Сraig. Dental Materials

424 Chapter 14 WAXES

F ew procedures in restorative dentistry can be completed without the use of wax in one of its many forms. Forming an inlay pattern, boxing

an impression before it is poured in dental stone, and making an impression for the registration of occlusal bite relationships each requires a specially formulated wax. Some uses for various dental waxes are shown in Fig. 14-1. These examples display how the tasks these waxes perform, and therefore their properties, vary greatly. Accuracy is a requisite for inlay or removable denture patterns, shown in the upper left of Fig. 14-1, whereas for the boxing of an impression, shown in the upper center, the ease and convenient manipulation of the wax are essential. Other applications, such as the denture forms shown in the lower left or the corrective impression wax shown in the upper right of Fig. 14-1, require equally varying qualities in the waxes, as do the applications of wax for the border and

palate of the metal tray or for attaching a plaster splint to the model, as shown in the lower right of the figure. Thus the specific use of the dental wax determines the physical properties that are most desirable for a successful application.

WMES, GUMS, FA=, A&

Dental waxes may be composed of natural and synthetic waxes, gums, fats, fatty acids, oils, natural and synthetic resins, and pigments of various types. The particular working characteristics of each wax are achieved by blending the appropriate natural and synthetic waxes and resins and other additives, some of which are shown in Table 14-1.

The chemical components of both natural and synthetic waxes impart characteristic physical properties to the wax, which are of primary

interest because the specific physical properties of a wax or wax blend determine its usefulness for intended applications. Natural waxes are distributed in nature, whereas synthetic waxes are produced by combination of various chemicals in the laboratory or by chemical action on natural waxes. The additives may be natural materials and synthetic products.

NATURAL WAXES

Historically, waxes have been classified according to their origin: (1) mineral, (2) plant,

(3) insect, and (4) animal; however, a better classification is based on their chemical composition. The two principal groups of organic compounds contained in waxes are hydrocarbons and esters, although some waxes contain free alcohols and acids as well.

The chief constituents of most mineral waxes are hydrocarbons ranging from 17 to more than 44 carbon atoms. a fact that accounts for odd and

even numbers in the chain, as shown in the following formula:

The hydrocarbons in plant waxes are saturated alkanes with from 19 to 31 carbon atoms present in odd numbers. Therefore, dental waxes contain molecules having a range of molecular weights that affect the melting and flow properties of the waxes.

Plant and animal waxes contain considerable concentrations of esters, and carnauba (a plant wax) contains 85% alkyl esters of various kinds. The principal ester in beeswax is myricyl palmitate,

which is the reaction product of myricyl alcohol and palmitic acid. Plant and animal waxes also contain acids, alcohols, hydrocarbons, and resins; whereas Montan wax (an earth wax) contains large amounts of esters, the main compound being

However, there are other esters composed of C,,-C,, acids and C,,-C,, alcohols.

This brief description of the composition of natural waxes indicates that they are complex combinations of organic compounds of reasonably high molecular weights. Also, the composition of these waxes varies, depending on the source and the time of collection; therefore, dental manufacturers must blend the particular batches of wax to obtain the properties desired for a particular application. Characteristics of various waxes used in dentistry and described below are summarized in Table 14-2.

Parafin waxesare obtained principally from the high-boiling point fractions of petroleum. The melting temperatures generally increase

with increasing molecular weights. The presence of oils in the wax, however, lowers the melting temperature; paraffin waxes used in dentistry are refined waxes and have less than 0.5% oil.

Paraffin waxes produced by current refining procedures can crystallize in the form of plates, needles, or malcrystals, but are usually of the plate type. Many hydrocarbon waxes undergo crystalline changes on cooling, and a transition from needles to plates occurs about 5" to 8" C below their melting temperature. During solidification and cooling, there is a volumetric contraction that varies from 11% to 15%. This contraction is not uniform throughout the temperature range from the melting temperature to room temperature, because the wax is a mixture of hydrocarbons and the wax passes through transition points accompanied by changes in physical properties.

Microcrystalline waxes are similar to paraffin waxes, except they are obtained from the heavier oil fractions in the petroleum industry and, as a result, have higher melting points. These waxes crystallize in small plates and are tougher and more flexible than paraffin waxes. They have an affinity for oil, and their hardness &ndtackiness may be altered by adding oil. Microcrystalline waxes have less volumetric change during solidification than paraffin waxes.

Barnsdahl is a microcrystalline wax%sed to increase the melting range and hardness and reduce the flow of paraffin waxes.

Ozokerite is an earth wax found near petroleum deposits in central Europe and the western United States. Ozokerite is similar to microcrystalline wax in that it is composed of straightand branched-chain hydrocarbons, but it also contains some closed-chain hydrocarbons. It also has great affinity for oil, and in quantities of 5%to 15% greatly improves the physical characteristics of paraffins in the melting range of 54" C.

Ceresin is a term used to describe waxes from wax-bearing distillates from natural-mineral petroleum refining or lignite refining. Like microcrystalline waxes, they are straightand branched-chain paraffins, but they have higher molecular weights and greater hardness than hydrocarbon waxes distilled from the crude

products. These waxes also may be used to increase the melting range of paraffin waxes.

Montan waxes are obtained by extraction from various lignites, and although they are mineral waxes, their composition and properties are similar to those of the plant waxes. Montan waxes are hard, brittle, and lustrous; they blend well with other waxes, and therefore are often substituted for plant waxes to improve the hardness and melting range of paraffin waxes.

Carnauba and ouricury waxes are composed of straight-chain esters, alcohols, acids, and hydrocarbons. They are characterized by high hzrdness, brittleness, and high melting temperatures. Both possess the outstanding quality of increasing the melting range and hardness of paraffin waxes; for example, adding 10% of carnauba wax to paraffin wax with a melting range of 20" C increases the melting range to 46' C. Adding ouricury waxes produces a similar effect, but they are less effective than carnauba wax.

Candelilla waxes consist of 40% to 60% paraffin hydrocarbons containing 29 to 33 carbon atoms, accompanied by free alcohols, acids, esters, and lactones. Like carnauba and ouricury wax, they harden paraffin waxes but are not so effective for increasing the melting range.

Japan wax and cocoa butter are not true waxes; they are chiefly fats. Japan wax contains the glycerides of palmitic and stearic acids and higher-molecular-weight acids; cocoa butter is completely fat and composed of glycerides of stearic, palmitic, oleic, lauric, and lower fatty acids.Japan wax is a tough, malleable, and sticky material that melts at about 51" C, whereas cocoa butter is a brittle substance at room temperatures. Japan wax may be mixed with paraffin to improve tackiness and emulsifying ability, and cocoa butter is used to protect against dehydration of soft tissues and to protect glass ionomer products temporarily from moisture during setting or from dehydrating after they are set.

Beeswax is the primary insect wax used in dentistry. It is a complex mixture of esters plus saturated and unsaturated hydrocarbons and high-molecular-weight organic acids. It is a brittle material at room temperature but becomes plastic at body temperature. It is used to modify the

428 Chapter I 4 WAXES

properties of paraffin waxes, and is the main component in sticky wax.

Animal waxes such as spermaceti wax, obtained from the sperm whale, are not used extensively in dentistry; like beeswax, they are mainly ester waxes. Spermaceti wax has been used as a coating in the manufacture of dental floss.

SYNTHETIC WAXES

In recent years synthetic waxes and resins have become available. Although the use of synthetic waxes and resins is increasing, it is still limited in dental formulations, and the natural waxes continue to be the priiwary components.

Synthetic waxes are complex organic compounds of varied chemical compositions. Although they differ chemically from natural waxes, they possess certain physical properties, such as melting temperature or hardness, which are akin to those of the natural waxes. They may differ from natural waxes in certain characteristics because of their high degree of refinement, in contrast with the contan~inationthat is common in natural waxes.

Synthetic waxes include (1) polyethylene waxes, ( 2 ) polyoxyethylene glycol waxes, ( 3 )halogenated hydrocarbon waxes, ( 4 )hydrogenated waxes, and ( 5 ) wax esters from the reaction of fatty alcohols and acids. Polyethylene polymers having molecular weights from 2000 to 4000 are waxes melting at 100" to 105' C . These waxes possess properties similar to high-molecular- weight paraffin waxes obtained from petroleum. Polyoxyethylene waxes are polymers of ethylene glycols and have melting temperatures from 37" to 63" C . They have limited compatibility with other waxes but do function as plasticizers and tend to toughen films of wax. The remaining synthetic waxes are prepared by reactions with natural waxes or wax products, as with chlorine in the preparation of halogenated waxes and hydrogen in the manufacture of hydrogenated waxes. The variability of various batches of synthetic wax is similar to that of natural waxes.

GUMS

Many waxes obtained from plants and animals resemble in appearance a group of substances described as gums. Many plants produce a variety of gums that are viscous, amorphous exudates that harden on exposure to air. Most gums are complicated substances. Many are mixtures containing largely carbohydrates; when they are mixed with water, they either dissolve or form sticky, viscous liquids. Gum arabic and tragacanth are two natural gums that do not resemble waxes in either their properties or composition.

FATS

As a class of substances, waxes are harder and have higher melting temperatures than fats, but in some ways they resemble fat. Both are tasteless, odorless, and colorless in the pure form, and they usually feel greasy. Chemically, fats are composed of esters of various fatty acids with glycerol and are known as glycerides, which distinguishes them from waxes. Some examples of fats are glycerides of stearic acid or tristearate, found in tallow, and the mixed glyceride of oleic, palmitic, and butyric acids, found in butter.

Glyceryl tristearate, the chief ingredient of beef tallow, is a fat with a melting temperature of about 43" C. It has a lustrous appearance and is a firm, slightly greasy solid that bears a resemblance to waxes. The fat may be used to increase the melting range and hardness of compounded wax. Oils have a pronounced effect on the properties of waxes, as mentioned earlier in connection with the discussion of paraffin waxes. Hydrocarbon oils may be used to soften mixtures of waxes, and small quantities of silicone oils may be added to improve the ease of polishing with waxes.

RESINS

In some respects natural resins resemble waxes in appearance and properties, although they form a distinct classification of substances. Many species of trees and other plants produce exu-

dates of natural resins, such as dammar, rosin, or sandarac. Natural resins are relatively insoluble in water, but vary in solubility in certain organic liquids. Generally, resins are complex, amorphous mixtures of organic substances that are characterized by specific physical behavior rather than by any definite chemical composition. Most natural resins are obtained from trees and plants; shellac, however, is produced by insects. Numerous natural resins are blended with waxes to develop waxes for dental applications.

Natural resins such as dammar and kauri may be mixed with waxes. They are compatible with most natural waxes and produce harder products. Synthetic resins, such as polyethylene and vinyl resins of various types, may be added to paraffin waxes to improve their toughness, filmforming characteristics, and melting ranges.

Natural and synthetic resins may also be used in organic solvents to produce film-forming materials that may be used as a cavity liner. Copal is a natural, brittle resin that has a melting range well above 149" C, but when deposited as a film from an organic solvent, it serves as a liner for prepared cavities. Polystyrene is a synthetic resin that may be used in a similar manner.

Useful and important properties of waxes include melting range, thermal expansion, mechanical properties, flow, residual stress, and ductility.

MELTING RANGE

Because waxes may contain several types of molecules, each having a range of molecular weights, they have melting ranges rather than melting points. The melting ranges of a paraffin wax, a carnauba wax, and a mixture of these two waxes are illustrated in Fig. 14-2. The curves are differential thermograms obtained in the manner

100% Paraffin

100% Carnauba

Temperature (" C)

Fig. 14-2 Differential thermograms of paraffin, carnauba, and a 75% paraffin-25% carnauba wax mixture.

described in the Thermal Properties section of Chapter 3. The melting range for paraffin wax is from 44" to 62" C, and for carnauba wax, from 50" to 90" C . When a mixture of 75% paraffin and 25% carnauba wax was prepared, the paraffin component melted at essentially the same temperatures, but the melting temperature of the carnauba wax was decreased slightly. Note that adding carnauba to paraffin wax dramatically increased the melting range to 44" C, compared with 18" C for paraffin alone.

The effect of the composition of paraffincarnauba mixtures on the melting range is shown in Fig. 14-3. The presence of 2.5% carnauba wax had little effect on the melting range, but the range increased rapidly as the concentration of carnauba wax was increased to 10%. Al-

Fig. 14-3 Melting ranges of mixtures of paraffin and carnauba wax.

though concentrations of carnauba wax greater than 10% had no further effect on the melting range, higher amounts are necessary for certain applications to control the flow and mechanical properties. The melting of mixtures of paraffin and higher-melting point waxes can be visualized as the melting of paraffin near its usual temperature, but the entire wax does not appear melted because the matrix of carnauba wax does not melt until a considerably higher temperature is reached.

THERMAL EXPANSION

Like other materials, waxes expand when subjected to a rise in temperature and contract as the temperature is decreased. This fundamental property may be altered slightly when various waxes are blended (Fig. 14-4), but the response to thermal changes cannot be reduced to negligible values. The expansion and contraction of dental waxes with changes in temperature are pronounced, as is illustrated in Table 14-3. In general, dental waxes and their components have the largest coefficient of thermal expansion of any material used in restorative dentistry.

The linear thermal expansion properties of waxes may be explained on the basis of the strength of secondary valence forces and the transition points. Mineral waxes generally have higher coefficients of linear thermal expansion than plant waxes. Mineral waxes expand more

Temperature (" C)

Fig. 14-4 Thermal expansion curves for four waxes.

because they have weak secondary valence forces, which are easily overcome by the energy absorbed during a rise in temperature. This permits more movement of the wax components, thus allowing a greater amount of thermal expansion.

Plant waxes, on the other hand, have high secondary valence forces because of their high concentrations of esters. Because the secondary valence forces restrict the movement of the wax components, small coefficients of thermal expansion are observed until the melting range of the wax is approached. This phenomenon is illustrated by beeswax; yellow beeswax has much higher coefficients of linear thermal expansion than bleached beeswax.

Many waxes exhibit at least two rates of expansion between 22" and 52" C. These changes in rate of expansion occur at transition points. At these points the internal structural parts become freer to move. For example, during this transition hydrocarbon chains of a mineral wax become free to rotate; consequently, after a wax has been heated through a transition point, it is

MINERAL

Paraffin

Litene

Barnsdahl

Ceresin

Montan

PLANT

Carnauba

Candelilla

Ouricury

Japan wax

INSECT

Beeswax (yellow)

Beeswax (bleached)

INLAY

Kerr blue inlay wax (hard)

Kerr blue inlay wax (regular)

freer to expand. Because the ingredient waxes are undergoing transitions that do not coincide with one another, certain inlay waxes exhibit more than two changes in rate of expansion.

. Paraffin\

Temperature (" C)

Fig. 14-5 Elastic modulus of various waxes as a function of temperature.

Different waxes may have decidedly different rates and amounts of thermal expansion, as shown in Table 14-3. For example, carnauba and Montan wax have approximately the same melting ranges, but from 22" to 52" C they have dissimilar expansion characteristics.

Some waxes have different rates of expansion in different temperature ranges, as seen by the change of shape of the curves for paraffin, beeswax, and an inlay wax in Fig. 14-4. Because the coefficient of thermal expansion of inlay wax is so great, temperature changes in wax patterns after the critical dimensional relationships are set may be a major contributing factor in inaccuracy of the finished restoration.

MECHANICAL PROPERTIES

The elastic modulus, proportional limit, and compressive strength of waxes are low compared with those of other materials, and these properties depend strongly on the temperature. The elastic moduli of various waxes between 23" and 40" C are shown in Fig. 14-5, with carnauba wax having the highest values and