This document discusses impression materials, beginning with definitions of key terms like impression, primary impression, secondary impression, cast, die, and tray. It covers the indications, requirements, and classification of impression materials. Specific materials discussed include impression compound, zinc oxide eugenol, and hydrocolloids. Hydrocolloids exist in sol and gel states and undergo an irreversible chemical reaction, examples being alginate which is used for making primary impressions, and agar.
3. 3
Definitions
Impression: negative replica (reproduction) of the oral tissues.
Primary impression
* Impression taken for the first time.
* Does not register (reproduce, record) the fine
details.
Secondary impression
* Impression taken for the second time.
* Register (reproduce, record) the fine details.
* More accurate than primary impression
4. 4
Definitions
Cast (model): positive replica (reproduction) of the oral
tissues.
Die: positive replica of single tooth.
5. 5
Definitions
Tray: a container that is used to hold (carry) the impression
material into the patient mouth.
Stock tray
* Universal tray * Ready-made
* Use: make primary impression
* Material: metal or plastic
Special tray
* Custom tray
* Use: make secondary impression
* Material: usually acrylic resin
8. 8
Indications
Any impression is poured to form a cast (primary or
secondary cast).
Functions of primary cast (diagnostic cast)
* Diagnosis & treatment planning
* Fabrication (making) of special tray
Function of secondary cast (master cast)
* Fabrication (construction) of indirect restoration,
such as crown & bridge
13. 13
Requirements of impression materials
Acceptable odor & taste, not toxic & not irritant
Suitable working & setting times and shelf life
Low cost and easy to use (with minimum equipment)
Easy to disinfect (without distortion)
14. 14
Requirements (continued)
Accurate (record fine details): by high flow & good wetting to the oral
tissues
Dimensional stability = no dimensional changes:
* No expansion or contraction
* No absorption or loss of water or fluids
* Not release byproduct
15. 15
Requirements (continued)
Elastic: to record undercuts
* Can be used with dentulous patient (having teeth)
* And good elastic recovery
Flexible: to be easily removed from undercuts
16. 16
Requirements (continued)
High tear strength: to resist tearing during removal from undercuts
Compatible with gypsum products (cast material):
* Not need separating medium
* Not release byproducts that affect the model surface
17. 17
Classification of impression materials
According to elasticity after setting
Rigid (non-elastic) Elastic
Impression plaster
Impression wax
Impression compound
Zinc oxide eugenol
Hydrocolloids Elastomers
Alginate
Agar
Polysulfide
Condensation silicone
Addition silicone
Polyether
18. 18
Classification of impression materials
According to the nature of setting reaction
Physical reaction
(reversible)
Impression plaster
Impression wax
Impression compound Zinc oxide eugenol
Elastomers
Alginate
Agar
Chemical reaction
(irreversible)
20. 20
Definition
Form
Types & uses
Composition
Reaction
Manipulation
Properties
Items to be covered
21. Thermoplastic material:
* Soften by heating
* Harden by cooling
Non-elastic → not record undercut
Used for making primary impression for edentulous
patient (without teeth)
21
Definition
23. Impression compound, low fusing type 45-55°C
Uses: * Primary impression for edentulous patient
* Tracing (border molding) of special tray
Tray compound, high fusing type 70°C
Use: Tray material which is rigid enough to support other impression
materials
23
Types & uses
Type I
Type II
27. Thermopastic materials
* To give thermoplasticity, flow and cohesion.
e.g. natural resins & waxes.
Fillers
* To give body and suitable working consistency.
e.g. talc.
27
Composition (continued)
28. Plasticizer
* To act as lubricants & control consistency (with
fillers), e.g., stearic acid or stearin.
Coloring agent
* To give characteristic color, e.g., rouge
28
Composition (continued)
30. The compound has low thermal conductivity, so
Soften the compound in thermostatically controlled water
bath at 45°C (just above the mouth temperature) for
sufficient (adequate) time
Kneading by fingers
Until complete (homogenous, uniform) softening
30
Manipulation
31. Loading the perforated stock tray
Insertion in the patient mouth
Cooling by water spray:
* To accelerate hardening
* For sufficient time → to ensure complete hardening
31
Manipulation (continued)
33. 33
Properties
Non-elastic
Not record fine details
Flow
Thermal conductivity
Thermal contraction
* Precautions to avoid contraction
Distortion (warpage)
Compatibility with model materials
34. Not record undercuts
Used only for edentulous patient (without teeth)
34
Properties
Non-elastic
Not record fine details
Used for making primary impression
35. At 45°C, type I has higher flow than type II:
Records better fine details than type II.
= More accurate than type II.
35
Properties
Flow
36. Impression compound is a bad thermal conductor.
During softening, the outside (surface) soften first before
the inside.
Softening must be done for sufficient time
To ensure complete softening
To avoid the internal stresses
36
Thermal conductivity
Why?
37. High coefficient of thermal expansion & contraction
Contraction (0.3–0.5%) occurs from the mouth temperature
(37°C) to room temperature (25°C)
37
Thermal contraction
38. Cooling the impression in the patient mouth by water
spray equal to room temperature
Heating the set impression surface & remaking the
impression
Pouring the impression immediately
38
Precautions to avoid contraction
40. Compatible with model & die materials
Not need separating medium
To separate the impression from the cast
→ use warm water
40
Compatibility with model materials
41. Can be added & reused after sterilization
Low cost & easy to use (ease of use)
Non-toxic & non-irritant
Compatible with model & die materials
* Not need separating medium
41
Advantages
42. Transfer infection (non hygienic)
Non-elastic → so …….
Not record fine details
Dimensional changes due to:
Thermal contraction
Relief of internal stresses
42
Disadvantages
46. Final (secondary) impression for edentulous patient
Wash impression over the compound
Impression in old denture for relining
46
uses
47. Two pastes in tubes
Base paste: white
Catalyst paste = accelerator = reactor: red or brown
47
Forms
48. Zinc oxide (80–85%): reactive ingredient
Inert oils (15–20%):
* Form paste
* Act as plasticizer
48
Composition
Base
49. Oil of cloves or eugenol (15%): oil of cloves, which contains
75% eugenol, is preferred than eugenol, because it
produces less burning sensation.
Filler (16%): form paste
Gum rosin & oils (65%): softened in hot water → facilitate
removal of the impression from the cast
MgCl2 or CaCl2: accelerator
Water
49
Composition (continued)
Catalyst
52. = ↓ setting time by
Increasing the catalyst
Drop of water & ↑ humidity
↑ Mixing time
Accelerator: zinc acetate
52
Factors affecting setting time of ZOE
Accelerate the reaction by
53. = ↑ setting time by
↑ Base
Cooling the glass slab
Waxes & oils
53
Factors affecting setting time of ZOE (continued)
Retard the reaction by
54. Two equal lengths of the base & catalyst are mixed
On a waxed paper pad or glass slab
Using a flexible stainless steel spatula
In a sweeping motion
Until a uniform (homogenous) color is obtained
54
Manipulation
55. Loading the acrylic special tray (with border tracing by green
compound).
Note: the tray should be dry
Because ZOE does not adhere well to wet surfaces.
Paint the patient lips with Vaseline before impression
To avoid sticking (adhering) of ZOE to the lips.
55
Manipulation (continued)
Why?
Why?
56. Non-toxic (but it is irritant & causes burning sensation)
High flow → can record fine details → secondary impression
High dimensional stability
Accurate
Compatible with model & die materials:
Not need separating medium
Not retard (affect) setting of gypsum
56
Properties
Advantages
57. To separate the ZOE impression from the cast
Use warm water at 60°C
Gum rosin: softened in hot water → facilitate removal of the
impression from the cast
57
Note
58. Non-elastic:
Not record undercuts
Used only for edentulous patient (without teeth)
Eugenol may cause burning sensation (irritation)
Eugenol-free zinc oxide can be used
Stick to dry skin (lips)
Paint the lips with Vaseline before impression
58
Disadvantages
59. Ethoxy benzoic acid is used instead of eugenol
To avoid burning sensation of eugenol
Irreversible chemical reaction called
Saponification reaction (instead of chelation reaction)
The reaction is less affected by water & humidity
59
Non-eugenol zinc oxide (eugenol-free)
Why?
Composition
Reaction
60. Two pastes in tubes
Base: white color
Catalyst: blue color (not red or brown)
60
Non-eugenol zinc oxide (eugenol-free)
Form
65. 65
Introduction
Solution, suspension, emulsion & colloids
Hydrocolloids
Sol-gel transformation
Types of hydrocolloids: alginate & agar
Items to be covered
66. 66
Solution Colloids Suspension
Homogenous (one phase) Heterogenous (more than one
phase)
Heterogenous
Particle size: 0.01-1 nm.
Atoms, ions or molecules.
Particle size: 1-1000 nm.
Intermediate in size between
those of solution & suspension.
Large molecules or aggregates.
Particle size: over 1000 nm.
Large particles or aggregates.
Often seen with naked eye.
Do not separate on standing.
Cannot be separated by filtration.
Do not separate on standing.
Cannot be separated by filtration.
Particles settle out.
Can be separated by filtration.
Do not scatter light Scatter light (Tyndall effect) May either scatter light or be
opaque
Sugar in water Alginate in water
Paint
Sand in water
68. The dispersed particles are intermediate in size between those
of solution & suspension.
68
Colloids
Emulsions
Liquid in a liquid
Such as oil in water
69. Colloids are called hydrocolloids when the dispersion medium
is water.
Used for making impression
Exist in sol & gel state
Inserted in the mouth in sol state, which is sufficiently fluid to
record details
Removed after the gel is formed, which has elastic properties
69
Hydrocolloids
71. Primary bond between fibrils
Irreversible Sol ––chemical reaction→ Gel
71
Types of hydrocolloids
Alginate
Agar
Secondary bond between fibrils
Reversible Gel ––heating→ Sol
Sol ––cooling→ Gel
73. Visco-elastic impression material
→ can record undercut
Used for making primary impression
→ less accurate than rubber impression
Irreversibe chemical reaction (sol → gel).
73
Definition
74. Used for making primary impression
Which is used for making diagnostic (primary) casts
Functions of primary cast (diagnostic cast)
* Diagnosis & treatment planning
* Fabrication (making) of special tray
* N.B: Not used for crown & bridge fabrication
- Because it does not produce fine details.
- Higher dimensional changes & lower tear strength
than rubber impression materials
74
Uses
Why?
75. Powder in container or individual packets
Packets are preferred,
- Less chance for contamination during storage
75
Form
Why?
76. Potassium or sodium alginate (soluble)
The main reactant
Reacts with calcium sulfate to form calcium alginate (insoluble)
Calcium sulfate dihydrate (CaSO4.2H2O)
Source of calcium for cross-linking of alginate chains
76
Composition
77. Sodium phosphate (Na3PO4): retarder
React with calcium to delay (retard) calcium reaction (cross-linking)
with alginate
According to its amount: alginate may be
- Regular set (3–4 min) - Fast set (1–2 min)
Potassium titanium fluoride (K.Ti.fluoride)
Gypsum accelerator
77
Composition (continued)
78. Fillers: such as diatomaceous earth or silicate powder
Strengthen the gel
Glycol: make the alginate dustless (dust-free)
Coloring & flavoring agents
Disinfecting agent: such as chlorhexidine
78
Composition (continued)
79. Irreversible chemical reaction
Called gelation reaction (sol → gel)
When the powder is mixed with water, a retardation reaction
occurs by reaction of calcium with phosphate (retarder).
After the retarder is consumed, a gelation reaction occurs by
reaction of calcium with alginate to form calcium alginate gel.
79
Reaction
80. Retardation reaction
Calcium + Phosphate (retarder)
Until the retarder is consumed
Gelation reaction
Calcium + Alginate → Calcium alginate gel
80
Reaction (simplified)
81. ↑ Water temperature → accelerate the reaction
↑ P/L ratio (within limits) → accelerate the reaction
↑ Mixing rate → accelerate the reaction
↓ Particle size of the powder → accelerate the reaction
↓ Amount of retarder → accelerate the reaction
Note: - ↑: increase - ↓: decrease
81
Factors affecting the setting time (reaction)
82. Shake the alginate container
The powder is mixed with water against the
wall of rubber bowl until homogenous
(uniform) creamy mix is obtained.
Perforated tray
Snap (rapid, sudden) removal
Pouring immediately
82
Manipulation
Why?
Why?
Why?
Why?
85. Visco-elastic impression material
→ can record undercut
Used for making primary impression
→ less accurate than rubber impression
85
Properties
86. Dimensional changes (instability) on storage
Due to syneresis or imbibition.
Syneresis: loss of fluid exudate → contraction (shrinkage)
Imbibition: uptake of water → expansion
86
Properties (continued)
Dimensional stability
Why?
87. How to reduce (decrease) dimensional changes?
Pouring the alginate impression immediately (within 10 min)
Or storing in sealed plastic bag
87
Dimensional stability (continued)
Reference: Ritter AV, Boushell LW, Walter R. Sturdevant's art and science
of operative dentistry. 7th ed. St. Louis, Elsevier; 2019. p. 485
89. Viscoelastic
Can record undercut
Elastic recovery: 97.3%
Significance (importance)
Snap (sudden, rapid) removal of alginate impression from the mouth →
↓ permanent deformation
89
Elasticity
Permanent
deformation
90. Flexible → easy to be removed from undercut
90
Flexibility
Means:
with low force
(low stress)
Will produce large
elastic deformation
(elastic strain)
91. Low tear strength
How to increase tear strength?
Snap removal
↑ Thickness of alginate impression (at least 4 mm)
Proper (correct) powder / water ratio
Note: ↑ Powder / water ratio → ↑ tear strength,
but this will ↑ viscosity & ↓ flow
91
Tear strength
92. Don’t need separating medium
Affect the setting & surface of gypsum
The water content of alginate inhibits (delays, retards) the setting of
gypsum surface.
Alginate is hydrophilic → absorbs water from the gypsum surface →
chalky appearance.
Note: potassium sulfate (2%) is a gypsum accelerator.
92
Compatibility with gypsum
93. Non-toxic
But inhalation of airborne particles from alginate powder can result in
pulmonary hypersensitivity
Glycol coat the alginate particles → to make the alginate dustless (dust-
free).
93
Biocompatibility
How to avoid?
94. Alginate is hydrophilic
Good wetting to oral tissues
Good wetting by gypsum
Imbibition (water uptake) → expansion (dimensional changes)
Cannot be electroplated
94
Hydrophilicity
Advantages
Disadvantages
95. Low cost & ease of use
Can record undercut
Stock tray
Hydrophilic → good wetting to oral tissues & by gypsum
95
Advantages of alginate
96. Dimensional changes: syneresis & imbibition
Low tear strength
Less accurate than rubber impression → primary impression
High permanent deformation
Hydrophilic → imbibition → cannot be electroplated
96
Disadvantages of alginate
How to avoid?
How to overcome?
How to overcome?