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4. Powder Flow

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yohosesi's version from 2016-07-01 12:39

Section 1

Question Answer
for tablets/ capsules good flow maintains...weight uniformity (particle packing), consistent properties, minimum air entrapment
minimising air entrapment...decreases the likelihood of tablet capping and lamination
for dusting powders/ powders for inhalation good flow maintains...application to the right site and desired quantity
how does flow affect material transfer?quicker and easier
how does flow affect sieving?powder needs sufficient flow in order to be characterised/ seperated
how well a powder flows is dependent on...applied forces (e.g. gravity), particle properties
powder flow is resisted by...adhesion/cohesion, friction, interlocking
extent of resistance is governed by...particle size, shape, density, nature of surface
what is adhesion/cohesion?molecular forces on solid particle surfaces mean they tend to stick together & to other surfaces. These forces can cause a resistance to movement and so flow. Cohesion occurs between like surfaces (e.g. same component). Adhesion occurs between unlike surfaces (e.g. walls & diff components)
what causes adhesion/cohesion?van der waals, adsorbed liquid layers on particle surfaces, electrostatic charges
what causes interparticulate friction?rough/pitted particle surfaces
what is interlocking?happens if particles are irregularly shaped. this may cause bridging or arching especially if flow is slow
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Section 2

Question Answer
how does particle size influence the extent of resistance to flow? smaller particles have greater SA so greater cohesion. particles greater than <250nm are generally free flowing, particles <100nm generally fairly cohesive with poorer flow, particles< 10nm extremely cohesive & unlikely to flow is single partilces
how does particle (true) density influence flow?dense flow better due to higher gravitational forces
how does particle shape influence flow?spheres have lowest contact so best flow, flakes have high SA and poorer flow.
how does nature of particle surface influence flow?small amount of water vapour can sometimes reduce cohesion/adhesion but excessive moisture likely to increase attractive forces (surface tension effect)
what factors resist movement?adhesion/cohesion, friction, interlocking
what factors promote movement?gravity, mass, angle of powder bed and applied forces
what is angle of repose?powders tipped onto flat surface tend to form a conical heap. particles slip/roll until gravitational and inter-particulate forces are balanced. the sides of the heap form an angle with the horizontal known as angle of repose (o)
what is the equation for angle of repose?tan o = h/r where o=angle, h=height, r= radius
what does angle of repose well you?o is high for powders with high interparticulate forces. generally <25=excellent, 25-30 good, 30-40 passable, 40-50 poor, >50 very poor. for many pharmaceuticals o=35-45
how does method of finding angle of repose affect results?diff method, how material handled before and during, moisture content
how is drained angle of repose found?powder poured into cylinder plugged at bottom containing a raised circular horizontal platform. plug removed and angle determined from heap on platform.
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Section 3

Question Answer
what does packing geometry govern?consistancy of weight and shell size required for capsules
what does a powder bed consist of?particles in contact with adjacent particles with varying amounts of free space between (voids)
what is porosity of the powder bed?% of bed volume occupied by voids
what causes variance in free space?shape and irregularity of the powder, packing (rhombohedral packing, cubical packing)
powders normally have a range of sizes, how does this affect packing?voids between larger particles can be partially filled by smaller particles reducing porosity.
what is bulk density (PB)?a characteristic of the powder bed/bulk defined as: mass of powder (M) occupying unit of bed volume (V) this includes void spaces
what is true/particle density (PT)characteristic of individual component particles. particle mass/volume PT always > PB
how can bulk density be changed?reducing air present due to vibration/compression
how does porosity affect cohesion?reducing porosity increases cohesion, this must occur extensively during tabletting to form tablets of sufficient strength.
is void space desirable in a tablet?yes, voids help liquid uptake and hence disintegration.
what factors affect porosity?particle size distribution, surface properties (e.g. electrostatic forces) shape and texture
arches/bridging is more likely to occur with what shapes of particles?irregular, needle or plate shaped, rough particles
vibration will promote...particle rearrangement, reduced porosity
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Section 4

Question Answer
a tapping/jolting volumeter is used to...monitor changes in bulk density when powder subjected to mechanical tapping. indicates how easily & to what extent a powder consolidates (packs down) and gives an indirect measure of flow properties
how is a tapping/jolting volumeter used?a known mass of powder (M) is poured into a cylinder using a funnel. initial bulk powder voume (VO) is measured, powder mechanically tapped by a rotating cam, vol is measured after known number of taps (typically 1250) or when equilibrium a reached (VF)
what is the equation for initial poured bulk volume?DO=M/V0
what is the equation for final bulk volume/ tapped density?DF=M/VF
how is Carr's index (%) calculated?(DF-D0)/DFx100
what does Carrs index show?5-12 excellent flow, 12-16 good, 17-21 fair, 22-35 poor, >35 very poor
how can Carrs be improved?<21% generally needed for filling so glidants can be added to improve it
what is the Hausner Ratio?DF/D0, <1.25=good flow, >1.5=unacceptable flow
what do Carr's index and Hausner NOT show?ease and speed of powder consolidation. Some materials have high Carrs index but pack down rapidly.
how can consolidation speed be measured?measuring the bulk density after an increasing number of taps
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Section 5

Question Answer
how can altering particle size dist. improve powder flow?coarse=less cohesive, however often an optimum for max flow rate. Can seive to reduce small particles or granulate
how can adding glidants improve powder flow?reduce adhesion, cohesion & friction by reducing ttractive forces or lodging in surface irregularities.
how can altering particle shape/texture improve powder flow?irregular shaped flow less well than spherical.spray drying produces near spherical particles & produces free-flowing excipients. Careful control of crystallisation conditions may produce drug/excipient particles which are more spherical/smoother
how can altering surface forces improve powder flow?reduces electrostatic forces by altering process conditions to reduce friction (e.g. reduce speed/distance), earthing containers sufficiently, Reducing effect of moisture by drying powders before processing and storing in controlled humidity- expensive and time consuming
how can altering process conditions improve powder flow?change hopper design by stressing powder within hopper by vibrating hopper by vibrating hopper, force movement of powder within the hopper by including stirrer paddles etc
how can altering mass flow and funnel flow improve powder flow?in hopper powders with high adhesion may flow out by funnel flow. This causes the last particles to leave first, a rat-hole to form, this can collapse causing flooding of powder leading to inconsistent tablet weight. Mass flow hoppers have steeper walls so powder empties freely.
any improvement in powder flow and therefore weight uniformity will tend to increase the potential for _____ to occursegregation. A formulator may need to aim for a balance between flow and segregation.
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