1. Dental Materials I Lesson I: Properties

2. Agencies: American Dental Association (ADA) Food and Drug Administration (FDA) Federation Dentaire Internationale International Standards Organization (ISO)

3. Properties 1.Dimensional change - change in size of a material can occur from a variety of reasons  Setting process  Thermal changes hot or cold  Handling Expansion- material becomes larger Contraction- material shrinks Teeth and materials contract and expand at different rates causing gaps between tooth structure and filling allowing leakage to occur. Percolation/Microleakage-leakage of oral fluids into space between tooth and material. This in and out motion could cause recurrent decay and pulpal irritation

4. Properties 2. Solubility – the degree to which a substance will dissolve in another substance 3. Sorption – uptake of fluid or substances by a material  Absorption- uptake of liquid by the bulk solid causing swelling  Adsorption- surface accumulation causing staining 4. Flow/creep- continuing deformation of a solid when subjected to a constant stress

5. Properties 5.Biting Force  Natural teeth: Molars 130 lbs. to 170 lbs. Bicuspids 70 lbs. Canines 50 lbs. Incisors 40 lbs.  Prosthetic: Bridge replacing 1 st molar 50 lbs.  Partial and complete dentures 25 lbs. 6. Force – any push or pull on an object

6. Properties 7. Stress-is the force per unit area Types of Stress  Compressive- being stepped on  Tensile- being pulled apart  Shear-uneven pull-friction 8. Strain- change in length, or deformation per unit produced by stress  Elastic strain- is reversible, disappears after stress is removed  Plastic strain- is a permanent deformation of the atoms inside the material

7. Properties Stress-strain curve- means of comparing the mechanical properties of materials and determines the corresponding value of stress and stra in. 1. Proportional limit- the value of stress on the stress/strain curve where stress ceases to be linear. Material no longer functions as an elastic solid. 2. Yield strength- point on the curve where permanent strain occurs, material stops functioning 3. Ultimate strength –point on the curve where material will fracture or rupture

8. Properties 9. Elastic modulus- measure of stiffness of a material  higher # indicates stiffer material  lower # indicates highly flexible material 10. Elongation- % of elongation = amount of deformation before material ruptures under tension (tensile stress)  Higher % of elongation the material is ductile  Lower % of elongation the material is brittle 11. Malleable-if material can be hammered or rolled into a sheet, it has high % of compression, it can tolerate a high value of compressive stress.

9. Properties 12.Hardness- is the resistance of a material to indentation and penetration. Tests of hardness  Knoop- diamond indentor  Brinell-steel ball  Rockwell – steel ball  Vickers- diamond pyramid

10. Properties 13. Wettability- is a measurement of a liquid wetting a solid as indicated by the spreading of a drop  Hydrophilic –low contact angle, drop spreads out, material likes water  Hydrophobic- high contact angle, drop doesn’t spread, material fears water 14.Viscosity-property which influences the way a material flows  High viscosity- does not flow easily  Low viscosity- flows very easily

11. Properties 15. Thermal Conductivity-the rate at which heat and cold flows through a material, how does the material conduct temperatures  Higher the # more conductive  Lower the # more protective, more insulating Metals are more conductive than plastics

12. Properties 16.Adhesion- is the force which causes unlike molecules to attach to each other 17.Galvanism- two different metals meet  Galvanic action- flow of electrical energy between dissimilar metals. Both metals are wet with saliva causing a little electrical shock (metallic taste)  Galvanism can lead to corrosion 18.Corrosion- is the dissolution of metal in the mouth (pitting and discoloration) 19.Tarnish- surface reaction of metal to saliva and food components.  This can be polished away.