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Electrostatic properties and characterization of two-layer paper sheets
Journal of Electrostatics 77 (2015) 51e57
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Electrostatic properties and characterization of two-layer paper sheets a nas a, *, Tadeus Lozovski a, b, Pranas Juozas Zilinskas , Ringaudas Rinku a Robertas Mald zius a b
Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, 3 korp, LT-10222 Vilnius, Lithuania University of Bialystok, Vilnius Branch, Kalvariju g. 143, LT-03202 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2014 Received in revised form 29 June 2015 Accepted 30 June 2015 Available online 18 July 2015
The aim of the research is to determine the electrostatic properties of two-layer paper sheets composed of laboratory filter paper and polyethylene. The volume resistivity and conductivity in a filter paper sheet, polyethylene film and a two-layer sheet consisting of them were studied. The experiment was done using two techniques. The investigated samples were exposed to static electric field and the surface and volume conductivity were measured in accordance with ASTM D257 standard. The same samples were also exposed to positive and negative air ion flux that allows a periodical deposition of a dosed amount of charge in order to measure electrostatic properties of the samples, i.e. to measure maximum surface voltage, sheet capacitance, surface voltage decay half time, volume resistivity, et cetera. A study of a twolayer sample consisting of laboratory filter paper and polyethylene film superimposed on the conductive surface in one position when the polyethylene film is on top and in other when the sample is in turned over position shows that the electrostatic properties of the top layer become dominant. The obtained results appear to be useful for more precise understanding of the phenomena occurring in multilayer sheets and to find a way to improve the multilayer sheets features. © 2015 Elsevier B.V. All rights reserved.
Keywords: Polyethylene Laboratory filter paper Electrostatic properties Volume resistivity measurement Volume conductivity measurement
1. Introduction Paper is one of the most commonly used visualization tool for transferring the image from electronic media onto a physical medium and is used in various types of printing machines [1]. The main features of paper are determined by the materials used for its production, by the manufacturing process and by the paper sheet final structure. When the image is created on a paper sheet by electrophotographic printing technology, the paper sheet's electrical properties become particularly important [2e4]. The increasing use of paper, ranging from daily printing, packaging and to its application as a carrier in printing electronics leads to deeper knowledge of electric and dielectric properties not only about paper sheets but also of complex structures containing low conductive plastic materials. Sheets of paper can be homogeneous or multilayered. Homogeneous paper sheets are those that consist of the uniform mass of
* Corresponding author. nas). E-mail address: [email protected] (R. Rinku http://dx.doi.org/10.1016/j.elstat.2015.06.011 0304-3886/© 2015 Elsevier B.V. All rights reserved.
substance (pulp) across the sheet volume. In order to modify mechanical, optical or electrical properties of the pulp mass, which is usually some variant of cellulose, various additives are added [5]. Properties of homogeneous paper sheets are highly dependent on environmental conditions, especially on the ambient humidity and temperature. In some cases, in order to achieve different mechanical, electrical and optical properties of paper, one or both sides of the sheet of paper are covered by different materials. Such a type of a paper sheet becomes a stratified structure and is usually referred to as a multilayer paper sheet [6e8]. A multilayer part composed of thin layers of metal and a dielectric material on a paper sheet, similarly to those proposed in Ref. [9], can also become a passive element for printing electronics. Some tests with a two-layer sample composed of paper and polyethylene placed on a conductive plate have been performed. Whereas significant differences in surface and volume conductivity and volume resistivity depending on the orientation of the sample were observed, a detailed study of this phenomenon was carried out.
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2. Experimental 2.1. Measurement technique The experiment was performed using two techniques. The investigated samples were affected by static electric field and by air ion flux in order to measure electrostatic properties of samples. Test sample affected by static electric field (contact method). A test method for conductivity measurement in accordance with ASTM D257 [10] with the measuring instrumentation described in Ref. [11] is applied. It consists of two types of electrodes: one is adapted for measuring surface conductivity of sheets or films and made using rake-type electrodes [11] (Fig. 1a), and the other, for measuring volume conductivity, is made using a circular conductive plate and ring electrodes [10] (Fig. 1b). The test samples, depending on the quantity measured, together with the same electrodes are pressed against a conductive plate. The test sample pressure was 1.5 kN/m2. For a rake-type electrode system, N ¼ 34, gs ¼ 400 mm and L ¼ 36 mm. For a circular conductive plate, d ¼ 30 mm and the distance between the circular electrode and the ring is gv ¼ 13 mm. The surface conductivity ssurf was obtained by placing a raketype conductive electrode system on the tested sample (Fig. 1c). In this case, the conductive plate electrode is grounded and a direct current voltage þUs to the electrode 1 is applied, and then a voltage Um on the resistor R connected to the electrode 2 is measured. The sample surface conductivity is calculated using the equation
ssurf ¼ ðUm =Us Þ$ðgs =RLÞ ½S; where Um is the measured voltage, Us is the applied voltage, gs is the distance between two adjacent lines of the rake-type electrode, R is the resistance of the external resistor, and L is the overall length of the rake-type electrodes (L ¼ l$N, see Fig. 1a).
a
b
2
gv
d gs
N 2
1 1 l
c
d
where Um is the measured voltage, Us is the applied voltage, h is thickness of the test sample, d is the diameter of the circular electrode, and R is the resistance of the external resistor. In both cases it is assumed that the resistance of the resistor R is much smaller than the sample resistance. Test sample affected by ion flux (non-contact method). Another test method is based on measurement of dielectric parameters when the test sample is placed on a conductive plate electrode and exposed to ion flux [12e14]. Detailed description of the measurement technique, the circuit diagrams and the action sequences for computing the main parameters of the investigated objects are presented in Ref. [12]. The measurement technique allows periodical deposition of a dosed amount of charge on a test sample and makes it possible to measure instantaneous values of deposited electric charge Q [C/m2] and surface voltage Usurf [V] (see Fig. 2). By measuring simultaneously the surface voltage and the deposited electric charge, the time dependence of surface voltage, the surface voltage dependence on deposited charge, the electric capacitance and the volume resistivity dependence on the surface voltage are obtained. From the obtained data the main parameters of the investigated test samples are calculated (maximum surface voltage Umax [V], sample capacitance per unit area C [F/m2], surface voltage decay half time t0.5 [s], volume resistivity rvol [U,m]). The measurement process consists of two stages. The first stage is deposition of electric charge. The deposited charge and the surface voltage are increasing. When the surface voltage stops to increase, the maximum surface voltage Us max is measured. In the initial part of this stage, i. e., at lower surface voltages, the sample capacitance C is calculated as follows: C ¼ DQ/DU, where DQ is the increment of the deposited charge, and DU is the corresponding change of the surface voltage. In the next stage the charge deposition is suspended. In this interval of time the surface voltage naturally begins to diminish and the surface voltage decay half time and the volume resistivity (also called specific insulation resistance) are calculated.
Um R
1 Um
1 2
Surface voltage meter Ion generator
Test sample
+Us 2
svol ¼ ðUm =Us Þ$ 4h=pd2 R ½S=m;
High voltage source
+Us
Test sample
1
The volume conductivity svol was measured by placing a circular conductive plate and ring electrodes on the tested sample (Fig. 1d). In this case, the conductive ring 1 is grounded and a direct current voltage þUs is applied to the conductive plate electrode, and then a voltage Um on the resistor R connected to the electrode 2 is measured. The sample volume conductivity is calculated using the equation
Test sample
Test sample
R
Charge meter Fig. 1. Electrode shape and circuit diagrams: a e rake-type electrode, b e circular conductive plate and ring electrodes, c e equivalent diagram for surface conductivity measurement with grounded conductive plate electrode, d e equivalent diagram for volume conductivity measurement with a connected conductive plate electrode to a direct current voltage.
Fig. 2. The circuit diagrams for volume resistivity measurement when the test sample is exposed to ion flux.
T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
For these calculations, it is assumed that the exponential discharging process is characterized by a single decay time constant. Hence, the volume resistivity rvol is calculated using the formula
rvol ¼ Dt=ðh$C lnðUt =U0 ÞÞ ½U$m; where U0 is the surface voltage at which the volume resistivity is calculated, Ut is the surface voltage after a certain time period Dt, C is the sample capacitance per unit surface area, and h is the thickness of the sample. If U0 is the initial surface voltage and Ut is the surface voltage at Dt ¼ t0.5, then
rvol ¼ 1:44t0:5 =ðh$C Þ ½U$m: In both measurements, i.e. when test samples are affected by static electric field or by ion flux during the experiment process, the environmental conditions were strictly controlled. 2.2. Test samples In order to compare the electric properties of homogeneous and two-layer paper products, several test samples have been prepared. The structure of the investigated test samples is shown in Fig. 3. The first test sample was made of laboratory filter paper (from Schlecher&Schuell 589/1 Rundfilter) (see Fig. 3a). The thickness was 140 (1 ± 5 %) mm. It should be noted that paper conductivity is highly dependent on environmental conditions, temperature and humidity. When the experiment was carried out, the environmental conditions were controlled. The temperature was 23 C and the relative humidity 50 %. The other test sample was made of primary low-density polyethylene film (from UAB Umaras) (see Fig. 3b). The thickness of these test sample was 40 (1 ± 5 %) mm and the conductivity was more then 1016 U1,m1. The conductivity of polyethylene film is many times smaller than the conductivity of laboratory filter paper and can differ more than 104 times. Using these two components (laboratory filter paper and polyethylene film), two-layer test samples have been produced e one with the polyethylene film above the paper sheet (see Fig. 3c) and another with the polyethylene film below the paper sheet (see Fig. 3d). The dimensions of the test samples that have been used for the experiments were 51 mm 46 mm. All test samples were placed on a conductive plate electrode.
53
volume conductivity on static electric field strength was measured with the measuring instrumentation described in Ref. [11]. The measurement results of four test samples (Fig. 3a, b. c and d) are presented in Fig. 4 and Fig. 5. Here the electric field strength E is estimated as Us divided by the electrode separation. The measurement data (Fig. 4) indicate that the surface conductivity of polyethylene and filter paper is quite different. The surface conductivity of filter paper in this case is about 102 times greater than the surface conductivity of polyethylene. The difference of surface conductivity measurement results for polyethylene (curve 1) and polyethylene on filter paper (curve 3) and filter paper (curve 2) and filter paper on polyethylene (curve 4) may be attributed to the electric field distribution across the investigated structure in the measuring process. The results of the volume conductivity measurement for the same test samples are presented in Fig. 5. When the electric field acting on the polyethylene test sample varies up to 60 kV/cm, the volume conductivity varies from 8,10e15 S,m1 up to 2,10e14 S,m1. When the electric field acting on laboratory filter paper test sample varies up to 50 kV/cm, the volume conductivity varies from 5,10e12 S,m1 to 5,10e11 S,m1. As it was expected, the given data show us that the volume conductivity of polyethylene is about 103 times smaller than the volume conductivity of filter paper and that is in accord with results of volume conductivity measurement. Completely different results are obtained when the polyethylene film is below or above the paper sheet. In two test samples (Fig. 3c and d) the volume conductivity varies about the value of 1,10e13 S,m1 and has approximately the same value for both configurations of the sample. The average value differs from volume conductivity of polyethylene or filter paper. This phenomenon needs more in-depth analysis. The presented data suggests that charge redistribution and accumulation effects can take place in such structures.
3. Experimental results 3.1. Surface and volume conductivity measurement Each measurement was started after the samples had been discharged by ionizing radiation. This process ensures that any possible residual effects related to the charge accumulation do not affect the measurement results. Dependence of the surface and
Polyethylene
180 μm
c
140 μm
b Filter paper
d Polyethylene Filter paper
180 μm
40 μm
a
Filter paper Polyethylene
Fig. 3. Structure and thickness of test samples on a conductive plate electrode.
Fig. 4. Dependence of the surface conductivity ssurf on electric field strength E for polyethylene (1), laboratory filter paper (2) and the two-layer structure of polyethylene and filter paper (3, 4).
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Fig. 6. The charging and discharging process of filter paper test sample: 1 e surface voltage Us, 2 e deposited electric charge Q.
Fig. 5. The dependence of the volume conductivity rvol on the electric field strength E for polyethylene (1), laboratory filter paper (2) and the two-layer structure of polyethylene and filter paper (3, 4).
3.2. Measurement of electrostatic properties of test samples exposed by ion flux For more detailed study of the observed phenomenon, a method based on measuring the electrostatic parameters when a sample is exposed to ion flux generated by a corona charging device [12,13] was applied. The instrumentation is able to measure the time dependences of the accumulated amount of electric charge of the sample and the surface voltage both when the sample is affected by ion flux and when it is not. The measured time dependences of four investigated samples are presented in Figs. 6e9. Paper sample chargingedischarging process (see Fig. 6). In this case, the investigated sample is a layer of porous filter paper (see Fig. 3a). It is known that under normal conditions the paper layer with uniform structure can be characterized by an ionic conductivity and that an ionic conductivity of paper is greater than the conductivity of polyethylene. High ionic conductivity of paper is associated with its low resistivity. Due to the relatively low paper resistivity during the charging process, a part of deposited charges on the paper surface moves through the paper layer. When the surface voltage is increasing, an electric field inside the paper induces effects that further reduce the paper resistivity and the charge leakage will increase until it reaches the equilibrium between the amounts of the deposited charges and the leaked charges. On the other hand, it must be taken into account that the paper is a porous material and generation of ions of both polarities may occur in such materials (it is believed that in any case this effect really exists) [15e18]. The deposited charge Q grows with a constant speed even when the paper surface voltage has already reached the maximum value (see Fig. 6 curve 2 exposed with positive ions). When the process of charge deposition is stopped,
Fig. 7. The charging and discharging process of polyethylene sample.
Fig. 8. The charging and discharging process of polyethylene on filter paper sample.
T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
Fig. 9. The charging and discharging process of filter paper on polyethylene sample.
the surface voltage begins to decrease. The slowing down of decrease of the surface voltage Us is associated with the volume resistivity of the tested sample. The generated ions also participate in the paper discharge process and accelerate it. We can see in Fig. 6 that the surface voltage of the tested filter paper sample decreases from 500 V to zero in less than a few seconds. The same phenomenon is observed when the tested sample of paper is exposed to negative ions generated in the air. As one can see a different sign of incident air ions causes a different value of deposited charge and different surface voltage of the sample. Polyethylene sample charging-discharging process (see Fig. 7). In this case, the investigated sample (see Fig. 3b) is a film with uniform structure of polyethylene. The polyethylene is an organic material with high volume resistivity. The polyethylene charging and discharging process is quite different than the same process of a paper sample (Fig. 6). We can see that when the surface voltage reaches the maximum value, the further charge deposition does not increase the surface voltage. The main reasons are that the electric field created inside the layer is sufficient for induced charge carrier formation but the surface voltage blocks the discharge process. When the charging process is stopped (see Fig. 7), the surface voltage decreases slowly over time (slow layer discharge) that occurs due to the small conductivity of polyethylene. The same conclusions can be made when the tested sample of polyethylene is exposed with negative ions. The actual measurement process goes on more than three hours but in Fig. 7 and in the next Fig. 8 the surface voltage dependences are shown truncated in time. By comparing the discharge process of a filter paper sample and a polyethylene film sample, one can see that the polyethylene film conductivity is many times lower than the tested filter paper sample conductivity. Polyethylene on paper sample charging-discharging process (see Fig. 8). In this case, the sample is composed of two layers: a polyethylene layer at the top and a paper layer at the bottom (see Fig. 3c). The upper polyethylene layer is affected by air ion flux. Each layer has different conductivity. It is known that the conductivity of polyethylene is many times lower than paper conductivity (see Fig 4). This is also reflected in the results of our measurements of the discharge process (Figs. 6 and 7). Experimental data shows (Fig. 8) that a charging process of a tested sample is very similar to the charging process of polyethylene (Fig. 7). We see also that the influence of paper on the sample charging process is almost invisible due to the relatively low resistance of paper. Thus, we can conclude that there are no
55
phenomena that can noticeably change the charging process in the paper layer of this structure. The discharge process in the initial stage is faster than the polyethylene discharge process. This may be attributed to the faster discharge process of paper layer due to the higher conductivity of paper. The subsequent discharge process is determined by low conductivity of the polyethylene layer. This suggests that a certain charge quantity is still accumulated on the paper layer. Paper on polyethylene sample charging-discharging process (see Fig. 9). In this case, the sample composed of two layers is turned over e the paper layer is at the top, and the polyethylene layer is at the bottom (see Fig. 3d). The upper layer made of filter paper is affected by air ions flux. We can see (Fig. 9) that the charge and the discharge processes are quite different in comparison with a non-overturned sample (Fig. 8). Comparison of chargingedischarging processes in the sample composed of paper on polyethylene and in the uniform paper sample (Fig. 6) suggests that the underlying processes are similar but there are some important differences. The similarity indicates that in this case the predominant effects are related to the effects that take place in the paper layer. The differences of charging and discharging processes should be discussed separately. The maximum surface voltage of the investigated samples in the charging stage may be compared. In the case of paper on polyethylene, the maximum surface voltage (~1600 V) is greater than in the case of the uniform paper sample (~450 V, see Fig. 6), but it is less than in the case of polyethylene on paper (~2100 V, see Fig. 8). The decrease rate of the surface voltage of the investigated samples in the discharging stage may be compared. It can be seen that the discharge process in the case of paper on polyethylene is faster than in the case of polyethylene on paper. In addition, it can be seen that the discharge process is slower than the discharge process of a paper with uniform structure and faster than the discharge process of a uniform polyethylene film. The slowing down of decrease of the surface voltage can be attributed to the influence of the accumulated charges in the polyethylene layer of the sample. The accumulated charge creates an electric field that hinders the charge carrier motion. It is also seen that the discharge process is incomplete and the accumulated charges on the polyethylene layer create a residual surface voltage on the surface of the investigated sample. This residual surface voltage decreases very slowly. So we can see that the discharge process consists of at least two stages e the fast stage, which can be attributed to the paper layer of the sample, and the slow stage, which can be attributed to the polyethylene layer of the sample. The slow stage of surface voltage decrease can be explained by the fact that only a part of the deposited ions reach the polyethylene layer of the sample, so that a smaller quantity of charge accumulates in the polyethylene layer. In the polyethylene layer, there is no significant phenomenon that could quickly reduce the surface voltage. So the discharge process starts at a lower electric field strength and lasts for a very long time. The main parameters of the tested samples were calculated from the data obtained. In Table 1 a maximum surface voltage Umax, a sample capacitance per unit area C, a surface voltage decay half time t0.5, and a volume resistivity rvol when the test sample was exposed to positive (þ) and negative () ions generated in air by the ion generator are presented. It is known that the compositions of positive and negative ions generated in air are different [19]. In this case, a different composition of incident air ions causes some different values of deposited charge and surface voltage of the sample so the relevant parameters of the investigated test samples differ but the sign of the ion charge does not have a substantial effect on the parameters of the tested samples.
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T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
Table 1 Measured and calculated parameters of test samples when the voltage of corona charging device was Ucv ¼ ±7.5 kV, ambient temperature 23 C and relative humidity 50 %. Test samples
d/mm
þUs
Filter paper Polyethylene Polyethylene on filter paper Filter paper on polyethylene
140 40 180 180
452 2018 2016 1577
max/V
eUs
max/V
461 2295 2216 1803
C/pF$m2 (þ)
C/pF$m2 ()
t0.5/s (þ)
t0.5/s ()
rvol/U,m (þ)
rvol/U,m ()
210 361 213 222
205 353 165 191
0.195 1770 553 3.6
0.191 1305 453 2.9
9.6 1012 1.7 1017 2.07 1016 1.29 1014
9.64 1.27 2.19 1.21
3.3. Comparative analysis of the obtained results The comparison of the data obtained by two different measuring methods shows that the previously-described phenomenon really exists. One can see that the volume conductivity of polyethylene on filter paper and filter paper on polyethylene, which was measured when the test samples were affected by static electric field (Fig. 10a) is between the polyethylene and filter paper volume conductivity. The same phenomenon can be observed for the volume resistivity measured by the method when the test samples were affected by ion flux generated by a corona charging device (Fig. 10b). The volume resistivity of polyethylene on filter paper and filter paper on polyethylene test samples also is between the polyethylene and filter paper volume resistivity. The calculation of the estimated volume conductivity se.vol and the estimated volume resistivity re.vol of two-layer samples using a series resistor model, based on the volume conductivity and volume resistivity of filter paper and polyethylene single layer measurements data, can be performed using the following equations. For calculation of the estimated volume conductivity the equation is:
1012 1017 1016 1014
se:vol ¼ s1 s2 ðl1 þ l2 Þ=ðl1 s2 þ l2 s1 Þ; where s1 is the volume conductivity of polyethylene, s2 is the volume conductivity of filter paper, l1 is the thickness of polyethylene and l2 is the thickness of filter paper. For calculation of the estimated volume resistivity re.vol the equation is:
re:vol ¼ ðl1 r1 þ l2 r2 Þ=ðl1 þ l2 Þ; where r1 is the volume resistivity of polyethylene, r2 is the volume resistivity of filter paper. The calculated values (se.vol ¼ 4.5$1014 S/m and re.vol ¼ 2.8$1016 U m) are shown in Fig. 10a and b. We can see that the calculated volume conductivity and the calculated volume resistivity also are between the measured values of polyethylene and filter paper. The difference of the obtained data in Fig. 10a and b can be explained by the fact that the volume conductivity was measured under a mechanical pressure (1.5 N/m2) of measuring electrodes while the volume resistivity was measured in the free state of the test sample. The capacitance measurement data (Table 1) show that the orientation of the sample does not affect the sample capacitance significantly but the surface voltage measurement data shows that the surface voltage depends on the orientation of the two-layer sample. If the upper layer is polyethylene, the surface voltage is greater than the surface voltage of filter paper but it is less than surface voltage of polyethylene film alone. If the upper layer is filter paper, the surface voltage is less than the surface voltage of polyethylene sample but it is greater than surface voltage of filter paper sample. While there are some differences, the same conclusions can be made when the tested sample of paper is exposed with negative ions. Also we can see that at the same measuring conditions when paper is on top the maximum surface voltage is about 400 V less than surface voltage in the case when polyethylene is on top, the sample capacitance in both cases is more or less the same and the surface voltage decay half-time when paper is on top is about 150 times shorter. This fast discharge part may be associated only with the paper discharge process. The slow discharge part may be associated with the discharge process of polyethylene. The resulting experimental data for samples composed of two layers when the samples are affected by air ions flux shows that the physical processes in the upper layer become dominant. 4. Conclusions The results of the performed experiments lead to the following conclusions:
Fig. 10. The comparison diagram of two measuring methods: a e the volume conductivity when the test samples were affected by static electric field (E ¼ 2.5 kV/cm), b e the volume resistivity when the test samples were affected by negative () ion flux.
1. A study of a two-layer structure sample consisting of laboratory filter paper and polyethylene film superimposed on the conductive surface when the polyethylene film is at the top and when the sample is in a turned-over position shows that the electrostatic properties of the top layer become dominant.
T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
2. An examination of volume resistivity of separate components of two-layer samples as well as of two-layer samples consisting of laboratory filter paper and polyethylene film show us that the two-layer sample does not acquire the volume resistivity of a component with maximum resistivity, but acquires values that are between the volume resistivity of a component with maximum resistivity, in our case the polyethylene film, and a component with minimum resistivity, in our case the laboratory filter paper. 3. The obtained results appear to be useful for more precise understanding of phenomena that occur in multilayer paper sheets and enable manufacturers to find a way to improve the features of multilayer paper sheets. Acknowledgements The research presented in this paper was supported by Vilnius University statutory grant “Investigation and application of charge carries transport in materials of disorder structures” and done in the Department of Solid State Electronics. We thank doc. dr. Andrius Poskus, for comments that helped us to improve our manuscript. References [1] B. Thompson, Printing Material: Science and Technology. Pira International, 1998, p. 591. [2] R.E. Mark, Handbook of Physical and Mechanical Testing of Paper and Paperboard, vol. 2, Marcel Dekker, 1984, p. 508. [3] F.A. Morsy, Dielectric properties of coated paper and the effect of various soluble thickeners, PolymerePlastics Technol. Eng. 44 (2005) 351e362. [4] N. Provatas, A. Cassidy, M. Inoue, Dielectric variation in paper and its effects on electrophotographic printing, in: IS&'sNIP18 International Conference of Digital Printing Technology, 2002, pp. 770e773. € , R. Mald [5] K. Backfolk, J. Sidaravi cius, P. Sirvio zius, T. Lozovski, J.B. Rosenholm, Effect of base paper grammage and electrolyte content on electrical and
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dielectric properties of coated papers, Nordic Pulp Pap. Res. J. 25 (no. 3) (2010) 319e327. M.J. Leskel€ a, Optical calculation of multilayer papers, TAPPI J. 78 (10) (October 1995) 167e172. C. Nutbeem, B. Hallam, Funnctional precoats for multilayer coating, Asian Pap. Conf. (2010) 17. Q.I. Xiaoguang, J. Ronzello, S. Boggs, Dielectric properties of metallized paperfilm capacitors, IEEE Trans. Dielectr. Electr. Insul. 12 (6) (December 2005) 1235e1240. T. Piasecki, K. Nitsch, A. Dziedzic, K. Chabowski, W. Ste˛ plewski, G. Kozioł, Electrical properties of dielectric foil for embedded PCB capacitors, Mater. Pol. 30 (4) (2012) 335e341. ASTM 257-07 Standard Test Methods for DC Resistance or Conductance of Insulating Materials. ASTM International Standard, West Conshohocken, PA, 19428-2959 USA, 18 pp. €, K. Backfolk, R. Mald P. Sirvio zus, J. Sidaravi cius, E. Montrimas, Dependence of paper surface and volume resistivity on electric field strength, J. Imaging Sci. Technol. 52 (3) (May-Jun, 2008) 30501-1e30501-8. P.J. Zilinskas, E. Montrimas, T. Lozovski, Measurement of Dielectric Layer Parameters by the Method of Periodic Charging of the Layer Surface, Tech. Phys. 51 (10) (October 2006) 1372e1378. Pleades Publishing Inc. (USA) Springer Science. €, J. Sidaravicius, T. Lozovski, S. Kuskevicius, K. Backfolk, Dosed P. Sirvio charging: application to the investigation of papers, J. Electrost. 67 (2009) 730e736. P.J. Zilinskas, T. Lozovski, J. Jurksus, Electrostatic properties and characterization of specific polymeric materials for building purposes, Mater. Sci. (Med ziagotyra) 16 (1) (2010) 57e62. D.A. Hays, J.C. Sheflin, Electrostatic adhesion of ion and triboelectric-charged particles, J. Electrost. 63 (2005) 687e692. C.H. Manoratne, R.M.G. Rajapakse, M.A.K.L. Dissanayake, Ionic conductivity of poly(ethyleneoxide) emontmorillonite nanocomposites prepared by intercalation from aqueous medium, Int. J. Electrochem. Sci. 1 (2006) 32e46. S. Sapieha, R. Seth, P. Lepoutre, Electrical conductivity and paper structure, Sven. Papperstidning (Svedish Pap. J. 87 (1984) R127eR132. J. Josefowitcz, Y. Deslandes, Electrical conductivity of paper: measurement methods and charge transport mechanisms, in: M. Hair, M. Croucher (Eds.), Colloids and Surfaces in Reprographic Technology, American Chemical Society, Washington, DC, 1981, pp. 493e531. M.M. Shahin, Nature of charge carriers in negative coronas, Appl. Opt. Suppl. 3: Electrophotogr. 8 (S1) (1969) 106e110.
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Electrostatic properties and characterization of two-layer paper sheets a nas a, *, Tadeus Lozovski a, b, Pranas Juozas Zilinskas , Ringaudas Rinku a Robertas Mald zius a b
Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, 3 korp, LT-10222 Vilnius, Lithuania University of Bialystok, Vilnius Branch, Kalvariju g. 143, LT-03202 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2014 Received in revised form 29 June 2015 Accepted 30 June 2015 Available online 18 July 2015
The aim of the research is to determine the electrostatic properties of two-layer paper sheets composed of laboratory filter paper and polyethylene. The volume resistivity and conductivity in a filter paper sheet, polyethylene film and a two-layer sheet consisting of them were studied. The experiment was done using two techniques. The investigated samples were exposed to static electric field and the surface and volume conductivity were measured in accordance with ASTM D257 standard. The same samples were also exposed to positive and negative air ion flux that allows a periodical deposition of a dosed amount of charge in order to measure electrostatic properties of the samples, i.e. to measure maximum surface voltage, sheet capacitance, surface voltage decay half time, volume resistivity, et cetera. A study of a twolayer sample consisting of laboratory filter paper and polyethylene film superimposed on the conductive surface in one position when the polyethylene film is on top and in other when the sample is in turned over position shows that the electrostatic properties of the top layer become dominant. The obtained results appear to be useful for more precise understanding of the phenomena occurring in multilayer sheets and to find a way to improve the multilayer sheets features. © 2015 Elsevier B.V. All rights reserved.
Keywords: Polyethylene Laboratory filter paper Electrostatic properties Volume resistivity measurement Volume conductivity measurement
1. Introduction Paper is one of the most commonly used visualization tool for transferring the image from electronic media onto a physical medium and is used in various types of printing machines [1]. The main features of paper are determined by the materials used for its production, by the manufacturing process and by the paper sheet final structure. When the image is created on a paper sheet by electrophotographic printing technology, the paper sheet's electrical properties become particularly important [2e4]. The increasing use of paper, ranging from daily printing, packaging and to its application as a carrier in printing electronics leads to deeper knowledge of electric and dielectric properties not only about paper sheets but also of complex structures containing low conductive plastic materials. Sheets of paper can be homogeneous or multilayered. Homogeneous paper sheets are those that consist of the uniform mass of
* Corresponding author. nas). E-mail address: [email protected] (R. Rinku http://dx.doi.org/10.1016/j.elstat.2015.06.011 0304-3886/© 2015 Elsevier B.V. All rights reserved.
substance (pulp) across the sheet volume. In order to modify mechanical, optical or electrical properties of the pulp mass, which is usually some variant of cellulose, various additives are added [5]. Properties of homogeneous paper sheets are highly dependent on environmental conditions, especially on the ambient humidity and temperature. In some cases, in order to achieve different mechanical, electrical and optical properties of paper, one or both sides of the sheet of paper are covered by different materials. Such a type of a paper sheet becomes a stratified structure and is usually referred to as a multilayer paper sheet [6e8]. A multilayer part composed of thin layers of metal and a dielectric material on a paper sheet, similarly to those proposed in Ref. [9], can also become a passive element for printing electronics. Some tests with a two-layer sample composed of paper and polyethylene placed on a conductive plate have been performed. Whereas significant differences in surface and volume conductivity and volume resistivity depending on the orientation of the sample were observed, a detailed study of this phenomenon was carried out.
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2. Experimental 2.1. Measurement technique The experiment was performed using two techniques. The investigated samples were affected by static electric field and by air ion flux in order to measure electrostatic properties of samples. Test sample affected by static electric field (contact method). A test method for conductivity measurement in accordance with ASTM D257 [10] with the measuring instrumentation described in Ref. [11] is applied. It consists of two types of electrodes: one is adapted for measuring surface conductivity of sheets or films and made using rake-type electrodes [11] (Fig. 1a), and the other, for measuring volume conductivity, is made using a circular conductive plate and ring electrodes [10] (Fig. 1b). The test samples, depending on the quantity measured, together with the same electrodes are pressed against a conductive plate. The test sample pressure was 1.5 kN/m2. For a rake-type electrode system, N ¼ 34, gs ¼ 400 mm and L ¼ 36 mm. For a circular conductive plate, d ¼ 30 mm and the distance between the circular electrode and the ring is gv ¼ 13 mm. The surface conductivity ssurf was obtained by placing a raketype conductive electrode system on the tested sample (Fig. 1c). In this case, the conductive plate electrode is grounded and a direct current voltage þUs to the electrode 1 is applied, and then a voltage Um on the resistor R connected to the electrode 2 is measured. The sample surface conductivity is calculated using the equation
ssurf ¼ ðUm =Us Þ$ðgs =RLÞ ½S; where Um is the measured voltage, Us is the applied voltage, gs is the distance between two adjacent lines of the rake-type electrode, R is the resistance of the external resistor, and L is the overall length of the rake-type electrodes (L ¼ l$N, see Fig. 1a).
a
b
2
gv
d gs
N 2
1 1 l
c
d
where Um is the measured voltage, Us is the applied voltage, h is thickness of the test sample, d is the diameter of the circular electrode, and R is the resistance of the external resistor. In both cases it is assumed that the resistance of the resistor R is much smaller than the sample resistance. Test sample affected by ion flux (non-contact method). Another test method is based on measurement of dielectric parameters when the test sample is placed on a conductive plate electrode and exposed to ion flux [12e14]. Detailed description of the measurement technique, the circuit diagrams and the action sequences for computing the main parameters of the investigated objects are presented in Ref. [12]. The measurement technique allows periodical deposition of a dosed amount of charge on a test sample and makes it possible to measure instantaneous values of deposited electric charge Q [C/m2] and surface voltage Usurf [V] (see Fig. 2). By measuring simultaneously the surface voltage and the deposited electric charge, the time dependence of surface voltage, the surface voltage dependence on deposited charge, the electric capacitance and the volume resistivity dependence on the surface voltage are obtained. From the obtained data the main parameters of the investigated test samples are calculated (maximum surface voltage Umax [V], sample capacitance per unit area C [F/m2], surface voltage decay half time t0.5 [s], volume resistivity rvol [U,m]). The measurement process consists of two stages. The first stage is deposition of electric charge. The deposited charge and the surface voltage are increasing. When the surface voltage stops to increase, the maximum surface voltage Us max is measured. In the initial part of this stage, i. e., at lower surface voltages, the sample capacitance C is calculated as follows: C ¼ DQ/DU, where DQ is the increment of the deposited charge, and DU is the corresponding change of the surface voltage. In the next stage the charge deposition is suspended. In this interval of time the surface voltage naturally begins to diminish and the surface voltage decay half time and the volume resistivity (also called specific insulation resistance) are calculated.
Um R
1 Um
1 2
Surface voltage meter Ion generator
Test sample
+Us 2
svol ¼ ðUm =Us Þ$ 4h=pd2 R ½S=m;
High voltage source
+Us
Test sample
1
The volume conductivity svol was measured by placing a circular conductive plate and ring electrodes on the tested sample (Fig. 1d). In this case, the conductive ring 1 is grounded and a direct current voltage þUs is applied to the conductive plate electrode, and then a voltage Um on the resistor R connected to the electrode 2 is measured. The sample volume conductivity is calculated using the equation
Test sample
Test sample
R
Charge meter Fig. 1. Electrode shape and circuit diagrams: a e rake-type electrode, b e circular conductive plate and ring electrodes, c e equivalent diagram for surface conductivity measurement with grounded conductive plate electrode, d e equivalent diagram for volume conductivity measurement with a connected conductive plate electrode to a direct current voltage.
Fig. 2. The circuit diagrams for volume resistivity measurement when the test sample is exposed to ion flux.
T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
For these calculations, it is assumed that the exponential discharging process is characterized by a single decay time constant. Hence, the volume resistivity rvol is calculated using the formula
rvol ¼ Dt=ðh$C lnðUt =U0 ÞÞ ½U$m; where U0 is the surface voltage at which the volume resistivity is calculated, Ut is the surface voltage after a certain time period Dt, C is the sample capacitance per unit surface area, and h is the thickness of the sample. If U0 is the initial surface voltage and Ut is the surface voltage at Dt ¼ t0.5, then
rvol ¼ 1:44t0:5 =ðh$C Þ ½U$m: In both measurements, i.e. when test samples are affected by static electric field or by ion flux during the experiment process, the environmental conditions were strictly controlled. 2.2. Test samples In order to compare the electric properties of homogeneous and two-layer paper products, several test samples have been prepared. The structure of the investigated test samples is shown in Fig. 3. The first test sample was made of laboratory filter paper (from Schlecher&Schuell 589/1 Rundfilter) (see Fig. 3a). The thickness was 140 (1 ± 5 %) mm. It should be noted that paper conductivity is highly dependent on environmental conditions, temperature and humidity. When the experiment was carried out, the environmental conditions were controlled. The temperature was 23 C and the relative humidity 50 %. The other test sample was made of primary low-density polyethylene film (from UAB Umaras) (see Fig. 3b). The thickness of these test sample was 40 (1 ± 5 %) mm and the conductivity was more then 1016 U1,m1. The conductivity of polyethylene film is many times smaller than the conductivity of laboratory filter paper and can differ more than 104 times. Using these two components (laboratory filter paper and polyethylene film), two-layer test samples have been produced e one with the polyethylene film above the paper sheet (see Fig. 3c) and another with the polyethylene film below the paper sheet (see Fig. 3d). The dimensions of the test samples that have been used for the experiments were 51 mm 46 mm. All test samples were placed on a conductive plate electrode.
53
volume conductivity on static electric field strength was measured with the measuring instrumentation described in Ref. [11]. The measurement results of four test samples (Fig. 3a, b. c and d) are presented in Fig. 4 and Fig. 5. Here the electric field strength E is estimated as Us divided by the electrode separation. The measurement data (Fig. 4) indicate that the surface conductivity of polyethylene and filter paper is quite different. The surface conductivity of filter paper in this case is about 102 times greater than the surface conductivity of polyethylene. The difference of surface conductivity measurement results for polyethylene (curve 1) and polyethylene on filter paper (curve 3) and filter paper (curve 2) and filter paper on polyethylene (curve 4) may be attributed to the electric field distribution across the investigated structure in the measuring process. The results of the volume conductivity measurement for the same test samples are presented in Fig. 5. When the electric field acting on the polyethylene test sample varies up to 60 kV/cm, the volume conductivity varies from 8,10e15 S,m1 up to 2,10e14 S,m1. When the electric field acting on laboratory filter paper test sample varies up to 50 kV/cm, the volume conductivity varies from 5,10e12 S,m1 to 5,10e11 S,m1. As it was expected, the given data show us that the volume conductivity of polyethylene is about 103 times smaller than the volume conductivity of filter paper and that is in accord with results of volume conductivity measurement. Completely different results are obtained when the polyethylene film is below or above the paper sheet. In two test samples (Fig. 3c and d) the volume conductivity varies about the value of 1,10e13 S,m1 and has approximately the same value for both configurations of the sample. The average value differs from volume conductivity of polyethylene or filter paper. This phenomenon needs more in-depth analysis. The presented data suggests that charge redistribution and accumulation effects can take place in such structures.
3. Experimental results 3.1. Surface and volume conductivity measurement Each measurement was started after the samples had been discharged by ionizing radiation. This process ensures that any possible residual effects related to the charge accumulation do not affect the measurement results. Dependence of the surface and
Polyethylene
180 μm
c
140 μm
b Filter paper
d Polyethylene Filter paper
180 μm
40 μm
a
Filter paper Polyethylene
Fig. 3. Structure and thickness of test samples on a conductive plate electrode.
Fig. 4. Dependence of the surface conductivity ssurf on electric field strength E for polyethylene (1), laboratory filter paper (2) and the two-layer structure of polyethylene and filter paper (3, 4).
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Fig. 6. The charging and discharging process of filter paper test sample: 1 e surface voltage Us, 2 e deposited electric charge Q.
Fig. 5. The dependence of the volume conductivity rvol on the electric field strength E for polyethylene (1), laboratory filter paper (2) and the two-layer structure of polyethylene and filter paper (3, 4).
3.2. Measurement of electrostatic properties of test samples exposed by ion flux For more detailed study of the observed phenomenon, a method based on measuring the electrostatic parameters when a sample is exposed to ion flux generated by a corona charging device [12,13] was applied. The instrumentation is able to measure the time dependences of the accumulated amount of electric charge of the sample and the surface voltage both when the sample is affected by ion flux and when it is not. The measured time dependences of four investigated samples are presented in Figs. 6e9. Paper sample chargingedischarging process (see Fig. 6). In this case, the investigated sample is a layer of porous filter paper (see Fig. 3a). It is known that under normal conditions the paper layer with uniform structure can be characterized by an ionic conductivity and that an ionic conductivity of paper is greater than the conductivity of polyethylene. High ionic conductivity of paper is associated with its low resistivity. Due to the relatively low paper resistivity during the charging process, a part of deposited charges on the paper surface moves through the paper layer. When the surface voltage is increasing, an electric field inside the paper induces effects that further reduce the paper resistivity and the charge leakage will increase until it reaches the equilibrium between the amounts of the deposited charges and the leaked charges. On the other hand, it must be taken into account that the paper is a porous material and generation of ions of both polarities may occur in such materials (it is believed that in any case this effect really exists) [15e18]. The deposited charge Q grows with a constant speed even when the paper surface voltage has already reached the maximum value (see Fig. 6 curve 2 exposed with positive ions). When the process of charge deposition is stopped,
Fig. 7. The charging and discharging process of polyethylene sample.
Fig. 8. The charging and discharging process of polyethylene on filter paper sample.
T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
Fig. 9. The charging and discharging process of filter paper on polyethylene sample.
the surface voltage begins to decrease. The slowing down of decrease of the surface voltage Us is associated with the volume resistivity of the tested sample. The generated ions also participate in the paper discharge process and accelerate it. We can see in Fig. 6 that the surface voltage of the tested filter paper sample decreases from 500 V to zero in less than a few seconds. The same phenomenon is observed when the tested sample of paper is exposed to negative ions generated in the air. As one can see a different sign of incident air ions causes a different value of deposited charge and different surface voltage of the sample. Polyethylene sample charging-discharging process (see Fig. 7). In this case, the investigated sample (see Fig. 3b) is a film with uniform structure of polyethylene. The polyethylene is an organic material with high volume resistivity. The polyethylene charging and discharging process is quite different than the same process of a paper sample (Fig. 6). We can see that when the surface voltage reaches the maximum value, the further charge deposition does not increase the surface voltage. The main reasons are that the electric field created inside the layer is sufficient for induced charge carrier formation but the surface voltage blocks the discharge process. When the charging process is stopped (see Fig. 7), the surface voltage decreases slowly over time (slow layer discharge) that occurs due to the small conductivity of polyethylene. The same conclusions can be made when the tested sample of polyethylene is exposed with negative ions. The actual measurement process goes on more than three hours but in Fig. 7 and in the next Fig. 8 the surface voltage dependences are shown truncated in time. By comparing the discharge process of a filter paper sample and a polyethylene film sample, one can see that the polyethylene film conductivity is many times lower than the tested filter paper sample conductivity. Polyethylene on paper sample charging-discharging process (see Fig. 8). In this case, the sample is composed of two layers: a polyethylene layer at the top and a paper layer at the bottom (see Fig. 3c). The upper polyethylene layer is affected by air ion flux. Each layer has different conductivity. It is known that the conductivity of polyethylene is many times lower than paper conductivity (see Fig 4). This is also reflected in the results of our measurements of the discharge process (Figs. 6 and 7). Experimental data shows (Fig. 8) that a charging process of a tested sample is very similar to the charging process of polyethylene (Fig. 7). We see also that the influence of paper on the sample charging process is almost invisible due to the relatively low resistance of paper. Thus, we can conclude that there are no
55
phenomena that can noticeably change the charging process in the paper layer of this structure. The discharge process in the initial stage is faster than the polyethylene discharge process. This may be attributed to the faster discharge process of paper layer due to the higher conductivity of paper. The subsequent discharge process is determined by low conductivity of the polyethylene layer. This suggests that a certain charge quantity is still accumulated on the paper layer. Paper on polyethylene sample charging-discharging process (see Fig. 9). In this case, the sample composed of two layers is turned over e the paper layer is at the top, and the polyethylene layer is at the bottom (see Fig. 3d). The upper layer made of filter paper is affected by air ions flux. We can see (Fig. 9) that the charge and the discharge processes are quite different in comparison with a non-overturned sample (Fig. 8). Comparison of chargingedischarging processes in the sample composed of paper on polyethylene and in the uniform paper sample (Fig. 6) suggests that the underlying processes are similar but there are some important differences. The similarity indicates that in this case the predominant effects are related to the effects that take place in the paper layer. The differences of charging and discharging processes should be discussed separately. The maximum surface voltage of the investigated samples in the charging stage may be compared. In the case of paper on polyethylene, the maximum surface voltage (~1600 V) is greater than in the case of the uniform paper sample (~450 V, see Fig. 6), but it is less than in the case of polyethylene on paper (~2100 V, see Fig. 8). The decrease rate of the surface voltage of the investigated samples in the discharging stage may be compared. It can be seen that the discharge process in the case of paper on polyethylene is faster than in the case of polyethylene on paper. In addition, it can be seen that the discharge process is slower than the discharge process of a paper with uniform structure and faster than the discharge process of a uniform polyethylene film. The slowing down of decrease of the surface voltage can be attributed to the influence of the accumulated charges in the polyethylene layer of the sample. The accumulated charge creates an electric field that hinders the charge carrier motion. It is also seen that the discharge process is incomplete and the accumulated charges on the polyethylene layer create a residual surface voltage on the surface of the investigated sample. This residual surface voltage decreases very slowly. So we can see that the discharge process consists of at least two stages e the fast stage, which can be attributed to the paper layer of the sample, and the slow stage, which can be attributed to the polyethylene layer of the sample. The slow stage of surface voltage decrease can be explained by the fact that only a part of the deposited ions reach the polyethylene layer of the sample, so that a smaller quantity of charge accumulates in the polyethylene layer. In the polyethylene layer, there is no significant phenomenon that could quickly reduce the surface voltage. So the discharge process starts at a lower electric field strength and lasts for a very long time. The main parameters of the tested samples were calculated from the data obtained. In Table 1 a maximum surface voltage Umax, a sample capacitance per unit area C, a surface voltage decay half time t0.5, and a volume resistivity rvol when the test sample was exposed to positive (þ) and negative () ions generated in air by the ion generator are presented. It is known that the compositions of positive and negative ions generated in air are different [19]. In this case, a different composition of incident air ions causes some different values of deposited charge and surface voltage of the sample so the relevant parameters of the investigated test samples differ but the sign of the ion charge does not have a substantial effect on the parameters of the tested samples.
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T. Lozovski et al. / Journal of Electrostatics 77 (2015) 51e57
Table 1 Measured and calculated parameters of test samples when the voltage of corona charging device was Ucv ¼ ±7.5 kV, ambient temperature 23 C and relative humidity 50 %. Test samples
d/mm
þUs
Filter paper Polyethylene Polyethylene on filter paper Filter paper on polyethylene
140 40 180 180
452 2018 2016 1577
max/V
eUs
max/V
461 2295 2216 1803
C/pF$m2 (þ)
C/pF$m2 ()
t0.5/s (þ)
t0.5/s ()
rvol/U,m (þ)
rvol/U,m ()
210 361 213 222
205 353 165 191
0.195 1770 553 3.6
0.191 1305 453 2.9
9.6 1012 1.7 1017 2.07 1016 1.29 1014
9.64 1.27 2.19 1.21
3.3. Comparative analysis of the obtained results The comparison of the data obtained by two different measuring methods shows that the previously-described phenomenon really exists. One can see that the volume conductivity of polyethylene on filter paper and filter paper on polyethylene, which was measured when the test samples were affected by static electric field (Fig. 10a) is between the polyethylene and filter paper volume conductivity. The same phenomenon can be observed for the volume resistivity measured by the method when the test samples were affected by ion flux generated by a corona charging device (Fig. 10b). The volume resistivity of polyethylene on filter paper and filter paper on polyethylene test samples also is between the polyethylene and filter paper volume resistivity. The calculation of the estimated volume conductivity se.vol and the estimated volume resistivity re.vol of two-layer samples using a series resistor model, based on the volume conductivity and volume resistivity of filter paper and polyethylene single layer measurements data, can be performed using the following equations. For calculation of the estimated volume conductivity the equation is:
1012 1017 1016 1014
se:vol ¼ s1 s2 ðl1 þ l2 Þ=ðl1 s2 þ l2 s1 Þ; where s1 is the volume conductivity of polyethylene, s2 is the volume conductivity of filter paper, l1 is the thickness of polyethylene and l2 is the thickness of filter paper. For calculation of the estimated volume resistivity re.vol the equation is:
re:vol ¼ ðl1 r1 þ l2 r2 Þ=ðl1 þ l2 Þ; where r1 is the volume resistivity of polyethylene, r2 is the volume resistivity of filter paper. The calculated values (se.vol ¼ 4.5$1014 S/m and re.vol ¼ 2.8$1016 U m) are shown in Fig. 10a and b. We can see that the calculated volume conductivity and the calculated volume resistivity also are between the measured values of polyethylene and filter paper. The difference of the obtained data in Fig. 10a and b can be explained by the fact that the volume conductivity was measured under a mechanical pressure (1.5 N/m2) of measuring electrodes while the volume resistivity was measured in the free state of the test sample. The capacitance measurement data (Table 1) show that the orientation of the sample does not affect the sample capacitance significantly but the surface voltage measurement data shows that the surface voltage depends on the orientation of the two-layer sample. If the upper layer is polyethylene, the surface voltage is greater than the surface voltage of filter paper but it is less than surface voltage of polyethylene film alone. If the upper layer is filter paper, the surface voltage is less than the surface voltage of polyethylene sample but it is greater than surface voltage of filter paper sample. While there are some differences, the same conclusions can be made when the tested sample of paper is exposed with negative ions. Also we can see that at the same measuring conditions when paper is on top the maximum surface voltage is about 400 V less than surface voltage in the case when polyethylene is on top, the sample capacitance in both cases is more or less the same and the surface voltage decay half-time when paper is on top is about 150 times shorter. This fast discharge part may be associated only with the paper discharge process. The slow discharge part may be associated with the discharge process of polyethylene. The resulting experimental data for samples composed of two layers when the samples are affected by air ions flux shows that the physical processes in the upper layer become dominant. 4. Conclusions The results of the performed experiments lead to the following conclusions:
Fig. 10. The comparison diagram of two measuring methods: a e the volume conductivity when the test samples were affected by static electric field (E ¼ 2.5 kV/cm), b e the volume resistivity when the test samples were affected by negative () ion flux.
1. A study of a two-layer structure sample consisting of laboratory filter paper and polyethylene film superimposed on the conductive surface when the polyethylene film is at the top and when the sample is in a turned-over position shows that the electrostatic properties of the top layer become dominant.
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2. An examination of volume resistivity of separate components of two-layer samples as well as of two-layer samples consisting of laboratory filter paper and polyethylene film show us that the two-layer sample does not acquire the volume resistivity of a component with maximum resistivity, but acquires values that are between the volume resistivity of a component with maximum resistivity, in our case the polyethylene film, and a component with minimum resistivity, in our case the laboratory filter paper. 3. The obtained results appear to be useful for more precise understanding of phenomena that occur in multilayer paper sheets and enable manufacturers to find a way to improve the features of multilayer paper sheets. Acknowledgements The research presented in this paper was supported by Vilnius University statutory grant “Investigation and application of charge carries transport in materials of disorder structures” and done in the Department of Solid State Electronics. We thank doc. dr. Andrius Poskus, for comments that helped us to improve our manuscript. References [1] B. Thompson, Printing Material: Science and Technology. Pira International, 1998, p. 591. [2] R.E. Mark, Handbook of Physical and Mechanical Testing of Paper and Paperboard, vol. 2, Marcel Dekker, 1984, p. 508. [3] F.A. Morsy, Dielectric properties of coated paper and the effect of various soluble thickeners, PolymerePlastics Technol. Eng. 44 (2005) 351e362. [4] N. Provatas, A. Cassidy, M. Inoue, Dielectric variation in paper and its effects on electrophotographic printing, in: IS&'sNIP18 International Conference of Digital Printing Technology, 2002, pp. 770e773. € , R. Mald [5] K. Backfolk, J. Sidaravi cius, P. Sirvio zius, T. Lozovski, J.B. Rosenholm, Effect of base paper grammage and electrolyte content on electrical and
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