![]() Use of a highly hydrophobic material under such conditions can produce an impression with microscopic defects (bubbles and holes) especially in areas near the gingival margin. Hydrophobia can also be a disadvantage if the field is not perfectly clean and traces of saliva or blood are present. (4) Condensation silicones are not adhesive, so suitable adhesives must be used when working with non-retentive impression trays. Contraction is caused by the release of alcohol molecules as a by-product of the condensation reaction. Dimensional stability is poorer than with addition silicones. They do, however, have their disadvantages too. (3)Īnother important benefit of these materials is that they are hydrophobic. They are therefore not liable to problems of syneresis or imbibition, meaning that their dimensional stability is not affected by ambient humidity. They are also cheap to buy, especially compared to addition silicones and other elastomers, and are extremely versatile: they can be used for fixed and removable prostheses, are compatible with standard and individual impression trays, and can be used in single and double step techniques. Mechanical characteristics of condensation siliconesĬondensation silicones have excellent mechanical characteristicsincluding very high elasticity, which allows them to overcome undercuts without suffering any significant or permanent deformation. Reaction time is also directly proportional to ambient temperature: reaction is faster in warm rooms and slower in cold rooms. Mixing and working times are generally short, so a certain amount of experience is needed to work the material properly within the given time frames. They have to be mixed manually using a metallic spatula in the case of the more fluid types. All the reactions involved occur at ambient temperature and for this reason condensation silicones are often referred to as RTV (room temperature vulcanisation) silicones. Volatile substances (alcohols) are released during the condensation reaction. Tin(ii) octoate is the most common activator. These substances react when a liquid or paste activator is added. They consist of a base molecule, dimethylsiloxane (a silicone polymer) with metal oxides and silica powders added to increase density. Condensation siliconesĬondensation silicones (polydimethylsiloxanes or first generation silicones) were first introduced for dental applications in the 1950s (2). ![]() Impression silicones come in two types: addition and condensation. ![]() Elastomers are also used to make other materials in addition to impression silicones. Some of these, like hydrocolloids, are intended only for preliminary impressions while others, like elastomers, offer better performance and can therefore be used for final, precision impressions. There are a many different impression materials on the market.
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