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.

1. Impression materials
1

2. 2
INTRODUCTION
Items to be covered
1. Definitions
2. Indications
3. Requirements
4. Classification

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

6. 6
Stock trays

7. 7
Special trays

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

9. 9
Pouring the impression with gypsum products → cast

10. 10
Notes
Stock tray → Primary impression → Primary cast (model) →
Special tray → Secondary impression (more accurate ….) →
Secondary cast (more accurate ….) →
Construction of crown, bridge or denture …..

11. 11
Crown Bridge

12. 12
Complete denture

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)

19. 19
Impression compound

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

22. Cakes
Sticks
22
Form

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

24. 24
Green compound sticks

25. 25
Special trays Border molding

26. Thermopastic materials
Fillers
Plasticizer
Coloring agent
26
Composition

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)

29. Physical reaction
Reversible
Thermoplastic:
* Soften by heating (45-55°C)
* Harden by cooling (37°C)
29
Reaction

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)

32. 32
Compound impression

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

39. Causes of distortion
Incomplete softening
Incomplete hardening
Delayed pouring of the impression
39
Distortion (warpage)

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

43. 43
Zinc oxide eugenol
(ZOE)

44. 44
 Definition
 Uses
 Form
Composition
 Reaction
Manipulation
Properties
Items to be covered

45. Non-elastic
Used for making secondary impression for edentulous
patient (without teeth)
45
Definition

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

50. Chemical reaction: called chelation reaction
Irreversible
Two steps
1. Hydrolysis
ZnO + H2O → Zn(OH)2
2. Formation of chelate
Zn(OH)2 + 2HE (eugenol) → ZnE2 (Zinc eugenolate)+ 2H2O
50
Reaction

51. Water:
* Initiate the reaction
* A byproduct of the reaction
51
Reaction (continued)

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

61. 61

62. 62

63. 63

64. 64
Hydrocolloids

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

67. 67
Tyndall effect

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

70. Sol (colloidal solution) → molecules agglomerate → fibrils → network
pattern → brush heap structure (gel)
70
Sol-gel transformation (gelation)

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

72. 72
 Definition
 Uses
 Form
Composition
 Reaction
Manipulation
Properties
Alginate

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?

83. 83
Manipulation

84. 84

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

88. 88
Storing in sealed plastic bag

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?

97.  Dust-free alginate (dustless)
 Chromatic alginate
97
Modified alginate

98.  Glycol → coats the alginate particles → dustless
98
Dust-free (dustless) alginate
Chromatic alginate
 pH indicator → change the alginate color during manipulation.
 Different colors during mixing, loading the tray & insertion.

99.  Compound & alginate are primary impressions.
 ZOE & rubber (elastomers) are secondary impressions.
99
Notes