Material Matters: A Review of Ethyl-Based Investments in Dental Practice

Abstract:

Ethyl-based investment materials have gained increasing attention in dentistry due to their favorable handling properties, thermal stability, and minimal expansion characteristics. These materials are primarily used during the casting procedures for dental restorations, playing a crucial role in achieving dimensional accuracy and material compatibility. This review aims to explore the composition, types, setting mechanisms, and clinical applications of ethyl-based investment materials. It further compares their performance with traditional investment materials like phosphate-bonded and gypsum-bonded alternatives. The review highlights recent advances, including the incorporation of modifiers to improve strength and reduce porosity. By analyzing current literature, this paper provides a comprehensive understanding of the advantages, limitations, and future potential of ethyl-based investments in restorative and prosthetic dentistry.

Keywords:

Ethyl-based investments, dental materials, casting procedures, investment materials, dental prosthetics, dimensional stability, restorative dentistry, material science in dentistry

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INTRODUCTION

The binding material for investments used with casting high-melting point alloys is a silica bonding ingredient. This type of investment may derive its silica bond from ethyl silicate, an aqueous dispersion of colloidal silica, or from sodium silicate. One such investment consists of a silica refractory, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid. The product of the hydrolysis is a colloidal solution of silicic acid and ethyl alcohol, which can be written as.1

Si(OC2H5) + 4H2O Si(OH)4+ 4C2H5OH

In practice, however, the reaction is more complicated, and instead of tetrasilicic acid, which is converted into SiO2.2H20, a polymerized compound of silicon is formed with the following structure.

This material has an even higher silica content and better refractory properties than the SiO2.2H20. Ethyl silicate has the disadvantage of giving off flammable components during processing, and the method is expensive; thus, other techniques and methods have been developed to reduce the use of this material. Sodium silicate and colloidal silica are more common binders of the silica type.1

TYPES

Based on the binder used two types may be seen.

1. One such investment consists of silica refractory, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid. The product of the hydrolysis is the formation of a colloidal solution of silicic acid and ethyl alcohol.

Si(OC2H5) 4 + 4H2 HCL Si(OH)4 + 4C2H5OH

1. Sodium silicate and colloidal silica are more commonly used as binders because ethyl silicate has the disadvantage of containing inflammable components which are required for manufacture.2

APPLICATIONS

Ethyl silicate is used for:

1. The construction of the high-fusing base metal partial denture (cobalt-chromium alloys)
2. Occasionally for the casting of nickel-based alloys.

COMPOSITION

It is Supplied as3:

• Powder
• Liquid-Available in two bottles

COMPOSITION

• REFRACTORY

Powdered quartz and cristobalite act as refractory materials. A small amount of magnesium oxide is added to the powder to reduce the pH of silica gel during manipulation.

• BINDER

Silica gel in the form of sodium silicate and colloidal silica is the commonly used binder that converts to silica at high temperatures. Silica gel can be produced by several methods, two of which are as follows:

1. By adding an acid or acid salt to sodium silicate, a bonding silicic acid gel is formed.

2. By mixing ethyl silicate with diluted hydrochloric acid to form silicic acid solution.9

Ethyl silicate is commonly used. It is the liquid component of this investment. It is supplied as two bottles of special liquid with which the investment powder is mixed. One bottle contains properly diluted water-soluble ethyl silicate solution. The other bottle contains properly diluted hydrochloric acid. The binder solution has to be prepared before manipulation of the investment material by mixing the contents of both the bottles. Ethyl alcohol can be used to improve the miscibility of both the components. Hydrolysis takes place between water present in the component and ethyl silicate in the presence of hydrochloric acid and ethyl alcohol. The formed product is called silicic acid solution; this solution is usually prepared and stocked in a dark bottle.

(C2H5 O)4 Si + 4H2O → Si(OH)4 + 4C2H5OH

A colloidal sol of polysilicic acid is formed since a polymerized form of ethyl silicate is used.

• LIQUID

Today this investment is usually supplied with two bottles of special liquid, instead of water, with which the investment powder should be mixed. In one of the bottles the manufacturer usually supplies a properly diluted water-soluble silicate solution. The other bottle usually contains a properly diluted acid solution, such as a solution of hydrochloric acid. The contents of each bottle can be stored almost indefinitely. Before use, mix an equal volume from each bottle and allow the mixed liquids to stand for a prescribed time, according to the manufacturer's instructions, so hydrolysis can take place and freshly prepared silicic acid forms.4

MANIPULATION

• MIXING

The silicic acid solution is mixed with the quartz or cristobalite, to which is added a small amount of finely powdered magnesium oxide to render the mixture alkaline5.

A sol (colloidal solution) is first formed by hydrolysing ethyl silicate in aqueous solution with dilute hydrochloric acid:

Si(OC2H5) + 4H2O Si(OH)4+ 4C2H5OH

When this is mixed with the investment powder, which contains magnesium oxide simply to neutralize the acid, the sol is converted to a gel. The silicic acid undergoes a condensation reaction (eliminating water) to form a three-dimensional network of high polymer and condensation silicone impression material.

2Si (OH)4 (OH)3SI-O-Si(OH)3+H2O

• GELATION

A coherent gel of polysilicic acid which is formed accompanied by a setting shrinkage. This soft gel is dried at a temperature below 168?C (334?F). During the drying process, the gel loses alcohol and water to form a concentrated, hard gel. As might be expected, a volumetric contraction accompanies the drying, which reduces the size of the mold.

This contraction is known as green shrinkage, and it occurs in addition to the setting shrinkage. This gelation process is likely to be slow and time-consuming. An alternative and faster method for the production of the silica gel can be used. Certain types of amines can be added to the solution of ethyl silicate so that hydrolysis and gelation occur simultaneously.

As the investment powder is nearly all silica, the expansion on heating is proportionately larger than for other investments, depending of course on the choice of allotrope or mixture (and because there is no shrinkage due to a decomposition reaction). But while higher mould temperatures may be tolerated by silica-bonded investments (up to ~1200?), compared with ~1000? for phosphate-bonded), there is a tendency to sinter at high temperature, and this may offset some of the expansion on heating.

• GELATION TIME

The gelation process is slow and time-consuming. An alternative and faster method for the production of the silica gel includes simultaneous hydrolysis and gelation, which can be promoted by amines such as piperidine.2

Disadvantage of this method:

• This procedure is accompanied by an unacceptable shrinkage, which is a result mainly of the hydrolysis reaction. Alloys, Casting and Investments It follows that with an investment of this type, the mould enlargement before casting must compensate not only for the casting shrinkage of the metal but also for the green shrinkage and for the setting shrinkage of the investment.

• INVESTING

In order that the material should have sufficient strength at the casting temperature, it is necessary to incorporate as much powder as possible into the binder solution.2

How can the Liquid/Powder ratio be reduced?

This process is aided by a gradation of particle sizes such that small grains fill in the spaces between the larger grains. The powder is added to the hydrolysed ethyl silicate liquid and mixed quickly. A very thick, almost dry mix of investment is used and vibrated into a mould that has an extra collar to increase the height. This allows the heavier particles to settle quickly while the excess liquid and some of the fine particles rise to the top. In about 30 min, the accelerator in the powder hardens the settled part, and the excess at the top is poured off. Thus, the L/P ratio in the settled part is greatly reduced, and the setting shrinkage is reduced to 0.1%.3

• BURNOUT

During burnout of ethyl silicate-bonded investment, extra care must be exercised during handling and burnout because flammable alcohol is given off. If the ethyl silicate-bonded investment is heated to a sufficiently high temperature, some silica converts to quartz and provides added expansion. This type of investment can be heated between lO50°C and 1180°C and is compatible with the higher fusing alloys. Thermal Stability of Silica-bonded materials have sufficient strength at the high temperatures used for casting base metal alloys.2

PROPERTIES

The various properties of ethyl silicate as proposed by ANSI/ADA Specification No. 91 (IS0 11246)

1. EXPANSION

Like other investments, ethyl silicate-bonded investment does not show setting expansion. It undergoes only shrinkage during setting. When these investments are heated to sufficiently high temperatures, considerable expansion takes place. This investment can be heated between 1090°C and 1180°C and this is compatible with high-fusing alloys. Continued heating causes considerable expansion which compensates for both the shrinkages. The total linear expansion is equal to the total linear thermal expansion3.

• The linear thermal expansion must not differ by more than 15% from the time stated by the manufacturer.
• A total expansion of 1.6% is attained at a temperature of about 600°C.

The factor influencing the expansion and strength of the investment is the amount of powder to be incorporated. This process is aided by gradation of small particle sizes. The smaller grains fill the space between larger grains giving a closer pack. This results in better expansion and strength. The investment procedure for ethyl silicate bonded investment is little complicated and time consuming. Ethyl alcohol vapor liberated during hydrolysis reaction can ignite at elevated temperatures. This is one of the reasons why the use of ethyl silicate-bonded investment in dentistry is limited.

1. SETTING TIME

The setting time must not differ by more than 30% from the time stated by the manufacturer.6

1. POROSITY

The silica-bonded materials are so closely packed that they are virtually porosity-free and there is a danger of back pressure building up which will cause:

1. The mould to be incompletely filled

2. The castings to be porous.

These problems can be overcome by making vents in the investment, which prevent the pressure from increasing.2

1. STRENGTH

Proposed ANSI/ADA Specification No. 91 (IS0 11246) for ethyl silicate casting investments

• The compressive strength is around 1.5 MPa.6

1. STORAGE

Stock solutions of the hydrolyzed ethyl silicate binder are normally made and stored in dark bottles. The solution gels slowly on standing and its viscosity may increase noticeably after three or four weeks. When this happens, it is necessary to make fresh solution.

ADVANTAGES AND DISADVANTAGES

Advantage

1. They have the ability to cast high temperature cobalt-chromium and nickel-chromium alloys. 7

2. They produce good surface finish.

3. They have high thermal expansion.

4. Low distortion.

5. Since they are more porous than phosphate bonded investments, thin and fine details can be reproduced.

6. They have low fired strength, which makes divestment easier in comparison to phosphate bonded investment.

Disadvantages 8

1. The extra precaution is required in handling the low strength fired moulds.
2. The silica powder (usually cristobalite) has to have a carefully-controlled particle size distribution to improve the strength, but settling and segregation occurs in transit due to vibration, and careful remixing is essential before use.
3. The silica sol is unstable and needs to be freshly prepared regularly.
4. Heating, likewise, has to be done very carefully, firstly to dry the gel, and then to avoid cracking on a ring liner allows expansion of the investment mould. Expansion on heating of a quartz-based, gypsum-bonded investment compared with that of a plain carbon steel casting ring. taking it up to the casting temperature. This type of investment is rather prone to cracking because of the large expansion on heating.
5. In addition, vacuum investing (ordinarily used to eliminate air bubbles, especially around the pattern) is not permissible because the ethanol would readily boil under the reduced pressure and disrupt the structure. In contrast, phosphate-bonded investments are simple to use and suffer none of these drawbacks.2

CONCLUSION

The binder is a silica gel that reverts to silica (cristobalite) on heating in case of ethyl based investment materials . Several methods may be used to produce the silica or silicic acid gel binders. When the pH of sodium silicate is lowered by the addition of an acid or an acid salt, a bonding silicic acid gel form. The addition of magnesium oxide strengthens the gel. An aqueous suspension of colloidal silica can also be converted to a gel by the addition of an accelerator, such as ammonium chloride.6 Ethyl silicate–bonded investment is losing popularity because of the more complicated and time-consuming procedures involved with its use; however, it is still used in the construction of the high-fusing base metal partial denture alloys.8

REFERENCES