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Hydrocolloids
Contents
1 Introduction
2 History
3 Ideal requirements
4 Classification of impression materials
5 Impression materials
Hydrocolloids
a) Alginate
b) Agar
7 Recent advances
8 References
Page 1
Hydrocolloids
INTRODUCTION
Constructing a model or cast is an important step in numerous dental procedures Various
types of cast amp models can be made from gypsum products using an impression mold or
negative likeness of a dental structure An impression is a negative replica of the tissue of the
oral cavity It is used to register or reproduce the form and relation of the teeth amp surrounding
tissue
HISTORY
1558 - Celline described a wax model to make impression in his book MEMORIES
1700 - Matheus G Purenam suggested that wax models can be used in prosthetic
work
1728 ndash Pierre Fauchard described various impression materials in his book LE
CHIRURGEEN DENTISTE
1756 ndash Persian Phillip Pfaff first used plaster models prepared from sectional wax
impression of the mouth
1810 ndash R C Skinner describe about many materials and techniques in his first
American book
1928 ndash ADA developed specification No 19 for impression material
1930 ndash Zinc Oxide Eugenol a rigid impression was introduced
1930 ndash Polysulfide was first used as a commercial synthetic rubber as a copolymer of
Ethylene Chloride amp sodium
1937 ndash Agar was introduced by Sears
1949 ndash Alginate was developed during World war II
Page 2
Hydrocolloids
1950 ndash Polysulfide was used in dentistry
1955 ndash Condensation Silicone was introduced in Germany
1975 ndash Addition Silicone was introduced
1977 ndash Costell introduced Dual tray technique
1980 ndash A Visible light cure Polyether Urethane Dimethacrylate rubber impression
material was introduced
1996 ndash Blare amp Wassed considered a no of solution used to disinfecting impression
material
IDEAL REQUIREMENTS OF IMPRESSION MATERIAL
1 Pleasant taste amp odor
2 Not contain any toxic amp irritating ingredients
3 Adequate shelf life
4 Satisfactory consistency and texture
5 Easy to disinfect with out loss of accuracy
6 Compatible with die and cast material
7 Dimensional stability
8 Good elastic properties
9 Easy manipulation
10 Adequate setting characteristics
11 High degree of reproduction details
12 Adequate strength
13 Should not release any gas
14 Economical
15 Should not be technique sensitive
Classification of impression materials
Impression materials can be classified into various types based on the following
characters
Page 3
Hydrocolloids
1 Based on rigidityelasticity -
a Rigid (non-elastic)
b Elastic
2 Based on viscosity -
a Mucostatic
b Mucocompressive
c Pseudoplastic
3 Based on setting of material -
1 a chemical reaction
b Physical change of state
2 a reversible
b Irreversible
4Based on interaction with salivawater -
1 Hydrophobic
2 Hydrophilic
5Based on chemistry-
1 Impression Plaster
2 Impression compound
3 Metal oxide (zinc oxide eugenol)
4 Reversible hydrocolloid
5 Irreversible hydrocolloid
6 Poly sulfides
7 Condensation silicones
8 Addition silicones
9 Polyether
10 Visible light curing polyether urethane dimethacrylate
Page 4
Hydrocolloids
6Based on use -
1 Primary impression materials
2 Secondary impression materials
3 Duplicating materials
HYDROCOLLOIDS
Colloids are often classified as fourth state of matter the Colloidal state In a solution
of sugar in water the sugar molecules are uniformly dispersed in the water amp there is
no visible physical separation between the solute amp the solvent molecules If sugar
molecules replaced with large amp visible particles such as sand the system is
Suspension or if molecules are liquid such vegetable oil then system is Emulsion
True solution exists as a single phase However both the colloid amp the suspension
have to phase- The dispersed amp Dispersion phase In the colloid the particles in the
dispersed phase consist of molecules held together either by primary or secondary
force The size of the particles range is 1 ndash 200nm
Gels
Colloids with a liquid as the dispersion medium can exist in two different forms
known as Sol amp Gel
A sol has the appearance amp many characteristic of a various liquid
A gel on the contrary is a semi solid amp produced from a soldering process of gelation
by the formation of fibrils or chains called Micelles
Gelation is the conversion of a sol to gel amp the temperature at which this occurs is
called Gelation Temperature
Gelation may be brought about in one of these ways
Page 5
Hydrocolloids
Lowering the temperature- It is done by reducing the thermal energy of effectively
These forces are secondary molecular forces The bond between the fibrils is weak amp they
break at slightly elevated temperature Gelation temp is 37-50degree centigrade
Liquefaction temperature- It is considerable higher than gelation temp amp this
property is known as Hysteresis This temp is between 70 -100 degree centigrade
Chemical reaction- Gelation may also induced by chemical reaction where in the
dispersed phase of soil is allowed to react with a substance to give a different type of
dispersed phase The process is not reversed by an increased temperature
Types of Hydrocolloids
REVERSIBLE HYDROCOLLOIDS - Reversible hydrocolloids are those sol can be
changed to gel but gel canrsquot be reversed back to the solution
Eg Alginate impression material
IRREVERSIBLE HYDROCOLLOIDS - Irreversible hydrocolloids are those materials
where the change from the sol to gel can be brought by lowering the temp of the sol amp the
gel can be converted back to sol condition by heating
Eg Agar ndash Agar
ALGINATE IRREVERSIBLE HYDROCOLLOID
bull The word alginate comes from Algin amp named by a chemist of Scotland It was identified
as a liner polymer with numerous carboxyl acid groups It is called as irreversible
hydrocolloids because gelation is induced by chemical reaction amp transformation is not
possible It is the most widely used dental materials
bull The principal factors responsible for the success of this type of impression materials are-
bull Easy to manipulate
bull Comfortable for the patients
bull Relatively inexpensive
Page 6
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
INTRODUCTION
Constructing a model or cast is an important step in numerous dental procedures Various
types of cast amp models can be made from gypsum products using an impression mold or
negative likeness of a dental structure An impression is a negative replica of the tissue of the
oral cavity It is used to register or reproduce the form and relation of the teeth amp surrounding
tissue
HISTORY
1558 - Celline described a wax model to make impression in his book MEMORIES
1700 - Matheus G Purenam suggested that wax models can be used in prosthetic
work
1728 ndash Pierre Fauchard described various impression materials in his book LE
CHIRURGEEN DENTISTE
1756 ndash Persian Phillip Pfaff first used plaster models prepared from sectional wax
impression of the mouth
1810 ndash R C Skinner describe about many materials and techniques in his first
American book
1928 ndash ADA developed specification No 19 for impression material
1930 ndash Zinc Oxide Eugenol a rigid impression was introduced
1930 ndash Polysulfide was first used as a commercial synthetic rubber as a copolymer of
Ethylene Chloride amp sodium
1937 ndash Agar was introduced by Sears
1949 ndash Alginate was developed during World war II
Page 2
Hydrocolloids
1950 ndash Polysulfide was used in dentistry
1955 ndash Condensation Silicone was introduced in Germany
1975 ndash Addition Silicone was introduced
1977 ndash Costell introduced Dual tray technique
1980 ndash A Visible light cure Polyether Urethane Dimethacrylate rubber impression
material was introduced
1996 ndash Blare amp Wassed considered a no of solution used to disinfecting impression
material
IDEAL REQUIREMENTS OF IMPRESSION MATERIAL
1 Pleasant taste amp odor
2 Not contain any toxic amp irritating ingredients
3 Adequate shelf life
4 Satisfactory consistency and texture
5 Easy to disinfect with out loss of accuracy
6 Compatible with die and cast material
7 Dimensional stability
8 Good elastic properties
9 Easy manipulation
10 Adequate setting characteristics
11 High degree of reproduction details
12 Adequate strength
13 Should not release any gas
14 Economical
15 Should not be technique sensitive
Classification of impression materials
Impression materials can be classified into various types based on the following
characters
Page 3
Hydrocolloids
1 Based on rigidityelasticity -
a Rigid (non-elastic)
b Elastic
2 Based on viscosity -
a Mucostatic
b Mucocompressive
c Pseudoplastic
3 Based on setting of material -
1 a chemical reaction
b Physical change of state
2 a reversible
b Irreversible
4Based on interaction with salivawater -
1 Hydrophobic
2 Hydrophilic
5Based on chemistry-
1 Impression Plaster
2 Impression compound
3 Metal oxide (zinc oxide eugenol)
4 Reversible hydrocolloid
5 Irreversible hydrocolloid
6 Poly sulfides
7 Condensation silicones
8 Addition silicones
9 Polyether
10 Visible light curing polyether urethane dimethacrylate
Page 4
Hydrocolloids
6Based on use -
1 Primary impression materials
2 Secondary impression materials
3 Duplicating materials
HYDROCOLLOIDS
Colloids are often classified as fourth state of matter the Colloidal state In a solution
of sugar in water the sugar molecules are uniformly dispersed in the water amp there is
no visible physical separation between the solute amp the solvent molecules If sugar
molecules replaced with large amp visible particles such as sand the system is
Suspension or if molecules are liquid such vegetable oil then system is Emulsion
True solution exists as a single phase However both the colloid amp the suspension
have to phase- The dispersed amp Dispersion phase In the colloid the particles in the
dispersed phase consist of molecules held together either by primary or secondary
force The size of the particles range is 1 ndash 200nm
Gels
Colloids with a liquid as the dispersion medium can exist in two different forms
known as Sol amp Gel
A sol has the appearance amp many characteristic of a various liquid
A gel on the contrary is a semi solid amp produced from a soldering process of gelation
by the formation of fibrils or chains called Micelles
Gelation is the conversion of a sol to gel amp the temperature at which this occurs is
called Gelation Temperature
Gelation may be brought about in one of these ways
Page 5
Hydrocolloids
Lowering the temperature- It is done by reducing the thermal energy of effectively
These forces are secondary molecular forces The bond between the fibrils is weak amp they
break at slightly elevated temperature Gelation temp is 37-50degree centigrade
Liquefaction temperature- It is considerable higher than gelation temp amp this
property is known as Hysteresis This temp is between 70 -100 degree centigrade
Chemical reaction- Gelation may also induced by chemical reaction where in the
dispersed phase of soil is allowed to react with a substance to give a different type of
dispersed phase The process is not reversed by an increased temperature
Types of Hydrocolloids
REVERSIBLE HYDROCOLLOIDS - Reversible hydrocolloids are those sol can be
changed to gel but gel canrsquot be reversed back to the solution
Eg Alginate impression material
IRREVERSIBLE HYDROCOLLOIDS - Irreversible hydrocolloids are those materials
where the change from the sol to gel can be brought by lowering the temp of the sol amp the
gel can be converted back to sol condition by heating
Eg Agar ndash Agar
ALGINATE IRREVERSIBLE HYDROCOLLOID
bull The word alginate comes from Algin amp named by a chemist of Scotland It was identified
as a liner polymer with numerous carboxyl acid groups It is called as irreversible
hydrocolloids because gelation is induced by chemical reaction amp transformation is not
possible It is the most widely used dental materials
bull The principal factors responsible for the success of this type of impression materials are-
bull Easy to manipulate
bull Comfortable for the patients
bull Relatively inexpensive
Page 6
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
1950 ndash Polysulfide was used in dentistry
1955 ndash Condensation Silicone was introduced in Germany
1975 ndash Addition Silicone was introduced
1977 ndash Costell introduced Dual tray technique
1980 ndash A Visible light cure Polyether Urethane Dimethacrylate rubber impression
material was introduced
1996 ndash Blare amp Wassed considered a no of solution used to disinfecting impression
material
IDEAL REQUIREMENTS OF IMPRESSION MATERIAL
1 Pleasant taste amp odor
2 Not contain any toxic amp irritating ingredients
3 Adequate shelf life
4 Satisfactory consistency and texture
5 Easy to disinfect with out loss of accuracy
6 Compatible with die and cast material
7 Dimensional stability
8 Good elastic properties
9 Easy manipulation
10 Adequate setting characteristics
11 High degree of reproduction details
12 Adequate strength
13 Should not release any gas
14 Economical
15 Should not be technique sensitive
Classification of impression materials
Impression materials can be classified into various types based on the following
characters
Page 3
Hydrocolloids
1 Based on rigidityelasticity -
a Rigid (non-elastic)
b Elastic
2 Based on viscosity -
a Mucostatic
b Mucocompressive
c Pseudoplastic
3 Based on setting of material -
1 a chemical reaction
b Physical change of state
2 a reversible
b Irreversible
4Based on interaction with salivawater -
1 Hydrophobic
2 Hydrophilic
5Based on chemistry-
1 Impression Plaster
2 Impression compound
3 Metal oxide (zinc oxide eugenol)
4 Reversible hydrocolloid
5 Irreversible hydrocolloid
6 Poly sulfides
7 Condensation silicones
8 Addition silicones
9 Polyether
10 Visible light curing polyether urethane dimethacrylate
Page 4
Hydrocolloids
6Based on use -
1 Primary impression materials
2 Secondary impression materials
3 Duplicating materials
HYDROCOLLOIDS
Colloids are often classified as fourth state of matter the Colloidal state In a solution
of sugar in water the sugar molecules are uniformly dispersed in the water amp there is
no visible physical separation between the solute amp the solvent molecules If sugar
molecules replaced with large amp visible particles such as sand the system is
Suspension or if molecules are liquid such vegetable oil then system is Emulsion
True solution exists as a single phase However both the colloid amp the suspension
have to phase- The dispersed amp Dispersion phase In the colloid the particles in the
dispersed phase consist of molecules held together either by primary or secondary
force The size of the particles range is 1 ndash 200nm
Gels
Colloids with a liquid as the dispersion medium can exist in two different forms
known as Sol amp Gel
A sol has the appearance amp many characteristic of a various liquid
A gel on the contrary is a semi solid amp produced from a soldering process of gelation
by the formation of fibrils or chains called Micelles
Gelation is the conversion of a sol to gel amp the temperature at which this occurs is
called Gelation Temperature
Gelation may be brought about in one of these ways
Page 5
Hydrocolloids
Lowering the temperature- It is done by reducing the thermal energy of effectively
These forces are secondary molecular forces The bond between the fibrils is weak amp they
break at slightly elevated temperature Gelation temp is 37-50degree centigrade
Liquefaction temperature- It is considerable higher than gelation temp amp this
property is known as Hysteresis This temp is between 70 -100 degree centigrade
Chemical reaction- Gelation may also induced by chemical reaction where in the
dispersed phase of soil is allowed to react with a substance to give a different type of
dispersed phase The process is not reversed by an increased temperature
Types of Hydrocolloids
REVERSIBLE HYDROCOLLOIDS - Reversible hydrocolloids are those sol can be
changed to gel but gel canrsquot be reversed back to the solution
Eg Alginate impression material
IRREVERSIBLE HYDROCOLLOIDS - Irreversible hydrocolloids are those materials
where the change from the sol to gel can be brought by lowering the temp of the sol amp the
gel can be converted back to sol condition by heating
Eg Agar ndash Agar
ALGINATE IRREVERSIBLE HYDROCOLLOID
bull The word alginate comes from Algin amp named by a chemist of Scotland It was identified
as a liner polymer with numerous carboxyl acid groups It is called as irreversible
hydrocolloids because gelation is induced by chemical reaction amp transformation is not
possible It is the most widely used dental materials
bull The principal factors responsible for the success of this type of impression materials are-
bull Easy to manipulate
bull Comfortable for the patients
bull Relatively inexpensive
Page 6
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
1 Based on rigidityelasticity -
a Rigid (non-elastic)
b Elastic
2 Based on viscosity -
a Mucostatic
b Mucocompressive
c Pseudoplastic
3 Based on setting of material -
1 a chemical reaction
b Physical change of state
2 a reversible
b Irreversible
4Based on interaction with salivawater -
1 Hydrophobic
2 Hydrophilic
5Based on chemistry-
1 Impression Plaster
2 Impression compound
3 Metal oxide (zinc oxide eugenol)
4 Reversible hydrocolloid
5 Irreversible hydrocolloid
6 Poly sulfides
7 Condensation silicones
8 Addition silicones
9 Polyether
10 Visible light curing polyether urethane dimethacrylate
Page 4
Hydrocolloids
6Based on use -
1 Primary impression materials
2 Secondary impression materials
3 Duplicating materials
HYDROCOLLOIDS
Colloids are often classified as fourth state of matter the Colloidal state In a solution
of sugar in water the sugar molecules are uniformly dispersed in the water amp there is
no visible physical separation between the solute amp the solvent molecules If sugar
molecules replaced with large amp visible particles such as sand the system is
Suspension or if molecules are liquid such vegetable oil then system is Emulsion
True solution exists as a single phase However both the colloid amp the suspension
have to phase- The dispersed amp Dispersion phase In the colloid the particles in the
dispersed phase consist of molecules held together either by primary or secondary
force The size of the particles range is 1 ndash 200nm
Gels
Colloids with a liquid as the dispersion medium can exist in two different forms
known as Sol amp Gel
A sol has the appearance amp many characteristic of a various liquid
A gel on the contrary is a semi solid amp produced from a soldering process of gelation
by the formation of fibrils or chains called Micelles
Gelation is the conversion of a sol to gel amp the temperature at which this occurs is
called Gelation Temperature
Gelation may be brought about in one of these ways
Page 5
Hydrocolloids
Lowering the temperature- It is done by reducing the thermal energy of effectively
These forces are secondary molecular forces The bond between the fibrils is weak amp they
break at slightly elevated temperature Gelation temp is 37-50degree centigrade
Liquefaction temperature- It is considerable higher than gelation temp amp this
property is known as Hysteresis This temp is between 70 -100 degree centigrade
Chemical reaction- Gelation may also induced by chemical reaction where in the
dispersed phase of soil is allowed to react with a substance to give a different type of
dispersed phase The process is not reversed by an increased temperature
Types of Hydrocolloids
REVERSIBLE HYDROCOLLOIDS - Reversible hydrocolloids are those sol can be
changed to gel but gel canrsquot be reversed back to the solution
Eg Alginate impression material
IRREVERSIBLE HYDROCOLLOIDS - Irreversible hydrocolloids are those materials
where the change from the sol to gel can be brought by lowering the temp of the sol amp the
gel can be converted back to sol condition by heating
Eg Agar ndash Agar
ALGINATE IRREVERSIBLE HYDROCOLLOID
bull The word alginate comes from Algin amp named by a chemist of Scotland It was identified
as a liner polymer with numerous carboxyl acid groups It is called as irreversible
hydrocolloids because gelation is induced by chemical reaction amp transformation is not
possible It is the most widely used dental materials
bull The principal factors responsible for the success of this type of impression materials are-
bull Easy to manipulate
bull Comfortable for the patients
bull Relatively inexpensive
Page 6
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
6Based on use -
1 Primary impression materials
2 Secondary impression materials
3 Duplicating materials
HYDROCOLLOIDS
Colloids are often classified as fourth state of matter the Colloidal state In a solution
of sugar in water the sugar molecules are uniformly dispersed in the water amp there is
no visible physical separation between the solute amp the solvent molecules If sugar
molecules replaced with large amp visible particles such as sand the system is
Suspension or if molecules are liquid such vegetable oil then system is Emulsion
True solution exists as a single phase However both the colloid amp the suspension
have to phase- The dispersed amp Dispersion phase In the colloid the particles in the
dispersed phase consist of molecules held together either by primary or secondary
force The size of the particles range is 1 ndash 200nm
Gels
Colloids with a liquid as the dispersion medium can exist in two different forms
known as Sol amp Gel
A sol has the appearance amp many characteristic of a various liquid
A gel on the contrary is a semi solid amp produced from a soldering process of gelation
by the formation of fibrils or chains called Micelles
Gelation is the conversion of a sol to gel amp the temperature at which this occurs is
called Gelation Temperature
Gelation may be brought about in one of these ways
Page 5
Hydrocolloids
Lowering the temperature- It is done by reducing the thermal energy of effectively
These forces are secondary molecular forces The bond between the fibrils is weak amp they
break at slightly elevated temperature Gelation temp is 37-50degree centigrade
Liquefaction temperature- It is considerable higher than gelation temp amp this
property is known as Hysteresis This temp is between 70 -100 degree centigrade
Chemical reaction- Gelation may also induced by chemical reaction where in the
dispersed phase of soil is allowed to react with a substance to give a different type of
dispersed phase The process is not reversed by an increased temperature
Types of Hydrocolloids
REVERSIBLE HYDROCOLLOIDS - Reversible hydrocolloids are those sol can be
changed to gel but gel canrsquot be reversed back to the solution
Eg Alginate impression material
IRREVERSIBLE HYDROCOLLOIDS - Irreversible hydrocolloids are those materials
where the change from the sol to gel can be brought by lowering the temp of the sol amp the
gel can be converted back to sol condition by heating
Eg Agar ndash Agar
ALGINATE IRREVERSIBLE HYDROCOLLOID
bull The word alginate comes from Algin amp named by a chemist of Scotland It was identified
as a liner polymer with numerous carboxyl acid groups It is called as irreversible
hydrocolloids because gelation is induced by chemical reaction amp transformation is not
possible It is the most widely used dental materials
bull The principal factors responsible for the success of this type of impression materials are-
bull Easy to manipulate
bull Comfortable for the patients
bull Relatively inexpensive
Page 6
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Lowering the temperature- It is done by reducing the thermal energy of effectively
These forces are secondary molecular forces The bond between the fibrils is weak amp they
break at slightly elevated temperature Gelation temp is 37-50degree centigrade
Liquefaction temperature- It is considerable higher than gelation temp amp this
property is known as Hysteresis This temp is between 70 -100 degree centigrade
Chemical reaction- Gelation may also induced by chemical reaction where in the
dispersed phase of soil is allowed to react with a substance to give a different type of
dispersed phase The process is not reversed by an increased temperature
Types of Hydrocolloids
REVERSIBLE HYDROCOLLOIDS - Reversible hydrocolloids are those sol can be
changed to gel but gel canrsquot be reversed back to the solution
Eg Alginate impression material
IRREVERSIBLE HYDROCOLLOIDS - Irreversible hydrocolloids are those materials
where the change from the sol to gel can be brought by lowering the temp of the sol amp the
gel can be converted back to sol condition by heating
Eg Agar ndash Agar
ALGINATE IRREVERSIBLE HYDROCOLLOID
bull The word alginate comes from Algin amp named by a chemist of Scotland It was identified
as a liner polymer with numerous carboxyl acid groups It is called as irreversible
hydrocolloids because gelation is induced by chemical reaction amp transformation is not
possible It is the most widely used dental materials
bull The principal factors responsible for the success of this type of impression materials are-
bull Easy to manipulate
bull Comfortable for the patients
bull Relatively inexpensive
Page 6
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Composition
Potassium or Sodium Alginate- 15 It is the chief active ingredient
Calcium Sulfate- 16 It is used as a reactor
Zinc Oxide amp Diatomaceous Earth- Zinc oxide ndash 4 amp Diatomaceous
earth 60
Potassium Titanium Fluoride - 3
Sodium Phosphate - 2
Gelation Process
The typical sol-gel reaction is a soluble alginate with calcium sulfate amp the formation
of an insoluble calcium alginate gel Calcium sulfate reacts rapidly to produce the in soluble
Ca alginate from the potassium or sodium alginate in a aqueous solution The production of
calcium alginate is rapid that it does not allow sufficient working time Thus third water
soluble salt such as a trisodium phosphate is added to the solution to prolong the working
time Thus the reaction between the calcium sulfate amp the soluble alginate is prevented as
long as there is uncreated trisodium phosphate
2Na3Po4 +n CaSo4 rarrnKaSo4 +Ca alginate
When the supply of the trisodium phosphate is exhausted the ca ions begin to react
with the potassium alginate to produce calcium alginate
Kzn Alg +n CaSo4rarr n KaSo4 + Cal alginate
The added salt is retarder
Controlling of gelation time-
An increase in the temp of water used for mixing shorten the working amp setting
time
The proportion of powder amp water also effect the setting time
PROPERTIES
Page 7
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Alginate is of two types
Type I - Fast setting
Type II - Normal setting
According to ADA Specification No 18 properties are-
1 Mixing time- Creamy consistency come in 45 ndash 60 sec
2 Working time-
- Fast setting material - 12 ndash 2 min
- Normal set material - 2 ndash 45 min
3 Setting or gelation time-
- Optimum gelation time - 3-4 min at room temp
- For fast setting material - 1 -2 min
- For normal setting material - 2 ndash 45min
4 Permanent deformation - ADA specification requires 97 recovery amp where 3 is
permanent deformation Alginate has 988 recovery amp 15 permanent deformation
5 Flexibility- ADA specification permits10 ndash 20 at the stress of 1000gm cm2 Hard
material has value of 5-8
6 Strength-
The compressive strength- 5000-8000gmcm2
Tear strength - 350-700gmcm2
7Viscoelasticity
Usually an alginate impression material does not adhere to the oral tissue as strongly as some
of the non aqueous elastomers so it is easy to remove the alginate impression rapidly
8Accuracy Most
alginate is not capable of reproducing the finer details that are absorbed in impression with
Agar amp other elastomeric impression material
9 Dimensional effects - The gel may loss water by evaporation from its surface amp it shrinks
- If the gel placed in the water it absorbed water amp gel swells
- Thermal change also contributes to dimensional change The alginate shrinks slightly due to
difference in temp between mouth temp(30c) amp room temp(23c)
10 Biocompatibility -
Page 8
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
No chemical or allergic reaction associated with alginate
11 Shelf life -
Alginate impressions have shorter shelf life Strong temp amp moisture contamination are two
factor which effect the self life of alginate
MANIPULATION OF ALGINATE
PREPARING THE MIX
A measured powder is shifted into pre measured water that has been placed in a clean
rubber bowl Care should be taken to avoid whipping air into the mix A vigorous figure 8
motion is best with the mix being swiped or stropped against the side of the mixing rubber
bowl with intermittent rotation ie180 degree of the spatula to press out air bubbles The
mixing time is 45sec -1 min
A variety of mechanical devices are also available for mixing impression material
There benefits are convenience speed amp elimination of human error
MAKING THE IMPRESSION
Before setting the impression the material should have developed sufficient body so
that it doesnrsquot flow out of the try A perforated tray is generally used If plastic try or metal
rim lock try is selected a thin layer of adhesive should be applied amp allowed to dry before
mixing and loading the try The thickness of the alginate between the tray amp tissue should
be at least 3mm
Recent developments
1 Dust-free alginates
Inhaling fine airborne particles from alginate impression material can cause
silicosis and pulmonary hypersensitivity
Page 9
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Dustless alginates were introduced which give off or no dust particles so avoiding
dust inhalation This can be achieved by coating the material with glycerine or
glycol This causes the powder to become more denser than in uncoated state
2 Siliconised alginates
It is a two component system in the form of two pastes one containing the
alginate sol and the second containing the calcium reactor
The components incorporate a silicone polymer component which makes material
tear resistant compared to unmodified alginates However the dimensional
stability is reported to be poor
3 Low dust alginate impression material
Introduced by Schunichi Nobutakwatanate in 1997
This composition comprises an alginate a gelation regulator and a filler as major
components which further comprises sepiolite and a tetraflouroethylene resin
having a true specific gravity of from 2-3
The material generates less dust has a mean particle size of 1-40microns
4 Antiseptic alginate impression material
Introduced by Tameyuki Yamamoto Maso Abinu patented in 1990
An antiseptic containing alginate impression material contains 001 to 7 parts by weight
of an antiseptic such as glutaraldehyde and chlohexidine gluconate per 100 parts by
weight of a cured product of an alginate impression material
The antiseptic may be encapsulated in a microcapsule or clathrated in a cyclodextrin
5 CAVEX Color change
Page 10
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
The alginate impression material with color indications avoiding confusion about setting
time
Color changes are visualizing the major decision points in impression making
end of mixing time
end of setting time ( tray can be removed from mouth)
it indicates two color changes
Violet to pink indicates the end of mixing time
Pink to white indicates end of setting time
Other advantages of this material are
-improved dimensional stability (upto 5 days)
Good tear and deformation resistance
Dust free
Smooth surface optimum gypsum compatibility
ADVANTAGES amp DISADVANTAGES
Advantages
1 Easy to mix and manipulate
2 Minimum requirement of equipment
3 Accuracy (if properly handled)
4 Low cost
5 Comfortable to the patient
6 Hygienic (as fresh material is used for each impression)
Disadvantages
1 Cannot be electroplated
2 Distortion occurs easily
3 Poor dimensional stability (poured within 15 min)
4 Poor tear strength
AGAR ndash REVERSIBLE HYDROCOLLOIDS
Page 11
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
When agar hydrocolloids heated they liquefy or go into the sol state amp on cooling they
return to the gel state Because this process can be repeated a gel of this type is
described as Reversible hydrocolloid The preparation of the agar hydrocolloids for
clinical use requires care full control amp yields accurate impression It has been largely
replaced by alginate hydrocolloids amp rubber impression materials
COMPOSITION
Borates - 02 ndash 05 Its works as retarder
water - 855 It is reaction medium
Agar- 13-17 It is main active constituent of reversible
hydrocolloid impression material
Sulfates - 1 ndash 2 Accelerators
Fillers - 05 -1 Diatomaceous earth silica wax rubber etc
used as filler
Bactericide - Thymol amp glycerin are used
Color amp flavor
GELATION PROCESS
The setting of the reversible hydrocolloid is called gelation The reaction can be
expressed a sol- gel reaction The physical changes from to gel does not return to the sol at
the same temp at which it solidified The gel must be heated at the liquefaction temp ie 70 -
100c to form sol Sol transform into a gel at 37-50c the exact gelation temp depended on
several factor including the molecular wt the purity of the material amp the ratio of agar to
other ingredients
The gelation is critical If the temp is high the heat from sol may injury to the soft
tissue or if the surface of the sol transforms to a gel as soon as the sol may injure the oral
tissues a high surface stress may develop If the gelation temp is too far below the oral
temp it will be difficulty or even impossible to chill the material sufficiently to obtain a firm
Page 12
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
gel to adjacent to the oral tissue The temp lag between the gelation amp liquefaction temp of
the gel makes it possible to use as a dental impression material
PROPERTIES
Gelation temperature -
After tempering the sol should be homogenous and should set to a gel between 37
ndash 500c when cooled
Viscoelastic properties -
It demonstrates the necessity of deforming the impression rapidly when it removed
from the mouth which reduces the amount of the permanent deformation The elastic
recovery of the hydrocolloid is never complete amp it does not return entirely to its original
dimension after deformation The amount of permanent deformation in clinical is
negligible provided that -
a) The material has adequately gelled
b) The impression has been removed rapidly
c) The under cuts present in the cavity preparation are minimal
Permanent deformation -
The ADA specification requires that the permanent deformation should be less than
15 after the material compressed 10 for 30sec This impression material readily meets
this requirement with the value of about 1
Distortion during gelation -
If the material is held rigidly to the tray then the impression material shrinks
toward the center of its mass Rapid cooling may cause a concentration of stress near the
tray where gelation first takes place
Flexibility -
The ADA specification requirement for flexibility allows a range of 4-14 amp most
agar materials meet this requirement
Strength -
The compressive strength of Agar impression material is 8000gmcm2
The tear strength of Agar is 7000gmcm2
Page 13
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Flow -
Agar is sufficiently fluid to allow detailed reproduction of hard amp soft tissue
Compatibility with gypsum -
Not all the agar impression materials are equally compatible with all gypsum
products The impression should be washed of saliva amp any trace of blood which
retarded the setting of gypsum
Manipulation of agar impression
The use of agar hydrocolloid involves special equipment called conditioning
unit for agar The hydrocolloid is usually supplied In two forms syringe and tray
materials The only difference between the materials is the color and the greater fluidity of
the syringe material
Impression tray
It is rim locked trays with water circulating device This types of trays should
allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth
After the tray has been properly positioned water is circulated at 13 degrees through the
tray until gelation occurs
Preparation of material
Proper equipment of liquefying and storing the agar impression material is essential
At first reverse the hydrocolloid gel to the sol stage Boiling water is a convenient
way of liquefying the material The material must be held at this temperature for a
minimum of 10mins Propylene glycol can be added to the water to obtain 100 degrees
After it has been liquefied the material must be stored in the tray The material can be
stored for several days Usually there are three compartments in the conditioning unit
making it possible to liquefy store and temper the material
Page 14
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Conditioning and tempering
Because 55 degree is the maximum tolerable temperature the storage temperature
of 65 degrees would be too hot for the oral tissues especially given the bulk of the tray
material Therefore the material that is used to fill the tray must be cooled and tempered
Eliminating the effect of imbibition is the purpose of placing the gauze pad over the
tempering tray materials When the tray material is placed into the tempering bath the
gauze is removed and the contaminating surface layer of material clings to the gauze and is
removed as well
Making the impression
The syringe material is taken directly from the storage compartment and applied to
the prepared cavities It is first applied to the base of the preparation and then the reminder
of the tooth is covered By the time the cavity preparation and adjoining teeth have been
covered the tray material has been properly tempered and is now ready to be placed
immediately in the mouth to form the bulk of impression Gelation is accelerated by
circulating cool water approximately 18 ndash 21 degrees through the tray for 3 ndash 5 minutes
Disinfection of hydrocolloid impression
As the hydrocolloid impression material must be poured within short time after
removal from the mouth The disinfection procedure should be relatively rapid to prevent
the dimensional change Most manufacturers recommended a specific disinfectant The
agent may be iodophor bleach or gluteraldehyde The distortion is minimal if the
recommended immersion time is followed and if the impression is poured properly The
irreversible hydrocolloid may be disinfected by 10 minute immersion inor spraying with
the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional
change The current protocol for disinfecting hydrocolloid impression is to use household
bleach iodophor and synthetic phenols as disinfectants After the immersion it is
thoroughly rinsed The disinfectant is sprayed liberally o the exposed surface The
immersion should not be submerged or soaked in the disinfectant solution
Page 15
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Recent techniques
Laminate technique
A recent modification of the conventional procedure is the combined agar alginate
technique The hydrocolloid in the tray is replaced with a mix of chilled alginate
that bonds with the agar expressed from a syringe
The alginate gels by a chemical reaction whereas the agar gels by means of
contact with cool alginate rather than with the water circulating through the tray
Since the agar not the alginate is in contact with the prepared teeth maximum
detail is reproduced
Advantages
syringe agar records tissues more accurately
Water cooled tray is not required
Sets faster
Disadvantages
Agar ndash alginate bond failure can occur
Viscous alginate may displace agar
Technique sensitive
Wet field technique
This is a recent technique
The oral tissues are flooded with warm water The syringe material is then
injected in to the surface to be recorded
Before syringe material gels tray material is seated
Page 16
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
The hydraulic pressure of the viscous tray material forces the fluid syringe
material down in to the areas to be recorded
The motion displaces the syringe material as well as blood and debris through out
the sulcus
ADVANTAGES amp DISADVANTAGES
Advantages
1 Hydrophilic Impression material
2 Good elastic properties Good recovery from distortion
3 Can be re-used as a duplicating material
4 Long working time and low material cost
5 No mixing technique
6 High accuracy and fine detail recording
Disadvantages
1 Only one model can be used
2 Extensive and expensive equipment required
3 It can not be electroplated
4 Impossible to sterilize for reuse
5 Low dimensional stability amp tear resistance
TYPES OF FAILURES
1 Distortion- due to
Delayed pouring of impression
Movement of tray during setting
Removal from mouth too early
2 Grainy impression- due to
Inadequate mixing
Page 17
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
Hydrocolloids
Prolonged mixing
Less water in mix
3 Tearing - due to
Inadequate bulk
Moisture contamination
Removal from mouth too early
Prolonged mixing
4 Bubbles- due to
Early gelation preventing flow
Air incorporated during mixing
5 Irregular voids- due to
Excess moisture ampdebris on
REFERENCES
1 Anusavice ldquoSkinners science of dental materialsrdquo Tenth Edition
2 Dental materials and their selection-willian jorsquo brien
3 Restorative dental materials-craig
4 Removable prosthodintics- stewart
Page 18
