(1) The flow properties of solids have great impact on the tableting processes since their manufacturing require the flow of powder from a storage container to tablet dies.
(2) Weight and content uniformity are also dependent on flow of powders.
(3) The flow properties of solids also greatly influence the mixing and demixing of powders.
(4) The speed of tablet production is also greatly affected by the formulation’s flow characteristics.
(5) For the final product, weight, content uniformity, hardness, disintegration and dissolution are affected by formulation flow.
Physical characteristics of the particles, such as size, shape, angularity, surface texture, porosity and hardness, will all affect flow properties. External factors such as humidity, conveying environment, vibration and, perhaps most importantly, aeration will compound the problem.
The flow properties of a material result from forces that can act between solid particles including (1) frictional forces,
(2) surface tension forces,
(3) mechanical forces caused by interlocking of particles of irregular shape, (4) electrostatic forces and
(5) cohesive or van der Waals forces. All of these forces can affect the flow properties of a solid. Most flow properties are significantly affected by changes in particle size, density, shape, electrostatic charge and adsorbed moisture, which may arise from processing or formulation.
In general, powders with large particles (>100 μm) will be noncohesive, permeable and will probably fluidize and will have low compressibility and relatively low shear strength. Conversely, fine powders, say <10 μm, are likely to be cohesive, compressible, contain much entrained air and yet have poor aeration characteristics.
CHARACTERIZATION OF POWDER FLOW:
Commonly used methods for characterizing powder flow are as follows:
- Compressibility index
- Angle of repose
- Flow rate through an orifice
A simple indication of the ease with which a material can be induced to flow is given by application of a compressibility index and Hausner ratio given by the following equation:
Carr’s compressibility index = (tapped density – bulk density) / tapped density X 100
Hausner ratio = tap density / bulk density
Table 1 – Scale of Flowability for compressibility index and Hausner ratio
|Very, very poor
Angle of repose
If a powder is allowed to flow onto a flat surface, a pile or heap of powder is formed. A material that is not cohesive and flows well, spreads out, forming a low heap. More cohesive materials form higher heaps, which are less spread out. The angle of repose (T) is defined as the angle of the free surface of a pile of powder to the horizontal plane and is represented by the following equation:
tanⱷ = h/r
where h is the height of pile, r the radius of pile and T the angle of repose.
It is the maximum angle that can be obtained between the freestanding surface of a powder heap and the horizontal plane.
Static angle of repose –
The fixed funnel method uses a funnel that is secured with its tip at a given height, h, above the graph paper that is placed on a flat horizontal surface. Powder or granulation is carefully poured through the funnel until the apex of the conical pile just touches the tip of the funnel. The radius of the base of the conical pile is then determined to calculate the angle of repose. The fixed cone method establishes the radius of the cone base, r, using a circular dish with sharp edges. Powder is poured onto the centre of the dish from a funnel that can be raised vertically until a maximum cone height, h, is obtained. The repose angle is calculated as before.
Dynamic or kinetic angle of repose –
Angle of repose methods, which result in a so-called dynamic angle, are preferred, since they most closely mimic the manufacturing situation, in which the powder is in motion.
Tilting box method: A sandpaper-lined rectangular box is filled with the powder and carefully tilted until the contents begin to slide. The maximum angle that the plane of a powder makes with the horizontal surface on rotation is taken as the angle of repose.
Rotating cylinder method: A typical dynamic test involves a hollow cylinder half-filled with the test powder, with one end sealed by a transparent plate. The cylinder is rotated about its horizontal axis until the powder surface cascades. The curved wall is lined with sandpaper to prevent preferential slip at this surface.
- T value < 20° exists rarely
- T value 25–30° indicates excellent flow
- T value 31–35° indicates good flow
- T value 36–40° indicates fair flow
- T value 41–45° indicates passable flow
- T value 46–55° indicates poor flow and such powder require agitation
- T value 56–65° indicates very poor flow
- T value t 65° indicates very, very poor flow
Flow Rate Through an Orifice
The simplest method of determining powder flowability directly is to measure the rate at which powder discharges from an orifice of the hopper. This method is not useful for cohesive materials and can be used only for materials that have some capacity to flow.
General guidelines for dimensions of the cylinder are as follows:
Diameter of opening > 6 times the diameter of the particle
Diameter of cylinder > 2 times the diameter of the opening
Multiple choice questions (MCQs)
1.Granule density is measured by
a)Coulter counter method
b)Kozeny carman equation
2.Porosity of powder is calculated by
a)Bulk volume – true volume/true volume X 100
b)Bulk volume – true volume/bulk volume X 100
c)True volume-bulk volume/true volume X 100
d)True volume-bulk volume/bulk volume X 100
3.For passable flow of powder angle of repose should be
a)Less than 20
b)Between 20 to 30
c)Between 30 to 40
4.The nominal mesh aperture size for sieve no. 10 is
5.Powder pass completely through sieve no. 44 and NMT 40% pass through sieve no. 85 is called
6.The rate of sedimentation of particle is directly proportional to
d)None of the above
7.One of the following apparatus is used to determine the particle size by the gravity sedimentation method
8.The sedimentation of particles in a suspension can be minimized by
a)Increasing the particle size of active ingredient
b)Decreasing the particle size of active ingredient
c)Increasing the viscosity of the suspension
d)Both b and c
9.Coulter counter method does not give information regarding the
d)Both b and c
10.Which one of these distributions is more important in the design of dosage forms?
11.Disadvantage of sieving method for particle size measurement of powder is
b)Attrition of powder is possible
c)Large number of sieves are required
d)Tedious and time consuming
12.If carr’s compressibility index value is in the range of 26-31 the flow will be
13.The powder having low density or large bulk volume is known as
14.Porosity is expressed in
15.Helium pycnometer is used to determine
- c) Pycnometer
- b) Bulk volume – true volume/bulk volume X 100
- c) Between 30 to 40
- b) 7mm
- c) Moderately fine
- a) Particle size
- c) Andreasen pipette
- d) Both b and c
- b) Particle shape
- d) Weight
- b) Attrition of powder is possible
- a) Poor
- c) Light powder
- a) Percentage
- b) True density
1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 58-62.
2. Martins Physical Pharmacy, 6th edition 2011, page no. 843-846.