Micromeritics is the science of small particles. It is the study of a number of characteristics, including paticle size and size distribution, shape, angle of repose, porosity, true volume, apparent demsity nd bulkiness.
Particles are any unit of matter having defined physical dimensions. Physical state of particles can be altered by physical manipulationParticle characteristics can alter therapeutic effectiveness.
MICROMERITICSFACTORS AFFECTING FLOW PROPERTIES1. PARTICLE SIZE AND SHAPE 250-2000m = free flowing 75 – 250 m = flow freely or cause problem depending on shape Very fine particles (less than 10 m) = do not flow freely as large particles Particle shape and flow properties Spherical shape flow better than needle particles Elongated or flat particles tend to pack resulting to high porosity powders2. POROSITY AND DENSITY High density, low porosity = FREE FLOWING3. SURFACE ROUGHNESS Leads to poor flow characteristics
MICROMERITICSTechniques of determining particle sizeMICROSCOPIC METHOD(OPTICAL MICROSCOPY) Uses an ordinary microscope for particle measurement in the range of 0.2 m to 100 m. Presence of agglomeration and particles of more than one component may be detected The diameter is obtained only from two dimensions: length and breadth, the thickness/depth in not measured.The microscopic method can include counting not fewer than 200 particles in a single plane using calibrated ocular on a microscope.
Particle SizeSIZE OF MIDDLE # OF “ND” 1. Given the followingPARTICLE VALUE PARTICLEIN uM “D”(um) PER data, what is the GR ”N” average diameter of40-60 50 15 750 the particles?60-80 70 25 1750 dav= ∑ nd / ∑n80-100 90 95 8550 dav = 36,850 / 355100-120 110 140 15400 dav = 103.8uM120-140 130 80 10400 ∑n=355 ∑nd= 36,800
PARTICLE SIZESIEVING - uses standard sieves; generally used for grading coarser particles. May be employed for screening materials as fine as 44 m (No. 325 sieve)POWDERS OF VEGETABLE AND ANIMAL DRUGS ARE OFFICIALLY DEFINED AS: VERY COARSE (#8) – all particles pass through no.8 sieve and not more than 20 % through sieve no. 60. COARSE (#20) – all particles pass through no.20 sieve and not more than 40 % through sieve no. 60. MODERATELY COARSE (#40) - all particles pass through no.40 sieve and not more than 40 % through sieve no. 80. FINE (#60) - all particles pass through no.60 sieve and not more than 40 % through sieve no. 100. VERY FINE (#80) – all particles pass through a no. 80 sieve. There is no limit as to greater fineness.
PARTICLE SIZEPOWDERS OF CHEMICAL DRUGS ARE OFFICIALLY DEFINED AS: COARSE (#20) – all particles pass through no.20 sieve and not more than 40 % through sieve no. 60. MODERATELY COARSE (#40) - all particles pass through no.40 sieve and not more than 60 % through sieve no. 60. FINE (#80) - all particles pass through no.80 sieve and there is no limit as to greater fineness.
PARTICLE SIZE 0.5 um – 10um – suspensions and fine emulsions 10 um – 50um - upper limit of subsieve range; coarse emulsion particles; flocculated suspension particles 50 um – 100um – lower limit of sieve range; fine powder range 150 um – 1000um – coarse powder range 1000 um – 3360um –average granule size
A stack of sieve is arranged in order, the powder placed in thetop sieve, the stack shaken, the quantity of powder resting oneach sieve weighed, and this calculation performed: dav= ∑(% retained)x(ave size) / 100 dav = 29.232 / 100 = 0.2923mm
PARTICLE SIZE Other ways of particle size determonation elutriation, centrifugation, permeation, adsorption, the Coulter Counter, and light obstruction and the use of Andreasen pipet. dst = 18 h /(i - e)gtAngle of repose A relative simple technique for estimating the flow properties of a powder. It can easily be determined by allowing a powder to flow though a funnel and fall feerly onto a surface. The height and diameter of the resulting cone are measured and the angle of repose calculated as: tan Ѳ = h/rh is the height of the powder cone and r is the radius of the powder coneLow angle of repose – flow freely; high angle of repose – flow poorly
PARTICLE SIZECharateristics used to describe powder – porosity, true volume, bulk volume, apparent density, true density, and bulkiness.Void= Vbulk - V ; Porosity = Vbulk - V x 100 Vbulk VbulkApparent Density pa = Weight of the sample ; Vbulktrue density p = Weight of the sample V Bulkiness, B = 1/pa
PARTICLE SIZEEXAMPLEA selected powder has a true density (p) of 3.5g/cc. Experimentally, 2.5 g of the powder measures 40 mL in a cylindrical graduate. Calculate the true volume, void, porosity, apparent density, and bulkiness.
PARTICLE SIZEClosest packing:rhombus-triangle – angles of 60 deg and 120 deg: void=o.26 ; porosity=20%Open type of packing:cubical – cubes packed at 90 deg angle: void=0.47; porosity=47%Importance of packing and flow:a. Affects the size of the containerb. The flow of granulationc. Efficiency of filling apparatues for making tablets and capsules.d. Ease of working with powders.
Particle Size ReductionComminution, reduction of the particle size of a solid substance to a finer state, is used :1. Facilitate crude drug extraction.2. Increase the dissolution rates of a drug3. Aid in the formulation of acceptable pharmaceutical forms.4. Enhance absorption of drugs.The reduction in the particle size of a solid is accompanied by a great increase in the specific surface area of the substance.
Particle Size ReductionEXAMPLEIncrease in number of particlesIf a powder consists of 1mm on edge and it is reduced to paricles 10 um on edge, what is the number of particles reduced?1. 1mm equals 1000um.2. 1000um/10um = 100 pieces produced on each edge; that is, if the cube is sliced into 100 pieces on the x-axis,each 10um long, 100 pieces result.3. If this repeated on the x- and y-axis, the result is 100x100x100 = 1 million particles produced, each 10um on edge, for each original particle 1mm on edge. This can also be written an 106 .
Particle Size ReductionIncrease in surface areaWhat increase in the surface area of the powder is produced by decreasing the particle size from 1mm to 10um?1. The 1mm cube has 6 surfaces, each 1mm on edge. Each face has a surface area of 1 mm2. Because there are 6 surfaces, this 6 mm2 surface area per particle.2. Each 10um cube has 6 surfaces, each 10um on edge. Each face has a surface area of 10x10 = 100 um2 . Because there are 6 faces, this is 6x100 um2 or 600 um2 surface area per particle. Since 106 particles resulted from comminuting the 1m cube, each 10um on edge, the surfaces are now is 600 um2 x 106, or 6 x 108um2 .3. To get everything in the same units for ease of comparison, convert the 6 x 108um2 in to square mm as follows.4. Since there are 1000um/mm, there must be 1000 2 ,or 1 million um2 / mm2 .This is more appropriately express as 106 um2 / mm2, 6 x 108um2 = 6 x 102mm2 The surface area have been increased from 106 um2 / mm2 6 mm2 to 600 mm by the reduction in Particle size of cubes 1mm to cubes 100um on edge, an hundred fold inc in surface area.
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