both hydrocarbons, paraffin wax being a simple straight-chain hydrocarbon whilst the microcrystalline material has a branched structure.

Paraffin waxes soften in the temperature range 37–55ºC and melt in the range 48–70ºC. They are brittle at room temperature. Microcrystalline waxes melt in the range 65–90ºC and when added to paraffin waxes they raise its melting point. At the same time they lower the softening temperature and render the material less brittle than paraffin wax alone.

Animal: Beeswax, derived from honeycombs, consists of a partially crystalline natural polyester and is often blended with paraffin wax in order to modify the properties of the latter. The effect of adding beeswax to paraffin wax is to render the material less brittle and to reduce the extent to which it will flow under stress at temperatures just below the melting point.

Vegetable: Carnauba wax and candelilla wax are derived from trees and plants. They are blended with paraffin wax in order to control the softening temperature and modify properties.

4.4 Properties of dental waxes

Waxes are generally characterised by their thermal properties such as melting point and solid–solid transition temperature which is closely related to the softening temperature observed in practice. The coefficient of thermal expansion is a major factor affecting accuracy. Dimensional stability is primarily a function of the magnitude of the stresses which become incorporated during thermal contraction after moulding. Important mechanical properties are brittleness and the degree of flow which a material will undergo in its working temperature range.

Thermal properties: All the waxes used in dentistry have a predominantly crystalline structure and are characterised by a well-defined melting point. On heating, a second endothermic peak exists at a temperature somewhat lower than the melting point. This peak is indicative of a solid– solid transition involving a change in the crystal structure of the wax. The change in crystal structure is accompanied by a change in mechanical properties and the wax is converted from a relatively brittle solid to a much softer, mouldable

material. For this reason, the solid–solid transition temperature is sometimes referred to as the softening temperature. For many applications of waxes the softening temperature should be just above mouth temperature. This is in order that the material may be introduced into the mouth in a mouldable state but will become relatively rigid at mouth temperature. The manufacturers can control the melting point and softening temperature by blending mixtures of various mineral, animal and vegetable components.

Waxes are very poor thermal conductors and must be maintained above the solid-solid transition temperature for long enough to allow thorough softening to occur throughout the material before moulding is attempted.

Following moulding, the waxes are allowed to cool. During this cooling period they may undergo potentially significant contraction due to the high values of coefficient of expansion exhibited by these products.

The thermal contraction may not be fully exhibited immediately after cooling. The low thermal conductivity values of the materials result in solidification of the surface layers of the wax well before the bulk becomes rigid. This reduces the magnitude of the thermal contraction and produces significant internal stresses. Dimensional changes may occur due to relief of the stresses. This is more likely to occur at elevated temperatures. Greater stresses may be incorporated if the wax is not properly softened before moulding.

Methods for softening wax prior to moulding include a water bath, an infra-red lamp and a bunsen burner. In order to achieve even heating with the latter it is important that the wax should be held in the warm rising air above the flame and not in the flame itself. If the surface becomes shiny it indicates that the wax is becoming too hot and the outer layers are beginning to melt.

Heating in warm water causes more regular softening although this method has been frowned upon since it was thought that some constituents may be leached out and small quantities of water may become incorporated causing an alteration in properties. These problems are probably overstated since most waxes contain few leachable components.

The method of softening used in standardization testing of waxes involves the use of a 250 W infra-red lamp. When using this method, the distance of the wax from the lamp must be carefully

controlled in order to cause softening but not melting.

The ideal method for softening wax is to use a wax annealer. This is a thermostatically controlled oven which keeps the wax at a constant temperature, just above the softening point, ready for use. The annealer is most useful for inlay waxes.

Mechanical properties: A major factor which determines the mouldability and stability of a wax is its flow value. This property is related to creep which is discussed on p. 17. Creep and flow are both measured by applying a load to a cylindrical specimen and measuring the extent to which the specimen becomes compressed after a given time. Materials should, ideally, exhibit considerable flow at the moulding temperature but should show little or no flow at mouth temperature or room temperature so that they are not easily distorted. The determination of flow is made on cylindrical specimens 10 mm diameter by 6 mm. They are loaded across their flat faces using a 2 kg weight for the specified time (10 minutes). The flow is recorded as the percentage change in the height of the cylinder. The nature of the materials

dictates that very precise temperature control be maintained (±0.1ºC) during these tests. Flow

values for some typical materials are given in Table 4.1.

Brittleness is another important property which the manufacturers can, to some extent, control. For some waxes, for example denture waxes, toughness is required since the wax denture base may have to be removed from a slightly undercut cast many times without fracturing. In other cases, such as inlay waxes, brittleness is preferred in order that the wax will fracture rather than distort on removal from an undercut cavity. This will

Table 4.1 Flow values of some dental waxes.

indicate to the dental surgeon that a modification to the cavity shape is required.

4.5 Applications

Apart from their uses as impression materials, the major applications of waxes in dentistry are as modelling waxes and inlay waxes, collectively termed pattern waxes.

Denture modelling waxes: The manufacture of dentures involves several stages with wax being used in at least two of these. (see Fig. 4.1)

Following the production of a stone model from an impression, a wax rim is constructed, either directly on the model, or on a denture base which has been adapted to the model. The rim is inserted into the patient’s mouth at the ‘registration’ stage in order to record a satisfactory occlusal relationship. The next stage is to mount artificial teeth on the wax rim and to check the suitability of the wax denture at the ‘try in’ stage.

Waxes used for this purpose should have a softening temperature well above mouth temperature so that they are not distorted at either the registration or try in stages. They should be tough in order to reduce the chances of fracture during removal from the stone model. Three types of material are available, designated as follows:

These products differ primarily in regard to their softening temperature. Only the type 3 material can be considered relatively stable at mouth temperature. Type 1 material is designed to be hard at room temperature but soft at mouth tempera-

Fig. 4.1 Dental modelling wax. This shows modelling wax being used to make a trial denture which the dentist can use for trying into the mouth of the patient. It will then become the ‘template’ for the acrylic denture. The artificial teeth have already been set up in these wax dentures. In this case the upper and lower trial dentures are mounted on models in an articulator. The models have been constructed from dental stone and dental plaster.

ture and is used only for building contours and veneers in the laboratory. The type 2 material is suitable for pattern production in temperate climates. Type 3 material is designed primarily for use in warmer climates.

Modelling waxes consist mainly of mixtures of paraffin wax and beeswax and have melting points in the range 49–58ºC. They are generally supplied in sheet form, the sheets being produced either by rolling or by cutting from a block. Rolled sheets often change shape on softening due to the relief of stresses which are introduced during rolling. Some denture modelling waxes are referred to as toughened by their manufacturers. This probably reflects attempts to control the rolling procedure to optimize crystal alignments which may influence mechanical properties. Sheets of toughened modelling wax can typically be bent without fracturing.

Softening of the sheets is normally carried out under controlled conditions using a water bath. This enables the thickness of the sheet (typically 1–2 mm) to be maintained which is important since the thickness of the wax will control the thickness of the denture base.

Although the softening temperatures of modelling waxes are above mouth temperature, even

type 2 and 3 materials will slightly soften and distort if left in the mouth for more than a few minutes. This should be taken into account during registration and try in.

Modelling waxes are tough enough to resist fracture when withdrawn from shallow undercuts. An important requirement of denture modelling waxes is that they can easily be trimmed with a sharp instrument without tearing, chipping or flaking. These characteristics are determined at room temperature since they dictate the ease with which trimming can be performed in the laboratory situation.

Waxes tend to have high values of coefficient of thermal expansion, which coupled with the manner in which the materials are used (softening with heat and cooling) suggests that a significant dimensional change can occur. An upper limit of 0.8% expansion on heating from 25ºC to 40ºC is allowed in the ISO specification. When the wax denture is invested, prior to formation of an acrylic denture base, the wax can be removed from the mould by melting in boiling water, leaving no detectable residue.

Some metal components of partial dentures are formed in wax on the model. Small sheets of casting wax are used, which have been rolled to a precise thickness, according to the metal gauge required. In manipulating this wax it is important that the thickness is maintained. It is usual to soften it in hot water and adapt it into position with a soft material such as cotton wool or rubber. Pre-formed polymeric components can be used as an alternative to waxes for modelling the metal components of partial dentures.

Temporary denture bases constructed from wax and used during denture construction are prone to distortions unless great care is taken. For this reason alternative materials/techniques are sometimes used.

Shellac, a wax-like resin which is more stable at mouth temperature, has been used for construction of the temporary denture base. The wax rim is then built on top of this more stable base. Shellac is a natural beetle exudate which has a considerably higher softening temperature than ordinary modelling wax. Care must be taken to ensure thorough softening prior to moulding, otherwise considerable stresses are introduced which eventually lead to distortion.

Another widely used approach to denture construction is to use either a temporary or perma-

nent acrylic base-plate on which to construct the wax rim. The advantages and disadvantages of the various approaches are beyond the scope of this book and are covered adequately in other excellent texts.

Inlay waxes: Wax patterns for inlays or cast posts can be produced either by a direct or indirect technique as mentioned previously. The use of direct wax patterns is normally reserved only for the most simple designs of cast posts/cores on anterior teeth. These patterns can be removed from the prepared tooth or root with a minimum of distortion. Inlay casting waxes are currently classified as follows:

This has caused some confusion because in the past the opposite classification was used. The type 1 materials are designed for use in the indirect technique, being suitable for making patterns outside the mouth for the production of inlays, crowns and cast pontics. The type 2 materials are suitable for use in the direct technique (and may also be suitable for indirect use) for the production of inlays and crowns. The difference between the two types of wax is characterised by their flow behaviour as shown in Table 4.1. Both types are commonly produced in the form of cones and sticks.

For direct wax patterns the softened wax is forced into the tooth cavity and held under pressure until it cools. The wax should soften just above mouth temperature and should not be raised to a higher temperature than necessary, in order to reduce the magnitude of thermal contraction and internal stresses. In addition, the softening temperature must be tolerated without pain by the patient. The thermal contraction occurring on removal of the direct pattern material from the mouth is of the order of 0.1–0.5%. This would seem sufficient to cause problems of fit with the final cast restoration although serious problems are rarely encountered in practice, possibly because of the relatively simple nature of the restorations produced by the direct technique. Dimensional changes caused by stress relief are minimized by investing the pattern as soon as possible.

The material should be hard at mouth temperature and when removed from the cavity should

fracture rather than flow if the cavity has unwanted undercuts. This enables the undercuts to be located and removed.

The wax should ideally have a good colour contrast with enamel and be easy to carve without flaking so that the exposed surfaces of wax can be easily contoured to shape and the margins readily observed. When a direct pattern for a two-surface restoration is removed there is a need to increase the bulk of material in the contact area between teeth to ensure that the correct pattern of tooth to tooth contact is recreated. This can be achieved either by using a very low fusing point wax and adding to the pattern or by soldering additional precious metal to the casting. Direct patterns for post cores are built up using a different approach. The post channel is usually prepared using a proprietary system and a pre-formed pattern for the post is installed into the canal. The core is then built up in wax by melting the wax on an instrument and carrying the molten wax into the tooth.

The required properties are produced by using a blend of several types of wax including paraffin wax, carnauba, candelilla and beeswax with small quantities of other resins.

The procedure for indirect patterns is similar to that described for direct patterns except that the softened wax is forced into a cavity on the gypsum die. Since this procedure is carried out at room temperature rather than mouth temperature, inlay waxes for indirect patterns may soften at a somewhat lower temperature than direct pattern waxes, although the softening temperature should not be so low that the wax will flow at room temperature (Table 4.1). The value of thermal contraction for the indirect technique is much lower as a result of the lower softening temperature. This may be considered a distinct advantage of the indirect technique. However, it should be appreciated that the die has been produced in an impression which was itself recorded at mouth temperature and then cooled to room temperature – thus undergoing a thermal contraction which may approach the value of a direct wax pattern material. The normal method of softening for inlay casting waxes is using dry heat. When using a flame, care should be taken not to overheat the wax whilst at the same time thorough softening throughout the whole bulk of material occurs. When material in stick form is used it is gently rotated above the flame until it becomes shiny. It is then removed