Some Electrical Properties of Wood Pulp

S. BOUTROS and A. A. HANNA, National Research Centre, Dokki, Cairo, Egypt

Synopsis

The variation with temperature and frequency of the dielectric constant c' and the dielectric loss 6'' for sheet wood pulp and ground wood pulp were measured. Also, the effect of the relative humidity on the dielectric behavior was measured for the ground sample a t 25C. For the dry ground wood pulp, the dielectric constant is larger than that for the dry sheet sample. This may be a result of the increase in the surface area, of the decrease in the size of crystals and/or of the decrease in the degree of crystallinity on grinding of the sheet sample. The variation of e'' with frequency passes through a maximum. From the shift of this maximum with temperature, it is found that the apparent activation energy LM for this relaxation is equal to 7.06 kcal/mole and i t is attributed to the polar-. ization of the OH groups in the cellulose molecule. From the relation between the dielectric constant and the specific resistivity R,, the dissociation energy UO for the ground wood pulp was calculated. UO for this sample below and above 52% RH is 0.315 and 5.13 X erg, respectively. Also, the dissociation energy of Egyptian Ashmouny cotton was calculated. The variation of the electrical conductivity u with humidity for different types of cellulosic materials is represented graphically.

INTRODUCTION

The dielectric properties of wood have been investigated by many authors.1-8 In a previous publication, the authors reported on the variation of the dielectric

properties with temperature, frequency, and relative humidity (RH) for Egyptian Ashmouny cottong and British cotton.lOJ1 It was observed that both the di- electric constant and the dielectric loss increased with increasing temperature and RH. The increase with temperature was attributed to an increase in the rotation of the polarized groups in the cellulose molecules, while the increase with the RH was attributed to the freeing of ions and groups in the cellulose as well as to the ionization of some water molecules, especially at high relative humidity. For the British cotton, the authors found that the dissociation energy Uo was 0.318 and 5.46 X

The present work is concerned with a further study of the dependence of the dielectric properties for the cellulosic materials upon the relative humidity. Also, a comparison between the dielectric behavior of sheet and ground wood pulp is discussed. From the relation between the dielectric constant and specific re- sistivity the dissociation energy is calculated.

erg, respectively below and above 52% RH.

EXPERIMENTAL

The starting material was wood pulp sheets supplied by Elof Hansson, Gote- borg, Sweden, and containing 91.3% cellulose. Two types of the wood pulp samples were studied: a number of sheets as received from the supplier and a ground wood pulp prepared from such sheets. To obtain the ground wood pulp, some of the sheets were cut into small pieces and ground in a hardened steel vessel containing two steel balls. The vessel was fitted to a Spex-Mixer mill which was

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 16, 1443-1448 (1978) 0 1978 John Wiley & Sons, Inc. 0360-6376/78/0016-1443$01.00

1444 BOUTROS AND HANNA

rotated for a sufficient time. The obtained powder was sieved through 0.2 standard mesh.

To study the dependence of the dielectric constant and the dielectric loss on the relative humidity, four disks of a ground wood pulp were exposed to 35,52, 76, and 92% RH at 25OC. The RH of the disk was followed by using a previously described technique.ll

A multidekameter WTW, type DK06, having frequency band from 0.1 to 12 MHz/sec and cell of the MFM5T type were used.9

RESULTS AND DISCUSSION The variation of the dielectric constant t' with temperature at various

frequencies for the sheet and the ground samples is shown in Figure 1. From the curves it is observed that the dielectric constant increases with increasing temperature at all frequencies; this may be due to the decrease in the effect of environment. The variation of the dielectric loss d' with log f passes through a maximum (Fig. 2).

The dielectric constant and the dielectric loss are tabulated in Table I. It is clear that both the dielectric constant and the dielectric loss increase with in- creasing the RH. Also, it is observed that the change in the dielectric loss with RH is more considerable at RH above 52%. This behavior was observed and discussed in detail previously for the Egyptian cotton9 and British cotton,ll but the value of the dielectric constant of thk wood pulp is smaller than that value for the Egyptian cotton, which contains 96.5% cellulose (Table 11). This may result from the presence of some lignin in the wood pulp (91.3% cellulose). Moreover, as the lignin compound has a relativity low dielectric constant, the polarization of the OH groups in the cellulose molecule is reduced.

From Figure 1, it is observed that the dielectric constant 'for the ground sample

-20 0 20 LO 60 80 -20 0 20 LO 60 80 P C

Fig. 1. Variation of the dielectric constant with temperature at different frequencies for sheet and ground wood pulp.

ELECTRICAL PROPERTIES OF WOOD PULP 1445

TABLE I Variation of Dielectric Constant t and Dielectric Loss 6 for Ground Wood Pulp with RH For

Different Frequencies at 25C ____.__

f (Mhd O%RH 35% RH 52% RH 76% RH 92% RH

sec) f t e CI f C c c c 6

0.2 3.35 0.188 4.91 0.199 5.71 0.241 6.97 0.831 8.31 3.02 0.5 3.26 0.196 4.80 0.203 5.52 0.241 6.81 0.681 7.80 1.70 1 3.20 0.213 4.70 0.217 5.40 0.242 6.62 0.592 7.01 1.21 2 3.13 0.221 4.61 0.224 5.34 0.242 6.48 0.470 6.66 1.04 5 2.96 0.217 4.40 0.225 5.12 0.242 6.04 0.421 6.42 0.96

10 2.81 0.230 4.20 0.231 4.84 0.243 5.50 0.391 5.56 0.78

TABLE I1 Variation of Dielectric Constant f and Dielectric Loss t for Egyptian Cotton with RH for

Different Frequencies at 25C

f (MHd O%RH 35% RH 52% RH 76% RH 92% RH

sec) t f f f! d f d c t t

0.2 2.82 0.102 3.78 0.220 4.36 0.343 5.49 1.584 8.97 13.600 0.5 2.76 0.134 3.69 0.215 4.22 0.309 5.06 1.032 7.70 6.330 1 2.73 0.131 3.63 0.222 4.14 0.310 4.85 0.820 6.62 3.550 2 2.67 0.165 3.55 0.258 4.03 0.326 4.59 0.691 5.47 2.518 5 2.57 0.185 3.39 0.292 3.82 0.329 4.20 0.591 4.21 1.900

10 2.46 0.207 3.22 0.315 3.61 0.367 3.89 0.571 3.90 1.400

is greater than that for the sheet. This behavior can be explained by assuming the dielectric constant of cellulose is that of a two-component system, one component being the crystalline fraction and the other the amorphous fraction.12 For the crystalline fraction, the dielectric constant depends to some extent upon the size of crystals; more properly, it depends upon the surface area to volume ratio of the crystals. This would be expected because the surface molecules will have some polar groups that are not bonded in the crystal lattice, and therefore have greater freedom of movement. Accordingly, the dielectric constant of very small crystals should be somewhat larger than that of large crystals by an amount corresponding to the polarization of the unbonded groups. For the amorphous fraction, the dielectric constant would appear to be influenced considerably by proximity and configuration of adjacent molecules, so that the decrease in crystallinity results in a greater chance for the amorphous chains to achieve random disorder, and therefore a higher dielectric constant value.

By grinding the wood pulp sheets, the ratio between the surface area to the volume increases, the size of the crystals and the degree of crystallinity decrease, and the polarization of the molecules increases, hence the dielectric constant increases.13

Figure 2 shows that there is a flat loss maximum which shifts towards a higher frequency with less broadening when the temperature is increased. The shift of the maximum loss with temperature for the ground wood pulp proceeds as follows: t = -2OoC, log f m , 5.58; OC, 6.03; 2OoC, 6.44; 4OoC, 6.80.

In general, two loss regions have been found in the high polymers in the solid

1446 BOUTROS AND HANNA

5 6 7 5 6 7 Log f

Fig. 2. Variation of the dielectric loss with frequency at different temperatures for sheet and ground wood pulp.

statel4-I6: the a relaxation which is characterized by a loss peak located at temperature above the glass-transition point and the /3 relaxation characterized by a flat loss maximum extending over a wide temperature range below the transition point. The primary or a relaxation, which has been characterized by a high apparent activation energy (70 kcal/mole) has been attributed to large-scale conformational rearrangements of the main chains. The secondary or p relaxation, whose activation energy is equal to 10 kcal/mole, has been at- tributed to the motion of the polar side groups, to chain twisting, or to small unit motion of chain.17

In order to calculate the apparent activation energy for the relaxation process AH, log f , is plotted against reciprocal temperature according to the equation18:

2.303 Rd log f, d ( l / T )

AH=

The resulting AH value is equal to 7.06 kcal/mole for ground wood pulp. This value is in agreement with the activation energy obtained for Egyptian c ~ t t o n , ~ which was attributed to the polarization of the OH groups according to Ishidel8 and Thurn.lg

For the moist samples, the electrical conductivity u and the specific resistivity R, were calculated by using the dielectric loss data11,20721 for Ashmouny Egyptian cotton, British cotton,ll and ground wood pulp (Table 111). From the relation between the electrical conductivity and the relative humidity (Fig. 3 ) , it is ob-

TABLE I11 Variation of Conductivity u, Specific Resistivity R,, and Dissociation Energy UO for Ground

Wood P u b and EevDtian Cotton with RH at 25OC

Ground wood pulp Egyptian cotton RH uo x 102 uo x 102 (%) x 107 log Rs (erg) x 107 log Rs (erg)

0 0.0611 8.2138 0.0833 8.0792 35 0.0777 8.0821 0.315 0.1100 7.9580 0.310 52 0.0944 8.0253 0.1390 7.870 76 0.5126 7.2900 1.0501 7.0210 92 2.2990 6.6835 5.13 13.6000 5.8630 3.510

ELECTRICAL PROPERTIES OF WOOD PULP 1447

- 6.0

- 6.5

-7.0

A m s -?.=

-8.0

-8.5

- 9.0

c 0 20 60 60 80 100

R H ' lo

Fig. 3. Variation of the electrical conductivity u with relative humidity: (- - -) Egyptian cotton; (-.-) British cotton; (--) ground wood pulp.

served that as the relative humidity increases, the electrical conductivity in- creases. This increase may be due to the increase in the number of ions available for conduction. At RH below 52%, the water was used in softening the cellulose molecules and then facilitated the polarization of the OH groups in the cellulose molecules, so that the dielectric constant increases markedly and the electrical conductivity increases slightly. Above 52% RH, the added water was ionized and the number of ions will increase, and hence the electrical conductivity in- creased markedly (Fig. 3).

The dissociation energy Uo was calculated from the following equation22:

A = Uo log e / 2 k T ,

where A is the slope of the line which represents the relation between log R, and l l d (t' measured at 0.2 MHz/sec), e is the electronic charge, k is the Boltzmann constant, and T is the absolute temperature (298'K).

erg below and above 52% RH, respectively (Table 111). These values are in good agreement with values obtained for British cellulose,l1 also they are of the same order of magnitude as the values obtained by other authors.22

In the same manner the dissociation energy was calculated for the Egyptian cottong (Table 111). It is found that Uo is 0.310 X and 3.51 X erg.

From the two values for the dissociation energy and the variation of the elec- trical conductivity (Fig. 3) for Egyptian cotton, British cotton, and ground wood pulp, it can be concluded that the effect of absorbed water on the electrical properties depends not only on the polarity of the polar groups in the cellulosic materials but also on the number of the ions resulting from the ionization of the water molecules.

The obtained value for the ground wood pulp is 0.315 X and 5.13 X

1448 BOUTROS AND HANNA

References

1. A. J. Stamm, Wood and Cellulose Science, Ronald, New York, 1964, p. 359. 2. J. H. Brown, R. W. Davidson, and C. Skaar, Forest Prod. J., 13,455 (1963). 3. S. T. Lee, Scientia Silvae, 9,233 (1964). 4. A. Venkateswaran and S. Y. Tiwari, Tappi, 47,25 (1964). 5. R. W. Peterson, The Dielectric Properties of Wood, Forest Prod. Lab. Tech. Note No. 16,

6. C. Skaar, The Dielectric Properties of Wood at Seueral Radio Frequencies, N.Y. State College

7. J. Tsutsumi and H. Watanabe, Nippon Mukuzai Gakkaishi, 11,232 (1965). 8. J. Tsutsumi and H. Watanabe, Nippon Mukuzai Gakkaishi, 12,115 (1965). 9. H. A. Rizk and S. Boutros, Chem. Scr., 4,111 (1973).

10. H. G. Shinouda and A. A. Hanna, Cellul. Chern. Technol., 9,317 (1975). 11. S. Boutros and A. A. Hanna, J . Polym. Sci., in press. 12. D. E. Kane, J . Polym. Sci., 18,405 (1955). 13. H. W. Verseput, Tappi, 34,572 (1951). 14. H. J. D. Sandiford, J. Appl. Chem. London, 8,188 (1958). 15. M. Takayanagi, Y. Ishida, and K. Yamfugi, J.J.S.T.M., 10,383 (1961). 16. Y. Ishida, J. Polym. Sci. A-2, 7,1835 (1969). 17. G. Williams, Molecular Relaxation Process (Chemical Soc. Spec. Publ. No. 20), Chemical

18. Y. Ishida, M. Toshino, and M. Takayanagi, J. Appl. Polym. Sci., 1/2,227 (1959). 19. H. Thurn and F. Wurstlin, Kolloid Z., 145,133 (1956). 20. M. Davies and G. Williams, Trans. Faraday Soc., 56,1651 (1960). 21. M. Davies, Some Electrical and Optical Aspects of Molecular Behavior, Pergamon Press,

22. J. W. S. Hearle, J. Text. Inst., 44, T177, 117 (1953).

Ottawa, Ont., Canada, 1960.

Forest. Technol. Publ. No. 69, 1948.

Society, London, 1966 p. 21.

New York-London, 1965, p. 84.

Received March 18,1977