Investment materialsbk/ dental implant courses by Indian dental academy

1

INVESTMENT MATERIALS CONTENTS 1.

History

2.

Ideal properties of investment materials

3.

Gypsum bonded investments

4.

Phosphate bonded investments

5.

Silica bonded investments

6.

Conclusion

7.

Bibliography HISTORY 1907 1907

Taggart proposed the lost wax theory for dental casting LeChatlier and Vant Hoff gave the crystal growth theory for

1930 1932

setting of gypsum products Taylor proposed usage of wet asbestos ring liner Scheu discovered that gypsum bonded investments expand

1955

when immersed in water Mahler and Ady proposed the hygroscopic expansion of

1959

gypsum products Fusayama proposed usage of dry water proofed ring liner

1981

made of asbestos Barnard proposed usage of ceramic ring liner.

Ideal properties of an investment 1 Easy to manipulate 2 Sufficient strength at room temperature 3 Stable at higher temperature 4 Sufficient expansion 5 Porous to allow venting 6 Reproduction of detail 7 Ease of divestment 8 Inexpensive Taggart's Lost Wax Technique (1907) After the wax pattern has been obtained a sprue former or sprue pin is attached to the wax pattern and this assembly is surrounded in suitable container (casting ring) over which a refractory material is poured and is allowed to set. The

2 mould is then heated to remove the wax and later a suitable molten alloy is forced into the vacant space left behind by the evaporated wax pattern.

The ring liner and the refractory material are then removed and the solidified metal is obtained is known as the casting.

Refractory materials or investment materials “Are those materials which are poured around the obtained wax pattern, upon elimination of the wax these materials form the confines in which a molten metal introduced to obtain a casting�.

Basically there are 3 types of investments a. Gypsum bonded investments b. Phosphate bonded investments c. Ethyl silica bonded investments GYPSUM BONDED INVESTMENTS Syn : Calcium sulphate bonded investments In the same ADA specification no. 2 three types of gypsum bonded investments are listed to be used with gold alloys.

Type I

To be used for casting of inlays and crowns where compensation for casting shrinkage is done by thermal expansion of investment.

Type II

Ex. Cristobalite Inlay (Kerr / Sybron) To be used for casting of inlays and crowns where compensation for casting shrinkage is done by hygroscopic expansion of

Type III

investment. Ex. Beauty Cast (Whip Mix Corp) To be used for construction of partial denture frame work R&R Gray (Dentsply)

3 Composition 1. Refractory filler (55% ~ 75%) 2. Binder (25%-45%) 3. Modifiers (Less than 5%) Refractory

Either

-

quartz

materials Binder

-

cristobalite or occasionally a mixture of the two Calcium

Modifers

-

hemihydrate either plaster or stone Accelerlators

or

sulphate or

retarders, colouring agents and reducing agents and Reducing

-

agents Expanders

-

finally materials that increase the thermal expansion Powdered graphite, copper Boric

acids,

soluble

halide salts ROLE OF INGREDIENTS Binders : When the investment powder is mixed in water the binder (hemihydrate) dissolves into water and sets as precipitated dihydrate. The refractory remains as such and doesn’t participate in the setting mechanism. The set investment consists of framework of set dihydrate with entrapped filler particles.

Refractory materials The two amorphous forms of silica namely quartz and cristobalite are used to offset the contraction that occurs in binder sitting. Their presence brings about a net expansion of the investment materials MECHANISM OF SETTING The investment materials sets exactly like the traditional gypsum products In mixing with water the hemihydrate turns into dihydrate, which precipitates and sets

4 There are several theories regarding this mechanism of setting including 1.

2.

Gel Theory Hydration

Setting occurs due t formation of gel Setting takes place via hydration of hemihydrate to form

3.

theory Crystalline

dihydrate By far the most popular and widely accepted theory which

theory

was proposed by Henry Louis Le Chatlier in 1887 and support by Vant Hoff in 1907. According to this theory there is a difference in the solubility of hemihydrate and dihydrate. Dihydrate being less soluble doesn’t dissolve once formed and thus the hemihydrate continues to dissolve in water to form dihydrate, till the water gets exhausted.

Microscopically there are two centers a. Centre for dissolution b. Centre of precipitation In

the

dissolution

centre

hemihydrate

predominates and in the other dihydrate predominates.

Upon dissolution of hemihydrate Ca+ and SO4+ ions are liberated and travel from this zone of high concentration to lower concentration (zone of precipitation).

Because dihydrate is 7% less in volume when compared to hemihydrate, this may lead us to believe that a contraction of 7% results however in reality a net expansion of 0.2% -0.4% occurs this is primarily due to the thrusting effect produced by the growing crystals of hemihydrate which results in volumetrically smaller but a surface area vise larger set characterized by its porous nature.

This porosity is beneficial as it provides the necessary venting of gases during wax elimination and investment during heating.

5 EFFECT OF COMPOSITION ON SETTING AND THERMAL BEHAVIOUR 1. Although silica is referred as the refractory component cast gypsum itself is sufficiently heat resistant and can withstand temperature upto 1,000°C. without decomposing. The fact at 900°C silica liberates gases.

CaSO4 + SiO2  CaSiO3 + SO3↑ Sulphur trioxide causes rapid corrosion of casting 870°C β quartz

1475°C β tridymite

1700°C β cristobalite

fused silica

160°C

573°C

Middle tridymite

220°C

105°C

α quartz

α- Tridymite

α- Cristobalite

All polymeric forms of silica i.e. quartz, tridymite and cristobalite occurs in α or β phases. However tridymite has a third stable polymeric forms named middle tridymite.

Either forms of silica can be converted into other by manufacturer to obtain a specific form by heating or by cooling. In an investment powder all forms of silica are in their α forms and on heating they turn into β forms.

The α forms being stable at room temperatures are known as low temperature phases and the β forms being stable at elevated temperature are known as high temperature phases. The temp at which the α forms transform into β are known as DISPLACIVE TRANSITION TEMPERATURE (DTT) Thus for quartz 573°C for cristobalite 220°C and for tridymite 105° and 160° are DTT. All α forms are more dense than their β forms thus inversion of α  β is always accompanied by expansion.

6 Cristobalite 

1.3% expansion at 220°C 2.2% expansion at net 700°C

Quartz 

0.7% expansion at net 573°C 1.6% expansion at net 700°C

Tridymite < 1% expansion at 260°C and 700°C Thus investments which are advocated for high thermal expansion usually contain cristobalite as refractory.

2. Binder  The gypsum in investment can be used in either forms When the gypsum in the investment is heated the binder shows marked contraction. It occurs at the following points.

1.

Contraction due to loss of water of crystallization as the dihydrate converts into hemihydrate and later into hexagonal calcium salts.

CaSO4, 2H2O110-136°C Dihydrate

2.

(Ca SO)4 H2O Hemihydrate

130-200°C CaSO4 Hexagonal anhydrite

A large contraction then accompanies its γ-β transformation between 300400°C.

3.

Another large contraction begins at 650°C which is a result of densification by sintering In all upon heating to 700°C Plaster of Paris shrinks by 3% (W/p = 0.50) and

high strength stone (Type IV) (w/p = 0.25) is about 1% Thus type IV gypsum product Dental stone within high strength is preferred due to its low shrinkage of 1% and with higher compressive strength of 35 µm.

EFFECTS OF VARYING COMPOSITION Within practical limits, increasing the proportion of silica in the investment powder increases the initial setting time, the setting expansion both in air and in water and the thermal expansion and reduces the compressive strength. There is however no effect on the rate of the setting reaction. The increase in manipulation time, initial setting time and a decrease in compressive strength occurs because of interfering by the refractory particles in the interlocking mechanism of growing gypsum crystals.

7 Setting expansion is increased because of interference by refractory particles in the crystal interlocking and shifting the crystal growth towards the periphery.

Thermal expansion occurs due to a net expansive effect of the refractory in comparison to the thermal contraction of the binder.

EFFECT OF MODIFYING AGENT Boric acid and soluble halide salts increase the thermal expansion of investment by reducing the two large contractions that occur in binder upto 300°C.Boric acid when heated above 150°C forms a viscous liquid which reduces the evaporation rate of last traces of water, delays the γ-B transformation of calcium sulfate and finally reduces the contraction due to high temperature sintering.

Investments containing boric acids show an increase in compressive strength by 40% - 50% mainly due to the stabilizing effect exerted by the viscous liquid boric acid.

Halide anions on the other hand reduces the first major shrinkage of binder that occurs at 300-400°C and completely eliminate the second contraction that starts at 650°C, however above 650°C they exert no effect and that contraction is resisted by silica content which forms a skeleton. Ironically halide anions decrease the strength of investment by 50% - 80% when heated to 650°C. PROPERTIES Particles size of powder •

If affects the smoothness of the mold cavity surface and then of the casting

•

It also affects the inherent porosity of the mold and thus the venting of the mold cavity Refractory is the major constituent and as it remains unchanged in whole

process, its particle size is of utmost importance.

75µm particle size of the refractory material is the most ideal keeping in mind the smoothness of the casting and the need of the adequate venting.

Manipulation time Mixing time: It is the time from the addition of the powder to the water until mixing is completed ~ 20-30 sec.

8 Working time : The time available to use a workable mix i.e. investment pouring and cleaning of equipment ~3 min.

Setting time : Time elapsed from time of mixing to loss of gloss till initial gilmore or the vicat ~ 13 min.

It indicates when the investment is strong enough for the sprue base and sprue former to be removed. Mold heating is generally delayed until setting expansion is complete ~ 1-2 hrs from the time of mixing.

SETTING EXPANSION, THERMAL EXPANSION AND HYGROSCOPIC EXPANSION OF GYPSUM BONDED INVESTMENTS. Setting expansion : 0.1% - 0.6% explained by the theory of crystal growth as proposed by. LeChatlier and Vant Hoff in 1907 Hygroscopic setting Expansion : 0.3% - 2% for the gypsum investments and the other gypsum products this theory was explained by Mahler and Ady in 1960.

Hygroscopic expansion can be facilitated by either immersing before the initial set investment in a water bath or by lining the casting ring with a wet liner.

There occurs a variation even in the number of saturated ring liners and the expansion so obtained.

Davis et al (IJP 1992 Vol 5 No. 2) compared the net expansion and the effect of ring volume / ring liner ratio and the saturation V/S dry treatment of the liner on gypsum bonded investments using xero-radiography.

Ring Dia. (mm) 28 51 51 51 51

Vr/Vl 10.3 19.6 9.31 19.6 9.31

Liner One saturated One saturated Two saturated One dry Two dry

Expansion (%) 1.3% 0.7% 1.4% 0.4% 0.8%

The technique of lining a casting using with wet asbestos was first described

by Taylor in 1930.

Asbestos is a health hazard thus the use of prewetted cellulose ring liner and ceramic ring liners is nowadays

preferred. Ceramic ring liners absorb negligible

amount of water at atmospheric pressure but absorb a significant amount of water

9 when investing is done in vacuum. This in turn reduces the W/p ratio of the investment material.

A low W/p ratio and a wet liner considerably increase the hygroscopic setting expansion. In this technique the investment is allowed to set normally first then the mold is heated to about 480°C for 20 min. At this temperature wax elimination takes place and the thermal expansion is negligible. Alternatively the true hygroscopic expansion can be done by immersing the investment before initial set, into a water bath at a controlled temperature.

FACTORS INFLUENCING SETTING AND HYGROSCOPIC SETTING EXPANSION OF GYPSUM BONDED INVESTMENTS 1.

2.

3.

4.

5.

6.

Factor High W/P ratio ↑ spatulation time ↑ rate of spatulation ↑ age of investment Delay before immersion high ↑ water bath temperature

Setting and hygroscopic expansion Decreased Increased Increased Decreased Decreased Decreased

1. W/P Ratio Increasing the W/p ratio (thinner mix) decreases the expansion because of a reduction in the no of nuclei of crystallization per unit volume.

2. Same as for 3 3. ↑ spatulation rate and time An increase in both results in more no of nuclei of crystallization and hence an increase in setting expansion and hygroscopic expansion.

4. ↑ Age of investment As the age of the investment material increases there occurs a decrease in SE and HSE values. As a consideration preweighed packets of investment materials should be used and use of large containers for storage should be discouraged.

10 5. & 6 ↑ delay in immersion and high water bath temperature In the hygroscopic expansion technique the freshly filled investment ring is immediately placed into a water bath at 38°C for 30 min. and in this process of immersion is delayed the value of HSE is decreased.If however the water temperature is increased beyond 50°C the setting reactions is impeded and at 100°C no setting reaction takes place.

THERMAL EXPANSION TECHNIQUE The mold is placed in a preheated oven at 480°C for 20 min and then the temperature is slowly increased to 700°C and held for 30 min at that temperature.

With this method the expansions of the refractory materials takes place, which produces a desired expansion for compensation of casting shrinkage.

An

expansion of 1.0% -1.6% is desirable when compensation is done by thermal expansion technique alone (Type I investment). An expansion of 0.6% at 500°C is required when compensation is done by hygroscopic expansion technique.

W/P ratio and TE Thinner the mix lower is the TE Modifiers and TE Boric acid because of viscous liquid formation aids in increasing the overall TE by decreasing the γ  β transformation shrinkage and decreasing the contraction of silica due to sintering at high temperatures.

PROPERTIES OF GYPSUM BONDED INVESTMENT MATERIAL W:P ratio  0.30-0.40 (Follow manufacturer instructions) Mixing time  upto 45 sec – 1 min Working time  3 + 1 min mixing time Setting time  30 min – 45 min Setting expansion  0.1% - 0.6% Hygroscopic S. Expansion  0.3% - 2% Thermal expansion  0.6% for HSE, 1-1.6% for TSE. Compressive strength  2.3 MPa min for type I

11 Reproduction of detail  50±8 µm MIXING AND PROPORTIONING Manufacturer’s recommended W/p should be used and usage according to scoops be avoided instead preweighed envelope should be used. The water should be free of contaminants and distilled water in properly gauged. Mixing can be done be hand or by mechanical and automatic mixers. When hand mixing is employed clean parabolic rubber bowl and a stiff bladed stainless steel spatula should be used.

Once water is poured in the bowl, powder is sifted in. mixing is commenced vigorously. Periodic wiping of the blade against the inside of the bowl with the spatula to ensure the wetting of all the powders and breaking up the lumps or agglomerates.

It should be continued till a smooth mix is obtained. At this point further mixing can be done in vacuum for elimination of air bubble. Vibrations can also be used but they compare poorly to vacuum mixing. With the spruing completed with centering of wax pattern and sprue in ring investing is done. With a paint-on technique a thin coat of a surfactant or debublizer is applied on the wax pattern and the sprue and the reservoir with spray or by sable hair brush.

With a sable hair and brush investment is picked up with the tip and teased ahead of the brushtip to avoid air entrapment. A thin coat of investment is applied.

The rest of the investment is poured along the sides of the ring, which is mounted on a vibrator or if a vibrator is not present then gentle thumping of the table or use of knurled handles on the table to produce vibrations is done.

The investing ring is tapped and before it sets the top most layer is wiped flush.

Alternatively with the mechanical vacuum investing technique can be used.

It has the advantage of eliminating air bubble formation very effectively. Vibrations should be set at low amplitude, high frequency setting to avoid dislodgment of the wax pattern and sprue alike.

STORAGE Set investment moulds should be stored at 70% humidity in sealed containers to avoid dimensional changes and drying of the investment materials.

12

WAX BURNOUT AND COMMENCING CASTING It should be done once the investment sets and both setting expansion or hygroscopic setting expansions are reached. Usually 45 min for setting expansion and 1½ - 2 hrs for hygroscopic setting expansion. Manufactures recommendation should be followed with regards to time of casting.

DIE STONE – INVESTMENT AKA DIVESTMENT Divestment is a commercial gypsum bonded material which is mixed with a colloidal silica liquid. The die is made from this mix and the wax pattern constructed on it. Then the entire assembly (die and pattern) is invested in the divestment, thereby eliminating the possibility of distortion of the pattern upon removal from the die or during the setting of the investment.

SE – 0.9% TE- 0.6% at 677°C. It is not recommended for high fusing alloys For this purpose DVP or divestment phosphate bonded investment is used.

DVP is also usable for making ceramic restorations.

PHOSPHATE BONDED INVESTMENT Eugene skinner in 1963 stated that : “I am inclined to predict that the dental investment of the future may be phosphate bonded and not gypsum bonded.”

Future is here and the prediction has come true especially with the advent of alloys requiring temperatures above 700°C. Composition Refractory : 80% usually cristobalite or quartz or mixture of the two.

Binder consists of magnesium oxide (basic) and a phosphate that is acidic in nature.

Historically phosphoric acid was used but now mono ammonium phosphate has been incorporated into the powder. Carbon is added to help produce clean castings and aid in dig out of the casting.

13 SETTING REACTION When the casting alloy is gold, carbon can be used, but when silver palladium or base metal alloys are to be cast, non carbon containing investment should be used.

Carbon makes such alloys brittle this reaction occurs below 1504°C hence if the temperature of casting of a high Pd exceeds this temperature phosphate bonded investments without carbon should be used. Even a carbon crucible should be not be used.

The binder system of MgO with NH4H2Po4 has two components a. A water soluble phosphate ion b. A component reacts and PO4 at room temperature The reaction on adding water occurs at these two interfaces, an acid base reaction follows between MgO and NH4H4PO4. Water reacts with magnesia at the surface of the water soluble phosphate ion forming a binding media with filler particles embedded in the matrix.

NH4H2PO4 + MgO + H2O  NH4MgPO4 .6H2O

+ H 2O

(Magnesium Ammonium Phosphate)

The water produced by this reaction at room temperature lowers the viscosity of the mix as spatulation continues. However phosphates are quite complex and he reaction is not a simple. One version is that magnesium ammonium phosphate is polymeric. This predominantly polymeric form coagulates around the excess MgO and fillers.

In heating this binder undergoes various thermal reactions NH4MgPO4 .6H2O + H2O (Colloidal type) Prolonged setting

at RT or dehydration at 50°C

(NH4 Mg PO4 .6H2O)n Dehydrated at 160°C (NH4 Mg PO4 . H2O)n Heated at 300-650°C (Mg2P2O7)n (Non crystalline polymeric phase) Heated above 690°C

14 Mg2P2O7 (Crystalline) + MgO Heated above 1040°C Mg3 (PO4)n

These investments can be mixed with water or silica sol in water. The normal setting reaction yields a slight contraction of 0.08% when water is used for mixing instead of special liquid. This normal setting contraction is replaced by a net setting expansion of 1.20%, when silica sol is used. Silica sol brings about a hygroscopic expansion and also increases its strength considerably.

ADA specification no 42 for dental phosphate bonded investments, specifies two types of investments for alloys having a solidus temperature above 1080°C. Type I

For inlays crowns and fixed prosthesis

Type II

For partial dentures and other cast removable restorations

Type I phosphate investments are now also used for casting of Co-Cr and Ni-Cr alloys along with alloys based on gold, platinum and palladium to which porcelain so fused for FPD. Some variations are used for cast titanium alloys. They are also used as brazing investments More recently modifications have been made for them to make refractory dies for the fabrication of custom veneer facings from porcelains.

ADVANTAGES 1. High strength materials making them resistant to impact of molten metals during casting 2. Higher values of thermal and setting expansions 3. Able to withstand temperature of 900°C for upto 1 ½ hours and temperature of upto 1000°C for short periods of time.

DISADVANTAGES 1. Cannot be used for c. temp greater than 1375°C 2. Divestment is difficult due to high strength

15 ADA SPECIFIED PROPERTIES 1. Casting investments a. Compressive strength

2.5 MPa / min Type I 10 MPa/min Type II

b. Setting expansion

0.4% normal 0.6% - 0.8% HSE

c. Thermal expansion

0.8% 50:50 water dilution of liq 1%-1.2% undiluted liq

2. Refractory die stone a. Compressive strength

13 MPa / min

b. Setting expansion

±30% manufactures claim

c. Thermal expansion

±15% manufactures claim

Market preparations GC fujivest (GC Belgium) Hi Temp (Whip – Mix) Ceramigold (Whip –Mix) Complete (J.F. Jelenko) Deguvest (Degussa) Mixing ratio 11 ml water / 60 gm powder 11 ml liquid / 60 gm powder Setting time -

SE (1 hr) 0.15% 0.30%

TE (at 700°C) 1.3% 1.45%-1.75%

4 min (initial set) 6-8 min (final set)

IMMERSION IN WATER In a study Hutton and Marshal in (JPD, 1995) the hygroscopic expansion so obtained by immersion in water bath at room temperature at different times i.e. 1 hour and at 24 hours, Time of immersion 1 hour 24 hour

Thermal expansion at 700°C ~1.66±0.12% ~ 1.71 ± 0.09%

16 Pointing towards a delayed hygroscopic expansion of phosphate bonded investments Setting expansion upto

0.3% completes within one hour

Hygroscopic expansion upto

0.4% (includes the SE)

Thermal expansion Time 1 hr 24 hr

700째C Distilled H2O Special Liq 1.06% 1.64% 1.17% 1.69%

800째C Distilled H2O Special Liq 1.03% 1.67% 1.18% 1.69%

EFFECT OF MANIPULATIVE TECHNIQUES Factor

Effect

High W/P ratio

S.E. increased

High colloidal silica Immersion in water after setting Delay before heating

T.E. Increased S.E. and T.E. increased Delayed hygroscopic expansion T. E. decreased

ETHYL SILICATE BONDED INVESTMENTS These investments have been used since the early 1930s to make the molds for the casting of removable partial dentures of cobalt chromium alloys. They are occasionally used the casting nickel based alloys.

Composition Powder

Refractory particles of silica and glass, calcined magnesium oxide and

Liquid

other refractory oxides in small quantities.

Either stabilized alcohol solution of silica gel or 2 liquids consisting of a. Ethyl silicate b. Acidified (HCl) denatured mix of ethanol

A three liquid system may also be supplied consisting of -

Stabilized colloidal silica

-

Ethyl silicate

-

Denatured ethyl alcohol

In the liquid following reaction takes place Si (OC2H5)4 + 4H2O

HCl

Si (OH)4

+

4C2H5OH

17 Ethyl silicate

Hydrated silica

Ethanol

The binding mechanism starts when the liquid is mixed with the powder nSi(OH)4 + MgO  MgO [Si (OH)4]n

A coherent mass (gel) of polysilicic acid is then formed in which the refractory silica (cristobalite) is trapped. This is accompanied by a setting shrinkage of very small value. The gel is dried at a temperature below 168°C, during drying the gel loses water and alcohol. Thus a net volumetric shrinkage occurs which reduces the size of the set investment mold. This occurs in addition to setting contraction and termed as GREEN SHRINKAGE. Thus before casting is done both the setting contraction and green shrinkage should be compensated for by thermal expansion technique by heating to 1,177°C. ADA specification 91 has specified the following properties for ESBI Compressive strength

 1.5 MPa min

Setting contraction

 0-0.4%

Thermal expansion

 1.5-1.8% at 1,177°C.

Setting time

 ± 30% claimed by manufacturer.

ADVANTAGES AND DISADVANTAGES 1. Castability of high fusing alloys Co-Cr. Ni-Cr. 2. Good surface finish 3. Low distortion 4. High thermal expansion 5. High reproducibility of fine details due to reduced density 6. Ease of divestment a. Added processing attention (resin – model reinforced dies, pattern coats). b. Extra precautions in handling low strength fired molds c. Precise casting temperatures to avoid cracking of investments

18

BRAZING INVESTMENTS When brazing (soldering) parts of a restoration such as clasps on a RPD, the parts must be surrounded with a suitable investment material before heating. The assembled parts are held together temporarily with sticky wax till the investment sets ADA spec. No. 93 lists two brazing investments Type I

Gypsum Bonded Brazing Investment

Type II

Phosphate Bonded Brazing investment

Brazing investments are required to posses low setting and thermal expansion so that the assembled parts do not shift in position during setting and heating of the investment.

Property Setting time Compressive strength Setting expansion Thermal expansion

Value ±30% claimed by manufacturer 2.0-10.0 MPa ±15% claimed by manufacturer ±15% claimed by manufacturer

INVESTMENT MATERIALS FOR TITANIUM CASTING A variety of investment formulations for the casting of titanium have been developed over the past several years. These might be classified as 1. Phosphate bonded 2. Ethyl silicate bonded

19

3.

Cemented …..according to the binder source Mold casting

1.

2.

3.

4.

5.

temp(°C) 800 RT 600-700 RT 600-700 600-700

100 6. 100 600-700

Binder

Refractory

Burnout temp

Al2O3, ZrO2 Al2O3, SiO2 ZrSiO4-SiO2 Al2O3, LiAlSi2O6 MgO, Al2O3

(°C) 1,150-1,200 900-1000 1,100 1,050 900

Ethyl silicate

MgO,SiO2, Al2O3

900

Aluminous cement

MgO, Zr. MgO, Zr.

850 850

MgO, Al2O3

900

MgO phosphate

Properties Standards do not exists, nor are they in development for investments for the casting of titanium and titanium alloys for dental purposes.

At temperature above 600°C titanium readily reacts with elements such as H, N2 and oxygen to form surface oxide layers measuring upto 100µm thick. These oxide layers reduce the properties of Ti or Ti alloys to make it more brittle.

Furthermore titanium may scavenge the oxygen in bound state of the investments leading to their rapid degradation. New methods of casting Ti and newer refractory formulations or modifications in the existing high heat investments have been made all in an order to prevent investment break down and titanium contamination.

One such method has been to reduce the amount of silica and add Zirconia and Alumina. The other has been to cool the mold to lower temperature to avoid, reduce the oxidation of Ti and reduction of investment which occurs at 600°C. however contamination was still encountered. Addition of Alumina and Zr as refractories solved the problem but the thermal expansion that was obtained was low and the presence of Silica still produced contamination of castings Further improvements came in the way of adding magnesia (MgO) and Alumina (Al2O3) to phosphate bonded investments. A very large thermal expansion of about ~1% was obtained due to formation of spinel of MgO and Al 2O3 at 1,150°C this temperature was reduced by adding magnesia acetate.

20 Another approach has been to use SPODUMEN (Li 2O-Al2O3-SiO2), which expands irreversibly upon heating to 900°C. Ethyl silicate bonded investments react less with titanium but are more complex in requirement for proper use.

A more recent development had been the development of an investment using magnesia bounded by an aluminous cement (CaO- Al 2O3) and containing 3% zirconium powder by weight. The aluminous cement serves as a binder for the refractory magnesia. The expansion is provided by the formation of zirconia oxide.

Other improvements such as vacuum mixing and investing, electric arc heating aid usage of alloyed titanium with lower casting temperature and lesser reactivity of Ti have been employed. Two such alloys being Ti-Pd, Ti-Cu and another one Ti-Al-V. CONCLUSION Irrespective of the material used binders in gypsum investments expand by 0.2% during setting contract at all elevated temp expand during hygroscopic conditions.

Refractories do not take part in the setting reaction. They expand during heating and during immersion in water.

Modifying agents such as boric acid and soluble halide anions negate the violent contractions that occur in gypsum binder during heating.

The temperature of gypsum investments at which degradation occurs is 700°C, and adequate thermal expansion also occurs at this range, while effective hygroscopic expansion occurs at 480°C will below the 700°C mark. Hence whenever possible compensation of casting shrinkage is needed HSE technique should be used if possible.

Phosphate bonded investments should be used when the casting temp of alloys is above 1000°C. The temperature of the investment mold should be as close to the melting temperature of the alloy during casting to avoid localized shrinkage and incomplete castings and cracking of the mold due to thermal shock.

Techniques such as vacuum mixing and investing should be employed in an order to reduce air bubble entrapment.

21 Finally the values of casting shrinkage for an alloy and the method of shrinkage compensation

should

be

calculated

and

determined

beforehand.

Manufacturers' recommendations regarding dispensing, mixing and investing should be strictly followed at all times.

ALLOY TYPES & THEIR CASTING SHRINKAGE VALUES

Type Type Type Type Type Type Type

Alloy I Au II Au III Au III low Au IV Au III Ag-Pd IV Ag-Pd

PFM (Au) PFM (Ag-Pd) PFM (High Pd) Co-Cr No-Cr Ti Ti-Cu

Melting range 943-960°C 924-960°C 932-960°C 843-916°C 921-943°C 1021-1099°C 930-1021°C

Shrinkage 1.56% 1.37% 1.42% ~1.3% ~1.5% ~ 2.03%

1271-1304°C 1232-1304°C 1155-1302°C 1350-1450°C 1300-1450°C > 1500°C ~1350°

~1.5% ~ 2.03% 2.3% 2.0% 1.0-1.6%

Ti-Pd

ALLOY TYPES, INVESTMENT TO BE USED & COMPENSATION OF SHRINKAGE Type Type Type Type Type Type Type

Alloy type I Au (1.56%) II Au (1.37%) III Au (1.42%) IV Au (1.5%) III low Au (1.3%) III Ag-Pd (~2%) III Ag-Pd (~2%)

PFM Au (~1.5%) PFM (Ag-pd) (~2%) PFM (High pd) (~2%) Ni-Cr (2%) Co-Cr (2.3%) Ti & Ti alloys (1.0 – 1.6%)

Investment

Gypsum

Phosphate(-c) Phosphate

Phosphate(-c) Ethyl silicate

Shrinkage Compensation <1%HSE + 0.6% TE 0.4% SE + 1.0% TE 0.4% SE + 1.0% TE <1% HSE + 0.6% TE 0.4% SE + 1.0% TE HSE 0.7% + 1.3% TE HSE 0.7% + 1.3% TE SE 0.2% + 1.3% TE HSE 0.7% + 1.3% TE HSE 0.7% + 1.3% TE HSE 0.7% + 1.3% TE HSE 0.7% + 1.3% TE 0.4% SE+1.5% TE

22 Ti-Cu (1.0 – 1.6%) Ti-Pd (1.0-1.6%)

Newer

SE 0.2% +1.3% TE

Phosphate Phosphate (- c)

SE 0.2% + 1.3% TE

BIBLIOGRAPHY 1.

Skinners science of dental materials 9th Ed

– Phillips

2.

Selection of dental materials 2ND Ed

– O Brian

3.

Restorative dental materials 11th Ed

– Craig

4.

Dental Lab Procedures vol 2

– Rudd and Marrow

5.

Dent mat 2002 vol 18 Pg 512-515

6.

Dent mat 2001 vol 17 Pg 247-252

7.

JPD 2000, vol 84 No 1 Pg 27-37

8.

JPD 1999, vol 81 No 2 Pg 387-391

9.

JPD 1999, vol 81 No 1 Pg 129-134

10. JPD 1996, vol 76 No 3 Pg 519-523 11. JPD 1995, vol 75 No 2 Pg 127-131 12. JPD 1993, vol 70 No 2 Pg 121-125 13. IJP 1992, vol 5 No 5 Pg 403-408 14. JPD 1984, vol 52 No 5 Pg 361-370