![]() ![]() – The hydrocolloid will absorb water, swell, and distort. – When disinfecting a hydrocolloid impression, however, it is important to limit the time that the impression is exposed to an aqueous disinfecting solution. – Syneresis occurs very slowly, but it is a second reason to pour hydrocolloid impressions as soon as possible. – Hydrocolloid materials contract slightly after setting and exude water. – To prevent evaporation of water from hydrocolloids (alginate and reversible hydrocolloid), impressions should be poured as soon as possible after disinfecting. – When the water evaporates, the impression shrinks and is no longer accurate. – The disadvantage of the material being predominantly water is that water evaporates from the surface of the impression if it is left exposed to air. – Hydrocolloid impression materials are quite hydrophilic, but the same cannot be said for all impression materials. – Again, the gypsum product, when mixed with water, easily wets the surface of the impression material, which is predominantly water. – The second advantage of the material being composed predominantly of water is that pouring the impression with gypsum products is easier than with elastomeric impression materials. – These two effects occur because water-based liquids mix together easily, like a mix of soda pop and fruit juice used to make punch. – Hydrocolloid materials will even absorb a limited amount of oral fluids. – Fewer air bubbles are trapped between the tooth and the impression material. – Reversible and irreversible hydrocolloid will wet a tooth surface that is contaminated by oral fluids. – Thus, several advantages and disadvantages result when their use is compared with that of other impression materials. – Both reversible and irreversible hydrocolloid materials are predominantly water. – The purpose is to reduce the microbial content of the powder as provided by the manufacturer more than to disinfect the impression after it has been taken. – Some alginate products have antimicrobial agents added to the powder. Several other chemicals are present in the powder: the reactor for the cross-linking reaction (calcium sulfate), the retarder for working time (sodium phosphate), colorants, and flavor. – Without filler, the mixed material would be too runny for use. – The filler gives the mixed material “body,” which allows acceptable handling. – Another main component of the powder is an inert filler, such as diatomaceous earth (silica). – Potassium alginate is derived from algae. – The reaction is similar to that of polycarboxylate and glass ionomer cements. – Carboxylate groups (–COOH) react with calcium ions and cross-link the material to form a gel. – It is a carbohydrate polymer that dissolves in water, forming a sol. – The powder contains potassium alginate. – Alginate materials are supplied as powders that are mixed with water. – Alginate materials are not as accurate as reversible hydrocolloid materials, but they are much easier to use. – They have advantages and disadvantages similar to those of reversible hydrocolloid materials because both types of materials are predominantly water. – Alginate Impression materials are termed “irreversible impression materials” because they will not reverse to the sol state once they react and become a gel.
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