Impression Materials PPT

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IMPRESSION MATERIALSPart 1 and Part 2

Stephen C. BayneDepartment of Operative Dentistry School of Dentistry University of North Carolina Chapel Hill, NC 27599-7450

What are tolerable limits for error in indirect procedures? a. b. c. d. e. f. g. Impressions Casts, Dies Waxing Investing Casting Finishing, Polishing Cementation = = = = = = = +/- 0 +/- 0 +/- 0 + 1.5% 1.5% +/- 0 +/- 0

PROBLEM ANALYSIS

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

CLASSIFICATION SYSTEMBased on Properties of Set MaterialsProperties: Impression Plaster Impression Compound Zinc Oxide/ Eugenol Rigid Rigid Rigid Flexible Flexible Flexible Flexible Flexible Flexible Reaction: Set: Irrev Rev Irrev Irrev Rev Irrev Irrev Irrev Irrev (Chem) (Phys) (Chem) (Chem) (Phys) (Chem) (Chem) (Chem) (Chem)

Rigid:

1. 2. 3.

Water-Based Gel: 4. Alginate (Irreversible Hydrocolloid) 5. Agar-Agar (Reversible Hydrocolloid)

Elastomers: 6. Polysulfide (Rubber Base, Thiokol) 7. Silicone (Conventional, Condensation) 8. Polyether 9. Polyvinyl Siloxane (Addition Silicone)

IMPRESSION MATERIALSKey Propertiesa. 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

GENERAL FORMULATIONFor 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) Filler or Extender (Dispersed Phase):Polymer (high shrinkage) Filled Polymer (low shrinkage)

2.

ELASTOMERIC IMPRESSIONSManagement of ShrinkageImpression Tray Impression

Use 2-step techniques: (a) Light-Heavy Body, OR (b) Wash-Putty

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.

ELASTOMERIC MATERIALSMixing and Delivery SystemsPolyether, Polyvinylsiloxane

Polysulfide Rubber, Silicone Rubber

2x2x2x2x2x2x2x2 = 256 folds

Mixing Options: 2 Pastes on Mixing Pad 2 Pastes in Mixing Gun 2 Pastes in Mixing Machine

IMPRESSION MATERIALSManagement of Distortion During Tray RemovalStrain rate sensitive elastomers !

Impression Tray Impression

Fast removal VERY FAST (SNAP)

Slow removal

IMPRESSION MATERIALSDistortion Time Related to Setting Reaction VPS Pe PS, Silicone

REACTION CONVERSION (%)

100

log TIME (minutes)

POLYSULFIDE RUBBERChemistry and Setting ReactionCONTINUOUS 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

POLYSULFIDE RUBBERManipulation and Technique Considerationsa. b. c.* d. e. f. Two-step techniques recommended: Reduces air entrapment and surface tension effects. Material adversely affected by H2O, saliva, and blood. Set impression should be removed quickly - do not rock tray. No syneresis or imbibition, but distortion due to continued reaction. Ideally need uniform thickness and at least 2 mm thick for accuracy. (1) Adhesive must be thin (2) Adhesive must be dry 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 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

g.

SILICONE RUBBERChemistry and Setting ReactionCONTINUOUS 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 H2O by-products

Manipulation and Technique Considerations

SILICONE RUBBER

a. b. c. d. e.

Limited shelf-life: Unstable in tubes. Requires mechanical retention or special tray adhesives No syneresis or imbibition, but continued polymerization shrinkage. Better dimensional stability than RHC but more expensive 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

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

POLYETHER RUBBER

POLYVINYL SILOXANEChemistry and Setting ReactionsCONTINUOUS 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 Considerationsa. b. c. BEST impression material for dimensional stability: Pouring should be delayed at least 4 hours for H2 out-gassing. Pouring can be delayed up to 7-to-10 days (or indefinitely). Stiffness makes removal difficult. Most material dispensed using auto-mixing gun and mixing tips

POLVINYLSILOXANE

COMMERCIAL PRODUCTS1980-1995

Predominantly POLYSULFIDE and SILICONE elastomers before 1995.

COMMERCIAL PRODUCTS1996-2004

Predominantly POLYETHER and PVS elastomers after 1996.

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