Dental Materials: Physical Properties

• Physical properties are unrelated to force application.

Mass Related Properties

• Density is mass per unit volume; the unit is gm/cm3.
• Lighter weight helps with the retention of upper dentures, improving comfort.
• Nonmetallic denture bases are lighter than metallic ones.
• Base metal alloys are lighter than gold alloys.
• Lighter alloys require more casting force for rapid mold filling.

Thermal Properties

• Melting and freezing temperature is where a material transitions between solid and liquid.
• Melting machine is used to determine the melting points of dental casting alloys.
• Investment type must be determined.
• Overheating materials should be avoided to prevent the evaporation of ingredients.
• Heat of fusion is the heat in calories to convert 1 gm of solid to liquid at the melting temperature.
• Latent heat of fusion is the heat in calories liberated to convert 1 gm of liquid to solid at the freezing temperature.
• Specific heat is the heat needed to raise 1 gram of a substance by 1°C.
• Metals have low specific heat, whereas non-metals have high specific heat.
• Dental gold alloys’ low specific heat means prolonged heating is unnecessary.
• Thermal conductivity measures heat in calories per second passing through a 1 cm thick body with a 1 cm cross section when the temperature difference is 1°C, and has the unit of Cal/Sec/cm2(°C/cm).
• Metals have better conductivity than non-metals.
• Thermal stimulation causes physiological responses such as vasodilation and vasoconstriction, which results in healthy tissues.
• Large Amalgam fillings may lead to thermal irritation of pulp.
• Cement base can be placed between the tooth and filling for insulation.

Coefficient of Thermal Expansion and Contraction

• Coefficient of thermal expansion and contraction signifies changes in length per unit length for a 1°C temperature change.
• Linear thermal coefficient of expansion and contraction is denoted by α.
• α = (L final - L original) / (L original X (°C final - °C original)).
• Differences in thermal expansion coefficients between teeth and restorative materials lead to marginal percolation.
• Marginal percolation can cause marginal leakage, discoloration, recurrent caries, and hypersensitivity.
• Porcelain and metal should have closely matched coefficients of thermal expansion in ceramometallic restorations for a successful bond.
• Matching the coefficient of thermal expansion (α) between Artificial teeth and denture bases to avoid crazing (fine cracks).
• High coefficient of expansion of the wax pattern prevents distortion during wax pattern construction, avoiding the production of a distorted cast.
• Investment material's thermal expansion is essential to compensate for the thermal shrinkage of molten metal.

Thermal Diffusivity

• Thermal diffusivity refers to the rate at which a body with non-uniform temperature approaches thermal equilibrium; unit: mm2/Sec.
• Thermal Diffusivity = Thermal conductivity / (Specific heat x Density)
• Low specific heat combined with high thermal conductivity of dental amalgam can create thermal shock.
• Adequate thickness of remaining dentine can prevent thermal pulp shock preserving tooth structure.

Electrical Properties

• Electrical conductivity and resistivity depend on the ability of the material to conduct electricity.
• Sound enamel is a relatively poor electric conductor compared to carious tooth structure.
• Cements and restorative materials replacing tooth structure should have low electric conductivity.
• Zinc oxide eugenol cements exhibit the highest resistivity.
• Glass ionomer cements have the most conductive values and are similar to dentine.

Other Properties

• Water sorption shows water amount adsorbed on the surface and absorbed into the material.
• High water sorption in acrylic denture bases can cause dimensional changes and warpage.
• Hydrocolloid impression material absorbs water upon immersion and will cause dimensional changes; can be addressed with 100% humidity storage.
• Fluidity shows liquids to flow.
• Viscosity shows the resistance of a liquid to flow.
• Plasticity is a characteristic of semi-solids materials getting deformed under different forces.
• Success of a given material depends on its properties in the fluid or solid state, and proper flow of impression materials can give dimensional accuracy.
• Self life is a term for the general deterioration and change in quality of materials during shipment and storage.