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Elastomeric Impression Materials

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Text of Elastomeric Impression Materials

Elastomers refer to a group of rubbery polymers, which are either chemically or physically cross-linked. They can be easily stretched and rapidly recover their original dimensions when the applied stress is released.

Chemically, there are four kinds of elastomeric impression materials: 1.polysulfide, 2. condensation-polymerizing silicone, 3.addition-polymerizing silicone, and 4.polyether

According to viscosity Each type is further divided into four viscosity classes, light-body medium-body or regular-body heavy body putty

ADA classification Type I Type II Type III

Based on cross-linking poly addition reaction of vinyl terminated poly siloxane polymer with methylhydrogen silicone cross-linking agent in the presence of platinum catalyst.

Pt may release H2 from water or OH groups from the reaction mixture causing bubbles on a gypsum model

Base paste: 1. Vinyl terminated polysiloxane polymer 2. Filler (cristobalite, talc) 3.Surfactant (hydrophilic agent)

Catalyst: 1. Vinyl terminated polysiloxane polymer 2. Cross-linking agent 3. Filler, silicone oil (adjust viscosity of duplicating materials) 4.Platinum salt

1. 2. 3. 4. 5. 6.

Accurate impressions, very low shrinkage Very good surface detail Highly elastic Perfect elastic recovery Dimensionally stable Non-toxic and non-irritant

1.Hydrophobic necessary to add a surfactant 2. Setting inhibited by latex gloves or some astringents (sulfur, heavy metals) 3. Hydrogen release which results in pinpoint voids in gypsum casts .This can be compensated by adding a noble metal or by waiting an hour before pouring the impression.

Based on cross-linking polycondensation reaction of hydroxy terminated polysiloxane prepolymer with tetra alkoxy silanes in the presence of stannous octoate.

Alcohol releases from the reaction mixture and contributes to contraction or shrinkage of the impression

Base paste: Hydroxyterminated polysiloxane polymer Filler (cristobalite, talc, starch)

Catalyst: Liquid catalyst: Ortho ethyl slicate and stannous octoate Paste catalyst: Cross-linking agent, activator, inert oil filler

1. Accurate impressions when poured soon 2. Good elastic recovery 3. Lower price

1. Hydrophobic 2. Shrinking of impression over time 3. Catalyst may cause allergic reaction 4. Difficult to dispense proper volumes of both components 5. Usually hand-mix version only

Based on cross-linking of polyether chains via cationic polymerization of aziridine rings using an aromatic sulfonate ester as an initiator

Base paste: Polyether Filler, plasticizer Catalyst paste: Sulphonic acid ester Inert oil filler

1. 2. 3. 4. 5.

Naturally hydrophillic Accurate and high dimensional stability Good elastic recovery Low setting contraction Excellent surface detail reproduction

1. Rather stiff when set material (difficult to remove from mouth) 2. Very expensive 3. May cause allergic reaction due to the sulphonic acid ester

Based on reaction between polymer with free mercaptan (-SH) groups and oxidizing agent PbO2 which lenghtens and cross-links chains via reaction of terminal and pendant -SH groups.

Supplied in a two paste system Base paste: polysulfide polymer, filler, plasticizer

Catalyst paste: lead dioxide, sulfur, inert oilVolume mixing ratio 1:1

Long working time Low price

1. Should be poured within 0.5 1 hour 2. Lead oxide may cause toxic effects 3. Unpleasant mercaptane smell 4. Long setting time app. to 10 min 5. Poor elastic recovery prone to plastic deformation

Excellent

reproduction of surface details

Dimensional

stability: sources of dimensional change 1.polymerization shrinkage 2.loss of by-product 3.thermal contraction from oral temperature to room temperature 4.imbibition

The dimensional change is greater in magnitude for the polysulfide and condensation silicone than for the poly ether and addition silicone[because poly sulfide and condensation silicone lose polymerization by-products ]

Tear strength of these materials are exellent,thus making it more resistant to tearing even when the impression is in thin sections Polysulfide has highest tear strength.

Shelf life: storage under cool conditions increases shelf life.The shelf life is about 2 years.The silicones have a reduced shelf life.

Hydrophobic material (>90o ) unable to wet humid Surfaces

Hydrophilic material able to wet humid surfaces

1. 2. 3. 4.

Preparing a tray Preparing the material Making an impression Removal of the impression

Impressions are usually made in special trays to minimize the effects of polymerization shrinkage .Perforated stock trays are used only for making impression in putty. Adhesion of impression material to trays also essential.Thus an adhesive should be applied onto the tray.

When the materials are provided in two paste tubes, dispense the same length of materials onto a mixing pad or glass slab. The catalyst paste is first collected on a stainless steel spatula and then distributed all the base. The mixture is then spread out over the mixing pad.

The mass is then scraped up with the spatula blade and spread uniformly back and forth on the mixing pad.

Single mix technique custom tray with 2-3mm spacing is used.regular body viscosity is used Method :The paste is mixed and part of it is loaded on to the tray and part in to a syringe.The syringe material is then injected on to the prepared area of impression.

Multiple mix technique: Method of using the syringe and tray materials. custom tray with 2-4 mm spacing is used.Light and heavy body viscosity are used Method: The two viscosities are mixed simultaneously but on separate pads.The tray is then filled with a uniform thickness of heavy material,and syringe is loaded with lighter material.

The filled tray is then inserted in the mouth and seated over the syringe material that has been extruded on hard or soft tissue.

Developed for condensation silicone to minimize the effect of polymerization shrinkage. Two approaches can be used:a two stage procedure and a single stage procedure

Two stage putty wash technique or reline technique: The thick putty material is placed in a stock tray and a preliminary impression is made.It acts as custom made tray.A mix of thin consistency wash material is placed in to the putty impression and then the putty wash combination and tray are seated in the mouth to make the final impression.

Single stage putty wash technique: In this the wash material is syringed in to place and then the unset putty is seated over the light body material.

The impression is dislodged from the mouth as quickly as possible for the following reason 1.Elastic recovery is better 2.Tear resistence is higher

Thank you