– The mixed material is loaded into the tray and placed in the mouth as a viscous paste. – The side of the spatula blade is used to scrape unmixed material off the paper pad and then to mix it into the rest of the material. – Mixing may take from 30 to 90 seconds depending on the amount and viscosity of the material. – The pastes are swirled and stropped together until a homogeneous paste is obtained. – The impression material spatula has a long (~4”), straight-sided blade. – An impression material spatula is used to mix the two pastes. – The two pastes are dispensed in equal lengths on a paper mixing pad. – A small percentage of sulfur is also included in the brown paste because it promotes the polymerization reaction. – The brown “accelerator” paste contains lead oxide and an “oily” organic chemical that does not react. – The white “base” paste contains a low-molecular-weight polysulfide polymer mixed with an inorganic filler, such as titanium oxide. – Typically, one paste is dark brown, and the other paste is white. – Polysulfide impression materials are supplied as two pastes in tubes. Composition of Polysulfide Impression Materials – The name “condensation” polymerization is based on the production of water. – Reaction by-products other than water are produced by other condensation polymerization reactions, but water is the most common by-product. – The functional groups of polysulfide impression material are mercaptan groups (sulfur and hydrogen atoms) and oxygen from lead oxide. – The functional groups of the monomers can be carboxylic acid and amine groups that produce proteins (or nylon). – In a typical condensation reaction, a hydrogen atom and a hydroxyl group (OH) are taken from monomers and are combined to form water (H2O). – Biologic polymers (proteins) are important components of the tissues of the body. – This is the same chemical reaction that joins together the building blocks of biologic polymers. – Polysulfide materials set via a condensation polymerization reaction. – Each impression material has its own adhesive, which will not work with other types of material. – One very important difference between types of elastomeric impression materials is the adhesive that is used to bond the impression material to a non perforated tray. – They have similar, but not identical, mixing and handling properties. – They all undergo cross-linking and chain lengthening polymerization reactions. Polysulfide, polyether, and silicone materials are also called non aqueous elastomeric impression materials. – Often, polysulfide materials are called “rubber” or “rubber base” materials, even though polyether and silicone materials are also rubber materials. – Polysulfide impression material was the first non aqueous elastomeric “rubber” impression material developed for dentistry.
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