L = 100

+ b

- a

+ a

- b

L = 0

Fig. 2.25 Three-dimensional representation of colour using the CIE system.

In many materials (e.g. resin-based products) the initial hue and chroma are controlled by the manufacturer through the incorporation of pigments and the use of fillers having varying translucency/opacity. The passage of light through a material having a composite structure is influenced by the degree to which the filler and matrix phases have a matched refractive index.

Changes in hue and chroma which occur over time can be due to either changes in the bulk of the material, i.e. through molecular transformations or reactions of one or more of the material components, or through the absorption of stains onto the surface of the material. To distinguish these two processes the terms intrinsic and extrinsic staining are often used. The latter is normally related to the surface roughness of the material.

2.7 Chemical properties

One of the main factors which determines the durability of a material used in the mouth is its chemical stability. Materials should not dissolve, erode or corrode, nor should they leach important or toxic constituents into the oral fluids.

Solubility and erosion: The solubility of a material is simply a measurement of the extent to which it will dissolve in a given fluid, for example, water or saliva. Erosion, on the other hand, is a process which combines the chemical process of dissolution with a mild mechanical action. Hence it is

possible to envisage a situation in which the surface layer of a material becomes weakened and undermined by dissolution and then becomes totally detached by mild abrasion.

There is occasionally some confusion over the term erosion. This is because in materials science the term is normally used to imply damage produced by the impingement of particles on an object. In dentistry the term has gained widespread use to describe the destruction of natural hard tissues by acids (either occurring naturally or present in food/drinks). This usage has been widened to include the degradation of restorative materials by a combination of chemical and mild mechanical action.

These properties are particularly important for all restorative materials since a high solubility or poor resistance to erosion will severely limit the effective lifetime of the restoration.

When assessing the solubility or erosion rate of materials it is important to consider the vast range of conditions which may exist in the mouth. The pH of oral fluids may vary from pH 4 to pH 8.5, representing a range from mildly acidic to mildly alkaline. Highly acidic soft drinks and the use of chalk-containing toothpastes extend this range from a lower end of pH 2 up to pH 12. It is possible for a material to be stable at near neutral pH values but to erode rapidly at extremes of either acidity or alkalinity. This partially explains why certain materials perform adequately with some patients but not with others.

Standard tests of solubility often involve the storage of disc specimens of materials in water for a period of time, the result being quoted as the percentage weight loss of the disc. Such methods, however, often give misleading results because they do not represent what occurs in the mouth. New methods of testing which have been incorporated in some ISO standards involve dripping or spraying sets of aqueous acidic solutions onto the surfaces of test specimens. This involves the use of a more aggressive fluid (typically pH 4) and the mildly abrasive effect of the impinging liquid droplets. The most appropriate method for measuring material loss is to estimate volume loss as this can be correlated with material loss from restorations as judged clinically. Judging material loss by weight change is less satisfactory as even though material is lost from the surface the weight of a specimen may increase if water absorption is great enough.

Leaching of constituents: Many materials, when placed in an aqueous environment, absorb water by a diffusion process. Constituents of the material may be lost into the oral fluids by a diffusion process commonly referred to as leaching. This may have serious consequences if it results in a change of material properties or if the leached material is toxic or irritant.

Some soft acrylic polymers used for cushioning the fitting surfaces of dentures rely on the presence of relatively large quantities of plasticizer in the acrylic resin for their softness. The slow leaching of plasticizer causes the resin to become hard and therefore ineffective as a cushion.

Occasionally leaching is used to the benefit of the patient. For example, in some cements containing calcium hydroxide, slow leaching causes an alkaline environment in the base of deep cavities. This has the dual benefit of being antibacterial and of encouraging secondary dentine formation.

Some restorations leach fluoride which is thought to have a beneficial effect on the surrounding dental hard tissues. It is important that this process does not have a deleterious effect on the properties of the material (e.g. cement). Much depends on the mechanism of leaching. If this occurs by ion exchange there is a good chance that

fluoride ions will be replaced by another anion (e.g. hydroxyl) and the integrity of the material is maintained. If the leaching occurs by washout, however, the process is likely to be accompanied by gradual degradation.

Corrosion: Corrosion is a term which specifically characterises the chemical reactivity of metals and alloys. The major requirement of any such material used in the mouth is that it should have good corrosion resistance.

Metals and alloys are good electrical conductors and many corrosion processes involve the setting up of an electrolytic cell as a first stage in the process.

The tendency of a metal to corrode can be predicted from its electrode potential. It can be seen from Fig. 2.26 that materials with large negative electrode potential values are more reactive whilst those with large positive values are far less reactive and are often referred to as noble metals. The electrode potential is a measure of the extent to which the reaction

M → M+ + electron

will occur.

In an electrolytic cell involving two metals, material is lost from the metal with the most

Contact

Zinc

Copper

Electrolyte

Solution

Zn2+

Fig. 2.27 Electrolytic cell involving two dissimilar metals in contact and an electrolyte. Corrosion of the most electronegative metal occurs. Arrows represent flow of electrons.

negative electrode potential. Thus, when zinc and copper come into contact in the presence of a suitable electrolyte (Fig. 2.27), material loss occurs from the zinc by the reaction:

Zn → Zn2+ + 2 electrons

Hydrogen is liberated at the copper by the reaction:

2H+ + 2 electrons → H2

In this simple type of electrolytic cell the zinc is referred to as the anode. This is the electrode at which positive ions are formed and therefore the electrode at which corrosion occurs. The copper is referred to as the cathode. The more negative a value of electrode potential which a metal possesses the more likely it is to form the anode in an electrolytic cell.

If a voltmeter is placed between the anode and cathode an electrical potential difference can be measured, thus illustrating the flow of electrons within the electrolytic cell.

The conditions under which an electrolytic cell may be set up in the mouth involve the presence of two or more metals of different electrode potential and a suitable electrolyte. Saliva and tissue fluids are good electrolytes. The two metals may be derived from restorations constructed from different metals or alloys, or from areas of different composition within one restoration, for example amalgam, as illustrated in Fig. 2.28.

Generally, the more homogeneous the distribution of metal atoms within an alloy the less tendency there is for corrosion to occur. Consequently,

Fig. 2.28 An electrolytic cell involving different phases within one alloy. Saliva acts as the electrolyte. Phase A is more electronegative than phase B. Arrows represent flow of electrons.

many manufacturers of alloys carry out homogenization heat treatments to help to eliminate the possibility of electrolytic corrosion.

It can be seen from Fig. 2.26 that chromium has a negative value of electrode potential and is at the reactive end of the series of metals shown. It is therefore surprising to learn that chromium is included as a component of many alloys in order to improve corrosion resistance. This apparent contradiction can be explained by the passivating effect. Although chromium is electrochemically active it reacts readily forming a layer of chromic oxide which protects the metal or alloy from further decomposition.

Other factors which can affect the corrosion of metals and alloys are stress and surface roughness. Stress in metal components of appliances produced, for example, by excessive or continued bending can accelerate the rate of corrosion and may lead to failure by stress corrosion cracking.

Pits in rough surfaces can lead to the setting up of small corrosion cells in which the material at the bottom of the pit acts as the anode and that at the surface acts as the cathode. The mechanism of this type of corrosion, sometimes referred to as concentration cell corrosion, is complicated but is caused by the fact that pits tend to become filled with debris which reduces the oxygen concentration in the base of the pit compared with the surface. In order to reduce corrosion by this mechanism, metals and alloys used in the mouth should be polished to remove surface irregularities.

An appreciation of the principles of electrolytic corrosion should help the dentist to design the arrangement of restorations and appliances in such a manner that dissimilar materials are not allowed to come into close contact. The conse-

quences of corrosion can be varied and include: pain due to the flow of galvanic current, metallic taste due to the release of ions, deterioration in appearance and mechanical properties and, most seriously, an increased body burden of metallic ions. There is increasing concern over the effects of increasing the body’s burden of heavy metals such as mercury and nickel.

Testing of corrosion and tarnish: According to ISO 10271:2001, the definition of corrosion is ‘physicochemical interaction between a metal or an alloy and its environment that results in a partial or total destruction of the material or in a change of its properties’. On the other hand, tarnish is defined as ‘surface discolouration due to interaction between a metal and its environment’.

An acceptable corrosion resistance of an alloy to be used in the mouth is an essential requirement for both function and freedom from biological hazard. Historically, experts have been unable to agree on a test method which is meaningful and would be suitable for inclusion in ISO standards for dental alloys. It is this factor amongst others which has required compositional limits for alloys to be part of the existing specifications. A standard on corrosion test methods published in 2001 (ISO 10271:2001) specifies methods for determining corrosion and tarnish by static or cyclic immersion or by an electrochemical test. Should this standard prove applicable to the testing of a variety of alloys it may alter the nature of a number of ISO standards.

The static immersion test involves soaking specimens of the test alloy in an aqueous solution of

lactic acid and sodium chloride for 7 days at 37ºC. The nature and concentration of the metallic ions is determined and reported with particular emphasis being placed on elements known to be hazardous such as nickel, cadmium and beryllium.

The electrochemical test involves setting up an electrolytic cell in which the test material forms the working electrode and the electrolyte comprises an aqueous solution of sodium chloride at a pH of 7.4. A potentiodynamic sweep is carried out in which the potential and/or the current density are increased and a plot of potential against logarithm of current density is obtained. The behaviour of the test material is characterised by the presence of active peak potentials and by the breakdown potential and the corresponding current densities of these events. The surface of the material is also examined for damage. A diagram of the test set-up used, as given in ISO 10271:2001, is shown in Fig. 2.29.

ISO 10271:2001 recommends that tarnish should be determined using a cyclic immersion test. The test sample is dipped into an aqueous solution of sodium sulphide hydrate for 10–15 seconds once every minute over a period of 3 days. Treated and untreated specimens are compared visually to determine if any discolouration has occurred. The test recognizes that tarnish is most commonly associated with the formation on the surface of metallic sulphides.

2.8 Biological properties

It is a primary requirement of any dental material that it should be harmless to the patient and to those involved in its manufacture and handling.

Fig. 2.29 Measuring circuit used in the electrochemical test for corrosion resistance. A potential is applied across the test and counter electrodes and the current measured indicates the extent to which corrosion occurs.