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
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.
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?
what is the equation for final bulk volume/ tapped density?
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 occur
segregation. A formulator may need to aim for a balance between flow and segregation.
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