powder flow Flashcards
what effects the powder flow?
particle size, shape, distribution, moisture, static charge
why is good flow important?
so good mix between excipient and drug
what increases flowability?
‘increase in particle size, regular shape, narrower size distribution’
why is the feeder important?
must ensure same rate and blend of the powder every time so each tablet has the same amount of powder
what is Carr’s index?
change in volume of powder from fluff state to agitation (tapped) state
what do powders flow thorugh?
orifice or opening
what are the types of flow from hopper?
mass flow
funnel flow
what is mass flow?
all material in hopper in motion but not always at same velocity
what is funnel flow?
core is moving and has a slow or non moving annular region leading to segregation of powders
what are the problems of hopper powder flow
arching and ratholing
what is arching?
cohesive arch stops material from leaving the hopper
what is ratholing?
powder collapses through orifice but only the centre core causes by poorly cohesive powders
how to design a hopper to prevent funnel flow?
the higher the angle of repose the worse the powder is flowing
the hopper flow should be in the mass flow region
in what case is the rate of flow of powder not dependent on the amount on powder present?
if powder bed height is about 3 times orifice diameter the rate of flow of powder isn’t dependent on the amount of powder (unlike water)
when does flow rate increase?
flow rate increases as average particle size decreases
how to increases size?
granulate powders
how to reduce size distribution?
removing finer particles
which particles do you not make the size smaller and why?
since coarse particles are less cohesive then don’t make the size smaller than you need
how to improve powder flow?
increase size
reduce size
alter shape to make it rounder
reduce interaction of forces
formulation additive added
lubricants
glidants
alter processing conditions
how to reduce the interacting forces ?
reduces electrostatic charging by electric earthing
if too humid reduce humidity
how do you reduce cohesion
use glidants
how to reduce adhesion?
use lubricants
what to use if flow is being impaired by moisture?
use magnesium oxide which stops water film formation
equation for Carr’s index?
Carr’s index (%) = (tapped density - fluff density) / tapped density
*100
what is the effect of particle size distribution on powder flow?
greater size distribution means more packing so more particle-particle contact so poorer flow
what particle shape is ideal for a good flow?
spherical because small surface area and less particle particle contact
what effects particle packing geometry and why?
Particle size and size distribution-empty space with coarse particles filled with finer particles making it more packed
Particle shape and texture-power bridges formed through rougher textures
Surface properties-some particles becomes electrostatically charged or absorb water
History of handling
what is the equation of packing density?
PB= M/V (kg/m-3)
what is packing fraction?
ratio of PB to true density
What does a pakcing fraction close to 1 mean about the flow?
the closer the packing fraction to 1 to poorer the flow
what causes powder segregation on pouring?
larger particles have more kinetic energy so travel the furthest over the smaller ones and will be found at the edges
percolation
