-10 mins with material loaded in the
tray.
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13. 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-infections
2. 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

14. 3. 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
Imbibition:
Agar material can absorb (Gain) water if present in wet
conditions - expansion
Synersis:
Agar material can exudates (Loss) water if present in
humid conditions-shrinkage
Dehydration:
Agar material can evaporate (Loss) water if present in dry
conditions-Shrinkage

15. ADVANTAGES
 1. Helps in preparing
dies accurately.
 2. Good elastic
property and
reproduces most
undercut areas.
 3. Good recovery from
distortion.
 4. Non hydrophobic.
 5. Palatable and well
tolerated.
 6. Cheap.
 7. Can be reused.
DISADVANTAGES
 1.Does not flow
properly.
 2.Cannot be
electroplated.
 3.Due to heat , it may
be painful.
 4.Tears relatively
easily.

16. -Combination of agar for tear strength
& alginate for case in manipulation
-Inject syringe material around the
prepared tooth
-Seat the tray loaded with alginate &
allow it to set
-Cool alginate gelates the agar
-There is specific alginate to be used
with specific agar for good bond
strength
LAMINATE TECHNIQUE

18. -Wet the tooth surface with warm water
-Syringe materials applied over the occlusal and
incisal portion
-Seat the tray with material
-Hydraulic pressure forces the fluid material down
the tooth displacing the blood & debris.
WET FIELD TECHNIQUE

19. Irreversible hydrocolloid impression material
During second world war it was developed
as substitute to Agar
Natural substance extracted from certain
brown seaweed
ADA Specification no. - 18
TYPES -
Type I – Fast setting.
Type II – Normal setting.

20. MODES OF SUPPLY
It is supplied as a powder that is packed:
a) In bulk or in tins or in sackets
(or)
b) In preweighed individual containers.
A plastic scoop is supplied for dispensing the
bulk powder, & a plastic cylinder is supplied
for measuring the water .

21. APPLICATIONS
1) Used for impression making :
a) when there are undercuts
b) In mouth with excessive flow of saliva
c ) For partial dentures with clasps.
2) For making preliminary impressions for
complete denture.
3) For orthodontic and study models.
4) For duplicating models.

22. COMPOSITION
 POTASSIUM ALGINATE (15%)
 CALCIUM SULFATE (16%)
 TRISODIUM PHOSPHATE (2%)
 DIATOMACIOUS EARTH(60%)
 ZINC OXIDE(4%)
 POTASSIUM TITANIUM FLUORIDE(3%)

23. PHYSICAL PHASES OF ALGINATE
 The first phase is a sol (as in solution). In
the sol phase, the material is in a liquid or
semiliquid form.
 The second phase is a gel. In the gel
phase, the material is semisolid, similar to
a gelatin dessert.

24. SETTING REACTION
 Reaction occurs by a chemical reaction.
 Soluble alginate reacts with calcium
sulphate
to produce insoluble calcium alginate as a gel.
 The production of calcium alginate is
delayed
by the addition of a third soluble salt to the
solution, with which the calcium sulphate will
react in preference to the soluble alginate to
form an insoluble calcium salt.

25. In a nutshell , when powder is mixed with water to obtain a
paste ,two main reaction occur during setting :
1) 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3Na2SO4
2)sodium alginate + CaSO4 + H2O -- > Ca alginate
+Na2SO4
(Powder ) (Gel)

26. MANIPULATION
•Powder is added to liquid to wet the
powder with water and to reduce
entrapment of air.
•One scoop of pre-dispensed
powder products is used with the
amount of water specified by
manufacturer (16g of powder to 38
ml of water)
•Once the powder is moistened,
rapid
spatulation by swiping, or stropping
against the side of the bowl.
•A vigorous figure of 8 motion
can also be used.

28. Filled tray is held passively and motionlessly and
material is left for 2 -3 minutes, till it becomes firm
because of adequate maturation of chemical set.
The thickness of the gel between tray and the
tissues
should be 3 mm .
Working time 1 – 2 min

29. Loading and Placement of tray
Remove after 2 to 3 minutes after gelation

30. Manual Technique
A mixing time of 45 seconds to 1 minute is sufficient
.Mixing should result in a smooth creamy mix that does
not drip off the spatula, when it’s raised from the bowl.

31. AUTOMATED TECHNIQUE

32. CAUSES FOR DISTORTION AND
DIMENSIONAL CHANGE OF
ALGINATE
- If an alginate impression is stored in
water or in a very wet paper towel, the
alginate will absorb additional water and
expand. This condition is called
imbibition.
-If an alginate impression remains in the
open air, moisture will evaporate from
the material, causing it to shrink and

33. RECENT ADVANCES IN ALGINATES
Dust free alginates-
Siliconized alginates
Alginates containing disinfectants
Hard and soft set alginates

34. PROPERTIES
1)FLEXIBILITY
It is about 14% at a stress of 1000g/cm2.
Lower w/p ratio results in lower flexibility.
2)ELASTICITY AND ELASTIC RECOVERY
alginate are highly elastic but less when compared to the
agar.
97.3% elastic recovery occurs.
Permanent deformation is less if the impression is
removed from the mouth quickly.3) STRENGTH
The compressive strengths of alginate gels range from
5000 to 8000 g/cm2.
The tear strengths vary from 350 to 700 g/cm2.
Both of these properties are time dependent with higher
values being obtained if the time of removal is delayed.

35. 4) DIMENSIONAL STABILITY :
Alginate impression loses water by evaporation
and shrinks on standing in air.
If it is placed in water it absorbs water and swells.
Therefore ,cast should be poured immediately
after making the impression.
5)SHELF LIFE
Alginate impression material have a short shelf
life.
They deteriorate rapidly when stored at elevated
temperatures in a humid environment.
Therefore ,it is better not to stock more than 1
year supply .

36. ADVANTAGES
It makes an accurate impression
It allows for undercuts
The process is not time consuming
It is easy to work with, has good viscosity & is
low in adhesive qualities
It is comfortable to the patient
Inexpensive to the dentist
Requires little armamentarium

37. DISADVANTAGES
- Can’t be corrected.
- Cannot be used alone for compressing tissues.
- It is not accurate enough for crown and bridge
Impressions.

38. DISINFECTION-
All the instrument i.e. rubber bowl, spatula are sterilized
or disinfected by spraying with 0.5%NaOCl or 2%
glutaraldehyde.
Spray technique -
Rinse under tap water to clean saliva or organic debris.
Trim excess alginate from non-critical areas to decrease
amount of Microbial contamination.
Spray the entire impression surface and the reverse side
of the tray as well. It can be kept in a plastic bag or
humidor for the recommended exposure time.

39. CONCLUSION

40. REFERENCE
-Philip’s science of dental materials (11thEdition)-
Anusavice KJ
-Restorative Dental Materials (11thEdition)- Craig
RG, Powers JM
-Fundamentals of Fixed Prosthodontics
(3rdEdition)
:Shillngberg Jr HT et al
-Basic dental materials – 1st edition – manapallil
JJ