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.

Introduction

Chemically, there are four kinds of elastomeric impression materials:

1.polysulfide, 2. condensation-polymerizing silicone,

3.addition-polymerizing silicone, and 4.polyether

Classification

According to viscosity

Each type is further divided into four viscosity classes,

light-body medium-body or regular-body heavy body putty

ADA classificationType IType IIType III

Supplied as

Automatic mixing device

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.

Addition silicone( vinyl siloxane, poly vinyl siloxane)

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 polymer2. Filler (cristobalite, talc)3.Surfactant (hydrophilic agent)

Composition:

Catalyst:1. Vinyl terminated polysiloxane polymer2. Cross-linking agent3. Filler, silicone oil (adjust viscosity ofduplicating materials)4.Platinum salt

1. Accurate impressions, very low shrinkage2. Very good surface detail3. Highly elastic4. Perfect elastic recovery5. Dimensionally stable6. Non-toxic and non-irritant

Advantages:

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.

Disadvantages

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

Condensation silicone

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

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

Composition:

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

1. Accurate impressions when poured soon2. Good elastic recovery3. Lower price

Advantages:

1. Hydrophobic2. Shrinking of impression over time3. Catalyst may cause allergic reaction4. Difficult to dispense proper volumes of

both components5. Usually hand-mix version only

Disadvantages

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

Polyether impression materials

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

Composition:

1. Naturally hydrophillic2. Accurate and high dimensional stability3. Good elastic recovery4. Low setting contraction5. Excellent surface detail reproduction

Advantages

1. Rather stiff when set material (difficult to remove from mouth)

2. Very expensive3. May cause allergic reaction due to the

sulphonic acid ester

Disadvantages

Based on reaction between polymer with freemercaptan (-SH) groups and oxidizing agent

PbO2 which lenghtens and cross-links chains via reaction of terminal and pendant -SH groups.

Polysulfide impression materials (Thiokol rubbers, mercaptan rubbers)

Supplied in a two paste system

Base paste: polysulfide polymer, filler, plasticizer

Catalyst paste: lead dioxide, sulfur, inert oil

Volume mixing ratio 1:1

Composition:

Long working time

Low price

Advantages

1. Should be poured within 0.5 – 1 hour2. Lead oxide may cause toxic effects3. Unpleasant mercaptane smell4. Long setting time app. to 10 min5. Poor elastic recovery – prone to plastic

deformation

Disadvantages

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

General properties

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.

They are generally hydrophobic

Hydrophilic material able to wet humid surfaces

Hydrophobic material(α>90o )unable to wet humidSurfaces

1. Preparing a tray2. Preparing the material3. Making an impression4. Removal of the impression

Making an 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.

Preparing a 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.

Preparation of Impression Materials

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 usedMethod :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.

Making an 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 usedMethod: 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

Putty wash technique

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 better2.Tear resistence is higher

Removal of impression

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