Dentophysics (part 2)

Thermal properties

The arrangement of atoms and molecules in materials is influenced by the temperature; as a result, thermal techniques are important in understanding the properties of dental materials

Thermal conductivity : thermal conductivity of a substance is the quantity of heat in calories or joules per second passing through a body 1cm thick with a cross section of 1 sq.cm when the temperature difference is 1°C.

Eg – a large amalgam filling or gold crown in proximity to the pulp may cause the patient discomfort when hot or cold foods produce temperature changes, this effect is mitigated when adequate tooth tissue remains or cavity liners are placed between the tooth and filling for insulation.

Specific heat : specific heat of a substance is the quantity of heat needed to raise the temperature of 1g of the substance by 1°C.

Eg – during the melting and casting process, the specific heat of the metal or alloy is important because of the total amount of heat that must be applied to the mass to raise the temperature to the melting point.

Thermal diffusivity : it is a measure of transient heat flow and is defined as the thermal conductivity, divided by the product of the specific heat, times the density.

Eg- for a gold crown or a dental amalgam, the low specific heat combined with the high thermal conductivity creates a thermal shock more readily than normal tooth structure does.

Coefficient of thermal expansion : the change in length per unit length of a material for a 1°C change in temperature is called the linear coefficient of thermal expansion.

Although the coefficient is a material constant, it doesn’t remain constant over wide temperature ranges. For eg, the linear coefficient of thermal expansion of a dental wax may be an average value of 300×10-6/°C upto 40°C, whereas it may have an average value of 500×10-6/°C from 40-50°C.

The coefficient of thermal expansion of a polymer changes as the polymer goes from a glassy state to a softer, rubbery material. This change in the coefficient corresponds to the glass transition temperature.

It is obvious that with the reduction in temperature, there is a contraction of a substance as much as of expansion tht occured during heating. Accordingly, tooth structure and restortive materials expand when warmed by hot food or beverages and contract when exposed to cold substances. Such expansions and contractions may break the marginal seal of a filling in tooth particularly when the difference between coefficient of thermal expansion of tooth and restorative material is too large.

Electrical properties

The ability of a material to conduct an electric current may be stated as conductivity or conversely as the specific resistance or resistivity. The conductivity by materials used to replace tooth tissues is of concern in restorative dentistry.

Dielectric constant : a material that provides electrical insulation is known as dielectric. The dielectric constant of a dental cement generally decreases as the material hardens. This decrease reflects a change from a paste that is relatively ionic and polar to one that is less.

Electromotive force : the electromotive series is a listing of electrode potentials of metals according to the order of their decreasing tendency to oxidise in solution. Those metals with a large negative electrode potential are more resistant to tarnish than those with a high positive electrode potential.

Galvanism : the presence of metallic restorations in the mouth may cause a phenomenon called galvnic action where saliva or bone fluids like electrolytes make up an electric cell.

Corrosion : the corrosion of gamma, gamma 1, gamma 2 phases in amalgam has been studied by electrochemical analysis. The dental amalgam specimens become pitted at the boundaries between the phases or in gamma 2 phase. The addition of copper to amalgam alloys to form copper-tin compounds during hardening has improved the resistance of amalgam to chloride and galvanic corrosion.

Tarnish : the process of steam sterilization of surgical instruments has long presented a serious problem of tenish and corrosion. Many non metallic materials such as cements and composites have shown a tendency to discolor in service because the colored substances penetrate the materials and continue chemical reactions in the composites.

Source : Craig’s textbook of restorative materials

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