dentistry. For these products setting often occurs very rapidly, leading to a more marked rise in temperature. In addition, the light sources used to activate setting produce heat. Hence, the combined effect of the setting reaction and the heating effect of the light can cause short-lived temperature rises in excess of 20ºC for even small quantities of some resin-based materials.

Coefficient of thermal expansion: The linear coefficient of thermal expansion is defined as the fractional increase in length of a body for each degree centigrade increase in temperature. Thus,

α = L / L0 °C−1

T

where the coefficient α is defined in terms of the change in length L, the original length L0 and the temperature change T.

Because the values of α are often very small numbers (typically 0.000025ºC−1 for amalgam) they are often quoted as parts per million (ppm). For example, the value for a typical amalgam specimen would be quoted as 25 ppm ºC−1. Values for some common materials are given in Table 2.5.

This property is particularly important for filling materials. When the patient takes a cold drink, both the filling material and tooth substance contract, the amount of contraction depending on the value of α for each. If the value of α for the material is significantly greater than that for tooth substance a small gap will develop down which fluids containing bacteria can penetrate, as illustrated in Fig. 2.20. The magnitude of the gap, shown as x in the diagram, is minimized for both hot and cold stimuli if the values of α for tooth substance and filling are matched. Hence, it can be seen from Table 2.5 that certain materials, for example silicate cements, perform very well in this respect, whilst others, such as acrylic resin, perform badly.

In practice, however, the situation is not so clear cut. Coefficient of thermal expansion is an equilibrium property and the expansion or contraction due to transient stimuli is a function of both coefficient of thermal expansion and thermal diffusivity. For filling materials, the most ideal combination of properties would be a low value of diffusivity combined with a coefficient of thermal expansion value similar to that for tooth substance.

Table 2.5 Coefficient of thermal expansion values of some selected materials.

Fig. 2.20 Diagram illustrating the production of a marginal gap due to thermal contraction.

2.5 Adhesion

Adhesion may be defined simply as an interaction between two materials at an interface where they are in contact. The nature of the interaction is such that their separation is prevented. The property of adhesion is recognized as being of major importance for filling materials, luting materials and fissure sealants. In each case the aim is to produce a tight seal between tooth substance and material with minimal destruction of tooth tissue.

Materials which are capable of bonding two surfaces together are called adhesives whilst the material to which the adhesive is applied is termed the adherend. In dentistry, an adhesive may, typically, be required to bond dentine and gold or, if the adhesive also acts as a filling material, may simply be required to attach to one surface, for example enamel or dentine. The latter is an ideal situation in which the restorative material possesses inherent adhesive characteristics which enable it to bond to dentine. This ideal is not always possible to achieve and dental adhesives are commonly used to form a thin layer between tooth substance and a restorative material.

Bonding may be achieved by one of two mechanisms – mechanical attachment or chemical adhesion.

In mechanical attachment the adhesive simply engages in undercuts in the adherend surface as shown in Fig. 2.21. When the surface irregularities responsible for bonding have dimensions of only a few micrometres the process is known as micromechanical attachment. This should be distinguished from macromechanical attachment which forms the basis of retention for many filling materials, using undercut cavities. In the case of chemical adhesion the adhesive has a chemical affinity for the adherend surface. If the attraction is caused by Van der Waals forces or hydrogen bonds, the resultant bond may be relatively weak. On the other hand, the formation of ionic or covalent links may result in a stronger bond.

Whichever mechanism of bonding is utilized the adhesive must be capable of wetting the adherend surface. In the case of mechanical attachment the adhesive must flow readily across the adherend surface and enter into all the surface undercuts in order to form the bond. For chemical adhesion the adhesive must wet the adherend surface in order that intimate contact between the adhesive and adherend may result in the formation of specific links which cause bonding.

The ability of an adhesive to wet an adherend surface is evaluated by measuring the contact angle which is formed when a drop of adhesive is applied to the surface. Figure 2.22 shows that for

good wetting, a low contact angle, ideally approaching 0º, is required. High contact angles indicate poor wetting and globule formation, and would probably result in poor adhesion.

The surface tension of the adhesive is the property which maintains it in the form of a droplet and acts to prevent wetting. There must be sufficient energy liberated through the forces of attrac-

Fig. 2.21 Diagram illustrating the difference between

(a) micromechanical attachment and (b) chemical adhesion.

tion between the adhesive and adherend in order to break down the surface tension of the adhesive and enable the materials to come into intimate contact. This affinity which is a prerequisite to adhesion has posed dental researchers with a great dilemma since the majority of resins used in dental fillings are relatively hydrophobic whilst dentine and enamel are relatively moist. This implies that there will be little natural affinity between the two materials which the dentist is trying to join together. The development of primers has helped to solve the problem. These materials change the nature of the adherend surface and improve affinity for resins used in restorative materials.

Adhesive forces are maximized if the adhesive and adherend are in intimate contact over a large surface area. This generally requires that adhesives should be applied to the adherend in the form of a low viscosity fluid or paste. Figure 2.23a illustrates the situation which arises if two solid surfaces are placed in contact. The rigid nature of the materials dictates that unless the two surfaces are perfectly flat (very difficult to achieve), they are in contact over only a very small proportion of their surface and the actual area of contact is only a fraction of the apparent area of contact. Hence, even if interactions between the two

surfaces are favourable the adhesive strength is unlikely to be great enough to maintain the adhesive and adherend in contact in the presence of even a small displacing force. If the adhesive is fluid but does not adequately wet the adherend surface the situation may not be much better, as illustrated in Fig. 2.23b. The ideal situation of a fluid adhesive which fully wets the adherend surface is illustrated in Fig. 2.23c. Here the two interacting materials take full advantage of the adhesive forces set up over the whole surface. In this case the actual area of contact is greater than the apparent area.

The way in which the preceding argument relates to certain dental adhesives is illustrated in Fig. 2.24. Figure 2.24a represents a cross-section through acid etched enamel. Figure 2.24b illustrates the situation which may exist when a resin is applied. Close adaptation and penetration of the resin into the enamel surface has not occurred due to poor wetting and/or the viscosity of the resin being too great. In Fig. 2.24c a resin of low

Fig. 2.23 Diagram illustrating adhesive (A) and adherend (B) surfaces in contact. (a) Two rigid surfaces make contact over a relatively small area. (b) The adhesive is fluid but does not wet the adherend surface.

(c) The adhesive is fluid and wets the adherend surface.

Fig. 2.24 Diagram illustrating application of a resin adhesive A to an etched-enamel surface B. (a) View of section through etched enamel. (b) The adhesive is too viscous or does not wet the enamel surface. (c) Good penetration of etched enamel by resin resulting from good wetting characteristics and relatively fluid resin.

viscosity and good wetting characteristics ensures good adaptation to and penetration of the etched enamel surface, probably resulting in good bonding characteristics.

2.6 Miscellaneous physical properties

Properties of materials which may influence their acceptability but which do not fall into any of the other categories are (1) dimensional changes during and after setting, (2) density, and (3) appearance.

Dimensional changes: Dimensional accuracy is an important requirement of many dental materials. The success of many restorative procedures depends on dimensional changes which occur during impression recording, casting of alloys or setting of direct restorative materials.

The manipulation of many materials involves the mixing of two or more components followed by a chemical reaction which brings about setting. Chemical reactions are invariably accompanied by dimensional changes. In the case of polymerisation reactions, a contraction normally occurs whereas other types of reaction may result in an expansion.

Where several stages are involved in the production of a restoration or appliance it is possible that dimensional changes occur at each stage. In such a case it is possible that an expansion at one stage can be used to partly counteract a contraction which occurs at another stage. For example, when constructing a cast metal restoration the setting expansion of the investment material partially compensates for the casting shrinkage of the alloy.

Dimensional changes may continue to occur in materials long after the apparent setting. There are many possible causes. Firstly, the changes may be due to continued slow setting or release of stresses set up during setting. Alternatively, they may be due to water absorption by, or loss of constituents from, the material. The degree to which the dimensions of a material alter after setting is said to be a measure of its dimensional stability.

Density: Density is a fundamental property which affects design aspects of dental appliances. If, for example, one were choosing an alloy with which to construct components of an upper denture, it would be necessary to consider density. A bulky

design in a heavy alloy would result in large displacing forces making retention difficult. In order to reduce such destabilizing forces one may choose to use a lower density alloy and to keep the alloy bulk to a minimum. These considerations become even more significant when other design parameters are taken into account. For example, if a rectangular cross-section beam is subjected to three-point bending, the force to cause a given deflection at the centre of the beam depends upon the square of the thickness. Hence, if a rigid (high modulus of elasticity), low density material is used equal performance can be achieved with a considerable saving in weight.

Appearance: One of the most demanding requirements of dental restorative materials is that they should match the natural hard and soft tissues in appearance.

Colour is a somewhat subjective phenomenon which may be judged differently by different observers. The majority who agree on the judgement of a colour refer to the minority who disagree as being colour-blind. Colour may, in fact, be produced in several different ways, including: selective reflection, selective absorption, diffraction, scattering and interference. Hence, the colour of an object or material is, in a sense, not an inherent property of that material but results from a number of factors including the composition of the material and its thickness and surface roughness as well as the nature of illuminating light.

Using the CIE (Commission International de l’Eclairage) method of colour measurement colour is defined by three parameters, L, a and b as illustrated in Fig. 2.25. From this, other commonly used terms can be explained as follows:

•The dominant wavelength or hue is represented by the relative values of a and b and their signs.

•The colour intensity or chroma is represented by the distance from the centre of the chart as indicated by the magnitude of the values of a or b.

•The brightness (sometimes called value) is represented by the value of L, which indicates the position on the vertical column.

The hue and chroma are inherent properties of materials whereas the brightness may be affected by factors such as surface finish.