L2: physical and chemical properties of dental materials Flashcards
what is creep and flow ?
creep: time dependent plastic strain of material under influence of static load ( small defomation by long lasting , large stress)
flow: potential to deform under minimal stress
low contact angle reflects _)___
good wetting
high contact angle reflects ____
poor wetting
what are the 3 important values regarding color and optical effect and what do they mean ?
value : brightness (L)
hue: dominant wavelength / color
chroma : intensity/ degree of saturation
what does +a , -a ,+b , and - b mean ?
a : measurement along Red ( +a) and green (-a) axis
b : measurement along yellow (+b) and blue (-b) axis
in increasing order , list the 3 major dental materials in terms of heat conductivity :
polymers < ceramics < metals
what is thermal diffusivity ?
rate at which a material approaches equilibrium by conduction of heat from higher temperature to lower temperature regions
what is the equation for thermal diffusivity ?
conductivity / specific heat x density
what is specific heat ?
quantity of heat needed to raise temperature of 1 gm of the substance by 1 degree centigrade
what is coefficient of thermal expansion ?
fractional increase in length of a body for each degree centigrade increase in temperature
what is erosion ?
process which combines chemical process of dissolution with a mild mechanical action
in dentistry erosion is described as destruction of natural hard tissue by acids
what is corrosion ?
chemical attack on material especially metals by environmental agents resulting in dissolution
tendency of corrosion can be measured by electrode potential
what is tarnish ?
- Surface discoloration on a metal or loss of
surface finish
what materials will be more reactive ?
materials with large negative electrode potential will be more reactive than those with large positive values ( corrosion)
???? wtf is this guy saying
- Chemical corrosion -> is seldom isolate
- Dry Corrosion -> dental gold alloy containing silver
- Stress Corrosion -> fatigue (cyclic loading) in mouth
- Crevice Corrosion -> margin of tooth/metal
restoration
what is leaching ?
The loss of constituents from a restorative material into oral fluids by
diffusion process.
in what cases is leaching beneficial ?
calcium hydroxide cements (antibacterial and secondary dentine formation)
and glass-ionomer cements (leaches fluoride ions to from
fluoroapatite which resist acid attack on tooth
what is adsorption >?
attachment of one substance to another surface
what is absorption ?
: The substance absorbed diffuses into the solid material
by diffusion process and the process is not noted for concentration of
molecules at the surface.
what is sorption ?
: Both adsorption and absorption exist and it is not clear
which process predominates, the whole process is known as sorption
What is dilatant and pseudoplasticity ?
Resistance to flow increases as the rate of deformation
(shear strain rate) increases. The more rapidly dilatant fluids are
stirred or forced through a syringe, the more viscous (thicker)
and more resistant to flow they become
pseudoplasticity is Viscous character that is opposite from dilatant
behavior in which the rate of flow decreases with increasing
strain rate until it reaches a nearly constant value. Thus the
more rapidly pseudoplastic fluids are stirred or forced through
a syringe, the less viscous (thinner) and more easily they flow.
what is Thixotropic?
c Property of gels and other fluids to become less
viscous and flow when subjected to steady shear forces through
being shaken, stirred, squeezed, patted, or vibrated. When the
shear force is decreased to zero, the viscosity increases to the
original value over a short time delay.
what is a concentration cell?
l Electrochemical corrosion cell in which the
potential difference is associated with the difference in concentration of a dissolved species, such as oxygen, in solution along
different areas of a metal surface. Pitting corrosion and crevice
corrosion are types of concentration cell corrosion.
what is electromotive series
s Arrangement of metals by their equilibrium values of electrode oxidation potential. Used to judge
the tendency of metals and alloys to undergo electrochemical
(galvanic) corrosion
