E LASTOMERIC IMPRESSION MATERIALS

Elastomers refer to a group of rubber 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.

Polymerization Is formation of large polymer molecule from smaller monomer molecules. 1. Condensation polymerization Is formation of large polymer molecule with the release of by-product. 2. Addition polymerization Is formation of large polymer molecule with no release of by-product.

T YPES OF ELASTOMERS Polysulfide Condensation polymerizing silicone Addition polymerizing silicone Polyether

Advantages of elastomeric impression materials over the hydrocolloid Elastomeric impression materials have higher tear strength than hydrocolloid. Elastomers have no synersis or imbibition like hydrocolloids. They are more dimensionally stable on storage. Most of elastomers can be electroplated.

C ONSISTENCIES Elastomeric impression materials are typically supplied in several consistencies (viscosities) to accommodate a range of impression techniques. Addition silicones are available: extra-low, low (syringe or wash), medium (regular), monophase, high (tray), and putty (extra-high) consistencies. Polyether impression materials are now available in low, medium, monophase, and high consistencies.

M IXING S YSTEMS Two types of systems are available to mix the catalyst and base thoroughly before taking the impression: static automixing and dynamic mechanical mixing

I MPRESSION T ECHNIQUES Three common methods for making impressions for fixed restorations are a simultaneous, dual- viscosity technique, a single-viscosity or monophase technique, and a putty-wash technique.

I. P OLYSULFIDE IMPRESSION MATERIALS Supplied as two pastes in collapsible tubes, one labeled base and the other labeled accelerator or catalyst. The polysulfide polymer has terminal and pendant mercaptan groups (-SH). The terminal and pendant groups of adjacent molecules are oxidized by the accelerator (lead dioxide) to produce chain extension and crosslinking, respectively. Water is formed as a byproduct.

Polysulfide impression materials are supplied in three consistencies: low (syringe or wash), medium (regular), and high (tray).

ADVANTAGES Good wettability Good surface detail High tear strength Long working time (although this may be a disadvantage in some clinical situations) Radiopaque

DISADVANTAGES High permanent deformation Staining of clothes due to the Lead oxide Messy to work with - unpleasant rubbery smell Can only be used in a special tray Must pour within 1 hour

II. C ONDENSATION SILICONE It is present as two components system: The base paste which present in a tube. The catalyst in the form of paste in a tube or liquid in a dropper.

Composition The base paste is composed of low molecular weight silicone polymer (Dimethyl siloxane) with terminal OH group and filler. The catalyst contains the alkyl silicate cross- linking and stannous octoate (or tin dilaurate) which is essential for activating the setting reaction. Supplied in low ( contain 35% of filler)and putty (75% of filler) consistencies.

S ETTING REACTION An exthothermic condensation polymerization reaction. Cross-linking occurs between the siloxane polymer and the alkyl silicate in the presence of the activator with the release of ethyl alcohol as reaction by-product. Evaporation of alcohol →higher degree of dimensional changes than that exhibited with polysulfide in storage. Alcohol could cause air bubbles in the stone cast.

A DDITION SILICONE Composition The base paste contains low molecular weight siloxane polymer (Hydrided or silane terminated silicone). Siloxane pre-polymers. Fillers. The accelerator contains Divinyl poly(dimethyl siloxane). Platinum salt activator. Siloxane pre-polymers. Fillers. Supplied in extra low, low, medium, heavy, and very heavy consistencies

S ETTING REACTION An exthothermic addition polymerization reaction with no release of reaction by-product. A side reaction releasing hydrogen gas may occur To overcome this  Manufacturers often add a noble metal as platinum or palladium to act as a scavenger.  To wait an hour or longer before pouring up the impression.

Silicones are hydrophobic, manufacturers have made the addition silicone more hydrophilic by adding surfactants (surface tension reducing agent) to the paste →this allows the impression material to wet soft tissue better. →to be poured in stone more effectively. The hydrophilization of addition silicones is gained with the incorporation of nonionic surfactants. These molecules consists of a hydrophilic part and a silicone compatible hydrophopic part.

It should be noted that latex gloves may interfere with setting of addition silicone impression materials. Vinyl and nitrile gloves do not have such an effect.

POLYETHER Composition Base paste: Polyether prepolymer with terminal ethylene- imine groups. Fillers Plasticizers Catalyst paste Aromatic sulfonic acid ester Fillers Plasticizers

A DVANTAGES Accurate and fine details Good dimensional stability during setting Good elastic recovery of 98.9% Compatible with cast & die materials Pour within one week (if kept dry) Good wettability (hydrophilic)

D ISADVANTAGES Stiff (difficult removal from the mouth) Expensive Short setting time (3-5 min) Bitter taste Absorbs water and changes dimension

Properties of elastomeric imp. materials Working and setting time Working time is measured at room temperature, while setting time is measured at mouth temp. Working and setting time are shortened by increases in temp. and humidity. Altering the base/catalyst ratio will change the curing rate but the mechanical properties can be adversely affected and this is not economic.

Dimensional changes on setting The order of dimensional changes is: condensation silicones > polysulfide > polyether > addition silicones. For greatest accuracy the models and dies should poured immediately for condensation silicones and polysulfide. Some addition silicones release hydrogen after setting, so one should not pour gypsum models and dies until after 1 to 2 hours

P ERMANENT DEFORMATION The addition silicones have the best recovery from deformation during removal from the mouth, followed by the condensation silicones, the polyether, and the polysulfide. Tear strength Elastomeric impression materials are viscoelastic, and their mechanical properties are time dependent. For example, the higher the rate of deformation, the higher the tear strength From greatest to least polysulfide > addition silicone > polyether > condensation silicone.

B IOCOMPATIBILITY These materials are highly biocompatible. Leaving a piece of material during the removal of the impression →severe gingival inflammation. Disinfection of Elastomeric Impressions All impressions should be disinfected upon removal from the mouth to prevent transmission of organisms to gypsum casts and to laboratory personnel.

What are the advantages and disadvantages of condensation and addition silicones?