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Triboelectric charging of pigmented polymer particles
Journal of Electrostatics, 19 (1987) 181--190
181
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TRIBOELECTRIC CHARGING OF PIGMENTED POLYMER PARTICLES MASOOD BAHKSHAEE, JOHN H. DALY, DAVID HAYWARD, RICHARD A. PETHRICK*, HARRAN U. RASHID and DAVID C. SHERRINGTON
Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, Cathedral Street, Glasgow G1 1XL (Great Britain) (Received February 6, 1986; accepted in revised form June 29, 1986)
Summary Triboelectric charging measurements are reported on melt mixed and suspension polymerized pigmented particles and illustrate the effects of surface chemistry and the nature of the dispersion of carbon black on the charge carried by the toner. The variation in the observed triboelectric charging measurements shows a strong correlation with changes in the nature of the dispersion of the carbon black as revealed by dielectric measurements and electron microscopy. It was also found that variations in the level of acidic functions at the carbon black surface significantly influence the charging capability of the toner particles.
Introduction Pigmented polymer particles of approximately 10/am diameter are widely used in a number of applications which include reprography [1--3] and electrostatic spraying. The performance of these particles, referred to as toners, depends critically upon the careful control of the charge generated during triboelectrification. A number of factors can influence the triboelectrification of toner particles. When the pigment is carbon black, parameters such as the chemical functionality of the surface, porosity of the particles, surface area and nature of the dispersion in the polymer matrix have all been identified as important variables in the system [4]. In a recent study of melt mixed toners [5] it was shown that a correlation exists for Regal 300 and 330 carbon blacks between the triboelectric charging levels and the dielectric constant of the composite. Comparison of these dielectric measurements with studies of carbon fibre composites and also with Maxwell Wagner Sillars calculations on chained carbon black structures indicates that a correlation also exists between the triboelectric charging levels and the way the carbon particles are distributed in the polymer matrix. Conventionally a dispersion of carbon black is achieved by the use of melt mixing, although it is possible to obtain similar dispersions by the use of a suspension poly*To whom communications should be addressed.
0304-3886/87/$03.50
© 1987 Elsevier Science Publishers B.V.
182
menzation route [6]. In this paper we compare the properties of suspension polymerized and melt mixed toner materials. It is well known that the triboelectric charging levels vary with the type of carbon black pigment used [7]. Comparison between various carbon blacks is however often not very instructive as factors such as chemical functionality, surface area and porosity may all be simultaneously varying. In order to understand the role of chemical functionality of the carbon black on the triboelectric charging levels we have carried out a study of the effects of oxidation on a basic carbon black. Basic carbon blacks are usually found to have low levels of surface acidic functions and oxidation will tend to increase the concentration of oxygen containing species present. Recently, Brewington [8] has examined the triboelectrification of toner materials charged by a biased blade in rubbing contact with a toner layer and has shown that the charging level varies linearly with the applied voltage and is also a function of the concentration of carbon black at the surface of the particle. It was also noted that the carbon black loading, type of black and degree of dispersion all have an influence on the charging characteristics of the toner, although these comments were not quantified.
Experimental Melt blended toner particles A series of toner materials (Table 1) were prepared by rubber milling of a copolymer with 10% carbon black. The copolymer was melted and the carbon black incorporated by passing the mixture through a set of heated rollers. The rubber milling was continued for a period of five minutes. The resultant slabs of composite material were mechanically crushed to form chips and then ground using a coffee mill to form comminute. The comminute was then further ground using a Fritsch planetary ball mill for a period of 60 minutes. The stainless steel mill was filled with approximately 100 g of comminute and 15 steel ball bearings of 20 mm diameter. The resultant TABLE
1
Characterization of the toners used in study Carbon black type
M e t h o d of preparation of toner
Particle size (~m)
Regal 300 (beaded) Regal 300 (fluffy) Regal 330 Raven 5750 Black Pearls L Black Pearls L
melt mixed melt mixed melt mixed melt mixed suspension polymerized melt mixed
15 14.5 15.2 16.0 14 15
-+ 1 -+ 1 + 1 + 1 + 1 + 1
183
material was then classified using a sieve shaker. The sieves were supplied by Endicott and were 5 - - 5 0 p m steel mesh. To assist classification, ball bearings were placed in the sieve shaker system to provide agitation of the powder. The fraction used in these studies was that retained by the 5 g m sieve and corresponded to approximately 10% of the material ground.
Suspension polymerized pigmented beads Another set of toner materials (Table 1) were prepared by suspension polymerization o f a m o n o m e r mix into which had been dispersed the carbon black [6]. By suitable choice of stabilizer and reaction conditions it was possible to generate beads of approximately 150--250~m diameter. These beads were dried in a vacuum oven to remove residual m o n o m e r and then subjected to the same grinding procedure as t h a t described above for the comminute from melt mixing. Classification was once more determined using a sieve shaker. Characterization of the toner particles -- molecular weight and molecular weight distribution In order to avoid damage to the gel permeation chromatography columns, the carbon black was removed from a solution of the toner in tetrahydrofuran by passing the liquor over a column packed with fragments of filter paper. This process was repeated until a clear solution was obtained. The molecular weight of the polymer was obtained using a Waiters Analytical GPC equipped with refractive index and UV detectors. The tracers were standardized by reference to narrow molecular weight distribution polystyrene standard samples. The value of the molecular weight and their distributions for the suspension polymerized toners were close to those of the copolymer used in the melt mixing procedure and the data for the materials are presented in Table 2. The molecular weight of the copolymer used in the production of the melt mixed toner was determined prior to the melt mixing process and by analysis of the final toner. The data on the molecular weights were found to be indistinguishable, indicating that no significant degradation o f the copolymer had occurred during the melt mixing process and also that the procedure for the removal of the carbon black does not influence the measured values of the distribution. TABLE 2 Molecular weights and molecular weight d i s t r i b u t i o n s o f suspension polymers Sample
Molecular Weight (Mn) X 10 s
Mw/M n
Melt mixed polymers Suspension polymers
2.7
2.2
- - average values
2.5
2.1
184
Particle size analysis The particle size of the final toner particles was measured using a Coulter TA instrument operating in the 2--500~m range. Volume average particle diameters were calculated and are presented in Table 1. The particle sizes of the milled and suspension polymerized materials were essentially identical and do not influence the measured values of the triboelectric charging. Characterization o f the dispersion of the carbon black The dispersion of the carbon black in the copolymer was characterized using a combination of optical and electron microscopy. The optical examination was performed using the m e t h o d described by Orzechowshi [9] and involved the swelling of the isolated toner particles with solvent. In our experience a more reproducible and satisfactory result is obtained if toluene is used rather than acetone, the solvent recommended by Orzechowshi [9]. The optical micrographs obtained do not, however, give a good impression of the dispersion in these toner materials. Similar observations on the limitations of this m e t h o d have been made by Brewington [8]. In order to obtain a better appreciation of the nature of the dispersions which were being studied, transmission electron micrographs (TEM) were obtained. The toners were embedded in epoxy resin and ultramicrotomed so as to obtain thin film sections. The specimens were then either partially oxidized or coated with gold to increase their conductivity. The data confirmed the initial impression obtained optically, but only the TEM data will be presented in this paper. Triboelectrification measurements The triboelectric charging of the toner was achieved using a procedure in which the toner was firstly rolled with a metal carrier in a glass jar. The mixture was then placed in a screened cage for separation of the toner from the beads with an air jet. The m e t h o d used is essentially t h a t described by Fabish and Duke [10], in which the ratio of the average toner charge to the average toner weight is a measure of the triboelectric charging. The toners were preconditioned by storage in a h u m i d i t y cabinet at 45--55% h u m i d i t y and a temperature of 25°C for a period of t w e n t y four hours prior to measurement of the triboelectric charging experiment. Values obtained using this m e t h o d for a 'standard' commercial toner material were in agreement with those supplied by the manufacturer. The electronics in the system were very stable, although changes in climatic conditions can lead to drifts in the values obtained from this experiment. In order to identify whether or n o t extraneous effects were influencing the data the apparatus was calibrated with a 'standard' toner at the beginning and end of each day. Data were rejected if the drift during the day or the reproducibility of the experimental observations for the triboelectric charging were greater than + 1 ~uC/g. In order to avoid the possible effects of variation in the mean surface area,
185 which occurs as a consequence o f variation in the size o f the t o n e r particles, the q u o t e d triboelectric charging values have been scaled to a nominal size o f 1 2 # m -- t he particle size for the commercial material. This norrealization p r o c e d u r e leads to less than a 10% variation in t he actual values o f the triboelectric charging values measured in this study. T he scaling was carried o u t on the basis o f a reduced surface area.
Dielectric measurements Th e permittivities for the c o m p a c t e d t o n e r powders were determined using a Wayne Kerr transformer ratio arm bridge apparatus operating at a f r eq u en cy o f 1 kHz. T he measurements were p e r f o r m e d in a cell which was placed inside an earthed screen. T he values obtained from a disc of polystyrene agreed well with those in the literature [ 1 0 ] . The e f f e c t of the pressure used in making the c o m p a c t e d disc on the permittivity has been investigated previously [ 5 ] . It was f o u n d t hat for t he toners used in this study, the permittivity is i n d e p e n d e n t of the compacting pressure above 3 5 , 0 0 0 p s i * . This pressure was therefore used as standard in subsequent preparation o f samples f or dielectric examination. Results
and discussion
A summary of the properties of the carbon blacks used in this study are presented in Table 3. The c o p o l y m e r used in this study was held constant in co mp o s itio n and therefore its role on the triboelectric effects, if synergism can be excluded, can be considered to be passive. This assumption is consistent with t h a t used by o t h e r workers [12, 14, 1 5 ] . TABLE 3 Characteristics of the carbon blacks and toners used in this study Carbon black type
Particle size (nm)
pH
Surface area (m2/g)
Triboelectric charging level (~C/g)
Regal (300) beaded Regal (300) fluffy Regal 300 Black Pearls L Raven 5750
27 26 27 24 25
7.5 7.0 7.2 3.0 2.0
80 90 94 138 575
18 8 17 --
25
Influence o f carbon black type on the triboelectric charging measurements The c o p o l y m e r was melt blended with Raven 5750, Regal 300 and 330 beaded and fluffy carbon blacks. T he data on the triboelectrification o f "1 psi ~ 6.9 X 10 3 Pa
186 these blends are summarized in Table 3. The Regal 300 carbon black required less energy than the others to disperse in the copolymer and optical micrographs indicated that a good dispersion had been obtained. All the data presented refer to a 10% carbon black loading. The surface of Raven 5750 is 575 m 2/g, whereas those for the Regal 300 and 330 are respectively 94 and 90 m 2/g. If the exposed surfaces of the carbon black particles were associated with the sites for charge storage in the toner particles, then it would be expected that the Raven 5750 would give a much larger tnboelectric charging value than that for Regal 300 or 330. The observed triboelectric charging is however very similar for these materials, indicating that the surface area of the individual carbon black particles does not primarily control the triboelectric charging. The particle size of the toners used in this study is essentially constant and therefore the differences observed must be associated with either differences in the degree of dispersion of the carbon black or the nature of its surface chemistry.
Influence of surface chemistry o f carbon black on the triboelectric charging measurements The characterization of carbon blacks, recently reviewed by Prest and Mosher [12] shows that various types of oxygen containing functionalities are essential for the generation of high negative triboelectric charging. In an a t t e m p t to explore the magnitude of the effects of the surface chemistry on the observed triboelectric charging, Regal 300 carbon black was oxidised using hydrogen peroxide. Varying the strength of the hydrogen peroxide and the oxidation time leads to the generation of carbon blacks with different values of pH (Fig. 1). The pH values of the carbon blacks were measured by back titration of an equilibrated solution of the toner and sodium hydroxide using a standard solution of hydrogen chloride. The variation of the pH values may be assumed to reflect changes in the concentration rather than the type of surface species present. The peroxide normally pC/g,
Q
70.
b
pH 6
10
*
50 .-t .-t
40
.-t
0
02 04 06 % peroxfde
08
O~ . . . . . 4 5 6 7 pH
8
Fig. 1. Effect of peroxide oxidation of Regal carbon black 300 upon the pH (a) and triboelectric charging level (b).
187 generates acid functionalities in the surface of the carbon black and this was confirmed by infrared spectroscopy. The change in the pH from 7 to 4, leads to a corresponding increase in the value of the triboelectric charging obtained from these materials. The oxidative m e t h o d used for the production of Regal 5750 leads to the generation of sulphate groups at the carbon black surface in addition to the expected carboxylic functionalities. This fact may in part explain the higher triboelectric charging and more acidic pH of this toner material. It is apparent, from this study and as reported by other workers [12--14] that the surface chemistry of the carbon black has a major influence on the triboelectric charging of toner materials. It was also f o u n d that the presence of tricalcium phosphate (TCP) at the surface of the toner can lead to reversal of the charge carried by the toner particles. This observation was made during the course of examination of a series of toners obtained from suspension polymerization in which TCP was used as stabilizing agent. The TCP used in the formation of the beaded polymer can be removed by washing the suspension polymerized beads with dilute nitric acid prior to grinding them to toner. The washing procedure was shown spectroscopically not to add any additional functionalities to the toner, although it does lead to a reduction of the adsorbed TCP in the suspension polymerized beads as indicated by elemental analysis. A series of samples were generated which had been subjected to increasing extents of washing and analysed as having decreasing amounts of TCP. It was found that the magnitude of the triboelectric charging decreased from approximately 20pC/g positive to almost zero and then started to increase in a negative sense.
Nature o f dispersion o f carbon black in suspension polymerized toner Examination of the TEM micrographs (Fig. 2) clearly reveals two features of the carbon black dispersions. Firstly, the suspension polymerization produces toners all of which have high concentrations of carbon black in the surface of the original bead and the resultant toner particles. The high concentration of carbon black in the surface is associated with its role in stabilization of the bead during the suspension polymerization process. As a consequence of the high concentration of carbon in the surface conducting tracks can be created which will contribute to the dielectric permittivity of the toner. Secondly, changes in the conditions used in the polymerization lead to a greater or lesser degree of dispersion and carbon black chaining within the toner particle. The suspension polymerized materials may be expected to display similar dielectric behaviour to melt mixed materials. For the latter materials it has been postulated [5] that the carbon black chains which occur give rise to the relatively large permittivity of the toners [13]. Thus the dielectric permittivity values indicate the extent of chaining of carbon black in the various toner materials and a comparison with the TEM data indicates whether the chains are at the surface or in the bulk of the toner.
188 Code -
Dielectric
perm~ttlvlty/
Magmflcatlon
I
I .
A-
59/10000
C - 41 / 10 000
.
.
.
B - 5 5 / 5.000
C - 3 2 / 10 000
Fig. 2. Transmission Electron Micrographs of the suspension polymerized toner samples presented in Fig. 3. The samples are identified by their dielectric permittivities. The toners with a significant surface excess of carbon black have high dielectric permittivities. It is difficult to give a comprehensive impression of the structure of the toners from the transmission electron micrographs although, the photographs presented do illustrate the type of variation observed.
Influence o f dispersion o f carbon black on the triboelectric charging Using a variety o f reaction conditions and stabilizer concentrations it was possible to generate a range of toners with different types of dispersion based on Black Pearls L carbon black, as revealed by dielectric permittivity measurements and TEM observations. It has been previously shown by Brewington [8] t ha t Black Pearls L behaves similarly to Raven 5750 when melt mix blended to form a toner, however its lower surface area makes it more attractive than the f or m er for incorporation into suspension polymerization processes. High surface area carbon blacks have a marked tendency to destabilize a suspension polymerization process and hence the Black Pearls L carbon black was used in this study. Black Pearls L is an acid black with a pH value close to that of Raven 5750. Measurements
189 of the permittivity (e') of the toners (Fig. 3) obtained from the suspension polymerization process are much higher than those obtained with melt mixed materials. However, comparison of the permittivity data with TEM observations reveals that a significant number of the conducting tracks in the high permittivity samples must be located at the surface in order to explain the values observed with those toners. Decrease in the concentration of carbon black in the surface layer and increases in the concentration of internally chained carbon black particles leads to a net lowering of the dielectric permittivity. If the carbon black chaining were to be removed altogether, as would occur in the case of a good dispersion, then the permittivity would drop to that observed with certain of the melt mixed samples. All the suspension polymerized toners have carbon black in their surfaces, whereas melt mixed toners may or may n o t have carbon black in their surfaces. In melt mixed toners, the increase in the permittivity is associated with an increased tendency for the carbon black particles to form chains within the bulk of toner particle. In suspension polymerized toners, chained carbon particles in the surface can add a further contribution to the permittivity. As a result, whereas it is possible to rank the melt mixed toners on the basis of permittivity, the suspension toners may not be classified in this way. However, in both cases a large extent of internally chained carbon black structures is essential for the development of a high level of triboelectric charging. In contrast to the proposition of Brewington [8] a high concentration of carbon black in the surface of the toner does not necessarily lead to a high level of triboelectric charging. In fact the toners produced by suspension polymerisation with the highest concentrations of carbon black in the surface had the lowest values of charging. The main conclusion which emerges from this study is t h a t the level of triboelectric charge is intimately connected to the extent to which carbon black particles exist in a chained structure within the bulk of the toner particle. -)uC/g
suspension 20
" melt m ~ x e d 10
0
2
.
.
.
.
3
.
.
.
.
E'
.
4
.
.
.
.
5
.
.
.
6
Fig. 3. Variation o f the triboelectric charging levels with measured dielectric permittivities o f the toner samples.
190
Conclusions C o m p a r i s o n o f d a t a o b t a i n e d f r o m m e l t m i x e d and s u s p e n s i o n polym e r i z e d t o n e r s indicates t h a t b o t h can p r o d u c e highly negative charging toners. T h e use o f T E M a n d dielectric p e r m i t t i v i t y m e a s u r e m e n t s indicates t h a t it is t h e e x t e n t to w h i c h the c a r b o n black particles f o r m c o n d u c t i v e chains w i t h i n t h e b u l k o f the t o n e r w h i c h is i m p o r t a n t in defining t h e level o f t r i b o e l e c t r i c charging r a t h e r t h a n t h e t o t a l a m o u n t w h i c h is present. It is also f o u n d t h a t t o n e r s with a high c o n c e n t r a t i o n o f c a r b o n b l a c k at t h e i r surface do n o t necessarily have higher levels o f charging t h a n t h o s e t h a t d o not. C h e m i c a l e f f e c t s are also clearly d e m o n s t r a t e d f r o m a c o m p a r i s o n o f t h e levels o f charging a c h i e v e d w i t h t o n e r s with d i f f e r e n t p H values. Acknowledgement
T h e a u t h o r s wish to t h a n k R a n k X e r o x f o r their s u p p o r t o f this fund a m e n t a l s t u d y o f t r i b o e l e c t r i f i c a t i o n a n d also f o r the m a n y h e l p f u l c o m m e n t s m a d e during t h e c o u r s e o f t h e p r o j e c t .
References 1 C.F. Carlson, History of electrostaticrecording. In: J.H. Dessauer and H.E. Clark (Eds.) Xerography and Related Processes, Focal Press, London and N e w York, 1964. 2 J.W. Weigl, In: M. Hair and M.D. Croucher (Eds.) A C S Symposium Series N o 200, American Chemical Society, Washington, DC, 1982, p. 139. 3 R.M. Schaffert, Electrophotography, 2nd edn. John Wiley and Sons, N e w York, 1975. 4 H.W. Gibson, F.C. Bailey, J.L. Mincer and W.H.H. Gunther, J. Polym. Sci. Polym. Chem., 17 (1979) 2961. 5 J.H. Daly, R.A. Pethrick and D. Hayward, J. Phys. D, 19 (1986) 885. 6 M. Bahkshaee, R.A. Pethrick, H.U. Rashid and D.C. Sherrington, Polymer, 26 (1985)
185. 7 T.J. Fabish, In: M. Hair and M.D. Croucher (Eds. } ACS Symposium Series No 200, American Chemical Society, Washington, DC, 1982, p. 197. 8 G.T. Brewington, In: M. Hair and M.D. Croucher (Eds.) ACS Symposium Series No 200, American Chemical Society Washington, DC, 1982, p. 182. 9 A. Orzechowski, Microscope, 27 (1979) 5. 10 T.J. Fabish and C.H. Duke, J. Appl. Phys., 49 (1978) 315. 11 Handbook of Physics and Chemistry, CRC Publications, 57 edn., 1976. 12 W.M. Prest and R.A. Mosher, In: M.L. Hair and M.D. Croucher (Eds.) ACS Symposium Series No 200, American Chemical Society, Washington, DC, 1982, p. 225. 13 V.E. Hanchett and R.H. Geiss, IBM J. Res. Dev., 27 (1983} 348--355. 14 T.J. Fabish and M.L. Hair, J. Colloid Interface Sci., 62 (1977) 16. 15 P.C. Julian, In: E. Sichel (Ed.), Carbon Black--Polymer Composites, Marcel Dekker Inc., New York, NY, 1982.
181
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TRIBOELECTRIC CHARGING OF PIGMENTED POLYMER PARTICLES MASOOD BAHKSHAEE, JOHN H. DALY, DAVID HAYWARD, RICHARD A. PETHRICK*, HARRAN U. RASHID and DAVID C. SHERRINGTON
Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, Cathedral Street, Glasgow G1 1XL (Great Britain) (Received February 6, 1986; accepted in revised form June 29, 1986)
Summary Triboelectric charging measurements are reported on melt mixed and suspension polymerized pigmented particles and illustrate the effects of surface chemistry and the nature of the dispersion of carbon black on the charge carried by the toner. The variation in the observed triboelectric charging measurements shows a strong correlation with changes in the nature of the dispersion of the carbon black as revealed by dielectric measurements and electron microscopy. It was also found that variations in the level of acidic functions at the carbon black surface significantly influence the charging capability of the toner particles.
Introduction Pigmented polymer particles of approximately 10/am diameter are widely used in a number of applications which include reprography [1--3] and electrostatic spraying. The performance of these particles, referred to as toners, depends critically upon the careful control of the charge generated during triboelectrification. A number of factors can influence the triboelectrification of toner particles. When the pigment is carbon black, parameters such as the chemical functionality of the surface, porosity of the particles, surface area and nature of the dispersion in the polymer matrix have all been identified as important variables in the system [4]. In a recent study of melt mixed toners [5] it was shown that a correlation exists for Regal 300 and 330 carbon blacks between the triboelectric charging levels and the dielectric constant of the composite. Comparison of these dielectric measurements with studies of carbon fibre composites and also with Maxwell Wagner Sillars calculations on chained carbon black structures indicates that a correlation also exists between the triboelectric charging levels and the way the carbon particles are distributed in the polymer matrix. Conventionally a dispersion of carbon black is achieved by the use of melt mixing, although it is possible to obtain similar dispersions by the use of a suspension poly*To whom communications should be addressed.
0304-3886/87/$03.50
© 1987 Elsevier Science Publishers B.V.
182
menzation route [6]. In this paper we compare the properties of suspension polymerized and melt mixed toner materials. It is well known that the triboelectric charging levels vary with the type of carbon black pigment used [7]. Comparison between various carbon blacks is however often not very instructive as factors such as chemical functionality, surface area and porosity may all be simultaneously varying. In order to understand the role of chemical functionality of the carbon black on the triboelectric charging levels we have carried out a study of the effects of oxidation on a basic carbon black. Basic carbon blacks are usually found to have low levels of surface acidic functions and oxidation will tend to increase the concentration of oxygen containing species present. Recently, Brewington [8] has examined the triboelectrification of toner materials charged by a biased blade in rubbing contact with a toner layer and has shown that the charging level varies linearly with the applied voltage and is also a function of the concentration of carbon black at the surface of the particle. It was also noted that the carbon black loading, type of black and degree of dispersion all have an influence on the charging characteristics of the toner, although these comments were not quantified.
Experimental Melt blended toner particles A series of toner materials (Table 1) were prepared by rubber milling of a copolymer with 10% carbon black. The copolymer was melted and the carbon black incorporated by passing the mixture through a set of heated rollers. The rubber milling was continued for a period of five minutes. The resultant slabs of composite material were mechanically crushed to form chips and then ground using a coffee mill to form comminute. The comminute was then further ground using a Fritsch planetary ball mill for a period of 60 minutes. The stainless steel mill was filled with approximately 100 g of comminute and 15 steel ball bearings of 20 mm diameter. The resultant TABLE
1
Characterization of the toners used in study Carbon black type
M e t h o d of preparation of toner
Particle size (~m)
Regal 300 (beaded) Regal 300 (fluffy) Regal 330 Raven 5750 Black Pearls L Black Pearls L
melt mixed melt mixed melt mixed melt mixed suspension polymerized melt mixed
15 14.5 15.2 16.0 14 15
-+ 1 -+ 1 + 1 + 1 + 1 + 1
183
material was then classified using a sieve shaker. The sieves were supplied by Endicott and were 5 - - 5 0 p m steel mesh. To assist classification, ball bearings were placed in the sieve shaker system to provide agitation of the powder. The fraction used in these studies was that retained by the 5 g m sieve and corresponded to approximately 10% of the material ground.
Suspension polymerized pigmented beads Another set of toner materials (Table 1) were prepared by suspension polymerization o f a m o n o m e r mix into which had been dispersed the carbon black [6]. By suitable choice of stabilizer and reaction conditions it was possible to generate beads of approximately 150--250~m diameter. These beads were dried in a vacuum oven to remove residual m o n o m e r and then subjected to the same grinding procedure as t h a t described above for the comminute from melt mixing. Classification was once more determined using a sieve shaker. Characterization of the toner particles -- molecular weight and molecular weight distribution In order to avoid damage to the gel permeation chromatography columns, the carbon black was removed from a solution of the toner in tetrahydrofuran by passing the liquor over a column packed with fragments of filter paper. This process was repeated until a clear solution was obtained. The molecular weight of the polymer was obtained using a Waiters Analytical GPC equipped with refractive index and UV detectors. The tracers were standardized by reference to narrow molecular weight distribution polystyrene standard samples. The value of the molecular weight and their distributions for the suspension polymerized toners were close to those of the copolymer used in the melt mixing procedure and the data for the materials are presented in Table 2. The molecular weight of the copolymer used in the production of the melt mixed toner was determined prior to the melt mixing process and by analysis of the final toner. The data on the molecular weights were found to be indistinguishable, indicating that no significant degradation o f the copolymer had occurred during the melt mixing process and also that the procedure for the removal of the carbon black does not influence the measured values of the distribution. TABLE 2 Molecular weights and molecular weight d i s t r i b u t i o n s o f suspension polymers Sample
Molecular Weight (Mn) X 10 s
Mw/M n
Melt mixed polymers Suspension polymers
2.7
2.2
- - average values
2.5
2.1
184
Particle size analysis The particle size of the final toner particles was measured using a Coulter TA instrument operating in the 2--500~m range. Volume average particle diameters were calculated and are presented in Table 1. The particle sizes of the milled and suspension polymerized materials were essentially identical and do not influence the measured values of the triboelectric charging. Characterization o f the dispersion of the carbon black The dispersion of the carbon black in the copolymer was characterized using a combination of optical and electron microscopy. The optical examination was performed using the m e t h o d described by Orzechowshi [9] and involved the swelling of the isolated toner particles with solvent. In our experience a more reproducible and satisfactory result is obtained if toluene is used rather than acetone, the solvent recommended by Orzechowshi [9]. The optical micrographs obtained do not, however, give a good impression of the dispersion in these toner materials. Similar observations on the limitations of this m e t h o d have been made by Brewington [8]. In order to obtain a better appreciation of the nature of the dispersions which were being studied, transmission electron micrographs (TEM) were obtained. The toners were embedded in epoxy resin and ultramicrotomed so as to obtain thin film sections. The specimens were then either partially oxidized or coated with gold to increase their conductivity. The data confirmed the initial impression obtained optically, but only the TEM data will be presented in this paper. Triboelectrification measurements The triboelectric charging of the toner was achieved using a procedure in which the toner was firstly rolled with a metal carrier in a glass jar. The mixture was then placed in a screened cage for separation of the toner from the beads with an air jet. The m e t h o d used is essentially t h a t described by Fabish and Duke [10], in which the ratio of the average toner charge to the average toner weight is a measure of the triboelectric charging. The toners were preconditioned by storage in a h u m i d i t y cabinet at 45--55% h u m i d i t y and a temperature of 25°C for a period of t w e n t y four hours prior to measurement of the triboelectric charging experiment. Values obtained using this m e t h o d for a 'standard' commercial toner material were in agreement with those supplied by the manufacturer. The electronics in the system were very stable, although changes in climatic conditions can lead to drifts in the values obtained from this experiment. In order to identify whether or n o t extraneous effects were influencing the data the apparatus was calibrated with a 'standard' toner at the beginning and end of each day. Data were rejected if the drift during the day or the reproducibility of the experimental observations for the triboelectric charging were greater than + 1 ~uC/g. In order to avoid the possible effects of variation in the mean surface area,
185 which occurs as a consequence o f variation in the size o f the t o n e r particles, the q u o t e d triboelectric charging values have been scaled to a nominal size o f 1 2 # m -- t he particle size for the commercial material. This norrealization p r o c e d u r e leads to less than a 10% variation in t he actual values o f the triboelectric charging values measured in this study. T he scaling was carried o u t on the basis o f a reduced surface area.
Dielectric measurements Th e permittivities for the c o m p a c t e d t o n e r powders were determined using a Wayne Kerr transformer ratio arm bridge apparatus operating at a f r eq u en cy o f 1 kHz. T he measurements were p e r f o r m e d in a cell which was placed inside an earthed screen. T he values obtained from a disc of polystyrene agreed well with those in the literature [ 1 0 ] . The e f f e c t of the pressure used in making the c o m p a c t e d disc on the permittivity has been investigated previously [ 5 ] . It was f o u n d t hat for t he toners used in this study, the permittivity is i n d e p e n d e n t of the compacting pressure above 3 5 , 0 0 0 p s i * . This pressure was therefore used as standard in subsequent preparation o f samples f or dielectric examination. Results
and discussion
A summary of the properties of the carbon blacks used in this study are presented in Table 3. The c o p o l y m e r used in this study was held constant in co mp o s itio n and therefore its role on the triboelectric effects, if synergism can be excluded, can be considered to be passive. This assumption is consistent with t h a t used by o t h e r workers [12, 14, 1 5 ] . TABLE 3 Characteristics of the carbon blacks and toners used in this study Carbon black type
Particle size (nm)
pH
Surface area (m2/g)
Triboelectric charging level (~C/g)
Regal (300) beaded Regal (300) fluffy Regal 300 Black Pearls L Raven 5750
27 26 27 24 25
7.5 7.0 7.2 3.0 2.0
80 90 94 138 575
18 8 17 --
25
Influence o f carbon black type on the triboelectric charging measurements The c o p o l y m e r was melt blended with Raven 5750, Regal 300 and 330 beaded and fluffy carbon blacks. T he data on the triboelectrification o f "1 psi ~ 6.9 X 10 3 Pa
186 these blends are summarized in Table 3. The Regal 300 carbon black required less energy than the others to disperse in the copolymer and optical micrographs indicated that a good dispersion had been obtained. All the data presented refer to a 10% carbon black loading. The surface of Raven 5750 is 575 m 2/g, whereas those for the Regal 300 and 330 are respectively 94 and 90 m 2/g. If the exposed surfaces of the carbon black particles were associated with the sites for charge storage in the toner particles, then it would be expected that the Raven 5750 would give a much larger tnboelectric charging value than that for Regal 300 or 330. The observed triboelectric charging is however very similar for these materials, indicating that the surface area of the individual carbon black particles does not primarily control the triboelectric charging. The particle size of the toners used in this study is essentially constant and therefore the differences observed must be associated with either differences in the degree of dispersion of the carbon black or the nature of its surface chemistry.
Influence of surface chemistry o f carbon black on the triboelectric charging measurements The characterization of carbon blacks, recently reviewed by Prest and Mosher [12] shows that various types of oxygen containing functionalities are essential for the generation of high negative triboelectric charging. In an a t t e m p t to explore the magnitude of the effects of the surface chemistry on the observed triboelectric charging, Regal 300 carbon black was oxidised using hydrogen peroxide. Varying the strength of the hydrogen peroxide and the oxidation time leads to the generation of carbon blacks with different values of pH (Fig. 1). The pH values of the carbon blacks were measured by back titration of an equilibrated solution of the toner and sodium hydroxide using a standard solution of hydrogen chloride. The variation of the pH values may be assumed to reflect changes in the concentration rather than the type of surface species present. The peroxide normally pC/g,
Q
70.
b
pH 6
10
*
50 .-t .-t
40
.-t
0
02 04 06 % peroxfde
08
O~ . . . . . 4 5 6 7 pH
8
Fig. 1. Effect of peroxide oxidation of Regal carbon black 300 upon the pH (a) and triboelectric charging level (b).
187 generates acid functionalities in the surface of the carbon black and this was confirmed by infrared spectroscopy. The change in the pH from 7 to 4, leads to a corresponding increase in the value of the triboelectric charging obtained from these materials. The oxidative m e t h o d used for the production of Regal 5750 leads to the generation of sulphate groups at the carbon black surface in addition to the expected carboxylic functionalities. This fact may in part explain the higher triboelectric charging and more acidic pH of this toner material. It is apparent, from this study and as reported by other workers [12--14] that the surface chemistry of the carbon black has a major influence on the triboelectric charging of toner materials. It was also f o u n d that the presence of tricalcium phosphate (TCP) at the surface of the toner can lead to reversal of the charge carried by the toner particles. This observation was made during the course of examination of a series of toners obtained from suspension polymerization in which TCP was used as stabilizing agent. The TCP used in the formation of the beaded polymer can be removed by washing the suspension polymerized beads with dilute nitric acid prior to grinding them to toner. The washing procedure was shown spectroscopically not to add any additional functionalities to the toner, although it does lead to a reduction of the adsorbed TCP in the suspension polymerized beads as indicated by elemental analysis. A series of samples were generated which had been subjected to increasing extents of washing and analysed as having decreasing amounts of TCP. It was found that the magnitude of the triboelectric charging decreased from approximately 20pC/g positive to almost zero and then started to increase in a negative sense.
Nature o f dispersion o f carbon black in suspension polymerized toner Examination of the TEM micrographs (Fig. 2) clearly reveals two features of the carbon black dispersions. Firstly, the suspension polymerization produces toners all of which have high concentrations of carbon black in the surface of the original bead and the resultant toner particles. The high concentration of carbon black in the surface is associated with its role in stabilization of the bead during the suspension polymerization process. As a consequence of the high concentration of carbon in the surface conducting tracks can be created which will contribute to the dielectric permittivity of the toner. Secondly, changes in the conditions used in the polymerization lead to a greater or lesser degree of dispersion and carbon black chaining within the toner particle. The suspension polymerized materials may be expected to display similar dielectric behaviour to melt mixed materials. For the latter materials it has been postulated [5] that the carbon black chains which occur give rise to the relatively large permittivity of the toners [13]. Thus the dielectric permittivity values indicate the extent of chaining of carbon black in the various toner materials and a comparison with the TEM data indicates whether the chains are at the surface or in the bulk of the toner.
188 Code -
Dielectric
perm~ttlvlty/
Magmflcatlon
I
I .
A-
59/10000
C - 41 / 10 000
.
.
.
B - 5 5 / 5.000
C - 3 2 / 10 000
Fig. 2. Transmission Electron Micrographs of the suspension polymerized toner samples presented in Fig. 3. The samples are identified by their dielectric permittivities. The toners with a significant surface excess of carbon black have high dielectric permittivities. It is difficult to give a comprehensive impression of the structure of the toners from the transmission electron micrographs although, the photographs presented do illustrate the type of variation observed.
Influence o f dispersion o f carbon black on the triboelectric charging Using a variety o f reaction conditions and stabilizer concentrations it was possible to generate a range of toners with different types of dispersion based on Black Pearls L carbon black, as revealed by dielectric permittivity measurements and TEM observations. It has been previously shown by Brewington [8] t ha t Black Pearls L behaves similarly to Raven 5750 when melt mix blended to form a toner, however its lower surface area makes it more attractive than the f or m er for incorporation into suspension polymerization processes. High surface area carbon blacks have a marked tendency to destabilize a suspension polymerization process and hence the Black Pearls L carbon black was used in this study. Black Pearls L is an acid black with a pH value close to that of Raven 5750. Measurements
189 of the permittivity (e') of the toners (Fig. 3) obtained from the suspension polymerization process are much higher than those obtained with melt mixed materials. However, comparison of the permittivity data with TEM observations reveals that a significant number of the conducting tracks in the high permittivity samples must be located at the surface in order to explain the values observed with those toners. Decrease in the concentration of carbon black in the surface layer and increases in the concentration of internally chained carbon black particles leads to a net lowering of the dielectric permittivity. If the carbon black chaining were to be removed altogether, as would occur in the case of a good dispersion, then the permittivity would drop to that observed with certain of the melt mixed samples. All the suspension polymerized toners have carbon black in their surfaces, whereas melt mixed toners may or may n o t have carbon black in their surfaces. In melt mixed toners, the increase in the permittivity is associated with an increased tendency for the carbon black particles to form chains within the bulk of toner particle. In suspension polymerized toners, chained carbon particles in the surface can add a further contribution to the permittivity. As a result, whereas it is possible to rank the melt mixed toners on the basis of permittivity, the suspension toners may not be classified in this way. However, in both cases a large extent of internally chained carbon black structures is essential for the development of a high level of triboelectric charging. In contrast to the proposition of Brewington [8] a high concentration of carbon black in the surface of the toner does not necessarily lead to a high level of triboelectric charging. In fact the toners produced by suspension polymerisation with the highest concentrations of carbon black in the surface had the lowest values of charging. The main conclusion which emerges from this study is t h a t the level of triboelectric charge is intimately connected to the extent to which carbon black particles exist in a chained structure within the bulk of the toner particle. -)uC/g
suspension 20
" melt m ~ x e d 10
0
2
.
.
.
.
3
.
.
.
.
E'
.
4
.
.
.
.
5
.
.
.
6
Fig. 3. Variation o f the triboelectric charging levels with measured dielectric permittivities o f the toner samples.
190
Conclusions C o m p a r i s o n o f d a t a o b t a i n e d f r o m m e l t m i x e d and s u s p e n s i o n polym e r i z e d t o n e r s indicates t h a t b o t h can p r o d u c e highly negative charging toners. T h e use o f T E M a n d dielectric p e r m i t t i v i t y m e a s u r e m e n t s indicates t h a t it is t h e e x t e n t to w h i c h the c a r b o n black particles f o r m c o n d u c t i v e chains w i t h i n t h e b u l k o f the t o n e r w h i c h is i m p o r t a n t in defining t h e level o f t r i b o e l e c t r i c charging r a t h e r t h a n t h e t o t a l a m o u n t w h i c h is present. It is also f o u n d t h a t t o n e r s with a high c o n c e n t r a t i o n o f c a r b o n b l a c k at t h e i r surface do n o t necessarily have higher levels o f charging t h a n t h o s e t h a t d o not. C h e m i c a l e f f e c t s are also clearly d e m o n s t r a t e d f r o m a c o m p a r i s o n o f t h e levels o f charging a c h i e v e d w i t h t o n e r s with d i f f e r e n t p H values. Acknowledgement
T h e a u t h o r s wish to t h a n k R a n k X e r o x f o r their s u p p o r t o f this fund a m e n t a l s t u d y o f t r i b o e l e c t r i f i c a t i o n a n d also f o r the m a n y h e l p f u l c o m m e n t s m a d e during t h e c o u r s e o f t h e p r o j e c t .
References 1 C.F. Carlson, History of electrostaticrecording. In: J.H. Dessauer and H.E. Clark (Eds.) Xerography and Related Processes, Focal Press, London and N e w York, 1964. 2 J.W. Weigl, In: M. Hair and M.D. Croucher (Eds.) A C S Symposium Series N o 200, American Chemical Society, Washington, DC, 1982, p. 139. 3 R.M. Schaffert, Electrophotography, 2nd edn. John Wiley and Sons, N e w York, 1975. 4 H.W. Gibson, F.C. Bailey, J.L. Mincer and W.H.H. Gunther, J. Polym. Sci. Polym. Chem., 17 (1979) 2961. 5 J.H. Daly, R.A. Pethrick and D. Hayward, J. Phys. D, 19 (1986) 885. 6 M. Bahkshaee, R.A. Pethrick, H.U. Rashid and D.C. Sherrington, Polymer, 26 (1985)
185. 7 T.J. Fabish, In: M. Hair and M.D. Croucher (Eds. } ACS Symposium Series No 200, American Chemical Society, Washington, DC, 1982, p. 197. 8 G.T. Brewington, In: M. Hair and M.D. Croucher (Eds.) ACS Symposium Series No 200, American Chemical Society Washington, DC, 1982, p. 182. 9 A. Orzechowski, Microscope, 27 (1979) 5. 10 T.J. Fabish and C.H. Duke, J. Appl. Phys., 49 (1978) 315. 11 Handbook of Physics and Chemistry, CRC Publications, 57 edn., 1976. 12 W.M. Prest and R.A. Mosher, In: M.L. Hair and M.D. Croucher (Eds.) ACS Symposium Series No 200, American Chemical Society, Washington, DC, 1982, p. 225. 13 V.E. Hanchett and R.H. Geiss, IBM J. Res. Dev., 27 (1983} 348--355. 14 T.J. Fabish and M.L. Hair, J. Colloid Interface Sci., 62 (1977) 16. 15 P.C. Julian, In: E. Sichel (Ed.), Carbon Black--Polymer Composites, Marcel Dekker Inc., New York, NY, 1982.