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Polarization behaviour of paper during corona charging
Journal of Electrostatics 71 (2013) 35e40
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Polarization behaviour of paper during corona charging J. Sidaravicius a, d, T. Lozovsky a, e, J. Jurksus a, R. Maldzius a, *, K. Backfolk b, P. Sirviö c a
Vilnius University, Department of Solid State Electronics, Sauletekio al. 9, Vilnius LT-10222, Lithuania Lappeenranta University of Technology, Laboratory of fiber and paper technology, FI-53851 Lappeenranta, Finland c Stora Enso Oyj, Imatra Research Centre, FI 55800 Imatra, Finland d Vilnius Gediminas Technical University, Department of Polygraphic Machines, J. Basanavicius str. 28, Vilnius LT-03224, Lithuania e University of Bialystok, Vilnius Branch, Kalvariju str. 143, Vilnius LT-08221, Lithuania b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2012 Received in revised form 12 September 2012 Accepted 1 November 2012 Available online 20 November 2012
The polarization phenomenon of papers containing different amounts and different types of inorganic fillers during corona charging was studied using a dose-charging method. This method enables to investigate the phenomena in the conditions close to the real ones. It revealed that the paper composition affects the polarization tendency during the corona charging of paper. The filler influences the paper’s dielectric constant and hence its electrical capacitance, charging potential and polarizing electric field. It was shown that paper polarization depends mainly on the electric field and this is caused by the orientation polarization and the role of the filler origin and its concentration in the polarization was surprisingly low. Polarization temperature dependences enable to propose the paper polarization mechanism. Paper polarizability is dominated by cellulose and the moisture content in it, and the liability of cellulose and hydrogen bonds hydroxyl groups. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Corona charging Paper Cellulose Polarization Paper filler
1. Introduction Paper is one of the main printing substrates and is used as electrotechnical material, as a background in printed electronic etc. Paper composition influences the print quality in both conventional printing and digital printing [1e3], its electrical and dielectric properties that are important in many applications. It was reported, for example, that the dielectric properties of paper clearly correlate with the print quality [4]. Morsy [5] investigated the relationship between paper dielectric constant and print gloss in gravure printing and found no clear correlation, although some indication of a correlation could be seen. Morsy tried also to find a correlation between dielectric properties and print gloss, but with a poor result. The electrical and dielectric properties of paper are more important in electrophotographic printing [1,5e7]. The electrical resistivity of the paper influences e.g. runnability or handling and the ink transfer efficiency. The effect on runnability arises from static electricity that can lead to double feeds, misfeeds, paper jams, and even sparks. If a paper is too conductive, a loss of image density can occur due to charge leakage, as proposed by Green [7]. In electrophotographic copiers, laser printers and printing presses, the toner image is transferred from a photoreceptor drum or * Corresponding author. Tel.: þ370 5 2366052; fax: þ370 5 2366003. E-mail address: [email protected] (R. Maldzius). 0304-3886/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2012.11.003
from an intermediate transfer belt to the paper through an electric field. The transfer field is created with a corona charger or a bias roll by depositing charges on the reverse side of the paper. The strength of the electric field and its time-dependence are influenced by the electrical and dielectric properties of the paper, which determine the amount of charges on the paper surface, their drift toward the photoreceptor surface and the velocity of this leakage. Sometimes, especially in high speed printers and at low RH, the electrical conductance may play a minor role because the charge leakage is relatively small during the paper transit through the toner transfer nip [1,6]. As has been reported in many publications [8e16], the electric field strength is in this case governed by the paper’s dielectric constant or specifically by the paper’s dielectric thickness, d/ε. Toner transfer efficiency is high when the transfer field is strong and the transfer is a maximum when the dielectric constant for a given paper caliper is as high as possible or the dielectric thickness d/ε is as small as possible. It must be noted that in all theoretical or experimental studies, the paper dielectric constant value determined at high frequencies in the MHz region is used although this value does not correspond to the conditions of toner transfer in the real electrophotographic printing devices. It can be estimated that the electric field changes with a change in printing speed of less than 10 Hz, whereas material polarization is atomic or electronic in the high frequency range polar groups and space charges are important at low frequencies. Cellulose materials contain space charges and
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J. Sidaravicius et al. / Journal of Electrostatics 71 (2013) 35e40
permanent polar groups, both of which are important at low frequencies. If water is present, however, even in relatively small amounts, it may make a major contribution due to its polar groups. In the analysis of toner transfer, the effective dielectric constant value that includes low frequency polarization input should then be used. Polarization effects of cellulose and its derivatives have been studied using the thermally stimulated depolarization method TSC [17]. Baum [18] studied dried regenerated cellulose films and reported depolarization bands at 58, 2 and þ25 C. The low temperature band was attributed to the rotation of the primary hydroxyl group on the glucose unit. The band at þ25 C was believed to be related to the transition occurring in dry cellulose at this temperature e hydrogen bond breakage. Sawatary et al. [19] studied cellulose powder and detected several depolarization bands, which are attributed to the primary hydroxyl group in the amorphous region (triple peak at 140 C), to adsorbed moisture (band at 70 C) and to the adsorbed moisture (peak at þ70 C). Okabe et al. [20] used the thermal depolarization method to study the interfacial polarization mechanism in oil-impregnated kraft paper. In Ref. [21] the polarization of paper during corona charging observed at room temperature was described. The polarization showed itself as an increase in the paper potential after forced paper discharge to zero by opposite polarity corona. This effect was attributed to the orientation polarization of the paper constituents and to the formation of space charges. The authors did not specify which paper constituents are responsible for the observed polarization. Because paper consists mainly of cellulose, filler and some amount of moisture, the question of which constituent is responsible for the polarization was not addressed. There is no doubt that polar groups on the cellulose fibers (primarily hydroxyl) and surface-bound and free water take part in the polarization. However, the role of filler and other additives for the polarization and charge behavior are unclear. While there is a significant difference in dielectric constant between paper constituents, they also possess different reactivities with water. Whereas fibers interact with water, the fillers interact to no great extent with water. Thermo-stimulated currents in corona-charged dielectric polymeric films and aramid paper have been investigated by several authors (see review in Ref. [11]). Unfortunately, the results of their work cannot be applied to paper because aramid paper includes synthetic aromatic polyamides. The aim of the present study was to investigate polarization effects during the corona charging of papers containing different types and amounts of inorganic fillers. Bearing in mind that the cellulose groups which are able to polarize can be frozen at different temperatures [18,20], the temperature-dependence of the polarization was studied. The effect of the presence of inorganic fillers in the matrix on the polarization and charging mechanisms is interesting since they further change the relative amount of cellulose.
flowing through the paper sample and the displacement current, which is proportional to the charge deposited on the paper, and the paper surface potential change and its decay after the charging is stopped were monitored and computer controlled. The example of papers’ charging transients and potential decay after the charging is stopped is presented in Fig. 1. For the investigation of the paper polarization during corona charging the paper was charged to the approximately maximum achievable value Us and then discharged by the deposition of the opposite polarity charges. The amount of the opposite charges was such that was needed to obtain paper zero potential. After such a forced discharge, a rise in paper potential was observed (Fig. 2). This potential reached its maximum value Usd (also called the depolarization potential) and then decayed slowly. This effect was observed for both charging polarities and for all the papers investigated. The absolute Us and Usd positive values are a little smaller than the Us and Usd negative values. This effect is due to the fact that the negative corona is more intense than the positive corona. The Usd value is considered in this work to characterize the paper polarization level during corona charging. Polarization effects were measured at temperatures from þ8 to 33 C. All measurements were performed at 30 3% RH. The papers used in the study were made on a pilot paper machine (Stora Enso Oyj, Imatra Research Centre) with controlled process conditions. Machine was running at speeds between 30 and 60 m/min depending on the grammage. Papers with three different grammages (90, 160 and 230 g/m2) and different amounts of added precipitated calcium carbonate were made (0, 15 and 30%). The furnish was Eucalyptus (80%) and pine (20%) refined to a Schopper Riegler value of 27. In addition to filler and cellulose fibers, cationic starch (8 kg/tn), alkyl ketene dimer, (1.5 kg/tn) and two component retention system were used (200 g/tn polyacryl amide, and 1.7 kg/tn Bentonite). The other fillers used in the study were ground calcium carbonate (Hydrocarb FF-LV 75, Omya AG), kaolinite (Hubertx, Omya), Talc (Finntalc F08, Mondo minerals Oy), SiO2 (PerkaSil SM 660, Grace Davison), and TiO2 (Kemira). The papers were further calendered to different thickness levels using a laboratory calender. The samples described in the table were obtained at (A) uncalendered (B) 1 25 kN, and (C) 2 25 kN. The moisture content of the papers was 4e5%. 3. Results and discussion The charging potential behavior and the polarization level characterized by the depolarization potential Usd on the 160 gsm
2. Experimental In this study, the dosed charging method which was proposed earlier in Ref. [21] for the investigation of the paper charging properties was used. A paper sample is placed on the disc. The disc rotates with a constant velocity, which can be changed in the range from 5 to 10 rps. Over the disc are placed a corona charger, an electrometer with a non-vibrating probe to measure the paper surface potential and a synchronization probe and sensor. The paper sample passes them successively and during the disc rotation the small charge dose is deposited in every rotation when the paper sample is under the charger. The charge dose depends on the rotation velocity and on the corona discharge intensity, which was controlled by the corona voltage. The charge dose DQ can be changed between 1010 C and 104 C. Both the electrical current
Fig. 1. Paper charging and paper potential decay.
J. Sidaravicius et al. / Journal of Electrostatics 71 (2013) 35e40
37
Fig. 2. Paper charging, discharging by opposite corona and depolarization potential rise and decay.
papers containing different amounts of precipitated calcium carbonate are shown in Figs. 3 and 4. The changes in charging potential (Fig. 3) are small and it can be concluded that the charging potential is independent of the filler content. The changes in the depolarization potential (Fig. 4) are also small and almost at the limit of the measurement accuracy. Nevertheless, the polarization level tends to decrease slightly with increasing PCC content from 0 to 30% at a temperature of 8 C, but to increase slightly at higher temperatures. The overall depolarization potential decreases significantly however with increasing temperature, particularly when no or only a small amount of precipitated calcium carbonate is present. The dependence on filler content evident in Fig. 4 was unexpected. The addition of precipitated calcium carbonate that has a higher dielectric constant than cellulose might be expected to increase the dielectric constant of papers, but Fig. 5 shows that the dielectric constant does not change as predicted, which is consistent with Simula’s data [22]. The differences in the dielectric constant of papers with different grammages are attributed to
Fig. 4. Depolarization potential versus PCC content at different temperatures: a e negative charging and positive decharging, b e positive charging and negative decharging.
Fig. 3. Charging potential versus PCC content at 23 C: 1 e negative, 2 e positive.
differences in paper density and structure. The influence of filler on the polarization effect is small, and it can be assumed that the cellulose and water orientation polarization is responsible for the observed effect. The depolarization should thus decrease with increasing filler content due to the decrease in the cellulose content, as was observed at 8 C. The fact that the dependence at higher temperatures was the opposite, although weak, means that some other factors dominate the mechanism, probably the formation of space charges. Space charges can be formed by mobile ions, and their concentration should increase with increasing filler content, while their mobility should be higher at higher temperature. There may also be an increase in the moisture content at higher temperatures. Papers with different fillers exhibited different charging and depolarization potentials (Fig. 6). The difference cannot be caused by the difference in the filler amount because differences are very
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J. Sidaravicius et al. / Journal of Electrostatics 71 (2013) 35e40 Table 1 Samples used in the study.
Fig. 5. Dielectric constant as a function of PCC content. Grammage: 1 e 90 gsm, 2e 160 gsm, 3 e 230 gsm.
small (Table 1). On the other hand the charging potential dependence on the filler amount is weak (Fig. 3) or even does not depend on the filler amount in the investigated range of filler concentration. The ground calcium carbonate and kaolin showed the same polarization behavior, whereas the paper containing silica possessed very low polarization and depolarization properties. This is interesting since both silica and kaolin contain SieOH, but silica has a considerably higher surface area and therefore possesses a larger amount of silanol groups compared to that of kaolin. Although silica is porous and may contain surface-bound water, it still has a very low polarization and depolarization potential. TiO2, which is considered to be a relative inert material, showed a lower polarization and depolarization behavior. Since the thicknesses of all the papers were approximately the same and since the potential decay speed does not essentially differ, the difference in charging potential is determined by the paper’s dielectric constant. Indeed, the maximum charging potential depends linearly on the dielectric constant, as shown in Fig. 7,
Fig. 6. Positive and negative max charging and depolarization potential for papers containing different fillers: grammage 160 gsm, filler content 15%.
Nr
Target grammage
Measured grammage
Filler
Filler%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 18
90 160 230 90 160 230 90 160 230 160 160 160 160 160 160
94.6 158.7 232.9 88.1 155.1 228.2 91.7 159.2 223.7
e e e PCC PCC PCC PCC PCC PCC GCC Kaolin Talc Gypsum SiO2 TiO2
0.5 0.4 0.4 14.6 15.0 14.4 27.6 29.2 28.8 16.0 17.1 14.8 22.1 13.6 19.8
it decreases with increasing dielectric constant due to the increase in the paper’s electrical capacity. The dielectric constant of TiO2 is high (over 80) and it is then understandable that paper containing TiO2 exhibits a low charging ability and low polarization level. The depolarization potential depends on the type of filler used. Since the influence of the filler content on the depolarization potential is weak, as is shown in the case of PCC filler (Fig. 4), it can be supposed that the observed difference is caused by some other factor. It can be supposed that origin of the observed polarization effects in papers is accumulation and dissipation of space charges or orientation polarization. Orientation polarization usually depends linearly on the electric field. This dependence was used by Baum [18] to prove that polarization of the regenerated cellulose films polarization is dipole. Baum used thermal depolarization current technique and determined the dependence of the depolarization band area on the polarization field strength. The linear dependence suggests that space charges are not responsible for the occurrence of depolarization current bands. In our case, depolarization potential is considered as a polarization level metric. In the charged paper the electric field is proportional to the charging potential. So the dependence of the polarization potential Usd on
Fig. 7. Maximal charging potential versus relative dielectric constant ε for papers with different fillers (Table 1), grammage 160 gsm. Dashed line e linear approximation, correlation coefficient e 0.869, SD e 83 V.
J. Sidaravicius et al. / Journal of Electrostatics 71 (2013) 35e40
39
the charging potential Us can be considered as an analog of the depolarization band area dependence on the polarization field. This dependence using the data for all the papers investigated independently on the filler origin is presented in Fig. 8. The linear dependence shows that polarization is due to orientation, and in accordance with [18] it can be attributed to the orientation of primary hydroxyl groups in the cellulose or/and hydrogen bonds, and the fillers’ origin played no significant role. The fact that the linear relationship does not pass through the origin means that the polarization is not due solely to orientation, but that there is an initial polarization caused by the space charges. It is well known that polarization depends on the temperature. The depolarization potential is plotted versus temperature for the papers with different contents of PCC in Fig. 9. The depolarization
Fig. 9. (a) Positive and (b) negative depolarization potential as a function of temperature. PCC content: 1 e 0%, 2 e 15%, 3 e 30%.
Fig. 8. The depolarization potential as a function of (a) the positive and (b) negative charging potential. Samples represent all fillers. Lines e linear approximation, correlation coefficient 0.92 (positive charging) and 0.96 (negative charging), SD 10 V (positive charging) and 7.3 V (negative charging).
potential of both polarities decreases gradually with temperature at first, but at a temperature in the vicinity of 30 C decreases faster (it was difficult to measure at higher temperature because of the fast potential decay). This drop in depolarization potential can be explained using Baum’s data [8]. He observed in cellulose films a depolarization maximum in the vicinity of 25 C, which was attributed to the labile hydroxyl groups of hydrogen bonds. If this is true, it can be expected that this polarization mechanism should be seen for example in the temperature dependence of the charging potential. In our previous article we did not pay attention to the unusual dependence for the paper with no salt and with NaCl addition [Ref. [23], Fig. 1b and c] e which showed shoulders at 23 C that were earlier attributed to the measurement inaccuracy. Having in mind these data (Fig. 10), we can conclude that these shoulders are real and that they are caused by changes on the hydrogen bonds in cellulose at temperatures above 20e25 C.
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Acknowledgment Tekes is acknowledged for financial support of the project and Dr. Anthony Bristow and Dr. R. Sidrys for linguistic revision of the manuscript. References
Fig. 10. Charging potential versus temperature. Papers with different salt contents [[23], Fig. 1b].
4. Conclusions Experimental data for papers with different fillers show that paper is polarized during corona charging. The role of the filler in the polarization was surprisingly low. Paper polarizability is dominated by cellulose and the moisture content and liability of cellulose and hydrogen bond hydroxyl groups. Filler influences the dielectric constant and therefore the electrical capacitance. This leads to a higher maximal charging potential and consequently a higher electric field. The increase in the electric field results in a higher polarization level, which depends mainly on the electric field and not on the type of filler. Together with the orientation polarization, space charges are also created. The dependence of depolarization potential on temperature reveals data which indicate possible changes in the hydrogen bond in the vicinity of 25 C, as reported by Baum [18].
[1] K. Johansson, P. Lundberg, R. Ryberg, A Guide to Graphic Print Production, John Wiley & Sons, 2007. [2] B. Thomson, Printing Material: Science and Technology, Pira Int, 1998. [3] R.E. Mark (Ed.), Handbook of Physical and Mechanical Testing of Paper and Paperboard, vol. 2, Marcel Dekker, 1984. [4] P. Himanen, Coating to meet expectations - a practical approach, in: Pira Coating Conference, 1977, Edinburg, UK, 11e12 Mar, 1997, pp. 143e148. [5] F.A. Morsy, Dielectric properties of coated paper and the effect of various soluble thickeners, Polym. Plast. Technol. Eng. 44 (2005) 351e362. [6] M. Alava, Paper surface and thermal electrical and friction characteristics, in: K. Niskanen (Ed.), Paper Physics, second ed., Paper ja Puu Oy, 2008. [7] Ch. J. Green, Functional properties in xerography, Tappi 64 (5) (1981) 79e81. [8] M. Zaretsky, J. Imaging Sci. Technol. 37 (1993) 187. [9] J. Kallunki, M. Alava, E.K.O. Hellen, J. Appl. Phys. 100 (2006) 023528. [10] N. Provatas, A. Cassidy and M. Inoue, Dielectric variation in paper and its effects on electrophotographic printing, in: IS&’sNIP18 Intern. Conf. Digital Printing Techn., 2002 pp. 770e773. [11] A. Cassidy, M. Grant, N. Provatas, Modelling dielectric heterogeneity in electrophotography, Model. Simul. Mater. Sci. Eng. 12 (2004) 91e107. [12] P. Sirviö, K. Backfolk, Effect of roughness of low-grammage coated papers on print quality in color electrophotography, J. Imaging Sci. Technol. 51 (1) (2008) 010505-1e010505-9. [13] M. Schleusener, 7th Intern. Congress on Advances in non-Impact printing Techn., in: Final Program Proceedings, 167, 1991. [14] T. N. Tombs, 14 Intern. Conf. Digital printing Techn., NIP14, 440, 1998. [15] Y. Furuya, J. Imaging Sci. Technol. 45 (6) (2001) 542. _ R. Mald˛ius, J. Sidaravi [16] J. Kalade, cius, P. Sirviö, K. Backfolk, Lithuanian J. Phys. 49 (2) (2009) 145e162. [17] G.G. Raju, Dielectrics in Electric Fields, Marcel Decker Inc, 2003, Ch.10. [18] G. Baum, Thermal depolarization currents of regenerated cellulose films, J. Appl. Polym. Sci. 17 (1973) 2858e2866. [19] A. Sawatari, H. Kurihara, T. Takashima, Thermally stimulated current in cellulose powder, in: Makuzai Gakkishi (Ed.), J. Jpn Wood Res. Soc. 24 (4) (1978) 224e229 (in Japanese). [20] Y. Okabe, H. Yamashita and H. Amano, Thermally stimulated current in oil impregnated kraft paper systems, in: Proc. 11th Symp. Electrical Insulating Materials in Japan, September 1978, pp. 113e116. [21] P. Sirviö, J. Sidaravicius, T. Lozovski, S. Kuskevicius, K. Backfolk, Dosed charging: application to the investigation of papers, J. Electrost. 67 (2009) 730e736. [22] S. Simula, in: Proceedings of 26th Research Conference of the Research Institutes for Printing, Information and Communication Industries, Pira Intern., Munich, Germany, 1999, pp. 35e45. [23] R. Maldzius, P. Sirviö, J. Sidaravicius, T. Lozovski, K. Backfolk, J.B. Rosenholm, Temperature dependence of electrical and dielectric properties of papers for electrophotography, J. Appl. Phys. 107 (2010) 114904.
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Polarization behaviour of paper during corona charging J. Sidaravicius a, d, T. Lozovsky a, e, J. Jurksus a, R. Maldzius a, *, K. Backfolk b, P. Sirviö c a
Vilnius University, Department of Solid State Electronics, Sauletekio al. 9, Vilnius LT-10222, Lithuania Lappeenranta University of Technology, Laboratory of fiber and paper technology, FI-53851 Lappeenranta, Finland c Stora Enso Oyj, Imatra Research Centre, FI 55800 Imatra, Finland d Vilnius Gediminas Technical University, Department of Polygraphic Machines, J. Basanavicius str. 28, Vilnius LT-03224, Lithuania e University of Bialystok, Vilnius Branch, Kalvariju str. 143, Vilnius LT-08221, Lithuania b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2012 Received in revised form 12 September 2012 Accepted 1 November 2012 Available online 20 November 2012
The polarization phenomenon of papers containing different amounts and different types of inorganic fillers during corona charging was studied using a dose-charging method. This method enables to investigate the phenomena in the conditions close to the real ones. It revealed that the paper composition affects the polarization tendency during the corona charging of paper. The filler influences the paper’s dielectric constant and hence its electrical capacitance, charging potential and polarizing electric field. It was shown that paper polarization depends mainly on the electric field and this is caused by the orientation polarization and the role of the filler origin and its concentration in the polarization was surprisingly low. Polarization temperature dependences enable to propose the paper polarization mechanism. Paper polarizability is dominated by cellulose and the moisture content in it, and the liability of cellulose and hydrogen bonds hydroxyl groups. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Corona charging Paper Cellulose Polarization Paper filler
1. Introduction Paper is one of the main printing substrates and is used as electrotechnical material, as a background in printed electronic etc. Paper composition influences the print quality in both conventional printing and digital printing [1e3], its electrical and dielectric properties that are important in many applications. It was reported, for example, that the dielectric properties of paper clearly correlate with the print quality [4]. Morsy [5] investigated the relationship between paper dielectric constant and print gloss in gravure printing and found no clear correlation, although some indication of a correlation could be seen. Morsy tried also to find a correlation between dielectric properties and print gloss, but with a poor result. The electrical and dielectric properties of paper are more important in electrophotographic printing [1,5e7]. The electrical resistivity of the paper influences e.g. runnability or handling and the ink transfer efficiency. The effect on runnability arises from static electricity that can lead to double feeds, misfeeds, paper jams, and even sparks. If a paper is too conductive, a loss of image density can occur due to charge leakage, as proposed by Green [7]. In electrophotographic copiers, laser printers and printing presses, the toner image is transferred from a photoreceptor drum or * Corresponding author. Tel.: þ370 5 2366052; fax: þ370 5 2366003. E-mail address: [email protected] (R. Maldzius). 0304-3886/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2012.11.003
from an intermediate transfer belt to the paper through an electric field. The transfer field is created with a corona charger or a bias roll by depositing charges on the reverse side of the paper. The strength of the electric field and its time-dependence are influenced by the electrical and dielectric properties of the paper, which determine the amount of charges on the paper surface, their drift toward the photoreceptor surface and the velocity of this leakage. Sometimes, especially in high speed printers and at low RH, the electrical conductance may play a minor role because the charge leakage is relatively small during the paper transit through the toner transfer nip [1,6]. As has been reported in many publications [8e16], the electric field strength is in this case governed by the paper’s dielectric constant or specifically by the paper’s dielectric thickness, d/ε. Toner transfer efficiency is high when the transfer field is strong and the transfer is a maximum when the dielectric constant for a given paper caliper is as high as possible or the dielectric thickness d/ε is as small as possible. It must be noted that in all theoretical or experimental studies, the paper dielectric constant value determined at high frequencies in the MHz region is used although this value does not correspond to the conditions of toner transfer in the real electrophotographic printing devices. It can be estimated that the electric field changes with a change in printing speed of less than 10 Hz, whereas material polarization is atomic or electronic in the high frequency range polar groups and space charges are important at low frequencies. Cellulose materials contain space charges and
36
J. Sidaravicius et al. / Journal of Electrostatics 71 (2013) 35e40
permanent polar groups, both of which are important at low frequencies. If water is present, however, even in relatively small amounts, it may make a major contribution due to its polar groups. In the analysis of toner transfer, the effective dielectric constant value that includes low frequency polarization input should then be used. Polarization effects of cellulose and its derivatives have been studied using the thermally stimulated depolarization method TSC [17]. Baum [18] studied dried regenerated cellulose films and reported depolarization bands at 58, 2 and þ25 C. The low temperature band was attributed to the rotation of the primary hydroxyl group on the glucose unit. The band at þ25 C was believed to be related to the transition occurring in dry cellulose at this temperature e hydrogen bond breakage. Sawatary et al. [19] studied cellulose powder and detected several depolarization bands, which are attributed to the primary hydroxyl group in the amorphous region (triple peak at 140 C), to adsorbed moisture (band at 70 C) and to the adsorbed moisture (peak at þ70 C). Okabe et al. [20] used the thermal depolarization method to study the interfacial polarization mechanism in oil-impregnated kraft paper. In Ref. [21] the polarization of paper during corona charging observed at room temperature was described. The polarization showed itself as an increase in the paper potential after forced paper discharge to zero by opposite polarity corona. This effect was attributed to the orientation polarization of the paper constituents and to the formation of space charges. The authors did not specify which paper constituents are responsible for the observed polarization. Because paper consists mainly of cellulose, filler and some amount of moisture, the question of which constituent is responsible for the polarization was not addressed. There is no doubt that polar groups on the cellulose fibers (primarily hydroxyl) and surface-bound and free water take part in the polarization. However, the role of filler and other additives for the polarization and charge behavior are unclear. While there is a significant difference in dielectric constant between paper constituents, they also possess different reactivities with water. Whereas fibers interact with water, the fillers interact to no great extent with water. Thermo-stimulated currents in corona-charged dielectric polymeric films and aramid paper have been investigated by several authors (see review in Ref. [11]). Unfortunately, the results of their work cannot be applied to paper because aramid paper includes synthetic aromatic polyamides. The aim of the present study was to investigate polarization effects during the corona charging of papers containing different types and amounts of inorganic fillers. Bearing in mind that the cellulose groups which are able to polarize can be frozen at different temperatures [18,20], the temperature-dependence of the polarization was studied. The effect of the presence of inorganic fillers in the matrix on the polarization and charging mechanisms is interesting since they further change the relative amount of cellulose.
flowing through the paper sample and the displacement current, which is proportional to the charge deposited on the paper, and the paper surface potential change and its decay after the charging is stopped were monitored and computer controlled. The example of papers’ charging transients and potential decay after the charging is stopped is presented in Fig. 1. For the investigation of the paper polarization during corona charging the paper was charged to the approximately maximum achievable value Us and then discharged by the deposition of the opposite polarity charges. The amount of the opposite charges was such that was needed to obtain paper zero potential. After such a forced discharge, a rise in paper potential was observed (Fig. 2). This potential reached its maximum value Usd (also called the depolarization potential) and then decayed slowly. This effect was observed for both charging polarities and for all the papers investigated. The absolute Us and Usd positive values are a little smaller than the Us and Usd negative values. This effect is due to the fact that the negative corona is more intense than the positive corona. The Usd value is considered in this work to characterize the paper polarization level during corona charging. Polarization effects were measured at temperatures from þ8 to 33 C. All measurements were performed at 30 3% RH. The papers used in the study were made on a pilot paper machine (Stora Enso Oyj, Imatra Research Centre) with controlled process conditions. Machine was running at speeds between 30 and 60 m/min depending on the grammage. Papers with three different grammages (90, 160 and 230 g/m2) and different amounts of added precipitated calcium carbonate were made (0, 15 and 30%). The furnish was Eucalyptus (80%) and pine (20%) refined to a Schopper Riegler value of 27. In addition to filler and cellulose fibers, cationic starch (8 kg/tn), alkyl ketene dimer, (1.5 kg/tn) and two component retention system were used (200 g/tn polyacryl amide, and 1.7 kg/tn Bentonite). The other fillers used in the study were ground calcium carbonate (Hydrocarb FF-LV 75, Omya AG), kaolinite (Hubertx, Omya), Talc (Finntalc F08, Mondo minerals Oy), SiO2 (PerkaSil SM 660, Grace Davison), and TiO2 (Kemira). The papers were further calendered to different thickness levels using a laboratory calender. The samples described in the table were obtained at (A) uncalendered (B) 1 25 kN, and (C) 2 25 kN. The moisture content of the papers was 4e5%. 3. Results and discussion The charging potential behavior and the polarization level characterized by the depolarization potential Usd on the 160 gsm
2. Experimental In this study, the dosed charging method which was proposed earlier in Ref. [21] for the investigation of the paper charging properties was used. A paper sample is placed on the disc. The disc rotates with a constant velocity, which can be changed in the range from 5 to 10 rps. Over the disc are placed a corona charger, an electrometer with a non-vibrating probe to measure the paper surface potential and a synchronization probe and sensor. The paper sample passes them successively and during the disc rotation the small charge dose is deposited in every rotation when the paper sample is under the charger. The charge dose depends on the rotation velocity and on the corona discharge intensity, which was controlled by the corona voltage. The charge dose DQ can be changed between 1010 C and 104 C. Both the electrical current
Fig. 1. Paper charging and paper potential decay.
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Fig. 2. Paper charging, discharging by opposite corona and depolarization potential rise and decay.
papers containing different amounts of precipitated calcium carbonate are shown in Figs. 3 and 4. The changes in charging potential (Fig. 3) are small and it can be concluded that the charging potential is independent of the filler content. The changes in the depolarization potential (Fig. 4) are also small and almost at the limit of the measurement accuracy. Nevertheless, the polarization level tends to decrease slightly with increasing PCC content from 0 to 30% at a temperature of 8 C, but to increase slightly at higher temperatures. The overall depolarization potential decreases significantly however with increasing temperature, particularly when no or only a small amount of precipitated calcium carbonate is present. The dependence on filler content evident in Fig. 4 was unexpected. The addition of precipitated calcium carbonate that has a higher dielectric constant than cellulose might be expected to increase the dielectric constant of papers, but Fig. 5 shows that the dielectric constant does not change as predicted, which is consistent with Simula’s data [22]. The differences in the dielectric constant of papers with different grammages are attributed to
Fig. 4. Depolarization potential versus PCC content at different temperatures: a e negative charging and positive decharging, b e positive charging and negative decharging.
Fig. 3. Charging potential versus PCC content at 23 C: 1 e negative, 2 e positive.
differences in paper density and structure. The influence of filler on the polarization effect is small, and it can be assumed that the cellulose and water orientation polarization is responsible for the observed effect. The depolarization should thus decrease with increasing filler content due to the decrease in the cellulose content, as was observed at 8 C. The fact that the dependence at higher temperatures was the opposite, although weak, means that some other factors dominate the mechanism, probably the formation of space charges. Space charges can be formed by mobile ions, and their concentration should increase with increasing filler content, while their mobility should be higher at higher temperature. There may also be an increase in the moisture content at higher temperatures. Papers with different fillers exhibited different charging and depolarization potentials (Fig. 6). The difference cannot be caused by the difference in the filler amount because differences are very
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J. Sidaravicius et al. / Journal of Electrostatics 71 (2013) 35e40 Table 1 Samples used in the study.
Fig. 5. Dielectric constant as a function of PCC content. Grammage: 1 e 90 gsm, 2e 160 gsm, 3 e 230 gsm.
small (Table 1). On the other hand the charging potential dependence on the filler amount is weak (Fig. 3) or even does not depend on the filler amount in the investigated range of filler concentration. The ground calcium carbonate and kaolin showed the same polarization behavior, whereas the paper containing silica possessed very low polarization and depolarization properties. This is interesting since both silica and kaolin contain SieOH, but silica has a considerably higher surface area and therefore possesses a larger amount of silanol groups compared to that of kaolin. Although silica is porous and may contain surface-bound water, it still has a very low polarization and depolarization potential. TiO2, which is considered to be a relative inert material, showed a lower polarization and depolarization behavior. Since the thicknesses of all the papers were approximately the same and since the potential decay speed does not essentially differ, the difference in charging potential is determined by the paper’s dielectric constant. Indeed, the maximum charging potential depends linearly on the dielectric constant, as shown in Fig. 7,
Fig. 6. Positive and negative max charging and depolarization potential for papers containing different fillers: grammage 160 gsm, filler content 15%.
Nr
Target grammage
Measured grammage
Filler
Filler%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 18
90 160 230 90 160 230 90 160 230 160 160 160 160 160 160
94.6 158.7 232.9 88.1 155.1 228.2 91.7 159.2 223.7
e e e PCC PCC PCC PCC PCC PCC GCC Kaolin Talc Gypsum SiO2 TiO2
0.5 0.4 0.4 14.6 15.0 14.4 27.6 29.2 28.8 16.0 17.1 14.8 22.1 13.6 19.8
it decreases with increasing dielectric constant due to the increase in the paper’s electrical capacity. The dielectric constant of TiO2 is high (over 80) and it is then understandable that paper containing TiO2 exhibits a low charging ability and low polarization level. The depolarization potential depends on the type of filler used. Since the influence of the filler content on the depolarization potential is weak, as is shown in the case of PCC filler (Fig. 4), it can be supposed that the observed difference is caused by some other factor. It can be supposed that origin of the observed polarization effects in papers is accumulation and dissipation of space charges or orientation polarization. Orientation polarization usually depends linearly on the electric field. This dependence was used by Baum [18] to prove that polarization of the regenerated cellulose films polarization is dipole. Baum used thermal depolarization current technique and determined the dependence of the depolarization band area on the polarization field strength. The linear dependence suggests that space charges are not responsible for the occurrence of depolarization current bands. In our case, depolarization potential is considered as a polarization level metric. In the charged paper the electric field is proportional to the charging potential. So the dependence of the polarization potential Usd on
Fig. 7. Maximal charging potential versus relative dielectric constant ε for papers with different fillers (Table 1), grammage 160 gsm. Dashed line e linear approximation, correlation coefficient e 0.869, SD e 83 V.
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the charging potential Us can be considered as an analog of the depolarization band area dependence on the polarization field. This dependence using the data for all the papers investigated independently on the filler origin is presented in Fig. 8. The linear dependence shows that polarization is due to orientation, and in accordance with [18] it can be attributed to the orientation of primary hydroxyl groups in the cellulose or/and hydrogen bonds, and the fillers’ origin played no significant role. The fact that the linear relationship does not pass through the origin means that the polarization is not due solely to orientation, but that there is an initial polarization caused by the space charges. It is well known that polarization depends on the temperature. The depolarization potential is plotted versus temperature for the papers with different contents of PCC in Fig. 9. The depolarization
Fig. 9. (a) Positive and (b) negative depolarization potential as a function of temperature. PCC content: 1 e 0%, 2 e 15%, 3 e 30%.
Fig. 8. The depolarization potential as a function of (a) the positive and (b) negative charging potential. Samples represent all fillers. Lines e linear approximation, correlation coefficient 0.92 (positive charging) and 0.96 (negative charging), SD 10 V (positive charging) and 7.3 V (negative charging).
potential of both polarities decreases gradually with temperature at first, but at a temperature in the vicinity of 30 C decreases faster (it was difficult to measure at higher temperature because of the fast potential decay). This drop in depolarization potential can be explained using Baum’s data [8]. He observed in cellulose films a depolarization maximum in the vicinity of 25 C, which was attributed to the labile hydroxyl groups of hydrogen bonds. If this is true, it can be expected that this polarization mechanism should be seen for example in the temperature dependence of the charging potential. In our previous article we did not pay attention to the unusual dependence for the paper with no salt and with NaCl addition [Ref. [23], Fig. 1b and c] e which showed shoulders at 23 C that were earlier attributed to the measurement inaccuracy. Having in mind these data (Fig. 10), we can conclude that these shoulders are real and that they are caused by changes on the hydrogen bonds in cellulose at temperatures above 20e25 C.
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Acknowledgment Tekes is acknowledged for financial support of the project and Dr. Anthony Bristow and Dr. R. Sidrys for linguistic revision of the manuscript. References
Fig. 10. Charging potential versus temperature. Papers with different salt contents [[23], Fig. 1b].
4. Conclusions Experimental data for papers with different fillers show that paper is polarized during corona charging. The role of the filler in the polarization was surprisingly low. Paper polarizability is dominated by cellulose and the moisture content and liability of cellulose and hydrogen bond hydroxyl groups. Filler influences the dielectric constant and therefore the electrical capacitance. This leads to a higher maximal charging potential and consequently a higher electric field. The increase in the electric field results in a higher polarization level, which depends mainly on the electric field and not on the type of filler. Together with the orientation polarization, space charges are also created. The dependence of depolarization potential on temperature reveals data which indicate possible changes in the hydrogen bond in the vicinity of 25 C, as reported by Baum [18].
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