Kinetics and mechanisms of the low-temperature degradation of cellulosewxjs.chinayyhg.com/upload/Files/Cellulose/1994-(01)/01/... · 2009. 11. 21. · Cellulose-based paper is widely

CELLULOSE (1994) 1, 26-56

REVIEW

Kinetics and mechanisms of the low-temperature degradation of cellulose

A. M. EMSLEY Department of Chemistry, University of Surrey, Guildford, Surrey, GU2 5XH

G. C. STEVENS Research & Engineering, National Power plc, Windmill Hill Business Park, Whitehall Way, Swinden, SN5 6PB

A critical review is given of the degradation of cellulose in the low-temperature region (below about 300 ~ of power transformer operation. The large number of kinetic studies, under a variety of environmental conditions from Kraft paper in insulating oil, to cotton and paper in oxygen, are considered in terms of a first-order polymer chain scission model. In many cases, the data are replotted to suit the model. A common activation energy of 111 +__ 6 kJmoi -1 is calculated and it is shown that the pre-exponential factor, rather than the activation energy, is sensitive to the oxidizing nature of the environment and the susceptibility to degradation of the material. The chemical mechanisms of degradation are reviewed, and conclusions and recommendations are made regarding chemical condition monitoring and life prediction of electrical insulation.

KEYWORDS: low-temperature degradation, kinetics, mechanisms, electrical insulation, transformers

S Y N O P S I S

Cellulose-based paper is widely used as electrical insulation in large electrical power transformers and cables. Its deterioration under the combined effects of thermal, oxidative and hydrolytic degradation determines the ultimate life of the insulation system, although other factors may cause it to fail earlier. In order to be able to predict insulation life, we need to model mathematically the degradation and failure processes, but first we must understand the detailed chemical mechanisms and kinetics of cellulose degradation. The use of chemical indicators of degradation is of vital importance to remote assessment of the condition of the material. Kinetics of cellulose depolymerization may be used to estimate insulation life, and degradation products, such as CO, H2, hydrocarbons and some furaldehyde derivatives can be used to assess insulation condition.

A general kinetic equation for depolymerization is derived for the degradation of cellulose over a wide range of material types and environmental conditions. The activation energy is 111 _ 6 kJ tool -1 and is independent of the reaction conditions. The degradation rate is significantly influenced by the pre-exponential factor, which increases with increasingly aggressive environments. Some general principles are 0969-0239 �9 1994 Blackie Academic & Professional

KINETICS A N D MECHANISMS OF CELLULOSE DEGRADATION 27

established regarding the degradation mechanisms, concentrating in particular on the formation of 2-furaldehyde, which is used as a specific chemical indicator of cellulose degradation in oil immersion applications.

I N T R O D U C T I O N

Cellulose degradation has been of interest to polymer chemists since polymer structures were first investigated. It is also of interest to the electrical power industry, because cellulosic insulation is widely used in oil-filled power transformers and high-voltage power cables. Degradation of the cellulose is critical in determining the ultimate life of the insulation system, although other factors may cause premature failure. It is essential that we understand the chemical and physical processes involved in its degradation, in order to enable us to predict insulation life and to assist in the development of insulation monitoring methods.

Early studies of cellulose degradation were in acidic and alkaline media and related to paper manufacture and cotton bleaching. Among the first theoretical works were studies by Kuhn and co-workers, based on statistical considerations (Kuhn, 1930; Freudenberg et al. , 1930). These were later developed into a kinetic model for the degradation of linear molecules by Ekamstam (1936) and it has been successfully applied in some later studies of cellulose degradation under a variety of conditions. Although a number of model experiments have been carried out in insulating oil, the Ekamstam equation has not previously been used to analyse the kinetics.

Measurements of rates of degradation have been made from changes in degree of polymerization (DP) in laboratory experiments in oil, with various levels of moisture in the paper and of oxygen in the oil. In all cases, DP was obtained from viscosity measurements of cellulose in solution. Model degradation experiments in vacuo, in inert gas, air and oxygen have also been carried out and are reported in the literature (Schultz, 1948; Madorsky et al. , 1956; Stamm, 1956; Golova and Krylova, 1957; Golova et al., 1957; Major, 1958; Pacault and Sauret, 1958; Fabre and Pichon, 1960; Ranby, 1961; Kilzer and Broido, 1965; Byrne et al. , 1966; Hino and Suganuma, 1967, 1972; MacKay, 1967; Fung, 1969; Bouvier, 1970; Fallou, 1970; Broido and Weinstein, 1971; Paloniem, 1972; Shafizadeh and Lai, 1972; Shivadev and Emmons, 1974; Miyo- shi, 1975; Hernadi, 1976; Shinouda, 1976; Saad et al., 1979; Shafizadei and Bradbury, 1979; Shafizadeh et al., 1979; Saad and E1-Khloy, 1980; Marx-Figini and Coun-Matus, 1981; Tamura et al. , 1981; Erofeev et al., 1982; Irklei et al., 1982; Zhilyaev et al., 1983; Krassig, 1985; Shafizadeh, 1985; Hatakeyama et al. , 1987; Yoshida et al. , 1987; Gibbons and Schroff, 1988; Moser and Dahinden, 1988). In some cases, a rigorous kinetic analysis has not been attempted. Where kinetic data are quoted a wide range of rates and activation energies are reported in the literature, including a possible change in activation energy above 140 ~ (see for instance Hino and Suganuma, 1967, 1972; Schroff and Stannett, 1985; Moser and Dahinden, 1988).

2-furaldehyde and related compounds are products of paper degradation that can be detected in insulating oil by high-performance liquid chromatography. They can therefore be used as specific chemical indicators of degradation (Burton et al., 1988; Unsworth and Mitchell, 1990; Le Guennec, 1992). Building an understanding of the mechanisms and kinetics into an overall model of degradation may eventually allow the accurate prediction of insulation life expectancy from a detailed knowledge of its past

28 EMSLEY AND STEVENS

operational history. Conversely, detailed forensic studies of failures may help to build an understanding of the chemical processes involved in cellulose degradation.

In this paper we critically review the available literature on the kinetics and mechanisms of cellulose degradation in a temperature range relevant to power plant operation (< 200 ~ Particular attention is paid to degradation of paper in oil, but other low-temperature degradation data are considered (in air, vacuo and inert gas) in order to produce generalized kinetics. We also review the structure and possible macroscopic and microscopic mechanisms of breakdown of cellulose, with some em- phasis on the likely mechanisms of formation of furfurals. Application to power transformers is discussed in a separate paper where we develop a simple ageing model for transformer insulation and discuss the errors and problems involved in life prediction (Emsley and Stevens, 1994).

CHEMICAL A N D M O R P H O L O G I C A L STRUCTURE OF CELLULOSE

Cellulose exists in four polymorphic forms, generally referred to as cellulose I, II, III and IV. Although cellulose I is not the most stable form, it is the only one which occurs naturally. It has recently been shown that cellulose I exists in two polymorphic forms designated Io~ and 1/3 (Sugiyama et al., 1991). The others are reconstituted forms generated by, for instance, dissolution in alkali and re-precipitation (cellulose II, mercerized cellulose). Only the structure and reactions of cellulose I will be considered in this paper.

Chemical structure

Cellulose is a natural polymer of cellubiose, which itself consists of two glucose molecules joined by a C1-C4 glycosidic oxygen linkage. In cellubiose the second glucose unit is inverted relative to the first. Both adopt a stable 1C 4 chair conformation and hydrogen bonding between adjacent oxygen and hydrogen atoms forces a linear arrangement. Cellubiose may be represented by the molecular model shown in Fig. 1, which also shows the hydrogen bonding. The bond angles and bond lengths have been determined by X-ray diffraction and the distance between glycosidic oxygens in the cellulose polymer is 0.544 nm, giving a repeat distance along the chain of 1.08 nm (see for instance Okamura, 1989). Six low-energy conformations have been identified in short chain polysaccharides, three of which are expected to dominate the cellulose structure. Strong hydrogen bonds (bond strength > 0.5 kJ/mole) straddle the glycosidic linkage and stabilize the chain structure (Simon et al., 1988a, b). The projected views of cellulose in Fig. 1 show one such conformation with associated hydrogen bonding.

Cellulose derives its strength from its fibrous composition, which results from the hydrogen bonding within the chain, maintaining a linear conformation and hydrogen bonding between cellulose chains, assisting in fibril formation. The initial degree of polymerization (DP) of natural cellulose (e.g. cotton) can be > 20 000. Wood consists of 40-55% cellulose, 15-35% lignins (polyaromatic compounds) and 25-40% hemicelluloses (water-soluble polysaccharides similar to cellulose, strictly described as pentosans). The Kraft pulping process, used in the production of electrically insulating paper, removes lignins and hemicelluloses. This increases the cellulose content in paper to about 90% (balance 3-7% lignin + residual pentosans) and reduces the DP to an

KINETICS AND MECHANISMS OF CELLULOSE DEGRADATION 29

~ _ Oxygen

(~ - - Hydrogen

1

.......... Hydrogen bond >0.5 Kcals/mole

. . . . . . Hydrogen bond


a

oL. C

%

n

@Oxygen

0 Hydrogen

FIGURE 2. Projection of the cellulose unit ceil, viewed along the c-axis (above) and the b-axis (below) (Okamura, 1989).

terms of a fringed fibrillar (fringed micellar) model, in which a cellulose chain meanders between areas of high crystallinity (micro-crystallites), passing through areas of low crystallinity (amorphous regions) in between (see Krassig, 1985 for further details). Some workers have suggested that the amorphous regions are degraded more rapidly than the crystalline regions and that, even within the amorphous regions, some areas are more reactive than others. The existence of 'weak links' in the polymer chain have been proposed to explain rapid reaction rates at the start of an experiment (Schultz, 1948; Michie et al., 1961; Ranby, 1961; Erofeev et al., 1982). During thermal and hydrolytic degradation, the DP decreases asymtopically towards a limiting value of about 30. Early structural work postulated that the chains of cellulose might fold back on themselves with a fold length DP of 30 repeat units (Manley, 1963; Chang, 1971, 1974). Such a model nicely explained both a basic crystallite length of about 30 nm


(Ranby, 1961; Chang, 1974) and the apparent existence of 'weak links', assuming that the folds will tend to break preferentially.

However, recent high-resolution electron microscopy studies (Sugiyama et al. , 1987) have shown fibrils longer than 50 nm. Also conformational analysis calculations on polysaccharides (Simon et al. , 1988a, b) indicate that the extra energy required to produce a fold is so large as to preclude the possibility except in the extreme case of very long molecules, where other interactions in the folded chain may compensate (Okamura, 1989). Chang's (1971) conclusions regarding chain folding were based on an analysis of gel permeation chromatography (GPC) peak shapes of nitrated cellulose. He found peaks with a low hydrodynamic volume, which he attributed to the folded molecules. More recent GPC studies have shown that nitration causes degradation of the sample (Lawther et al . , 1990), which could have explained his observations.

C R Y S T A L L I N I T Y , W E A K L INKS A N D D E G R A D A T I O N

The most significant consequence of cellulose degradation is the loss of mechanical strength. This can be understood in terms of the three structural characteristics which account for the mechanical strength of cellulose fibres:

(a) hydrogen bonding of polymer chains within micro-crystallites (b) interlinking of micro-crystallites by the direct sharing of polymer chains by

adjacent crystallites within a fibril (c) the pinning of chain ends within the crystalline and amorphous regions by

hydrogen bonding and physical entanglements.

It is the breakdown of chain length and of hydrogen bonding that causes the mechanical weakening and embrittlement of fibres during thermal and hydrolytic degradation. Consequently, it is necessary to know how these factors change during degradation and which structural characteristics dominate bulk strength.

Degradation of cellulose involves the breaking of covalent bonds within and between monomer units in the chain and the loss of inter- and intra-chain hydrogen bonds. In some cases, true random chain scission has been reported to be preceded by a short period of rapid reaction, which has been attributed to the scission of weak links in the polymer chain. These occur approximately every 500 monomer units and are considered to react up to 10000 times faster than the highly ordered crystalline regions. They are also reported to be sensitive to far UV light (Feller et al. , 1986).

Weak links apart, degradation occurs most rapidly in the amorphous regions between areas of high crystallinity (Erofeev et al. , 1982). The degradation rate is not uniform within the amorphous regions and some areas clearly react more rapidly than others (Rowland and Roberts, 1972). The degree of crystallinity of cellulose samples is reported to affect the overall kinetics of degradation. The activation enthalpy reportedly decreases and the activation entropy increases with increasing crystallinity (Hanna et al. , 1984). The activation energy, obtained from differential thermal analysis, was reported to decrease from 55 to 31 kcal/mole for a change in crystallinity of only 88-91%. However, sample preparation involved cutting, shredding and sieving to produce samples with particle size ranging from < 0.05 mm to > 0.5 mm. It seems likely that the surface area differences, which appear not to have been taken into account, may have influenced the results.


Two models for the macroscopic degradation of cellulose have been proposed. Both make assumptions that cannot easily be substantiated and neither can be quantified kinetically. The first model (Chang, 1971 and 1974) has already been discussed in the previous section and assumes a chain-folded crystal structure. The folds are considered to be weak links where degradation might be expected to occur initially. The limiting DP, observed during degradation, is then the average fold length (estimated as 30 monomer units). Degradation is assumed to occur in three stages: first-order scission at chain folds, zero-order peeling of the exposed chain ends, and finally first-order lateral scission (unzipping) down the chains.

A second approach (Elema, 1973) has been taken to explain the apparently high reactivity every 500 monomer units (Schultz, 1948). An initial DP in native cellulose of 14 000 is assumed and critical assumptions about the size of the elementary micro fibrils are made, v i z that each has 30 molecules per cross-section (calculated on the basis of a fibril cross-sectional diameter of 10 nm 2 and a molecular cross-section of 0.33 rim2). If the micro-fibril is assumed to be constructed from 30 randomly bunched, linear polymer chains, then the chain ends will also be randomly distributed along the length of the fibril. In an average chain length of fibril there will be 30 break positions where one chain terminates and the next one starts. Such breaks will cause strain in adjacent chains and could be points of weakness where attack might be expected to occur more rapidly. According to the initial assumptions, these weak points will be 14000/30 or approximately 500 monomer units apart. The assumptions made are rather convenient and arbitrary. In particular, there appears to be an inherent assumption that individual chains start and end in the same micro-fibril, which is unlikely to be true. However, the premise that chain ends act as stress raisers in adjacent chains is a valuable one worthy of further consideration.

KINETICS OF D E G R A D A T I O N OF L INEAR POLYMERS

In the mid 1930s Kuhn and co-workers (Kuhn, 1930; Freudenberg et a l . , 1930) investigated the degradation of cellulose from a theoretical statistical viewpoint. Their work was later developed into a kinetic model for the degradation of linear polymer molecules by Ekamstam (1936). The derivation of the model equation is as follows:

For a first-order reaction of the form:

A - + B + C (1)

the rate might be described by the equation:

dA - k x [ A ] (2)

dt

from which

[A] = [Aole -kt (3)

where k = the reaction rate constant, [A] = concentration of reactant chains at time t, and [A0] = initial concentration of reactant chains.

For a simple first-order reaction a plot of loge [A] (or a parameter proportional to it such as sample weight) against time will yield a straight line of slope - k.

In the case of a linear polymer undergoing random degradation, [A] can be replaced


by the total number of unbroken inter-monomer bonds remaining (number of chains times the number of monomer units in the chain - 1). If the initial number of molecules of polymer is M0 and the initial, total number of monomer units is N0, then the total number of bonds initially is 10, where

l o = N o _ M o = N o ( l _ 1 ) DPo (4)

where DP o = initial degree of polymerization -- No/Mo. Similarly, the number of unbroken bonds remaining at time t is:

l t = N o _ M t = N o ( l _ 1 ) DPt (5)

where M t = the number of polymer molecules at time t, DP t = the degree of polymerization at time t = Nt/Mt, and l t = the number of inter-monomer bonds per molecule at time t.

For first-order kinetics of scission, the rate is proportional to the number of unbroken bonds remaining.

dl - - - - = k x [ t (6)

dt

SO

[t = /0 e-kt -

Substituting for It and lo

1

If DPt are DPo are large this simplifies to:

1 1 = kt. DPt DPo

This approach is strictly applicable only in the following circumstances:

(a) the polymer chain is linear and of high molecular weight (b)

(c) (d)

(7)

(8)

(9)

the polymer is monodisperse and the products of scission are themselves long chain molecules there is a low degree of chain end-chopping there is no loss of monomer units during scission.

APPLICATION OF FIRST-ORDER KINETIC MODELS TO CELLULOSE

Although the idealized Ekamstam relationship is unlikely to apply fully to cellulose, it has been used by a number of workers to describe the degradation of cellulose in a number of different atmospheres (Major, 1958; Fung, 1969; Bouvier, 1970; Shafizadei and Bradbury, 1979; Marx-Figini and Coun-Matus, 1981; Irklei et al., 1982). In some cases, linear kinetics are preceeded by a period of rapid reaction and the linear relationship breaks down as the degradation process approaches completion. Neverthe-


less, it provides a simple kinetic analysis, which will be shown to be applicable to a wide range of experimental data.

While kinetic studies and DP measurements indicate that degradation occurs primar- ily by chain scission, a large number of different chemical by-products are formed according to the temperature and conditions of degradation. Some of the reported products of degradation are listed in Table 1, where an indication is given of the range of temperatures over which they have been identified. From this it is convenient to separate degradation studies into high- and low-temperature regions about 200 ~ and to consider, especially, the formation of levoglucosan, which plays a pivotal role in understanding the degradation mechanisms.

Levoglucosan formation

At temperatures above about 200 ~ levoglucosan (formally 1,6-anhydro-/3-D-glucopyranose) is a key intermediate product in the degradation of cellulose and one that could lead directly to the formation of furfural products. At temperatures around 200 ~ it is found in yields that range from a few percent to as high as 50-60%, depending on conditions (MacKay, 1967; Broido and Weinstein, 1971). At 300 ~ in air, the initial decomposition of cellulose involves chain scission to a fragment DP of about 200. During this stage only 5-20% of the product is levoglucosan. The DP remains at 200 for 4-80% of the reaction, but the yield of levoglucosan increases (Golova and Krylova, 1957; Golova et al. , 1957). Levoglucosan formation is considered to be favoured in regions of dense chain packing.

TABLE I. Products of degradation of cellulose (other than carbon dioxide and water)

Degradation products Temperature Conditions References range (~

levoglucosan 250-400 vacuum pyrolysis

Shafizadei and Bradbury, 1979; Golova and Krylova, 1957; Golova et al., 1957; Madorsky et al., 1956

acetaldehyde, acetic acid, acetone, acrolein, acrylaldehyde, 3-butandione, 2-butenal, butyraldehyde, formaldehyde, furan, 2-furaldehyde, furfuryl alcohol, glyoxal, 5-hydroxymethyl-furfural, I-hydroxy-2-propanone, lactic acid, laevulinic acid, propionaldehyde, pyrluvic acid, methanol, 5-methyl-2-furaldehyde,

180-400 vacuum pyrolysis of levoglucosan

Shafizadeh and Lai, 1972; Kosik et ah, 1983; Shaflzadeh et al., 1971; Glassner and Pierce, 1965; Pavlath and Gregorski, 1988; Schwenker and Beck, 1963

acetaldehyde, formic acid, 2-furfural, glucose, 5-hydroxymethyl-furfural, 5-methyl-2-furfural, laevulinic acid, saccharinic acid

100-200 hydrolytic Fukuchi et al., 1977; Nevell, 1985


Formation of levoglucosan, and subsequently furfural, from glucose is discussed in detail later. The processes can be summarized as:

(a) rupture of glycosidic bonds in the cellulose molecule releases the rnonomeric product glucose (Shafizadeh et al . , 1979)

(b) dehydration of glucose across the 1C and 2C atoms with the elimination of water is followed by rearrangement to a stable 1C to 6C oxygen bridge structure, which is levoglucosan (Golova et al . , 1957; Kilzer and Broido, 1965; Byrne et al . , 1966)

(c) decomposition to furfural then proceeds by a series of rearrangement and elimination reactions which result in the loss of two further molecules of water and one of formaldehyde (Shafizadeh and Lai, 1972).

Adding glucose to cellulose depresses the yield of levoglucosan during degradation (Golova et al. , 1957), so glucose may not be the primary source. In cellulose, therefore, there may exist a co-operative mechanism, involving fragment migration between adjacent monomer units in the same or adjacent chains, which may account for its formation (Kilzer and Broido, 1965). Such a mechanism might involve the stronger hydrogen bonds, that exist within the chains, encouraging inter-monomer dehydration reactions and even the direct formation of five-membered furfural-like structures. However there is no direct evidence for this mechanism.

High-temperature degradation of cellulose (> 200 ~

A large number of high-temperature degradation studies of cellulose have been made in connection with the fire performance and combustion behaviour of thermal cellulosic insulation, cellulosic waste and cigarette paper. The conditions and processes involved are far removed from the low-temperature conditions of interest here and are only briefly discussed for completeness.

At temperatures above 300 ~ volatilization of cellulose begins to occur (Shafizadeh, 1985). Rates of pyrolysis are generally obtained from measurements of weight loss in air or inert environments and the kinetics are then analysed in terms of Equation 3. Similar rates are obtained in all environments (Shafizadeh, 1985), dependent only on the form of the sample material. Below about 380 ~ the apparent activation energy for pure, thin samples of cellulose is about 110 kJ tool -1 (Shivadev and Emmons, 1974; Calahorra et al. , 1989). This increases with increasing crystallinity and alpha cellulose content (Hanna et al . , 1984), molecular weight (Calahorra et al . , 1989) and thickness to a high value of 170 kJmol -I. At this level, the reaction is diffusion controlled (Rogers and Ohlemiller, 1980). Above 380~ the activation energy may increase to 225 kJ tool -I (Shivadev and Emmons, 1974), although the change may only be apparent in experiments where diffusion is not a controlling factor.

Although most weight loss results approximate to first-order kinetics (Equation 3), a number of workers have analysed combustion results in terms of multiple first-order reaction schemes (Shivadev and Emmons, 1974; Lewellen et al. , 1977; Jackson et al. , 1988a, b). Others have found a linear relationship between the activation energy and the pre-exponential factor of the Arrhenius expression, the so-called compensation-law effect (Chornet and Roy, 1980; Agrawal, 1985), which is common to a number of different polymer systems (David, 1987; Montanari, 1990).


Between 200 and 300 ~ the overall reaction is a mixture of pyrolysis and degradation (depolymerization). Volatile hydrocarbons, CO2, CO and water are formed as product, s. The apparent activation energy, from first-order weight loss kinetics, ranges from 110 to 220 kJmol -I (Shafizadeh, 1985; Kilzer, 1971). Kilzer and Broido (1965) proposed a general scheme for the high-temperature formation of char (solid carbon- containing products) water, carbon monoxide and carbon dioxide on the one hand and volatile tars, including levoglucosan on the other. The reactions occur in parallel, the first reaction proceeding via a dehydration process and the second by de-polymerization as illustrated below:

CELLULOSE

I I

dehydration

dehydro-cellulose

I depolymerization

kB

volatile 'tars'

kc

char + water etc

Broido and Weinstein (Broido and Weinstein, 1971) calculated values for the rate constants kA, kB and kc at 226 ~ from a thermogravimetric analysis, assuming parallel and independent first-order reactions. However, the reliability of their values is questionable, because a more detailed study of the thermogravimetric data indicated that extra steps were required to fully explain the experimental observations.

Low-temperature degradation of cellulose (< 200 ~

Most of the studies in this temperature range have been carried out to support an understanding of degradation and failure of paper insulation in electrical power transformers. Studies extend over the so-called normal (60-90,~C), overheated (> 90 ~ or hot-spot (> 120 ~ temperature conditions of transformers. A short review of the data available in the literature, under different reactions conditions, will be given before drawing them all together into a single model.

Degradation of cellulose in oil

Most reported studies have used insulation-grade (Kraft) paper immersed in high- quality paraffinic and/or naphthyfinic insulating oil. In the absence of oxidation or hydrolysis of the oil, it simply acts as an immersion medium. However, degradation of the oil may influence the degradation behaviour of the cellulose.

In the first reported studies of degradation of paper in oil, 50 ~m thick paper was degraded at temperatures in the range 90 ~ ~ (Fabre and Pichon, 1960). In 1960,


the data were originally analysed in terms of the time required to reach a particular DP value. They were subsequently re-analysed by Fallou (1970) using an analysis similar to the random chain scission method described above to show that the data could be represented by a plot of the number of bonds broken per thousand against time. However, the resultant graph displayed a discontinuity, requiring two straight lines. Since 1960, a number of experiments of DP change in oil have been reported, but no attempts have been made to relate the changes to the type of kinetics described by Equation 10.

A systematic study of change in DP of paper degraded in oil and the effects of water and oxygen was published by Schroff and Stannett (1985). It has since been extended, but not reported externally, by Gibbons and Schroff (1988). In the original report the data were plotted as log DP vs time. The lines obtained were not entirely linear and did not extrapolate through the origin. Some of the original data, supplemented by the new data, are replotted in Figs 3a-e according to Equation 10. Good straight lines, which now pass through the origin, are obtained in all cases, confirming the potential applicability of the random chain scission model.

(a) ~ S ~20oc / 110o C (b)

3p// 21-11 o

S_ lIP ~/100c �9 ~ 1

12000 0 4000

o 5 o4f t,,ooc ~3F ' / /140~

0 4000 8000 Time, h

5 o ~ 160% o4x / ~ ~ ~ I ~

02 o_ "O 1

0 4000 8000 12000 Time, h

5 - ~ 4% H20 (e) o _ ~ / 2 % H20

"~~ , ~ J ~ ~ 1 % H 2 0 C~ 3 ~ a p e r

.-~ g a:0~"~ I- I I I I I

0 4000 8000 12000 Time, h

(c) S 8 04

~3 121

-~2 o

._o_ 1

~0 0

I _ I F J 8000 12000

Time, h

4000 8000 Time, h

(d)

I 12000

F I G U R E 3. Reassessment of Schroff and Stannett 's (1985) data and more recent unpublished data (Gibbons and Schroff, 1988). The effect of t empera tu re on (a) al l -wood Kraft paper, (b) al l -wood Kraft paper loaded wi th 4% water , (c) thermally upgraded, al l -wood paper, (d) the effect of air and copper, and (e) the effect of water Ioadings of I, 2 and 4% on Kraft paper at 120 ~


Figs 3a-c show the effects of temperature on: (a) dry all wood paper; (b) paper loaded with 4% water; and (c) thermally upgraded paper. The detrimental effects of air and water in accelerated degradation at 120 ~ are obvious in Figs 3d and e respectively.

Other available data, for degradation under oil, are replotted in the same form in Figs 4a-i. Throughout the data there is, on the whole, good agreement with Equation 10. Some data do not extrapolate through the origin and, on occasions, the rates tend to increase continuously at the highest temperatures. The Fabre and Pichon data (1960) referenced above are replotted in Fig. 4a. It can be seen that good straight lines are obtained, without discontinuities. Detailed examination reveals that deviations to lower reciprocal DP values occur at long times and there is a slight upward curvature to the data at 140 ~

The data of Miyoshi (1975), Moser and Dahinden (1988), Tamura et al. (1981) and Zhilyaev et al. (1983) produce good straight lines. The data of Hino and Suganuma (1967, 1972) generate reasonably good lines but they do not extrapolate to the origin. The data of Yoshida et al. (1987), like those of Fabre and Pichon (1960), deviate to lower values of reciprocal DP at long times. There is also a clear upward curvature in some data at 140 ~ and above, which is discussed further below.

The observed deviations to lower values of reciprocal DP can probably be explained by inhomogeneity of degradation of the paper, e.g. rapid degradation of the amorphous regions, or of the larger molecules, followed by a gradual reduction in reaction rate as the relative degree of crystallinity increases. The results of Hino and Suganumaa (1967, 1972 and Figure 4b) are more difficult to explain. It is possible that degradation, occurring during the drying process at 150 ~ prior to oil emersion, may have reduced the initial DP. Alternatively, degradation during sample preparation for DP measurement may have introduced errors.

Thermal and oxidative degradation

Measurements of cellulose degradation have been made in vacuo, in nitrogen, air and oxygen. Observed rates increase with increasing oxidation potential of the environment and activation energies are quoted which fall in the range 76-148 kJ mol -I (see Stamm, 1956 and Table 2). Results have generally been analysed according to, and shown to be in good agreement with, Equation 10. However, some results show an initial rapid change in DP before linear kinetics are established (Major, 1958; Pacault and Sauret, 1958; Bouvier, 1970; Hernadi, 1976; Shafizadei and Bradbury, 1979), which has been attributed to rapid scission of a small number of weak bonds (Schultz, 1948; Michie et al., 1961; Ranby, 1961; Elema, 1973; Feller et al., 1986). Not all results show this initial rapid rise (e.g. Major, 1958; Saad et al., 1979; Saad and E1-Khloy, 1980) and it is clear that this is an area that requires further investigation. The DP eventually levels off as it reaches a value approximately equal to the crystallite size (Shafizadei and Bradbury, 1979).

Hydrolytic degradation

No data are available on the hydrolytic degradation of cellulose in the relevant temperature range (which is too high for hydrolytic degradation) and the reaction


15. g r "~ 10 ~.- C~ ~ 5 . 0 g O"

5 o o 4

3 o % 2 o. 1

0

3

x o-2

o 1 Q_

of 0

5 o 04

~3 ~2 o .O 1 c~ 0

5 g o4 x ~3 ,m -~2 o .O 1

" 0

(a) / j ~ Z ~ , ~ 140~

130~

100oc 10000 20000 30000

Time, h

170~ / 5 0 O c (c)

~ ' - I - 1 I I I I I ~ 400 800 1200 1600

Time, h

(e) / ~ ~,~.13 5~

120~

0 2000 4000 6000 8000 10000 Time, h

I60% (g)

140~

~ ~ I

2000 4000 6000 8000 Time, h

- 1 8 0 % (i)

200 4000

140~ . . . . 0

I I I I / P t 600 800 1000 1200

Time, h

0.8 oo o ,~ 0.6

20.4

~: 0

uC A160~ 150~ / ( b )

[] ~ ~ 1 4 0 ~

100 200 300 400 Time, h

5 _ o (d) # | /145C ~ 4 ~rJ~.J160~ ~3 135~

! i ~ f 120~ 100oC :0c_ c

0 2000 8000 12000 16000 Time, h

5 2-- , (f) ~4~ /Transboard, 50 g H,O

/ T,.~ %' 2 L /'~ Transboard, 20 g H20

~.1 ~ y ~ a r d d r i e d ~163 0 I I J_ I

0 1000 2000 3000 4000 5000 Time, h

5-- o ~ �9 x 140~ C3

._~ 1 120~ A 1 0 C

'~" 0 1100% 2000 4000 6000 8000

Time, h

(h)

FIGURE 4. Reassessment of literature data of the degradation of cellulose paper in transformer oil: (a) Fabre and Pichon, 1960; (b) Hino and Suganuma, 1967, 1972; (c) Miyoshi, 1975; (d) Moser and Dahinden, transformer board, 1988; (e) Moser and Dahinden, cotton board, 1988; (f) Moser and Dahinden, effects of water on transformer board at 135 ~ 1988; (g) Tamura et al., 1981; (h) Yoshida et al., 1987; (i) Zhilyaev et al., 1983.


TABLE 2. Low-temperature (< 200 *C) kinetic data from the literature

Material Temperature Gas Activation Source (~ energy

(kJ//mole)

Kraft Paper 200-280 vacuum 148 Kraft Paper 100-130 vacuum 84 Sulphate paper 130-190 oxygen 80 Cotton linters 170-230 vacuum 108 Kraft paper 105-145 air 76 Cotton linters 150-190 oxygen 88 Cotton linters 150-190 nitrogen I 13

Fung, 1969 Bouvier, 1970 Hernadi, 1976 Pacault and Sauret, 1958 Saad et al., 1979; Saad and EI-Khloy, 1980 Shafizadei and Bradbury, 1979 Shafizadei and Bradbury, 1979

conditions have generally been too extreme (i.e. concentrated acid or alkali). Acid degradation data in potassium bisulphate fits Equation 10 (Marx-Figini and Coun- Matus, 1981). Alkaline degradation fits LOG DP vs time better, whereas 1//DP plots have positive curvature (to larger changes in DP) (Irklei et al. , 1982). Extrapolated rates from the linear sections are similar to those for acid degradation.

Two-stage kinetics, with a rapid initial period of reaction before linearity is established, are common for pyrolytic, oxidative and hydrolytic degradation of cellulose. There are also numerous subjective reports that the amorphous regions of the cellulose react more rapidly than the crystalline regions (eg Rowland and Roberts, 1972; Shinouda, 1976; Erofeev et al. , 1982; Hatakeyama et al. , 1987).

C O R R E L A T I O N OF D E G R A D A T I O N RATE D A T A FROM DP M E A S U R E M E N T S

It is useful to establish a correlation between DP measurements and the kinetics of degradation from the available literature data, to establish if a common mechanism exists. We have done this for data in the temperature range 90-290 ~ over the wide range of conditions explored by different authors. To this end, the logarithms of the first-order reaction rate constants of data replotted in Figs 3 and 4 have been plotted against reciprocal absolute temperature in Fig. 5. Additional points from literature data originally analysed according to Equation 10 are also included.

The data were divided into five sets according to increasing 'susceptibility to degradation' of the cellulose or 'potential for oxidation' of the environment. Thus, antioxidants in the paper (as in thermally upgraded paper) reduce its susceptibility to degradation. The presence of water in oil, or the use of air or oxygen increase the potential for degradation. An analysis of covariance on reaction conditions was then carried out to estimate the activation energy of reaction. Data were grouped in such a way as to minimize the heterogeneity of slope in the analysis. The optimum arrangement of sets is shown in Fig. 5 and was found to be, in order of increasing susceptibility to degradation:

(a) thermally upgraded paper in dry insulating oil (b) dry Kraft paper in dried insulating oil (c) dry paper or cotton in vacua or in nitrogen + moist Kraft paper containing

1-2% water in vacua + dry Kraft paper in dry oil saturated with oxygen


1. VThermally upgraded paper in dry oil 2. A Kraft paper in dry oil 3. n Paper/cot-ton in vacuo or nitrogen; or

Kraft paper and 1-2% H20 in vacuo 4. �9 Kraft paper and I-2% H20 in oil;

or paper/cotton in air 5. O Kraft paper and 4% H20 in oil or vacuo;

or cotton / paper in oxygen x Cotton in acid or alkali solution

-2 •

-4 .% x -6 5C~. ~ x

.o_ c -8 ~ 4 ** D

s V 10"~ ~ O.

_io

-18 I ; I t I I I r i Z~ t 1.8 2.0 2.2 2.4 2.6 2.8

Reciprocal temperature, IO00/K

FIGURE 5. Analysis of covariance of the first-order rates of degradat ion of cellulose as a funct ion of 'potent ia l for ox idat ion ' . I. V Therma l l y upgraded Kraf t paper in dry insulat ing oi l (Zhi lyaev et al., 1983; Schroff and Stannett , 1985). 2. A Kraf t paper in dr ied insulat ing oi l (Fabre and Pichon, 1960; Hino and Suganuma, 1967, 1972; Miyoshi, 1975; Tamura et al., 1981 ; Zh i lyaev et al., 1983; Schroff and Stannett , 1985; Yoshida et al., 1987; Gibbons and Schroff, 1988; Moser and Dahinden, 1988). 3. [ ] Kraf t paper and I - 2 % moisture in vacuo (Bouvier, 1970); paper or cotton in vacuo (Fung, 1969) or in n i t rogen (Major, 1958; Shafizadei and Bradbury, 1979). 4. �9 Kraf t paper and I - 2 % mois ture in in i t ia l ly dry insulat ing oil (Schroff and Stannett , 1985; Gibbons and Schroff, 1988); paper or cot ton in air (Major, 1958, Shafizadei and Bradbury, 1979). 5. O Kraf t paper and 4% moisture in vacuo (Bouvier, 1970) or in in i t ia l ly dry insulat ing oi l (Schroff and Stannett , 1985); paper or cotton in oxygen (Major, 1958; Shafizadei and Bradbury, 1979). X Cot ton in acid (Marx-Fig in i and Coun-Matus, 1981) or alkal i ( I rk le i et al., 1982).

(d) dry paper or cotton in air + Kraft paper containing 1-2% water in insulating oil (e) dry paper or cotton in oxygen + moist Kraft paper containing 4% water in oil or

in vacuo .

The analysis yielded an activation energy of 111 kJ mo1-1, with 95% confidence limits on the mean of 105 and 117 kJ/mole. The pre-exponential factors and their 95% confidence limits are given in Table 3. Extrapolated hydrolytic rates are also plotted in Fig. 5 as crosses. They are high compared with other data because the pre-exponential factor is high, but the activation energy is similar.


TABLE 3. Pre-exponential factors from analysis of covariance of degradation rates

Data set Pre-exponential 95% confidence limits

Upgraded paper in oil Dry Kraft paper in oil Kraft paper + 1% H20 in oil. Paper or cotton in vacua or in nitrogen Kraft paper + 2% HzO in oil. Paper or cotton in air Kraft paper + 4% moisture in oil. Paper or cotton in oxygen

3.65 x 107 7.93 x 106 1.68 x 108 1.07 x 108 2.41 )< 107 4.71 x 108 3.50 x 108 8.41 x 107 1.46 x 109

7.78 x 108 1.83 x 108 3.30 x 109

3.47 x 109 7.66 x 108 1.57 x 1010

C H E M I C A L M E C H A N I S M S A N D I N D I C A T O R S O F D E G R A D A T I O N

A number of papers have been published detailing the products of degradation of cellulose and proposing chemical reaction schemes to explain the observations (e.g. Madorsky et al., 1956; Murphy, 1962; MacKay, 1967; Conley, 1970; Kilzer, 1971; Freedman, 1978; Kosik et al., 1983; Blazej and Kosik, 1985; Shafizadeh, 1985). Much of the data however refer to high-temperature pyrolysis, or aqueous acid or alkaline degradation.

The products that are of technical importance in power transformers are 2-furaldehyde (furfural) and related compounds, because they can be distinguished from oil degradation products. They can be identified in the oil by high-performance liquid chromatography and can be used as chemical indicators of paper degradation (Burton et al., 1988; Unsworth and Mitchell, 1990; Le Guennec, 1992; Emsley and Stevens, 1994). The remaining discussion will therefore concentrate on furfural formation and related compounds. Other potentially interesting chemical indicators are phenol and related compounds which result from the degradation of phenol-formaldehyde resins in the transformer, but these are discussed elsewhere (Emsley and Stevens, 1993).

Furfural formation mechanisms

There is no published information available on the mechanisms of furfural formation from cellulose. Some work has been done on the decomposition of the monomeric products of degradation, based on the assumption that primary chain scission occurs at the glycosidic link to release glucose. It has also been suggested recently (Vergne et al., 1991; Le Guennec, 1992) that furfural originates from the minor pentosan components of paper and is therefore of lesser value as a chemical indicator.

Pyrolysis o f glucose

Pyrolysis of glucose to furfural requires the loss of two molecules of water and one of formaldehyde (or CO and H2). Studies at 225 ~ of the pyrolysis of glucose labelled with carbon-14 in different positions have clearly shown that furfural is formed as a product. Pyrolysis occurs by one of two preferred routes. 80% of reaction occurs with preferential elimination of 6-C and 20% with preferential elimination of 1-C. In both


cases, 2-C is fully retained (Houminer and Patai, 1969). The same experiments showed that more than the theoretical yield of water (10% by weight of glucose) is formed and it was proposed that degradation is preceded by condensation dimerization of glucose, which would yield a further 5% water. Other compounds such as ene-diols, epoxides and glycosones have also been proposed as intermediates of furfural during the pyrolysis and have been isolated from pyrolosates of both glucose and cellulose (Kato and Komorita, 1968).

A mechanistic scheme compiled from a number of the sources referenced above is presented in Fig. 6a. The detailed chemistry involved is as follows:

Mechanism I (Fig. 6a) �9 Molecule (I) is D-glucose where R could be hydrogen or a cellulose chain; for clarity,

hydrogen atoms are not shown unless they are involved in the reaction. �9 The first stage of degradation involves the formation of the epoxy-bridged molecule

(II) (1,2-anhydro-cr-D-glucopyranose) by elimination of water. �9 This rearranges internally to the more stable 1,6 oxygen bridged molecule (III)

(levoglucosan). �9 Further internal rearrangement to the 1,4 oxygen bridged molecule (IV), 1,4-

anhydro-fi-•-glucopyranose, is followed by conversion of the aldehyde (V) to the furan substituted enediol (VI).

�9 Thereafter rearrangement of the ketone (VII) is followed by three condensation reactions, eliminating two molecules of water and one of formaldehyde (or hydrogen + carbon monoxide) to yield the furfural product (IX).

Hydrolysis of glucose

Most mechanistic work has been carried out on the hydrolytic degradation of glucose in acid solution, where the process is proton catalysed. Labelled carbon experiments have shown that the 5-carbon ring linkage can occur from glucose or levoglucosan, from the 1-C and the 5-C atoms under acid catalysed conditions and from 2-C and the 6-C atoms under alkali catalysed conditions (Shafizadeh and Lai, 1972). Anhydroglucopyranoses are thought to be the primary intermediates of hydrolysis. They are formed initially by a dehydration reaction between 1-C and 5-C followed by internal rearrangement and further dehydration and elimination of formaldehyde.

The mechanistic schemes in Fig. 6b represent the possible processes occurring under hydrolytic conditions, as follows.

Mechanism 2 (Fig. 6b, Route A) �9 The formation of the 1-C to 2-C bridge is accompanied by scission of the 1-C oxygen

bond to form 1,2-anhydroglucose (X). �9 Recyclization at 2-C with the elimination of water gives 2,5-anhydro-D-mannose

(XI). �9 Decomposition, with the elimination of two molecules of water, yields the hydroxy-

methyl substituted furaldehyde (XVI) and then furfural by elimination of formaldehyde.


Mechanism (a)

/ H20 alo 5, a o

I I

(I) D-Glucose (II) 1,2-Anhydro- i !3_D_Glucopyranose i i

i OH '

I I

1

RO RO

(111) 1,6-Anhydro- (IV) 1,4-Anhydro- !3-D-Glucopyranose j3-D-Glucopyranose (Levog[ucosan)

OH OF{

"1 H R RO OH

(V) 3,6-Anhydro- (Vl) 2,3-Hydroxy- D-Galactose Tetrahydrofuryl-

Ethylene-Diol

- 2 H20

(VII) 2,3-Hyd roxy- Tetrahydrofuryl- HydroxymethyI-Ketone

(VIII) FuryI-Hydroxymethyl- Ketone

W (iX) 2-Furaldehyde

Mechanism (b)

H ~ O R

OH (L) D-Glucose

%o_, A _ , ~ o R

" H20 1

R o u t ~ B ~ H "OR

(XlII) 3-Deoxy-Hexapyranos- 2-Ene

(XIV) 3-Deoxy-Erythro- Hexasulos-2-Ene

" H20 1

(XV) 3-Deoxy-D-Erythro- Hexasulos 3-Ene

Route A

~ " R ~ OH OH .o.A ~z} .

(X) 1,2-Anhydroglucose

(Xl) 2,5-Anhydro-D- Mannose - H2O I

(Xll) 4, 5-Dihydro-4-H yd roxy -5-(Hydroxymet hyl) -2-Furaldehyde

- H20 1

- H20

) (XVI) 5-(Hydro•

2-Furaldehyde

HCHO 1

2 Furaidehyde

FIGURE 6. (a) Possible pyrolytic mechanisms of formation of furaldehyde from cellulose (glucose) via levoglucosan (Kilzer and Briodo, 1965; Byrne et al. , 1966; Shafizadeh and Lai, 1972); (b) Possible mechanisms of degradation of cellulose (glucose) to furaldehyde via an epoxide (Route A) or eneol (Route B) (Anet, 1961 and 1964; Noller, 1965; Gardiner et al. , 1966; Kato and Komorita, 1968; Kilzer, 1971). In cellulose, R = H, or R = cellulose chain. R = H is assumed in the names. H atoms are not shown for clarity.


Mechanism 2 (Fig. 6b, Route B) �9 an internal dehydration reaction across the 2-C to 3-C bond yields the unsaturated

hexapyranose (XIII) �9 scission of the glycosidic linkage to the adjacent monomer unit (XIII) and breaking

of the 1-C oxygen bond with the elimination of water (XIV) is followed by two condensation reactions

�9 the hydroxymethyl substituted furaldehyde (XVI) yields furfural by the elimination of formaldehyde.

It may be possible to differentiate mechanism 1 from mechanism 2 if intermediate species can be identified, e.g. 5-hydroxy-2-furaldehyde from hydrolytic degradation and levo-glucosan or furyl-hydroxymethyl-ketone from pyrolytic degradation. However, it is unlikely that the other intermediates of mechanism 2 will be sufficiently stable to differentiate between Routes A and B.

All these mechanisms are tentative in nature and, because of the complexity of the process, impossible to prove conclusively. Further experiments with carbon-14 or tritium labelled materials and the use of techniques such as nuclear magnetic resonance spectroscopy might help to clarify the chemistry and also identify intermediate products.

Furfural production

It is clear that furfural can be formed during the pyrolytic degradation of cellulose at high enough temperatures. In the low-temperature region, however, more work is required to establish the precise source. It is normally manufactured by the acid hydrolysis of pentosans, 5-carbon sugars (Noller, 1965; Maciejewski et al., 1981; Butsena and Kulkevits, 1986; Vergne et al., 1991; Roze et al., 1988; Le Guennec, 1992), whereas 5-hydroxymethyl-2-furfural and 5-methyl-2-furfural are the usual products of hydrolysis of a hexose (6-carbon sugar) such as glucose. The former does decompose to furfural on heating (Moye 1964; Noller 1965).

It is feasible that the furfural products of paper degradation arise from the hydrolytic component of degradation and may even come mainly from the pentosan component of paper. If the former, their presence may be more a measure of increasing moisture and/or acidity in the oil, rather than of thermal degradation processes. If the latter, they cannot be said to be representative of degradation of the bulk material. It is important that the precise role of furfural is understood, to enable accurate interpret- ation of furfural in oil measurements.

Furfural formation data reported by Schroff and Stannett (1985) for degradation in insulating oil are discussed in detail elsewhere (Emsley and Stevens, 1993). It is shown that, when suitably replotted, they tend to support the premise that furfural is largely a product of hydrolytic degradation. The lowest rates of formation are attained where the oil is continuously flushed with gas which will help remove any moisture formed by the degradation process. The amount of furfural formed is even low when air is used as the flushing gas, although DP measurements on the paper showed enhanced rates of degradation. Highest rates of formation are found where moisture was allowed to accumulate or deliberately added. The possibility that furfural formation is largely a hydrolytic process needs further investigation in acid- and moisture-free conditions. Similarly the accumulation of sugars in the paper (they are insoluble in the oil) should be investigated.


DISCUSSION

Despite the volumes of literature on cellulose structure and degradation, there are still a number of grey areas where further work is required before a full degradation mechanism can be expounded:

(a) it is still not clear whether or not the chains are folded in the cellulose structure. The evidence in favour is, at best, circumstantial (Manley, 1963; Chang, 1971 and 1974) and seems to have been superseded by positive, conformational analysis (Simon et al. , 1988a, b) and microscopic evidence (Sugiyama et al. , 1991) against

(b) there is some evidence for weak links in the cellulose chain approximately every 500 monomer units (Schultz, 1948). They may be sensitive to far UV light (Feller et al. , 1986) and may be created by chain ends acting as stress raisers in adjacent chains (Elema, 1973), but the precise nature and location of these higher reactivity sites has not been established

(c) crystallinity is said to have an important effect but it has not been quantified. Degradation occurs more rapidly in the amorphous and less crystalline regions (Rowland and Roberts, 1972; Erofeev et al. , 1982) and an attempt has been made to relate rates of degradation to the degree of crystallinity (Hanna et al., 1984), but the results are ambiguous and need further clarification

(d) further work is required to relate cellulose degradation to paper strength and to devise a model of cohesion and failure

(e) there is a shortage of positive information on the chemical mechanisms of degradation of cellulose. It is generally agreed that the primary chain scission processes will occur at the glycosidic linkages joining the monomeric (glucose) units. Studies have been made of the degradation of glucose and a related sugar, levoglucosan, which has been observed during cellulose degradation, but other direct cellulose degradation mechanisms may exist

(f) furaldehyde and related compounds are currently considered to be the most promising degradation products to use as chemical indicators of paper ageing in oil immersion applications, because they can be differentiated from oil degradation products. They are synthesized commercially from pentose (5-membered ring) sugars and this has led to suggestions that they may arise as a result of degradation of the minor constituents of paper and hence could be unrepresen- tative of the bulk (Vergne et al. , 1991; Le Guennec, 1992). However, degradation of 14C labelled glucose (Houminer and Patai, 1969) and levoglucosan (Shafizadeh and Lai, 1972) clearly shows that furfural can be formed from hexose (6-membered ring) sugars

(g) in the low-temperature regime, the hydrolytic degradation reaction may dominate, in which case the formation of furfural would be more indicative of the acidity of the oil than of bulk degradation of the insulation. Work is required to establish the relative rates of thermal, oxidative and hydrolytic degradation

(h) the kinetics and mechanisms of furfural formation and decomposition, and its partition between oil and paper, have yet to be established before its cumulative concentration in oil can be used to make definitive assessments of the insulation condition.


Cellulose degradation kinetics

Fig. 5 shows that it is possible to rationalize degradation rate data for different forms of cellulose and different experimental conditions to a single simple model. In so doing, the pre-exponential of the Arrhenius relationship is allowed to vary according to the reaction conditions, but a constant activation energy of reaction is assumed. This implies, but does not prove, a similar rate-controlling step in all the breakdown processes.

Some workers, e.g. Bouvier (1970) have used inert conditions (vacuum or nitrogen) to model degradation kinetics in oil. Clearly this approach is not totally satisfactory, because, although a similar activation energy might be obtained (in fact the Bouvier activation energy is slightly lower than average), actual rates may be high, because of the higher pre-exponential factor. An improved mechanistic understanding of the reaction processes involved in degradation might define a relationship between the pre-exponential term and particular experimental conditions. This would quantify the model environment approach and obviate the need to carry out all experiments under oil.

To minimize the heterogeneity of the slopes in the analysis of covariance it was found necessary to re-allocate some data from SET 2, where they obviously ought to lie into SET 3. In other cases, data were split between the two sets. The following is a list of data concerned:

(a) Bouvier (6) data in vacuo at 100 ~ Data at 130 ~ fit in SET 2. Here the water level in the paper was reportedly < 0.2%

(b) Fabre and Pichon (1960) data at 120 ~ and below. Higher-temperature data fit in SET 2. The quoted moisture level was 0.5%, which would put the data between SET 2 and SET 3

(c) Miyoshi (1965) data. Here, although the paper was supposedly dried, no moisture measurements were recorded, and the rates are clearly too high for SET 2

(d) Moser and Dahinden (1988) data for transformerboard at 135 ~ and above, except for data at 135 ~ where the oil was dried continuously with a molecular sieve. Also cottonboard data at 90 and 100 ~ (cottonboard is produced from a cotton cellulose, as opposed to transformerboard which is made from wood cellulose)

(e) all wood paper data of Gibbons and Schroff (1988) at 150 and 160 ~ Earlier data of Schroff and Stannett (1985) at 140 ~ and below fit into SET 2.

The data of Bouvier, Fabre, Moser and Dahinden and of Schroff and Stannett are reproduced separately in Figs 7a-d respectively to the same axes as Fig. 5 for comparison. The high rates of Bouvier (1970), Fabre and Pichon (1960) and Moser and Dahinden (1988) (cottonboard but not transformerboard) at 120 ~ and below are difficult to explain at this stage. Perhaps the most obvious reason is that the paper was not as dry as it should have been. This is clearly an area that needs further investigation in future experiments, because, if the deviation to higher rates at temperatures below about 110 ~ is real, it makes a Significant difference to the extrapolation of data to lower-temperature conditions. The fact that not all results show this deviation - e.g.


- 2 -

o - 4 -

-6

�9 CO -8 -

-10 - t o

~-~2 CO

-14 -

g -16

-18 t- 1.8

Mater ia l :o Kraf t paper dry o Kraf t paper 1% H20 A Kraft paper 4% H20

/al I I 1 I I I / 1 T 1

2.0 2.2 2.4 2.6 2.8

Reciprocal t e m p e r a t u r e , 1000/K

-2

o -4 4~ to

-6

D 4o7_

~-12 -

-14F- 03 o -16 (b)

-18 ~- 1 1.8

Mater ia l :o Kraft paper 0.5% H20

2.0 2.2 2.4 2.6 2.8

Reciprocal temperature, 1000/K

-2

-4 ro L -6

-~ -~o ro &-12 04

-0 -14

-18 1,8

Material: o Cot tonboard �9 Transformer board a Transformer board dried

Transformer board 20gh H20 v Transformer board 50g H2Q

. .

I l I I I I I I , t I 2.0 2.2 2.4 2.6 2.8

Reciprocal temperature, 1000/K

-2

-4 c~

-6 Co -8

-10 r ~-12

-o -14

-16

-18 ~- 1.8

Material: o A l l dry wood �9 Manil la 60140 a Up graded a l l wood dry q A l l wood 1% H20 �9 All wood 2% H20 o A l l wood 4% H20 A Upgraded a l l wood a i r f low

(d) "

I I J I I I I I ~ 1 2.0 2.2 2.4 2.6

Reciprocal t e m p e r a t u r e , IO00/K

I

28

FIGURE 7. Comparison of the analysis of covariance of first-order rates (dashed lines) with the data of: (a) Bouvier for degradation in vacuo (1970)-I:3 dry Kraft paper; O Kraft paper 4- I% water; A Kraft paper -I- 2% water; (b) Fabre and Pichon for degradation in oil (1960)-O Kraft paper + 0.5% water; (c) Moser and Dahinden for degradation in oil (1988)-O cottonboard; �9 transformerboard; A transformerboard maintained dry; [] transformerboard + 20 g of water; V transformerboard -I- 50 g of water; (d) Schroff and Stannett for degradation in oil (Schroff and Stannett, 1985; Gibbons and Schroff, 1988)-O dry all-wood Kraft paper; �9 60/40 manilla Kraft paper; [] upgraded Kraft paper; V Kraft paper -I- I% water; �9 Kraft paper + 2% water; ~ Kraft paper + 4% water; A Dry Kraft paper + air.

transformerboard data of Moser and Dahinden (1988) - indicates a possible experimental artefact, which would again affect extrapolation to transformer conditions.

A useful pointer to the validity of this whole approach to rationalizing the rate data from different sources, is the excellent agreement of the data in vacuo of Bouvier (1970) for paper containing 1% and 4% water with those of Shroff et al. (1985, 1988) obtained under oil at the same moisture levels. Conversely, the disagreement with some data obtained under closed reactions conditions, such as those used by Schroff and Stannett, Fabre and Pichon and others, highlights the dangers of attempting to simulate real conditions too closely, at the expense of maintaining tight control over experimental parameters. On the one hand, there is the need to model real environments, but on the other there is the need to understand the underlying chemistry under


well defined conditions. Ultimately, a mathematical model of degradation requires this fundamental data to construct a picture of the processes occurring under real conditions. There is therefore an absolute need for both 'model' and 'fundamental' experiments.

Is there a change in rate above 140 ~

A number of workers (Hino and Suganuma, 1967; Bouvier, 1970; Fallou, 1970; Hino and Suganuma, 1972; Paloniem, 1972; Yoshida et al., 1987; Gibbons and Schroff, 1988; Moser and Dahinden, 1988) have suggested that a change in rate occurs above about 140 ~ It has also been suggested that the reaction might be auto-accelerated by water, since it is a product of degradation.

Some support for the concept of auto-acceleration can be drawn from the data of Schroff et al. (1985, 1988) and of Moser and Dahinden (1988). In Fig. 5, their data above 140 ~ appear to fit on a higher Arrhenius line than equivalent data below 140 ~ except in the case where moisture was continuously removed from the oil by molecular sieve (1988). In addition, a detailed examination of all the published data shows that nearly all exhibit a tendency to positive curvature at high temperatures when plotted according to Equation 10 (see Fabre and Pichon, Hino and Suganuma, Miyoshi, Zhilyaev et al. in Figs 4a, b, c and i). Only the data of Moser and Dahinden and Tamura et al. do not show this trend in Figs 4d-g. However, the timescales of their experiments are short compared with those of other workers and the effect may not have had time to develop. It is proposed that the observed high rates of degradation at high temperatures and long times in oil can be explained by the accumulation of water product in the paper, which increases the rate by increasing the pre-exponential factor.

Hino and Suganuma's evidence (1967, 1972) for a change in rate comes from measurement of gas evolution. It was clearly related to a step increase in the pre-exponential factor as well as an apparent increase in the activation energy above 140 ~ They proposed that the change was due to residual moisture in the paper. Unfortunately, they did not make DP measurements below 140 ~ to enable a comparison with other work. Their DP results at and above 140 ~ plotted according to Equation 10 (Fig. 4b), show an increasing deviation from the origin and are distinctly curved above 150 ~

So what is the effect o f water?

Unravelling the effects of moisture accumulation and its partition between paper and oil will be a key issue in devising a model for the degradation of cellulose paper in insulating oil applications.

The heterogeneity of slope in Fig. 5 was reduced by moving the low-temperature (< 120 ~ low-moisture (< 1%) data of Fabre and Pichon (1960) and of Bouvier (1970) to a higher pre-exponential set. In effect, the low-temperature reaction rates were too high to fit SET 2. At the same time the higher-temperature data of Moser and Dahinden (1988) and Schroff et al. (1985, 1988) were also moved to a higher rate set. Accumulation of water in the paper could explain at least the high-temperature effect. The effects of high water levels are clearly shown in Figs 3e and 4f, taken from the data of Schroff et al. (1985, 1988) and of Moser and Dahinden (1988) respectively.


Increasing the initial water content increases the rate of degradation. Decreasing the water level by drying the oil brings the rate back to its expected level in SET 2 (1988).

Schroff et al. (1985, 1988) and Moser and Dahinden (1988) have shown that the water level in the paper, degraded under oil, increases as degradation proceeds. The increase is particularly rapid at temperatures above about 120 ~ It is reasonable to assume that it is this accumulation of water which causes both the observed increases in rate and non-linearity of rate at temperatures above about 140 ~ However, we propose that it is not the activation energy which is increased, as suggested by other workers, but the pre-exponential factor. No curvature of data or rate increases have been reported (or noted when replotting data) in experiments carried out in vacuo, or in inert or oxidizing gases, or during hydrolysis. However, a vacuum or a hot dry gas would be expected to extract water and dump it to waste, thus preventing accumulation in the cellulose. It will therefore be interesting to carry out experiments in which the inert gas is recirculated and the water level is allowed to build up.

DP measurement and degradation kinetics

Throughout the reported literature of cellulose degradation, DP change, determined in solution, is used as the reaction parameter. DP, however, is strictly only a viscosity average measure of the molecular weight distribution and does not give detailed information about molecular weight changes occurring during degradation. Measure- ment of the true changes in the distribution, as obtained from gel permeation chromatography (GPC) for instance, could assist modelling of the kinetics and mechanisms of degradation. Three reports of GPC studies of cellulose degradation in the literature have shown measurable changes in molecular weight distribution as a result of degradation (Cosgrove et al . , 1985; Burgess, 1986; Darveniza et al. , 1992). However, Cosgrove et al. and Darveniza et al. derivatized their samples to the water-soluble carbamate to facilitate the analysis. Both reported low molecular weight tails to the distribution, attributed, in one case (Darveniza et al . , 1992), to hemicelluloses in the paper. However, other studies using non-derivatizing solvents have shown that carbammation degrades the cellulose (see for instance (Lawther et al . , 1990)). The low molecular weight end of the distribution will therefore be affected by the degradation products of sample preparation, unless care is taken to avoid this influence.

Insulation life prediction

We do not currently have adequate models to predict insulation lifetimes from degradation rates with any degree of accuracy. Detailed information is required on temperature distribution and the accumulation of moisture and oxygen with time. In addition, the initial and final end of life conditions of the insulation are important and the relationship of mechanical strength to DP is required before a life prediction model can be formulated more precisely.

Using the conventional method of relating life to the time to reach a limiting DP value, the following equation can be formulated:

1 1 - k(life) (10) D Pfinal D Pinitial


where

k = Ae -AE/RT (11)

T is the absolute temperature, R is the gas constant (8.314), A E -- activation energy = 111 kJ/mole, and A is obtained from Table 3 for the relevant conditions.

For the case of paper insulation in a transformer, the initial and final DP values can most simply be taken as 1000 and 200 respectively. The equation then simplifies to:

life - 0.004 e13350/r hours. (12) A

A plot of insulation life against temperature, calculated from Equation 12, is shown in Fig. 8 for standard, Kraft insulating paper in dried insulating oil, in the temperature range 80--110 ~ which represents the upper limits of operation of a typical transformer. The dashed lines are the errors one standard deviation from the mean.

500 >,400

~- 300

200

~" 100

0 80

- (a) 160 _q (b) >\ 140 -\

>` \ 95% confidence limits ~ 120 \ ~ \

" ~ '- 100

r I I 1 I 1 0 85 90 95 100 105 110 80 85 90 95 100 105 110 Temperature, ~ Temperature, ~

60 (c)

5o '\ >`

d 40 ~ \ \

~ 3o N 20

o T I i / 80 85 90 95 100 105 110

o Temperature, C

30[ (d) ~ - 2 0 ~ \ \ = \ \

80 85 90 95 100 105 110 Temperature, ~

4

43 -O

(e) \\

\ % \ , ~ "\ \ \ \

80 85 90 95 100 105 110 Temperature, ~

F I G U R E 8. Predicted insulation life expectancy f rom f irst-order rates of degradat ion of cellulose in dried insulating oil.


There has been much discussion in the literature over what is an acceptable limiting value for the final DP of insulating paper. 150-250 is generally accepted as the ultimate limit because below this the paper loses all mechanical strength (Schroff and Stannett, 1985). Little consideration has been given to the initial DP. 1000-1200 is generally accepted prior to transformer conductor winding but values of 850-950 are common after the initial drying out period (Schroff and Stannett, 1985). The effect of the choice of starting and final DP on the transformer life is significant and is discussed in detail elsewhere (Emsley and Stevens, 1993).

Best estimates of insulation life from Equation 12, for any closely defined condition, vary by less than a factor of 2, but the total variation over all environmental and cellulose conditions is a factor of 200. This highlights the need for improved ageing models to be developed. We consider that this can only be achieved from a more detailed understanding of cellulose degradation mechanisms and the influence of operational parameters upon them, together with an accurate determination of the corresponding kinetics.

A C K N O W L E D G E M E N T S

This paper is published by permission of National Power plc.

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