4 Waxes

Chapter 4


4.1 Introduction

The waxes used in dentistry normally consist of two or more components which may be natural or synthetic waxes, resins, oils, fats and pigments. Blending is carried out to produce a material with the required properties for a specific application.

Waxes are thermoplastic materials which are normally solids at room temperature but melt, without decomposition, to form mobile liquids. They are, essentially, soft substances with poor mechanical properties and their primary uses in dentistry are to form patterns of appliances prior to casting.

Following the production of a stone model or die (Chapter 3), the next stage in the formation of many dental appliances, dentures or restorations is the production of a wax pattern of the appliance on the model. The wax pattern defines the shape and size of the resulting appliance and is eventually replaced by either a polymer or an alloy using the lost-wax technique. Methods which involve the production of a model followed by the laying down of a wax pattern are known as indirect techniques. Some dental restorations, such as inlays, may be produced by a direct wax pattern technique in which the inlay wax is adapted and shaped in the prepared cavity in the mouth. Waxes used in the production of patterns by either the direct or indirect technique must have very precisely controlled properties in order that well-fitting restorations or appliances may be constructed. Other waxes used in dentistry have less rigorous property requirements. One such material is used by manufacturers for attaching denture teeth to display sheets (carding wax). Another product is used for boxing in impressions prior to making a gypsum model (boxing-in wax). A third material is used for temporarily joining two components of an appliance, for example, during soldering (sticky wax).

An important group of waxes used in dentistry are the impression waxes. These are discussed in Section 17.4.

4.2 Requirements of wax-pattern materials

The major requirements of waxes used to construct wax patterns by either the direct or indirect technique are as follows.

(1) The wax pattern must conform to the exact size, shape and contour of the appliance which is to be constructed.
(2) No dimensional change should take place in the wax pattern once it has been formed.
(3) After formation of the casting mould, it should be possible to remove the wax by boiling out or burning without leaving a residue.

The ability to record detail depends on the flow of the material at the moulding temperature, which is just above mouth temperature for direct techniques and just above room temperature for indirect techniques. Accuracy and dimensional stability depend on dimensional changes which occur during solidification and cooling of the wax. Distortions may also occur if thermal stresses are introduced.

4.3 Composition of waxes

Dental waxes are composed of mixtures of thermoplastic materials which can be softened by heating then hardened by cooling. The major components may be of mineral, animal or vegetable origin.

Mineral: Paraffin wax and the closely related microcrystalline wax are both obtained from petroleum residues following distillation. They are 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– 55°C and melt in the range 48–70°C. They are brittle at room temperature. Microcrystalline waxes melt in the range 65– 90°C 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 />

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Jan 1, 2015 | Posted by in Dental Materials | Comments Off on 4 Waxes
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