Advances in Powder Flow Characterization by Freeman Technology using Ft4 Powder Rheometer
Vijaya Gayathri Ambadipudi, Girish Pai Kulyadi, Vamshi Krishna Tippavajhala*
Department of Pharmaceutics, Manipal College of Pharmaceutical Sciences, Manipal Academy of Higher Education, Manipal, Karnataka, India.
*Corresponding Author E-mail: krissrcm@gmail.com
ABSTRACT:
Powders are challenging materials to work with. Flow properties have critical significance in the pharmaceutical industry, which is why understanding powder behaviour in various unit operations like blending, compression, volume filling and various other scale-up techniques as well as in the storage and transportation is utmost important. Although there are many well established tests to evaluate flow properties, they lack to interpret the effect of process and test conditions on the flow behaviour. With the help of Powder Rheometer, the process conditions can be simulated thereby enabling one to evaluate the powder flowability in consolidated, aerated and confined states. Rheometer is also called as the ‘Universal Powder Tester’ as it evaluates powder flow in bulk, dynamic and shear states with accuracy and sensitivity. This paper provides insights about the powder characterization in dynamic flow, bulk and shear by FT4 Powder Rheometer.
KEYWORDS: FT4 powder rheometer; Powder flow characterization; Freeman technology.
INTRODUCTION:
As the technology advanced, many inventive methods to measure powder flow such as cohesivity [8], avalanching [9], shear cell [10,11], dielectric imaging [12], atomic force microscopy [13] and penetrometry [14] have come up. Angle of repose, Carr’s index and Hausner ratio have been estimated using Near-IR spectroscopy in few case [15].
Recently powder rheometers are being used to quantify powder flow [16,17] and for advanced investigation Novel Electrical Capacitance Tomography (ECT) is being used along with powder Rheometers [18]. FT4 uses a novel patented technology with which it measures the resistance experienced to flow while the powder is in motion. It has the potential to mimic the powder processing conditions thereby testing the samples in consolidated, aerated, stressed states [19]. The results obtained using FT4 have accuracy and reproducibility. In this review, we will summarize the basic operating principles involved in the FT4 for the evaluation of powder characteristics and also their importance in the pharmaceutical industry.
2. METHODOLOGY:
FT4 Powder rheometer (Freeman Technology Ltd) characterizes powder flow in Bulk, Dynamic, Shear in accordance with ASTM D7891 [20] and Processing conditions. This Versatility of FT4 makes it a Universal Powder tester.
FT4 powder rheometer consists the following accessories which are used to perform multiple tests:
Split vessels and Extension vessels (calibrated), Blade/Impeller, Vented piston, Shear cell, Standard base, aeration base, Clamp ring, Split Assembly, aeration unit and funnel. Based on the sample size blade and the vessel combination can be chosen like 25mm vessel with 23.5mm blade, 50mm vessel with 48.0mm blade and 62mm vessel with 60.0mm blade.
3. DYNAMIC METHOD:
Powder rheometer has a unique technology which is patented that measures the resistance to flow shown by the powders when they are in motion. As the blade moves through the powder which is in a calibrated glass vessel rotational and vertical resistances are measured in the form of Torque and Force respectively[1,19,21]. Powder’s resistance to flow is the combination of measured torque and force (Figure1a). Total Energy can be measured by finding out the work done, which is total resistance faced by the blade for each small distance travelled. As the force and torque differ for each mm of travel by the blade, Energy Gradient is measured in mJ/mm.
Total Energy = (Resistance x Distance)
Total Flow Energy can be obtained by calculating area under the Energy Gradient curve (Figure 1b).
Figure1: Dynamic powder characterization (a) Torque & Force measurement to calculate the resistance to flow. (b) Total flow energy determination.
3.1 Confined and unconfined powder flow:
Specific flow patterns are used to measure flowability, which are of two types:
3.1.1 Forced (confined) Flow:
Measure of powder’s flowability when it is forced to flow, wherein it is restricted by the closed bottom of test vessel. This property is defined as the BFE and is measured during the downward movement of the blade [19, 22].
3.1.2 Low Stress (unconfined) Flow:
Measure of powder’s flowability when it is free or not restricted. The resistance to flow is measured as the blade travels across the powder bed from bottom to top [19,22]. This property is defined as the SE.
3.2 Conditioning:
The density of the powder’s changes randomly during handling, transportation, processing etc. This is due to stress applied on the powder during the processing conditions which will have a major impact on the powder behaviour. Therefore, the powder has to be prepared for any test by applying uniform stress in the powder bed by removing air spaces or air pockets. This initial step before starting all other tests is called conditioning which uses the dynamic methodology[19,23] to gently displace the whole sample to loosen and slightly aerate the powder.
3.3 Stability and Variable Flow Rate Test:
This is a simple analysis done to understand if the powder being evaluated is going to change because of the stresses faced during flow and to evaluate the sensitivity of powder to flow rate as they are rate of powder handling is different for all processes. Split vessel, blade are required for this test. An initial conditioning cycle is performed to stabilise the powder followed by the flow rate testing. The blade with a fixed helix angle of 5o and an initial tip speed of 100mm/s goes till 2mm from bottom of the vessel and comes up to 55mm from the bottom during which the change in flow energy is measured. Different tip speeds 100, 70, 40, 10 are tested and change is flow energy is recorded. This test helps in the evaluation of parameters like BFE, SE, FRI, SI and Total Energy. The interpretation of the obtained data is mentioned in Table:1.
NOTE: the tip speed is constant at 100mm/s in the stability test whereas in the Variable Flow Rate test the tip speed will change like 100,70,40,10mm/s). The data typically represents the flow energy for downward blade movement in both the tests. These tests are done individually or in combination as a single test.
Table 1. Typical Stability and Flow Rate Powder Behaviour
|
Flow rate index (fri) |
Powder nature |
Stability index |
Powder nature |
|
FRI>3.0 |
High flow rate sensitivity |
SI=1 |
No effect on being made to flow |
|
1.5<FRI<3.0 |
Average flow rate sensitivity |
SI>1 |
Powder affected by being made to flow. Possible causes are: • De-aeration • Agglomeration • Segregation • Moisture uptake and Electrostatic charge |
|
FRI=1 |
Flow rate insensitive |
SI<1 |
Powder affected by being made to flow. Possible causes are: Attrition and De-agglomeration |
|
FRI<1.0 |
Pseudoplastic or Newtonian flow rate |
|
|
Figure 2: Dynamic power characterization – Aeration test (Freeman Technology, UK)
The BFE of the powder is employed to evaluate the variability in the flow properties caused by the external variables.
3.3.1 Aeration:
A controlled air supply unit is introduced into the powder column through a porous mesh base which will aid to measure the influence of air. This method is to understand the cohesive forces that exist among the particles. The cohesive forces are generally a combination of Wander Vaal’s and electrostatic forces which is difficult to measure. By this aeration test it is possible to measure the cohesive forces through the changes in powder’s bulk properties (Figure.2). The air flowing through the powder column will try to overcome the cohesive forces by separating the particles nearby. The recorded resistance to flow shown by the powder equals the powders cohesive forces which is depicted as AE. Comparing BFE of the powder with the AE gives the AR, which is the powder’s sensitivity to air flow [19, 24].
ARyy = BFE / AEyy
Where ‘yy’ defines the air velocity in mm/s
To measure the effect of air on the flow properties of the powders and also check the cohesivity existing between the particles. Calibrated glass vessel with an aeration base supplied to an aeration unit is required to carry out the test. Initial Conditioning is done at a tip speed of 40mm/s at a helix angle of 5o with no air where the blade moves up to 5mm from the bottom of vessel to 55mm. In the next cycle goes down with a helix angle of 2o with the same tip speed. Cycle 3 helix angle remains 5o tip speeds change from 100mm/s while going down to 40mm/s while coming up. From Cycle 4 to 13 for each air pressure two sets of tip speeds 100mm/s and 40mm/s, 40mm/s and 40mm/s with a helix angle of 5o and height up to 75mm from the bottom are performed. Air pressure range are as follows 2, 4, 6, 8, 10 mm/s. AE and AR are the parameters evaluated by this test. The aeration energy should be low, and the aeration ratio should be high for the powder to become less cohesive (Table:2).
Table 2. Typical Aeration data
|
AERATION RATIO |
POWDER NATURE |
|
AR=1 |
Not sensitive to aeration |
|
2<AR<20 |
Average sensitivity to aeration |
|
AR>>20 |
Very sensitive to aeration |
3.3.2 Consolidation:
This test is similar as that of BFE, but the powder has to be in a consolidated state. The blade moves through the powder column to measure the flow. After conditioning, consolidation can be achieved by Tapping or by applying normal stress with help of a piston. Therefore, the resistance which will be faced by the blade is measured as the consolidation energy. Comparing the CE with BFE gives the CI, which is the change in flow of the powder due to consolidation [19, 24].
CIyy = CEyy / BFE
Where ‘yy’ can be either Number of taps, or Applied normal stress (kPa)
4 BULK PROPERTIES:
These properties have the potential to influence the process and product quality attributes, though they are not the direct measure for flowability. FT4 has the capability to measure three different types of bulk properties.
4.1 Conditioned Bulk Density:
Density can be defined as the relation between the weight and volume. The density of the powder keeps varying easily and it is essential that the value should be reproduced to obtain accurate results. In order to achieve reproducibility, the packing structure of the powder should be well known. With the help of the conditioning cycle the packing structure can be understood which when combined with the in-built balance (measures mass), split vessel (measures volume) accurate measurement of the CBD can be achieved [19, 25] (Figure 3).
CBD = Split mass after conditioning / Sample volume
Figure 3:Bulk powder characterization- Conditioned Bulk Density determination(Freeman Technology, UK)
4.2 Compressibility:
The percentage change in the volume recorded with the application of increasing compressive force by a pistonon the conditioned powder is termed as compressibility (Figure: 4). The piston used is a vented one which helps in the removal of entrapped air in the powder. This data can also be presented as a work of applied stress in the form of Compressibility Index and BD [19, 25].
Density after compression
Compressibility Index= –––––––––––––––––––––––
CBD
Bulk density= Split mass / Compressed Powder Volume
Figure 4: Bulk powder characterization- Compressibility determination (Freeman Technology, UK).
To measure the change in density on application of normal stress. Split vessel, blade and vented piston which is used to compress the sample are required to perform this test. Initial conditioning cycle is carried out at a tip speed of 40mm/s at 5o helix angle then the vessel is splitted and the split mass is recorded. Blade is now replaced with a vented piston which applies stress on the powder at increasing levels. Each stress level is applied for a certain period of time (60 sec) to make sure the powder reaches equilibrium. The helix angle now is 90o, stress is applied at 0.5,1,2,4,6,8,10,12,15 kPa with a holding force for 60sec and carriage speed 0.05mm/s, distance travelled by the piston is measured which gives % change in volume. The higher the percentage change in volume better is the compressibility.
Figure 5: Bulk powder characterization- Permeability determination (Freeman Technology, UK).
4.3 Permeability:
Powder’s
resistance to air flow through it is called as the permeability. This test is
similar to the compressibility wherein vented piston applies increasing levels
of stress on the conditioned powder while air is passed through from the mesh
base connected to an aeration unit (Figure:5). Various ranges of stresses
and air flow rates can be applied to perform this test. The 
(pressure drop) or difference between the top
and bottom of the powder column measures the permeability [19, 25].
To measure the extent of fluid transmission from a powder when it is in the bulk state. Consolidation stress influences the permeability by altering the porosity and contact surface areas. Split vessel with aeration base supplied to an aeration unit are required to perform the test. Initial conditioning is done at a tip speed of 40mm/s at 5o helix angle which moves 5mm from the bottom of vessel to a height of 50mm. The vessel is split, the split mass is recorded, followed by the replacement of the blade with vented piston. Each stress level is applied for a certain period of time to make sure the powder reaches equilibrium. Now the helix angle is 90o, in the first cycle holding time is 20sec with a normal stress of 0.05kPa, 0.5N compression force. From cycle 2 the holding time is 60 sec, air pressure, normal stress and compression force range as follows 1kPa/0.45N, 2kPa/0.9N, 4kPa/1.81N, 6kPa/2.71N, 8kPa/3.62N, 10kPa/4.52N, 12kPa/5.43N, 15kPa/6.79N. PD will be evaluated. Lower the pressure drop higher is the permeability. This test can be performed in two ways, one by keeping the air pressure constant with increased stress ranges and vice-versa.
5. SHEAR PROPERTIES:
5.1 Shear Cell:
This test measures the powder behaviour as it transitions from a non-flowable state to a flowable state. To perform the shear test the powder should always be in a consolidated state. This technique of the FT4 is issued by the ASTM International Committe D18 on Soil and Rock (20). Shear (horizontal) force will be applied on the top layer of the powder while the immediate bottom layer of the powder is stopped from moving (vice-versa) at lower levels of speed. The shear force is gradually increased to a level until the powder shows movement, where the shear force exceeds the shear strength of the powder at this point powder bed yields leading to slipping of the top layer over the lower. There are several designs available for shear cell over the years, but the rotational design (Figure:6a) is the most commonly used, yet all work on the same operating principle.
Figure 6: Shear Cell characterization (a) Rotational shear cell (b) construction of Mohr’s circles to the yield locus to obtain Flow Function.
The test sequence of the shear cell includes many shear stresses which will be carried out against various levels of normal stresses. By plotting shear stress against normal stress powder’s yield locus can be obtained. Numerous mathematical models can be applied to obtain the flow function of the powder, as proposed by Jenkie[10,11] for the description of flowability of the powders. One such method is applying Mohr’s circles (Figure:6b) to the obtained yield locus, this gives UYS and MPS values. Ratio of these is the FF, which is one of the criteria used to evaluate flowability(Table:3). Values below 4 indicate poor flow, 10 and above for a good flow [26, 27].
To measure the flowability of a consolidated powder which is at rest. Split vessel, blade, vented piston and shear cell are required to perform the test. Initial conditioning is done at a tip speed of 40mm/s and a helix angle of 5o which moves 5mm from bottom of the vessel to a height of 55mm. The blade is replaced with vented piston which will compress the powder from 0.05-9 kPa at 60sec holding time and with 0.5mm/s carriage speed. Now the vessel is split, the split mass is recorded. Replace the vented piston with shear cell, Shear test is performed at 6,7and 9 kPa, pre-shearing is done to make the powder reach to a steady state by applying higher normal stresses, to determine the steady state software identifies maximum torque and checks for 20 secs, if reaches steady state shear stress specified is applied until yield locus is obtained. Flow Function is evaluated to understand the flowability by constructing Mohr’s circles.
Table 3. Classification of powder flowability
|
TYPE OF FLOW |
FLOW FUNCTION VALUE |
|
Free-flowing |
10<FF |
|
Easy-flowing |
4<FF<10 |
|
Cohesive |
2<FF<4 |
|
Very cohesive and non-flowing |
FF<2 |
5.2 Wall friction:
Measure resistances that exists between the powder and the process equipment’s surface. It also investigates if a powder will be adhering to the process equipment wall and other surfaces like packing material, capsules and sachets [28,29].
The
operating principle is much like the shear cell, where a test coupon of the
material acts as the surface of equipment which will be sheared opposing the
powder bed. Data obtained will be plotted as shear stress vs. normal stress,
which will determine the Wall Friction Angle (
). More the value, higher the resistance between the wall
coupon and powder.
5.3 Hopper Design:
Automated Hopper design software which is in built in the powder rheometer utilises the data obtained from the shear and the wall friction tests. Hopper design algorithms are made to run to obtain suitable design within 3 hours. [26-29].
6. APPLICATIONS:
Based on the data interpretation obtained from various tests performed by the powder rheometer insights on the powder behaviour in different processing conditions can be obtained. Although compressibility doesn’t directly measure flowability, it relates to the conditions in storage hoppers and also roller compacter. Aeration test is all about the evaluation of cohesive forces between the particles, which relate to the conditions where the powder made to flow under gravity as seen in die filling during tabletting. Few more processes where cohesive forces play a vital role are during blending, flow into capsule or vial and inhalations. Permeability can also be used to understand powder flow in hopper storage and its outer flow, vacuum transfer, pneumatic transfer, vial filling and dry powder inhalations. In every process powder undergo stress because of consolidation which changes density, in such cases to make them flow it is important that the yield point is overcome. Consolidation stress can be faced at storage in hopper or barrel, through an IBC( Intermediate bulk Container)on the top of tablet press, handling and roller compactor. Shear test is such a test that provides information on the yield point of powders and help in understanding the flow when there are bridging, stoppings and blockages.
7. DISCUSSION AND CONCLUSION:
Existing powder characterization methods fail to produce the detailed results as these tests doesn’t mimic the processing conditions. Moreover, the compendial tests are operator handled, lack predictability and reproducibility. All established methods are designed in such a manner that they evaluate free-flowing powders with smaller particle size when compared to powders that are cohesive and also granular particles with large particle size. As seen in the measurement of angle of repose, powder has to flow through a funnel, cohesive powder will be made to flow mechanically causing variations that are unavoidable. In semi-quantitative tests like Tapped density, tap force cannot overcome the cohesive forces that exist between the particles leading to false values. Shear tests are generally carried out at higher stress rates for powders contrastingly the actual processing unit operations will be at a lower stress rate. Therefore, particle size and cohesive nature of the powders are the major drawbacks for compendial test methods.
With the advancement of powder rheometer these limitations can be overcome as the instrument is capable of evaluating both free-flowing and cohesive powders with varying particle size. Rheometer allows direct characterization of powders at various process and environmental factors by simulating the unit operation conditions through various range of tests. Finally, FT4 Powder rheometer is an instrument that can evaluate the powder behaviour in the process chain from raw material till a finished product with sensitivity, predictability and reproducibility.
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Received on 13.07.2019 Modified on 18.08.2019
Accepted on 20.09.2019 © RJPT All right reserved
Research J. Pharm. and Tech. 2019; 12(11):5536-5542.
DOI: 10.5958/0974-360X.2019.00960.0