Сraig. Dental Materials

Increases in hardness were obtained for model plaster but not for dental stone or high-strength dental stone. Increases in scratch resistance of 15% to 41% were observed for a high-strength dental stone impregnated with epoxy resins or a light-cured dimethacrylate resin. Generally, impregnating set gypsum with resin increases abrasion resistance, but decreases compressive strength and surface hardness.

The idea of drying molds, casts, or dies in an oven to obtain a quick, dry compressive strength and dry surface hardness of a material is not practical, because the gypsum would be dehydrated, which would reduce the strength instead of increasing it. Soaking the gypsum dies or casts in glycerin or different oils does not improve the surface hardness but rather makes the surface smoother, so that a wax carver or other instrument will not cut the stone as it slides over the surface. Mixing high-strength dental stone with a commercial hardening solution containing colloidal silica (about 30%) improves the surface hardness of the set gypsum. The Knoop hardnesses of two commercial high-strength dental stones were 34 and 77 kg/mm2 when mixed with water. When the hardening solution was used, these values increased to 62 and 79 kg/mm2, respectively. Increased surface hardness does not necessarily mean improved abrasion resistance, because hardness is only one of many factors that can affect wear resistance. Two-body abrasion studies suggest that the commercial hardening solutions do not improve the abrasion resistance of high-strength dental stones. However, the clinical relevancy of the two-body abrasion test on gypsum has not been established. Further studies of abrasion resistance and methods of measurement are needed. As discussed in the chapter on impression materials, gypsum dies abrade more readily than epoxy dies, even though the gypsum dies are harder.

Although the use of disinfectant chemicals on gypsum dies effectively destroys potentially dangerous organisms, some can damage the surface of a die. Surfaces can be eroded, and the surface hardness can be adversely affected by treatment with some commonly used disinfectants. Other disinfectants, including sodium hypochlorite so-

lutions, have very little effect on the surfaces of gypsum dies.

TENSILE STRENGTH

The tensile strength of model plaster and dental stone is important in structures in which bending tends to occur because of lateral force applications, such as the removal of casts from flexible impressions. Because of the brittle nature of gypsum materials, the teeth on the cast may fracture rather than bend. The diametral compression test for brittle materials is normally used to determine the tensile strength of gypsum products.

Some significant observations have resulted from these studies. First, the 1-hour wet tensile strength of model plaster, 2.3 MPa, is about one

model plaster in either the wet or dry condition is about one half that of high-strength dental stone in the comparable condition. Third, the tensile strength of model plaster in the wet or dry condition is about one fifth the compressive strength in the same condition (dry, 4.1 MPa in tension; 20 MPa in compression). Fourth, the high-strength dental stone shows an increased

Fig. 13-5A detail reproduction block.

difference between tensile and compressive values; for example, about 8 MPa in tension and 80 MPa in compression for the dry condition.

REPRODUCTION OF DETAIL

ANSI/ADA Specification No. 25 requires that Types 1 and 2 reproduce a groove 75 pm in width, whereas Types 3, 4, and 5 reproduce a groove 50 pm in width (see Table 13-31, An example of a detail reproduction block is shown in Fig. 13-51,Gypsum dies do not reproduce surface detail as well as electroformed or epoxy dies because the surface of the set gypsum is porous on a microscopic level (Fig. 13-6). Air bubbles are often formed at the interface of the impression and gypsum cast because freshly mixed gypsum does not wet some rubber impression tmterials (e.g., some silicone types) well. The incorporation of nonionic surfactants in polysulfide and silicone impression materials improves the wetting of the impression by slurry water. The use of vibration during the pouring of a cast reduces the

presence of air bubbles. Contamination of the impression in which the gypsum die is poured by saliva or blood can also affect the detail reproduction. Rinsing the impression and blowing away excess water can improve the detail recorded by the gypsum die material.

SETTING EXPANSION

When set, all gypsum products show a measurable linear expansion. The percentage of setting expansion, however, varies from one type of gypsum material to another. Under ordinary conditions, plasters have 0.2% to 0.3% setting expansion, lowto moderate-strength dental stone about 0.15% to 0.25%, and high-strength dental stone only 0.08% to 0.10%.The setting expansion of high-strength/high-expansiondental stone ranges from 0.10% to 0.20%. Typically, over 75% of the expansion observed at 24 hours occurs during the first hour of setting.

The setting expansion may be controlled by different manipulative conditions and by the ad-

dition of some chemicals. Mechanical mixing decreases setting expansion. As shown in Table 13-6, a vacuum-mixed high-strength stone expands less at 2 hours than when mixed by hand. Power mixing appears to cause a greater initial volumetric contraction than is observed for hand mixing. The W/P ratio of the mix also has an effect, with an increase in the ratio reducing the setting expansion. The addition of different chemicals affects not only the setting expansion of gypsum products, but may also change other properties. For example, the addition by the manufacturer of sodium chloride (NaC1) in a small concentration increases the setting expansion of the mass and shortens the setting time. The addition of 1% potassium sulfate, on the other hand, decreases the setting time but has no effect on the setting expansion.

If during the setting process, the gypsum materials are immersed in water, the setting expansion increases slightly. This is called hygroscopic expansion. A typical, high-strength dental stone has a setting expansion of about 0.08%. If during the setting process the mass is immersed in water, it expands about 0.10%. Increased expansion is observed when dental stone hardens as it comes in contact with a hydrocolloid impression.

When any of the gypsum products is mixed with water, it should be spatulated properly to obtain a smooth mix. Water is dispensed into a mixing bowl of an appropriate size and design (Fig. 13-7).The powder is added and allowed to settle into the water for about 30 seconds. This technique minimizes the amount of air incorporated into the mix during initial spatulation by hand. Spatulation can be continued by hand using a metal spatula with a stiff blade (see Fig. 13-7), a hand-mechanical spatulator (Fig. 13-B), or a power-driven mechanical spatulator (Fig. 13-9). A summary of the effect of various manipulative variables on the properties of gypsum products is presented in Table 13-9.

Spatulation by hand involves stirring the mixture vigorously while wiping the inside surfaces of the bowl with the spatula. Spatulation to wet and mix the powder uniformly with the water requires about 1 minute at 2 revolutions per second.

Spatulation with a power-driven mechanical spatulator requires that the powder initially be wet by the water as with hand mixing. The mix is then spatulated for 20 seconds on the low-speed drive of the mixer. Vacuuming during mixing reduces the air entrapped in the mix. Vibration immediately after mixing and during pouring of the gypsum minimizes air bubbles in the set mass.

Pouring an impression with gypsum requires care to avoid trapping air in critical areas. The mixed gypsum should be poured slowly or added to the impression with a small instrument such as a wax spatula. The mass should run into the rinsed impression under vibration in such a manner that it pushes air ahead of itself as it fills the impressions of the teeth. Commonly, the teeth of a cast are poured in dental stone or high-strength dental stone, whereas the base is poured in model plaster for easier trimming.

Once poured, the gypsum material should be allowed to harden for 45 to 60 minutes before the impression and cast are separated and disinfected. Models can be disinfected by immersion in 1:10 dilution of sodium hypochlorite for 30 minutes or with a spray of iodophor following manufacturer's instructions.

The adoption of the casting practice in dentistry for making gold alloy inlays, crowns, bridges, and other restorations represents one of the major advances in restorative dentistry. In recent years, alloys with higher melting points, the palladium and base metal alloys, have been cast into crowns, bridges, and removable partial denture restorations by using basically the same lost-wax technique used for dental gold alloys. All such

casting operations involve ( 1 ) a wax pattern of the object to be reproduced; (2) a suitable mold material, known as investment, which is placed around the pattern and permitted to harden; ( 3 ) suitable furnaces for burning out the wax patterns and heating the investment mold; and ( 4 ) proper facilities to melt and cast the alloy. An investment can be described as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast. The operation of forming the mold is described as investing. Details of the casting technique are described in Chapter 17.

metal and should not have reacted chemically with it.

9. Inexpensive.

These represent the requirements for an ideal investment. No single material is known that completely fulfills all of these requirements. However, by blending different ingredients an investment can be developed that possesses most of the required qualities. These ideal qualities are the basis for considering the behavior and characteristics of casting investments.

COMPOSITION

PROPERTIES REQUIRED OF AN INVESTMENT

Easily manipulated: Not only should it be possible to mix and manipulate the mass readily and to paint the wax pattern easily, but the investment should also harden within a relatively short time.

Sufficient strength at room temperature: To permit ease in handling and provide enough strength at higher temperatures to withstand the impact force of the molten metal. The inner surface of the mold should not break down at a high temperature.

Stability at higher temperatures: Investment must not decompose to give off gases that could damage the surface of the alloy.

Sufficient expansion: Enough to compensate for shrinkage of the wax pattern and metal that takes place during the casting procedure.

Beneficial casting temperatures: Preferably the thermal expansion versus temperature curve should have a plateau of the thermal expansion over a range of casting temperatures.

Porosity: Porous enough to permit the air or other gases in the mold cavity to escape easily during the casting procedure. Smooth surface: Fine detail and margins on the casting.

Ease of divestment: The investment should break away readily from the surface of the

In general, an investment is a mixture of three distinct types of materials: refractory material, binder material, and other chemicals.

Refractory Material This material is usually a form of silicon dioxide, such as quartz, tridymite, or cristobalite, or a mixture of these. Refractory materials are contained in all dental investments, whether for casting gold or highmelting point alloys.

Binder Material Because the refractory materials alone do not form a coherent solid mass, some kind of binder is needed. The common binder used for dental casting gold alloy is a-calcium sulfate hemihydrate. Phosphate, ethyl silicate, and other similar materials also serve as binders for high-temperature casting investments. These latter investments are described later in conjunction with investment for casting high-melting point alloys.

Other Chemicals Usually a mixture of refractory materials and a binder alone is not enough to produce all the desirable properties required of an investment. Other chemicals, such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder, or magnesium oxide, are often added in small quantities to modify various physical properties. For example, small amounts of chlorides or boric acid enhance the thermal expansion of investments bonded by calcium sulfate.

CALCIUM SULFATE-BONDED INVESTMENTS

The dental literature and patent references describe the quantity and purpose of each of a variety of ingredients in dental casting investments. In general, the investments suitable for casting gold alloys contain 65% to 75% quartz or cristobalite, or a blend of the two, in varying proportions, 25% to 35% of a-calcium sulfate hemihydrate, and about 2% to 3% chemical modifiers. With the proper blending of these basic ingredients, the manufacturer is able to develop an investment with an established group of physical properties that are adequate for dental gold casting practices. A list of specific compositions is of little value, however, because the final product's properties are influenced by both the ingredients present in the investment and the manner in which the mass is manipulated and used in making the mold.

Investments with calcium sulfate hemihydrate as a binder are relatively easy to manipulate, and more information about the effect of different additives, as well as various manipulative conditions, is available for this type than for other types, such as those that use silicates or phosphates as binders. The calcium sulfate-bonded investment is usually limited to gold castings, and is not heated above 700" C. The calcium sulfate

portion of the investment decomposes into sulfur dioxide and sulfur trioxide at temperatures over 700" C, tending to embrittle the casting metal. Therefore, the calcium sulfate type of binder is usually not used in investments for making castings of palladium or base metal alloys.

PROPERTIES OF CALCIUM SULFATE-BONDED INVESTMENTS

ANSI/ADA Specification No. 2 (IS0 7490) for gypsum-bonded casting investments applies to two different types of investments suitable for casting dental restorations of gold alloys:

Type 1: For casting inlays and crowns

Type 2: For casting complete and partial denture bases

Both types have calcium sulfate as a binder material. The physical properties included in this specification are appearance of powder, fluidity at working time, setting time, compressive strength, linear setting expansion, and linear thermal expansion. The testing consistency is the W/P ratio recommended by the manufacturer. The values allowed for these properties are summarized in Table 13-10. The test specimens and methods of testing, which also apply to other

I

I

types of casting investments, are described in the specification.The manipulation of investments in the inlay casting procedure is discussed in detail in Chapter 17.

EFFECT O F TEMPERATURE ON INVESTMENT

In casting with the lost-wax process, the wax pattern, after being invested, is melted and removed from the investment, leaving a mold cavity into which the molten metal is cast. In this way, regardless of whether the high-heat or hygroscopic casting technique is used, the investment is heated to an elevated temperature. This temperature varies from one technique to another, but in no case is it lower than 550" C or higher than 700" C for calcium sulfate-bonded investments. During the heating process, the refractory material is affected differently by the thermal changes than is the binder.

Effect of Temperature on Silicon Diox-

cristobalite-expands when heated, but the percentage of expansion varies from one type to another. Pure cristobalite expands to 1.6% at 250" C, whereas quartz expands about 1.4% at 600' C, and the thermal expansion of tridymite at 600" C is less than 1%. The percentage of expansion of the three types of silica versus temperature is shown in Fig. 3-12. As seen in Fig. 3-12, none of the three forms of silica expands uniformly; instead they all show a break (nonlinearity) in their thermal expansion curves. In the case of cristobalite the expansion is somewhat uniform up to about 200" C. At this temperature its expansion increases sharply from

0.5% to 1.2%, and then above 250" C it again becomes more uniform. At 573" C quartz also shows a break in the expansion curve, and tridymite shows a similar break at a much lower temperature.

The breaks on the expansion versus temperature curves indicate that cristobalite and quartz each exist in two polymorphic forms, one of which is more stable at a higher temperature and the other at a lower temperature. The form that is more stable at room temperature is called the a-form, and the more stable form at higher temperatures is designated as the P-form. Tridymite has three stable polymorphic forms. Thus the temperatures of 220" C for cristobalite, 573" C for quartz, and 105" and 160" C for tridymite are displacive transition temperatures. A displacive change involves expansion or contraction in the volume of the mass without breaking any bonds. In changing from the a-form (which is the more stable form at room temperature) to the p-form (which is stable at higher temperatures), all three forms of silica expand. The amount of expansion is highest for cristobalite and lowest for tridymite.

The quartz form of silica is found abundantly in nature, and it can be converted to cristobalite and tridymite by being heated through a reconstructive transition during which bonds are broken and a new crystal structure is formed. The a-quartz is converted to P-quartz at a temperature of 573" C. If the P-quartz is heated to 870" C and maintained at that temperature, it is converted to P-tridymite. From P-tridymite, it is possible to obtain either a-tridymite or P-cristobalite. If P-tridymite is cooled rapidly to 120"C and held at that temperature, it is changed to a-tridymite, which is stable at room temperature. On the other hand, if P-tridymite is heated to 1475"C and

held at that temperature, it is converted to P-cristobalite. Further heating of P-cristobalite produces fused silica, but if it is cooled to 220" C and held at that temperature, a-cristobalite is formed. These transitions are shown in the equation on p. 408.

All forms of silica are in their a-forms in the investment, and during the heating process they are converted completely or in part to their corresponding p-forms. This transition involves an expansion of the mass, which helps to compensate for the casting shrinkages.

Effect of Temperature on Calcium Sulfate Binders The binder used for gold alloy casting investments in dentistry is a-calcium sulfate hemihydrate. During the investing process, some of the water mixed with the investment reacts with the hemihydrate and is converted to calcium sulfate dihydrate, whereas the remainder of the water is uniformly distributed in the mix as excess water. During the early stages of heating, the excess water evaporates. As the temperature rises to about 105"C, calcium sulfate dihydrate starts losing water. The investment mass is then heated still further to the proper temperature for casting the metal. In this way, anhydrous calcium sulfate, silica, and certain chemical additives remain to form the mold into which the gold alloy is cast.

It has been observed experimentally that investment expands when it is first heated from room temperature to about 105" C, then contracts slightly or remains unchanged up to about 200" C, and registers varying degrees of expansion, depending on the silica composition of the investment, between 200 and 700" C. These properties are explained as follows. Up to about 105" C, ordinary thermal expansion occurs. Above 105" C, the calcium sulfate dihydrate is converted to anhydrous calcium sulfate. Dehydration of the dihydrate and a phase change of the calcium sulfate anhydrite cause a contraction. However, the a-form of tridymite (which might be present as an impurity) is expanding and compensates for the contraction of the calcium sulfate sufficiently to prevent the investment from registering a serious degree of contraction.

At elevated temperatures, the a-forms of silica present in the investment are converted to the p-forms, which cause some additional expansion.

The thermal expansion curves for a currently available hygroscopic type of investment containing quartz (A) and a thermal expansion type of investment containing cristobalite (B) are shown in Fig. 13-10, which illustrates the expected degree of expansion at different temperatures. The expansion of the silica content of the investment must not only be sufficiently high to overcome all the contraction, but should also take place at temperatures close to the temperature at which contraction of the hemihydrate occurs.

Cooling of the Investment When the investment is allowed to cool, the refractory and binder contract according to a thermal contraction curve that is different from the thermal expansion curve of the investment (Fig. 13-11). On cooling to room temperature, the investment exhibits an overall contraction compared with its dimensions before heating. On reheating to the temperature previously attained, the investment

Temperature ("C)

Fig. 13-10 Thermal expansion curves for calcium sulfate-bonded investments.A, Hygroscopic type; B, thermal expansion type.

(Adapted from Asgar K: Casting restorations. In Clark JW, editor: Clinical dentistr): vol 4, New York, 1976, Harper & Row.)

Temperature ("C)

Fig. 13-11 Thermal expansion and contraction curves for calcium sulfate-bonded investment (thermal expansion type). Curve 1 is first heating, curve 2 is cooling, and curve 3 is reheating.

does not expand thermally to the previous level; moreover, the process of cooling and reheating causes internal cracks in the investment that can affect the quality of the casting.

SETTING AND HYGROSCOPIC EXPANSION OF CALCIUM SULFATE-BONDED INVESTMENT

All the calcium sulfate-bonded investments currently available for casting gold alloys have thermal expansion, setting expansion, and hygroscopic expansion. The total dimensional change is a very important property of dental casting investments. The setting expansion of an investment, like that of other gypsum products discussed earlier in this chapter, is the linear expansion that takes place during the normal setting of the investment in air. Hygroscopic expansion, on the other hand, is the linear expansion of the investment that occurs if the investment is in contact with water from any source during the setting process. Such contact with water can be achieved by placing the casting ring, after invest-

ing the wax pattern, in a water bath or by using a wet liner inside the casting ring.

Distinguishing between a setting expansion and hygroscopic expansion is difficult, because both take place almost at the same time and end at the same time. In practice, a sum of the hygroscopic and setting expansions of the investment is obtained, which is higher than the setting expansion alone.

The mechanism of hygroscopic expansion has been studied by a number of investigators. According to some of the theories, the addition of water during the setting of an investment increases the surface-film thickness on the inert particles and gypsum crystals, thereby forcing them apart. Some believe that the added water during the setting process permits further hydration of calcium sulfate, thus causing the expansion of the investment, whereas others believe that the added water may force gypsum gel to swell. Another theory states that hygroscopic expansion and setting expansion are the result of the same phenomenon as that occurring with dental stone, which is the outward growth of gypsum crystals. The addition of water or any other liquid provides additional volume into which the gypsum crystals can grow.

Although some question is raised about the exact mechanism of hygroscopic expansion, the effects of many of the manipulative conditions have been studied in detail. The effects of some of these conditions on the setting, hygroscopic, and thermal expansion of calcium sulfatebonded investment are summarized in Table 13-11. These conditions also have a significant influence on the properties of the casting investment and the quality of the casting.

WatedPowder Ratio As with the setting expansion of gypsum products, the more water in the mix (the thinner the mix or the higher the W/P ratio), the less the setting and hygroscopic expansions. Notice that less thermal expansion is also obtained with a thinner mix.

Spatulation The effect of spatulation on the setting and hygroscopic expansion of the investment is similar to that on the setting expan-

Water/powder ratio increased

Time of spatulation increased

Rate of spatulation increased

Age of investment increased

Delay before immersion increased

Water bath temperature increased

Strength of wax for some patterns

Location of sprue

sion of all gypsum products. As shown in Fig. 13-12, curve C, less spatulation produces lower expansion with hygroscopic investment.

Age of Investment Investments that are 2 or 3 years old do not expand as much as freshly prepared investments. Fig. 13-12,curve D, shows the effect of aged investment on hygroscopic expansion. For this reason, the containers of investment must be kept closed as much as possible, especially if the investment is stored in a humid atmosphere.

Delay Before Immersion After the wax pattern is invested, the investment mass may be immersed in a water bath to obtain a better fit of the casting. The time between mixing and immersion has an effect on the total amount of expansion. In general, the hygroscopic expansion decreases with increased time between mixing and immersion. However, some investigators claim that if the investment is immersed at about the initial setting time, it expands more than if it is immersed earlier. However, more reproducible results are obtained from one test to another if the immersion is made before the gloss of the investment is lost.

Water-Bath Temperature The temperature of the water bath has a measurable effect on the wax pattern. At higher water-bath temperatures, the wax pattern expands, requiring less

0.5 -

Water added (ml)

Fig. 13-12 Linear expansion of a hygroscopic investment as a function of the amount of water added and mani~ulativeconditions.

expansion of the investment to compensate for the total casting shrinkage. In addition, higher water-bath temperatures soften the wax. The softened wax then gives less resistance to the expansion of the investment, thus making the setting and hygroscopic expansions more effective. The net effect is higher expansion of the mold with higher water-bath temperatures.

Particle Size of Silica The particle size of calcium sulfate hemihydrate has little effect on hygroscopic expansion, whereas the particle size of silica has a significant effect. Finer silica produces higher setting and hygroscopic expansions.