BY Dr. Reham Mohammed Abdallah Physical properties BY Dr. Reham Mohammed Abdallah

Items to be covered I. Definition II Items to be covered I. Definition II. Classification of physical properties 1. Optical properties • Color • Color parameters • Factors affecting color matching • Color matching in dentistry 2. Thermal properties • Glass transition temperature • Melting temperature

• Heat of fusion • Thermal conductivity • Thermal diffusivity • Coefficient of thermal expansion 3. Electrical properties • Electrical conductivity and resistivity • Electromotive series • Corrosion

Physical property is a property that can change without involving a change in the chemical composition. Unlike mechanical, physical properties are not concerned with the application of force to a body.

A. OPTICAL PROPERTIES Color One of the most desirable characteristics of dental restoration is a natural tooth-like color. For an object to be visible, it must emit, reflect or transmit light. Light is an electromagnetic radiant energy of certain wavelength. Human eye is sensitive to wavelengths ranging between 400 nm(violet) to700 nm (red).

Color parameters The eye can distinguish between 3 color parameters: a. Hue It is the dominant wavelength of light (ex: red, blue or green). b. Value It is the lightness or darkness of color (lighter shade has higher value). c. Chroma It is the degree of saturation (intensity) of a particular hue.

Factors affecting color matching 1. Surface finish Rough surface appears lighter than smooth surface of the same material. 2. Pigmentation Esthetic effects are sometimes produced by incorporating colored pigments in dental materials such as composite resin and acrylic denture.

3. Metamerism A phenomena in which objects appear color-matched under one type of light, and different under another light source. Clinical Significance Color matching should be done under 2 light sources, one of which is sun light (in the dental clinic, color matching could be done using either white light or sun light).

4. Fluorescence It is the ability of a material to absorb color of certain wave length and reemit light of a different wave length. Example Natural teeth can absorb U.V. and near U.V. radiation (invisible light) and emit visible light of different wavelength, so teeth appear as a light source. (Photoflash lamps, vapor lamps, and sunlight contain near U.V radiation).

Clinical Significance Dental porcelains are formulated with fluorescing agents (as Cerium oxide) to produce the natural appearance of tooth structure, while acrylic resin doesn’t have the same property. 5. Opacity, translucency and transparency Opacity: The property of a material that prevents passage of light (by absorption, reflection or both).

Translucency: Is the property of a material that permits passage of light but with dispersion, so objects cannot be seen clearly through it, e.g. Tooth enamel, dental ceramics. Transparency: Is the property of a material that allows passage of light with only little distortion that objects can be seen clearly through it. e.g. Glass

Color matching in dentistry The most common method used for color matching in dentistry is the “shade guide”(see the following figure). Figure 1: Color matching by shade guide selection.

B. THERMAL PROPERTIES 1. Glass Transition temperature Tg This property is characteristic to non-crystalline structures (as polymers & waxes) that do not have definite melting point but rather gradually soften as their temperature is raised.

So, glass transition temperature (Tg) can be defined as: The temperature at which there is an abrupt increase in the coefficient of thermal expansion. 2. Melting temperature It is the temperature at which the material transforms from solid to liquid (for crystalline materials).

3. Heat of fusion It is the amount of heat in calories or joules that is required to convert 1 gram of a material from solid to liquid state at the melting temperature. 4. Thermal conductivity It is the quantity of heat in calories or joules, per second passing through a body 1 cm thick with a cross section of 1 cm2 when the temperature difference is 1° C.

Clinical Significance 1 Clinical Significance 1. In case of metallic restorations with high thermal conductivity as amalgam, extremes in temperature would produce pain & injury to the tooth pulp. Thus, an insulating base is needed under metallic restorations to prevent such thermal shocks. 2. Low thermal conductivity of resin denture base materials tends to prevent heat exchange between the oral cavity and soft tissues.

This exchange is important to keep vitality of soft tissues. Furthermore, these materials don’t allow the patient to have the normal sense of heat and cold while swallowing food and drinks. So metallic denture base would be better is this instance.

5. Thermal diffusivity It is the rate at which a body of non-uniform temperature approaches thermal equilibrium. Clinical Significance Gold or amalgam restorations may cause high thermal shock due to their high thermal diffusivity (as they have high thermal conductivity and low specific heat).

6. Coefficient of thermal expansion (CTE) It is the change in length per unit length of a material for 1° C change in temperature. Clinical Significance 1. Ideally, filling materials should have similar CTE to that of the tooth structure to avoid breaking the marginal seal and subsequent leakage between the filling & the tooth during thermal changes.

2. During denture construction, matching in CTE between denture base and artificial teeth is essential to avoid their separation during thermal changes.

C. ELECTRICAL PROPERTIES Electrical conductivity and resistivity Electrical conductivity It is the ability of the material to conduct an electric current. Electrical resistivity Is the resistance of material to conduct electric current.

Clinical Significance Cements to be used as insulating bases under metallic restorations, should have high electrical resistivity to protect the underlying tooth tissues from galvanism. 2. Electromotive series It is a listing of electrode potentials of metals according to their decreasing tendency to oxidize in solution.

Metal as Zn, tends to oxidize & corrode relatively easily, while those such as Pt& Au resist oxidation. 3.Corrosion It is the deterioration of a metal caused by reaction with its environment. Galvanic corrosion It is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte.

Significance Avoid using different metallic restorations (e.g. gold and amalgam) in opposing or adjacent teeth as they can form an electric cell in conjunction with saliva, leading to galvanic corrosion of the more anodic metal.