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Determination of pigment dispersion microstructure by magnetic measurements
Journal of Magnetism and Magnetic Materials 155 (1996) 120-122
Journalof -,7-" magnetism and magnetic ~ H materials
ELSEVIER
Determination of pigment dispersion microstructure by magnetic measurements S.J. Greaves *, K. O'Grady Magnetic Materials Research Group, SEECS, UCNW, Bangor, Gwynedd LL57 I UT, UK
Abstract Magnetic measurement techniques have been used to determine the microstructure of metal particle dispersions with the eventual aim being to find a method of assessing dispersion quality during the manufacturing process of magnetic tape. We find that there are three processes which contribute to the magnetisation processes in a dispersion and that the effect of milling is to produce a more uniform dispersion.
I. Introduction
The assessment of dispersion quality is of major importance in the tape manufacturing industry. Magnetic measurements of the coercivity and remanence show only small changes with dispersion quality and affect the dispersion microstructure. In addition to single particles the dispersion consists of: aggregates of single particles, these are tightly bound and difficult to break down, agglomerates, which are collections of aggregates formed whilst the pigment is dry; and flocculates, groups of aggregates formed once the pigment has been wetted. Current measurements to determine the state of a dispersion rely on the observation of the gloss of a trial coating or microscopic observation of the agglomerates present [ 1]. These methods lack accuracy and are subjective in nature. Measurements of isothermal remanence magnetisation (IRM) curves have been made in which a field is applied and then removed and the remanence measured [2]. Beginning with small fields the dispersion microstructure, in terms of the number of agglomerates present, should remain unaffected. Furthermore, at low fields the changes in remanence brought about by the applied field are due to rotation of particles and aggregates in the dispersion. In well-dispersed high coercivity pigment systems, e.g. metal particles, it has been shown previously that the magnetisation mechanism is generally one of pure physical rotation [1]. The differential of the isothermal remanence curve may then he used to determine the number and size of aggregates in the dispersion [3]. The IRM curve shown in Fig. 1 depicts how the remanence of a dispersion is
acquired. At low fields there is a distortion of flocculates and networks with the result that they acquire a net magnetic moment. At slightly higher fields it is the rotation of single particles and small aggregates that provide the remanence as they align with the applied field. This process is generally reversible, when the field is removed the magnetisation will relax on a moderate time scale. The next stage is when switching of the moments in the particles results in an increase in the remanence, this is an irreversible process and relaxation is now much slower. Some of the flocculates may also change shape in the large field. The reversal of aggregates by the applied field, at fields below 1 kOe, is mainly due to rotation, the probability of reversal by switching being very low. A model of aggregate rotation has been used by Mayo and O'Grady [3] to interpret IRM curves in terms of the average size and standard deviation of the aggregates in the dispersion. The
M/M s 0.8 0.6 Reversal by "
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 6 7 3 - 7
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* Corresponding author.
s
Rotation of aggregates and particles
Distortion o f flocculates and networks 0
1
2 3 4 Applied Field (kOe)
Fig. I. An IRM curve of a dispersion.
S.J. Great'es, K. O'Grad3"/Journal of Magnetism and Magnetic Materials 155 (1996) 120 122
equation describing the motion of the aggregates in the applied field is given by
l(t)=l~(~)f, I-exp(-t/'r~lf(y)dy,
(11
where t is the measurement time, r ± the torque acting on the aggregates, l,.(~c) the remanence after saturation of the dispersion and f ( y ) the log-normal distribution of aggregate sizes. The degree of dispersion of a sample is also reflected in the behaviour of the magnetisation during time-dependence measurements. Well-dispersed samples result in time-dependence curves due to rotation of single particles and aggregates, whereas the convoluted phenomena of rotation, alignment within agglomerates and change in the shape of the agglomerates contribute to the time dependence of the less well dispersed samples.
2. Experiment Samples of metal pigment dispersion were prepared according to a commercial formulation. Samples were obtained at three stages; after initial mixing of components (Premix), after milling for four hours in a recirculating high shear mill (Milled) and after the addition of extra solvents and resin in order to reduce the dispersion viscosity (Letdown). Magnetic measurements were made using a VSM. Time-dependence measurements were made on dispersion samples which had not previously been magnetised. A field was applied for 15 min during which time the magnetisation of the sample was monitored. After the measurement the relaxation of magnetisation in zero field was studied for a further 15 rain.
3. Results IRM differentials for premix, milled and letdown samples are shown in Fig. 2. We see that the main peak in the
Zirr
1.0
,~ 3.2
0.8
~lbO • /
0.6
i •
•
•
~ \
/
•
v 0., []
v Letdown o IV~ll [] l ~ r ~
• \
.
• Tape .
.
.
0.4 0.2
0
1
2 3 4 Applied Field (kOe)
Fig. 2. IRM diflerentials for the three dispersions.
5
121
M/M 0 105
0
0
0 090
~"
" 2nd Measurement
lstMeasuremenl _ f~i
:1I
....
0.060 ~
"
~
~
0.045 L~ ~-~°~. . . .
0.030 [. . . .
10
_ ~
:
~
"rime(s)
100
~ -~-~
1000
Fig. 3. Time-dependence curves for the letdown sample. H = 10 Oe. letdown curve occurs at a very low field (about 80 Oe). This is due to rotation of the particles and aggregates and there is hardly any switching. There is a second small, broad peak centred around 1800 Oe which is attributable to switching of moments in agglomerates. The two peaks are distinct and do not overlap. For the premix and milled samples the difference is less pronounced with two convoluted peaks corresponding to reversal by rotation and switching. As expected, the IRM differential for the tape sample shows only one peak due to switching. Its position is slightly lower than that of the dispersion switching peaks, this is presumably because the dispersion aggregates are aligned by the lower applied fields in the measurement, whereas the tape microstructure is fixed. In the tape the particles are also more closely packed and so interactions could also be a factor. The position of the first peak in the IRM differential is to a large extent determined by the time the applied field is switched on, and the time after removal of the applied field at which the remanence measurement is made. As an example, Fig. 3 shows how the magnetisation of the letdown dispersion sample varies with time in an applied field of just 10 Oe. At the start of the first measurement at t = 0, m = 0. The magnetisation jumps rapidly in the first second after application of the field to about half of its value after 900 s. There is a large change in magnetisation over the measuring period both with the field applied and once it has been removed. Two measurements were made with this sample, which had not previously been magnetised. The second measurement was made immediately after the first and shows that once the dispersion has been subjected to a magnetic field it is much easier to remagnetise than from the initial state. There is also some degree of irreversibility in the magnetisation process, the magnitude of the irreversibility is related to the size of the applied field. We may explain this irreversibility in terms of the deformation of flocculates and structures prior to rotation taking place. Once deformed the change to the dispersion microstructure is permanent. Thus the relaxation curves in
122
S.J. Greaves, K. 0 'Grady / Journal of Magnetism and Magnetic Materials 155 (1996) 120-122
MIRM/'/VIIRMMAX 0.5
0.553 for the premix sample to 0.373 for the milled sample and 0.116 for the letdown sample. The average aggregate size remains almost unchanged. Thus the effect of the milling is to produce a more uniform dispersion. The viscosity of the letdown dispersion, reduced by the addition of extra solvents, is also found to decrease in the theoretical fits between the mill and letdown by 30%.
---
0.4 0.3 0.2 O. l
0
'7
0
v
0.2
4. Discussion a n d conclusions
Letdown
0.4 0.6 0.8 Applied Field (kOe)
1.0
Fig. 4. Experimental and theoretical IRM curves for the three dispersion samples.
Fig. 3 are due to rotation alone. Fig. 4 shows fitted and experimental data for the IRM curves of the three dispersion samples. The upper part of the curves, where switching is predominant, has been omitted as the model assumes acquisition of magnetisation by Brownian motion of the aggregates. We find good agreement between the model of Ref. [3] and experiment at high fields with some deviation at lower fields. It is possible that in a previously unmagnetised dispersion the aggregates initially possess only a small net moment. Therefore, it would be necessary for some deformation of the flocculates and networks of particles in the dispersion to take place before there is a sufficient net moment to allow the applied field to rotate them, overcoming the dispersion viscosity. The parameters used for the theoretical curves show a decrease in the standard deviation of particle sizes from
The magnetisation process of a dispersion may be considered to be made up of three distinct processes. At low fields the main mechanism is deformation of flocculates and structures of particles in the dispersion. This process results in only a small change in the magnetisation as the changes due to deformation are small. The process is irreversible. By increasing the magnitude of the applied field there will be sufficient force exerted on the aggregates and particles to produce rotation. There is now a rapid increase in the magnetisation of the dispersion with both field and time. Upon removal of the applied field the magnetisation will relax as the rotation process is a reversible one. The final process occurs at higher fields and is due to N~el reversal of moments and is irreversible. References
[1] K. O'Grady, R.G. Gilson and P.C. Hobby, J. Magn. Magn. Mater. 95 (1991) 341. [2] P.E. Kelly, K. O'Grady, P.1. Mayo and R.W. Chantrell, IEEE Trans. Magn. 25 (1989) 3881. [3] P.I. Mayo and K. O'Grady, IEEE Trans. Magn. 29 (1993) 3640.
Journalof -,7-" magnetism and magnetic ~ H materials
ELSEVIER
Determination of pigment dispersion microstructure by magnetic measurements S.J. Greaves *, K. O'Grady Magnetic Materials Research Group, SEECS, UCNW, Bangor, Gwynedd LL57 I UT, UK
Abstract Magnetic measurement techniques have been used to determine the microstructure of metal particle dispersions with the eventual aim being to find a method of assessing dispersion quality during the manufacturing process of magnetic tape. We find that there are three processes which contribute to the magnetisation processes in a dispersion and that the effect of milling is to produce a more uniform dispersion.
I. Introduction
The assessment of dispersion quality is of major importance in the tape manufacturing industry. Magnetic measurements of the coercivity and remanence show only small changes with dispersion quality and affect the dispersion microstructure. In addition to single particles the dispersion consists of: aggregates of single particles, these are tightly bound and difficult to break down, agglomerates, which are collections of aggregates formed whilst the pigment is dry; and flocculates, groups of aggregates formed once the pigment has been wetted. Current measurements to determine the state of a dispersion rely on the observation of the gloss of a trial coating or microscopic observation of the agglomerates present [ 1]. These methods lack accuracy and are subjective in nature. Measurements of isothermal remanence magnetisation (IRM) curves have been made in which a field is applied and then removed and the remanence measured [2]. Beginning with small fields the dispersion microstructure, in terms of the number of agglomerates present, should remain unaffected. Furthermore, at low fields the changes in remanence brought about by the applied field are due to rotation of particles and aggregates in the dispersion. In well-dispersed high coercivity pigment systems, e.g. metal particles, it has been shown previously that the magnetisation mechanism is generally one of pure physical rotation [1]. The differential of the isothermal remanence curve may then he used to determine the number and size of aggregates in the dispersion [3]. The IRM curve shown in Fig. 1 depicts how the remanence of a dispersion is
acquired. At low fields there is a distortion of flocculates and networks with the result that they acquire a net magnetic moment. At slightly higher fields it is the rotation of single particles and small aggregates that provide the remanence as they align with the applied field. This process is generally reversible, when the field is removed the magnetisation will relax on a moderate time scale. The next stage is when switching of the moments in the particles results in an increase in the remanence, this is an irreversible process and relaxation is now much slower. Some of the flocculates may also change shape in the large field. The reversal of aggregates by the applied field, at fields below 1 kOe, is mainly due to rotation, the probability of reversal by switching being very low. A model of aggregate rotation has been used by Mayo and O'Grady [3] to interpret IRM curves in terms of the average size and standard deviation of the aggregates in the dispersion. The
M/M s 0.8 0.6 Reversal by "
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 6 7 3 - 7
/
w
i
t
c
~
/ 0.4
//J~
0.2 0
* Corresponding author.
s
Rotation of aggregates and particles
Distortion o f flocculates and networks 0
1
2 3 4 Applied Field (kOe)
Fig. I. An IRM curve of a dispersion.
S.J. Great'es, K. O'Grad3"/Journal of Magnetism and Magnetic Materials 155 (1996) 120 122
equation describing the motion of the aggregates in the applied field is given by
l(t)=l~(~)f, I-exp(-t/'r~lf(y)dy,
(11
where t is the measurement time, r ± the torque acting on the aggregates, l,.(~c) the remanence after saturation of the dispersion and f ( y ) the log-normal distribution of aggregate sizes. The degree of dispersion of a sample is also reflected in the behaviour of the magnetisation during time-dependence measurements. Well-dispersed samples result in time-dependence curves due to rotation of single particles and aggregates, whereas the convoluted phenomena of rotation, alignment within agglomerates and change in the shape of the agglomerates contribute to the time dependence of the less well dispersed samples.
2. Experiment Samples of metal pigment dispersion were prepared according to a commercial formulation. Samples were obtained at three stages; after initial mixing of components (Premix), after milling for four hours in a recirculating high shear mill (Milled) and after the addition of extra solvents and resin in order to reduce the dispersion viscosity (Letdown). Magnetic measurements were made using a VSM. Time-dependence measurements were made on dispersion samples which had not previously been magnetised. A field was applied for 15 min during which time the magnetisation of the sample was monitored. After the measurement the relaxation of magnetisation in zero field was studied for a further 15 rain.
3. Results IRM differentials for premix, milled and letdown samples are shown in Fig. 2. We see that the main peak in the
Zirr
1.0
,~ 3.2
0.8
~lbO • /
0.6
i •
•
•
~ \
/
•
v 0., []
v Letdown o IV~ll [] l ~ r ~
• \
.
• Tape .
.
.
0.4 0.2
0
1
2 3 4 Applied Field (kOe)
Fig. 2. IRM diflerentials for the three dispersions.
5
121
M/M 0 105
0
0
0 090
~"
" 2nd Measurement
lstMeasuremenl _ f~i
:1I
....
0.060 ~
"
~
~
0.045 L~ ~-~°~. . . .
0.030 [. . . .
10
_ ~
:
~
"rime(s)
100
~ -~-~
1000
Fig. 3. Time-dependence curves for the letdown sample. H = 10 Oe. letdown curve occurs at a very low field (about 80 Oe). This is due to rotation of the particles and aggregates and there is hardly any switching. There is a second small, broad peak centred around 1800 Oe which is attributable to switching of moments in agglomerates. The two peaks are distinct and do not overlap. For the premix and milled samples the difference is less pronounced with two convoluted peaks corresponding to reversal by rotation and switching. As expected, the IRM differential for the tape sample shows only one peak due to switching. Its position is slightly lower than that of the dispersion switching peaks, this is presumably because the dispersion aggregates are aligned by the lower applied fields in the measurement, whereas the tape microstructure is fixed. In the tape the particles are also more closely packed and so interactions could also be a factor. The position of the first peak in the IRM differential is to a large extent determined by the time the applied field is switched on, and the time after removal of the applied field at which the remanence measurement is made. As an example, Fig. 3 shows how the magnetisation of the letdown dispersion sample varies with time in an applied field of just 10 Oe. At the start of the first measurement at t = 0, m = 0. The magnetisation jumps rapidly in the first second after application of the field to about half of its value after 900 s. There is a large change in magnetisation over the measuring period both with the field applied and once it has been removed. Two measurements were made with this sample, which had not previously been magnetised. The second measurement was made immediately after the first and shows that once the dispersion has been subjected to a magnetic field it is much easier to remagnetise than from the initial state. There is also some degree of irreversibility in the magnetisation process, the magnitude of the irreversibility is related to the size of the applied field. We may explain this irreversibility in terms of the deformation of flocculates and structures prior to rotation taking place. Once deformed the change to the dispersion microstructure is permanent. Thus the relaxation curves in
122
S.J. Greaves, K. 0 'Grady / Journal of Magnetism and Magnetic Materials 155 (1996) 120-122
MIRM/'/VIIRMMAX 0.5
0.553 for the premix sample to 0.373 for the milled sample and 0.116 for the letdown sample. The average aggregate size remains almost unchanged. Thus the effect of the milling is to produce a more uniform dispersion. The viscosity of the letdown dispersion, reduced by the addition of extra solvents, is also found to decrease in the theoretical fits between the mill and letdown by 30%.
---
0.4 0.3 0.2 O. l
0
'7
0
v
0.2
4. Discussion a n d conclusions
Letdown
0.4 0.6 0.8 Applied Field (kOe)
1.0
Fig. 4. Experimental and theoretical IRM curves for the three dispersion samples.
Fig. 3 are due to rotation alone. Fig. 4 shows fitted and experimental data for the IRM curves of the three dispersion samples. The upper part of the curves, where switching is predominant, has been omitted as the model assumes acquisition of magnetisation by Brownian motion of the aggregates. We find good agreement between the model of Ref. [3] and experiment at high fields with some deviation at lower fields. It is possible that in a previously unmagnetised dispersion the aggregates initially possess only a small net moment. Therefore, it would be necessary for some deformation of the flocculates and networks of particles in the dispersion to take place before there is a sufficient net moment to allow the applied field to rotate them, overcoming the dispersion viscosity. The parameters used for the theoretical curves show a decrease in the standard deviation of particle sizes from
The magnetisation process of a dispersion may be considered to be made up of three distinct processes. At low fields the main mechanism is deformation of flocculates and structures of particles in the dispersion. This process results in only a small change in the magnetisation as the changes due to deformation are small. The process is irreversible. By increasing the magnitude of the applied field there will be sufficient force exerted on the aggregates and particles to produce rotation. There is now a rapid increase in the magnetisation of the dispersion with both field and time. Upon removal of the applied field the magnetisation will relax as the rotation process is a reversible one. The final process occurs at higher fields and is due to N~el reversal of moments and is irreversible. References
[1] K. O'Grady, R.G. Gilson and P.C. Hobby, J. Magn. Magn. Mater. 95 (1991) 341. [2] P.E. Kelly, K. O'Grady, P.1. Mayo and R.W. Chantrell, IEEE Trans. Magn. 25 (1989) 3881. [3] P.I. Mayo and K. O'Grady, IEEE Trans. Magn. 29 (1993) 3640.