الفهرس 
AIM OF THE STUDYOnly 14 pages are availabe for public view
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Abstract
Elastomers are rubber-based materials used for dental impressions taking that can be classified into four groups according to the polymer constituent:
Poly sulfide (PS), polyether (PE), polyvinyl siloxane (PVS) and condensation silicone (CS). Poly sulfides present high tear resistance, good reproduction of details, and low cost. Poly ethers exhibit intermediate tear resistance, excellent detail reproduction, and high cost. PVS provides excellent tear strength, a good working time, and excellent elastic recovery, whereas condensation silicones have low tear strength and demonstrate greater distortion (1).
An ideal impression material should present, among other characteristics, dimensional stability over long periods, which allow the production of precise cast models at any time. However, the materials commonly used in dental impressions present alterations in their dimensional behavior. The release of water and ethanol as by-products of the polymerization of polysulfide and condensation silicone, respectively, and polyether hydrophilic behavior may affect their dimensional stability. Due to the absence of by-products in PVS polymerization, it presents the most favorable dimensional behavior (1-5).
The dimensional changes of the impression materials may affect the quality of fit and retention of dental prostheses, which influence the success of indirect restorative procedures (4,6-10) .The dimensional behavior of impression material, is influenced by humidity, the time interval from mixing to pouring, and the thickness of the layer of material in the tray. In addition, impression materials contract with the temperature change from the oral cavity to the external environment due to their linear expansion thermal coefficient (9, 11).
Volumetric changes are also related to the type of tray, the degree of adhesion between the tray and materials, and the type of polymer comprising the elastomers (11). The impression technique can be performed using single or double steps, which can lead to different outcomes with respect to the dimensional accuracy (8, 12).
New generation of impression material keep immersing in dental market every day, so more investigations are needed to study their properties and quality their uses.