3-14. TYPES OF IMPRESSION MATERIALS
Impression materials are of three types: the rigid type, the thermoplastic type,
and the elastic type. Thermoplastic materials soften when warmed and harden when
cooled with no change in chemical makeup. Elastic materials expand and contract with
no change in structure or shape. All three types have advantages and disadvantages.
The dental officer determines which material best meets the requirements of each
particular case. He frequently will use two or more materials to make a single
impression.
3-15. ALGINATE-TYPE HYDROCOLLOID -- CHARACTERISTICS
a. General. The alginate-type hydrocolloids are an elastic type impression
material. An alginate is a salt of alginic acid (an extract from seaweed). Alginate-type
hydrocolloids gel by chemical action. Once the gelation process begins, it is
irreversible.
b. Composition. The composition of the alginate-type hydrocolloids varies with
different manufacturers. The basic components are a soluble alginate (either potassium
alginate or sodium alginate) and a reactor (calcium sulfate) that causes the alginate to
gel. The product also contains a retarder (sodium or potassium sulfate, oxalate, or
carbonate) to prevent gelation from occurring too rapidly. A fluoride is usually added to
prevent retardation of the setting time of the casts. The remainder of the material is
composed of fillers that increase the strength and stiffness of the gel.
c. Usage. Alginate-type hydrocolloids are supplied in powder form, either in
bulk or in measured portions packaged in foil envelopes. The powder must be stored in
a cool place. The bulk form must be kept in a tightly closed container to protect it from
contamination and to prevent it from absorbing moisture from the air. The containers
are agitated to loosen the bulk powders before they are measured, thus preventing use
of an excessive proportion of the powder. The powder is mixed with a measured
amount of water. Further discussion follows below.
3-16. DISPENSING
Water and powder measuring cups are provided by the manufacturer in each can
of alginate-type material. Lines on the water measurer correspond to the number of
scoops of powder used. The ratio of use is 1 to 1 (for example, three scoops of powder
to three units of water). The actual amount will vary, depending on the size of the
impression tray. Prior to mixing, it is necessary to tumble the container in order to fluff
the powder.
MD0502
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