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Copyright 2008, Dr. Stephen Bayne.
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Part 1 and Part 2
University of Michigan
School of Dentistry
Ann Arbor, Michigan sbayne@umich.edu

What are tolerable limits for “error” in indirect procedures?
a. Impressions b. Casts, Dies c. Waxing d. Investing e. Casting f. Finishing, Polishing = g. Cementation =
=
=
= +/- 0
=
=
+/- 0
+/- 0
+ 1.5%

1.5%
+/- 0
+/- 0
Prepped
Tooth
Width
8 mm (=8,000
 m)
8,000
 m x 0.5% = 40
 m = 20
 m/side
Typical clinical error = >100
 m/side

Based on Properties of Set Materials
Properties: Reaction: Set:
Rigid:
1.
Impression Plaster
2.
Impression Compound
3.
Zinc Oxide/ Eugenol
Rigid
Rigid
Rigid
Water-Based Gel:
4.
Alginate (Irreversible Hydrocolloid) Flexible
5.
Agar-Agar (Reversible Hydrocolloid) Flexible
Irrev
Rev
Irrev
Irrev
Rev
(Chem)
(Phys)
(Chem)
(Chem)
(Phys)
Elastomers:
6.
Polysulfide (Rubber Base, Thiokol) Flexible
7.
Silicone (Conventional, Condensation) Flexible
8.
Polyether Flexible
9.
Polyvinyl Siloxane (Addition Silicone) Flexible
Irrev
Irrev
Irrev
Irrev
(Chem)
(Chem)
(Chem)
(Chem)

Key Properties a.
Accuracy = ability to replicate the intraoral surface details.
b. Dimensional Stability = ability to retain its absolute dimensional size over time.
c.
Tear Resistance = ability to resist tearing in thin sections (such as through the feather-edged material within the gingival sulcus.
Impression Tray
Impression
Accuracy
Dimensional
Stability
Tear
Resistance
Image source: Steve Bayne, University of Michigan, 2008

For Elastic Impression Materials
1.
Flexible Matrix (Continuous Phase): a.
Multifunctional Pre-Polymer or Polymer b.
Crosslinking Agent c.
Curing Agent (Catalyst or Initiator) d.
Modifiers (Accelerators, Retarders,
Plasticizers, Flavoring Agents, Colorants)
2.
Filler or Extender (Dispersed Phase):
Polymer (high shrinkage)
Filled Polymer (low shrinkage)
Image source: Steve Bayne, University of Michigan, 2008

Management of Shrinkage
Use 2-step techniques:
(a) Light-Heavy Body, OR
(b) Wash-Putty
Impression Tray
Impression
Load tray with heavy-bodied IM
Cover prep with light-bodied IM from syringe
During setting shrinkage, distortion of impression is toward tray.
Impression
Dies tend to be oversize and so the casting is oversize.
Images source: Steve Bayne, University of Michigan, 2008

Mixing and Delivery Systems
Polyether,
Polyvinylsiloxane
Polysulfide Rubber,
Silicone Rubber
Mixing Options:
 2 Pastes on Mixing Pad
 2 Pastes in Mixing Gun
 2 Pastes in Mixing Machine
Graph source: Steve Bayne, University of Michigan, 2008
Images source: Undetermined
2x2x2x2x2x2x2x2 = 256 folds

Management of Distortion During Tray Removal
Impression Tray
Impression
Strain rate sensitive elastomers !
Fast removal
VERY
FAST
(SNAP)
Slow removal
Image and graph source: Steve Bayne, University of Michigan, 2008

Distortion Time Related to Setting Reaction
100
VPS Pe PS, Silicone log TIME (minutes)
Graph source: Steve Bayne, University of Michigan, 2008

Chemistry and Setting Reaction
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
DISPERSED PHASE:
Fillers
= Mercaptan Functional Polysulfide
= Sulfur and/or Lead Peroxide
= PbO2 or Copper Hydroxides (Type I)
Zinc Peroxide or
Organic Hydroperoxide (Type II)
= TiO2 or Zinc Sulfate or
Lithopone or Calcium Sulfate Dihydrate
SETTING REACTION:
Stepwise (relatively slow), Exothermic, Affected by temperature

Manipulation and Technique Considerations a.
Two-step techniques recommended: Reduces air entrapment and surface tension effects.
b.
Material adversely affected by H
2
O, saliva, and blood.
c.* Set impression should be removed quickly - do not rock tray.
d.
No syneresis or imbibition, but distortion due to continued reaction.
e.
Ideally need uniform thickness and at least 2 mm thick for accuracy.
(1) Adhesive must be thin
(2) Adhesive must be dry f.
Paste-Paste Mixing Recommendations:
(1) Dispense pastes at the top of the mixing pad
(2) Mix pastes with tip of spatula only for 5 seconds
(3) Transfer mass to fresh surface at center of mixing pad
(4) Wipe spatula off with paper towel; Strop mass for 15s to constant color
(5) Load syringe or tray
(6) Use pad excess to monitor setting time g.
Pouring of models:
(1) Wait 20-30 minutes before pour for stress relaxation to occur
(2) RB is non-reactive with model and die materials
(3) Be careful of glove powder contamination of impression
(4) RB can be electroplated
Images source: Steve Bayne, University of Michigan, 2008

Chemistry and Setting Reaction
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
Modifiers
DISPERSED PHASE:
Fillers
= Polydimethyl Siloxane
= Alkyl Orthosilicate or Organo H-Silane
= Organo Tin Compounds (e.g., tin octoate)
(but not dibutyl tin dilaurate)
= Colorants, Flavorants
= Silica
SETTING REACTION:
Stepwise (relatively slow), Exothermic, Affected by temperature
H
2
O by-products

Manipulation and Technique Considerations a.
Limited shelf-life: Unstable in tubes.
b.
Requires mechanical retention or special tray adhesives c.
No syneresis or imbibition, but continued polymerization shrinkage.
d.
Better dimensional stability than RHC but more expensive e.
Pouring of models:
(1) More flexible so more chance for distortion during removal
(2) Wait 20-30 minutes before pour for stress relaxation to occur

Chemistry and Setting Reactions
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
Modifiers
DISPERSED PHASE:
Fillers
= Amine-terminated Polyether
= Aromatic Sulfonate
=
= Colorants, Glycol Plasticizers, Flavorants
= Silica
SETTING REACTION:
Stepwise (relatively slow), Exothermic, Affected by temperature

Manipulation and Technique Considerations a.
Excellent impression accuracy and dimensional stability.
b.
Stiff and therefore difficult to remove without rocking.
c.
Break seal and rock slightly to prevent tearing: Low tear resistance.
d.
Negatively affected by H2O, saliva, and blood.
(1) Since hydrophobic, moisture increases marginal discrepancy
(2) Increased water absorption occurs if use thinning agents e.
Can be dispensed from automated extruder and mixer (ESPE PentaMix)
Image source: Undetermined

Chemistry and Setting Reactions
CONTINUOUS PHASE:
Polymer
Crosslinking Agent
Catalysts
Modifiers
DISPERSED PHASE:
Fillers
= Double-bond-functional Silicone Polymer
= Chloroplatinic Acid
=
= Colorants, Flavorants, Plasticizers
= Silica
SETTING REACTION:
Chain (very fast), Exothermic, Affected by temperature
Hydrogen gas released by decomposition of crosslinking agent.

Manipulation and Technique Considerations a.
BEST impression material for dimensional stability:
Pouring should be delayed at least 4 hours for H
2 out-gassing.
Pouring can be delayed up to 7-to-10 days (or indefinitely).
b.
Stiffness makes removal difficult.
c.
Most material dispensed using auto-mixing gun and mixing tips

1980-1995
Predominantly
POLYSULFIDE and
SILICONE elastomers before 1995.

1996-2004
Predominantly POLYETHER and
PVS elastomers after 1996.

Comparison of Key Properties
Poor >---------------------------------> Good
1.
Accuracy (Reproduction of Detail): a.
Wetting of Tissues: b.
Wetting by Dental Stone:
ALG < SIL, PS < PE <= RHC, PVS
SIL, PVS < PS < PE <= ALG, RHC
ALG <= RHC < PS, SIL < PE, PVS 2.
Dimensional Stability:
(Resistance to Distortion): a.
Polymerization Shrinkage b.
Thermal Shrinkage c.
Loss of Components d.
Stress Relaxation
3.
Tear Resistance (Elasticity): a.
Elastic Deformation b.
Strain Rate Sensitivity
ALG, RHC, PE < PVS , SIL < PS

Dimensional Stabilty
PRODUCT EXAMPLES