Chapter 16 Waxes and occlusal registration materials
Waxes are a group of ubiquitous materials that play a significant part in both restorative dentistry and in the dental laboratory. They are used extensively in the denture-making process, in making interocclusal records, as a splinting material in denture repairs, for boxing impressions when casting models, and forming the wax pattern in the lost wax technique to produce removable and fixed prostheses. A range of waxes with different properties are required to carry out these varying functions. A number of natural and mineral waxes, gums, fatty acids, oils and resins (both synthetic and natural) are blended together to produce the wax with specific properties required for its intended use (Table 16.1).
Constituents of Dental Waxes
The common constituents of most dental waxes are hydrocarbons and esters with a high molecular weight. The use of natural waxes does present the manufacturer with a problem as these waxes have variable properties depending on their source and also the time at which they were obtained. The common plant waxes include carnauba, ouricury, candelilla (Figure 16.1) and Japan wax. Japan wax is really a fat containing glycerides of palmetic and stearic acid. It is derived from the berries of the sumac tree found in Japan and China and is regarded as a form of lacquer. The animal and insect waxes include spermaceti from the sperm whale and beeswax (Figure 16.2).
Fig. 16.1 Carnauba wax (A) is derived from the leaf of the Copernicia palm (B) found primarily in Brazil. Ouricury wax is obtained from the leaves of the feather palm (C) by scraping the surface of the leaves while candelilla is sourced from the leaves of a shrub Euphorbia antisyphilitica (D).
Mineral waxes are more consistent in their properties and presentations. They are usually derived from the refining of petroleum products. One of the most commonly available is paraffin wax (Figure 16.3) with a melting point of 40–70 °C. This wax has a tendency to show substantial volumetric contraction (in the region of 10–16%) during the cooling and solidification process.
The primary source of microcrystalline wax is the heavier oil fractions produced during the petroleum distillation process. These have higher melting points and are formed into plates. They are much tougher than paraffin waxes and show much less volumetric change when changing from liquid to solid. Their properties may be adapted by adding oil, making the wax less hard. The common microcrystalline waxes include ozokerite, ceresin (Figure 16.4), montan and barnsdahl. The first three waxes are all derived from shale and lignites found near petroleum deposits. Other waxes which may be incorporated into dental wax include synthetic waxes which have been produced for other purposes, such as polyethylene waxes and polyethylene glycol waxes. While the use of synthetic waxes in dental waxes is increasing, the natural waxes still predominate.
The handling properties of the individual waxes may be adjusted by the addition of gums, fats and resins. The resins may be either synthetic or naturally occurring. Dammar, rosin and sandarac (Figure 16.5) are three examples in the latter group and are derived from plants. Another common naturally occurring material is shellac (Figure 16.6), which is produced by insects. It is commonly obtained from India and Thailand. Shellac is a thermoplastic material and is used to make baseplates, which are used in the denture-making process. It is a rigid material when solid and other waxes may be added to it, for example to make occlusal rims. As it is more stable at mouth temperature, it is less likely to distort when used in the mouth.
It is already apparent that the melting temperatures of the various constituents of the wax will affect the melting range of the final product. By adding waxes of high melting temperatures, the melting range of the paraffin wax-based materials can be increased significantly. This is of importance if the wax is to be used in the mouth as the melting range needs to be higher than mouth temperature. In the dental laboratory the wax needs to change rapidly from solid to liquid to in order to manipulate it. The melting range can be narrowed as well as raised. To manipulate the wax effectively, it must be maintained at the correct temperature. This may be done using a flame or induction coil (Figure 16.7).
Thermal expansion characteristics
The thermal expansion characteristics must be carefully judged as the mode of expansion as well as the amount which occurs is important. In some cases the waxes go though phase changes so that at a given temperature a marked dimensional change will occur or the rate of thermal expansion will change. This must be taken into account in the wax blending to achieve the properties which are suitable for each application.
None of the waxes have particularly high mechanical properties, in fact their compressive strengths and elastic moduli are relatively low. However, within the group of waxes there is some variation with beeswax having the lowest and carnauba wax the highest compressive strength and elastic modulus. The properties are also dependent on the temperature and the composition of the wax mixture is dependent on the intended use.
The flow characteristics of each individual component wax will vary, with some showing a steady increase in flow with temperature while others show minimal flow until close to their melting point. By blending and varying the composition of the dental wax, its flow can be controlled until a short time before its melting point is reached. This is useful in the dental laboratory when making a wax pattern.
Control of residual thermal stressing
It is important that once waxes have been formed into a specific shape, they are not permitted to stress relieve by being left for long periods of time between processes in the laboratory. For example, the time between the completion of the wax pattern and investment should be kept as short as possible. Similarly if the wax is compressed into a mould, it may rebound or alter its shape on removal of the pressure if left for too long. Again, the properties of the wax over a given temperature range may be modified by the manufacturer by the blending the constituents of the wax to create a product for a specific purpose.
Applications of Waxes used in Dentistry
• In the construction of fixed or removal appliances, both of a temporary or permanent nature. These uses include waxes used to form the wax pattern for cast restorations (inlay waxes) and wax patterns for cast clasps. Waxes are also used to make baseplates of dentures and occlusal rims. These are sometimes called pattern waxes.
• In the construction of auxiliary aids in the manufacture of appliances and implements for both clinical and laboratory use. These uses require processing waxes, that is, waxes to extend special trays, box the impression trays and support the plaster during the pouring process and those used to temporarily splint components or fragments of appliances.