3-17. MIXING AND SETTING TIME
The best method of controlling the gelation time of alginate-type hydrocolloid
materials is to alter the temperature of the water used in the mix. The higher the
temperature of the water, the faster gelation will occur. Higher temperatures accelerate
the chemical reaction. The temperature of the water must be regulated carefully within
a few degrees of that recommended by the manufacturer to obtain a constant and
reliable gelation time. The average recommended temperature of water is 70 F (21 C).
Changing the water-powder ratio and the mixing time will alter the gelation time, but
these methods of control also impair certain properties of the material. The amount of
the retarder in the material can be regulated only by the manufacturer since the action
of the retarder changes the nature of the material.
The water-powder ratio recommended by the manufacturer must be used. Too
little or too much water will weaken the gel. Mixing must be timed. Undermixing may
prevent the chemical reaction from occurring evenly, and overmixing may break up the
gel. Either can decrease the strength of the material by as much as 50 percent. The
strength of alginate-type hydrocolloids increases for several minutes after the initial
gelation. Consequently, the impression must not be removed from the mouth for at
least 2 or 3 minutes after gelation has occurred.
3-19. DIMENSIONAL STABILITY
Alginate-type hydrocolloid impression materials are influenced by syneresis,
imbibition, strain, and stress. Hence, for the most accurate results, the impression
should be fixed and the cast poured soon after the impression is removed from the
mouth. If the impression must be stored for a short period of time, it should be placed in
a humidor in which the relative humidity is 100 percent.
3-20. EFFECT ON GYPSUM PRODUCTS
Alginate-type hydrocolloid impression materials affect gypsum products in the
same manner as the agar-type materials affect them. Some alginates do not require
the use of hardening solutions because the manufacturer has incorporated these
materials in the powder. However, the hardness of the surfaces of the cast can always
be improved by "fixing" the impression with a hardening solution.
3-21. ELASTOMERIC IMPRESSION MATERIALS-- CHARACTERISTICS
a. General. Elastomeric impression materials have greater dimensional
stability with time and ability to record precise details. The first materials were
polysulfides (rubber base), followed by condensation silicones, polethers and addition
silicones (polyvinyl siloxanes). Polyvinyl siloxane impressions are currently the most
used, followed by polyether.