Сraig. Dental Materials

392 Chapter 13 GYPSUM PRODUCTS AND INVESTMENTS

G ypsum products probably serve the dental profession more adequately than any other materials. Dental plaster, stone, high-strength/ high-expansion stone, and casting investment materials constitute this group of closely related products. With slight modification, gypsum products are used for several different purposes. For example, impression plaster is used to make impressions of edentulous mouths or to mount casts, whereas dental stone is used to form a die that duplicates the oral anatomy when poured into any type of impression. Gypsum products are also used as a binder for silica, gold alloy casting investment, soldering investment, and investment for low-melting-point nickel-chromium alloys. These products are also used as a mold material for processing complete dentures. The main reason for such diversified use is that gypsum materials are unique in nature and their properties can be easily modified by physical and

chemical means.

The dihydrate form of calcium sulfate, called gypsum, is usually white to milky yellowish in color and is found in a compact mass in nature. The mineral gypsum has commercial importance as a source of plaster of paris. The term plaster of paris was given this product because it was obtained by burning the gypsum from deposits near Paris, France. Deposits of gypsum, however, are found in most countries.

CHEMICAL AND PHYSICAL

OF GYPSUM PRODUCTS

Most gypsum products are obtained from natural gypsum rock. Because gypsum is the dihydrate form of calcium sulfate (CaSO, . 2H,O), on heating, it loses 1.5 g mol of its 2 g mol of H,O and is converted to calcium sulfate hemihydrate (CaSO, . %H,O), sometimes written (CaSOJ2 . H,O. When calcium sulfate hemihydrate is mixed

with water, the reverse reaction takes place, and the calcium sulfate hemihydrate is converted back to calcium sulfate dihydrate. Therefore, partial dehydration of gypsum rock and rehydration of calcium sulfate hemihydrate constitute a reversible reaction. Chemically, the reaction is expressed as shown below.

The reaction is exothermic, and whenever 1 g mol of calcium sulfate hemihydrate is reacted with 1.5g mol of water, 1 g mol of calcium sulfate dihydrate is formed, and 3900 calories of heat are developed. This chemical reaction takes place regardless of whether the gypsum material is used as an impression material, a die material, or a binder in the casting investment.

MANUFACTURE OF DENTAL PLASTER, STONE, AND HIGH-STRENGTH STONE

Three types of base raw materials are derived from partial dehydration of gypsum rock, depending on the nature of the dehydration process. Plasters are fluffy, porous, and least dense, whereas the hydrocalvariety has a higher density and is more crystalline. Densite is the most dense of the raw materials. These three types of raw materials are used to formulate the four types of relatively pure gypsum products used in dentistry. They are classified as plasters (model and laboratory), lowto moderate-strength dental stones, high-strength/low-expansiondental stones, and high-strength/high-expansiondental stones, or alternatively as Types 2, 3 , 4, and 5 in ANSI/ADA Specification No. 25 (IS0 6873).

Although these types have identical chemical formulas of calcium sulfate hemihydrate, CaSO, . l/zH,O, they possess different physical properties, which makes each of them suitable for a different dental purpose. All four forms are derived from natural gypsum deposits, with the main difference being the manner of driving off part of the water of the calcium sulfate dihydrate.

Synthetic gypsum can also be used to formulate some products, but is less popular because of higher manufacturing costs.

Plasters are produced when the gypsum mineral is heated in an open kettle at a temperature of about 110" to 120" C. The hemihydrate produced is called p-calcium sulfate hemihydrate.

Such a powder is known to have a somewhat irregular shape and is porous in nature. These plasters are used in formulating model and lab plasters. Crystals of model plaster are shown in Fig. 13-1.

If gypsum is dehydrated under pressure and in the presence of water vapor at about 125" C, the product is called hydrocal. The powder particles of this product are more uniform in shape and more dense than the particles of plaster. Crystals of a dental stone are shown in Fig. 13-2. The

Fig. 13-1 Crystals of model plaster

calcium sulfate hemihydrate produced in this manner is designated as a-calcium sulfate henzihydrate. Hydrocal is used in making lowto moderate-strength dental stones.

Types 4 and 5 high-strength dental stones are manufactured with a high-density raw material called densite. This variety is made by boiling gypsum rock in a 30% calcium chloride solution, after which the chloride is washed away with hot water (100" C) and the material is ground to the desired fineness. The calcium sulfate hemihydrate in the presence of 100" C water does not react to form calcium sulfate dihydrate because at this temperature their solubilities are the same. The powder obtained by this process is the densest of the types. These materials are generally formulated as high-strength/low-expansion dental stone or high-strength/high-expansion dental stone.

Gypsum products may be formulated with chemicals that modify their handling characteristics and properties. Potassium sulfate, K2S04, and terra alba (set calcium sulfate dihydrate) are

Fig. 13-2 Crystal structure typical of dental stone.

effective accelerators. Sodium chloride in smdl amounts shortens the setting reaction but increases the setting expansion of the gypsum mass. Sodium citrate is a dependable retarder. Borax, Na,B,O,, is both a retarder and accelerator. A mixture of calcium oxide (0.1%) and gum arabic (1%) reduces the amount of water necessary to mix gypsum products, resulting in improved properties. Type 4 gypsum differs from Type 5 in that Type 4 contains extra salts to reduce its setting expansion.

CHEMICAL REACTION

The chemical reaction that takes place during the setting of gypsum products determines the quantity of H,O needed for the reaction. The reaction of 1 g mol of plaster with 1.5 g mol of water produces 1 g mol of gypsum material. In other words, 145 g of plaster requires 27 g of water to react and form 172 g of gypsum. Therefore 100 g of plaster requires 18.6g of water to form calcium sulfate dihydrate. As seen in practice, however, model plaster cannot be mixed with such a small amount of water and still develop a mass suitable for manipulation. Table 13-1 shows the recommended mixing water, required water, and excess water for model plaster, dental stone, and high-strength dental stone.

For example, to mix 100 g of model plaster to a usable consistency, use 45 g of water. Note that only 18.6 g of 45 g of water reacts with 100 g of model plaster, and the excess is distributed as free water in the set mass without taking part in the chemical reaction. The excess water is necessary to wet the powder particles during mixing. Naturally, if 100 g of model plaster is mixed with

50 g of water, the resultant mass is thinner and mixes and pours easily into a mold, but the quality of the set gypsum is inferior and weaker than when 45 g of water is used. When model plaster is mixed with a lesser amount of water, the mixed mass is thicker, is more difficult to handle, and traps air bubbles easily when it is poured into a mold, but the set gypsum is usually stronger. Thus, careful control of the proper amount of water in the mix is necessary for proper manipulation and quality of the set mass.

Water/PowderRatio of Dental Stone and High-Strength Dental Stone The principal difference among model plaster, dental stone, and high-strength dental stone is in the shape and form of the calcium sulfate hemihydrate crystals. Some calcium sulfate hemihydrate crystals are comparatively irregular in shape and porous in nature, as are the crystals in model plaster, whereas the crystals of dental stone and the two high-strength dental stones are dense and more regular in shape, as shown in Figs. 13-1 and 13-2. This difference in the physical shape and nature of the crystals makes it possible to obtain the same consistency with less excess water with dental stone and high-strength dental stones than with model plaster.

In comparison, dental stone requires only about 30 ml of water, and high-strength dental stones require as little as 19 to 24 ml. The difference in their watedpowder ratios has a pronounced effect on their compressive strength and resistance to abrasion.

When mixed with water, model plaster, dental stone, or high-strength dental stones set to a hard mass of gypsum. The gypsum products known as

high-strength dental stones (Types 4 and 5) are the strongest, the mass produced as model plaster is the weakest, and dental stone produces an intermediate strength material. Note, however, that all gypsum products have the same chemical formula, and that the chemical nature of the masses produced by mixing them with water is also identical; the differences among them are primarily in their physical properties.

Mechanism of Setting The most important and well-recognized theory for the mechanism of the setting is the crystalline theory. It was originated in 1887 by Henry Louis Le Chatelier, a French chemist; in 1907, the theory received the full support of Jacobus Hendricus van't Hoff, a famous Dutch chemist in Berlin at the turn of the century. According to the explanation of van't Hoff, the difference in the solubilities of calcium sulpate dihydrate and hemihydrate causes the setting differences in these materials. Dissolved calcium sulfate precipitates as calcium sulfate dihydrate, because calcium sulfate dihydrate is less soluble than hemihydrate. However, scanning electron microscope and x-ray diffraction studies have shown that not all hemihydrate converts to dihydrate. The residual hemihydrate affects the properties of the set gypsum.

If in a setting mass of plaster two types of centers are distinguished, one for dissolution and the other for precipitation, the dissolution centers are located around the calcium sulfate hemihydrate, and the precipitation centers are around the calcium sulfate dihydrate. The concentration of calcium sulfate is also different in these two centers; it is highest around the dissolution ten- ters and lowest close to the precipitation centers. Calcium and sulfate ions travel in solution by diffusion from the area in which the concentration is greatest to the area in which the concentration is lowest. By understanding the basic concept of the crystalline theory, the effect of different manipulative conditions on some physical properties can be explained.

The effect of manipulative variables and certain chemicals on setting has been studied recently by a kinetic model. An induction time and a reaction constant for crystal growth have been

observed. Other theories include the gel theory and the theory of hydration.

Volumetric Contraction Theoretically, calcium sulfate hemihydrate should contract volumetrically during the setting process. However, experiments have determined that all gypsum products expand linearly during setting. As indicated earlier, when 145.15 g of calcium sulfate hemihydrate reacts with 27.02 g of water, the result is the production of 172.17 g of calcium sulfate dihydrate. However, if the volume rather than the weight of calcium sulfate hemihydrate is added to the volume of water, the sum of the volumes will not be equal to the volume of calcium sulfate dihydrate. The volume of the calcium sulfate dihydrate formed is about 7% less than the sum of the volumes of calcium sulfate hemihydrate and water. Instead of 7% contraction, however, in practice about 0.2% to 0.4% linear expansion is obtained. According to the crystalline theory of Le Chatelier and van't Hoff, the expansion results from the thrusting action of gypsum crystals, CaSO, . 2H,O, during their growth from a supersaturated solution. The fact that the contraction of gypsum is not visible does not invalidate its existence, and when the volumetric contraction is measured by a dilatometer, it is determined to be about 7%. Because of the linear expansion of the outer dimensions, which is caused by the growth of calcium sulfate dihydrate, with a simultaneous true volumetric contraction of calcium sulfate dihydrate, these materials are porous when set. The gel theory (setting through colloidal state to formation of a gel) may explain the validity of contraction. The theory of hydration interprets expansion through the formation of a hydrate of calcium sulfate.

Effect of Spatulation The mixing process, called spatulation, has a definite effect on the setting time and setting expansion of the material. Within practical limits an increase in the amount of spatulation (either speed of spatulation or time or both) shortens the setting time. Obviously when the powder is placed in water, the chemical reaction starts, and some calcium sulfate dihydrate is formed. During spatulation

the newly formed calcium sulfate dihydrate breaks down to smaller crystals and starts new centers of nucleation, around which the calcium sulfate dihydrate can be precipitated. Because an increased amount of spatulation causes more nuclei centers to be formed, the conversion of calcium sulfate hemihydrate to dihydrate requires somewhat less time.

Effect of Temperature The temperature of the water used for mixing, as well as the temperature of the environment, has an effect on the setting reaction of gypsum products. The setting time probably is more affected by a change in temperature than any other physical property. Evidently the temperature has two main effects on the setting reaction of gypsum products.

The first effect of increasing temperature is a change in the relative solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate, which alters the rate of the reaction. The ratio of the solubilities of calcium sulfate dihydrate and calcium sulfate hemihydrate at 20" C is about 4.5. As the temperature increases, the solubility ratios decrease, until 100" C is reached and the ratio becomes one. As the ratio of the solubilities becomes lower, the reaction is slowed, and the setting time is increased. The solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate are shown in Table 13-2.

The second effect is the change in ion mobility with temperature. In general, as the temperature increases, the mobility of the calcium and sulfate ions increases, which tends to increase the rate of the reaction and shorten the setting time.

Practically, the effects of these two phenomena are superimposed, and the total effect is observed. Thus, by increasing the temperature from 20" to 30" C , the solubility ratio decreases from 0.90/0.200 = 4.5 to 0.72/0.209 = 3.44, which ordinarily should retard the reaction. At the same time, however, the mobility of the ions increases, which should accelerate the setting reaction. Thus, according to the solubility values, the reaction should be retarded, whereas according to the mobility of the ions, the reaction should be accelerated. Experimentation has shown that in-

creasing the temperature from room temperature of 20" C to body temperature of 37" C increases the rate of the reaction slightly and shortens the setting time. However, as the temperature is raised over 37" C, the rate of the reaction decreases, and the setting time is lengthened. At 100' C the solubilities of dihydrate and hemihydrate are equal, in which case no reaction occurs, and plaster does not set.

Effect of Humidity In the manufacture of plaster it is not practical to convert all the calcium sulfate dihydrate (CaSO, . 2H,O) to calcium sulfate hemihydrate (CaSO, . '/zH,O). During the calcination process most of the gypsum particles are changed to the hemihydrate, although a small portion may remain as the dihydrate, and possibly some particles may further dehydrate completely to form anhydrous soluble calcium sulfate (CaSO,). Soluble calcium sulfate, to a greater degree, and plaster, to a lesser degree, are hygroscopic materials by nature and can easily absorb water vapor from a humid atmosphere to form calcium sulfate dihydrate, which changes the original proportion of each form of calcium sulfate. The presence of small amounts of calcium sulfate dihydrate on the surface of the hemihydrate powder provides additional nuclei for crystallization. Increased contamination by

CaS04 %H,O CaS04 2H,O

Temperature (g/100g (€dl00g

water) water)

Adapted from Partridge EP, White AH:J A m Chem Soc 51: 360, 1929.

moisture produces sufficient dihydrate on the hemihydrate powder to retard the solution of the hemihydrate. Experience has shown that the common overall effect of contamination of gypsum products with moisture from the air during storage is a lengthening of the setting time. For the best results in practice, all gypsum products should be kept in a closed container and well protected from atmospheric humidity. Practically, warmer temperatures and high moisture accelerate the rate of setting of gypsum during mixing.

Effect of Colloidal Systems and pH

Colloidal systems such as agar and alginate retard the setting of gypsum products. If these materials are in contact with CaS0, . 1/2H,O during setting, a soft, easily abraded surface is obtained. Accelerators such as potassium sulfate are added to improve the surface quality of the set CaSO, . 2H20 against agar or alginate.

These colloids do not retard the setting by altering the solubility ratio of the hemihydrate and dihydrate forms, but rather by being adsorbed on the CaS04 . l/2H2O or on the CaSO, . 2H20 nucleation sites and thus interfering in the hydration reaction. The adsorption of these materials on the nucleating sites retards the setting reaction more effectively than adsorption on the calcium sulfate hemihydrate.

Liquids with low pH, such as saliva, retard the setting reaction. Liquids with high pH accelerate setting.

The important properties of gypsum products include quality, fluidity at pouring time, setting time, linear setting expansion, compressive strength, tensile strength, hardness and abrasion resistance, and reproduction of detail. Some of these property requirements, described by ANSI/ ADA Specification No. 25 (IS0 6873), are summarized in Table 13-3.

SETTING TIME

Definition and Importance The time required for the reaction to be completed is called the final setting time.If the rate of the reaction is too fast or the material has a short setting time, the mixed mass may harden before the operator can manipulate it properly. On the other hand, if the rate of reaction is too slow, an excessively long time is required to complete the operation. Therefore, a proper setting time is one of the most important properties of gypsum materials.

The chemical reaction is initiated at the mo-

ment the powder is mixed with water, but at the early stage only a small portion of the hemihydrate is converted to gypsum. The freshly mixed mass has a semifluid consistency and can be poured into a mold of any shape. As the reaction proceeds, however, more and more calcium sulfate dihydrate crystals are produced. The viscosity of the mixed mass increases, and the mass can no longer flow easily into the fine details of the mold. This time is called the working time.

The final setting time is defined as the time at which the material can be separated from the impression without distortion or fracture. The initial setting time is the time required for gypsum products to reach a certain arbitrary stage of firmness in their setting process. In the normal case, this arbitraiy stage is represented by a semihard mass that has passed the working stage but is not yet completely set. Even at final setting, however, the conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate is only partially completed. In high-strength stones, the conversion to dihydrate is never complete. The presence of residual hemihydrate in the set gypsum increases the final strength of the set mass.

Measurement The initial setting time is usually measured arbitrarily by some form of penetration test, although occasionally other types of test methods have been designed. For example, the loss of gloss from the surface of the mixed mass of model plaster or dental stone is an indication of this stage in the chemical reaction and is sometimes used to indicate the initial set of the mass. Similarly,the setting time may be measured by the temperature rise of the mass, because the chemical reaction is exothermic.

The Vicat apparatus shown in Fig. 13-3 is commonly used to measure the initial setting time of gypsum products. It consists of a rod weighing 300 g with a needle of 1 mm diameter. A ring container is filled with the mix, the setting time of which is to be measured. The rod is lowered until it contacts the surface of the material, then the needle is released and allowed to penetrate the mix. When the needle fails to penetrate to the bottom of the container, the material has reached the Vicat or the initial set-

ting time. Other types of instruments, such as Gillmore needles, can be used to obtain the initial and final setting times of gypsum materials.

Control of Setting Time The setting time of gypsum products can be altered rather easily. For example, plaster, which is able to take up water readily, can absorb moisture from the atmosphere and change to gypsum; this absorbance alters the setting time and the other properties of the plaster. The rate of the chemical reaction also can be changed by the addition of suitable chemicals, so it may take from a few minutes to a few hours for the reaction to be completed. According to the crystalline theory discussed earlier, the difference between the solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate causes the set of the gypsum mass. At a temperature of 20" C, about

Fig. 13-3 Vicat penetrometer used to determine initial setting time of gypsum products.

4.5 times as much hemihydrate is dissolved in a given amount of water as is dihydrate. If this ratio of 4.5 is increased by the addition of certain salts, the chemical reaction progresses faster and the setting time is shortened. The salt that causes such a change is called an accelerator. On the other hand, if by the addition of some salt the ratio of the solubilities of hemihydrate to the dihydrate is decreased, the rate of the reaction is slowed, and the setting time is lengthened. Then the salt is considered a retarder.

Although not all accelerators and retarders work on this principle, the change of the solubility ratio may be considered as one way of changing the setting time. In general, setting time can be controlled by manufacturers when they add different chemicals to model plaster or other gypsum products and by operators when they change the manipulative conditions.

Factors Controlled by the Manufacturer

The easiest and most reliable way to change the setting time is to add different chemicals. Potassium sulfate, K,SO,, is known as an effective accelerator, and the use of a 2% aqueous solution of this salt rather than water reduces the setting time of model plaster from approximately 10 minutes to about 4 minutes. On the other hand, sodium citrate is a dependable retarder. The use of a 2% aqueous solution of borax to mix with the powder may prolong the setting time of some gypsum products to a few hours.

If a small amount of set calcium sulfate dihydrate is ground and mixed with model plaster, it provides nuclei of crystallization and acts as an accelerator. The set gypsum used as an accelerator is called terra alba, and it has a pronounced effect at lower concentrations. The setting time changes significantly if the amount of terra alba present in the mix is changed from 0.5% to 1%. However, terra alba concentrations above 1% have less effect on the setting time. Manufacturers usually take advantage of this fact and add about 1% terra alba to plaster. Thus, the setting time of model plaster is altered less in normal use because of opening and closing the container. When not in use, the model plaster container should be closed tightly to reduce the possibility

of moisture contamination, which lengthens the setting time.

Water/Powder Ratio The operator also can change the setting time of model plaster to a certain extent by changing the water/powder (W/P) ratio or the extent of spatulation.

The W/P ratio has a pronounced effect on the setting time. The more water in the mix of model plaster, dental stone, or high-strength dental stone, the longer the setting time, as shown in Table 13-4. The effect of spatulation on setting time of model plaster and dental stone is shown in Table 13-5. Increased spatulation shortens the setting time. Properties of a high-strength dental stone mixed by hand and by a power-driven mixer with vacuum are shown in Table 13-6. The setting time is usually shortened for power mixing compared with hand mixing.

The viscosities of several high-strength dental stones and impression plaster are listed in Table 13-7. A range of viscosities from 21,000 to 101,000 centipoises (cp) was observed for five different high-strength stones. More voids were observed in casts made from the stones with the

*Adaptedfrom Garber DK,. Powers JM, Brandau HE: Mich Dent Assoc J 67:133, 1985. Stones were mixed with 1% sodium citrate solution to retard setting. Viscosity was measured 4 min from the start of mixing.

From Garber DK, Powers JM, Brandau HE: Mich Dent AssocJ 67:133, 1985.

higher viscosities. Impression plaster is used infrequently, but it has a low viscosity, which makes it possible to take impressions with a minimum of force on the soft tissues (mucostatic technique).

COMPRESSIVE STRENGTH

When set, gypsum products show relatively high values of compressive strength. The compressive strength is inversely related to the W/P ratio of the mix. The more water used to make the mix, the lower the compressive strength.

Model plaster has the greatest quantity of excess water, whereas high-strength dental stone contains the least excess water. The excess water is uniformly distributed in the mix and contributes to the volume but not the strength of the material. The set model plaster is more porous than set dental stone, causing the apparent density of model plaster to be lower. Because highstrength dental stone is the densest, it shows the highest compressive strength, with model plaster being the most porous and thus the weakest.

The 1-hour compressive strength values are approximately 12.5 MPa for model plaster, 31 MPa for dental stone, and 45 MPa for highstrength dental stones. These values are representative for the normal mixes, but they vary as the W/P ratio increases or decreases. The effect of the W/P ratio on the compressive strength of these materials is given in Table 13-8. As shown in Table 13-6, the compressive strength of a high-strength dental stone is improved slightly by vacuum mixing. Evidently, when stone is mixed with the same W/P ratio as model plaster, the compressive strength of dental stone is almost the same as that of model plaster. Similarly, the compressive strength of high-strength dental stone with W/P ratios of 0.3 and 0.5 is similar to the normal compressive strength of dental stone and model plaster.

All mixes spatulated 100 turns and tested 1 hr after the start of mixing.

At 1 or 2 hours after the final setting time, the hardened gypsum material appears dry and seems to have reached its maximum strength. Actually, this is not the case. The wet strength is the strength of gypsum materials with some or all of the excess water present in the specimen. The dry strength is the strength of the gypsum material with all of its excess water driven out. The dry compressive strength is usually about twice that of the wet strength. Notice that as the hardened mass slowly loses its excess water, the compressive strength of the material does not increase uniformly. The effect of drying on the compressive strength of dental stone is shown in Fig. 13-4. Theoretically, about 8.8%of excess water is in the hardened mass of the stone. As the mass loses up to 7% of the water, no appreciable change develops in the compressive strength of the material. When the mass loses 7.5% of the excess water, however, the strength increases sharply, and when all of the excess (8.8%) is lost, the strength of the material is over 55 MPa.

The drying time for gypsum materials varies according to the size of the gypsum mass and the temperature and humidity of the storage atmo-

Weight loss ('10)

Fig. 13-4 Effect of loss of excess water on compressive strength of dental stone.

sphere. At room temperature and average humidity, about 7 days are necessary for an average denture flask filled with gypsum materials to lose the excess water.

SURFACE HARDNESS

AND ABRASION RESISTANCE

The surface hardness of unmodified gypsum materials is related in a general way to their compressive strength. High compressive strengths of the hardened mass correspond to high surface hardnesses. After the final setting occurs, the surface hardness remains practically constant until most excess water is evaporated from the surface, after which its increase is similar to the increase in compressive strength. The surface hardness increases at a faster rate than the compressive strength, because the surface of the hardened mass reaches a dry state earlier than the inner portion of the mass.

Attempts have been made to increase the hardness of gypsum products by impregnating the set gypsum with epoxy or methyl methacrylate monomer that is allowed to polymerize.