Properties of Materials DHYG 113 Restorative Dentistry I

Objectives Describe the physical, chemical, biologic, and mechanical properties of commonly used dental materials Explain the application of properties in the use of materials to insulate pulp Define coefficient of thermal expansion Define viscosity and wetting and apply to the science of dental materials Explain how hardness is measured and relate to biocompatibility of various materials

Objectives Define modulus of elasticity and its relationship to stress and strain Explain how plastic deformation, elastic limit, proportional limit, and yield strength affect materials Describe the different types of stress in relation to materials Explain the differences between resilience and toughness

Physical Properties Based on Laws of Physics Mass Energy Force Light Heat Electricity Other physical phenomena

Density Mass of a material in a given volume Common value is grams/cubic centimeter Depends on the type of atoms present High density metals feel heavy High atomic numbers Atoms closely packed

Density In the example to the right, both cans have Comparison of:  Classic Coke  Diet Coke  355 mL  Water = 355 g  Sugar = 39 g  Sugar = 0 g  Nutra Sweet = 0 g Nutra Sweet = 0.1g  Tot. Wgt. = 394 g Tot. Wgt. = 355.1 g In the example to the right, both cans have the same volume, but the classic coke is more dense because the sugar weighs more than nutrasweet. The density of classic coke is 1.11 g/ml, and the density of diet coke is 1.00 g/ml http://www.elmhurst.edu/~chm/vchembook/121Adensitycoke.html

Properties of Materials Physical Properties Based on laws of physics Mechanical Properties Material’s ability to resist forces Chemical Properties Setting reactions, setting & degradation Biologic Properties Effects on living tissues

Thermal & Electrical Properties Materials that conduct electricity need to have insulation from the pulp Electrical current generation Usually by means of different metals in contact with each other (Galvanism) Saliva facilitates flow of electrons between metals, producing an electrical current like a battery Pain reaction to electrical current in tooth with deep filling (little insulating dentin) Normally, remaining dentin in a cavity preparation insulates the pulp. When little dentin remains (within 1 mm of pulp), cement bases can be used to insulate pulp. Composite and ceramic restorations are nonconductive and do not need insulators.

Pulp Insulation Normally, remaining dentin in a cavity preparation insulates the pulp. When little dentin remains (within 1 mm of pulp), cement bases can be used to insulate pulp. Composite and ceramic restorations are nonconductive and do not need insulators.

Boiling and Melting Points Help identify chemicals Mixtures have boiling range rather than a specific boiling point Atomic bonds broken by thermal energy Some materials don’t melt or boil… Decompose (burn) – wood, cookie dough

Vapor Pressure Measure of tendency to evaporate Higher temperature increases vapor pressure Molecules escape from liquid to form gas Useful as solvents Solvent evaporates, leaving a film of desired material (Copal varnish, etc.)

Thermal Conductivity The rate that heat flows through a material Metals have low heat capacity Readily warms up and transmits heat Example: Temperature change of hot food (55°C) and pulp (37°C) provides strong stimulus Insulating base of .75 – 1mm minimize effects of rapid temperature change

Thermal Conductivity Measurement depends on: Distance the heat travels Difference in temperature between source and destination (water pipe) Measured in heat flow over time Calories/second·meter·degree Insulating material needed to protect pulp with deep metal restoration

Thermal Conductivity Measure of heat transfer Rate of heat flow Thermal Conductivity of Dental Materials Material Thermal Conductivity (cal/sec/cm2 [ºC/cm]) Human enamel 0.0022 Amalgam 0.055 Gold alloy 0.710

Heat Capacity Amount of energy it takes to raise the temperature of that object 1° Specific heat capacity is the amount of energy it takes to raise the temperature of 1 unit of mass of that material 1°

Heat of Fusion & Vaporization Amount of energy needed to melt a material = heat of fusion Need 80 times more energy to melt ice than to raise the temperature of water 1° Amount of energy needed to boil a material = heat of vaporization Need 540 times the energy to boil the same quantity of water

Coefficient of Thermal Expansion Measurement of change in volume in relation to a change in temperature Cooling results in shrinkage/contraction Compare dental material to tooth Restoration will shrink with cold and expand with heat Opens gaps between restoration and tooth = microleakage (may cause recurrent decay) Opening and closing gap = percolation Dental amalgam – percolation decreases over time due to corrosion products from the amalgam filling the space

K ([mcal · cm]/[cm2 · sec · °C]) Thermal Properties Material Thermal Expansion α(x10-6/°C) Thermal Conductivity K ([mcal · cm]/[cm2 · sec · °C]) Tooth 8-11 1-2 Porcelain 6-15 2-3 Dental Cement 10-12 1-3 Gold 14-16 710 Amalgam 22-28 55 Composite 20-50 Wax 250-400 1

Electrical Conductivity Metals are good conductors Polymers and ceramics are poor conductors – insulators Affects corrosion of metals Electric pulp testing – need to know what material is in or on the tooth

Viscosity Ability to flow Measured in grams/meter·second, or poise (P) Temperature-dependent property Thick = flow poorly (cold syrup) Thin = flow easily (warm syrup) Water at 20ºC = 0.01 P (1 cP) Impression materials between 100,000 and 1,000,000 cP

Wetting Low viscosity and ability to wet a surface are important in dental materials Measured by determining the contact angle of a liquid or solid Low contact angle = good wetting Example: drop of water on ice cube High contact angle = poor wetting Example: drop of water on plastic

Wetting Partial wetting-contact angle around 90 degrees Non wetting-close to 180 degree contact angle http://en.wikipedia.org/wiki/Contact_angle

Hardness Measured by pressing a hard shaped tip into the surface of a material Brinell, Rockwell, Vickers, Knoop Calculated based on: Size of indentation Load on the tip Shape of the tip Knoop (KHN): enamel = 350, dentin = 70, porcelain = 400-500, acrylic denture teeth = 20

Abrasion Resistance Goldilock’s Principle (Just Right!) Wear resistance of dental materials to food and opposing teeth Hard enough to wear well, but not wear away opposing teeth

Solubility Calculated by amount of material that dissolves in a given amount of liquid in a given time Test by immersing in water Sample weighed before and after Weight difference is solubility Dental material should be nearly zero

Water Sorption Ability to absorb water Measured much like solubility Weight gained is the water sorption

Color Complex phenomenon Psychologic response to a physical stimulus Perception of color may differ between people Color depends on light (hard to match restorative material to adjacent teeth) Measured by matching against color tabs Spectrophotometer (not useful in clinical dentistry) Fluorescence is important Color of teeth is in the yellow range

Interaction with X-Rays Some materials are radiolucent Not seen in XR Radiopaque – metals Some materials match radiopacity of enamel to allow diagnosis of recurrent caries – makes them hard to detect on XR, though!

Mechanical Properties Subgroup of physical properties Describe a material’s ability to resist forces Elasticity, stress, strain

Biting Forces Force: Any push or pull upon matter Stress: The reaction within the material to an externally applied force Strain: The change produced within the material as the result of stress

Types of Forces Compression – pushing or crushing stress Average biting force in posterior is ~170 lbs. or about 28,000 psi on a single cusp of a molar

↑ ↓ Forces Tension – pulling stress (tug of war) Shear – parts of an object slide by each other Torsion – twisting force Bending – combination of several types of stresses One side stretched, other side compressed ↑ ↓

Forces in single dimensions Compression Tension Shear Torsion Flexure Diametrical tension/compression This video also available separately on ANGEL

Stress and strain When force is applied to an object, it deforms Stress-load per unit of cross-sectional area (eg. pounds per square inch); the resistance a material makes to an applied load Strain-deformation per unit length; the change in shape (deformation) a material makes in response to stress

Stress and strain If a pile of books is placed on a shelf, the weight of the books exerts a downward force on the shelf. The shelf does not fall down, the shelf resists the weight of the books. This resisting force is stress. If the shelf were to change shape (eg. sag in the middle) as a result of the weight of the books, the amount of change would be the strain. http://www.shelvingcompany.co.uk/media/heavy-duty%20floating_shelf.jpg

Strain & Stress Strain : Change in length divided by the original length Fractions (0.02) or percent (2%) Stress : Force that develops in loaded object (load) Stress = load/area Pounds/square inch (psi) Stress and strain are proportional

Elasticity When force is removed, the object returns to its original shape Atomic bonds = microscopic springs Bending = stretching + compression of atomic bonds Compression or elongation of a loaded object – measured in terms of change in length

Young’s modulus Modulus of elasticity Measure of the material’s rigidity or stiffness Resistance of the material to strain or deformation High modulus = stiff material (enamel) Low modulus = more flexible (rubber band) Units are psi, but larger (psi x 106 or gigapascals)

Strain Elastic strain: deformation/strain that is reversible (eg. stretching an elastic band a little and it bounces back to its original shape and size) Plastic strain: some permanent deformation caused (eg. stretching an elastic band really wide to the point that when it relaxes, it remains a little stretched out) The tipping point between elastic and plastic strain is the elastic limit

Plastic Deformation Stress no longer proportional to strain Spring doesn’t return to original length Elastic limit, proportional limit, yield point Ultimate tensile strength – point where material breaks (failure occurs) Highest stress on the graph Bad for bridges – road or dental ones Ultimate strength = highest stress measured Compressive test shows compressive (tensile) strength

Mechanical Properties of Dental Materials Resilience – ability to absorb energy and not be deformed (mouthguard) Toughness – energy absorbed up to the failure point on stress/strain diagram (helmet) Fracture toughness – energy it takes to fracture a material when a crack is present Glass = low Metals = high

Fatigue Fatigue – testing replicates real world applications Materials used multiple times – things fail eventually Testing predicts amount of stress the material can endure without breaking

Time-dependent Properties Creep – very slow flow Small change in shape when an object is under continuous compression (amalgam) Takes place over a long period of time Temperature dependent Stress relaxation – similar to creep Slow decrease in force over time (ortho elastics)

Stress Concentration Stress focuses around defect Glass cutter scratches surface (defect) Bending stress applied; fracture occurs Control the defects – it’s important in dentistry to handle materials properly Remove surface defects that concentrate stress Polish restorations, proper design, glaze porcelain

Chemical Properties Decay or degradation Setting Reactions Gypsum products set by precipitation Composites polymerize

Biologic Properties Effects of a material on living tissue