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A shear cell to characterise internal friction in high-pressure compacted powder beds
75
Powder Technology, 65 (1991) 75-19
A shear cell to characterise compacted powder beds
internal friction in high-pressure
G. Butters*, S. A. Leng and A. F. Thomas* BP Chemicals, R & D Department, P.O. Box 21, Bo’ness Road, Grangemouth, Stirlingshire, FK3 9XH (U.K.)
Abstract Many processes submit bulk particulate materials to high pressure, for example, polymeric powders can be subjected to pressures of the order of 100 MPa in an extruder. However, methods to characterise the shear properties of powders at these high pressures have not been available. This paper describes a novel shear cell designed to measure the internal friction properties of powders at up to 100 MPa. The cell is
designed to induce movement of an annulus of the compacted powder in contact with the stationary central cylindrical core and the resulting shear stress is recorded. A key feature is that no bearings are involved in the shearing process. Coefficient of friction data measured at high pressure on a family of polymer powders show correlation with extrusion output, confirming the potential value of this high pressure shear cell in developing understanding of this and other processes involving powders at high pressure.
Introduction
Various designs of powder shear cell are established in the simulation of powder flow in silos and similar powder handling situations. The pressures involved are relatively low and often below 0.1 MPa. Schwedes [l] has published a useful comparison of the various designs of shear cell available. Their most common application is in Jenike’s silo design procedure [2], which is standard chemical engineering practice. There are, however, several powder processing situations where the pressures involved are beyond the scope of these established shear cells. One such example is the extrusion of a thermoplastic polymer powder, when the pressure involved in the extruder may exceed 100 MPa. Knowledge of the shear behaviour of the powders involved at these high pressures and the effects of speed, temperature and other parameters would be of obvious benefit in increasing understanding, design and control of these processes.
*Present address: Department of Chemical and Process Engineering, Heriot-Watt University, Edinburgh, U.K.
0032-5910/91/%3.50
There have been several unsuccessful attempts to design a shear cell to operate under these demanding conditions. Some workers have attempted to modify one of the low-pressure shear cells and one of the problems encountered has been in modifying the bearings to accommodate the high pressures. This paper describes the design and operation of a novel concept of shear cell geometry, which successfully operates over the pressure range 50100 MPa. Design and operation The concept
The cell design is shown in Fig. 1. After compaction to the required level, the central core of the compacted bed, between the upper and lower pistons remains stationary, while the surrounding annulus of compacted powder is moved upwards. This is achieved by moving the main body of the cell upwards over the stationary upper and lower pistons. A key feature of this design is that there are no bearings involved in the shearing process and the body of the cell can move with negligible metalto-metal friction.
0 Elsevier Sequoia/Printed in The Netherlands
76 LOAD APFtEJ BY MT!-iON
TOP PSTON 9-l
Houow
being sheared is quite different from the original powder. Once equilibrium has been reached, the main body of the cell is then raised hydraulically using a hollow ram cylinder and pump. Compaction and shear data are recorded from the Instron load cell and the pressure transducer. If r~ is the normal stress recorded on the pressure transducer and z is the shear stress obtained from the shear load, then the coefficient of internal friction p, is given by ,u = Z/O. Results
RAM
lMOvEsBOOYoFcaL
k!zYiF ToA-
Fig. 1. High-pressure
shear cell.
Practical and theoretical considerations The internal dimensions of the cell are Length Diameter Piston diameter
52 mm 27 mm 19 mm
The cell was constructed to withstand a pressure of 100 MPa. The studies described here were based upon high-density poly( ethylene) powders. Compaction of the powder is achieved by mounting the cell in an Instron Dynamic Tester and applying the compaction load via the top piston. A compaction procedure was developed whereby the cell is repeatedly compacted and topped up with more powder till full. The system is then allowed time to relax and reach equilibrium under load before the next stage of the operation, During this compaction procedure, the relaxation process is monitored using the pressure transducer reading. Relaxation to a steady plateau value typically requires 45 min and 1 h was routinely used to ensure complete relaxation. The importance of allowing the powder bed to relax to equilibrium is clear from the fact that the yield stress of solid poly(ethylene) is of the order of 20 MPa. Thus, once 20 MPa has been achieved during the compaction process and until the compaction stress has reached equilibrium, the material may deform. It should be noted that compaction under these high pressures can cause significant particle deformation so that the particle size, shape and structure of the material
Typical Instron load cell and shear cell pressure transducer readings for the loading and compaction process followed by the shear readings are illustrated in Fig. 2 (the long relaxation periods have been eliminated from this figure). An example of the original shear cell pressure transducer and Instron load cell data together with the derived coefficient of friction data are presented in Table 1. The coefficient of friction data are also shown graphically in Fig. 3. It has been estimated that the friction coefficients are measured to an experimental precision of approximately 10%. For a given set of nine determinations on one material, the variation of the results around the mean was found to be approximately 6%. Direct comparison of these data from the highpressure cell with equivalent data from a traditional low-pressure cell is difficult. Firstly, low-pressure cells at best cover the pressure range O-0.05 MPa compared to 50-100 MPa for the high-pressure cell so that considerable extrapolation is required. Secondly, low-pressure cells usually operate at the much lower shear speeds, typically less than 0.25 mm s-i compared with 1.5 mm SK’ for the high-pressure cell. Thirdly, the low-pressure shear data available on materials studied to date show only limited discrimination. However, it is interesting to note that the coefficient of friction values on the six powders measured at 50 MPa do rank in the same order as the coefficient of friction measured on a traditional low-pressure shear cell. A more critical comparison between data measured at low and high pressures should be attempted, but is not possible using the existing high-pressure cell. Removal and examination of the powder from the cell after shearing gives useful information about the testing procedure. It confirms that the shear plane is situated as expected and as shown in Fig. 1. Also, the compaction of the powder bed appears uniform throughout the cell. However, there is a need to improve the facility for removal and examination of the material.
71
Ist. COMPACTION
WSTRONLOAD CELLTRACE kN1
2nd. COWACTlON
3rd, COMPACtION
4th. COMPACTlON
20
IO
PRESSLM TRANSDUCER Wa)
oL=i=ii/~-0
I
2
3
0
I
23012301
2
3
07-E
Fig. 2. Illustration of shear cell traces.
TABLE 1. Raw experimental and derived data for poly(ethylene) powder A Instron load (kN)
Normal stress (MPa)
Area of shear (m*)
Derived shear stress (MPa)
Derived powder coefficient of friction
17.36 20.70 23.25 22.07 22.66 23.05 26.39
56.44 67.08 77.61 71.58 77.61 83.20 85.18
0.00253 0.00255 0.00254 0.00252 0.00259 0.00264 0.00269
6.862 8.118 9.154 8.758 8.149 8.731 9.810
0.1215 0.1210 0.1179 0.1224 0.1127 0.1049 0.1152
Validation and application High-pressure coefficient of friction and other data on a family of high-density, high-molecular weight poly(ethylene) powders are given in Table 2. These powders are similar in molecular weight and molecular structure and have a mean particle size of approximately 800 pm. There are differences in particle structure, which give rise to differences in bulk density, internal friction and other powder properties. These materials are of the type typically used in the production of large
hollow containers, which are normally extruded into finished product in an extruder having longitudinal grooves in the powder feed section of the barrel designed to enhance the output rate. The output rate data given in Table 2 were obtained for such an extruder. The authors have proposed that high output in a grooved feed zone extruder demands a powder capable of withstanding the pressures in the early stages of the feed zone without internal slippage in order that plug flow is initiated. The pressure involved in the early stages of the feed zone is
78 Shear Stress versus Normal Stress
Coefficient
versus 0.15
of Shear
Friction Stress
T
0.14 t 0.15 0.1
Normal
Stress
4
(MPa)
Normal
Stress
(MPe)
Fig. 3. Shear behaviour of a poly(ethylene) powder.
TABLE 2. High-pressure friction, density and extrusion output data on poly(ethylene) powders Powder
A B C D E F
Rate of change of coefficient of internal friction with compaction from SO- 100 MPa (GPa-‘)
Coefficient of friction at 50 MPa
-0.44 - 1.43 -1.27 -1.77 - 1.54 -0.73
0.127 0.135 0.145 0.152 0.147 0.126
estimated to be of the order of 50 MPa. Once plug flow has been established, it is postulated that the powder bed is required to break down under the higher pressures prevailing in the later stages of the grooved feed zone (estimated to be in excess of 100 MPa), leading to the onset of the fusion process. Therefore, what is required is a powder with a high coefficient of friction at pressures up to about 50 MPa, corresponding to the early stages of the feed section, and then a rapidly
Bulk density (kg m-?
508 483 510 448 465 466
Extrusion output rate (kg h-‘) Run 1
Run 2
137.7 155.8 136.4 143.0 145.5 123.7
129.3 157.0 138.3 144.5 145.6 118.6
falling coefficient at the higher pressures, corresponding to the later stages. Table 2 gives both the coefficient at 50 MPa and the rate of change at higher pressures. It is clear from these data that high output is favoured by a high coefficient at 50 MPa and by a rapidly falling coefficient as the pressure is increased between 50 and 100 MPa. High powder bulk density (measured by BS 2782 Part 5 1970) also favours high output rate and the correlation coefficient
79
LOAD AFFIJED BY NSTRON
TOP PISTON
I
I
/
TOP PBTON NSERT
THREADED RNG DOW3
PIN
Special design of piston with a movable central core to allow compaction over the entire area of the cell. - Greater distance of travel to allow measurement at lower pressures. - Higher operating pressure and compression load. - Larger cell capacity to accommodate materials of larger particle size (e.g., up to 4 mm particle diameter).
Conclusions
BOlTOM
FISTON
SPACER BLOCK
1 Fig. 4. Mark
&-BASE~=~ATE
2 high-pressure
shear cell.
relating output rate to these coefficient of friction variables and bulk density is 0.997. Thus, the high-pressure cell is seen to be capable of providing data relevant to the behaviour of the powder in this processing situation. The technique clearly has considerable potential as a tool to study the behaviour of polymer powder in this application.
Further development
Whilst’the validation and application results to date do show considerable promise, experience with the cell has identified design limitations which could be overcome. A proposed Mark 2 version is shown in Fig. 4. Some of the key features are - Screwed-on top to the cell for easier removal and examination of the sheared material.
Whilst only limited practical studies have been carried out to date, there are, nevertheless, clear indications that this novel design of shear cell is capable of producing meaningful powder friction data at pressures up to 100 MPa. Comparison with a traditional low-pressure cell shows agreement in terms of ranking the coefficient of friction of different powders. Absolute agreement cannot be confirmed with the present cell because of differences in shear rate and cell geometry. A family of high-density poly(ethylene) powders with similar molecular weight, but having significantly different extrusion processing output rates, has been characterised for high-pressure internal friction. The coefficient of friction data show a strong correlation with extrusion output, clearly demonstrating the potential of the shear cell as a tool to develop understanding of this extrusion process, which could lead to optimisation of the design of the powder and the process and hence maximise the extrusion output rate. It seems reasonable to conclude that this shear cell could be used effectively to study other processes in which powders are processed at similar high pressures. Whilst these initial results are encouraging, a number of design weaknesses have been identified which the Mark 2 has been designed to overcome.
References 1 J. Schwedes, Proc. 2nd Eur. Symp. on Particle Characterisation, Z&26/09/1979, Heinrich Schuster, Nuremberg, pp. 278-300. 2 A. W. Jenike, Bulletin No. 108, Utah Engineering Experimental Station, Salt Lake City, 1962.
Powder Technology, 65 (1991) 75-19
A shear cell to characterise compacted powder beds
internal friction in high-pressure
G. Butters*, S. A. Leng and A. F. Thomas* BP Chemicals, R & D Department, P.O. Box 21, Bo’ness Road, Grangemouth, Stirlingshire, FK3 9XH (U.K.)
Abstract Many processes submit bulk particulate materials to high pressure, for example, polymeric powders can be subjected to pressures of the order of 100 MPa in an extruder. However, methods to characterise the shear properties of powders at these high pressures have not been available. This paper describes a novel shear cell designed to measure the internal friction properties of powders at up to 100 MPa. The cell is
designed to induce movement of an annulus of the compacted powder in contact with the stationary central cylindrical core and the resulting shear stress is recorded. A key feature is that no bearings are involved in the shearing process. Coefficient of friction data measured at high pressure on a family of polymer powders show correlation with extrusion output, confirming the potential value of this high pressure shear cell in developing understanding of this and other processes involving powders at high pressure.
Introduction
Various designs of powder shear cell are established in the simulation of powder flow in silos and similar powder handling situations. The pressures involved are relatively low and often below 0.1 MPa. Schwedes [l] has published a useful comparison of the various designs of shear cell available. Their most common application is in Jenike’s silo design procedure [2], which is standard chemical engineering practice. There are, however, several powder processing situations where the pressures involved are beyond the scope of these established shear cells. One such example is the extrusion of a thermoplastic polymer powder, when the pressure involved in the extruder may exceed 100 MPa. Knowledge of the shear behaviour of the powders involved at these high pressures and the effects of speed, temperature and other parameters would be of obvious benefit in increasing understanding, design and control of these processes.
*Present address: Department of Chemical and Process Engineering, Heriot-Watt University, Edinburgh, U.K.
0032-5910/91/%3.50
There have been several unsuccessful attempts to design a shear cell to operate under these demanding conditions. Some workers have attempted to modify one of the low-pressure shear cells and one of the problems encountered has been in modifying the bearings to accommodate the high pressures. This paper describes the design and operation of a novel concept of shear cell geometry, which successfully operates over the pressure range 50100 MPa. Design and operation The concept
The cell design is shown in Fig. 1. After compaction to the required level, the central core of the compacted bed, between the upper and lower pistons remains stationary, while the surrounding annulus of compacted powder is moved upwards. This is achieved by moving the main body of the cell upwards over the stationary upper and lower pistons. A key feature of this design is that there are no bearings involved in the shearing process and the body of the cell can move with negligible metalto-metal friction.
0 Elsevier Sequoia/Printed in The Netherlands
76 LOAD APFtEJ BY MT!-iON
TOP PSTON 9-l
Houow
being sheared is quite different from the original powder. Once equilibrium has been reached, the main body of the cell is then raised hydraulically using a hollow ram cylinder and pump. Compaction and shear data are recorded from the Instron load cell and the pressure transducer. If r~ is the normal stress recorded on the pressure transducer and z is the shear stress obtained from the shear load, then the coefficient of internal friction p, is given by ,u = Z/O. Results
RAM
lMOvEsBOOYoFcaL
k!zYiF ToA-
Fig. 1. High-pressure
shear cell.
Practical and theoretical considerations The internal dimensions of the cell are Length Diameter Piston diameter
52 mm 27 mm 19 mm
The cell was constructed to withstand a pressure of 100 MPa. The studies described here were based upon high-density poly( ethylene) powders. Compaction of the powder is achieved by mounting the cell in an Instron Dynamic Tester and applying the compaction load via the top piston. A compaction procedure was developed whereby the cell is repeatedly compacted and topped up with more powder till full. The system is then allowed time to relax and reach equilibrium under load before the next stage of the operation, During this compaction procedure, the relaxation process is monitored using the pressure transducer reading. Relaxation to a steady plateau value typically requires 45 min and 1 h was routinely used to ensure complete relaxation. The importance of allowing the powder bed to relax to equilibrium is clear from the fact that the yield stress of solid poly(ethylene) is of the order of 20 MPa. Thus, once 20 MPa has been achieved during the compaction process and until the compaction stress has reached equilibrium, the material may deform. It should be noted that compaction under these high pressures can cause significant particle deformation so that the particle size, shape and structure of the material
Typical Instron load cell and shear cell pressure transducer readings for the loading and compaction process followed by the shear readings are illustrated in Fig. 2 (the long relaxation periods have been eliminated from this figure). An example of the original shear cell pressure transducer and Instron load cell data together with the derived coefficient of friction data are presented in Table 1. The coefficient of friction data are also shown graphically in Fig. 3. It has been estimated that the friction coefficients are measured to an experimental precision of approximately 10%. For a given set of nine determinations on one material, the variation of the results around the mean was found to be approximately 6%. Direct comparison of these data from the highpressure cell with equivalent data from a traditional low-pressure cell is difficult. Firstly, low-pressure cells at best cover the pressure range O-0.05 MPa compared to 50-100 MPa for the high-pressure cell so that considerable extrapolation is required. Secondly, low-pressure cells usually operate at the much lower shear speeds, typically less than 0.25 mm s-i compared with 1.5 mm SK’ for the high-pressure cell. Thirdly, the low-pressure shear data available on materials studied to date show only limited discrimination. However, it is interesting to note that the coefficient of friction values on the six powders measured at 50 MPa do rank in the same order as the coefficient of friction measured on a traditional low-pressure shear cell. A more critical comparison between data measured at low and high pressures should be attempted, but is not possible using the existing high-pressure cell. Removal and examination of the powder from the cell after shearing gives useful information about the testing procedure. It confirms that the shear plane is situated as expected and as shown in Fig. 1. Also, the compaction of the powder bed appears uniform throughout the cell. However, there is a need to improve the facility for removal and examination of the material.
71
Ist. COMPACTION
WSTRONLOAD CELLTRACE kN1
2nd. COWACTlON
3rd, COMPACtION
4th. COMPACTlON
20
IO
PRESSLM TRANSDUCER Wa)
oL=i=ii/~-0
I
2
3
0
I
23012301
2
3
07-E
Fig. 2. Illustration of shear cell traces.
TABLE 1. Raw experimental and derived data for poly(ethylene) powder A Instron load (kN)
Normal stress (MPa)
Area of shear (m*)
Derived shear stress (MPa)
Derived powder coefficient of friction
17.36 20.70 23.25 22.07 22.66 23.05 26.39
56.44 67.08 77.61 71.58 77.61 83.20 85.18
0.00253 0.00255 0.00254 0.00252 0.00259 0.00264 0.00269
6.862 8.118 9.154 8.758 8.149 8.731 9.810
0.1215 0.1210 0.1179 0.1224 0.1127 0.1049 0.1152
Validation and application High-pressure coefficient of friction and other data on a family of high-density, high-molecular weight poly(ethylene) powders are given in Table 2. These powders are similar in molecular weight and molecular structure and have a mean particle size of approximately 800 pm. There are differences in particle structure, which give rise to differences in bulk density, internal friction and other powder properties. These materials are of the type typically used in the production of large
hollow containers, which are normally extruded into finished product in an extruder having longitudinal grooves in the powder feed section of the barrel designed to enhance the output rate. The output rate data given in Table 2 were obtained for such an extruder. The authors have proposed that high output in a grooved feed zone extruder demands a powder capable of withstanding the pressures in the early stages of the feed zone without internal slippage in order that plug flow is initiated. The pressure involved in the early stages of the feed zone is
78 Shear Stress versus Normal Stress
Coefficient
versus 0.15
of Shear
Friction Stress
T
0.14 t 0.15 0.1
Normal
Stress
4
(MPa)
Normal
Stress
(MPe)
Fig. 3. Shear behaviour of a poly(ethylene) powder.
TABLE 2. High-pressure friction, density and extrusion output data on poly(ethylene) powders Powder
A B C D E F
Rate of change of coefficient of internal friction with compaction from SO- 100 MPa (GPa-‘)
Coefficient of friction at 50 MPa
-0.44 - 1.43 -1.27 -1.77 - 1.54 -0.73
0.127 0.135 0.145 0.152 0.147 0.126
estimated to be of the order of 50 MPa. Once plug flow has been established, it is postulated that the powder bed is required to break down under the higher pressures prevailing in the later stages of the grooved feed zone (estimated to be in excess of 100 MPa), leading to the onset of the fusion process. Therefore, what is required is a powder with a high coefficient of friction at pressures up to about 50 MPa, corresponding to the early stages of the feed section, and then a rapidly
Bulk density (kg m-?
508 483 510 448 465 466
Extrusion output rate (kg h-‘) Run 1
Run 2
137.7 155.8 136.4 143.0 145.5 123.7
129.3 157.0 138.3 144.5 145.6 118.6
falling coefficient at the higher pressures, corresponding to the later stages. Table 2 gives both the coefficient at 50 MPa and the rate of change at higher pressures. It is clear from these data that high output is favoured by a high coefficient at 50 MPa and by a rapidly falling coefficient as the pressure is increased between 50 and 100 MPa. High powder bulk density (measured by BS 2782 Part 5 1970) also favours high output rate and the correlation coefficient
79
LOAD AFFIJED BY NSTRON
TOP PISTON
I
I
/
TOP PBTON NSERT
THREADED RNG DOW3
PIN
Special design of piston with a movable central core to allow compaction over the entire area of the cell. - Greater distance of travel to allow measurement at lower pressures. - Higher operating pressure and compression load. - Larger cell capacity to accommodate materials of larger particle size (e.g., up to 4 mm particle diameter).
Conclusions
BOlTOM
FISTON
SPACER BLOCK
1 Fig. 4. Mark
&-BASE~=~ATE
2 high-pressure
shear cell.
relating output rate to these coefficient of friction variables and bulk density is 0.997. Thus, the high-pressure cell is seen to be capable of providing data relevant to the behaviour of the powder in this processing situation. The technique clearly has considerable potential as a tool to study the behaviour of polymer powder in this application.
Further development
Whilst’the validation and application results to date do show considerable promise, experience with the cell has identified design limitations which could be overcome. A proposed Mark 2 version is shown in Fig. 4. Some of the key features are - Screwed-on top to the cell for easier removal and examination of the sheared material.
Whilst only limited practical studies have been carried out to date, there are, nevertheless, clear indications that this novel design of shear cell is capable of producing meaningful powder friction data at pressures up to 100 MPa. Comparison with a traditional low-pressure cell shows agreement in terms of ranking the coefficient of friction of different powders. Absolute agreement cannot be confirmed with the present cell because of differences in shear rate and cell geometry. A family of high-density poly(ethylene) powders with similar molecular weight, but having significantly different extrusion processing output rates, has been characterised for high-pressure internal friction. The coefficient of friction data show a strong correlation with extrusion output, clearly demonstrating the potential of the shear cell as a tool to develop understanding of this extrusion process, which could lead to optimisation of the design of the powder and the process and hence maximise the extrusion output rate. It seems reasonable to conclude that this shear cell could be used effectively to study other processes in which powders are processed at similar high pressures. Whilst these initial results are encouraging, a number of design weaknesses have been identified which the Mark 2 has been designed to overcome.
References 1 J. Schwedes, Proc. 2nd Eur. Symp. on Particle Characterisation, Z&26/09/1979, Heinrich Schuster, Nuremberg, pp. 278-300. 2 A. W. Jenike, Bulletin No. 108, Utah Engineering Experimental Station, Salt Lake City, 1962.