Alginate is an irreversible hydrocolloid impression material made from seaweed. It consists of gelatin particles suspended in water. When the powder and water are mixed, it forms a sol that transitions to a gel through a chemical reaction where soluble alginate reacts with calcium sulfate to form insoluble calcium alginate. This reaction is delayed by adding trisodium phosphate. Alginate has adequate working and setting times and is used for impressions where there are undercuts or excess saliva. It provides detailed impressions but is prone to dimensional changes with moisture changes.

1. Alginate
IRREVERSIBLE HYDROCOLLOID
FIRST BDS LECTURE

2. IMPRESSION MATERIAL
DEFINITION
A negative likeness or copy in reverse of the
surface or object.
An imprint of the teeth and adjacent structures
for use in dentistry

3. Non-elastic
Elastic
Aqueous
Hydrocolloids
Non-aqueous
Elastomers
Polysulfide
Silicones
Condensati
on
Addition
Agar
(reversible)
ALGINATE
(IRREVERSIBLE)
Plaster
Compound
Waxes
ZnO - Eugenol
Polyether
O‟Brien, Dental Materials & their Selection

4. ALGINATE
🞭 Alginate is classified as irreversible hydrocolloid.
🞭 Hydrocolloid because it consists of particles of a gelatinous
(colloidal) state in water (hydro) and irreversible because once
it has jelled it cannot be returned to a liquid solution.
They consist of gelatin particles suspended in water (Lyosol). Since
water is the dispersion medium it is known as hydrocolloid.

5. IRREVERSIBLE HYDROCOLLOID—ALGINATE
The word alginate comes from ‘alginic acid’ (anhydro-β-d-
mannuronic acid) which is a mucous extract yielded by
species of brown seaweed (Phaeophyceae)
Type I — Fast setting.
Type II — Normal setting

6. 🞭 The Word Alginate comes from the term
“ALGIN”.
🞭 It is a peculiar mucous extract yielded by
certain brown seaweed ( algae ) .
🞭 The term „ALGIN‟ was coined by chemist
from scotland @ the end of 19th century.

7. TYPES -
Type I – Fast setting.
Type II – Normal settin g.

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

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

11. COMPOSITION :
🞭 Potassium alginate(15%)
🞭 Comes from seaweed; is used as a thickening agent.
🞭 Calcium sulfate(16%)
🞭 Reacts with the potassium alginate to form the gel.
🞭 Trisodium phosphate (2%)
🞭 Added to slow down the reaction time for mixing.

12. 🞭 Diatomaceous earth( 60 %)
🞭 A filler that adds bulk to the material.
🞭 Zinc oxide (4%)
🞭 Adds bulk to the material.
🞭 Potassium titanium fluoride(3%)
🞭 Added so as not to interfere with the setting and
surface strength.

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

14. Setting reaction
When alginate powder is mixed with water a sol is formed which later sets to a gel by a chemical
reaction.
The final gel, i.e. insoluble calcium alginate is produced when soluble sodium alginate reacts
with calcium sulfate (reactor). However, this reaction proceeds too fast. There is not enough
working time. So the reaction is delayed by addition of a retarder (trisodium phosphate) by the
manufacturer.
Initially the sodium phosphate reacts with the calcium sulfate to provide adequate working
time.
Reaction 1 2Na3 PO4 + 3CaSO4 Ca3 (PO4 ) 2 + 3Na2 SO4

15. Next after the sodium phosphate is used up, the remaining calcium sulfate reacts with sodium
alginate to form insoluble calcium alginate which forms a gel with water.
Reaction 2 Sodium alginate + CaSO4 + H2 O Ca alginate + Na2 SO4
 (Powder) (Gel)

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

17. Setting reaction
 Chemical reaction named gelation
Calcium sulfate  Sodium phosphate
 Calcium phosphate  + Sodium sulfate   (1)
Calcium sulfate  Sodium alginate
 Insoluble calcium alginate  (2)
 The speed of the reaction is affected by;
2. Temperature
4. powder particle size
1. L/P ratio
3. Mixing rate
5. Additives

18. In a nutshell , when powder is mixed with water to obtain a paste
,two main reaction occur during setting :
1) 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3 Na2SO4
• First , sodium phosphate reacts with the calcium sulphate to provide adequate
working time.
2) sodium alginate + -------- > Ca alginate +
CaSO4 + H2O
Na2SO4
(Powder )
(Gel)
• Second ,after the sodium phosphate has reacted, the remaining calcium
sulphate reacts with sodium alginate to form an insoluble calcium alginate which
forms a gel with water.

19. WATER-TO-POWDER RATIO
🞭 An adult mandibular impression generally requires two
scoops of powder and two measures of water.
🞭 An adult maxillary impression generally requires three
scoops of powder and three measures of water.

20. Manipulation of Alginate
🞭 Fluff or shake the powder to distribute the powder particles
evenly
🞭 Use the specific measuring devices (water & powder) provided
by the manufacturer for mixing
🞭 Follow the manufacturer‟s direction regarding the ratio of water
to powder.
🞭 Use exact measurements

21. 🞭 After the water and powder have been measured, place the water in
a clean dry bowl.
Sift the powder into the water.
Adding the powder to the water ensures the powder particles are wet
evenly.
If mixed in reverse (the water is added to the powder) the chemical
reaction will start early with some particles setting faster than
others.

22. 🞭 Mix the alginate for the specific amount of time and using a stiff
spatula “swipe” the alginate mass against the sides of the bowl to
avoid entrapment of air in the mix.
figure of
“8”
motion
1. Remove most of the
air bubbles.
2. Wipe dissolved
algin from the surface
of the yet undissolved
algin thereby
promoting complete
dissolution

23. Alginate
Mixing Time
For fast set alginate—45 seconds.
 For normal set alginate—60 seconds.
Working Time
Fast set alginate—1¼ minutes.
Normal set alginate—2 minutes.

25. SETTING TIME OF ALGINATE
Since alginate reacts or gels chemically, temperature is a
major factor in the setting time.
The colder the temperature of the water the longer it takes
to set; conversely, the higher the water temperature the
faster it sets.
Ideal water temperature is 68 degrees or room temperature.

26. LOADING OF TRAY
🞭 A perforated tray is used so that the
material is forced out slightly through
the holes in the tray during loading,
thereby locking itself mechanically
into the tray.
🞭 The surface of the alginate in the tray
may be smoothened out by
moistening the finger with water and
running it over the surface of the
alginate.

27. SEATING THE TRAY
🞭 Before making the upper impression, saliva should be wiped off
from the palatal region.
🞭 In order to prevent the material from flowing along the throat and
causing nausea to the patient, the posterior portion may be seated
first and then the anterior portion seated properly.
🞭 Since the material sets from tissues towards periphery,any
movement during gelation may result in distortion.
🞭 So once the tray is seated ,it must be held in place firmly without
any movemnets.

29. REMOVAL OF THE IMPRESSION
🞭 An alginate impression when set develops a very effective
peripheral seal.
🞭 This seal should be freed by running the finger around the
periphery.
🞭 The impression must be removed suddenly
with a jerk.or a snap
🞭 After removal from the mouth,
- washed with water to remove saliva.
-cast should be poured as soon as possible.

30. ALGINATE SETTINGS
🞭 Normal set alginate
Working time of 2 minutes and a setting time of up to 41/2
minutes after mixing.
🞭 Fast set alginate
Working time of 11/4 minutes and a setting time of 1 to
2 minutes.
Working time
The time allowed for mixing the alginate, loading the tray, and
positioning the tray in the patient's mouth.
Setting time
🞭 The time required for the chemical action to be completed.

31. MAKING AN ALGINATE IMPRESSION
🞭 Explain the procedure to the patient:
🞭 The material will feel cold, there is no unpleasant taste,
and the material will set quickly.
🞭 Breathe deeply through your nose to help you relax
and be more comfortable.
🞭 Use hand signals to communicate any
discomfort.

32. AN ACCEPTABLE ALGINATE IMPRESSION
🞭 There is a complete "peripheral roll," which includes
all of the vestibular areas.
🞭 The tray is not "overseated," which would result in
exposure of areas of the impression tray.
🞭 The impression is free from tears or voids.
🞭 There is sharp anatomic detail of all teeth and soft
tissues.
🞭 The retromolar area, lingual frenum, tongue
space, and mylohyoid ridge are reproduced in the
mandibular impression.
🞭 The hard palate and tuberosities are recorded in the
maxillary impression.

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

35. RECENT ADVANCES IN ALGINATES
Dust free alginates-
De-dusting agent (glycerin or glycol)
Siliconized alginates-
silicon polymers are added to strengthen the material
Alginates containing disinfectants-
eg quaternary ammonium salts or chlorohexamine are added
to alginate powder.
Hard and soft set alginates-
by adjusting the amount of fillers

36. PROPERTIES
1)FLEXIBILITY
It is about 14% at a stress of 1000g/cm2.
Lower w/p ratio results in lower flexibility.
2)ELASTICITY AND ELASTIC RECOVERY
alginate are highly elastic but less when
compared to the agar.
97.3% elastic recovery occurs.
Permanent deformation is less if the impression is
removed from the mouth quickly.

37. 3) STRENGTH
The compressive strengths of alginate gels
range from 5000 to 8000 g/cm2.
The tear strengths vary from 350 to 700 g/cm2.
Both of these properties are time dependent
with higher values being obtained if the time
of removal is delayed.

38. 4) DIMENSIONAL STABILITY :
Alginate impression loses water by
evaporation and shrinks on standing in air.
If it is placed in water it absorbs water and
swells.
Therefore ,cast should be poured immediately
after making the impression.

39. 5)SHELF LIFE
Alginate impression material have a short shelf
life.
They deteriorate rapidly when stored at
elevated temperatures in a humid
environment.
Therefore ,it is better not to stock more than 1
year supply .

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

41. DISADVANTAGES
1) Can‟t be corrected.
2) Cannot be used alone for compressing the
tissues.
3) It is not accurate enough for crown and
bridge impressions.

43. Definition
A dental impression is a negative record of the tissues of the mouth
It is used to reproduce the form of teeth and surrounding tissues

44. PROPERTIES OF IMPRESSION MATERIAL
 Nontoxic and Nonirritant
 Acceptable to the patient – pleasant taste, odor,
consistency & color.
Should record accurate surface detail, elastic properties
with no deformation & dimensionally stable
Adequate shelf life for storage and distribution

45. PROPERTIES OF IMPRESSION MATERIAL
Economical
Handling properties- sufficient working time, set quickly in
mouth & minimum equipment.
Have adequate strength so that it will not break or tear
while removing from mouth.
Should be compatible with die and cast material .

46. CLASSIFICATION OF IMPRESSION MATERIAL
A. According to mode of setting and elasticity.
B. According to tissue displacement during impression .
C. According to uses in dentistry.

47. According to mode of setting and elasticity
MODE OF
….SETTING
RIGID ELASTIC
Set by chemical reaction
Irreversible / Thermoset
Impression plaster Zinc oxide
eugenol
Alginate hydrocolloid
Nonaqueous elastomers -e.g.
polysulfide, silicone
Set by temperature change
Reversible/Thermostatic
Compound, Waxes Agar hydrocolloid

48. ACCORDING TO TISSUE DISPLACEMENT
1. Mucostatic
2. Mucocompressive (Mucodisplacive)

49. Mucostatic materials produce minimal displacement of the tissue during
impression, e.g. plaster, zinc oxide eugenol, low viscosity alginates, low
viscosity elastomeric materials, etc.
Mucocompressive materials are more viscous and displace the tissues
while recording them, e.g. compound, high viscosity alginates, high
viscosity elastomers, etc

50. ACCORDING TO THEIR USES IN DENTISTRY
1. Impression materials used for complete denture
prosthesis.
2. Impression materials used for dentulous mouths .

51. Impression materials used for complete denture
prosthesis Impression plaster, impression compound and
impression paste set to a hard rigid mass, and hence cannot
be removed from undercuts without the impression being
fractured or distorted. Therefore these materials are best
suited for edentulous mouth.

52. Impression materials used for dentulous mouths
On the other hand alginates and rubber base impressions
are sufficiently elastic to be withdrawn from undercut areas.
Such elastic impression materials are suitable for
impressions for fabrication of removable and fixed partial
denture prostheses, where the impressions of the ridge and
teeth are required.

53. RIGID IMPRESSION MATERIALS
1. IMPRESSION COMPOUND- rigid, reversible impression material which sets by
physical change. On applying heat, it softens and on cooling it hardens. It is
mainly used for making impressions of edentulous ridges.
CLASSIFICATION
Type I - Impression compound
Type II - Tray compound

54. APPLICATIONS
1. For making a preliminary impression in an edentulous
mouth (mouth without teeth).
2. For impressions of full crown preparations where gingival
tissues must be displaced.
3. Peripheral tracing or border molding.
4. To check undercuts in inlay preparation.
5. To make a special tray

55. REQUIREMENTS OF IMPRESSION COMPOUND
1. Harden at or little above mouth temperature.
2. Be plastic at a temperature not injurious or harmful to oral tissues.
3. Not contain irritating or toxic ingredients.
4. Harden uniformly when cooled without distortion.
5. Have a consistency when softened which will allow it to reproduce
fine details.
6. Be cohesive but not adhesive.

56. ZINC OXIDE EUGENOL IMPRESSION PASTE
Zinc oxide and eugenol based products are widely used in dentistry.
ADA specification No. 16
 Available as
 In paste form in two tubes
1.Base paste (white in color)
2.Accelerator or reactor or catalyst paste (red in color)
CLASSIFICATION
Type I or Hard
Type II or Soft

57. COMPOSITION
Base paste:
Zinc Oxide 87%
Vegetable or mineral oil:13%
Accelerator paste:
Oil of cloves or eugenol: 12%
Gum or polymerized rosin 50%
Filler 20%
Lanolin 3%
Calcium chloride and color 5%

58. SETTING REACTION
SETTING REACTION The setting reaction is a typical acid-base reaction to form a
chelate. This reaction is also known as chelation and the product is called zinc
eugenolate.
1. ZnO + H2O Zn(OH)2
2. Zn(OH)2 + 2HE ZnE2 + 2H2O

59. ELASTIC IMPRESSION MATERIALS
Two systems are used
1. Hydrocolloids
2. Elastomeric materials
Hydrocolloids :
They consist of gelatin particles suspended in water
(Lyosol). Since water is the dispersion medium it is
known as hydrocolloid.

60. Based on the mode of gelation
Reversible hydrocolloids They are called reversible because their physical state can be reversed.
This makes them reusable.
Irreversible hydrocolloids Once these set, it is usually permanent, and so are known as
irreversible.
Reversible hydrocolloids—Agar
 Classification BASED ON VISCOSITY
Type 1 — Heavy bodied (for use as tray material)
Type 2 — Medium bodied (for use as tray or syringe material)
Type 3 — Light bodied (for syringe use only)
Type 3A — Light bodied for agar-alginate combination technique

61. AGAR
SUPPLIED AS
Gel in collapsible tubes (for impressions)
As cartridges or gel sticks (syringe material)
In bulk containers (for duplication)

62. AGAR
INGREDIENT FUNCTIONS
AGAR 13-17% Basic constituent
BORATES 0.2-0.5% Improves strength and retards setting of
plaster
Potassium sulphate 1–2% It counters retarding effect of borates
Wax, hard 0.5–1% It acts as a filler
Thixotropic materials 0.3–0.5% It acts as plasticizer
Alkylbenzoates 0.1 % It acts as preservative.
Coloring and flavoring agents Traces For patient comfort and acceptance
Water B alance (around 84%) It acts as the dispersion medium

63. Gelation or setting of agar
Agar changes from the sol to the gel state (and vice versa) by a
physical process.
As the agar sol cools the dispersed phase groups to form fibrils
called micelles.
The fibrils branch and intermesh together to form a brush-heap
structure.

65. AGAR
The fibrils form weak covalent bonds with each other which break easily at
higher temperatures resulting in gel turning to sol.
The process of converting gel to sol is known as liquefaction which occurs at a
temperature between 70 and 100 °C.
On cooling agar reverses to the gel state and the process is called gelation.
Gelation occurs at or near mouth temperature which is necessary to avoid
injury to oral tissue.

66. PROPERTIES OF AGAR HYDROCOLLOIDS
Gelation, liquefaction and hysteresis
Syneresis and imbibition (dimensional stability)
Flexibility
Elasticity and elastic recovery
Gel strength including tear and compressive strengths

67. ADVANTAGES
1. Accurate dies can be prepared, if the material is properly
handled.
2. Good elastic properties help reproduce most undercut
areas.
3. It has good recovery from distortion.
4. Hydrophilic, moist mouth not a problem. It also gives a
good model surface.

68. Disadvantages of agar
1. Does not flow well when compared to newly available
materials.
2. It cannot be electroplated.
3. During insertion or gelation the patient may experience
thermal discomfort.
4. Tears relatively easily. Greater gingival retraction is
required for providing adequate thickness of the material.

69. IRREVERSIBLE HYDROCOLLOID—ALGINATE
The word alginate comes from ‘alginic acid’ (anhydro-β-d-
mannuronic acid) which is a mucous extract yielded by
species of brown seaweed (Phaeophyceae)
Type I — Fast setting.
Type II — Normal setting

70. ALGINATE
As a two paste system One contains the alginate sol, while the second contains
the calcium reactor.
One product is supplied in low density for use with syringe
Dust free alginates Concern over the inhalation of alginate dust have prompted
manufacturers to introduce ‘dust free alginates’
Chromatic alginates Alginates which change color on setting
Commercial names: Zelgan (DPI), Jeltrate (Dentsply), Hydrogum (Zhermack),
etc.`

71. APPLICATIONS
1. It is used for impression making – When there are undercuts. – In
mouths with excessive flow of saliva. – For partial dentures with
clasps.
2. For making preliminary impressions for complete dentures.
3. For impressions to make study models and working casts. 4. For
duplicating models.

72. COMPOSITION -AGAR
Ingredients % wt Function
Sodium or potassium alginate Dissolves in water and reacts with
calcium ions
Calcium sulfate (reactor) Reacts with potassium alginate and
forms insoluble calcium alginate
Zinc oxide Acts as a filler
Potassium titanium fluoride Gypsum hardener
Diatomaceous earth Acts as a filler
Sodium phosphate (retarder) Reacts preferentially with calcium sulfate
Coloring and flavoring agent e.g. wintergreen, peppermint

73. Setting reaction
When alginate powder is mixed with water a sol is formed which later sets to a gel by a chemical
reaction.
The final gel, i.e. insoluble calcium alginate is produced when soluble sodium alginate reacts
with calcium sulfate (reactor). However, this reaction proceeds too fast. There is not enough
working time. So the reaction is delayed by addition of a retarder (trisodium phosphate) by the
manufacturer.
Initially the sodium phosphate reacts with the calcium sulfate to provide adequate working
time.
Reaction 1 2Na3 PO4 + 3CaSO4 Ca3 (PO4 ) 2 + 3Na2 SO4

74. Next after the sodium phosphate is used up, the remaining calcium sulfate reacts with sodium
alginate to form insoluble calcium alginate which forms a gel with water.
Reaction 2 Sodium alginate + CaSO4 + H2 O Ca alginate + Na2 SO4
 (Powder) (Gel)

75. PROPERTIES OF ALGINATE HYDROCOLLOID
1. Pleasant Taste and Odor
2. Flexibility
3. Elasticity and Elastic Recovery
4. Reproduction of Tissue Detail
5. Strength -Compressive strengths •
Ranges from 0.5 to 0.9
MPa.
6. Syneresis and Imbibition

76. Manipulation of Alginate
Fluff or aerate the powder by inverting the can several times. This ensures
uniform distribution of the filler before mixing.
The proper W/P ratio as specified by the manufacturer should be used (usually
one measure water with two level scoops of powder.
The water is taken first. The powder is sprinkled in to the water in the rubber
mixing bowl and the lid of the metal can is replaced immediately.
The mixing is started with a stirring motion to wet the powder with water.

77. Once the powder has been moistened, rapid spatulation by swiping or stropping
against the side of the bowl is done.
A vigorous figure-eight motion can also be used.
 This helps
1. Remove most of the air bubbles.
2. Wipe dissolved algin from the surface of the yet undissolved algin thereby
promoting complete dissolution.

78. Alginate
Mixing Time
For fast set alginate—45 seconds.
 For normal set alginate—60 seconds.
Working Time
Fast set alginate—1¼ minutes.
Normal set alginate—2 minutes.

79. ELASTOMERIC IMPRESSION MATERIALS
Elastomers are essentially polymers with elastic or rubber-like properties.
They contain large molecules with weak interaction between them. They are
tied together at certain points to form a three-dimensional network.
On stretching, the chains uncoil, and on removal of the stress they snap back
to their relaxed entangled state
They are amorphous polymers existing above their glass transition
temperature, so that considerable segmental motion is possible.

80. Types of polymerization reactions:
In elastomers 3 types of polymerization reactions are seen:
1. Addition polymerization
2. Condensation polymerization
3. Ring opening polymerization

81. Classification :
According to their chemistry,
1. Polysulfide
2. Condensation polymerizing silicones
3. Addition polymerizing silicones
4. Polyether

82. According to viscosity (ISO 4823:2015) they are further divided
based on consistencies determined immediately after completion of
mixing,
Type 0—Putty consistency (very heavy)
Type 1—Heavy-bodied consistency (tray consistency)
Type 2—Medium-bodied consistency (regular bodied)
Type 3—Light-bodied (syringe consistency)

83. According to wetting ability or contact angle,
1. Hydrophilic, if their contact angle is from 80 to 105°.
2. Hydrophobic, if their contact angle is from 40 to 70°

84. Uses:
1. In fixed partial dentures for impressions of prepared teeth.
2. Impressions of dentulous mouths for removable partial dentures.
3. Impressions of edentulous mouths for complete dentures.
4. Polyether is used for border molding of edentulous custom trays.
5. For bite registration.
6. Silicone duplicating material is used for making refractory casts
during cast partial denture construction

85. Supplied as :
1. Two component (base and catalyst) systems,
2. Cartridges—light and regular body material are also supplied in
cartridges to be used with static mixing tips and
dispensers
3.Putty consistency is supplied in jars

86. THANK YOU