Сraig. Dental Materials

432 Chapter 14 WAXES

beeswax the lowest. The elastic modulus of carnauba wax decreased from 1790 to 760 MPa from 23" to 37" C . Paraffin wax showed a sharp decrease in modulus from 310 to 28 MPa between 23" and 30" C . The inlay wax, which siinulates a mixture of 75% paraffin and 25% carnauba wax, had intermediate changes in modulus of 760 to 48 MPa between 23" and 40" C .

The modulus of the inlay wax is important in the hygroscopic expansion of casting investments, in which the wax pattern is subjected to stresses resulting from the expansion of the investment during setting. Nonuniform deformation of wax patterns, such as crowns, can be minimized by using waxes having different elastic moduli for particular parts of the pattern. For example, in a crown the lateral walls can be prepared with inlay wax, and the occlusal surfaces can be constructed of soft green casting wax (see Fig. 14-5).At the investing temperature, the modulus ratio for the inlay and soft green casting wax is 7 : 1 ,which is the approximate ratio needed for many patterns to obtain uniform expansion in the occlusal compared with the marginal areas.

The proportional limits and the con~pressive strengths of the waxes shown in Fig. 14-5 exhibit the same trends as their elastic moduli. The proportional limit of carnauba wax decreased from 11 to 5.5 MPa over the range of 23" to 37" C . Inlay casting wax experienced a decrease in proportional limit of 4.8 to 0.2 MPa from 23" to 40" C . The compressive strength of inlay wax decreased from 83 to 0.5 MPa over the same temperature range, and the percent compression at rupture varied from 2.7% to 4.3%. Hence the inlay wax would be considered a brittle material, although it possesses flow or viscous properties at stresses below its proportional limit.

FLOW

The property of flow results from the slippage of molecules over each other. A measure of flow in the liquid state of wax would be synonymous with viscosity. Below the melting point of the wax, however, a measure of the flow actually

would be a measure of the degree of plastic deformation of the material at a given temperature. Flow is decidedly dependent on the temperature of the wax, the force bringing about the deformation, and the time the force is applied, as shown in Fig. 14-6. Flow greatly increases as the melting point of the wax is approached. Although a high percentage of flow at a given temperature may be required for a specific wax, it may be extremely deleterious at a temperature a few degrees lower. This is especially true for the direct inlay wax. This material must have a relatively high flow a few degrees above mouth temperature so it is workable but not uncomfortably warm when placed in the mouth of the patient. At mouth temperature, an inlay wax to be used for a direct pattern must have essentially no flow to minimize the possibility of distortion of the pattern during removal from the tooth cavity.

The flow of wax at different temperatures is shown in Fig. 14-6. Yellow beeswax does not flow extensively until it reaches 38" C, and at 40" C it flows about 7%. From these data it is easy to understand why beeswax has been used as a major ingredient in dental impression wax. Many mineral waxes have about a 10" C range between 1% and 70% flow, which indicates that these waxes soften gradually over a broad temperature

mately 20" C below their melting range. This can be explained by the fact that the mineral waxes are straightor branched-chain hydrocarbons. The secondary valence forces in these waxes are rather weak and are gradually dissipated as the temperature is increased.

Montan wax, another mineral wax, requires a temperature of 71" C, or 8" C below its melting range, to flow 50%. However, this wax is similar to the plant waxes in that it is composed mainly of esters formed in nature by the union of higher alcohols with higher fatty acids. The plant waxes likewise require temperatures close to their melting range to produce 50% flow. As a result of the presence of ester groups in these waxes, the secondary valence forces are rather strong, and a

high temperature is necessary to overcome these forces. Once the secondary valence forces are overcome, these waxes flow rapidly. Below this point they often appear to fracture in a manner similar to a brittle material.

Yellow beeswax, which is also primarily an ester wax, flows extensively 24" C below its melting range (61" to 63" C)and displays an 8" C temperature difference between 1% and 70% flow. This wax contains a large number of impurities, which interfere with the secondary valence forces. As beeswax goes through the bleaching process and some of these impurities are removed, the secondary valence forces increase. Flow data illustrate this point, because bleached beeswax requires a temperature closer to its melting range to produce a large amount of flow, and the temperature difference between 1% and 70% flow is only 4" C. Note that the flow of various batches of yellow beeswax shows that significant differences may exist between batches. A similar observation is seen with paraffin and carnauba wax.

A plot of percent flow versus time for a hard inlay wax shows that at 40" C the amount of flow in relation to time is linear (Fig. 14-7). The total amount of flow after 10 minutes at this temperature is only 2%. At 42" C , the flow increases enough to cause an increase in the rate of flow. At 43" and 45" C , the rate of flow is very large at the beginning of the test, and the rate decreases rapidly as a result of the increase in diameter of the specimen.

The flow of dental waxes is influenced by the presence of solid-solid and melting transformations that occur in the component waxes. The transformation temperatures can be related to flow indirectly by studying the resistance of the wax to penetration as a function of temperature. In Fig. 14-8, penetration thermograms are compared with a differential thermal analysis curve for an inlay casting wax for annealed, A, and unannealed, 9 specimens tested at two stress levels. At the lower stress level, the high-melting point ester component of the wax influenced penetration. However, at a high stress level the

Fig. 14-7 Flow curves at various temperatures for Kerr hard (Type 2) wax.

temperature of the solid-solid transformation associated with the hydrocarbon component of the wax determined the resistance to penetration. Annealing the wax in an oven at 50' C for 24 hours before testing had the effect of increasing the resistance of the wax to penetration.

RESIDUAL STRESS

Regardless of the method used to prepare a wax pattern, residual stress exists in the completed pattern. The presence of residual stress can be demonstrated by comparing the thermal expansion curves of annealed wax with wax that has been cooled under compression or tension. The thermal expansion of an annealed inlay wax is shown in Fig. 14-9, in which the same curve is obtained on heating or cooling. When the wax specimen is prepared by holding the softened

Fig, 14-8 Penetration thermograms for annealed,

A, and unannealed, U, inlay wax compared with a differential thermal analysis (DTA) curve.

wax under compression during cooling, followed by the determination of the thermal expansion, the thermal expansion is greater than for the annealed specimen. The extent of the deviation from the curve for the annealed wax is a function of the magnitude of the residual internal stress and the time and temperature of storage of the specimen before the thermal expansion curve is determined. Therefore a shaded area is shown rather than a specific curve. When the wax specimen is cooled while being subjected to tensile stress and the thermal expansion is determined, the curve for the wax specimen is lower than that for the annealed specimen. If sufficient residual stress is introduced, a thermal contraction may result on heating; again a shaded area indicates the direction of the effect.

The changes in dimensions resulting from the heating of wax specimens formed under com-

Fig. 14-9 Dimensional change on heating annealed in- lay wax patterns, and that of waxes formed under compression and tension.

pression or tension can be explained as follows: When the specimen is held under compression during cooling, the atoms and molecules are forced closer together than when they are under no external stress. After the specimen is cooled to room temperature and the load removed, the motion of the molecules is restricted; this restriction results in residual stress in the specimen. When the specimen is heated, the release of the residual stress is added to the normal thermal expansion, and the total expansion is greater than normal. When the specimen is cooled while under tensile stress and expansion as a result of heating is measured, the release of the residual tensile stress results in a dimensional change that is opposite to the thermal expansion. The sum of these two effects results in a lower thermal expansion curve than for annealed wax. As seen in Fig. 14-9, if a sufficientlylarge amount of residual

stress is introduced, the overall dimensional change on heating results in a contraction of the specimen.

DUCTILITY

Like flow, ductility increases as the temperature of a wax specimen is increased. In general, waxes with lower melting temperatures have a greater ductility at any given temperature than those w-it11 higher melting temperatures.

The ductility of a blended wax is greatly influenced by the distribution of the melting temperatures of the component waxes. A blended wax with components that have wide melting ranges generally has greater ductility than blended waxes that have a narrow range. Whenever a wide range of melting temperatures is present, the softening point of the lowest component is approached first. A further temperature rise begins to liquefy this component and approach still closer to the softening points of the higher-softening point components. This tends to plasticize the entire wax mass, thereby enhancing ductility.

Generally, highly refined waxes are quite brittle. The lower-melting point, microcrystalline mineral waxes, which contain appreciable amounts of occluded oil, are moderately soft and exhibit a high degree of plasticity or ductility, even with their comparatively high melting temperatures.

A variety of natural waxes and resins have been used in dentistry for specific and well-defined applications. In some instances, the most favorable qualities can be obtained from a single wax, such as beeswax, but more often a blend of several waxes is necessary to develop the most desirable qualities.

A classification of dental waxes according to their use and application is given in Table 14-4. Pattern waxes are used to form the general predetermined size and contour of an artificial dental restoration, which is to be constructed of a

436 Chapter I 4 WAXES

more durable material such as cast gold alloys, cobalt-chromium-nickel alloys, or acrylic resin. All pattern waxes have two major qualities, thermal change in dimension and tendency to warp or distort on standing,which create serious problems in their use whether an inlay pattern, crown, or complete denture is being constructed.

Processing waxes are used primarily as auxiliary aids in constructing a variety of restorations and appliances, either clinically or in the laboratory. Processing waxes perform numerous tasks that simplify many dental procedures in such operations as denture construction or soldering.

One of the oldest recorded uses of wax in dentistry is for taking impressions within the mouth. Because a wax formulated for use as an impression material exhibits high flow and ductility, it distorts readily when withdrawn from undercut areas. Therefore, the use of wax has been limited to the non-undercut edentulous portions of the mouth. Recently, specially formulated addition-silicone and polyether impression materials have replaced wax as a bite registration material.

INLAY PATTERN WAX

Gold inlays, crowns, and bridge units are formed by a casting process that uses the lost-wax pattern technique. A pattern of wax is first constructed that duplicates the shape and contour of the desired gold casting. The carved wax pattern is then embedded in a gypsum-silica investment material to form a mold with an ingate or sprue leading from the outer surface of the investment

mold to the pattern, as described in Chapter 17. The wax is subsequently eliminated by heating and softening, and the mold is further conditioned to receive the molten gold by controlled heating in a furnace.

Composition The principal waxes used to formulate inlay waxes are paraffin, microcrystalline wax, ceresin, carnauba, candelilla, and beeswax. For example, an inlay wax may contain 60% paraffin, 25% carnauba, 10% ceresin, and 5% beeswax. Therefore, hydrocarbon waxes constitute the major portion of this formulation. Some inlay waxes are described as hard, regular (medium), or soft, which is a general indication of their flow. The flow can be reduced by adding more carnauba wax or by selecting a highermelting point paraffin wax. An interesting example is that a hard inlay wax may contain a lower percentage of carnauba wax than a regular inlay wax, but the flow of the hard inlay wax is less than the regular wax because of the selection of a higher-melting-point paraffin in the formulation of the hard wax. Resins in small amounts, such as 1%, also affect the flow of inlay waxes. Inlay waxes are usually produced in deep blue, green, or purple rods or sticks about 7.5 cm long and 0.64 cm in diameter. Some manufacturers supply the wax in the form of small pellets or cones or in small, metal ointment jars.

Properties The accuracy and ultimate usefulness of the resulting gold casting depend largely on the accuracy and fine detail of the wax pattern. A wax that is able to function well in the gold casting technique must possess certain, very important physical properties.

Revised ANSI/ADA Specification No. 4 (IS0 1561) for dental inlay casting wax has been formulated for waxes used in direct and indirect waxing techniques. A summary of flow requirements of this specification is given in Table 14-5. Because the wax patterns are to be melted and vaporized from the investment mold, it is essential that no excessive residue remain in the mold because of incomplete wax burnout. Excess residue may result in the incomplete casting of inlay

margins. The specification therefore limits the nonvolatile residue of these waxes to a maximum of 0.10% at an ignition temperature of 700" C.

Types 1 (soft) and 2 (hard) dental inlay casting waxes are recognized by Revised ANSVADA Specification No. 4. Type 1 wax is a soft wax used as an indirect technique wax. Type 2 wax is a harder wax prescribed for forming direct patterns in the mouth, where lower flow values at 37" C tend to minimize any tendency for distortion of the pattern on its removal from the cavity preparation. Type 1wax shows greater flow than Type 2 wax at temperatures both below and above mouth temperature. The lower flow of Type 2 wax and the greater ease of carving the softer Type 1 waxes are desirable working characteristics for the techniques associated with each.

The specification also requires the manufacturers to include instructions regarding the method of softening and the working temperature for the wax preparatory to forming a direct pattern. Both types should soften without becoming flaky, and when trimmed to a fine margin during the pattern-carving operation, they should not chip or flake. Thermal expansion data for the Type 2 wax are no longer required by the specification.

Flow When forming a wax pattern directly in the mouth, the wax must be heated to a temperature at which it has sufficient flow under compression to reproduce the prepared cavity walls in great detail. The working temperature, suggested by the manufacturer, which should be satisfactory for making direct wax patterns, must

not be so high as to cause damage to the vital tooth structure or be uncomfortable to the patient. Insufficient flow of the wax caused by insufficient heating results in the lack of cavity detail and introduces excess stress within the pattern. An overabundant amount of flow resulting from excessive heating makes compression of the wax difficult because of a lack of "body" in the material.

The values listed in Table 14-5 represent minimum or maximum values of percent flow that occur at various temperatures when Types 1 and 2 wax specimens are subjected to a 19.6 N load for 10 minutes. The temperature that the Type 2 wax must attain to register cavity detail is usually somewhat above 45" C. As seen from these values, the flow of the hard wax is no more than 1% at body temperature. The flow of the Type 1 wax is about 9% at this temperature. Low flow at this temperature tends to minimize distortion of a well-carved pattern as it is withdrawn from an adequately tapered cavity in the tooth.

Thermal Coefficient of Expansion

The curve in Fig. 14-10 shows that the rate of expansion of the Type I inlay wax is greatest from just below mouth temperature to just above 45" C. Knowing the amount of wax expansion or contraction allows one to judge the compensation necessary to produce an accurate casting. Data sufficient to show the thermal contraction of the wax from its working temperature to room temperature may be included in each package of inlay wax. Once the wax pattern is carved, its removal from the tooth cavity and transfer to the laboratory bring about a reduction in tempera-

438 Chapter 14 WAXES

Temperature ( O C )

Fig. 14-10 Percent expansion of inlay and casting waxes from 20" to 50"C, showing the percent dimensional change from mouth temperature to average room temperature.

ture and subsequent thermal contraction. A decrease of 12" to 13" C in temperature, from mouth temperature to a room temperature of about 24" C, causes a 0.4% linear contraction of the wax, or about 0.04% change for each degree change in temperature.

Warpage of Wax Patterns Inlay pattern wax has a high coefficient of expansion and tends to warp or distort when allowed to stand unrestrained. The distortion is increased generally as the temperature and time of storage are increased. This quality of wax patterns is related to the release of residual stress developed in the pattern during the process of formation. This characteristic of stress release and warpage is

present in all dental waxes, but is particularly troublesome in inlay patterns because of the critical dimensional relations that must be maintained in inlay castings.

Because warpage of the pattern is related to the temperature during pattern formation and storage, the rules related to the pattern temperature must be understood. In general, the higher the temperature of the wax at the time the pattern was adapted and shaped, the less the tendency for distortion in the prepared pattern. This is reasonable, because the residual stress in the pattern causing the distortion is associated with the forces necessary to shape the wax originally. The incorporation of residual stress can be minimized by softening a wax uniformly by heating at j O O C for at least 15 minutes before use, by using warmed carving instruments and a warmed die, and by adding wax to the die in small amounts.

Because the release of internal stress and subsequent warpage are associated with the storage temperature, it follows that greater warpage results at higher storage temperatures. Lower temperature does not completely prevent distortion, but generally the amount is reduced when the storage temperature is kept to a minimum. If inlay wax patterns must be allowed to stand uninvested for a time longer than 30 minutes, they should be kept in a refrigerator. Although some distortion may take place at this temperature, it will be less than at normal room temperature. Such a practice of storage for long periods is not recommended if freedom from warpage is desired. The best way to minimize the warpage of inlay wax patterns is to invest the pattern immediately after it is completely shaped. A refrigerated wax pattern should be allowed to warm to room temperature before it is invested. During spruing, distortion can be reduced by use of a solid wax sprue or a hollow metal sprue filled with sticky wax. If the pattern was stored, the margins should be readapted. Temperature of formation, time and condition of storage, and promptness of investing the pattern are major factors related to all techniques of pattern formation.

CASTING WAX

The pattern for the metallic framework of removable partial dentures and other similar structures is fabricated from casting waxes. These waxes are available in the form of sheets, usually of 28- and 30-gauge (0.40 and 0.32 mm) thickness, ready-made shapes, and in bulk. As shown in Fig. 14-11, the ready-made shapes are supplied as round, half-round, and half-pear-shaped rods and wires of various gauges in approximately 10-cm lengths. Although casting waxes serve the same basic purpose as inlay waxes in the formation of patterns for metallic castings, their physical properties differ slightly. Little is known of the exact composition of these sheet and shaped waxes, but they include ingredients similar to those found in inlay waxes, with various combinations and proportions of paraffin, ceresin, beeswax, resins, and other waxes being used.

The casting wax sheets are used to establish minimum thickness in certain areas of the partial denture framework, such as the palatal and lingual bar, and to produce the desired contour of the lingual bar. A partial denture framework in the process of being waxed is shown in the left center of Fig. 14-1. The physical nature and form in which the sheet casting wax is supplied re-

sulted in its use for post damming of con~plete maxillary denture impressions, checking high points of articulation, producing wax bites of cusp tips for the articulation of stone casts, and many other uses.

Physical Characteristics The casting sheets and ready-made shapes of certain types of casting waxes rnay possess a slight degree of tackiness, which helps to maintain their position on the cast and on each other during assembly of the pattern. This tackiness is not sufficient to prevent changes in position from being made with relative ease, and when the waxes are in final position, they are sealed to the investment cast with a hot spatula.

There is no ANSI/ADA specification for these casting waxes, but a federal specification has been formulated that includes values for softening temperature, amount of flow at various temperatures, general working qualities, and other characteristics. A summary of the properties included in Federal Specification No. U-W-140 is given in Table 14-6. In general, the characteristics most desired include a certain degree of toughness and strength, with a true gauge dimension, combined with a minimum of dimensional

change with change in temperature, and the ability to be vaporized completely from the investment mold.

Because the pattern for the removable partial denture framework is constructed on and sealed to an investment cast (from which it is not separated subsequently) at room temperature, there is little need for the casting wax to exhibit low flow at body temperature. The flow characteristics of the casting wax, when measured similarly to the inlay wax, show a maximum of 10% flow at 35" C and a minimum of 60% flow at 38" C. These characteristics are significantly different from the flow values for inlay waxes that comply with the requirements of ANSI/ADA Specification No. 4.

The requirement for ductility of the casting waxes is high. The federal specification requires that the casting wax be bent double on itself without fracture at a temperature of 23" C and that the waxes be pliable and readily adaptable at 40" to 45" C. Heating over a flame and the compression to adapt either the ready-made shapes or the sheet casting wax easily may alter their thickness and contour because of their relatively high ductility and flow.

Because these materials are casting pattern waxes for partial denture cast restorations, as is the inlay wax, they too must vaporize at about 500" C with no residue other than carbon. The mold cavity thus produced will result in moredesirable casting surfaces, because it will be free of foreign materials. Pattern waxes are being

replaced to some extent by preformed plastic patterns.

RESIN MODELING MATERIAL

Light-curingresins are available as lowand highviscosity pastes and as a liquid for the fabrication of patterns for cast metal or ceramic inlays, crowns and bridges, and precision attachments (Fig. 14-12). The modeling pastes are based on diurethane dimethacrylate oligomers with 40% to 55% polyurethane dimethacrylate or poly(methy1 methacrylate) fillers. The liquid consistency is mostly urethane dimethacrylate. These resins have a camphorquinone activator. Self-cured acrylic plastics used as inlay patterns are described in Chapter 21.

Modeling resins are characterized by lower heat of polymerization and shrinkage than acrylics, higher strength and resistance to flow than waxes, good dimensional stability, and burnout without residue. Average marginal discrepancies of light-cured resin and self-cured acrylic patterns are similar to those of wax for full-crown patterns but less than those of wax for inlay patterns (Table 14-7). Dimethacrylate resin patterns do not result in cracked investment from heating during burnout, which can occur with acrylic patterns.

Gypsum and resin dies must be treated with a separator and have undercuts blocked out. The modeling resin is applied in layers 3- to 5-mm thick, with each layer cured separately in a

Fig. 14-12 Sculpted anterior crowns made from lightcured resin modeling material.

(Courtesy Heraeus Kulzer, GmbH, Wehrheim, Germany.)

Adapted from Iglesias A, Powers JM, Pierpont HP:J Prosthodont 5:201, 1996.

high-intensity, light-curing chamber for 90 seconds or by using a hand-held, light-curing unit for 20 to 40 seconds per area of irradiation. The liquid material is used first to obtain close adaptation to the die and last to provide a smooth surface. Complete elimination of m o d e h g resins occurs between 670" to 690" C and requires about 45 minutes.

BASEPLATE WAX

Baseplate wax derives its name from its use on the baseplate tray to establish the vertical dimension, plane of occlusion, and initial arch form in the technique for the complete denture restoration. This wax also may be used to form all or a portion of the tray itself. The normally pink color provides some esthetic quality for the initial stage of construction of the denture before processing. Baseplate wax is the material used to produce the desired contour of the denture after teeth are set in position. As a result, the contour wax establishes the pattern for the final plastic denture. Patterns for orthodontic appliances and prostheses other than complete dentures, which are to be constructed of plastics, also are made of baseplate wax. Although these are the primary functions of baseplate wax, it has also been widely used in many phases of dentistry to check the various articulating relations in the mouth and to transfer them to mechanical articulators.

Composition A few formulas are found in the literature for baseplate wax. Baseplate waxes may contain 70% to 80% paraffin-based waxes or commercial ceresin, with small quantities of other waxes, resins, and additives to develop the specific qualities desired in the wax. A typical composition might include 80% ceresin, 12% beeswax, 2.5%carnauba, 3% natural or synthetic resins, and 2.5% microcrystalline or synthetic waxes. Differential thermal analysis and penetration curves of a typical baseplate wax are shown in Fig. 14-13.

Physical Characteristics Baseplate waxes are normally supplied in sheets 7.60 x 15.00x 0.13 cm in pink or red. The manufacturer usually formulates two types of wax to accommodate the varying climates in which they will be used, because the flow of the wax is influenced greatly by the temperature.

The requirements for dental baseplate wax are listed in Table 14-8, which summarizes ANSI/ ADA Specification No. 24 (IS0 12163). Three types of wax are included: Type 1 is a soft wax