REFRACTORY MATERIALS

Dr Khushboo Racca

I MDS

CONTENTS Introduction

Refractory materials- Properties

Investment materials1. Gypsum bonded investment

- Classification- Composition- Properties

2. High temperature investmentsi. Phosphate bonded investments

- for all ceramic restorations- for titanium based alloys

ii. Silica bonded investments

Divestments

Soldering investments

Dental ceramics

Conclusion

INTRODUCTION

Refractory materials-This material is usually a form of silicon dioxide, such as Quartz, Tridymite, Cristobalite, or a mixture of these. Refractory materials are contained in all dental investments, whether for casting gold or high melting point alloysRecently Magnesia, Alumina, Zirconia are also added in the list of refractory materials

The main properties due to which they are known as refractory materials

1. Poor thermal conductors & high heat capacity2. Change in crystalline structure on heating which decreases

density & increases volume without breakage of bonds

Example - 1. Quartz when heated at 575⁰C (1067⁰F), α form gets

converted to β form2. Cristobalite when heated at 200-270⁰C (392-518 F), α ⁰

crystalline form gets converted to β crystalline form3. Tridymite undergoes inversions over a range of

temperature.Two inversions at 117⁰C & 163⁰C4. Fused quartz does not undergo inversion at any

temperature. Also it has low co-efficient of thermal expansion

Materials that utilizes refractory materials-1. Investment materials2. Divestment3. Soldering investment4. Dental ceramics

INVESTMENT MATERIALSThese are the materials in which the wax patterns are casted.

They should posses the following requirements-- Sufficient mold expansion to compensate casting shrinkage- Fine particle size for smooth surface- Easy manipulation- Smooth consisting on mixing- Porous for gas escape- Must not decompose on high temperature- Adequate strength- Casting temperature should not be critical- Should break easily on casting- Should not react with metal- Economical

Composition

Refractory material-It constitutes 60-65% of the investment material. It withstands high temperature without decomposition. It resists heat to which it is subjected and force applied during casting. It compensates casting shrinkage

BinderIt constitutes 20-30% of the investment material. It helps forming a coherent solid mass. Calcium sulfate hemihydrate, Sodium silicate, Ethyl silicate, Ammonium sulfate, Sodium phosphate acts as binder

Other Chemicals

It constitutes 5% of the total mass.Sodium chloride, boric acid, potassium sulphate, graphite, copper powder etc are added to modify physical properties

CLASSIFICATION

A. BASED ON PROCESSING TEMPERATURE1. High Temperature

- Phosphate Bonded Investment- Silica Bonded Investment

2. Low Temperature- Gypsum Bonded Investment

B. BASED ON REFRACTORY MATERIAL1. Quartz Investment2. Cristoballite Investment

C.BASED ON TYPE OF BINDER USED 1. Gypsum bonded investments 2. Phosphate bonded investments3. Silicate bonded investments

GYPSUM BONDED INVESTMENTS

ADA Specification No.2 for casting investments for dental gold alloys encompasses three types of investments

1. TYPE 1 – for Inlays or crowns. Undergoes thermal expansion

2. TYPE 2 - for Inlays or crowns. Undergoes hygroscopic expansion

3. TYPE 3 - Fixed Partial Dentures with Gold alloys

Composition

1. Refractory materialIt constitutes about 60-65%.Silica is used in the form of Quartz/ Cristobalite. It regulates thermal expansion and also provides mold expansion

2. α hemihydrate/ dental stoneIt is 30-35% in the investment. It acts as a Binder. Imparts strength to the material. It Contributes to mold expansion by setting expansion 0.06-0.5%. if it is heated above 700⁰C, disintegrates into Sulphur di oxide and Sulphur tri oxide

3. Modifiers 5%Carbon added acts as a reducing agent by providing a non oxidizing atmosphere. Coloring agent is added in the form of powdered Copper.Boric acid, NaCl regulates setting expansion and setting time. It prevents most of the shrinkage when gypsum heated above 300˚C

Setting time A.D.A specification no 2 states that for inlay casting investments setting time should not be less than 5 min & not more than 25 min. Modern inlay investments should have setting time of 9-18 min

Effect of temperature on investment

1. On heating-Quartz expands to 1.4% at 600⁰C. Pure Cristobalite expands to 1.6% at 250⁰C. Expansion of Tridymite at 600⁰C is less than 1%. α form stable at room temperature while β form stable on heating. All three forms of silica expand on heating.

Highest expansion is shown by Cristobalite while the lowest by Tridymite

2. On cooling On cooling to room temperature, investment exhibits an overall contraction.On reheating, investment does not expand to the previous level. Reheating of investment causes internal cracks

ExpansionExpansion is needed for compensation of casting shrinkage of alloy .It is of 3 types :

1. Normal setting expansion. 2. Hygroscopic setting expansion3. Thermal expansion

Setting expansionSilica & gypsum results in greater setting expansion than gypsum alone. Silica helps in expansion by interfering with the intermesh creating outward thrust

Hygroscopic expansionThere are two ways of achieving hygroscopic expansion

1. Water immersion hygroscopic techniqueIt is achieved when Investment mould is placed in water during initial set. It is five times the normal setting expansion. Certain investments are specially designed for hygroscopic expansion. According to ADA Specification, type II investments should have a minimum setting expansion of 1.2% & maximum permissible is 2.2%. Investments used in the hygroscopic technique usually contain Quartz or Tridymite which have lower thermal expansion but higher hygroscopic expansion

Factors Affecting hygroscopic Setting Expansiona) Effect of compositionb) Water powder ratioc) Effect of temperatured) Effect of time of immersione) Effect of spatulationf) Shelf life of investmentg) Effect of confinementh) Effect of amount of added water

2. Water added techniqueIn this, water is placed on the upper surface of the investment within the casting ring. It produces readily controlled expansion

Thermal expansionIt is directly related to the amount & type of Silica used. Contraction of gypsum is entirely balanced when Silica content is 75% . Investments containing Cristobalite expand earlier than those containing Quartz because of lower inversion temperature of Cristobalite compared to Quartz. For Type I investment thermal expansion should neither be less than 1% nor greater than 1.6%. If hygroscopic expansion is used, thermal expansion of 0.5-0.6% sufficient. If thermal expansion used with normal setting expansion, then thermal setting expansion should be 1-2%. Investments used in the thermal technique contains cristobalite which has a high thermal expansion.

Factors affecting thermal expansion1. water powder ratio2. Effect of thermal modifiers

STRENGTH

According to ADA Specification No 2, compressive strength for inlay investment should not be less than 2.5 MPa 2 hrs after testing. This is adequate to prevent fracture or chipping of mould during heating and casting of gold alloy

HYGROSCOPIC- THERMAL INLAY CASTING INVESTMENTIt is used for hygroscopic or thermal type of expansion. It consists of blend of Quartz & Cristobalite. For hygroscopic casting technique it is heated to 482⁰C while for thermal casting technique it is heated to 649⁰C

HIGH TEMPERATURE INVESTMENTS

ISO classification

Type 1- fabrication of inlays, crowns, & other fixed restorationType 2- fabrication of partial dentures or other removable appliancesType 3- fabrication of casts for soldering, brazing & weldingType 4- for refractory dies

PHOSPHATE BONDED INVESTMENTSIt is the most common type of investment for casting high melting alloys

Types-Type I - Inlays, crowns & other fixed restorationType II- Partial dentures & other cast removable restorations

Composition1. Binder- 20%

Acidic part is ammonium diacid phosphate while the basic part is magnesium oxide

2. Refractory- 80%Silica (Cristobalite or Quartz or mixture of two) also combination of different particle sizes are used. It provides high temperature thermal shock resistance & thermal expansion at high temperature

3. Modifiers- Carbon acts as reducing agent. It is added when the casting alloy is gold. It is not used for Silver-palladium alloys, base metal alloys. Carbon crucible should also be avoided when these metals needs to be casted. Gold alloys used with porcelain are not premelted in presence of carbon.

4. Colloidal solution of silica preferred over water due to dual effect of increasing setting expansion & strengthening the set material.

SETTING & THERMAL EXPANSIONPhosphate bonded investments when mixed with water exhibit shrinkage between 2000C to 4000C. Colloidal silica when used for mixing replaces this contraction. Predominantly for base metal alloys, 33% dilution of colloidal solution is used. Early thermal shrinkage of phosphate bonded investments is because of decomposition of binder and is accompanied by evolution of ammonia

Setting & working time-Working time - 2 minutes Setting Time - 1 hour

Warmer the mix, faster it sets.Increasing mixing time, fastens the setting reaction. Increasing L/P ratio, increases working time

Advantages1. High green strength. It is the prefired strength of an

investment acquired through some chemical reaction at or near room temperature. It is compared with fired strength.

2. High firing strength. Hence less mold cracking and few fins on casting

3. Can withstand temperature up to 1000˚C for short period of time

Disadvantages1. When temperature is raised to 1375˚C, mold breaks down

producing roughen surface of casting2. Special liquid that is used for mixing produces less porous

mold which leads to incomplete casting3. Produces oxides that is difficult to remove from castings

INVESTMENTS FOR ALL CERAMIC RESTORATION

Two types of investment are used1. For cast glass technique

It is provided by the manufacturer & it utilizes phosphate bonded refractories

2. Refractory dieIt is obtained by pouring the investment in the impression directly. Die ovtained is then heated to remove gasses.

These materials must accurately produce the details, remain undamaged during firing & have a thermal expansion compatible with ceramic. These materials are also phosphate bonded & they generally contain fine grained refractory fillers.

INVESTMENTS FOR TITANIUM BASED ALLOYS

Molten Titanium is highly reactive with Oxygen. It also reduces oxides in investments. It dissolves Oxygen, Nitrogen & Carbon present in investments which embrittles the casting. Hence conventional investments are not used for casting titanium alloys.

CLASSIFICATION

A. Phosphate bonded investmentsB. Ethyl silicate bonded investmentsC.Cemented investments

Refractories used are Silica, Alumina, Magnesia, Zirconia

Modifications in Phosphate bonded investments for titanium based alloys are-

1. Investment of Phosphate binder, Magnesia & Quartz. It provides little advantage of use of one form of silica over other.

2. Investment of Phosphate binder, Alumina & magnesia- Thermal expansion provided by these oxides is low

3. Investment with Phosphate bonded investments that contains both Magnesia & Alumina. It attains large expansion by the spinal reaction (MgO+Al2O3---MgO-Al2O3) at 1150-1200⁰C. Spinal forming temperature can be reduced by mixing with magnesia acetate

4. PBI with Spodumen ( LiO2-Al2O3-SiO2). Spodumen expands irreversibly from 900-1100⁰C

Ethyl- silicate bonded investments

Reaction between these investments & liquid Titanium is less due to highly refractory oxides in the powder. It is not used due to complex procedure

Cemented investmentsIt includes Investment of Magnesia refractory bonded to Aluminous cement( CaO-Al2O3) with mass fraction of 5% Zirconia powder. Oxidation of Zirconia powder during burnout produces irreversible expansion. FEF (Gibbs free energy of formation of Zirconia is 728 kj & hence it does not contaminate Titanium

SILICA BONDED INVESTMENTS

These investment materials are being used since 1930 but are slowly loosing there popularity due to complicated and time consuming procedures involved

COMPOSITIONBinder ---- Sodium silicate, Colloidal silica & Ethyl silicateRefractory ---- Silica (80%)Additives---- Magnesium oxideWetting agent

Silica bonded investments are supplied in 2 bottles- One bottle contains diluted water soluble silicate sol as sodium silicate, while the other contains diluted acid solution

Green shrinkageDuring manufacturing, Polysilicic acid gel is dried at temperature less than 168⁰C. It Leads to volumetric contraction losing alcohol & water and this is known as green shrinkage.

DIVESTMENT

It is a die stone-investment combination. Commercially obtained gypsum-bonded material is mixed with concentrated colloidal silica liquid to obtain divestment.

Die preparation-14-16ml liquid is mixed with 50 gm powder. It is used with Polysulphide or Silicone rubber impression material. Rigid impression tray is used & there should be adequate clearance between teeth & tray. Divestment is poured in the impression & it should be separated from Polysulphide impression within 3 hrs. Aerosol dry lubricant spray by Dupont- SLIP-SPRAY can be used for easy separation

Investment-Entire assembly (die and pattern) is invested in the divestment. The only concern is that of sprue attachment. It should be secured properly.

Setting expansion is 0.9% . while thermal expansion is 0.6% on heating 677°C

AdvantagesHighly accurate technique

DisadvantagesNot recommended for high fusing alloys

DIVESTMENT

Divestment phosphate or DVP is a phosphate bonded investment that is used in the same manner for high fusing metal ceramic alloys

SOLDERING INVESTMENT

It is used In the process of assembling parts of RPDADA Specification No 93 (ISO 11244) for dental brazing investments defines 2 types:-

1. Type I- Gypsum bonded dental brazing investments2. Type II- Phosphate bonded dental brazing investments

For low melting alloys the investment is similar to casting investments containing Quartz & Calcium Sulphate hemihydrates.For high melting alloys, Phosphate bonded investments are used they are designed to have lower setting & thermal expansion to prevent shifting of assembled parts

DENTAL CERAMICS

CompositionFeldspar- 60-80%- basic glass formerKaolin 3-5%- binderQuartz 15-25% - filler & refractoryAlumina 8-20%- glass formerBoric oxide 2-7%- glass former & fluxOxides of Na, K & Ca%- glass modifiersMetallic pigments

Refractory material added to ceramic helps it in withstanding high temperature

Conclusion Research is needed in laboratory material for casting procedure to help producing better fitting castings

Bibliography

1. Science of Dental Materials- Anusavice- 10th edition2. Dental materials & their selection- William 0’Brien3. Restorative Dental Materials- Craig-12th edition4. Clinical aspects of Dental Materials- Galdwin & Bagby5. Operative dentistry, modern clinicals & practise- Marzouk6. Applied Dental Materials- McCabe