POWDER FLOW. Prepared by: Dr. Geeta Patel

Transcription

1 POWDER FLOW Prepared by: Dr. Geeta Patel 1

2 Significance for including free-flowing powders Powdres are generally considered to be composed of solid particles of the same or different chemical compositions having diameters <1000 µm. Pharmaceutically, the largest use of powders is to produce tablets and capsules, in addition to mixing and compression properties, the flowability of a powder is of critical importance in production. Success or failure in many pharmaceutical operations can be directly linked to the flow properties of the powder being processed. Flowability is critically important when assessing how material moves around the plant. Industry needs the ability to develop formulations with tailored flowability, and first step is identifying suitable techniques for powder characterization. 2

3 Significance for including free-flowing powders 1. Uniform feed from bulk storage container or hoppers into feed mechanisms of tableting or capsule-filling machine. 2. Uniform particle packing and a constant volume/mass ratio which maintains tablet weight uniformity. 3. Reproducible filling of tablets dies and capsule body, which improves weight uniformity and uniform physico-mechanical properties of tablets. 4. Uneven flow leads to excess air entrapment within powders, which may produce capping or lamination problems in tablets, also increase particle die-wall friction, causing lubrication problems, and increase dust contamination risks during powder transfer. 3

4 Capping tablet (Top) and Laminated tablet (Right) 4

5 Factors influencing the flow of powders Under stress condition powder can flow like a liquids, they don t flow if the stresses are to small. Many manufacturing problems are attributed to powder flow, including non-uniformity (segregation) in blending, under-or0over dosage, inaccurate filling, and stoppages. Storage, handling, production, packing, distribution and end use can all be negatively affected by common powder flow problems. The factors associated with the nature of the particles and their surface area. Particle size, particle size distribution & specific area. Particle shape Moisture content Adhesion and cohesion 5

6 Particle size, particle size distribution & specific area All matter interacts, as the dimensions of particles increase, the forces acting on them change. With relatively small particles, the flow through an orifice may be restricted b cos the cohesive forces between the particles are of the same magnitude as gravitational forces. Particle size - <100 µm acted upon primarily by surface force and >1000 µm governed by gravitational force Balance of interaction forces determines powder behavior. Fine particles (<74 µm) with very high surface area are more cohesive than coarse particles which are influenced more by gravitational forces. Particles > 250 µm free flowing, but as the size fall below 100 µm powders become cohesive and flow problems are likely to occur. 6

7 Particle size, particle size distribution & specific area Very fine particles (<10 µm) - extremely cohesive and resist flow under gravity, except possibly as large agglomerates. PS has been systemically investigated flat-bottom meter. Particle shape Irregular shapes can have markedly different flow properties due to difference contact areas. A group of spheres has minimum interparticle contact and generally optimum flowability, whereas flakes or irregular particles have a very high surface area and poor flow properties. 7

8 8

9 Moisture content Absorbed moisture in solids can exist either in the unbound state or as part of crystal structure. Its effect directly change surface properties of the particles. It can also affect flow properties indirectly and permanently through the granules formulation, which are held together by solid bridges generated by hydration and dehydration. At higher moisture content and higher packing densities liquid bridges may progress. The effect of moisture varies, depending on the degree of packing or the porosity of the powder bed. In a porous and cohesive material, flowability is not affected by moisture because the moisture can penetrate to the inside of particle without the formation of liquid bridge. 9

10 Packing property A set of particles can be filled into a volume of space to produce a powder bed which is in static equilibrium due to he interaction of gravitational and adhesive/cohesive forces. The change in bulk volume has been produced by rearrangement of the packing geometry of the particles. In general, such geometric rearrangements result in a transition from loosely packed to more tightly pack. More tightly packed powders require a higher driving force to produce powder flow than more loosely packed particles. 10

11 Adhesion and cohesion Cohesion occurs between like surfaces, such as component particles of a bulk solid, whereas adhesion occurs between two unlike surfaces like between a particle and a hopper wall. Cohesion and other particle properties affect flow property of powder. Vander Waals, surface tension and electrostatic forces are important properties related with powder flow. Adhesive/cohesive forces acting between a single pair of particle and substrate can be accurately determined using a ultracentrifuge to apply very high forces strong enough to separate the two surfaces. 11

12 Figure : Example of (a) free flowing and (b) weakly cohesive powder blends 12

13 Surface charges Higher the electrostatic charges, poor the flow. Friction generates electrostatic charges, so minimize the friction to reduce the electrostatic charges. Humidity Relative humidity of the air (interstitial as well as head space) in a storage container, such as a bin or silo, also affects properties of bulk materials. Many bulk materials are hygroscopic and thus the expose to humid conditions results in increased moisture content of the bulk. This can leads to an increased in bulk strength and also to an increase in angle of repose, flowability and cohesiveness of granular powder. 13

14 Temperature Temperature also has a sustainable effect on bulk solid flowability. The most drastic temperature effect is the freezing of the moisture contained within the granular materials and on particle surfaces. The resulting ice bonds weaken the flow. However, the temperature from 30 to 40 C does not usually have a great impact on powder flowability; if there is the component having melting point exceeds its glass transition temperature. Pressure Compacting pressure is also an important factor that affects the flow properties of bulk solids. The increased pressure leads to a larger number of larger contact points between particles thus causing more inter-particle adhesion and increased compaction produces a significant increase in critical arching dimensions. 14

15 Measurement of powder flowability It is useful to be able to quantify the type of behavior of powder. Various methods are - Direct using dynamic or kinetic methods. - Indirect measurements on static bed. Indirect Methods Angle of Repose (AR) It is indirect method used in many branches of science to quantify powder flowability. AR is a characteristic related to interparticulate friction or resistance to movement b/w particles. Many methods may produce different values for the same powder despite its difficulties, the method continues to be used in Pharma. industry. 13

16 Measurement of powder flowability Basic Methods for AR :- The AR measured by keeping the powder static bed condition is called static angle of repose and the AR if measured by keeping the powder in motion iscalled dynamic angle of repose. Dynamic AR is preferred measurement as it is better correlated with the actual tablet or capsule manufacturing, in which powder is usually in motion. Static AR can be classified two experimental variables 1. The height of the funnel may be fixed relative to the base or the height may be varied as the pile forms. 14

17 Measurement of powder flowability 2. The base may be of fixed diameter or the diameter of the powder cone may be allowed to vary as the pile forms. Variations in AR Methods Following variations have been used to some extent 1. Drained AR is determined by allowing an excess quantity of material. Formation of the cone of powder on the fixed diameter base allows determination of the drained AR. 2. Dynamic AR is determined by filling a cylinder (Flat bottom) and rotating it at a specified speed. It is the angle (relative to the horizontal plane) formed by the flowing powder. 15

18 Various Methods for Measurement of AR 1. Funnel Method (Static AR) Allow the material to flow through a funnel or orifice on to a horizontal surface below. The angle of conical heap so formed can be determined from simple geometry. Figure Fixed Funnel Method 16

19 17

20 Measurement of powder flowability Drawbacks :- It s suitable only for free flowing powders. It doesn't give reproducible results, since the cone shape is distorted by the impact of the particles. 2. Tilting Box Method (Dynamic AR) A sandpaper lined rectangular box is fixed with the powder and carefully tilted until the contents begin to slide. The repose angle is the angle formed by surface of the box with the horizontal plane. 18

21 Various Methods for Measurement of AR Figure Titling Box Method 3. Rotating Cylinder Method (Dynamic AR) A hollow cylinder half-filled with the test powder with one end sealed by transparent plate. Rotate it on horizontal plane, until the powder surface cascades. 19

22 Various Methods for Measurement of AR Figure Rotating Cylinder Method The curved wall is lined with sandpaper to prevent slippage. The angle formed by cascading line with the horizontal plane is the AR. 20

23 Figure : Dynamic angle of repose instrument 21

24 Various Methods for Measurement of AR 4. Drained AR It is obtained with flow devices like rectangular vessel, cubic vessel or cubical box containing circular disc platform. Drained AR is usually larger than the poured AR for the same powder. a) Ledge Method :- A rectangular vessel (ledge), which contains a movable or sliding shutter at the bottom side. A vessel is filled with the sample to be tested. Then the shutter is opened to allow the drainage of material itself. 22

25 Various Methods for Measurement of AR Figure Ledge Method The angle formed by material left in the ledge with horizontal plane is know as drained AR. 23

26 Various Methods for Measurement of AR b) Crater Method :- A cubic vessel, which contains a movable or sliding shutter at the center of the bottom. A vessel is filled with the sample to be tested. Then the shutter is opened to allow the drainage of material itself. Figure Crater Method 24

27 Various Methods for Measurement of AR The angle formed by material left in the cubic vessel with horizontal plane is know as drained AR. c) Platform Method :- A large container with built-in-platform is used. The bottom of the container is provided with a sliding shutter. The container is filled with the sample to be tested. Then the shutter is opened and the material is allow to flow out at the bottom, leaving an undisturbed conical heap on the platform. 25

28 Various Methods for Measurement of AR This method eliminates wall friction from the measurement and also avoids cone distortion due to falling particles. Figure Platform Method 26

29 Experimental consideration for AR AR is not an intrinsic property of powder, very much dependent upon type of method. 1. The cone peak of the powder can be distorted by the impact of powder from above. 2. The nature of base upon which the powder cone is formed influences the AR. Recommended, cone be formed on a common base. This can be done by using a base of fixed diameter with a protruding outer edge to retain a layer of powder. 27

30 Pharmacopoeial specifications Angle of Repose (AR) The angle of repose is the constant, three dimensional angle (relative to the horizontal base) assumed by a cone like pile of material formed by any of several methods discuss below. If more material is added to the pile, it slides down the slides until the common friction of the particles producing a surface at an angle is in equilibrium with the gravitational force. The tangent of the angle of repose is equal to the coefficient of friction between the particles. 30

31 tan = ; = tan -1 Or we can write as: Where, h = height of the pile r = radius of base of pile = angle of repose 31

32 Factors affecting angle of repose Rough and irregular surface of the particles give higher angle of repose. Decrease in particle size leads to higher angle of repose. Lubricants at low concentration decreases angle of repose whereas high concentration increases angle of repose. So, optimum concentration of lubricants required to maintain angle of repose for good powder flow. Fines increases angle of repose. 32

33 Angle of Repose (AR) Table : Types of flow properties and corresponding AR Flow property Angle of Repose ( ) Excellent 25 to 30 Good 31 to 35 Fair 36 to 40 Passable may hang up 41 to 45 Poor must agitate, vibrate 46 to 55 Very poor 56 to 65 Very, very poor > 66 50

34 28 Official procedure for density measurement (USP) USP recommends two methods Method-I Measurement in a Graduated Cylinder Method-II Measurement in a Volumeter Apparatus:

35 Official procedure for density measurement (USP) Procedure: Allow an excess of powder to flow through the apparatus into the sample receiving cup until its overflows (using a minimum 25 ml powder with square cup, 35 ml cylindrical cup). Carefully scrape excess powder from the top of the cup (smoothly spatula). Take care to keep the spatula perpendicular to prevent packing or removal of powder. Determine the weight, M of the powder to nearest 0.1%. Calculate the bulk density by the formula : M/V O. In which V o is the volume in ml of the cup. 29

36 Tapped density measurement (Official) Procedure: Mechanically tapping the measuring cylinder containing powder. After observing the initial volume, the cylinder mechanically tapped and volume reading are taken until little further volume change is observed. Tapping is achieved by raising the cylinder and allowing it to drop under its own weight a specified distance. Two methods. Method I : Manual Tapping Method II: Tapped density tester (BP) 30

37 Method-I- Tapped density measurement Procedure: Unless otherwise specified, pass a quantity of material through a 1.0 mm (No. 18). Into a dry 250 ml glass graduated cylinder (readable to 2 ml) weighing 220±44 gm & mounted on a holder weighing 450±10 gm, approximately 100 gm of test sample, M, weighed with 0.1% accuracy. If not possible, the amount of the test sample may be reduced and the volume of the cylinder may be modified. Carefully level the powder without compacting, if necessary and read the unsettled apparent volume, Vo. 31

38 Method-I- Tapped density measurement Mechanically tap the cylinder, using a suitable tapped density tester that provides a fixed drop of 14±2 mm at a nominal rate of 300 drops/min. Unless otherwise specified, tap the cylinder 500 times, measure tapped volume, V a. Repeat the tapping an additional 750 times and measure tapped volume, V b. If the difference between two volumes is < 2%, V b is the final tapped volume, V f. Repeat in increments of 1250 taps, until the difference between succeeding measurements <2%. Calculate the tapped density M / V f (gm/ml). Generally replicate determinations. 32

39 Method-II Tapped density tester (BP) It provides a fixed drop of 3 mm±10 % at a nominal rate of 250 drops/ min. Apparatus: Consists of the following components. 1. A settling apparatus, capable of producing 205 ± 15 taps in 1 min from a height of 3 ± 0.2 mm. 2. The support for the graduate cylinder, with its holder that has a mass of 450 ± 5 gm. 3. A 250 ml graduate cylinder with a mass of 220 ± 40 gm. Method : Into a dry cylinder, pour 100 gm (M) sample. If not possible, select a test sample with an apparent volume between 50 & 250 ml. Secure the cylinder in its holder, read the unsettled apparent volume (V o ). Carry out 10, 500, and 1250 taps and read the corresponding volumes V 10, V 500 and V

40 34

41 Method-II Tapped density tester (BP) If the difference b/w V 500 and V 1250 is greater than 2 ml, carry out another 1250 taps. Expression of results: Apparent volumes: Apparent volume before settling or bulk volume is Vo in ml and apparent volume after settling is V 1250 and V Ability to settle: It is the difference b/w V 10 & V 500 (ml). Apparent densities: Apparent density before settling or bulk density is M / V 0 (gm/ml) and apparent density after settling or tapped density is M / V 1250 or M / V 2500 (gm/ml). Marketed Equipments Tapped density tester from Electrolab ETD-1020 Tapped density tester from Quantachrome Tapped density tester from Varian 35

42 Hausner s Ratio (HR) It is used to measure both bulk volume and tapped volume of powder. Where, V 0 = unsettled apparent volume and V f = final tapped volume It can also be measured in terms of density. Where, tapped = bulk density bulk = tapped density 42

43 Carr s Compressibility Index (CI) This property of powder is also known as compressibility or Carr s consolidation index. It is simple, fast and popular method of predicting powder flow characteristics. It is an indirect measure of bulk density, size, shape, surface area, moisture content and cohesiveness of material because all of this can influence observed CI. It is used to measure bulk volume and tapped volume of powder. 43

44 Where, V 0 = unsettled apparent volume V f = final tapped volume It can also be measured in terms of density. Where, tapped = bulk density bulk = tapped density 44

45 Pharmacopoeial specifications Compressibility Index & Hausner ratio Table : Scale of flowability Carr s Index (%) Flow property Hausner ratio 10 Excellent 1 to to 15 Good 1.12 to t0 20 Fair 1.19 to to 25 Passable 1.26 to to 31 Poor 1.35 to to 37 Very poor 1.46 to 1.59 > 38 Very, very poor >

46 36

47 Flow through an orifice Monitoring the flow rate of material through an orifice has been proposed as a better measure of powder flowability. Flow through an orifice useful - for monitoring flow continuously b cos pulsating flow patterns have been observed for free flowing material. - Changes in flow rate can also be observed. - Empirical equations have been determined (opening diameter, particle size and density). - The flow rate is generally measured as the mass/flowing time from any types of containers (cylinders, funnels, hoppers). 37

48 Basic Methods for flow through an orifice Flow rate through an orifice can be classified.. 1. The type of container used. 2. The size and shape of the orifice used. 3. The method of measuring powder flow rate. It can be measured continuously using an electronic balance with some sort of recording device (computer camera) or in discrete samples, example the time taken for 100 gm of powder or the amount of powder passing in 10 sec. 38

49 Funnel method for flow rate measurement (BP) Apparatus: The funnel with or without stem, with different angles and orifice diameters are used. The funnel is maintained upright. The assembly must be protected from vibrations. 39

50 Funnel method for flow rate measurement (BP) Method: Introduce a test sample weighed with 0.5% accuracy in to a dry funnel, The bottom opening of which has been blocked by suitable means, without compacting the sample. Unblock the bottom opening of the funnel and measure the time required for entire sample to flow out. Expression of results: The flowability is expressed in seconds per 100 gm of sample. Variations in methods Either mass flow rate or volume flow rate can be determined, but it biases the results in favor of high-density materials. A vibrator is occasionally attached to facilitate flow from container; however this appears to complicate prediction of the results. 40

51 Direct method for powder flow measurement Hopper flow rate: The simplest techniques, to measure the rate at which powder discharges from a hopper. The shutter is placed over the hopper outlet and the hopper is filled with powder. The shutter is then removed and the time taken for te powder to discharge completely is recorded. By dividing the discharged powder mass by time, a flow rate is obtained. Hopper or discharge tube outlets should be selected to provide a good model for a particular flow application. Example If a powder discharges well from a hopper into a tablet machine feed frame, but does not flow reproducibly into the tablet die. 41

52 Recording flowmeter A recording flowmeter is essentially similar to the previous method except that powder is allowed to discharge from hopper or container on to a balance. In case of analogue balances a chart recorder is used to produce a permanent record of the increase in powder mass with time. Recording flowmeters allow mass flow rates to be determined and also provide a means of quantifying uniformity of flow. A. Hosokawa powder characteristics tester B. Aero-flow 42

53 43

54 Improvement of powder flowability A. Alteration of particle size & size distribution Coarse particles are less cohesive than fine particles and an optimum size for free flow exists. The flowability problem can be solved by removing a proportion of the fine particle fraction or by increasing the proportion of coarser particles. B. Alteration of particle Shape and texture Spherical particles better flow properties than more irregular particles. Spray-drying can be used (spray dried lactose). Temp cycling crystallization. Very rough surface will be more cohesive and greater tendency to interlock than smooth-surfaced particles. Both controlled by crystallization and granulation. 44

55 55

56 Improvement of powder flowability C. Alteration of surface forces Reduction of electrostatic charges can be achieved by reducing frictional contacts or altering process conditions. Electrostatic charges can be prevented ordicharged by efficient earth connections. Moisture content as absorbed surface moisture films tend to increase bulk density and reduce porosity. In cases where moisture content is excessive, powder should be dried and if hygroscopic stored properly. 46

57 Improvement of powder flowability D. Flow enhancers or flow promoters:glidants Small amount of glidant is often used. Like talc, corn starch, silicon dioxide and colloidal silica (Cab-O-Sil, Aerosil). Praposed mechanism for glidant action Distribution of glidant in the host particles. Dispersion of static charges from the host paticles surface. Preferential adsorption of gases and moisture. Physical separation of particles and subsequent reduction in Van-der walls interaction. Reduce friction between granules and surface roughness is minimized. 47

58 Improvement of powder flowability E. Alteration of process conditions Use of vibration-assisted hoppers In cases where the powder arch strength within a bin or hopper is greater than the stresses in it, due to gravitational effects, powder flow will be broken up or prohibited. The poor powder flow may result because of either rate holing or arching/bridging that may take place. 48

59 Improvement of powder flowability Powder flow can be encouraged by adding stresses due to gravitational interactions by vibrating the hopper mechanically. Use offorce feeders The powder discharge irregularly or flood out can be improved by fitting vibrating baffles, known as live-bottom feeders, at the base of the conical section within a hopper. Force feeders are usually made up of a single or two counter-rotating paddles at the base of the hopper just above the die table in place of feed frame. The paddles presumably act by preventing powder arching over dies, improve die filling at high speeds. 49

60 60