L2: physical and chemical properties of dental materials

Question 1

what is creep and flow ?

Answer 1

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

Question 2

low contact angle reflects _)___

Answer 2

good wetting

Question 3

high contact angle reflects ____

Answer 3

poor wetting

Question 4

what are the 3 important values regarding color and optical effect and what do they mean ?

Answer 4

value : brightness (L)

hue: dominant wavelength / color

chroma : intensity/ degree of saturation

Question 5

what does +a , -a ,+b , and - b mean ?

Answer 5

a : measurement along Red ( +a) and green (-a) axis

b : measurement along yellow (+b) and blue (-b) axis

Question 6

in increasing order , list the 3 major dental materials in terms of heat conductivity :

Answer 6

polymers < ceramics < metals

Question 7

what is thermal diffusivity ?

Answer 7

rate at which a material approaches equilibrium by conduction of heat from higher temperature to lower temperature regions

Question 8

what is the equation for thermal diffusivity ?

Answer 8

conductivity / specific heat x density

Question 9

what is specific heat ?

Answer 9

quantity of heat needed to raise temperature of 1 gm of the substance by 1 degree centigrade

Question 10

what is coefficient of thermal expansion ?

Answer 10

fractional increase in length of a body for each degree centigrade increase in temperature

Question 11

what is erosion ?

Answer 11

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

Question 12

what is corrosion ?

Answer 12

chemical attack on material especially metals by environmental agents resulting in dissolution

tendency of corrosion can be measured by electrode potential

Question 13

what is tarnish ?

Answer 13

• Surface discoloration on a metal or loss of
  surface finish

Question 14

what materials will be more reactive ?

Answer 14

materials with large negative electrode potential will be more reactive than those with large positive values ( corrosion)

Question 15

???? wtf is this guy saying

Answer 15

• 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

Question 16

what is leaching ?

Answer 16

The loss of constituents from a restorative material into oral fluids by
diffusion process.

Question 17

in what cases is leaching beneficial ?

Answer 17

calcium hydroxide cements (antibacterial and secondary dentine formation)
and glass-ionomer cements (leaches fluoride ions to from
fluoroapatite which resist acid attack on tooth

Question 18

what is adsorption >?

Answer 18

attachment of one substance to another surface

Question 19

what is absorption ?

Answer 19

: The substance absorbed diffuses into the solid material
by diffusion process and the process is not noted for concentration of
molecules at the surface.

Question 20

what is sorption ?

Answer 20

: Both adsorption and absorption exist and it is not clear
which process predominates, the whole process is known as sorption

Question 21

What is dilatant and pseudoplasticity ?

Answer 21

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.

Question 22

what is Thixotropic?

Answer 22

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.

Question 23

what is a concentration cell?

Answer 23

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.

Question 24

what is electromotive series

Answer 24

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