The document discusses the various types of impression materials used in dentistry, focusing on hydrocolloids such as agar and alginate. Key characteristics, classifications, and the gelation process of these materials are detailed, as well as their applications and limitations. It also covers the disinfection processes, dimensional stability issues, and factors affecting the accuracy and effectiveness of dental impressions.

1. Presented by –
Piyali bhattacharya
1st year PGT
Dept . Of Prosthodontics Crown & Bridge
HIDSAR

2. a negative likeness or copy in reverse of the surface of
an object; an imprint of the teeth and adjacent
structures for use in dentistry (GPT-9)
Impression area :The surface that is recorded in an impression
Impression Material: Any substance or combination of
substances used for making an impression or negative
reproduction.

3. 1. Dimensional Stability
2. Adequate Strength
3. SatisfactoryTexture And Consistency
4. EasyTo Disinfect Without Loss Of Accuracy
5. Compatible With Die And Cast Material
6. Good Elastic Properties
7. Easy Manipulation
8. Adequate Setting Characteristics

4. 9.High Reproduction Of Details
10.Should Not Release Any Gas
11.Should Not BeTechnique Sensitive
12.Economical
13.Not Contain AnyToxic And Irritating Component
14.Adequate Shelf Life
15.Pleasant Smell AndTaste

5. CLASSIFICATION OF IMPRESSION MATERIAL
RIGID ELASTIC
A.IMPRESSION
PLASTER
B.IMPRESSIONCOMPOUND
C.METALLIC PASTE
HYDROCOLLOIDS
A. REVERSIBLE
B. IRREVERSIBLE
SYNTHETIC ELASTOMERS
A.POLYSUFIDE
B.POLYETHER
C.SILICONE
(ADDITION,
CONDENSATION)
Ref: Anusavice 11th edition 209

6. INTRODUCTION

8. Reversible hydrocolloid was introduced to the dental
profession in 1925 by Alphons Poller , an
Austrian as impression material .
Agar was first introduced into dentistry for recording
crown impressions in 1937 by Sears .
Sanjay Madhavan et al /J. Pharm. Sci. & Res.Vol. 7(9), 2015, 704-707

9. Type I: High Consistency(Tray Material)
Type II: Medium Consistency (Tray/Syringe Material)
Type III: Low Consistency (Syringe Material)
Classification (ISO 1564:2001)

10. 1.Full mouth impression without deep undercuts
2.As a tissue conditioner
3.As duplicating material
4.Earlier it was extensively used for FPD impression
prior to elastomers

11. Composition & Functions Of Each
Components:
Agar (13%-17%)- Brush-heap structure
Borate(0.2%-0.5%)- Strength
Sulfate(1%-2%)- Gypsum Hardener
Wax(0.5%-1.0%)- Filler
Thixotropic materials(0.3%-0.5%)-Thickener
Water –Reaction medium

12. Brush –Heap StructureOf Hydrocolloid

15. Tray Selection for Agar
http://www.juniordentist.com/agar-reversible-hydrocolloid-impression-material.html

16. 1) Preparation And Conditioning Of The Material
2) TemperingThe Material
3) MakingThe Agar Impression

19. Working time: 7-15 minutes
Setting time: 5 minutes

20. Accuracy:
One of the most accurate impression materials
which has a long history of successful use in
dentistry.
Viscosity ofThe Sol:
The material must be viscous enough even after it
has been liquefied in order to prevent itself from
flowing out of the tray when it’s inverted e.g.;
while making a mandibular impression.

21. Even when the material is viscous enough to stabilize the
tray it does not offer much resistance to seating Also
patient may “bite through” the impression material..
With a “TripleTrayTechnique ” the oral impression of the
maxillary ,mandibular arches as well as occlusal
relationship can be recorded although it is technique
sensitive procedure.

22. Distortion during Gelation:
As the material undergoes physical change from
Sol to Gel contraction occurs (usually in the center
of the tray, resulting in larger dies)
As the Sol is a poor conductor of temperature,
rapid cooling of the tray causes stress concentration
near the tray where the gelation first occurs.
Ideally water at 20℃ is most suitable for cooling the
impression tray than ice water.

23. 1.Gelation, Liquefaction And Hysterisis
Gelation at 37 ℃
Liquefaction at 60 ℃ - 70 ℃
2.Syneresis And Imbibition
Whenever immersed into water agar doesn’t imbibe the
original amount of water as it loses by evaporation
3.Flexibility:
Between 4-15% with application of 14.2 Psi pressure

24. 4.Elasticity and elastic recovery
Highly elastic in nature, elastic recovery is about 98.8%
Strength:
Tear strength of agar 0.8-0.9 N/mm
Shelf-life:
One year.

25. Introduction:
In 1921 in England a chemist , sirWilliam Wilding
received a basic patent for using Algin as a dental
impression material
Was introduced as a substitute for agar when its’ supply
became scarce after WW II.
The material is based on substances extracted from
certain brown seaweed (anhydro –β-δ- mannuronic
acid/ alginic acid).

26. Properties of Alginate Impression Material:
1. Pleasant taste and smell
2. Highly flexible
3. good elasticity and elastic recovery
4. Fine reproduction of details
5. Compressive strength 0.33-0.70 MPa
6. Tear strength 350-700 gm/cm²
7. Poorly adheres to the tray

27. Uses:
Used for making preliminary impression to construct
custom tray, make study models , help in treatment
planning.
As alginate is hydrophilic in nature, impression can be
recorded even on moist oral tissues

28. Potassium Alginate (15%)- Soluble Alginate
(Or Sodium Alginate)
Calcium Sulfate(16%)- Reactor
Zinc Oxide(4%)-Filler Particle
PotassiumTitanium Fluoride(3%)- Accelerator
Diatomaceous earth (60%)- Filler Particles
Sodium Phosphate (2%)- Retarder

29. Gelation Process:
Typical Sol-Gel reaction .
1.Reaction between soluble alginate with calcium sulfate
& formation of insoluble calcium alginate .
This is a rapid reaction and thus doesn’t allow much
working time.
2.Trisodium phosphate (water soluble salt) is added to
prolong the working time. Calcium sulfate first reacts with
this salt & reaction with alginate is deferred as long as the
unreacted trisodium phosphate is left.

30. 1. K2n- alginate +n CaSO4 Ca n-(alginate) +n k2SO4
(powder) (gel)
2 . Na3PO4 + 3 CaSO4 Ca3(PO4)2 + 3 Na2SO4

31. The final structure
Sodium/potassium
alginate cross-
linked with calcium ions.
The Final Gel Structure

32. SettingTime:
Depending on setting time
i) Fast setting (1.5-3 min)
ii) Normal setting Alginate(3-4.5 min)
Controlling the setting time:
i) W/P ratio: affects properties e.g.; tear strength
and elasticity
ii) Amount of retarder
iii) Alteration of temperature of water: a 1 – min
reduction in setting time occurs for each
10℃ of temperature increase

33. * Setting time in minutes & temperature in ℃

35. 1.Measured powder is sifted into premeasured
water into a clean rubber bowl
2.The powder is incorporated into water by
carefully mixing with a metal spatula in vigorous
figure of 8 motion with intermittent rotations (180◦)
of spatula to press out air bubbles
powder should always be weighed not measured.
Mixing time: 45 sec-1 min
Ideal w/p ratio: 38 ml water/16 gm. powder

36. A perforated tray is usually used .if plastic or metal rim-lock
tray is used , a thin layer of alginate tray adhesive is applied
and allowed to dry before mixing and loading the tray.
As alginate is weak the tray must fit into the patient’s mouth
to provide sufficient bulk of material.
The thickness of alginate between tray and oral tissue should
be 3 mm.

37. The compressive strength of alginate doubles the first 4 mins
but after 4 mins it does not increase much
Alginate impression material should be kept in the mouth
for at least 3 min after the gelation has already occurred.
Removal of Impression:
The tear strength of alginate increases if the impression is
removed with a snap
Avoid torqueing or twisting of impression

39. The tray must be filled with the
impression
material up to the tray border
The tray is filled by forcing the
material firmly
into the tray to make sure voids are
not trapped.
A useful method is to start at the
posterior of the tray and push the
mix anteriorly by adding more and
more to the posterior .
The excess unsupported material at
the periphery must be removed
with the mixing spatula
The surface of the mix should be
wiped with gloved finger
Tray Selection

40. Impression tray is positioned
in the mouth by retracting
the patient’s lip on one side
with a mouth mirror /gloved
finger ; and on the other side
by rotating the tray into the
mouth.
The tray has to be centered
in position in the mouth ;
and with light pressure, so
that alginate can flow
properly and impression
held in place.
IMPRESSION MAKING

42. While making the
impression,
the tray should cover the
entire impression area &
provide a space of at
least 3 mm on all sides.

45. Factors affecting the strength of the gel
1.Adequate w/p ratio: Too much or too less water
weakens the gel
2.Adequet spatulation: over mixing breaks up the
calcium alginate network and reduces the strength.
Accuracy
Alginates are sufficiently accurate for making impression
for removable partial dentures.
However, as compared to elastomeric impression materials,
alginate do not produce finer details

46.  The tear strength is a measure of
force/thickness ratio needed to initiate or
continue tearing.
 Ranges from- (0.37 to 0.69)N/mm
 Tearing occurs in the thin sections of the
impressions and the probability of tearing
decreases with the increase in rate of removal

47. Effect Of Rate Of Deformation OnThe
Tear Strength Of Alginate Material

48. 1.Flexibility:About 14% at stress of 1000gm/cm2 .Some of
the hard set materials have lower elasticity(5-8%)lower
w/p ration results in lower flexibility
2.Elasticity & Elastic Recovery:Less elastic when compared to
agar impression material.it Is about 98.2%.thus permanent
deformation is more for alginate, but when set impression is
removed early from mouth , permanent deformation is less
3.Reproduction of tissue detail: Lower when compared to
agar.ADA sp. Requires this material to reproduce a line that is
0.075 mm in width
4.Strength:
Compressive strength: 0.33-0.70MPa

53. Duplicating Material
Both hydrocolloids can be used for duplicating casts or
models to construct prosthetic appliances & orthodontic
models
But agar is more popular as it can be used many times
With intermittent stirring agar can be kept liquid for 1-2
Weeks at a constant pouring temperature.
Duplicating materials have the same composition as the
impression material except, they have a higher water content
& a lower agar / alginate content compromising its
compressive strength & percent permanent set.
This property requirement is identified as ADA specification
no. 20

54. Hydrocolloids should be disinfected for a limited time period .
Immersion is more secure than spraying and self-disinfecting
materials are efficacious, but better accompanied by immersion.
Most manufacturers recommend iodophor, gluteraldehyde,
phenol as disinfectants
E . k o t s i omi t i et al., Dimensional changes of disinfected i m p r e s s i o n s, Journal of Oral
Rehabilitation 2008 35; 291–299
Disinfection of hydrocolloids

55. The addition of 1% chlorhexidine di acetate to
Alginate has decreased its compatibility with
the dental stones when compared with its non-
antimicrobial counterpart. In terms of gypsum
compatibility, the non-antimicrobial Alginate
was compatible with all disinfectant and
gypsum combinations.
King, Norling, and Seal, Compatibility /Antimicrobial Alginates, Journal of prosthodontics.Vol3,
No 4 (December), 1994:pp219-227

56. The current protocol from the CENTER for DISEASE CONTROL
is to use house hold bleach, (1-10 dilution), Iodophor ,
synthetic phenol as disinfectants . After impression is rinsed
thoroughly under water, disinfectants can be sprayed . then
impression is wrapped in disinfectant soaked paper towel &
placed in a plastic bag for 10 minutes. Finally, impression
Is taken out of bag unwrapped, rinsed, shaken to remove
excess water and poured with stone.

57. Dimensional stability of Hydrocolloids
In case of Agar, if the impression is left exposed in the air at
room temperature shrinkage occurs due to syneresis.
If the impression is immersed into water, swelling occurs due to
imbibition
2% Potassium sulfate or 100% relative humidity are suggested
to prevent dimensional change of agar.
Alginate impression materials shrink slightly because of the
thermal differential between mouth temperature
(37˚c) and room temperature(20 ˚c).While agar impression
material shows a temperature shift in opposite direction , from
chilled water to warmer room temperature makes the
impression to expand and become less accurate

58. Linear contraction of reversible hydrocolloid
in air(31%-42% relative humidity) & subsequent
Expansion in water
Percentage change in water content
according to weight of agar hydrocolloid in
Various storage medium, at 100% humidity
The percent change is minimal

59. Borax , which is a gypsum retarder is used as a filler in agar
impression material which may cause surface of gypsum
casts prepared from agar to be soft for using as dies.
The gelation process of alginate impression produces
calcium alginate & sodium sulfate which is a gypsum
accelerator at low concentration & a retarder at high
concentration .The amount of sodium sulfate produced
becomes enough to retard the setting of gypsum cast

60. 1. By immersing impression in accelerator such as 2%
potassium sulfate solution prior to pouring impression with
gypsum product
2. By using products that incorporate gypsum hardener or
accelerator e.g.; sulfate in agar & potassium titanium fluoride
in alginate.
The Stone cast/Die should be in contact with the impression
material for at least 30min or preferably 60 min.

61. Separation of Cast : Separation of cast should be
done within a reasonable period of time . As
desiccation of the hydrocolloid may cause abrasion
of gypsum during removal

62. Biocompatibility
Silica particles present in alginate may cause silicosis ,pulmonary
hypersensitivity,
The most common side effect of reversible hydrocolloid is
thermal injury due to improper tempering or faulty equipment
during impression making

63. The Bio Hazard from Alginate Impression Materials
BRUNE AND BELTESBREKKE, Airborne Particles of Alginate, Scand.J. Dent. Res. 1978: 86: 206-210

64. BRUNE AND BELTESBREKKE, Airborne Particles of Alginate, Scand.J. Dent. Res. 1978: 86: 206-210

65. Dust -FreeAlginate
Dust –Free Aginate:
In attempt to minimize the
risk from the dust , alginate
is coated with Glycerin ,
Glycol, Polypropylene
Glycol or Polyethylene
Glycol

66. 1.Extended-pourAlginate:
enables to delay pouring impression under specified
condition for up to 100 hrs without much dimensional change.
Some extended pour alginate allows to delay pouring up
to 4 weeks
Kaur et. Al., Alginate Impression Material: from then till now..
HealTalk | November-December 2012|volume 05| issue 2

68. 2. HighViscosityAlginates:
By incorporation of hydrophobic material(.01-.1 wt %) or
surfactants (0.05-0.1 wt %) permeation of water is speeded,
Kneading time is shortened , incorporation of polysaccharides
prevent gagging
3.Color Changing Alginate: By adding pH indicator.
Although most pH indicator changes color little above pH 8
before the actual gelation time.

69. 4.Two Paste system:
Base paste contains soluble mix of Alginate , water and fillers .
With paste forming materials e.g.; Gum arabican , carrageenan
Reactor/ catalyst paste contains calcium salt with viscous liq.
like paraffin & a pH stabilizing agent (magnesium hydroxide)
5.Tray Adhesive for Alginate:
A tray adhesive in the form of liquid or spray containing
polyamide or diethylene tetra amine ether , ester gum and rosin
containing isopropyl alcohol.

70. Two Paste System

72. Infection Free Alginate: Alginate incorporated with
disinfectant materials such as quaternary ammonium
compounds , bisguanides , chlorhexidine without
compromising its’ properties
Epimax showed highest disinfection action after 10 mins

73. EFFECT CAUSE
Grainy material •Improper mixing
•Prolonged mixing
•Excessive gelation
•water/powder ratio too low
Tearing •Inadequate bulk
•Moister contamination
•Premature removal from mouth
•Prolonged mixing
External bubbles Undue gelation preventing flow air
incorporated during mixing

74. Irregularly shaped voids Moisture or debris on tissue
Rough or chalky stone model •Inadequate cleaning of impression
•Excess water left in impression
•Premature removal of impression
•Model left in impression too long
•Improper manipulation of stone
Distortion •Impression not poured immediately
•Movement of tray during gelation
•Premature removal from mouth
•Improper removal from mouth

75. Shelf Life
Two factors affecting the shelf life of alginate:
1.Storage temperature
2.Moisture contamination
It is best not to stock more than 1 year’s supply in dental
office.
The material should be stored in a cool, dry environment

76. 1. Ref: Anusavice 11th edition 209
2. Sanjay Madhavan et al /J. Pharm. Sci. & Res.Vol. 7(9), 2015, 704-707
3. http://www.juniordentist.com/agar-reversible-hydrocolloid-
impressionmaterial.html
4. E . k o t s i omi t i et al., Dimensional changes of disinfected i m p r e s s i o n s,
Journal of Oral Rehabilitation 2008 35; 291–299
5. King, Norling, and Seal, Compatibility /Antimicrobial Alginates, Journal of
prosthodontics.Vol3, No 4 (December), 1994:pp219-227
6. BRUNE AND BELTESBREKKE, Airborne Particles of Alginate, Scand.J. Dent. Res.
1978: 86: 206-210
7. Kaur et. Al., Alginate Impression Material: from then till now..
HealTalk | November-December 2012|volume 05| issue 2