DENTAL MATERIALS Dr Waseem Mushtaha Al Azhar University Gaza Lama El Banna

1. Investment materials
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics

2. Introduction
When a restoration or appliance is being made
by a (loss wax) process, the wax pattern is
embedded in an investment material. The
wax is then removed from this mould, and
the space which it occupied is filled by the
material of which the restoration or
appliance is to be made

3. Investment materials

5. Requirements for investment
materials
All investment materials contain:
1(A refractory substance: a material that will
not decompose or disintegrate on heating.
2(A binder: a material which will set and
bind together the particles the refractory
substance.

6. In addition the following properties are desirable:
a) The mould must expand to compensate for the
shrinkage on cooling of the alloy.
b) The power should be of a fine particle size to
ensure a smooth surface on the casting.
c) The mixed unset material should have a smooth
consistency.
d) The material should have a suitable setting time.
e) The set material should be permeable to allow air
to escape as the molten alloy enters the mould.
f) The strength of the material should be sufficient to
withstand the force of the molten alloy entering
the mould.

7. Types of investment material
Three types are available. They all contain
silica (SiO2) as the refractory constituent.
The chief difference between them is the
type of binder used, as follows:
a) gypsum-bonded investments are widely
used for gold alloys, but are unsuitable for
alloys which melt at temperatures
approaching 1200C

8. b) Phosphate- bonded materials are used for
casting cobalt-chromium alloys, since they
can withstand higher temperatures
c) Silica-bonded investments are an
alternative to the phosphate-bonded
materials for high temperature casting

9. Gypsum-bonded investments

10. Gypsum-bonded investments
constituents
a. Silica is present in one of its allotropic
forms (cristobalite or quartz) to:
1(Act as refractory.
2(Provide mould expansion by thermal
expansion and inversion

11. B- Autoclaved calcium sulphate hemihydrate, for the
following purposes:
1(To react with water and on hydration to bind the
silca together
2(To impart sufficient strength to the mould.
3(To contribute to the mould expansion by the
setting expansion which occurs.
C- A reducing agent such as powdered charcoal, to
reduce any oxide formed in metal.
D- Modifying chemicals such as boric acid or
sodium chloride to inhibit shrinkage on heating.

12. Manipulation
a) The mixing of an investment material is similar to
that of dental stone. Use of the correct
water/powder ratio is important to ensure that the
correct strength, setting time and expansion are
obtained.
b) Before investing the wax pattern , it is washed
with a non-foam detergent to remove any oil or
grease, and to facilitate the wetting of the pattern
by the investment mix.

13. C) The casting ring is usually lines with a wet
asbestos strip. This does two things:
1-It facilitates mould expansion, since it can be
compressed as expansion occurs, whereas a rigid
ring on its own cannot do this.
2-It contributes the pattern may be carried out
a) Under vacuum to prevent trapping air on the
surface of the pattern
b) painting investment material on to the pattern with
a brush before carfully inserting it into the filled
casting ring

15. E- The mould is heated through 150-200C;
this dries out the excess water and burns off
the wax. The mould is then slowly heated to
above the temperature of inversion- usually
to 700C it is held at this temperature for 30
minutes to ensure that the wax is
completely burn out.

16. Dimensional changes of the mould
A) Setting expansion is caused by the crystal growth of
gypsum.
b) Hygroscopic expansion. In one technique the investment is
immersed in water after setting has begun. A gratly
increased setting expansion occurs. So less thermal
expansion is required. Increased hygroscopic expansion is
obtained in the following cases:
1(When a lower water/powder ratio is used
2(For an investment material of greater silica content.
3(If water of higher temperature is used.
4(For longer immersion in water.

17. c) Thermal expansion. Investment materials containing cristobalite and
quartz show rapid expansions between 200-300C and 500-600C
respectively, due to displacive transformation of the silica. The amount
of thermal expansion therefore depend on:
1-Temperature.
2-Quantity of silica in the material
3-Allotropic form of silica used, for example the thermal expansion of
cristobalite is greater than that of quartz at most temperatures.
4-water/powder ratio- thicker mixes have greater thermal expansion.
d) Shrinkage on heating. This occurs due to the dehydration of the set
gypsum in two stages:
2caso4,2H2O (CaSO4)2,H2O + 3H2O
)CaSO4)2,H2O 2CaSO4 + H2O
The investment shrinkage is eliminated or reduced by the presence of
small quantities of additives such as sodium chloride or boric acid.
Other properties:
a) The total expansion of the mould is generally sufficient to compensate
for the shrinkag on cooling of cold alloys ( about 1.5% by volume(.

18. b) The investments containing finer particles of silica and
calcium sulphate hemihydrate give smoother surfaces on
the finished casting.
c) gypsum-bonded investments are easy to manipulate giving
a smooth consistency mix.
d) The setting time of theses materials can be easily
controlled, as for stone and dental plaster
e) The set investment is porous, as is set gypsum. This helps
to prevent back-pressure porosity in castings.
f) The strength of these materials, when set, if mixed at the
correct water/powder ratio, is sufficient to withstand the
forces of the molten alloy as it enters the mould. The
autoclaved hemihydrate is used in preference to the
calcined material for this reason

19. Limitations
Above around 1200C, a reaction can occur between
calcium sulphate and silica:
CaSO4 + SiO2 CaSiO3 + SO3
The sulphur trioxide gas that is evolved:
1(Causes porosity in the casting.
2(Contributes to the corrosion of the casting.
For this reason gypsum-bonded investments are not
used for the higher fusing dental alloys, such as
cobalt-chromium. In this case phosphate or silica
bonded materials are chosen.

20. Phosphate-bonded investments

21. Phosphate-bonded investments
composition and setting
Magnesium oxide can react with a phospharic
such as an ammonium phosphate in an
aqueous system as follows:
MgO + NH4H2PO4 MgNH4PO4 +H2O
The crystals of the magnesium ammomium
phosphate bind together the particles of the
silica refractory.

22. Manipulation
These materials are mixed with water, similar to
gypsum-bonded investments. However, the
following difference in manipulation should be
noted:
a) Because of the strength of the set material, metal
casting rings are not needed. In their place, plastic
rings can be used, they are removed after the
material has set, but before the investment is
heated.
b) The investment is heated to 1000-1100C

23. Properties
a) Expansion. The setting reaction is accompanied
by an expansion, analogous to the crystal growth
of gypsum. Also, thermal expansion occurs in
heating.
b) Porosity : a set phosphate-bonded material shows
a certain degree of porosity, again similar to the
gypsum containing investments.
c) strength: the set materials increases in the strength
during heating, possibly due to chemical
interaction between silica and the binder, giving
complex silicophoshates.

24. Silica-bonded investments
Setting reaction:
a) Stage 1 : hydrolysis. Ethyl silicate can be
hydrolysed to silica acid, with liberation of
ethyl alcohol:
Si(OC2)4 + 4H2O Si(H)4 + 4C2H5OH
In practical, a polymerised form of ethyl
silicate is used, yielding a sol of polysilicate
acid.

25. b) Stage 2: gelation. The sol is mixes with
cristobalite or quartz, then gel formation is made
to occur under alkaline conditions by adding
magnesium oxide. There is a slight shrinkage at
this stage.
c) Stage 3: drying. on heating, considerable
shrinkage occurs and there is a loss of alcohol and
water, leaving a mould made of silica particles
tightly packed together.
As alternative of the above, simultaneous hydrolysis
and gel formation can occur, when an amine such
as piperidine is incorporated.

26. Properties
a) Dimensional changes. There is a large amount of
the thermal expansion, due to the very large
percentage of silica in the final material. This
expansion is usually sufficient to compensate for:
1-The setting shrinkage of the investment material.
2-The casting shrinkage of the alloy.
b) porosity. The particles of the set material are
packed so closely together that the porosity is
negligible. Air spaces or vents must be left in the
investment to permit escape of the air from the
mould.