A nonionic surfactant can be added to the paste in the manufacturing process to render a degree of hydrophilicity to the surface of the material.
One of the disadvantages of the silicone impression materials (including condensation silicones) is their inherent hydrophobic nature. The impression should be left overnight if epoxy will be used for pouring models. Manufacturers may add a noble metal, such as palladium, as a scavenger for the released hydrogen gas. Nonetheless, the hydrogen gas evolved can result in pinpoint voids in the gypsum casts poured soon after removal of the impression from the mouth. Technically, hydrogen gas is a reaction by-product that does not affect the dimensional stability of the impression. However, the residual polymethylhydrosiloxane in the material can lead to a secondary reaction with each other or with moisture, to produce hydrogen gas. No reaction by-products are formed as long as the correct proportions of divinylpolysiloxane and polymethylhydrosiloxane are used and there are no impurities. The zigzag line at the other end of divinylpolysiloxane represents repeating units of dimethylsiloxane with a vinyl terminal. Final structure after the platinum salt has initiated the addition polymerization reaction ( bottom). Hydrogen atoms along the backbone structure of the vinyl silicone chain move to the vinyl groups during addition polymerization ( top). Loss of this small molecule from the set material has a significant effect on the dimensional stability of the impression.įIGURE 8-5 Polymerization of addition silicone.
The reaction yields water as a by-product. In particular, hot and humid conditions will accelerate the setting of polysulfide impression material. Moisture and temperature have a significant effect on the course of the reaction. During the final set, a material of adequate elasticity and strength is formed that can be removed past undercuts quite readily. At this stage, a resilient network has started to form. The reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete ( Figure 8-3). Since the composition of the material in the tube is balanced with that of the accelerator, the matched tubes supplied by the manufacturer should always be used. Reactor is a more appropriate term for the reactions associated with polysulfide and other types of impression materials.Įach paste is supplied in a dispensing tube with appropriately sized bore diameters at the tip so that equal lengths of each paste are extruded from each tube to provide the correct ratio of polymer to cross-linking agent. The terms catalyst and accelerator used here and with other impression materials are actually misnomers.
Lead dioxide is the component that gives polysulfide impression material its characteristic brown color. The catalyst (or accelerator) paste contains lead dioxide, filler, and plasticizer as in the base paste, and oleic or stearic acid as a retarder to control the rate of the setting reaction. The base paste, is a polysulfide polymer that contains a multifunctional mercaptan (-SH) called a polysulfide polymer, a suitable filler (such as lithopone or titanium dioxide) to provide the required strength, a plasticizer (such as dibutyl phthalate) to confer the appropriate viscosity to the paste, and a small quantity of sulfur, approximately 0.5%, as an accelerator. This chapter discusses the unique properties of currently used impression materials and describes how these characteristics affect the quality of an impression and of the cast or model made from the impression. To produce accurate replicas of intra- and extraoral tissues, the impression materials should be (1) sufficiently fluid to adapt to the oral tissues, (2) viscous enough to be contained in a tray, (3) able to transform (set) into a rubbery or rigid solid in the mouth in a reasonable time (less than 7 min), (4) resistant to distortion or tearing when removed from the mouth, (5) dimensionally stable long enough to allow one or more casts to be poured, (6) biocompatible, and (7) cost-effective in terms of time as well as the expense of the associated processing equipment.Įnvironmental conditions and the type of tissue dictate the choice of materials, quality of the impression, and quality of the cast. Which of the seven criteria that ensure accurate impression making are related to the time the impression material is in the mouth? Which one is related primarily to the properties of a set impression?
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