impression materials in dentistry specially those used in Prosthodontics. Impression compound Zinc oxide Eugenol impression paste Alginate Agar-agar Elastomeric impression compounds like Polysulfides, Condensation silicones, Addition silicones(PVS), Polyether detailed description with properties, mixing time, working time, setting time ,physical and properties etc

1. IMPRESSION MATERIALS
Presented by- Dr Sakshi
II Yr PG
Dptmnt of Prosthodontics
1

2. WHAT IS AN IMPRESSION???
• negative replica of the hard and soft oral
tissues
• Register or reproduce the form and
relationship of the teeth and oral tissues
2

3. Purpose of Impression Materials
• Impression –negative reproduction of tissues
• Positive cast made by filling the impression
with dental stone or other model material
3

4. Desirable qualities of Impression
Materials
A pleasant odour, taste and acceptable color
Absence of toxic or irritant constituents
Adequate shelf life for requirements of
storage and distribution
4

5. Desirable qualities contd…
Easy to use with minimum of equipment
Setting characteristics which meet clinical
requirements
Low enough viscosity to adapt to the oral
tissues,yet be viscous enough to be contained
in the impression tray
5

6. Desirable qualities contd…
Should have adequate wettability of the oral
tissues
Set impression should show adequate elastic
recovery with no permanent deformation
upon removal from mouth
6

7. Desirable qualities contd….
Dimensionally stable after setting over
temperature and humidity ranges normally
found in clinical and lab procedures until the
pouring of the cast
Compatible with cast and die materials
7

8. Desirable qualities contd….
Biocompatible ,non-toxic
Must be of color and opacity that allows the
dentist to evaluate the impression
Readily disinfected without significant loss of
accuracy or loss of mechanical properties
8

9. Desirable qualities contd….
Materials ,associated processing time and
equipment –cost effective
No release of gas or other by-products during
setting of the impression materials
9

10. 10

11. CLASSIFICATON:
Classified on the basis of setting and elasticity.
Mode of setting rigid Elastic
Set by chemical
reaction(irreversible)
Impression plaster, zinc
oxide eugenol.
Alginate, polysulfide,
polyether, silicone,
Set by temperature
change(reversible)
Compound, waxes Agar hydrocolloid.
11

12. 12

13. Impression
Materials
Non-elastic
Elastic
Aqueous
Hydrocolloids
Non-aqueous
Elastomers
Polysulfide
Silicones
Polyether
Condensation
Addition
Agar (reversible)
Alginate (irreversible)
Plaster
Compound
ZnO - Eugenol
Waxes
O’Brien, Dental Materials & their Selection 199713

14. NON-ELASTIC IMPRESSION
MATERIALS
NON-ELASTIC
IMPRESSION
MATERIALS
Impression
plaster
Zinc-oxide
eugenol
impression paste
Impression
compound
14

15. Impression Plaster
• used as mucostatic impression material for
making final impressions for edentulous
patients
• Doesn’t compress and displace tissues during
seating of tray due to its fluidity
15

16. Impression plaster contd…
• Applicable to patients with displaceable soft
tissues that should be recorded in a passive
state
16

17. Composition
• ß-calcium sulphate hemihydrate
• Reacts with water to form calcium sulphate
dihydrate
• W/P ratio– 0.5-0.6
• Expansion and setting times controlled by
incorporating compounds designed to mediate
handling properties
17

18. • Potassium sulphate added as an anti-setting
expansion agent
• Borax(retarder)- added to the powder to
balance the setting acceleration caused by
Pot. Sulphate and to bring the setting time
under control
18

19. • Alzarin red-to make clear distinction between
the impression and model
19

20. • Custom tray constructed using 1-1.5mm
spacer with acrylic resin or shellac
• Impression plaster can be used as wash
material
• Techinque- “Puddling” the impression into
place
20

21. • With remaining plaster in
tray, the tray is seated in
single movement
• Then tray is gently
moved from side-to side
and antero-posteriorly to
take advantage of fluidity
of material
21

22. • Hemihydrate particles absorb moisture from
the surface of the oral tissues allowing
intimate contact between impression material
and the tissues
22

23. • Plaster impression material –very brittle and
fractures easily
• When undercut is involved, fracture the
impression to facilitate removal from mouth
• Fragments reconstructed to form completed
impression
23

24. • Beading of the impression done
• Coated with separating medium and cast in
fresh plaster
• Disinfection- achieved with a 10 min soak in
sod hypochlorite solution
24

25. • Not used regularly due to mechanical
limitations
• Used frequently as occlusal registration
material
25

26. Zinc-oxide Eugenol Impression
Paste
• Composition-2 separate pastes dispensed in
tubes
• One tube contains zinc oxide and vegetable or
mineral oil
• Other tube contains eugenol and rosin
26

27. Composition
Components Percentage
Tube no 1 (base)
Zinc-oxide 87
Fixed vegetable or mineral oil 13
Tube no 2 (accelerator)
Oil of cloves or eugenol 12
Gum or polymerised rosin 50
Filler(silica type) 20
Lanolin 3
Resinous balsam 10
Accelerator solution(CaCl2) and color 5 27

28. Setting reaction of ZOE
• Ionic in nature
• Requires ionic medium in which it can proceed
at any desired rate
• 1st reaction-hydrolysis of zinc oxide to its
hydroxide form
28

29. Zinc Oxide Eugenol
• When the 2 pastes are mixed,the phenol –OH
of the eugenol acts as a weak acid and
undergoes an acid-base reaction with zinc
hydroxide
• Forms a salt- zinc eugenolate
29

30. ZOE contd…
• Two further coordinate bonds are formed by
donation of pairs of electrons from methoxy
oxygen to zinc
30

31. ZOE contd…
• Disadvantage-
• Stinging or burning sensation caused by
eugenol
• Orthoethoxybenzoic acid -substitute
31

32. Manipulation
• Mixed on oil impervious paper or glass mixing
slab
• Proper proportion of two pastes obtained by
squeezing 2 strips of paste of the same
length,one from each tube ,onto the mixing
slab
32

33. Manipulation of ZOE contd…
• Flexible stainless steel spatula used for mixing
• 2 strips of contrasting colors combined with
the first stroke of the spatula ,mixing is
continued for approx 1 min, until a uniform
color achieved
33

34. Types of ZOE
• Classified as Hard paste(type I)
soft paste(type II)
• Final set for type I paste-10 min
type II paste-15min
Actual time shorter when setting occurs in
mouth
34

35. • Shorten the setting time – by adding small
amount of Zinc acetate or additional
accelerator or a drop of water in the paste
before mixing or by extending mixing time
• Prolonging the setting time- cool spatula and
mixing slab
35

36. • Paste of thick consistency –compresses the
tissues
• Thin,fluid material results in little or no
compression
Advantage of heavier consistency – increased
strength
36

37. Dimensional stability
• Negligible shrinkage(less than 0.1%) may
occur during hardening
• No significant dimensional change
• Impression can be preserved indefinitely
without change in shape provided the tray
material is dimensionally stable
37

38. Disinfection
• 2 % alkaline glutaraldehyde solution
• Immersed in solution for required time,rinsed
and poured immediately
38

39. Applications of ZOE
• Final impression of edentulous ridges
• As a wash impression with other impression
• As an interocclusal registration material
• As a temporary liner material for dentures
• As a surgical dressing
39

40. Impression compound
• Also called “modelling plastic”
• Thermoplastic material
• Supplied in the form of cakes(red) and sticks
(green, gray or red)
40

41. Composition
• Mixture of –waxes(principal ingredient)
thermoplastic resins
filler(increase viscosity and
rigidity)
coloring agent
• Shellac, stearic acid and gutta-percha added
to improve plasticity and workability
41

42. Types of
Impression
compound
Type-I
(Lower fusing)
Type- II
(Higher fusing)
42

43. Type I (Lower fusing material)
• Cakes- as an
impression material for
completely edentulous
patients, the material is
softened by heat,
inserted into the tray
and placed against the
tissues before it cools
to a rigid mass
43

44. • Sticks- as a border
molding material for
the custom tray ,the
material is used
before making the
final impression
44

45. Type II( Higher fusing material)
• Used as an adaptation material which requires
more viscous properties
• Used for making primary impression of the
soft tissues and then used a tray to support a
thin layer of a second impression material
such as ZnOE paste, hydrocolloids or
nonaqueous elastomers
45

46. Manipulation
• Setting mechanism– reversible physical
process
• Softening by heat – prerequisite
• Preheated and used warm (~450C)
• Then cooled to the intraoral temperature(370C)
at which it is fairly rigid
46

47. • Once the impression tray is seated , it should
be held gently(passively) in position until the
impression cools below the fusion
temperature
47

48. • Softened by heat over flame(green stick) or in
a temperature controlled water bath
• When direct flame is used, the material
should be moved over the flame in such a
manner that it will not be allowed to boil or
ignite so that the constituents are volatilized
48

49. • Prolonged immersion or overheating in water
bath makes the compound brittle or grainy
due to leaching of low molecular weight
ingredients
49

50. • Dimensional stability- allow thorough cooling
of the impression before removal from the
mouth and to construct the cast or die as soon
as possible after the impression has been
obtained(at least within the hour)
• Disinfection – 2% alkaline glutaraldehyde
solution
50

51. Thermal Properties
a)Thermal conductivity-low thermal conductivity
Significance
• During softening of the material, the outside will
soften first and inside last, so to ensure uniform
softening, the material should be kept immersed
for a long time in water bath
• The layer adjacent to tissues will remain soft .
Thus it is important to cool the compound
thoroughly before removing the impression
51

52. b)Coefficient of Thermal Expansion- high COTE
(0.3% acceptable)
c)Glass Transition Temperature
• The temperature at which the material loses
its hardness or brittleness on heating or forms
a rigid mass upon cooling
• Approx 39°C
52

53. d)Fusion temperature– corresponds to a definite
reduction in plasticity of the material during
cooling
• Above this temperature, the material remains
plastic while the impression is being made
• Approx 43.5°C
53

54. Significance of Fusion temp and Glass
transition temp
• Above Fusion temp,the fatty acids are liquid
and lubricate the softened material to form a
smooth plastic mass while the impression is
being obtained.
• Thus all impressions with compound should
be made above this temperature
54

55. • Once the impression tray is seated,it should
be held firmly in position until first fusion
temperature and later the glass transition
temperature is reached
• Thus, impression is made above the fusion
temperature and removed after it cools down
to its glass transition temperature
55

56. Important considerations for proper
use of impression compound
• Low thermal conductivity- adequate time
needed to attain thorough heating and cooling
• Incorporation of water(wet kneading)-
excessive flow of the material at mouth temp.
producing distortion as the impression is
removed from mouth
56

57. • Tray used for impression must be strong and
rigid enough to support the material and to
avoid distortion of the impression
• Relatively high viscosity limits its ability to
record fine details
• Cast should be poured as soon as possible to
minimise distortion due to relaxation of the
compound
57

58. • Safely disinfected by immersion in sod
hypochlorite, iodophors or phenolic
glutaraldehydes
58

59. Elastomeric Impression
Materials
ELASTOMERIC
IMPRESSION
MATERIALS
Aqueous
hydrocolloids
Agar-agar Alginate
Non-aqueous
elastomers
Polysulfides Silicones Polyethers
59

60. AQUEOUS HYDROCOLLOIDS
60

61. Agar
• Reversible hydrocollloid
• Physical change of agar from sol to gel
induced by lowering temperature
• Gel liquefies to sol when heated to a
temperature known as liquefaction
temperature(700C-1000C)
61

62. • When sol is cooled, it becomes gel at a point
known as the gelation temperature( btw 370C
and 500 C)
• Thus called reversible hydrocolloid
62

63. • Gelation temp- critical for impression making
• If too high,heat from the sol may injure the
oral tissues
• If too low,below oral temperature,impossible
to make impression because the sol will not
convert to a gel
63

64. • Polysaccharide- extracted from certain types
of seaweed
• Water major constituent
• Supplied as gel
• Available in tray and syringe consistencies
• Tubes used to fill water cooled trays and
cartridges used with syringes
64

65. Composition
Agar Gelling agent
Borax Improves strength
Potassium sulphate Gypsum hardener
Alkyl benzoates Preservatives
Water Reaction medium(>80%)
Coloring agents
Flavouring agents
65

66. • Fillers such as diatomaceous earth,wax,
clay,silica,rubber and similar inert powders–
used to control strength , viscosity and rigidity
• Thymol and glycerine added as bactericidal
agent and plasticiser
66

67. Making the Agar impression
• Process requires a 3 compartment
conditioning unit for the agar tray material
• Allows liquefaction,storage and tempering
• Syringe material used only in liquefaction and
storage compartments
67

68. Liquefy the hydrocolloid gel in the tube in
boiling water at 1000C for minimum 10 min
tube then placed in a storage bath at 65°C to
retain the sol condition until needed
impression tray filled with hydrocolloid sol
from the tube taken from storage bath , gauze
pad placed over the top of the tray material
Tray placed in water filled tempering
compartment(at abt 45°C)
68

69. 69

70. Just before tempering completed,syringe material
taken directly from storage compartment and
applied to the prepared teeth
• Note--- tempering time-3-10 min
• if >10 min,partial gelation occurs
• syringe material doesn’t require
tempering bcoz maintained in fluid state to
enhance adaptation to tissues
70

71. • Syringe material first applied to the base of
the preparation,then remainder of the
prepared tooth is covered
• Tip of the syringe is held close to the tooth
and it remains embedded below the surface
of the syringe material to prevent entrapment
of air bubbles
71

72. • Water soaked outer layer of hydrocollloid
loaded tray and the gauze covering the tray
impression material are removed to ensure
firm bonding to the syringe hydrocolloid
• Tray immediately brought into position,seated
with light pressure and held with a very light
force
72

73. • Gelation accelerated by circulating cool (18-
21°C)through tray for 3-5 min
• During gelation process, tray must be held in
mouth until gelation has proceeded to a point
at which gel strength is sufficient to resist
deformation or fracture
• Tray removed with a snap
73

74. Distortion during gelation
• Some contraction occurs due to physical
change (sol gel)
• If held rigidly in the tray,shrink towards the
center of its mass,thus creating larger dies
• Rapid cooling may cause stress concentration
near the tray
74

75. 75

76. Dimensional stability
• Storage medium- 2% potassium sulfate
solution or 100% relative humidity
76

77. Compatibility with Gypsum
• Contains borax- retarder for setting of gypsum
products
• Deficiency of gypsum setting can be overcome
by--Immersing agar impression in a solution
containing a gypsum accelerator(2% pot
sulfate solution) prior to pouring of the
impression
• By incorporating gypsum surface
hardener in the material such as sulfate
77

78. • Disinfection
House hold bleach or iodophors
• Accuracy
Most accurate of impression materials
78

79. Alginate
• Irreversible hydrocolloid
• Most widely used material in dentistry
• Developed as a substitute for agar
79

80. Advantages
• Ease of manipulation
• No need of expensive
equipments
• Relatively low cost
• Comfort to patients
• High viscosity
• Ability to displace
tissues
Disadvantages
80

81. Potassium or sodium alginate dissolves in water and reacts with calcium ions
Calcium sulphate dihydrate A reactor ,reacts with potassium alginate to form a
dihydrate insoluble alginate gel
Zinc oxide Filler particles, affects properties and setting time
Potassium titanium fluoride Accelerator ,counteracts the inhibiting effect of the
hydrocolloid on the setting of stone,ensures good
quality surface of the cast
Diatomaceous earth Filler particles, controls the consistency of the mix
and the flexibility of the set alginate
Trisodium phosphate Retarder,controls the settting time to produce
either regular or fast set alginates
Coloring agents
Flavoring agents
Composition
81

82. Modified alginates
• Dustless alginates
• Include polyethylene glycol or polypropylene
glycol on the alginate powder to agglomerate
the particles
• Color indicators added to reveal the stage of
setting reaction
82

83. Modified alginates
• Two paste alginate materials
• One paste contains sol of
alginate,fillers,retarders and other ingredients
like glycols and dextrose
• Other paste contains gypsum dihydrate, fillers,
retarder , glycerol or glycol, gypsum surface
modifier and some silicone oil
83

84. Gelation process
• Typical sol-gel reaction
• Soluble alginate reacts with calcium ions from
calcium sulphate and forms insoluble calcium
alginate
• Production of calcium alginate- rapid,doesn’t
allow sufficient working time
• Retarder trisodium phosphate added to
extend working time
84

85. Setting reaction of alginate
(a) K2n-Alginate+ n CaSO4 nK2SO4 + Can-
alginate
(b)2Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4
85

86. 86

87. Controlling Setting Time
• Ideal W/P ratio- 20 ml water/8gms of powder
40 ml water/16gms of powder
• Powder should be weighed not measured
• Approx 2.5:1
• Slight modification in W/P ratio affects 2
important properties--- tear strength
elasticity
87

88. • Thus setting time best regulated by amount of
retarder added during manufacturing
• Fast-set alginate- 1.5-3min
• Regular set alginate- 3-4.5 min
• Can also be influenced by altering the
temperature of water
88

89. • Cool water in hot weather
• Precool mixing bowl and spatula
• Tap water-contains certain levels of metallic
ions(Ca,Mg)
• Tap water with a high hardness may accelerate
setting time
89

90. Preparation of Alginate
Impression Materials
• Measured powder added
slowly to premeasured water
already poured into clean
rubber bowl
• Powder incorporated into
water by carefully mixing
with a metallic spatula
flexible enough to adapt well
to the wall of the mixing
bowl
90

91. • Avoid incorporating excessive air into the mix
• Vigorous figure of 8 stropping motion
• Mixing time- 45sec to 1 min
• Result should be a smooth creamy mixture
that doesn’t drip off the spatula when raised
from bowl
91

92. • Mechanical mixing devices
• Include rotating mixing bowl , mechanical
mixer with time-control unit, a vaccum mixer
for water/powder mixing
• Advantages- convenience, speed and
reduction of human error
92

93. Making the alginate
impression
• Perforated metal tray preferred
• Thickness of alginate impression between the
tray and the tissues should be at least 3mm
• Compressive strength doubles during first 4
min after gelation,but doesn’t increase
appreciably thereafter
• Improve elasticity over time which minimizes
distortion of the material during impression
removal(undercut areas)
93

94. Compressive strength of an Alginate
gel as a function of Gelation time
Time from Gelation(min) Compressive strength(KPa)
0 330
4 770
8 810
12 710
16 740
94

95. • Alginate impression should
not be removed from the
mouth for at least 3 min after
gelation has occurred
• Tear strength increased when
the impression is removed
along a vertical path with a
snap
• Speed of removal- between
rapid movement and a slower
rate
95

96. STRENGTH
• Manufacturer’s directions should be followed
• Any deviation from instructions can have
adverse effects on the gel strength
SHELF LIFE
• 2 factors affecting shelf life- storage
temperature and moisture contamination
96

97. Dimensional stability
• Syneresis- loss of water when exposed to air
at room temperature associated with
shrinkage
• Imbibition- swelling of the impression if
immersed in water
97

98. How can distortion be minimised??
• Poured immediately after making impression
• If pouring delayed, then rinsed in tap
water,disinfected wrapped in a surgical paper
towel saturated with water and placed in a
sealed plastic bag or humidor
98

99. Compatibility with Gypsum
• Poatssium titanium fluoride-surface hardener
or gypsum hardener
• Solubility-1.3gm per 100 ml of water at 20°C
• Fluoride on surface of alginate will form fine
calcium fluoride precipitates with the calcium
from the gypsum
• Fine calcium fluoride particles become nuclei
that accelerate the setting of gypsum
99

100. • Rough stone surface will result if excess rinsing
water collected on the surface of the
impression at the time of pouring the stone
mixture
• A dried gel results in its adherence to the
surface of the cast which results in tearing
upon removal
100

101. • Surface of impression should be shiny but
with no visible water film or droplets at the
time of pouring
• Stone cast or die should be kept in contact
with the impression for a minimum of 30 min,
preferably for 60 min before the impression is
separated from the cast
101

102. Disinfection
• Household bleach(1-10 dilution) iodophors or
synthetic phenols
• After rinsing,disinfectants sprayed on exposed
surface
• Impression immediately wrapped in a
disinfectant-soaked paper towel and placed in
a sealed plastic bag for 10 min
102

103. • Wrapped impression removed from the bag,
unwrapped, rinsed and shaken to remove
excess water
• Then poured with stone of choice
Accuracy
Not capable of reproducing the finer details
compared with other impression materials
103

104. 104

105. Laminate technique(Alginate -Agar
method)
• Modification of agar procedure
• Agar in tray replaced with a mix of chilled
alginate that bonds to the agar expressed
from a syringe
• Alginate gels by chemical reaction while agar
gels by means of contact with the cool
alginate rather than water circulating through
the tray
105

106. • Agar contacts the prepared teeth ,maximum
detail reproduced
• Equipment cost low, less preparation time
needed
• Main disadvantages-
bond btw agar and alginate not always
sound
higher viscosity of alginate displaces agar
during seating
dimensional inaccuracy of alginate limits
its use to single units
106

107. NON-AQUEOUS ELASTOMERS
107

108. Elastomeric impression materials
• Comprise a group of synthetic polymer-
based impression materials that are
chemically cross-linked when set and can
be stretched
• Rapidly recover to their original
dimensions
108

109. Based on the backbone of polymer
chains
Elastomers
Polysulfide Silicones Polyethers
109

110. Elastomers
• Supplied in 2 components: base paste
catalyst paste
• Formulated in several consistencies in increasing
content of filler
Extra light body
Light body
Medium or regular body
Heavy body
Putty(extra heavy)
110

111. • Extra low and putty available only for
condensation and addition silicones
• Polysulfide provided only in light body and
heavy body
• No heavy body for condensation silicone
111

112. • Different colored pastes dispensed either
through a spiral mixing tip or in equal lengths
on a mixing pad
• Setting occurs through a combination of
chain-lengthening polymerization and
chemical cross-linking by either a
condensation or addition reaction
112

113. • 3 types of systems available to mix the catalyst
and base : hand mixing, static automixing,
dynamic mechanical mixing
• To record soft tissues for edentulous patients
under minimum compression: polysulfide or
addition silicones
(free flowing, minimum viscosity)
113

114. • If moderate compression required: medium-
viscosity polysulfides, addition silicones,
polyethers
• Undergo shrinkage upon polymerisation
• Condensation type silicones undergo
additional contraction
114

115. • Polysulfides and condensation silicones :
highest dimensional changes during setting
• Addition silicones and polyethers: lesser
dimensional changes
115

116. Polysulfides
• First synthetic elastomeric impression
material introduced in 1950
• 2 paste system
• Available in low, medium and high
consistencies
• Made up of a base and accelerator/reactor
• Brands- COE-FLEX,PERMALASTIC,NEOPLEX etc
116

117. Composition
The Base
Polysulfide polymer(-SH,mercaptan gp) Principal ingredient
Titanium oxide and Zinc Fillers
Sulphate ,copper carbonate or silica Strengthener
Dibutyl phthalate Plasticizer (confers viscosity to base)
The Accelerator
Lead dioxide, hydrated copper oxide or
organic peroxide
Reactor
Sulfur Promoter, accelerates the reaction
Oleic acid or Stearic acid Retarder, controls setting reaction
117

118. • Working time: 5-7 min( longest among
elastomers)
• Setting time: 8-12 min
• Pouring the cast: impression must be poured
within 30 min to 1 hr
118

119. • Each paste supplied in a dispensing tube with
approx sized bore diameters at the tip
• Equal lengths of paste extruded from each
tube to provide the correct ratio of polymer to
cross-linking agent
119

120. • Reaction starts at the beginning of mixing and
reaches its maximum rate soon after
spatulation is complete
• Resilient network started to form
• During final set, a material of adequate
elasticity and strength is formed that can be
removed past undercuts
120

121. • Polymerisation results in chain lengthening
and cross –linking with an increase in
molecular weight
• Setting indicated by change of the color of the
paste to dark- brown or gray- brown
• Color- presence of lead oxide
121

122. 122
Polymerisation of ploysulfide impression material
SH groups interact with oxygen released from lead dioxide
Completion of the condensation reaction results in water as a by-product
Pendant –SH for cross-linking and terminal for chain lengthening

123. • Hot and humid conditions accelerate the
setting of polysulfide impression material
• Reaction slightly exothermic and yields water
as a by-product
123

124. • Good flexibility
• High tear strength
• Hydrophobic
• Messy, stains clothes and has an offensive
odor
• Uses—impression for crown and bridge
edentulous impressions
124

125. Condensation silicone
• Followed in 1955
• Supplied as two-paste system or base-paste and a
low viscosity liquid catalyst or a two-putty system
• Putty used as tray material in conjunction with a
low-viscosity silicone
• Referred to as the Putty-wash technique
• Brands- Speedex
125

126. Composition
The Base paste
- -hydroxyl-terminated
polydimethyl siloxane
High molecular weight polymer
Silica or calcium carbonate Fillers
The Liquid Accelerator
Tin octoate Metal organic ester
Orthoalkyl silicate
Oil-based diluents
Thickening agents Increase viscosity 126

127. Condensation silicone
• Working time: 3 min
• Setting time: 6-8 min
• Impression must be poured as soon as
possible within first 30 min
127

128. • Curing involves a reaction of tri- and tetra-
functional alkyl silicates in the presence of
stannous octoate as a catalyst
• Sets by cross-linking between terminal groups
of the silicone polymers and the alkyl silicate
to form a 3-D network
128

129. 129
Condensation polymerisation of alpha-omega hydroxy –
terminated poly(dimethyl siloxane) with tetraethyl
orthosilicate in the presence of stannous octoate(catalyst)
This reaction results in the release of ethanol molecules

130. • By-product: Ethyl alcohol
• Subsequent evaporation accounts for much of
the contraction that takes place in the setting
impression
• Extra-heavy or putty consistency developed to
counteract the large polymerisation shrinkage
130

131. Addition silicone
• Commonly referred to as Polyvinyl
siloxanes(PVS)
• Supplied as low, medium, high and very high
consistencies
• Based on silicone prepolymers that carry vinyl
and hydrogen side groups which can
polymerize by addition polymerization
• Brand name- Aquasil
131

132. Composition
The Base
Polymethylhydrosiloxane Low molecular weight polymer
Fillers
The Accelerator
Divinyl polymethyl siloxane
Other siloxane pre-polymers
Platinum salt Catalyst
Reatrder Controls working and setting times
132

133. • Working time: 2-4.5 min
• Setting time: 3-7 min
• Cast can be poured upto 1 week after making
the impression
133

134. • Reaction activated by a platinum salt catalyst
(chloroplatinic acid) without the release of by-
products
• In presence of impurites or moisture,
secondary reaction takes place between the
residual hydrides and moisture leading to
evolution of hydrogen gas
134

135. 135
Hydrogen atoms along the backbone str of PVS chain move to the vinyl group
during addition polymerisation(top)
Final str after platinum salt has initiated the addition polymerisation reaction
(bottom)
The zigzag line at the other end of divinylpolysiloxane represents repeating units of
dimethylsiloxane with a vinyl terminal

136. • This can cause minute gaseous voids in the
gypsum casts and reduce the effectiveness of
cross-linking polymer structure
• Automatic mixing systems simplified their
manipulation, reduced voids in impressions,
reduced the amount of material wasted and
reduced the sensitivity of their mixing
technique
136

137. 137

138. Advantages
• Most elastic of currently available materials
• Virtually negligible distortion upon removal
from undercuts
• Exceptional accuracy in reproducing anatomic
details
• Dimensional stability allows pouring long after
impression making
• Excellent occlusal record registration material
138

139. Disadvantages
• Inherent hydrophobic nature
Non-ionic surfactant wetting agent added to
silicone paste rendering the surface of the
impression more hydrophillic and called
hydrophilized addition silicone
139

140. • Sulfur contamination from natural latex gloves
inhibits the setting of addition silicone
• Touching the tooth with latex gloves before
seating the impression can inhibit the setting
of critical surface next to tooth
140

141. Polyether
• Supplied as two-paste system in low, medium
and high consistencies
• 2 types: 1. based on ring-opening
polymerization of aziridine rings which are at
the end of branched polyether molecules
2. based on an acid-catalyzed
condensation polymerization of polyether
prepolymer with alkoxysilane terminal groups
• Brands- IMPREGUM
141

142. 1st type
• Main chain probably a copolymer of ethylene
oxide and tetrahydrofuran
• Cross-linking and setting promoted by an
initiator and an aromatic sulfonate ester
• R alkyl group
• Produces cross-linking by cationic
polymerisation via the imine end groups
• Supplied as 2 pastes: base and accelerator
142

143. Composition
The Base
Polyether polymer
Colloidal silica Filler
Glycol ether or pthalate Plasticizer
The Accelerator
Alkyl aromatic sulfonate Initiator
Filler
Plasticizer
143

144. • Working time: 2.5 min
• Setting time: 4.5 min
• Poured upto 1 week of storage
144

145. 145
Initiator ,aromatic sulfonate ester dissociates and forms alkyl cations that bind the nitrogen
atoms of the azridine ring terminals of the prepolymer(top,left)
The arrows indicate binding between the cations (R) with nitrogen atoms
This action opens up the ring,and the reacted pre-polymer (center) now has 2 ethylene
imine terminals(-NR-CH 2-C+H2) which can react with nitrogen atoms of adjacent unreacted
prepolymers.(R2 Aziridine ring
Chain propagation polymerization reaction yields a larger molecule(right) which continues
growing by binding with aziridine rings of additional unreacted prepolymers
Polymerisation reaction terminates when the growing chain combines with a counter ion

146. 2nd type
• Based on an acid-catalyzed condensation
polymerization of polyether prepolymer with
alkoxysilane terminal groups
• Mechanism similar to condensation silicones
• Material often called hybrid
• Behave very much like the 1st type due to
ether linkages
146

147. • High degree of
wettability
• Inherent hydrophillic
nature
• Relative stiffness
Excellent material for good
duplication of fine details
and rigid support for
pick-up copings
147

148. Making impressions with elastomeric
materials
• Fabrication of gypsum models ,casts and dies
involves 6 major steps:
1. Preparing a tray
2. Managing tissue
3. Preparing the material
4. Making the impression
5. Removing the impression
6. Preparing stone casts and dies
148

149. 1.Impression trays
• Custom tray recommended to reduce the
quantity of material required
• In case of severe undercuts, custom tray
avoided
• Prior to impression making, uniform thickness
of tray adhesive applied
149

150. 2.Tissue management
• Displace the gingival tissues, control gingival
haemorrhage and control sulcular fluids to
ensure access for the tooth preparation and
making impression
• Gingival retraction cord- most commonly used
150

151. 3.Manipulation of impression
materials
• Supplied for 3 modes: hand mixing, static
mixing and dynamic mechanical mixing
151

152. Hand mixing
• Dispense the same length of materials onto a
mixing pad or glass slab
• Catalyst paste first collected on stainless steel
spatula and then spread over base paste
• Mixture is then spread over the mixing pad
• Mass is then scraped up with the spatula
blade and spread uniformly back and forth on
the mixing pad
152

153. • Process continued until the mixed paste is
uniform in color with no streaks of the base or
catalyst appearing in the mixture
• 2 putty systems(condensation and addition
silicone) dispensed by volume using equal
number of scoops of each material
• Knead the material with fingers until a
uniform color is obtained
153

154. Static mixing
• Transforms 2 fluid(or paste-like)
materials into a homogenous
mixture without mechanical
mixing
• Device used- gun for
compressing materials into a 2-
cylinder cartridge, which
contains the base and catalyst
separately, as well as mixing tip
154

155. • Mixing tip is made of helical mixer elements in
a cylindrical housing
• Mixer elements are series of alternating right
and left –turn 180°helixes positioned so that
leading edge of one element is perpendicular
to the trailing edge of the next
• Length of each material is the same as the
inner diameter of the cylinderical housing
155

156. Dynamic mechanical
mixing
• Device uses motor to drive
parallel plungers,forcing the
materials into a mixing tip and
out into an impression tray or
syringe
• Motor driven impeller mixes the
materials as they are extruded
through the tip
• Materials supplied in collapsible
plastic bags housed in cartridge
• Polyether and addition silicone
156

157. 4.Making an impression
• 3 techniques:
• Multiple-mix technique
• Monophase technique
• Putty wash technique
157

158. Multiple-mix technique
• Syringe material(light body)and tray
material(heavy body)
• Lighter material injected within or around the
tooth preparation
• Filled tray then inserted in the mouth and
seated over the syringe material
• Tray material force the syringe material to
adapt to the prepared tissues
158

159. Monophase technique
• Medium body polyether and addition silicone
• Only one mixture is made and a part of the
material is placed in the tray and another
portion in syringe for injection in the prepared
tissues
• Success depends on pseudoplastic (shear
thinning) property of material
159

160. Putty –wash technique
• Originally developed for condensation silicone
to minimize the effect of associated
dimensional changes
• Thick putty material placed in stock tray and a
primary impression made
• Space for light-body “wash” material provided
• Mixture of thin consistency wash material
placed into putty impression and preparation
160

161. 5.Removal of the impression
• Shouldn’t be removed until curing progressed
sufficiently to provide adequate elasticity ,so
distortion doesn’t occur
• Typically impression should be ready for
removal within at least 10 min from time of
mixing,allowing 6-8 min for impression to
remain in mouth
161

162. • Mechanics of removing impression–
separation at the impression/tissue interface
and stretching of the impression
• 1st step to break the physical adhesion
between the tissue and the impression
• Polyether requires extra effort
• 2nd step stretches the impression enough to
pass under the height of contour of hard
tissue to remove impression
162

163. 6.Preparation of stone casts and dies
• Silicones– hydrophobic
• Surfactant sprays- debubblizers improve
surface wettability of silicone impression
material for stone slurry
• Dilute solution of soap also acts as surfactant
• Polyether n polysulfide- don’t require
surfactant
163

164. Properties of elastomeric impression
materials
1. Working and setting time
2. Reproduction of oral structure detail
3. Rheological properties
4. Elasticity and visco-elasticity
5. Tear strength
6. Dimensional stability
7. Disinfection
8. Wettability and hydrophillization
9. Biocompatibility
164

165. 1. Working and setting time
Mean working time(min) Mean setting time(min)
Impression material 23°C 37°C 23°C 37°C
POLYSULFIDE 6.0 4.3 16.0 12.5
CONDENSATION SILICONE 3.3 2.5 11.9 8.9
ADDITION SILICONE 3.1 1.8 8.9 5.9
POLYETHER 3.3 2.3 9.0 8.3
165

166. 2.Reproduction of detail
• Record detail to the finest degree
• When stone poured on the surface, finest
details not always reproduced
166

167. 3.Rheological properties
• Viscosity and flow behaviour depends upon
- ease of mixing
-air entrapment during mixing
-tendency of trapped air to escape before the
impression is made
• All elastomers exhibit shear-thinning
characteristics before setting
167

168. • 2 categories of shear thinning phenomena—
• Pseudoplasticity
• Thixotropy
• Pseudoplastic material- displays decreasing
viscosity with increasing shear stress and
recovers its viscosity immediately upon a
decrease in shear stress
168

169. • Thixotropic material- doesn’t flow until
sufficient surface energy in the form of an
impact force or vibration force is applied to
overcome the yield stress of material
• Extreme shear thinning- material retains an
immobile state at rest but flows freely under
stress
169

170. Significance of shear thinning
• Exhibited by addition silicone and polyether
impression materials
• Enable the clinician to use a monophase
impression making technique to capture
details needed for fixed prostheses
170

171. 4. Elasticity and viscoselasticity
• Explained well by Maxwell- Voigt model
• Relative amount of permanent deformation
• Addition silicone<polyether<polysulfide
• Recovery of elastic deformation following
strain is less rapid for the polysulfide material
than for other three types of impression
materials
171

172. MAXWELL-VOIGT MODEL
172
A-Maxwell-Voigt model in a stress-free state
B-during loading,only S1 spring contracts in response to load
C-when loading continues,the pistons in dashpot D1 and D2 move proportionally to the
duration of loading . S2 spring contracts alongwith dashpot D2. No change in S1
D-the moment the load is released,S1 spring recovers instantly, whereas rest of the
elements remain unchanged. S2 should also recover instantly but retarded by the
sluggishness of dashpot D2
E- as time passes, S2 spring recovers and extends dashpot D2 slowly near to its original
position. Dashpot D1 remains unchanged

173. • Polyvinyl siloxane- exhibit most elastic
recovery
• Distortion on removal from undercuts is
virtually non-existent
• If material is in advanced stage of elasticity,
and compressed excessively during seating of
impression,distortion can occur when the
material elastically rebounds
173

174. • MOE in increasing order
• Polysulfide<condensation silicone<addition
silicone< polyether
174

175. 5.Tear strength
• Low viscosity materials used in interproximal
and subgingival areas
• Measures the resistance of an elastomeric
material to fracture when subjected to a
tensile force acting perpendicular to a surface
flaw
• Tear strength in increasing order
• Silicones<polyethers<polysulfides
175

176. • Influenced by consistency and manner of
removing the materials
176

177. 6. Dimensional stability
• Dimensional accuracy v/s dimensional stability
• 6 major sources of dimensional change
1. Polymerisation shrinkage
2. Loss of condensation reaction by-product
3. Thermal contraction from oral temp to room temp
4. Absorption of water or disinfectant over a period of
time
5. Incomplete recovery of deformation because of
plastic deformation
6. Incomplete recovery of deformation because of
viscoelastic behaviour
177

178. • Absorption of water or fluids- negatively
affects polyether impression
• Simultaneous leaching of water soluble
plasticizer
• Stored in dry(relative humidity<50%),cool
environment to maintain its accuracy
• Should never be left for protracted periods in
disinfecting solutions
178

179. 7.Disinfection
• Polysulfides and silicones--Glutaraldehydes,
chlorine compounds ,iodophors,phenolics
• Polyether- chlorine compounds or iodophors
179

180. 8.Wettability and hydrophilization
• Silicones-most hydrophobic
• Polyethers-hydrophillic
• Spray surfactant on hydrophobic impressions
prior to pouring with gypsum
• Non-ionic surfactant added during
manufacturing
180

181. • Hydrophyllized PVS- ether group –hydrophillic
and oriented towards the surface when the
surfactant migrates by diffusion to the surface
region
• Depends on the contact angle made by water
droplets with the impression surface and
surface energy
181

182. • As soon as hydrophillized PVS encounters oral
fluids during impression making, begins to
release surfactant to its surroundings
• Adequate amount of surfactant trapped on
the surface of set PVS impressions ,thus
providing hydrophilicity for pouring of gypsum
dies
182

183. 9.Biocompatibility
• Tests covered in ISO 10993-5,Biological
Evaluation of Medical Devices:Tests for in-vitro
cytotoxicity
• Polysulfide – lowest cell death count
• Polyether –highest cell toxicity scores
183

184. • Elastomer induced biocomaptibility problem–
fragment of impression material trapped in
patient’s gingival sulcus
• Causes severe gingival imflammation
• Can also occur in 2nd stage implant surgery
• Contact dermatitis from Polyether in dentists
or dental technicians
184

185. 10. Shelf life
• Don’t deteriorate appreciably in tube or
container before the expiration date when
stored in dry , cool environment
• Clear liquid expressed along material-
plasticizer segregation
• Indicates manufacturing error or excessive
temperature extremes during storage
185

186. Effects of mishandling
• Failure to produce an accurate epoxy or
gypsum die or cast
186

187. Common failures occurring with use of
elastomeric impression materials
187
Type of failure Causes
Rough or uneven impression surface Incomplete polymerization,improper ratio or mixing of
components,presence of oil or plaque on teeth
Surface agents (latex for PVS) inhibit polymerization
Too rapid polymerization from high humidity or temp
Excessively high accelerator /base ratio(cond silicone)
Bubbles Air incorporated during mixing
Irregularly shaped voids Moisture debris on teeth surfaces
Rough or chalky stone casts Inadequate cleaning of impression
Excess water not blown off impression
Excess wetting agent left on impression
Premature removal of cast,improper w/p ratio of stone
Failure to delay pour(PVS) that doesn’t contain
Palladium salt for 20 min

188. Distortion Resin tray not aged sufficiently , still undergoing polymerisation
shrinkage
Lack of adhesion of elastomer to tray
Excessive bulk of material
Lack of mechanical retention to the impression tray
Excessive bulk of material
Insufficient relief for the reline material
Development of elastic properties in material before tray is fully
seated
Continued pressure against impression material that developed
elastic properties
Movement of tray during polymerization
Premature/improper removal from mouth
Delayed pouring of polysulfide or condensation silicone
impression
188

189. Comparative properties of elastomeric
impression materials
Property Polysulfide Condensation
silicone
Addition
silicone
Polyether
WT(min) 4-7 2.5-4 2-4 3
ST(min) 7-10 6-8 4-6.5 6
Tear strength(N/m) 2500-7000 2300- 2600 1500-4300 1800-4800
Percent contraction(at 24h) 0.40-0.45 0.38-0.60 0.14-0.17 0.19-0.24
Contact angle(°) 82 98 98/53 49
Hydrogen gas evolution N N Y N
Automatic mixing N N Y Y
Custom tray Y N N N
Unpleasant odour Y N N N
Multiple casts N N Y Y
Stiffness 3 2 2 1
Distortion on removal 1 2 4 3189

190. s
190