After the retarder (the phosphate ions) has reacted, the setting reaction becomes the dominant reaction. The calcium ions do not react with the dissolved alginate until all the phosphate ions have reacted. Initially, the calcium ions react with the phosphate ions and precipitate out of solution. When the powder is mixed with water, the alginate powder dissolves, as does the calcium sulfate and sodium phosphate. Working time is provided by a competing reaction that initially delays cross-linking. – Alginate Impression materials set by a chemical reaction that cross-links a carbohydrate polymer. – 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.

Agar dental impression material