Duplicating and refractory materials used in removable partial/endodontic courses

DUPLICATING ANDDUPLICATING AND
REFRACTORY MATERIALSREFRACTORY MATERIALS
USED IN REMOVABLEUSED IN REMOVABLE
PARTIAL DENTURESPARTIAL DENTURES
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.comwww.indiandentalacademy.com

 ContentsContents
1.1.      DEFINITIONDEFINITION
oo          DuplicationDuplication
oo          Refractory castRefractory cast
2.2.      DUPLICATION MATERIALSDUPLICATION MATERIALS
oo          Agar agarAgar agar
oo          AlginateAlginate
oo          SiliconesSilicones
oo          Aqueous acrylamide gelAqueous acrylamide gel
oo          Polyvinyl chloridePolyvinyl chloride


3.3.      REFRACTORY MATERIALSREFRACTORY MATERIALS
oo          Phosphate bondedPhosphate bonded
oo          Ethyl silicate bondedEthyl silicate bonded
oo          Gypsum bondedGypsum bonded
4. Review of literature4. Review of literature
5.5.      SUMMARY AND CONCLUSIONSUMMARY AND CONCLUSION
6.6.      BIBLIOGRAPHYBIBLIOGRAPHY

INTRODUCTIONINTRODUCTION
In preparing partial dentures,a duplicate shouldIn preparing partial dentures,a duplicate should
be made of the plaster or stone cast of thebe made of the plaster or stone cast of the
patient mouth. This duplicate is required for twopatient mouth. This duplicate is required for two
reasons:reasons:
1)1) The cast on which the wax pattern of the metalThe cast on which the wax pattern of the metal
framework is to be formed must be made fromframework is to be formed must be made from
the refractory investment,because it mustthe refractory investment,because it must
withstand the the casting temperatureswithstand the the casting temperatures
2)2) 2) The original cast is needed for checking the2) The original cast is needed for checking the
accuracy of the metal framework and foraccuracy of the metal framework and for
processing the plastic portion of the partialprocessing the plastic portion of the partial
denture.denture. www.indiandentalacademy.comwww.indiandentalacademy.com

DuplicationDuplication
 The procedure of accurately reproducing aThe procedure of accurately reproducing a
cast.cast.
Refractory Cast:Refractory Cast:
 A cast made of materials that willA cast made of materials that will
withstand high temperatures withoutwithstand high temperatures without
disintegrating and, when used in partialdisintegrating and, when used in partial
denture casting techniques, hasdenture casting techniques, has
expansion to compensate for metalexpansion to compensate for metal
shrinkage.shrinkage.

Materials used for duplication:Materials used for duplication:
 Agar agarAgar agar
 AlginateAlginate
 Polyvinyl siloxanePolyvinyl siloxane
 Aqueous acrylamide gelAqueous acrylamide gel
 Polyvinyl chloridePolyvinyl chloride

Agar agar:Agar agar:
 Agar is the material of choice to make the moldAgar is the material of choice to make the mold
for duplication, as it is a reversible hydrocolloidfor duplication, as it is a reversible hydrocolloid
and can be used many times.and can be used many times.

Composition :Composition :
 The basic constituent of the hydrocolloidThe basic constituent of the hydrocolloid
impression material is agar.impression material is agar.
 Agar is an organic hydrophilic colloidAgar is an organic hydrophilic colloid
extracted from certain types of seaweed.extracted from certain types of seaweed.
 It is a sulfuric ester of a linear polymer ofIt is a sulfuric ester of a linear polymer of
galactose.galactose.

 The principle ingredient by weight is waterThe principle ingredient by weight is water
(>80%)(>80%)
 A small percentage (0.2% - 0.5%) of boraxA small percentage (0.2% - 0.5%) of borax
is added to strengthen the gel.is added to strengthen the gel.
 To counteract the effects of water andTo counteract the effects of water and
borax, potassium sulfate is addedborax, potassium sulfate is added
because it accelerates the setting ofbecause it accelerates the setting of
gypsum.gypsum.

 Some products contain filler particles for the controlSome products contain filler particles for the control
of strength, viscosity, and rigidity. Eg, fillers used areof strength, viscosity, and rigidity. Eg, fillers used are
diatomaceous earth, clay, silica, wax, rubber anddiatomaceous earth, clay, silica, wax, rubber and
similar inert powders.similar inert powders.
 Other ingredients like thymol as bactericide andOther ingredients like thymol as bactericide and
glycerin as plasticizer may be added pigments andglycerin as plasticizer may be added pigments and
flavours can also be added.flavours can also be added.

Gelation process:Gelation process:
 The setting of a reversible hydrocolloid oftenThe setting of a reversible hydrocolloid often
called gelation is a solidification process.called gelation is a solidification process.
SolSol GelGel
 The physical change from the sol to gel and viceThe physical change from the sol to gel and vice
versa is induced by a temperature change.versa is induced by a temperature change.
However the hydrocolloid gel does not return toHowever the hydrocolloid gel does not return to
the sol at the same temperature at which itthe sol at the same temperature at which it
solidifies.solidifies. www.indiandentalacademy.comwww.indiandentalacademy.com

 The gel must be heated to a higherThe gel must be heated to a higher
temperature known as the liqueficationtemperature known as the liquefication
temperature (70° to 100°C) to return it totemperature (70° to 100°C) to return it to
the sol condition.the sol condition.
 When cooled, the material remains aWhen cooled, the material remains a
solution for below the liquefactionsolution for below the liquefaction
temperature. It transforms into a geltemperature. It transforms into a gel
between 37°C to 50°Cbetween 37°C to 50°C

 The temperature lag between the gelationThe temperature lag between the gelation
temperature and the liquefactiontemperature and the liquefaction
temperature of the gel makes it possible totemperature of the gel makes it possible to
use agar as a impression material.use agar as a impression material.
 The molten colloid can be stored for a fewThe molten colloid can be stored for a few
days in a hydrocolloid storage unit at 57°Cdays in a hydrocolloid storage unit at 57°C
– 60°C.– 60°C.

AlginateAlginate
 A Chemist from Scotland noticed that a certainA Chemist from Scotland noticed that a certain
brown seaweed (algae) yielded a peculiarbrown seaweed (algae) yielded a peculiar
mucous extraction and named it as ALGIN.mucous extraction and named it as ALGIN.
 This natural substance was later identified as aThis natural substance was later identified as a
linear polymer with numerous carboxyl acidlinear polymer with numerous carboxyl acid
groups and named an hydro-b-d- mannuronicgroups and named an hydro-b-d- mannuronic
acid (also called alginic acid).acid (also called alginic acid).

The advantage of alginate over agar is:The advantage of alginate over agar is:
 Its easy to manipulateIts easy to manipulate
 Comfortable for the patient.Comfortable for the patient.
 Relatively inexpensive because it does notRelatively inexpensive because it does not
require elaborate equipment like agar.require elaborate equipment like agar.

Modified alginates:Modified alginates:
 The traditional alginate is used as a two componentThe traditional alginate is used as a two component
system, a powder and liquid.system, a powder and liquid.
 However the alginate is also available in the form of aHowever the alginate is also available in the form of a
solution containing the water but no source of calciumsolution containing the water but no source of calcium
ions.ions.
 A reactor of plaster of Paris can then be added to theA reactor of plaster of Paris can then be added to the
solution. In this instances, the second component issolution. In this instances, the second component is
the reactor, not the water.the reactor, not the water.

 There is yet another form that is available. TheThere is yet another form that is available. The
two component system which is in the form oftwo component system which is in the form of
two pastes : one contains the alginate solutiontwo pastes : one contains the alginate solution
and the other contains the calcium reactor.and the other contains the calcium reactor.
 Impression materials of this type may alsoImpression materials of this type may also
contain silicone and may be supplied both in acontain silicone and may be supplied both in a
tray viscosity and in a syringe viscosity.tray viscosity and in a syringe viscosity.

Composition:Composition:
 The chief active ingredient of theThe chief active ingredient of the
irreversible hydrocolloid is one of theirreversible hydrocolloid is one of the
soluble alginate such as sodium,soluble alginate such as sodium,
potassium or triethanolamine alginates.potassium or triethanolamine alginates.
 Other components like diatomaceousOther components like diatomaceous
earth, and zinc oxide act as filler particlesearth, and zinc oxide act as filler particles
are added which can increase the strengthare added which can increase the strength
and stiffness of the alginate gel, produce aand stiffness of the alginate gel, produce a
smooth texture and ensure a firm gelsmooth texture and ensure a firm gel
surface that is not tackysurface that is not tacky

 CaSOCaSO44 dihydrate or hemihydrate can bedihydrate or hemihydrate can be
used as a reactor.used as a reactor.
 A fluoride, such as potassium titaniumA fluoride, such as potassium titanium
fluoride, is added as an accelerator for thefluoride, is added as an accelerator for the
setting of the stonesetting of the stone
 NaPONaPO44 acts as a retarder.acts as a retarder.

Gelation processGelation process::
 The typical sol-gel reaction can beThe typical sol-gel reaction can be
described simply as a reaction of solubledescribed simply as a reaction of soluble
alginate with calcium sulphate and thealginate with calcium sulphate and the
formation of an insoluble calcium alginateformation of an insoluble calcium alginate
gel.gel.
 CaSO4
reacts rapidly to form insoluble calcium
alginate from the potassium or sodium alginate
in an aqueous solution.

The production of the calcium alginate is so rapid that
does not allow sufficient working time. Thus, a third
water soluble salt, such as trisodium phosphate, is
added to the solution to prolong the working time.

 Thus the reaction between CaSO4
and the soluble
alginate is prevented as long as there is unreacted
trisodium phosphate.
2Na3
PO4
+ 3CaSO4
 Ca5
(PO4
)2
+ 3Na2
SO4
 When the supply of trisodium phosphate is
exhausted, the calcium ions begin to react with
the potassium alginate to produce calcium alginate.
K2
n Alg + n CaSO4
 n K2
SO4
+ Can Alg

Controlling gelation time:
 It is the time measured from the beginning of
mixing until gelation occurs.
 The optimal gelation time is between 3-4 min at
room temperature.
 Gelation time is best regulated by the amount of
retarder or by altering the temperature of the
water.

Silicones:
Two types of silicones are used as rubber
impression materials and are identified on
the basis of their chemical reactions as
 Condensation and
Addition

Condensation silicones
 They are supplied as a base and an accelerator.
The base contains a moderately low molecular
weight silicone called a dimethyl siloxane, which
has reactive terminal hydroxyl groups.
 Fillers like silica or copper carbonate are added
to the liquid silicone polymer to form a paste.
 Accelerators like stannous octate or alkyl silicate
are used. www.indiandentalacademy.comwww.indiandentalacademy.com

The condensation polymerization of this material
involves a reaction with trifunctional and tetrafunctional
alkyl silicates, commonly tetraethyl orthosilicate, in
the presence of stannous octate.
These reactions can take place at ambient
temperatures; thus, the materials are often called room
temperature vulcanization silicones.

• Ethyl alcohol is a by product of the
condensation setting reaction.
Its subsequent evaporation probably
accounts for condensation silicones biggest
disadvantage which is poor dimensional
stability.

Addition silicones
 They are frequently called as polyvinyl siloxane.
 In contrast with the condensation silicones, the
addition reaction polymer is terminated with vinyl
groups and is cross linked with hydride groups
activated by a platinum salt catalyst.
 Both base and catalyst pastes contain a form of the
vinyl silicone.
 The base paste contains polymethyl hydrogen
siloxane as well as other siloxane prepolymers.

 The catalyst paste contains divinyl polydimethyl
siloxane and other siloxane prepolymers.
 If the catalyst paste contains the platinum salt
activator, then the paste labeled base must contain
the hybrid silicone. Both pastes contain fillers, and
retarders are present in paste containing the
activator.
 One of its disadvantage is its inherent hydrophobicity.

Aqueous acrylamide gel
The time necessary for performing the duplicating
procedure using agar which is used successfully in
dental laboratory duplicating procedures is 2 to 3
hours.
There are situations however in which a duplicating
material is needed that can produce a cast in a very
short period of time.

Aqueous acrylamide gel was produced from a
mixture of two organic monomers, acrylamide
and N, N – methylenebis acrylamide in water.
 When these substances are catalysed by
the addition of ammonium persulfate and b-
dimethylaminoprionitrile, a transparent gel
forms by a polymerization – cross linking
reaction.

 This material is commercially available under the
trade name of CYANOGUM 41 or AM-9 chemical
group.
Potassium alginate is added for the purpose of
preventing sticking between the acrylamide gel and
the master or refractory cast.
 It is a duplicating material that allows a mold to be
made of a master cast within 15 min.

Polyvinylchloride
 It is a reversible plastic gel.
The main advantages of this material is the high
strength properties and the high chemical stability,
which permit a large number of duplications before
replacement.

Properties and requirements of
duplicating material
ANSI / ADA specification no. 20 for dental duplicating
materials includes 2 types
1.   Thermoreversible (Type I)
-  Hydrocolloidal (Class I)
-  Nonaqueous organic (Class II)
2.   Non reversible (Type II)
-  Hydrocolloidal (Class I)
-  Nonaqueous organic (Class II)

The specification sets requirements of freedom from
foreign material and impurities and that it shall be
suitable for taking impressions of plaster, stone or
investment casts of the oral tissues.
Pouring temperature and the temperature of gel
formation are defined for the thermoreversible products.
Working and setting times are specified for the non-
reversible material. Compatibility with at least one type
of investment and the ability to reproduce detail
satisfactorily are required.www.indiandentalacademy.comwww.indiandentalacademy.com

 Type I products are required to show no mold growth
after inoculation under controlled conditions.
Requirements for permanent deformation, strain in
compression, and resistance to tearing are described.
 Packaging must include instructions that indicate the
type of investment that can be used with the material
and for type I products, must also include (1) method of
liquefying (2) tempering or storing temperature and (3)
pouring temperature.

Refractory materials
The rapid growth in use of cast removable partial
dentures , the increased use of higher melting
alloys and the use of less expensive base metal
alloys have resulted in an increased use of
phosphate or silicabonded refractory material.

Phosphate bonded
Composition
These investments consist of refractory fillers and a binder.

The filler is silica, in the form of Cristabolite, Quartz, or a
mixture of the two and in a concentration of approx 80%.
The binder consists of magnesium oxide and a phosphate
that is acid in nature..

It is fortunate that the colloidal silica suspensions became
available in time for use with the phosphate investments in
place of water to compensate for the greater contraction of the
high fusing alloys during solidification, as it increases the
setting expansion of the investment.

 Carbon is often added to the powder to produce clear
castings and facilitate the divesting of the casting from the
mold.
 But the latest evidence indicates that palladium does react
with carbon at temperature above 1504°C. Thus if the
temperature exceeds this temperature during casting, a
carbon free investment should be used.

Setting reactions
 Magnesium ammonium phosphate formed is polymeric.
Although the stoichiometric quantities are equal molecules of
magnesia and monoammonium phosphate, an excess of
magnesia is usually present, and some of it is never fully
reacted.
 What is thus formed is a predominantly colloidal
multimolecular aggregate around excess MgO and fillers. On
heating, the binder of the set investment undergoes thermal
reactions. www.indiandentalacademy.comwww.indiandentalacademy.com

Setting and thermal expansion:
 Theoretically, the reaction should entail a shrinkage, but
in practice there is a slight expansion by using a colloidal
silica solution instead of water.
 The early thermal shrinkage of phosphate investments is
associated with the decomposition of the binder, magnesium
ammonium phosphate, and is accompanied by evolution of
ammonia.
 Some of the shrinkage is masked because of the
expansion of the refractory filler especially in the case of
cristobalite www.indiandentalacademy.comwww.indiandentalacademy.com

Working and setting time:
Phosphate investments are affected by temperature.
The setting reaction itself gives off heat, and this
further accelerates the rate of setting.
Increase in mixing time and mixing efficiency results in
faster set.
Increase in w:p ratio, increases the working time.

Ethyl silicate-bonded:Ethyl silicate-bonded:
It is losing popularity because of the marc complicatedIt is losing popularity because of the marc complicated
and time consuming procedures involved.and time consuming procedures involved.
BBinder is a silica get that reverts to silica on heatinginder is a silica get that reverts to silica on heating
several methods may be used to produce the silica orseveral methods may be used to produce the silica or
silicic acid gel binders.silicic acid gel binders.
Another system for binder formation is based on ethylAnother system for binder formation is based on ethyl
silicate. A colloidal silicic acid is formed b hydrolyzingsilicate. A colloidal silicic acid is formed b hydrolyzing
ethyl silicate in the presence of hydrochloric acid, ethylethyl silicate in the presence of hydrochloric acid, ethyl
alcohol and water.alcohol and water.

 A polymerized form of ethyl silicate is used a
colloidal solution of polysilicic acid is formed. This
solution is mixed with quartz or cristabolite to which is
added a small amount of finely powdered MgO to
render the mixture alkaline.
 A coherent gel of poly silicic acid then forms,
accompanied by a setting shrinkage. This soft gel is
dried at a temperature below 168°C. During the drying
process, the gel loses alcohol and water to form a
concentrated, hard gel.

This contraction is known as “green shrinkage” and
it occurs in addition to the setting shrinkage.
Investments of this type are designed to reduce the
layer of silica gel around the particles.
This type of investment can be heated to 1090°C to
1180°C and is compatible with the higher fusing
alloys.

Gypsum bonded
 When casting gold alloys gypsum bonded investment
material can be used. Most materials now contain a-
hemihydrate because greater strength is obtained.
 This gypsum product serves as a binder to hold the
other ingredients together and to provide rigidity.
 The strength of the material is dependent on the
amount of binder present. The material may contain
25% to 45% of gypsum.
 In addition to silica certain modifying agents, coloring
matter and reducing agents, such as carbon and
powdered copper are present.www.indiandentalacademy.comwww.indiandentalacademy.com

 Some of the modifiers such as boric acid and sodium
chloride not only regulate the setting expansion and the
setting time but they also prevent most of the shrinkage
of gypsum when it is heated above 300°C.
 But all forms of gypsum dehydrate between 200-
400°C. A slight expansion than occurs between 400°C –
700°C and then a large contraction occurs.
 This latter shrinkage is most likely caused by
decomposition, and sulfur gases, such as sulfur dioxide

 This decomposition not only causes shrinkage but
also contaminates the castings with the sulfides of
the non-noble alloying elements, such as silver and
copper.
Thus it is imperative that gypsum bonded material
not be heated above 700°C.
Silica is added to provide a refractory during the
heating of the material and to regulate the thermal
expansion. If proper forms of silica like quartz or
crystabolite are used in the material, the contraction
during the heating can be eliminated and changed to
an expansion.

Summary and Conclusion
In this seminar the commonly used duplicating and
refractory materials were described. Agar agar and
phosphate bonded materials are used extensively as
duplicating and refractory materials respectively because
of their mentioned advantages over the other materials.
Knowledge of these materials is a pre-requisite for
obtaining acceptable refractory casts.

Duplicating and refractory materials used in removable partial/endodontic courses

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Duplicating and refractory materials used in removable partial/endodontic courses