Dental Materials

Lec. 4 ‫ ضياء رشيد‬.‫د‬

Setting Time
It is the time from the beginning of mixing the powder with water until
the material hardens.
Setting time of gypsum products are often measured in term of their
ability to resist penetration and divided into:
Mixing time from addition of powder to the water until a homogeneous
mix.
Working time (3 minutes) time available for mixing, and it´s usually
before initial setting.
Initial setting time (12 minutes) beginning of the mixing until partial
setting occurs, during this stage, the material will not flow; it is rigid but
not hard. It is possible to carve away the excess material.
Final setting time (Several hours) beginning of mixing until complete
setting takes place. The model or die will be strong and hard or
completely set and can be separated from the impression without
deterioration or fracture.

Measuring Setting time:

1- Loss of gloss: To judge proper working time. It indicates partial
setting.
2- Temperature rise: Since the reaction is exothermic, it indicates
setting.
3- Penetration tests: depend on resistance of the set gypsum material for
penetration by VICAT NEEDLE APPARATUS (measuring initial
setting time) & GILMORE NEEDLE APPARATUS (measuring
initial and final setting times), in which needle of different weight and
thickness is used, penetration of these needles being measured at various
times during setting.
Factors affecting setting time
1. Factors controlled by the operators.
2. Factors controlled by the manufacturers.

1. By operator:
A. Water, powder ratio: the more water used for mixing (thin mix), the
fewer nuclei will be per unit volume; consequently, the setting time will
be prolonged.
B. Mixing time and rate: the longer and the more rapidly the gypsum is
mixed, the shorter is the setting time. As mixing begins, more particles
will be exposed to water and thus form more crystals at the same time
the crystals are broken up by mixing and they are distributed throughout
the mixture and result in the formation of more nuclei for crystallization,
thus the setting time is decreased.
C. Temperature: there is a little change in the setting time when the
temp. is between 0-50Cº, the setting time will be retarded when the
temp. exceeds 50Cº, no setting will take place when the temp.
approaches 100 Cº.

2.By manufacturers
A. Fineness: the finer the particles size of CSH, the faster the mixture
will be hardened. The gypsum nuclei are more numerous and therefore a
rapid crystallization will occur, this is because the solubility of CSH will
be increased and increases growing CaSO4.2H2O
B. Impurities: a small amount of gypsum (Calcium sulphate dihydrate
0.5-1%) added to the powder of plaster or stone, will lead to increase in
the potential nuclei of crystallization and fastening (shortening) setting
time.
C. Accelerators & Retarders: the most effective and practical way to
control the S.T. of gypsum products is by adding of accelerator &
retarder.
Retarders: e.g. (Borax, gum Arabic, glue, acetate)
The retarder is a chemical material added to the gypsum product to
prolong (increase) S.T., these materials adhere on the surface of the CSH
or the growing gypsum crystals thereby reduce the dissolution of CSH &
might deposit on the nuclei of crystallization and effectively reduce the
rate of crystallization, so S.T. will be retarded.
***Blood, saliva, hydrocolloid also prolong the setting time. So, the
impression should be washed before pouring the model.
Accelerators: e.g. (Nacl, K2SO4 in certain concentrations)
The accelerator is a chemical material added to the gypsum product to
decrease S.T., these salts increase the rate of dissolution of CSH, and
thus the saturation of solution occur more rapidly, increases number of
nuclei of crystallization and increases growing CaSO4.2H2O

Properties of gypsum products

1. Setting Expansion: regardless of the type of gypsum product, an
expansion of the mass can be detected during the change from CSH to
CSD after mixing with water. This expansion could be explained on the
basis of the mechanism of crystallization. There is an outward growth of
crystals from nuclei of crystallization, as a result of the growth, there is
an entanglement and there is interception between the crystals. If one
crystal intercepts another crystal, there will be stress at the point of
interception in the direction of the growth of the impinging crystals. If
the process is repeated by thousands of the crystals during growth, it is
possible that the outward stress or thrust could produce an expansion of
the mass. The final structure immediately after setting is composed of
interlocking crystals between which are micro pores and pores
containing excess water. On drying the excess water is lost and the total
empty space is greatly increased.
Factors effecting the Setting Expansion:
A. water/ powder ratio: the setting expansion is inversely related to W/P
ratio. The higher the W/P ratio, the less the expansion to due to the
fewer nuclei of crystallization per unit volume are present than the
thicker mixes and since it can be assumed that the space between the
nuclei will be greater in such a case, follows that there will be less
growth interaction of the dihydrate crystals with less outward thrust
resulting.
B. Addition of chemicals (accelerators & retarders): the S.E. can be
reduced by the adding of a balance amount of accelerators and retarders
to CSH.
C. Mixing time and rate: increasing mixing time and rate more nuclei of
crystallization in a given volume, more growing CaSO4.2H2O crystals
in this volume, increasing the outward thrust of the growing crystals,
increasing the setting expansion.
**Hygroscopic expansion
If the setting process is allowed to occur under water, the S.E.
may be more than doubled in magnitude, this is called
hygroscopic expansion. This increased expansion is due to the
additional growth of crystals permitted and not to any difference
in the chemical reaction.
2. Strength & Hardness: the strength of gypsum products is usually
determined by terms of compressive strength although tensile strength is
also considered. In general, factors affecting decrease or increase the
strength have the same effect on surface hardness of gypsum Products.

Factors affecting strength:

A. water, powder ratio: w/p ratio is inversely related to the strength. The
greater w/p ratio the less strong or weaker product because the less
intermeshing between the crystals. Since w/p ratio of plaster is the
highest and more than that of stone, its cast is weaker than the cast of
stone.
B. Dryness: the strength of plaster or stone increase rapidly as the
material harden after I.S.T, the excess water presents in the set mass
affect the strength, it is of two types
Wet strength: is the strength when there is excess water left in the mass.
Dry strength: is the strength when the excess water has been dried in air
or oven in warm temperature. The dry strength may be two times greater
than the wet. The effect of drying is to remove the excess water between
the crystals. The excess water reduces the cohesion between the crystals
themselves. 40% of the strength is due to the cohesive forces between
the crystals in addition to the strength which can be attributed to the
interlocking of the crystals during growth.
Drying period Compressive strength kg/cm²
2h. 98
8h. 119
24h. 238
C. porosity: set gypsum is always somewhat porous, this porosity of two
types:
*Micro porosity: due to the setting reaction.
*Air bubbles porosity: caused by incorporation of air during mixing.
Both of these types are affected by W/P ratio but in opposite way. When
W/P ratio increased, air bubbles porosity increased, but micro porosity is
decreased, and vice versa. The vacuum treatment of mixing will produce
a reduction in porosity & cause an increase in strength about 20%.
D. Mixing time and rate: affect the strength of gypsum. Increase mixing
time within limits will provide more nuclei of crystallization in a given
volume. So, increase the strength of the material. But over mixing will
reduce the strength because it will break up the crystals which are
formed & result in less crystal interlock.
E. Chemicals: the addition of accelerator or retarder lowering both wet
& dry strength due to the reduction in the inter-crystalline cohesion.
F. Type of gypsum products: strength of Improved stone > Strength of
dental stone > Strength of model plaster.

Storage of gypsum products

Plaster and stone powder absorb moisture, which causes gradual
deterioration. Hydration begins on the surface of the hemihydrate
particles forming fine coat of gypsum and this will act as effective nuclei
for crystallization and this shortens the setting time. To avoid
deterioration, plaster and stone powder should be stored in airtight
waterproof containers in a dry region of the laboratory.