Basic Properties of Dental MaterialsProf. K.A. Wettasinghe Biological Properties • Non-toxic - Patient - Dentist - Staff • Non-irritant • Non-allergenic • Non-mutagenic / carcinogenic • Mercury • Acids - Phosphoric - Hydrochloric - Hydrofluoric • Nickel containing alloys • Dusts • H 2O 2 Mechanical Properties • Strain – Is the change in length that a material undergoes when a force is applied to it – It is dimensionless because it has length per units of measurement – Is often expressed as a percentage – Tension / Compression – Elastic / Plastic A F Stress =F/A l l Strain = Δ l/ l . • Stress – It is the internal reaction to an externally applied load and is equal in magnitude but opposite in direction to the external load. – Force / area – N/m2 – Compression / tension / shear . tension compression . Fixed end .torque Torsion results in shear strains. • The relationship between stress and strain is often used to characterize the mechanical properties of a material.Stress-strain relationship • Stress and strain are not independent and unrelated properties. . Stress-Strain relationship stress E Y P T strain Stress-Strain plot obtained from a simple compressive or tensile test . Modulus of Elasticity • Measure of the relative stiffness or rigidity of a material The unit values are those of force per area • Modulus of Elasticity=Stress/Strain This only applies the elastic portion of the stress-strain diagram On the diagram the modulus is indicated by the slope of the linear part of the line Young’s modulus of elasticity . . 0 80.Material Young’s Modulus GN/m2 11.3 20.00 20.0 2.1 11.00 78-103 36-50 Enamel Composite filling material Dental ceramic Denture base acrylic Stainless steel Co-Cr alloy Gold alloy Amalgam . Stress-Strain relationship stress E Y P T strain Stress-Strain plot obtained from a simple compressive or tensile test P – Proportional limit E – Elastic limit Y – Yield point T – Total failure . Modulus of Elasticity • Measure of the relative stiffness or rigidity of a material. The unit values are those of force per area. • Modulus of Elasticity=Stress/Strain This only applies to the elastic portion of the stress-strain diagram On the diagram the modulus is indicated by the slope of the linear part of the line • Young’s modulus of elasticity . . 0 80.00 20.00 78-103 36-50 Enamel Composite filling material Porcelain Denture base acrylic Stainless steel Co-Cr alloy Gold alloy Amalgam .0 2.Material Young’s modulus GN/m2 11.1 11.3 20. the ability of a material to undergo permanent tensile deformation without fracture • Malleability .• Ductility .the ability of a material to undergo permanent compressive deformation without fracture or rupture . Ultimate tensile/ compressive strength Material Tensile strength (MPa) 98 10 48-69 414-828 34-62 28 40 421 Compressive strength (MPa) 297 400 310-483 ---200-345 97 150 ---- Human dentin Human enamel Dental amalgam Gold alloys Composite Unfilled acrylic plastic Porcelain (feldspathic) Nickel-chromium alloy . before fracture .Resilience The amount of energy a material can absorb before permanent deformation .Total area upto T .Area under P Toughness ……. Izod Notched specimens .Impact strength Application of a sudden force .Charpy . • Fracture toughness Resistance of a material to fracture under a sudden impact or the amount of energy absorbed by a material when it is stressed to a point just shy of it’s fracture point • Tear strength Resistance to tearing • Fatigue strength Repetitive application of a small load . Hardness • Resistance to scratching or denting • The value of hardness is referred as the hardness number • Depends on the method used for evaluation • Brinell • Vickers • Knoop . Hardness tests Brinell Vickers Knoop Increasing hardness for Vickers test . Abrasion resistance Time dependent properties Flow/Creep • Is an undesirable permanent deformation • When a constant force is maintained. certain materials continue to permanently deform • Compressive stresses that can cause flow is created during biting . Change of viscosity • Working time .RHEOLOGICAL PROPERTIES • Science of flow & deformation Mix Fluid paste Solidify .Reach initial / final set Command Setting .Time for manipulation • Setting time . THERMAL PROPERTIES Extremes of temperature inside mouth • Thermal conductivity • Thermal diffusivity .Denture bases .Filling materials .Rapidity of warming up of a material • Thermal expansion Clinical implications . green.LIGHT.Blue. red . COLOUR AND AESTHETICS Visible light Blue light 380-780nm 470-480nm for LC units Primary colours . • Transmission of light • Refraction • Absorption • Scattering . • Colour of the light source • Nature of the object .Colour: physical aspects A colour of an object will depend on. Teeth absorb UV (400nm) & emit light of 400-450nm .• Metamerism .A colour appear different depending on the light source • Fluorescence Absorb and emit a different wave length . Colour: Physiological and psychological aspects • Defective colour vision • High sensitivity to green • Colour fatigue • Optical illusions . .Shade CHROMA VALUE . 2.Saturation of a colour 3. HUE .Measurement of colour 1.Darkness or lightness of a colour . Polymers Polymer Monomer Polymerization . No molecules are eliminated .A smaller molecule is eliminated Free –Radical Addition Reactions .Condensation Reactions . chemicals .Activation and Initiation Free radicals – very reactive molecules with unpaired electrons H2O2 H2O + O.Heat.) Activation of initiator . UV light. (R. Examples .Heat / chemicals for acrylic .Benzoil Peroxide and P-toluidine for LC composites . Degree of polymerization .Molecular weight .Physical properties 1) Elastomers or rubbers 2) Hard amorphous polymers Tg – Glass transition temperature .Polymer Structure & Properties . Effect of molecular weight MW Entanglement Rigidity Strength Tg. Effect of cross linking Cross linking Harder Brittle Solvent resistant Tg. . Plasticizers Liquids that penetrate and make it more flexible and rubbery Co-polymers Two or more monomeric units are there .Tailor made properties . conductors. lustrous . opaque. dense.Metals Properties of metals Hard. ductile & malleable Electro positive .+ in solution . TC burs Electro forming .Forming & Shaping of Metals 1) 2) 3) 4) Casting Cold working Powder Metallurgy .Electro plating . 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