Сraig. Dental Materials

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Sticky wax

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for building contours and veneers, Type 2 is a hard wax to be used for patterns to be tried in the mouth in temperate climates, and Type 3 is an extra-hard wax for patterns to be tried in the mouth in tropical weather. The flow values are listed for 23", 37", and 45" C when they are applicable. The maximum flow allowed at any given temperature decreases ra$dly from Type 1 to Type 3. The flow requirements of Type 3 baseplate wax are comparable to those of the Type 2 (hard) inlay wax, with less flow allowed for the baseplate wax at 45" C.

Because baseplate wax is used both to set denture teeth and to adapt around these teeth to develop proper contour, the dimensional changes that may take place because of variations in temperature are important. Although the need for dimensional stability is not as critical as with the inlay wax, the maintenance of good tooth relationship is important. Although no shrinkage value from a molten state to room temperature is required by the specification, the linear thermal expansion from 26" to 40" C should be less than 0.8%.

A summary of practical requirements is also given in Table 14-8. Baseplate waxes should be easily trimmed with a sharp instrument at 23" C and should yield a smooth surface after gentle flaming. These waxes should not leave any residue on porcelain or plastic teeth, and the coloring agents in the wax should not separate or impregnate the plastic mold during processing.

There is residual stress within the baseplate wax that holds and surrounds the teeth of a wax denture pattern. This stress results from differential cooling, "pooling" the wax with a hot spatula, and physically manipulating the wax below its most desirable working temperature. Remember that both time and temperature affect the relief of these residual stresses; the waxed and properly articulated denture should not be allowed to stand for long periods of time, especially when subjected to elevated temperatures. Such treatment often results in distortion of the wax and movement of the teeth. The waxed denture should be flasked soon after completion to maintain the greatest accuracy of tooth relations.

BOXING WAX

To form a plaster or stone cast from an impression of the edentulous arch, first a wax box must be formed around the impression, into which the freshly mixed plaster or stone is poured and vibrated. This boxing procedure is also necessary for some other types of impressions. The boxing operation usually consists of first adapting a long, narrow stick or strip of wax around the impression below its peripheral height, followed by a wide strip of wax, producing a form around the entire impression, as seen in the upper center of Fig. 14-1.

The dental literature occasionally refers to carding wax for use in the boxing operation. Carding wax was the original material on which porcelain teeth were fixed when received from the manufacturer. The terms carding wax and boxing wax have been used interchangeably, although boxing wax is more acceptable.

The requirements of Federal Specification No. U-W-138 for boxing wax, which are summarized in Table 14-9, stipulate that this wax should be pliable at 21" C and should retain its shape at 35" C. This broadly defines its lower temperature limit of ductility and flow. Because the impression may be made from a viscoelastic material that is easily distorted, a boxing wax that is readily adaptable to the impression at room temperature is desirable. This property reduces the likelihood of distorting the impression, from the standpoint of both the temperature and stress involved in the boxing procedures. In general, boxing wax should be slightly tacky and have sufficient strength and toughness for convenient manipulation.

UTILITY WAX

An easily workable, adhesive wax is often desired. For example, a standard perforated tray for use with hydrocolloids may easily be brought to a more desirable contour by such a wax, as shown in the center view of Fig. 14-1. This is done to prevent a sag and distortion of the impression material. A soft, pliable, adhesive wax may be used on the lingual portion of a bridge pontic to stabilize it while a labial plaster splint is

poured. These and many other tasks are performed by the utility wax, justifying its name.

The utility wax is usually supplied in both stick and sheet form in dark red or orange. The ductility and flow of utility waxes, as indicated by the summarized requirements of Federal Specification No. U-W-156 in Table 14-9, are the highest of any of the dental waxes. The utility wax should be pliable at a temperature of 21" to 24" C, which makes it workable and easily adaptable at normal room temperature. The flow of this wax should not be less than 65% nor more than 80% at 37.5" C . Because building one layer on top of another is often desirable, the specification requires a sufficient adhesiveness at 21" to 24" C . Utility wax probably consists of beeswax, petrolatum, and other soft waxes in varying proportions.

STlCW WAX

A suitable sticky wax for prosthetic dentistry is formulated from a mixture of waxes and resins or

other additive ingredients. Such a material is sticky when melted and adheres closely to the surfaces on which it is applied. However, at room temperature the wax is firm, free from tackiness, and brittle. Stickywax should fracture rather than flow if it is deformed during soldering or repair procedures. Although this wax is used to assemble metallic or resin pieces in a fixed temporary position, it is primarily used on dental stones and plasters. The lower right portion of Fig. 14-1 shows an application of sticky wax to seal a plaster splint to a stone model in the process of forming porcelain facings.

According to Federal Specification No. U-W- 00149a (DSA-DM), sticky wax should have a dark or vivid color so it is readily distinguishable from the light-colored gypsum materials. The specification, summarized in Table 14-9, also limits the shrinkage of sticky wax to 0.5% at temperatures between 43" and 28" C.

The literature contains several formulas for sticky wax, representing both a high and a low resin content. In addition to rosin and yellow

beeswax, which are the usual major constituents, coloring matter and other natural resins such as gum dammar may be present. Differential thermal analysis and penetration curves for a typical dental sticky wax are shown in Fig. 14-13.

CORRECTIVE IMPRESSION WAX

Corrective impression wax is used as a wax veneer over an original impression to contact and register the detail of the soft tissues. It is claimed that this type of impression material records the mucous membrane and underlying tissues in a functional state in which movable tissue is displaced to such a degree that functional contact with the base of the denture is obtained. Corrective waxes are formulated from hydrocarbon waxes such as paraffin, ceresin, and beeswax and may contain metal particles. There are no ANSI/ADA or federal specifications for corrective impression waxes. The flow of several corrective waxes measured by penetration at 37' C is 100%. Differential thermal analysis and penetration curves for a typical corrective impression wax are shown in Fig. 14-14. These waxes are subject to distortion during removal from the mouth.

BITE REGISTRATION WAX

Bite registration wax is used to accurately articulate certain models of opposing quadrants. The wax bite registration for the copper-formed die

Temperature (O C)

Fig. 14-14 Differential thermal analysis and penetration curves for a dental corrective impression wax. The stresses for TMA-1 and TMA - 2 are 0.015 and 0.25 MPa, respectively. Observe that the solid-solid transition temperature occurs below 37" C.

must provide proximal and occlusal relations. Bite registrations are often made from 28- gauge casting wax sheets or from hard baseplate wax, but waxes identified as bite waxes seem to be formulated from beeswax or hydrocarbon waxes such as paraffin or ceresin. Certain bite waxes contain aluminum or copper particles. There are no ADA or federal specifications for bite waxes. The flow of several bite waxes as measured by penetration at 37" C ranges from 2.5% to 22%, indicating that these waxes are susceptible to distortion on removal from the mouth.

I SELECTED PROBLEMS

Problem 1

The investing of a wax pattern was delayed for a day, and the gold alloy casting that was produced did not fit. What probably caused this problem and can it be corrected?

Solution a

Residual stresses are always present in wax patterns from manipulation, and they may be released during storage. To minimize distortion, invest the wax pattern promptly.

446 Chapter 14 WAXES

Solution b

Distortion of the wax pattern may have occurred during spruing or removal of the pattern from the die. A sprued pattern may be replaced on the die to evaluate and readapt the margins.

Problem 2

A wax pattern that had been refrigerated was invested, and the gold alloy casting that was produced did not fit. What probably caused this problem, and how can it be corrected?

Solution

Inlay waxes have high coefficients of thermal expansion. Cooling and heating of a pattern can result in a nonuniform dimensional change. A refrigerated wax pattern should be allowed to warm to room temperature, and its margins should be readapted before proceeding with investing.

Problem 3

The internal surface of a wax pattern prepared on a silver-plated die showed a wrinkled appearance. What caused this problem, and how can it be corrected?

Solution

When molten wax is flowed incrementally onto a cool metal die, the wax next to the die solidifies rapidly. Solidification of adjacent wax occurs more slowly and pulls the previously congealed wax away from the die, resulting in poor surface adaptation. A metal die should be warmed to about 32" C under an electric light or on a heating pad during waxing. An alternative solution is to use a soft wax at the cervical margins.

Problem 4

A wax-up of a complete denture was completed and stored overnight on an articulator. The next day the posterior teeth were no longer in contact. What happened during the storage period, and how can the problem be corrected?

Solution

The esthetic portion of the wax-up is done after the teeth have been articulated. A hot spatula and an alcohol torch are used for shaping the wax, causing it to expand. Sub- sequent contraction of the wax can occur overnight, thereby moving the teeth out of articulation. This contraction can be minimized by not working in any one area for too long and by not applying too much heat. The waxed denture should be flasked soon after completion of the wax-up to minimize distortion caused by the release of residual stresses in the wax.

Problem 5

The impression wax forming a posterior palatal seal to a maxillary denture impression pulled away from a rubber impression material in several areas when the impression was removed from the mouth. What caused this problem, and how can it be corrected?

Solution

Impression wax adheres poorly to polysulfide and silicone impression materials. The separation can be prevented by applying a very thin layer of sticky wax to the area of the impression material where the impression wax is to be applied. The impression wax will adhere to the sticky wax.

Problem 6

The broken pieces of a denture were joined with baseplate wax, flasked, and processed with heat-cured acrylic. However, the repaired denture did not fit. What may have caused this problem, and how can it be corrected?

Solution

Baseplate wax will flow somewhat if subjected to stress. The denture pieces held together by wax may move relative to each other during the repair process. Pieces to be repaired should be joined with sticky wax. It is brittle and will break rather than distort, thus readily indicating that the pieces have moved.

Anderson, JN: Applied dental materials, ed 5, Oxford, 1976, Blackwell Scientific.

Bennett H: Industrial waxes, vols 1 and

2, Brooklyn, NY, 1963, Chemical Publishing. Coleman RL: Physical properties of dental materials, US Bureau of Standards, Research Paper No 32, J Res Nat Bur Stand 1:867, 1928.

Council on Dental Materials, Instruments, and Equipment: Revised ANSI/ADA Specification No 4 for inlay wax, J A m Dent Assoc lO8:88, 1984.

Craig RG, Eick JD, Peyton FA: Properties of natural waxes used in dentistry, J Dent Res 44:1308, 1965.

Craig RG, Eick JD, Peyton FA: Flow of binary and tertiary mixtures of waxes, J Dent Res 453397, 1966.

Craig RG, Eick JD, Peyton FA: Strength properties of waxes at various temperatures and their practical application, J Dent Res 46:300, 1967.

Craig RG, Powers JM, Peyton FA: Differential thermal analysis of commercial and dental waxes, J Dent Res 46:1090, 1967.

Craig RG, Powers JM, Peyton FA: Thermogravimetric analysis of waxes, J Dent Res

50:450, 1971.

Dirksen LC: Composition and properties of a wax for lower impressions, J Am Dent Assoc 26:270, 1939.

Farah JW, Powers JM, editors: Bite registration materials, Dent Advis 15(4):1, 1998.

Grajower R: A new method for determining the thermal expansion of dental waxes, J Dent Res 57:659, 1978.

Grossman LI: Dental formulas and aids to dental practice, Philadelphia, 1952, Lea & Febiger.

Hollenback GM, Rhodes JE: A study of the behavior of pattern wax, J South Calif State Dent Assoc 27:419, 1959.

Iglesias A, Powers JM, Pierpont HP: Accuracy of wax, autopolymerized, and lightpolymerized resin pattern materials, J Prosthodont 5201, 1996.

Ito M, Yamagishi T, Oshida Y et al: Effect of selected physical properties of waxes on investments and casting shrinkage, J Prosthet Dent 75211, 1999.

Lasater RL: Control of wax distortion by manipulation, J A m Dent Assoc 27:518, 1940.

Ludwig FJ Jr: Modern instrumental wax analysis, Soap G Chem Spec 42:70, 1966.

Markley MR: The wax pattern, Dental Clinics of North America, Symposium on Dental Materials, Philadelphia, Nov 1958, Saunders.

Maves TW:Recent experiments demonstrating wax distortion on all wax patterns when heat is applied, J A m Dent Assoc

19606, 1932.

McCrorie JW: Dental modelling waxes A new approach to formulation, Br Dent J 132:189, 1972.

McCrorie JW: Corrective impression waxes, Br Dent J 152:95, 1982.

Morrison JT, Duncanson MG Jr, Shillingburg HT Jr: Wetting effects of surface treatments on inlay wax-investment combinations,

J Dent Res 60:1858, 1981.

Nelson EA: Practical applications of crowns and inlay casting technics, J A m Dent Assoc 27:588, 1940.

Ohashi M, Paffenbarger GC: Melting, flow, and thermal expansion characteristics of some dental and commercial waxes, J A m Dent Assoc 72:1141, 1966.

Ohashi M, Paffenbarger GC: Some flow characteristics at 37" C of ternary wax mixtures that may have possible dental uses, J Nihon Univ Sch Dent 11:109, 1969.

Phillips RW: Skinner's science of dental materials, ed 8, Philadelphia, 1982, Saunders.

Phillips RW, Biggs DH: Distortion of wax patterns as influenced by storage time, storage temperature, and temperature of wax manipulation, J Anz Dent Assoc 41:28, 1950.

Powers JM, Craig RG: Penetration of commercial and dental waxes, J Dent Res

53:402, 1974.

Powers JM, Craig RG: Thermal analysis of dental impression waxes, J Dent Res 57:37, 1978,

448 Cha~ter14 WAXES

Powers JM, Craig RG, Peyton FA: Calorimetric analysis of commercial and dental waxes, J Dent Res 48:1165, 1769.

Smith DC, Earnshaw R, McCrorie JW: Some properties of modelling and base plate waxes, Br Dent J 118:437, 1765.

Taylor NO: Progress report. Research on dental materials: the research program; cooperative work on inlay casting technic, J A m Dent Assoc 18274, 1731.

Taylor NO, Paffenbarger GC: A survey of current inlay casting technics, J A m Dent Assoc 17:2058, 1730.

Taylor PB: Inlay casting procedure-its evolution and the effect of manipulatory variables. In Anderson GM, editor: Proceedings of the Dental Centenay Celebration, Baltimore, 1940, Maryland State Dental Association.

US General Services Administration, Federal Supply Service, Federal Specification No U-W-135,July 31, 1747, Wax, base plate, dental; No U-W-138, May 12, 1747, Wax, boxing, dental; No U-W-140, March 16, 1753, Wax, casting, dental; No U-W-14la, Dec 6, 1956, Wax, dental (casting inlay); No U-W-00147a (DSA-DM), Sept 7, 1766, Wax, sticky, dental; No U-W-156, Aug 17, 1948, Wax, utility, dental.

Warth AH: m e chemisty and technology of waxes, ed 2 , New York, 1756, Reinhold.

Washburn KC: Inlay wax and its manipulation, Ill Dent J 16:407, 1747.

450 Chapter 15 NOBLE DENTAL ALLOYS AND SOLDERS

Noble dental alloys have undergone a tremendous evolution because the U.S. government lifted its support on the price of gold in 1969. Before then, more than 95% of fixed dental prostheses in the United States were made of alloys containing a minimum of 75% gold and other noble metals by weight. However, when the price of gold increased from $35 per ounce to more than $800 per ounce in the early 1980s, the development of alternative alloys increased dramatically to reduce the cost of cast dental restorations. These alternatives included alloys with reduced gold content, but also alloys that contain no gold and alloys that contain no noble metal. Today, the majority of alloys used in the US. and a significant portion of those used in other countries are alternative alloys. In 1999, the tremendous increase in the price of palladium, from about $100 to more than $350 per ounce, again encouraged the development of new types of

casting alloys.

This chapter will focus on dental casting alloys that have a noble metal content of at least 25% by weight. In addition, wrought noble alloys and solders will be discussed. The limit of 25 wt% is somewhat arbitrary, but represents the limit established by the ADA for alloys that can be classified as noble alloys. Base-metal alloys will be discussed in Chapter 16. The first section of the current chapter will survey the metallic elements classified as noble dental alloys. In subsequent sections, the physical and chemical properties of noble dental alloys, both cast and wrought, will be presented. Finally, the compositions and properties of noble solders will be discussed. Techniques for casting and soldering will be discussed in Chapter 17.

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For dental restorations, it is necessary to combine various elements to produce alloys with adequate properties for dental applications because none of the elements themselves have properties that are suitable. These alloys may be used for dental restorations as cast alloys, or may be

manipulated into wire or other wrought forms. The metallic elements that make up dental alloys can be divided into two major groups, the noble metals and the base metals. A general discussion of the principles of alloys and metallurgy that are used in the current chapter can be found in Chapter 6.

NOBLE METALS

Noble metals are elements with a good metallic surface that retain their surface in dry air. They react easily with sulfur to form sulfides, but their resistance to oxidation, tarnish, and corrosion during heating, casting, soldering, and use in the mouth is very good. The noble metals are gold, platinum, palladium, iridium, rhodium, osmium, and ruthenium (Table 15-1, see Fig. 6-11, These metals can be subdivided into two groups. The metals of the first group, consisting of ruthenium, rhodium, and palladium, have atomic weights of approximately 100 and densities of 12 to 13 g$cm3. The metals of the second group, consisting of osmium, iridium, platinum, and gold, have atomic weights of about 190 and densities of 19 to 23 g/cm3. The melting points of members of each group decrease with increasing atomic weight. Thus, ruthenium melts at 2310" C, rhodium at 1966" C, and palladium at 1554" C. In the second group the melting points range from 3045" C for osmium to 1064" C for gold. The noble metals, together with silver, are sometimes called precious metals. The term precious stems from the trading of these metals on the commodities market. Some metallurgists consider silver a noble metal, but it is not considered a noble metal in dentistry because it corrodes considerably in the oral cavity. Thus the terms noble and precious are not synonymous in dentistry.

Gold (Au) Pure gold is a soft, malleable, ductile metal that has a rich yellow color with a strong metallic luster. Although pure gold is the most ductile and malleable of all metals, it ranks much lower in strength. The density of gold depends somewhat on the condition of the metal, whether it is cast, rolled, or drawn into wire. Small amounts of impurities have a pro-

nounced effect on the mechanical properties of gold and its alloys. The presence of less than 0.2% lead causes gold to be extremely brittle. Mercury in small quantities also has a harmful effect. Therefore scrap of other dental alloys, such as technique alloy or other base-metal alloys, including amalgam, should not be mixed with gold used for dental restorations. The addition of calcium to pure gold improves the mechanical properties of gold foil restorations (see subsequent discussion).

Air or water at any temperature does not affect or tarnish gold. Gold is not soluble in sulfuric, nitric, or hydrochloric acids. However, it readily dissolves in combinations of nitric and hydrochloric acids (aqua regia, 18 vol% nitric and

82 vol% hydrochloric acids) to form the trichloride of gold (AuC1,). It is also dissolved by a few other chemicals, such as potassium cyanide and solutions of bromine or chlorine.

Because gold is nearly as soft as lead, it must be alloyed with copper, silver, platinum, and other metals to develop the hardness, durability, and elasticity necessary in dental alloys, coins, and jewelry (Table 15-2). Through appropriate refining and purification, gold with an extremely high degree of purity may be produced. Such highly refined ingots of pure gold (99.99% by weight) serve as the starting material for gold foil.

Gold foil is formed by a process known as gold beating. High-purity gold is first passed through a series of rollers and then annealed until