A study of degradation of cellulosic insulation materials in a power transformer, part 1. Molecular weight study of cellulose insulation paper

Degradation and Stability 48 (1995) 79-81 0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0141-3910/95/$09.50

Polymer

ELSEVIER

0141-3910(95)00023-2

A study of degradation of cellulosic insulation materials in a power transformer, part 1. Molecular weight study of cellulose insulation D. J. T. Hill,“* T. T. Le,” M. Darvenizab & T. Sahab Polymer Materials and Radiation Group, ” Department of Chemistry and ’ Department of Electrical Engineering, The University of Queensland, Brisbane, Q4072, Australia (Received

3 November

1994: accepted

17 December

1994)

As a transformer ages, the chemical and physical properties of the cellulose insulation paper will change. For example, the average molecular weight of the cellulose chains decreases with age, as does the strength of the insulation paper. The molecular weight changes can be studied by several methods. However, the GPC method reported in this paper has an advantage over the other methods because it yields the total molecular weight distribution. In order to understand the mechanism of the degradation of the cellulosic insulation materials in a power transformer, a study of the accelerated ageing of cellulosic insulation paper in insulation oil has been performed in the temperature range 129-1&X, under vacuum. The molecular weight and molecular weight distribution of the cellulose chains present in the insulation paper were measured by GPC techniques. The results provide information on the kinetics of degradation of the insulation paper. The results of the molecular weight study in the accelerated ageing experiments were found to be most useful for monitoring the condition of the insulation paper in an operational transformer.

INTRODUCTION

the insulation materials can undergo slow degradation with concurrent loss in mechanical and electrical properties. Assessing the state of cellulose insulation in a functioning transformer is difficult. For this reason many studies have been directed towards monitoring the cellulose degradation products in the transformer insulation oil. But this method is indirect, and thus suffers from the major disadvantage that it is dependent on a knowledge of the history of the transformer and its components. The accelerated ageing of insulation paper has been reported by several workers over the last century.‘-” These studies have been designed chiefly to develop methodologies for predicting the useful life of a transformer insulation system. Accelerated ageing studies have been carried out in evacuated systems, under an inert atmosphere

Cellulosic materials have been used in the insulation system of power transformers for a long time. Because they are more economical and easier to manufacture than other materials, cellulose products are used in a variety of forms, such as insulation press-board, paper, transformer-board, etc. Cellulosic insulation materials have been proven to have desirable chemical and physical properties for use as electrical insulators, but they degrade as the materials age. Therefore, the degradation of the cellulosic materials is an important factor in determining the life of a transformer. Typical operating temperatures for power transformers lie between 65 and 100°C. At these temperatures, * To

whom correspondence

should

be addressed. 79

80

D. J. T. Hill et al.

such as nitrogen, or in the presence of controlled amounts of oxygen and moisture. These studies have demonstrated that the degradation of cellulose above a temperature of 200°C follows a different mechanism from that below 200°C. However, while the mechanism of the degradation reaction above 200°C is relatively well understood,’ the kinetics and mechanism of degradation reactions below 200°C are less well known, despite power transformers being designed to operate in this low temperature region. Current knowledge of the degradation of insulation materials has been the subject of an excellent recent review by Emsley.s In order to make predictions about the life-time of transfomer insulation, it is necessary to obtain information about the kinetics of the degradation processes. As indicated above, such studies are most conveniently carried out using accelerated ageing. However, in order to carry out such a study it is necessary to identify the of the polymer which can be properties conveniently used to monitor the degradation process. It is also important to ensure that the mechanism of the ageing reaction over the temperature range investigated is the same as that for the operational transformer. Bond scission of the cellulose main chains results in a decrease in the average molecular weight of the chains and, if end effects are ignored, an increase of one chain per scission in the number of chains present. Thus, an investigation of the time dependence of the molecular weight of the cellulose provides information about the rate of main chain bond scission. Previous studies of the kinetics of cellulose insulation paper degradation have been based upon measurements of the limiting viscosity number of solutions of a cellulose derivative, often the copper-amine complex. Thus, these measurements yield a viscosity average molecular weight, M,, for the aged cellulose, although to obtain the rate of bond scission, the number average molecular weight, M,, is required. M, is always greater than M,, and for cellulose it is close to the weight average molecular weight, M,. However, M, is dependent on a number of factors, including the choice of solvent and the nature of the molecular weight distribution of the sample. For &aft insulation paper, the nature of the molecular weight distribution changes on ageing. Thus, while straight line plots have been

reported for the time dependence of the reciprocal of M, (or of DP,),’ the slope of these plots is not a reliable estimate of the true rate of bond scission. Therefore, in this study the molecular weight and molecular weight distributions of the aged samples have been determined by gel permeation chromatography, so that reliable values of M, may be obtained. These then allow calculation of the true rate of bond scission. Most of the previous studies on cellulose insulation paper ageing have concentrated on accelerated ageing experiments performed under simulated operational conditions for transformers, because water and oxygen have been shown to be important factors that may accelerate the degradation of the insulation. However, before the roles of water and oxygen can be properly assessed, it is important to understand some of the features of the degradation in the initial absence of these molecules. Therefore, in this study thermal ageing experiments have been performed in Shell Diala B insulation oil, under vacuum, at temperatures in the range 130-170°C.

EXPERIMENTAL Accelerated

ageing experiments

Thermal ageing experiments have been performed by many authors over the last few decades, and the results of these experiments have been used to make predictions about transformer lifetime. There have been several reports of accelerated ageing experiments in the presence of air and oxygen*%’ and under vacuum.‘.’ These experiments can be divided into two types: long-term accelerated ageing experiwhich have been conducted at low ments, temperatures in the range 90-145°C’ and short-term accelerated ageing experiments, which have been conducted at high temperatures in the range 120-200°C.” In this study, a series of accelerated ageing experiments was performed on Kraft insulation paper, press-board and transformer-board in the temperature range 130-170°C under vacuum. They were performed in transformer oil in order to avoid any oxidation by oxygen or hydrolysis by water initially present. The study will provide a basis for later studies under controlled conditions

Degradation

found in operational simulating those transformers. A section of new insulated conductor (20cm) was placed in a glass ampoule and evacuated (
chromatography

81

of cellulosic insulation materials-l

(GPC)

Cellulose is not readily soluble in simple organic solvents. For this reason the cellulosic components of the insulation papers were modified to improve their solubility. Various modification procedures have been suggested, but the phenyl isocyanate derivatives have been shown by Evans et al.” to offer several advantages. In particular, they have shown that, through the use of mild reaction conditions in pyridine solvent, degradation of the cellulose chains during derivatization can be avoided. Such degradation has proved to be a problem if some other derivatization procedures are used.” In this project, the procedure of Evans et af.“’ has been modified for use in preparing the tricarbanilate derivatives of the cellulosic polymers present in Kraft paper. The procedure used was as follows: 40 mg of dry paper was placed in a 50 ml reaction bottle with 10ml of dry pyridine, together with 1 ml of phenyl isocyanate, and was heated to 80°C for 40 h, after which 1 ml of methanol was added to stop the reaction. The resulting clear, viscous solution was cooled and the cellulose tricarbanilate (CTC) precipitated by adding the solution to 100 ml of stirred methanol. The CTC precipitate was collected by centrifugation of the sample for 5-10min. The solid precipitate was further purified by dissolution in acetone, followed by precipitation in a water: methanol mixture (75 : 25) and centrifugation to separate the precipitate. After collection, the CTC derivative was dried in a vacuum oven at 50°C overnight. The degree of substitution (DS), determined by solution 13C NMR, was

found to be 3, indicating substitution of each hydroxyl group of the anhydroglucose units by the phenyl isocyanate.12 For the GPC measurements, the cellulose tricarbanilate samples were dissolved in tetrahydrofuran at a concentration of 0.1% w/v, the sample solutions being filtered through a O-45 pm the GPC prior to filter immediately measurement. GPC analysis

The molecular weights of derivatized cellulose using a Waters samples were determined Associates Chromatograph, equipped with two detectors connected in series, a variable wavelength Waters 484 tunable absorbance detector (wavelength 236 nm), and a differential refractometer Waters 410 detector (sensitivity range 256-512). For the cellulose tricarbanilate samples, a set of four Ultrastyragel columns (lo”-10h A) was used in series at room temperature in the chromatograph, and tetrahydrofuran (THF) was used as the eluent. The elution profile was acquired through interfacing to a PC computer. Typically, data for analysis were collected at intervals of 1 s. The Ultrastyragel columns were calibrated using a series of 12 monodisperse polystyrene standards in the range 9002 X lo6 g mall’, and a universal calibration curve was prepared, based on the equations given below for polystyrene and CTC in THF:‘” PSTY[n] = 1.800 CTC[n] = 0.201

X 1o-4 X 1o-4

M0.74 M”‘y2

The reliability of the calibration procedure was confirmed by a study of several secondary standard cellulose samples, whose molecular weights had been previously determined by light scattering measurements.

RESULTS

AND DISCUSSION

Figure 1 shows the GPC chromatograms of new The and used cellulose insulation papers. chromatogram of the new cellulose insulation paper shows the presence of two components. One component at lower elution volume or high molecular weight is due to cellulose, while the smaller, lower molecular weight component, is

82

D. J. T. Hill et al.

I”“““‘l”r’l~~“I”“~‘~“I”‘~~~“‘l”“””’l

20

25

Elution Fig. 1. GPC chromatograms

Volume

/

of the CTC derivatives

due to hemicellulose. In the chromatogram of the cellulose insulation paper taken from an aged transformer, the molecular weight of the cellulose component has decreased significantly. The molecular weight distribution of the cellulose has also broadened considerably, and the peak due to the hemicellulose has become barely discernible, suggesting that the hemicellulose component of the paper may have been largely degraded. Figure 2 shows chromatograms of artificially aged cellulose insulation papers, which reveal trends similar to those observed for the cellulose insulation papers obtained from aged

45

40

35

30

mL of new and used insulation

paper.

power transformers. The GPC chromatograms of the cellulose obtained from ageing experiments on insulation paper can be analysed in a manner similar to that adopted for the cotton linter, reported elsewhere.14 Molecular

weight distribution

As the cellulose chains undergo scission, the number of polymer chains present in the sample increases. The concentration of cellulose chains in a given mass of sample can be calculated from the GPC analysis. In this calculation, it has been

l’~“““‘l”“““‘l”““‘r’l~‘~‘~““l”“~’~~’l

20

25

Elution Fig. 2. GPC chromatograms

40

35

30 Volume

of the CTC derivatives

/

45

mL

of the artificially

aged insulation

paper.

Degradation

assumed that any products of chain degradation of the cellulose, including water, CO, COZ, and furans, have a negligibly small effect on the mass of the material present. A check of this assumption by weighing samples before and after degradation confirmed its validity. The conventional GPC chromatogram can be modified and presented as a plot of the number of polymer chains versus the molecular weight of the CTC, as shown in Fig. 3. This figure shows that the distribution of cellulose chains extends to a very high molecular weight and it indicates the presence of two well defined peaks. One peak has a maximum molecular weight of 1-OX 10’ gmol-’ (DP = 193) and is due to the hemicellulose component; the other has a maximum molecular weight of 5 X 10’ g mol-’ (DP = 963) and is due to the cellulose. This second peak is characterized by having a long tail to high molecular weight. The hemicellulose peak in the distribution plot is sharp compared with the broad cellulose peak. A typical set of molecular weight distribution plots for artificially aged insulation paper is shown in Fig. 4. As the insulation paper ages, the high molecular weight tail in the distribution is gradually lost and the concentration of chains of lower molecular weight increases, particularly in the molecular weight range 2-5 X lo’ g mol-’ (DP = 385-963). The hemicellulose peak also becomes less distinct as the degradation time increases, and the degraded hemicellulose com-

distribution

of the curve.

Kinetic model for chain scission

Various workers have developed kinetic models for chain scission to account for the observed changes in the average molecular weight (or average DP) of the cellulose. Shafizadeh and Bradbury’” have reported that thermal chain scission in air or nitrogen at low temperatures follows a zero order model, while Emsley” has suggested that the degradation of cellulose in transformer oil is best represented by a first order model. If chain scission occurs without significant depolymerization, then the number of bonds (I) between anhydroglucose groups for 1 g of polymer is given by I = N,IM, - NC (1) where NA is Avogadro’s number, MG is the molecular weight of the anhydroglucose unit and NC is the number of polymer chains per gram of sample. Zero order model

If chain scission follows a zero order model, then the rate of bond scission is constant. Thus, -dlldt

3

Molecular weight

ponents increasingly overlap some cellulose components in the distribution

2

1

Fig. 3. Molecular

83

of cellulosic insulation materials-l

Weight

(2)

5

6

of new insulation

paper.

4 x mol

=k

/ g 1O-6

plot of the CTC derivative

D. J. T. Hill et al.

1

0

3

2

Molecular Fig. 4. A typical

set of molecular

which on integration

weight

Weight

x mol

(UDP,), (3) where

where I,, and 1, are the number of bonds present per gram at time 0 and time t, respectively, and k is the rate constant expressed in terms of bond scissions per gram of cellulose per second. Substitution from eqn (1) then leads to the relationship; (NC), = (NC),, +

k.t

(4)

If the number average molecular weight of the polymer is M,, then the number of chains present per gram of polymer is: NC = h’,IM,, Substitution into eqn (4) then relationship below, provided no tion takes place:

(5) leads to the depolymeriza-

(6)

(NAM,), = (N&K),, + k . t of polymerization

or

of the artificially

aged paper.

obtained

at

or

1, = I,, - k . t

DP, = MnIM,,

6

/ g 10m6

plots of the CTC derivatives 153°C.

distribution

gives

Also, since the degree given by the expression.

5

4

M, = I&.

DP, is

DP,

(7)

then (NAIM
(8)

= (1/DP&

+ k’ . t

(9)

k’ is given by the equation: k’ = k. M,IN,

(10)

Figure 5 shows the linear relationships between the number of polymer chains per gram and the degradation time for the thermal degradation of cellulose insulation paper. These straight-line relationships indicate that the thermal degradation of insulation paper can be represented by a zero order kinetic equation. According to eqn (4), the zero order rate constant k for chain be obtained from the linear scission can relationship between the number of polymer chains and the degradation time. The rate constants k were obtained and are presented in Table 1. Relatively few of the glycidic bonds in the cellulose are broken over the time of the experiments carried out in this study, so the agreement between the experimental data for the time dependence of Nr and the predictions of the zero order kinetic model, given by eqn (4), should not be taken to mean that the bond scission reaction is inherently zero order. Indeed, the rate constants calculated from the data should be seen only as ‘pseudo’ zero order rate constants, applicable over the region of bond scission covered by this study. However, the

85

Degradation of cellulosic insulation materials-l

15

10

5

0

Time Fig. 5. Plot of the number

of polymer

Table 1. The zero order rate constant k for the thermal degradation of cellulose from insulation paper Temperature 129 138 153 166 “Values data.

(“C)

k

(10” chains/g/s) 1.5 2.9 6.1 12.0

f 0.2 f 0.6 zk 0.8 It 2.0

k (10"

chains/g/s)” 2.3 4.8 15.4 39.4

are calculated from Emsleys and are based on viscosity

25

30

/ days

chains vs degradation

changes in the average molecular weight of the polymer chains over this region are significant, and a decrease by a factor of two is considered to render the cellulose insulation in a transformer unsuitable for further use. Thus, for predictive purposes these zero order rate constants are of practical use. Emsley’ has evaluated the rate constant for bond scission based upon the viscosity data of Shroff and Stannett.’ The values of the rate constants for bond scission calculated from their data at the temperatures studied here are also given in Table 1. While the kinetic parameters of Emsley are of similar magnitude to the values reported herein, they are all higher. This systematic difference can be attributed to the approximations associated with the use of M,

20

time for artificially

aged insulation

paper.

rather than M, to estimate the number of bonds present per gram of polymer. Activation

energy

If the

mechanism of the scission reaction is the same at all temperatures studied, the temperature dependence of the rate constant may be expected to follow the Arrhenius equation: k = A. exp(-Es/R lnk=lnA-EaIR.

. T) T

W) (12)

An Arrhenius plot of the data in Table 1 is shown in Fig. 6 and leads to an activation energy of 79&3kJmoll’. This value is in close agreement with the values previously reported by Fallowl and Bouvier” for thermal degradation of cellulose in oil of 85 kJ mall’. It is slightly smaller than the value reported by Lawson et ~1.~ of 92 kJ mall’, which was based on the viscosity average molecular weight measurements. However, it is smaller than the activation energy for the hydrolytic degradation reactionlx of 120 kJ mall’, and is also smaller than the value reported by Emsleg of 113 kJ mall’, based on the data of Schroft and Stannett.’

D. J. T. Hill et al.

86

26 -

1 0.0022

0.0024

0.0023 1 /

Fig. 6. Temperature

dependence

Temp.

of the rate of chain scission

CONCLUSION The cellulose chain scission reaction in Kraft insulation paper results in a decrease in the average molecular weight of the cellulose, and particularly in a decrease in the concentration of the high molecular weight chains relative to those of lower molecular weight. The loss of the high molecular weight chains is accompanied by a significant change in the nature of the molecular weight distribution of the polymer, which has implications with respect to investigations of the kinetics for bond scission. The rate constant for bond scission can be determined from the time dependence of the number average molecular weight or the number In the average degree of polymerization. previous literature, several workers have reported estimates of the rate constant for bond scission based on the time dependence of the viscosity average molecular weight. Such studies would be valid only if the cellulose was monodisperse or if the relationship between the values of M, and M, remained fixed as the polymer degrades, and the relationship between them was known and taken into account in the kinetic analysis. However, neither of these possibilities is true for the cellulose in Kraft insulation paper. Studies of the thermal scission of cellulose reported in this work have been based upon GPC measurements of M,. These studies indicate that

0.0025

0.0026

(K) for the thermal

degradation

of the insulation

paper.

for the extent of bond scissions studied, which is the region of practical importance, the reaction can be represented by a zero order kinetic model, and that the activation energy for bond scission is about 79 kJ mall’. ACKNOWLEDGMENTS The authors would like to thank the Australian Electricity Supply Industry Research Board (AESIRB) f or supporting and funding this research project, and to express their gratitude to Mr D. Allan and Mr C. Jones from the Queensland Electricity Commission and Dr A. Wallis of CSIRO, Division of Forest Products, for their assistance throughout this research. REFERENCES 1. Schroff. D. H. & Stannett, A. W., IEE Proc., 132 (1985) 312. 2. Moser, H. P., Dahinden, V.. Brupbacher, P.. Schneider, E., Hummel, H., Potocnik, O., Ammann, R., Brechan, H. & Zuger, F., paper 12-12, presented at Int. Conf. Large High-Voltage Elec. Sys. (CIGRE), Paris, 1986. -7 Burton, P. J., Carballiera, M., Duval, M., Fuller, C. W., Graham, J., de Pablo Samat, J. & Spicar, E., paper 15-08, presented at Int. Conf. Large High-Voltage Elec. Sys. (CIGRE), Paris, 1988. 4. Unworth, J. & Mitchell, F., IEEE Trans. Elec. Insul., 25 (1990) 737. 5. Emsley, A. M., Polym. Deg. Stab., 44 (1994) 343. 6. Shafizadeh, F., In Cellulose Chemistry and Its Applications, ed. T. P. Nevell & S. H. Zeonian. Ellis Horwood, New York, 198.5, Chapter 11.

Degradation

of cellulosic

7. Fabre, H. & Pitchon, A., paper 137, presented Conf. Large High-Voltage Elec. Sys. (CIGRE), 1960. 8. Lawson, W. G., Simmons, M. A. & Cale, P. S., Trans. Elect. Znsul., El-l2 (1977) 61. 9. Morrison, E. L., IEEE Trans. Elect. Insul., El-3 76. 10. Evans, R., Wearne, R. H. & Wallis, A. F. A., J. Polym. Sci., 37 (1989) 3291. 11. Wood, B. F., Conner, A. H. & Hill, C. G., Jr, J. Polym. Sci., 32 (1986) 3703.

at Int. Paris, IEEE (1968) Appl. Appl.

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materials-l

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12. Le, T. T., PhD thesis, The University of Queensland, Brisbane, 1994. 13. Ashmawy, A. E. E., Dannhelka, J. & Kossel, I., Suensl Pupperstidning, 16 (1974) 603. 14. Hill, D. J. T., Le, T. T., Darveniza, M., Saha, T. K. & Williams, B., Mater. Forum, 16 (1992) 141. 15. Shafizadeh, F. & Bradbury, A. G. W., J. Appl. Polym. Sci., 23 (1979) 1431. 16. Fallow, B., Rev. Gen. Elect., 79 (1971) 645. 17. Bouvier, B., Rev. Gen. Elect., 79 (1975) 489. 18. Skubala, W. & Roganwicz, J., Przegl Puier, 31(1975) 357.