Elastomeric Impression Materials Irreversible; Non-aqueous

Impression materials – elastic impression materials hydrocolloids (aqueous) agar (reversible) alginate (irreversible) elastomers (nonaqueous) polysulfide polyether silicone (condensation, addition)

Elastomeric polymers: elasticity Undisturbed (unstretched) polymer molecules naturally curl up covalent bonds along polymer back-bone are randomly directional, imparting a random curvature to the molecule. The curled configuration is the lowest energy state of the molecule.

Elastomeric polymers: elasticity After stretching, the polymer molecules will return to the curled (lowest energy) configuration. completely elastic (in principle) not completely elastic because polymer molecules slide with respect to one another while the ensemble is stretched

Elastomeric polymers: elasticity When stretched, polymer molecules line up parallel to one another. Sliding between polymer molecules is resisted by: van der Waals forces; hydrogen bonding tangling among polymer molecules cross links between polymer molecules

Polysulfide Elastomer Impression Materials

Polysulfide & other elastomeric impression materials were traditionally supplied in two tubes, one containing a “base” paste and the other containing a “catalyst” paste.

Polysulfides - development used in industry since 1929 liquid polysulfides developed by Thiokol labs in 1943 used in dentistry since early 1950s

Polysulfides – base polymer low molecular weight polymer (2000 – 4000 amu) terminal mercaptan groups – SH condensation reaction

Base paste Catalyst paste

Polysulfide – Base Paste Polysulfide Polymer Filler 74% balance

Polysulfide – Catalyst Paste Lead dioxide Dibutyl phthalate Sulfur Other ingredients 78% 17% 3% 2%

Polysulfide – reactions scheme: polysulfide base + lead dioxide polysulfide rubber + water

polymerization of polysulfide oligomers C S R m H 3 n S H SH H S R n S H H S H C 3

Rm = - CH2 - CH2 - O - CH2 - CH2 - Rn = - CH2 - CH2 - O - CH2 - S - S -

polymerization of polysulfide oligomers C S R m H 3 n oxygen from PbO 2 O S H SH H S R n mercaptan S H H S H C 3

forms sulfur – sulfur bonds C H H C H S S R S S C S S R S S R S S S S 3 H C H S S R S S C S S R S S R n m n S S S S C S S + 3 H 2 O H C H by-product cross-link C H 3

The dimensional stability of polysulfide impressions along the mesial and distal dimensions of a crown die is poor. PS impressions release water as a by-product of the polymerization reaction. Evaporation of the water causes the dimensional change. After making a PS impression, one must pour a stone model within the 1st hour to minimize the loss of accuracy due to evaporation of the by-product. Lacy AM, Fukui H, Bellman T & Jendresen MD (1981) Journal of Prosthetic Dentistry 45(3) 329-33.

The dimensions of stone dies are being measured here. Note that the die appears to expand with time. Lacy AM, Fukui H, Bellman T & Jendresen MD (1981) Journal of Prosthetic Dentistry 45(3) 329-33.

Dies are larger than the tooth when the impression material is bonded to the tray with adhesive. Shrinkage is toward the custom tray. If the contraction of the impression material continues, dies poured later will be larger than dies pour earlier.

Condensation Silicone Elastomeric Impression Materials Condensation silicones are no longer used in U.S. dental practices.

Condensation Silicones Wash Putty

Elastomeric Impression Materials Polyether Elastomeric Impression Materials

Polyether Impression Pastes Composition of Polyether Impression Pastes Base Catalyst polyether prepolymer aromatic alkyl sulfonate silica filler glycoether phthalate (plasticizer) glycoether phthelate

polyether prepolymer Like addition silicones, the reaction occurs 3 N 2 O x R n p ethyleneimine rings rings open during polymerization – possibly less polymerization shrinkage. polyether prepolymer Like addition silicones, the reaction occurs without a by-product.

S O R C l C l alkyl dichlorobenzene sulfonate 3 Catalyst for polyether polymerization reaction – difficult to handle – carcinogen when separate

Polyethers – setting time, stiffness Polyether will set more quickly than polysulfide and is stiffer when set. The setting of both impression materials is sensitive to temperature, but polysulfides are more sensitive to temperature. Reciprocating rheometer

Dimensional change with time: 2 polysulfides & 2 polyethers condensation silicone low humidity: Storage at high humidity will expand a PE impression. Resulting stone will be smaller than actual tooth. A crown built from this stone die will be too small. 2 polyethers condensation silicone high humidity:

Electrical mixer - dispensers: Pentamix 2 dispenser photos: 3M Espe; 2005 Polyether is very viscous – difficult to stir. Electrical mixer makes sense.

Polyethers – brands: varying viscosity Impregum Penta Soft (3M Espe) * Permadyne (3M Espe) * Impregum (3M Espe) P2 (Hereaus Kulzer) * Not sold as monophase

Polyethers - advantages less hydrophobic than others – inherently hydrophobic not affected by latex gloves dimensional stability good, except at high humidity

Polyethers - disadvantages very stiff – can loosen periodontally compromised teeth poor taste & smell may distort during disinfection (keep immersion brief)

Elastomeric Impression Materials Addition Silicone Elastomeric Impression Materials

Composition of Addition Silicone Pastes Base Catalyst poly(methyl siloxane) vinyl groups are substituted for a small percentage of the methyl groups poly (methyl siloxane) hydrogens are substituted for a small percentage of the methyl groups platinum and palladium powder chloroplatinic acid (or some other platinum-containing compound) filler

H2PtCl6 increases the reaction rate 106 times Addition Silicones - catalyst Chloroplatinic Acid: H2PtCl6 increases the reaction rate 106 times

Polymerization of addition silicone oligomers H H 3 H Si C H 3 O Si C C H + 2 O H C H 3 C H 3 H C Si H + H C C Si O 3 2 C H H O C H 3 3 O Si C C H + H Si C H 2 3 C H 3 Some products release hydrogen during and after setting.

Polymerization of addition silicone oligomers siloxane backbone C H H H 3 O Si C C Si C H 3 C H H H O H H C H 3 3 H C Si C C Si O 3 chloroplatinic acid catalyst C H H H O H H C H 3 3 O Si C C Si C H 3 C H H H 3 Notice that there is no by-product. The siloxane backbone is cross-linked in three dimensions.

Addition silicone elastomeric impression materials: Current material: Extrude TM Kerr Corp. dual paste cartridge mixing tip

Addition silicone elastomeric impression materials:

Addition silicone elastomeric impression materials:

Addition silicones – mixing pseudoplastic – becomes less viscous as shear rate increases automix syringes reduces voids by 25%

The dimensional stability of addition silicone impressions along the mesial and distal dimensions of a crown die is relatively good. AS impressions do not release a by-product during polymerization and, consequently, are very stable. Lacy AM, Fukui H, Bellman T & Jendresen MD (1981) Journal of Prosthetic Dentistry 45(3) 329-33. When using addition silicone impression materials, one does not have to pour the stone immediately. It is possible to ship the impression to the lab and allow the lab to pour the stone model.

Addition silicones – brands: varying viscosity hydrophilic brands*: Exafast NDS (GC America) Affinis (Coltene / Whaledent) Imprint II (3M Espe) Aquasil (Dentsply / Caulk) * All the above are sold in light, medium, heavy, and sometimes putty viscosity * Not sold as monophase

Addition silicones – brands: varying viscosity hydrophilic brands*: Honigum (Zenith/DMG) Standout (Kerr) * Virtual (Ivoclar Vivadent) Aqualsil Ultra (Dentsply / Caulk) All the above are sold in light, medium, heavy, and sometimes putty viscosity * Not sold as monophase

Addition silicones – disinfection impressions can be immersed for 30 minutes in aqueous solution of either 1:10 hypochlorite or 2% glutaralde-hyde without ill effect

Addition silicones – setting retardation setting of putties may be retarded by some latex gloves & rubber dams sulfur in the latex is the problem – vinyl gloves do not retard setting

Addition silicones - advantages tasteless; odorless dimensional stability good relatively easy removal from mouth can be disinfected without dimensional changes (immerse briefly)

Addition silicones - disadvantages set may be retarded by latex gloves not inherently hydrophilic; moisture from saliva, blood etc. can interfere; surfactant-containing overcome this problem, but how well?

Elastomers Comparing Properties

Properties of Elastomeric Impression Materials - Working Times Good while learning, longer than desirable in practice.

Mechanical properties of Elastomeric Impression Materials - Permanent Deformation Least elastic.

Reducing permanent deformation: Mechanical properties of Elastomeric Impression Materials - Permanent Deformation Reducing permanent deformation: Reduced if deformation is brief: remove impressions quickly Reduced if there is less deformation: larger undercuts will produce more permanent deformation

Mechanical properties of Elastomeric Impression Materials - Strain in Compression Very flexible. Some very stiff Very stiff

Mechanical properties of Elastomeric Impression Materials - Tear Strength an advantage for polysulfides

Dimensional Accuracy of Stone Casts Poured One Hour after an Impression was Made ** All these are acceptable. ** Johnson & Craig. J Prosthet Dent 1985;53:484-490. Custom trays were used to make the PS, PE & AS impressions.

Dimensional Accuracy of Stone Casts Poured One Hour after an Impression was Made This is a concern. The stone die is too small, so the casting made on the die will not seat. Johnson & Craig. J Prosthet Dent 1985;53:484-490. * Custom trays were used to make the PS, PE & AS impressions.

Properties of Elastomeric Impression Materials - Dimensional Change at 24 hours very stable; note that there is still polymerization shrinkage.

Properties of Elastomeric Impression Materials - Contact Angles / Stone Castability Low contact angles are associated with more complete fill of elastomer molds by stone.

Properties of Elastomeric Impression Materials - Contact Angles / Stone Castability Adding surfactants to addition silicones elastomers decreases the contact angle and increases fill of elastomer molds by liquid stone

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