Kinetics of the production of chain-end groupsand methanol from the depolymerization of celluloseduring the ageing of paper/oil systems.Part 1: Standard wood kraft insulation

Roland Gilbert Æ Jocelyn Jalbert ÆPierre Tetreault Æ Brigitte Morin ÆYves Denos

Received: 14 April 2008 / Accepted: 25 September 2008 / Published online: 15 October 2008

� Springer Science+Business Media B.V. 2008

Abstract Recently, the existence of a relation

between the rupture of 1,4-b-glycosidic bonds in

the cellulose during thermal-ageing of paper/oil

systems and the detection of methanol in the oil has

been reported for the first time in this journal (Jalbert

et al. 2007). The present study addresses the rate

constants of the reaction for standard wood kraft

papers, two immersed in inhibited naphthenic oil

under air (paper/oil weight–volume ratio of 1:18) and

one in non-inhibited paraffinic oil under nitrogen

(paper/oil weight–volume ratio of 1:30). The iso-

therms in the range of 60–130 �C show that the initial

rate of methanol production markedly increases with

temperature and to a lesser extent with the moisture

of the specimens (initially between 0.5 and

2.25% (w/w)), similarly to what is noted for the

depolymerization through the Ekenstam’s pseudo-

zero order model. The Arrhenius expression of the

rate constants reveals linear relationships that confirm

the dominance of a given mechanism in both cases. A

very good agreement is also noted for the activation

energy over the entirely paper/oil systems studied

(106.9 ± 4.3 and 103.5 ± 3.7 kJ mol-1 for methanol

and scissions, respectively). Furthermore, a compar-

ison of the rate constants kCH3OH=kscissionsð Þ shows

approximately constant values indicating an apparent

yield for the methanol of about one-third molecule

per every scission for the tests under air (0.27 ± 0.04

for Clupak HD75 and 0.37 ± 0.14 for Munksjo

TH70) and even lower for the ones under N2

(0.12 ± 0.03 for Munksjo E.G.). As expected from

a pseudo-zero order model, these values were shown

to be consistent with a similar comparison of the

amount of CH3OH and chain-end groups produced

under specific time–temperature ageing conditions

(168 h at 120 �C). Finally, an additional test carried

out with unaged cellulose in contact with a fresh

solution of methanol in oil (cellulose/oil weight–

volume ratio of 1:18) shows that at equilibrium, over

58% of the species is lost from the solution due to

penetration into the fibres. Such results reveal the

importance of the species partitioning in establishing

the true correspondence between the molecules of

CH3OH produced and the scissions.

Keywords Cellulose degradation �1,4-b-Glycosidic bond scission � Methanol �Ageing indicator � Kinetics � Activation energy �Frequency factor � Insulating paper �Moisture content � Insulating oil � Transformer �Remaining life

R. Gilbert (&) � J. Jalbert � P. Tetreault � B. Morin

Institut de recherche d’Hydro-Quebec (IREQ), 1800,

boulevard Lionel-Boulet, Varennes, QC J3X 1S1, Canada

e-mail: gilbert.roland@ireq.ca

Y. Denos

Electricite de France, EDF R&D, 1, avenue du General de

Gaulle, BP 408-92141, Clamart, France

123

Cellulose (2009) 16:327–338

DOI 10.1007/s10570-008-9261-1

Introduction

A few years ago, thermal-ageing tests on paper/oil

systems revealed a relationship between the scission

of 1,4-b-glycosidic bonds in cellulose (paper depo-

lymerization) and the detection of specific species in

oil, regardless of the amount of stabilizing nitroge-

nous agents added to the paper (upto 3.9% (w/w) N2).

Among the detected species, methanol, the subject of

a recent paper in Cellulose (Jalbert et al. 2007),

proved to be of particular interest for monitoring

paper depolymerization under normal transformer

operating conditions. The present study deals more

specifically with the kinetics of the reaction. If

methanol is effectively formed at the chain break, a

close kinetic correspondence should be seen between

the two processes, which is of central importance to

qualify the species for indirect field monitoring. In

this paper, the methanol evolution in the oil measured

through the ageing of three paper/oil systems will

then be compared to the depolymerization, according

to the cellulose degradation models depicted below.

The first concepts on the degradation of cellulose

were established in 1930 through the work of Kuhn.

This author studied the degradation of cellulose using

a statistical approach, which was later translated by

Ekenstam (1936) into a kinetic depolymerization

model. The model, which covers the degradation of

all linear-type polymers, is based on the premise that

matter is monodispersed and the bonds between

monomers have an equal probability of scission. If

one considers the number of 1,4-b-glycosidic bonds

between monomer units in a molecular chain that is

broken by unit of time, expressed in terms of the

number of unbroken bonds, N, it could then be

assumed that the random scission of these bonds will

follow a first-order kinetic law:

dN=dt ¼ �k0N ð1Þ

in which t is the time, and k0 the first-order rate

constant. This law, which is often used to model the

degradation of cellulose in air, applies equally to

paper immersed in transformer oil. It can also be

expressed in terms of the fraction of the total number

of unbroken bonds, 1 - 1/DPn, in which DPn is

defined as the number-average degree of polymeri-

zation per molecular chain, thus giving a direct

indication of the ageing of the macromolecule:

d 1� 1=DPnð Þ=dt ¼ �k0 1� 1=DPnð Þ ð2Þ

The integration of Eq. 2 gives a first-order equation

in which the subscripts o and t refers respectively, to

the initial value of DPn and to the value at any time, t:

ln 1� 1=DPnðoÞ� �

� ln 1� 1=DPnðtÞ� �

¼ k0t ð3Þ

When the number of ruptured bonds is very low, the

results can be modelled by a zero-order rate law (rate

with a zero or quasi-zero dependence on the bonds

not yet ruptured):

1=DPnðtÞ � 1=DPnðoÞ ¼ akt ð4Þ

in which the left side corresponds to the average

number of scissions per b-D-glucopyranose unit. The

factor a then becomes a measure of the accessibility

of the bonds, and k the specific rate constant of the

scission of these bonds (k0 of Eqs. 1–3 equals ak). As

reported by Emsley et al. (1997), Eq. 4 describes

most of the literature data on the insulating paper in

oil, air, vacuum and oxygen in the range of 100–

200 �C, except for tests carried out to a high degree

of molecular chain deterioration. In the latter situa-

tion, the pseudo-zero order approximation tends to

overestimate the changes in the degree of polymer-

ization, and consequently to underestimate the useful

life of the paper insulation. These authors then

proposed a new model based on the arguments

presented by Zou et al. (1996): k of Eqs. 3 and 4

would not be constant over the entire duration of the

reaction. The model requires two constants: k1o, the

initial degradation rate constant, and k2, the rate at

which k1 changes:

k1ðrate of depolymerizationÞ ¼ k1oe�k2t ð5Þ

in which k1o is the initial value of k1. Various

substitutions can then be used to demonstrate that:

1=DPnðtÞ � 1=DPnðoÞ ¼ k1o=k2 1� e�k2t� �

ð6Þ

The evolution of k1 over time would basically be

caused by modifications in the substrate. Contrary to

what occurs in a simple chemical reaction, the nature

of the reagent changes as the reaction proceeds, along

with the molecular weight distribution of the poly-

mer. In addition, cellulose is a semicrystalline matter

for which greater reactivity is expected in amorphous

regions. The authors concluded that by using this

328 Cellulose (2009) 16:327–338

123

model rather than the above approximation, greater

accuracy would be obtained in the prediction of the

time required to reach low DP values. Alternately,

considering that the physical basis of Eq. 6 was not

sufficiently clear, Calvini (2005) (also in Calvini

et al. 2008) proposed another approach in which the

value of the levelling-off degree of polymerization

(LODP) is introduced for heterogeneous systems in

the first-order kinetic law:

1=DPnðtÞ � 1=DPnðoÞ ¼ 1=LODP� 1=DPnðoÞ� �

1� e�kt� � ð7Þ

the same formal results as Eq. 6 are obtained,

although with a different meaning of the pre-expo-

nential factor, without introducing a change in the

rate constant with the progression of the reaction.

As the main objective of the present study is to

further confirm the existence of a correlation between

the rupture of 1,4-b-glycosidic bonds in cellulose and

the production of methanol, the pseudo zero-order

Ekenstam’s relationship (Eq. 4) would then be

applied to obtain the rate constants at the early stages

of paper degradation under different temperature,

moisture and atmosphere conditions. The rate con-

stants of the methanol production will be obtained by

assuming, analogically with depolymerization, that a

pseudo zero-order reaction is involved at a low level

of production of the species. The authors are aware

that a more powerful model should be used, such as

the one in Eq. 6 or 7, when extending the investiga-

tion of an advanced stage in order to obtain the

kinetic parametres needed to predict the life expec-

tancy of such insulating papers.

Experimental section

Ageing cells for tests under air

Two standard wood kraft papers were used for the

tests: Clupak HD75 (wood cellulose containing up to

7% (w/w) lignin and hemicellulose) and Munksjo

Thermo 70 (namely Munksjo TH70) also known as

ThermoKraft (obtained from the ultra-purification of

a neutral or slightly alkaline 100% kraft pulp). The

thermal resistance of the latter paper meets the

requirements for transformers with a 65 �C winding

temperature rise without the need for chemical

treatment (such as a cyanoethylation reaction or

addition of organic amines). 180 strips of 0.5 g

(17.2 cm 9 1.9 cm for the Clupak HD75 and

17.7 cm 9 2.7 cm for the Munksjo TH70 giving an

exposed specimen surface of 32.7 and 47.8 cm2,

respectively) were cut from the sheets that were

supplied. These loosely rolled strips were transferred

to a moisture-controlled glove box along with an

equal number of 20-mL glass ampoules and 3-L

bottle of Nynas Nytro 10CX naphthenic oil. The oil

contains about 3,000 mg/kg of 2,6-di-tert-butyl-p-

cresol (oxidation inhibitor). The glove box was under

air at 6.3% RH. On attaining the target moisture in

the specimens (*2% (w/w) H2O), 180 ampoules

were prepared by inserting a 0.5-g specimen and

9 mL of oil giving a paper/oil weight–volume ratio of

1:18. After being withdrawn from the glove box, the

ampoules were sealed under air in order to model the

conditions prevailing in transformers with inhibited

oil under an air conservator. The cells were then

distributed in seven forced-air ovens operating at 60–

120 �C in 10 �C increments. The temperature of each

oven was measured using a Hart 1522 thermometer

equipped with a platinum probe (±0.05 �C in a range

of 50–120 �C). The cells were removed from the

ovens after varied lengths of time depending upon the

ageing temperature. The same procedure was applied

to prepare an equivalent number of ageing cells with

paper specimens equilibrated at *1.0 and

*0.5% (w/w) H2O. Additional details on the prep-

aration of the cells and equipment can be found

elsewhere (Jalbert et al. 2007).

Ageing cells for tests under nitrogen

Some of the characteristics of the Munksjo Electrical

Grade (namely Munksjo E.G.) paper used for these

tests are given in Table 1 together with the ones of

the previous papers and the Hi-Val kraft pressboard

(used for partitioning tests at 40 �C). Although this

paper is marketed for an application in electrical

equipment, it differs from the above Munksjo not

only from the standpoint of thickness and grammage,

but also on the basis that it cannot meet the 65 �C rise

standard (suspected to be obtained from a pulp that is

not ultrapurified). Because of a lower grammage, the

0.5-g strips used to manufacture the specimens had a

greater exposed surface (35 cm 9 4 cm for

140 cm2). The loosely rolled strips were transferred

Cellulose (2009) 16:327–338 329

123

to a moisture-controlled glove box under nitrogen

([O2] \ 1,000 ppm (v/v)) at 4.2% RH along with an

equal number of 20-mL glass ampoules and 3-L

bottle of Shell Diala S paraffinic oil (uninhibited oil).

20 lg kg-1 of 2-furfural was measured in the oil

along with an indeterminate amount of toluene and

about 300 mg kg-1 of 2,6-di-tert-butyl-p-cresol. On

attaining the target moisture in the specimens

(*2.25% (w/w) H2O), 155 ampoules were prepared

by inserting a 0.5-g specimen and 15 ml of oil, which

gave a paper/oil weight–volume ratio of 1:30. The

ampoules were then temporarily plugged (to prevent

air from being introduced) and removed from the

glove box to be sealed. Four additional ampoules

were prepared using 15 ml of oil (cell without paper).

These ampoules subjected to the same procedure as

the previous ones showed 148 ± 86 ppm (v/v)

(n = 4) of O2 in the oil, thus validating the method

used to preclude this gas in order to model the

conditions prevailing in transformers with uninhib-

ited oil under a nitrogen conservator. The ageing cells

were then distributed in seven forced-air ovens

operating at 70–130 �C in 10 �C increments. The

same procedure was applied to prepare an equivalent

number of cells with specimens equilibrated at

*0.6% (w/w) H2O.

Apparatus and methods

The ageing of the papers was monitored over time by

following two parametres: (1) the viscosity-average

degree of polymerization of the fibres (DPv) which

is obtained by measuring the kinematic viscosity of

the papers in solution (the viscosity is related to

the molecular weight (Mv) by the Mark-Houwink

Sakurada’s equation), and (2) the accumulation of

methanol in the oil in contact with the paper speci-

mens. After being removed from the ovens, the cells

were stabilized for 3 h at 20 �C to equilibrate all the

systems at a given reference temperature. After the seal

was broken, an aliquot portion of oil was transferred

into a 10-ml headspace vials (Supelco #27295) and the

paper specimen stored in the dark for subsequent

measurement. A Model 7694 static headspace sampler

coupled with a Model 6890N gas chromatograph

equipped with a Model 5973N mass selective detector

at 70 eV ionization energy in the electron impact mode

(Agilent Technologies) was used to assess the meth-

anol in oil. Quantification was done by processing the

chromatograms offline in selected ion monitoring

(SIM) mode (for details, see Jalbert et al. 2007). The

viscosity-average degree of polymerization was

obtained based on measurements at 20.00 �C using

an automatic viscometer (Polyvisc) equipped with a

two-sphere Ubbelohde tube (0.5–50 cSt). Each paper

specimen was first de-oiled with a Soxhlet (Soxtec

Avanti 2050) using fresh distilled hexane, then defi-

bered in a water-cooled grinder (Janke&Kunkel, IKA-

Werk). The loose fibres were then dissolved in the

viscosimetric solvent bis (ethylenediamine) copper(II)

hydroxide (Anachemia #29072-360). The procedure is

based on ASTM D4243. The moisture content of the

specimens is determined by titration using a Karl

Fischer 756 KF coulometer (Brinkmann) according to

ASTM D1348 Method C. Lastly, the amount of

nitrogenous compounds in the paper is determined

with the Kjeldahl method using Brinkman digestion

and distillation units, models Buchi B-435 and B-323,

and a Methrom Titrino titration unit, Model 716BMS,

based on ASTM D982-05.

Table 1 Characteristics of standard wood kraft insulating papers and pressboard

Producta Thickness

(lm)

Grammage

(g m-2)

Apparent

density

(g/cm-3)

N2

(% w/

w)b

DPv(o)c

Clupak HD75 75 ± 2 NAd 1.00–1.15 0.073 1224.9

Munksjo TH70 63 ± 3 61–67 0.75–0.85 0.013 1175.5

Munksjo E.G. 45–55 36–44 0.80 0.013 1208.4

Hi-Val kraft transformer board 1,788 ± 9 NA 0.90–1.05 \0.03 1169.8

a Except for Munksjo TH70 meeting the 65�C, all the other products met the transformer winding rise standard of 55 �Cb As per Kjeldahl methodc Measured in our laboratoryd NA not available

330 Cellulose (2009) 16:327–338

123

Results and discussion

Kinetics of the depolymerization (production

of chain-end groups)

Typical examples of the decrease in the viscosity-

average degree of polymerization of the paper

specimens with time for the three paper/oil systems

according to the expression of the data in the form of

Eq. 4 are illustrated in Fig. 1. It is interesting to note

that the isotherms show fairly good linearity, thus

justifying the Ekenstam’s zero-order kinetic approach

for modelling the early stage of the reaction. For the

tests carried out under N2 (Fig. 1c), a larger scattering

is seen for the data of the low-ageing temperature

side (T = 70, 80, 90 and 100 �C). Such variability

could have been introduced by some leaking cells.

With respect to the high-temperature isotherms, the

defective units detectable by the colour of the oil

were discarded. This section of the depicted relation

in Fig. 1 appears to result from the scission of the so-

called ‘‘normal’’ 1,4-b-glycodisic bonds in the amor-

phous regions of the microfibrils. Nevertheless, some

isotherms showed a slight initial deviation from

linearity that could be tentatively attributed to the

presence of ‘‘abnormal’’ 1,4-b-glycodisic bonds. This

is especially noticeable for the Munksjo TH70 (for

example, see T = 80 �C in Fig. 1b). Although it is

unusual to report such deviations for paper/oil

systems (possibly due to a lack of experimental

points), they were noted by several authors during the

acid hydrolysis of the cellulose under heterogeneous

conditions (Sharples 1971; Daruwalla and Narsian

1966; Agarwal et al. 1991). These bonds, which are

sometimes identified as ‘‘weak bonds’’ or ‘‘bonds

sensitive to acid’’ could be natural in origin or the

result of modifications to the b-D-glucopyranose units

during the pulp manufacturing. The Munksjo TH70

paper is known to result from a special purification

process that could lead to a larger number of such

modified units in the finished product (no attempts

have been made in this paper to show the presence of

side carbonyl or carboxyl groups that could lead to

initial deviations from Ekenstam’s relationship).

Furthermore, when the degradation proceeds over

time, the 1/DPv(t) - 1/DPv(o) graphs tend to level off,

as shown in Fig. 2 (downward curvature in the

kinetic plot), to reach an asymptotic value that would

correspond to the DPv of the cellulose microcrystals.

The zero-order reaction then shifts to a higher order

(increase dependence of k on the number of bonds

available for the scission). The rupture of the bonds

that initially occurred in the amorphous regions is

now extended over the more ordered domains.

The initial rate constants of the reaction obtained

from a forced-through-zero linear regression of the

data are compiled in Table 2 (corresponding to the

dashed lines on the isotherms in Fig. 1). As expected,

k is seen to increase markedly with T and to a lesser

extent with the moisture content of the specimens. To

0 4000 8000 12000 16000

(1/D

P v(t

)-1/D

P v(o

)) x

103

Clupak HD75 in Nynas Nytro 10CX under air

120oC

110oC 100oC

90oC

80oC

70oC60oC

1.92% (w/w) H2O

0.0

0.8

1.6

2.4

3.2

60oC

70oC

80oC

120oC

110oC

100oC

(1/D

P v(t

)-1/

DP v

(o))

x 1

03

90oC

1.89% (w/w) H2OMunksjö TH70 in Nynas Nytro 10CX under air

2.4

1.6

0.8

0.0

3.2

(1/D

P v(t

)-1/

DP v

(o))

x 1

03

2.4

1.6

0.8

0.0

3.2

90oC 80oC

70oC

100oC

110oC120oC

130oC

2.25% (w/w) H2OMunksjö E.G. in Shell Diala S under nitrogen

Ageing time (h)

0 4000 8000 12000 16000

0 4000 8000 12000 16000

(a)

(b)

(c)

Fig. 1 Typical examples of 1/DPv(t) - 1/DPv(o) versus time

graphs

Cellulose (2009) 16:327–338 331

123

compare the data in Table 2 with the methanol data in

the next section, these rate constants should first be

expressed in the same concentration units as for the

CH3OH production, i.e. lmol g-1 h-1. As shown by

Whitmore and Bogaard (1994), this could be

achieved by applying the following definitions: (1)

the number-average degree of polymerization is

about half the measured viscosity-average degree of

polymerization (DPn = DPv/2) and (2) the total

number of molecular chains in a given weight of

sample, i.e. the amount of the reducing end groups, is

equal to the total number of monomer units in a given

sample weight divided by DPn. The increasing

amount of cellulose chains due to depolymerization

(i.e. the number of scissions) could then be expressed

as 6,170 9 (2/DPv(t) - 2/DPv(o)), where the first

coefficient takes into account the reciprocal molec-

ular weight of the b-D-glucopyranose units. The

above authors showed that this relationship, which is

valid for an ideal polymer (polydispersity index of

two) subjected to purely random scissions, gives a

good one-to-one correspondence with the amount of

reducing end groups (lmol g-1) produced by acid

hydrolysis, independently of the order of the kinetics

of reaction. The re-calculated k-values compiled in

Table 3 were used to plot the Arrhenius expressions

(ln k vs 1/T), of which typical examples are provided

in Fig. 3.

In general, this relation shows a good linearity

over the entire range of temperatures, confirming that

the overall degradation process is dominated by a

given mechanism. Nevertheless, a slight deviation

from the linearity is sometimes noticeable at one end

or both ends of the temperature range, especially for

the low-moisture specimens for which the hydrolysis

contribution to the degradation is being minimized.

Such deviations could be indicative that at low-

ageing temperatures (T & 60–70 �C), oxidation con-

tributes to a greater extent to the overall process

whereas at the opposite end (T & 120–130 �C), the

pyrolytic contribution may be more pronounced (a

break in this expression has effectively been reported

by Knosp and Fallah (1961) between 130 and

140 �C). It is important to remind here that for the

papers ageing in transformers, k is an apparent rate

0 400 800 1200 1600 2000

0

1

2

3

4

5

6

2.25% (w/w) H2O

Munksjö E.G. in Shell Diala Sunder nitrogen

transition to a higher order

DPv = 298

(1/D

P v(t

)-1/

DP v

(o))

x 1

03

Ageing time (h)

T = 130oCpseudo zero-order

Fig. 2 Transition of the depolymerization reaction from a

zero-order to a higher order kinetic law

Table 2 Initial rate constants of depolymerization as established from 1/DPv(t) - 1/DPv(o) versus time relationships

System Clupak HD75 in Nynas

Nytro 10CX under air

Munksjo TH70 in Nynas

Nytro 10CX under air

Munksjo E.G. in Shell

Diala S under nitrogen

k (h-1)

% (w/w) H2O % (w/w) H2O % (w/w) H2O

T (8C)a 1.92 1.11 0.47 1.89 1.15 0.45 2.25 0.62

61.93 1.015E-08 1.086E-08 8.262E-09 1.815E-08 1.812E-08 1.725E-08 NA NA

71.2 2.532E-08 1.977E-08 1.707E-08 4.002E-08 3.677E-08 3.015E-08 1.067E-08 NA

79.95 7.109E-08 5.742E-08 3.287E-08 7.541E-08 7.313E-08 5.445E-08 2.289E-08 NA

92.04 2.601E-07 1.871E-07 1.247E-07 3.000E-07 2.047E-07 1.798E-07 7.724E-08 3.965E-08

103.21 7.838E-07 6.848E-07 3.800E-07 7.332E-07 6.853E-07 4.241E-07 1.897E-07 6.322E-08

113.23 1.362E-06 1.481E-06 1.041E-06 1.931E-06 1.518E-06 1.173E-06 6.734E-07 1.492E-07

120.26 3.697E-06 2.079E-06 1.876E-06 2.989E-06 1.631E-06 1.520E-06 2.219E-06 3.652E-07

130.98 NA NA NA NA NA NA 5.141E-06 1.063E-06

a T measured in ovens with the platinum probe

332 Cellulose (2009) 16:327–338

123

constant that corresponds to a combination of ki,

where i refers to the ith degradation mechanism

(oxidative, hydrolytic or pyrolytic in nature) contrib-

uting to the overall process.

The Arrhenius parametres obtained from a linear

regression of the data (corresponding to the dashed

lines in Fig. 3) are listed in Table 4. As recently

reminded by Calvini and Gorassini (2006), the

activation energy (Ea) is dependent upon the mech-

anism that imposes its characteristics on the

degradation process (oxidative, hydrolytic or pyro-

lytic), while for the frequency factor (Aa), it is

influenced by all the other experimental parametres

(humidity, acidity, physical structure of the paper).

An averaged value of 103.5 ± 3.7 kJ mol-1 (n = 8)

Table 3 Initial rate constants of depolymerization as expressed in terms of production of chain-end groups

System Clupak HD75 in Nynas

Nytro 10CX under air

Munksjo TH70 in Nynas

Nytro 10CX under air

Munksjo E.G. in Shell

Diala S under nitrogen

k (lmol g-1 h-1)

% (w/w) H2O % (w/w) H2O % (w/w) H2O

T (8C)a 1.92 1.11 0.47 1.89 1.15 0.45 2.25 0.62

61.93 1.252E-04 1.341E-04 1.020E-04 2.239E-04 2.237E-04 2.129E-04 NA NA

71.2 3.124E-04 2.439E-04 2.107E-04 4.938E-04 4.537E-04 3.721E-04 1.317E-04 NA

79.95 8.772E-04 7.086E-04 4.056E-04 9.306E-04 9.025E-04 6.718E-04 2.825E-04 NA

92.04 3.210E-03 2.309E-03 1.539E-03 3.702E-03 2.527E-03 2.218E-03 9.531E-04 4.893E-04

103.21 9.672E-03 8.450E-03 4.689E-03 9.048E-03 8.457E-03 5.233E-03 2.340E-03 7.801E-04

113.23 1.681E-02 1.828E-02 1.284E-02 2.383E-02 1.873E-02 1.448E-02 8.310E-03 1.841E-03

120.26 4.562E-02 2.565E-02 2.315E-02 3.688E-02 2.012E-02 1.876E-02 2.738E-02 4.507E-03

130.98 NA NA NA NA NA NA 6.344E-02 1.312E-02

a T measured in ovens with the platinum probe

Table 4 Arrhenius parametres of the depolymerization reaction (based on data in Table 3)

System H2O %

(w/w)

Range of T covered

by the linear

regression (�C)

r Activation

energy

(kJ mol-1)

Frequency

factor

(lmol g-1 h-1)

Clupak HD75 in Nynas

Nytro 10CX under air

1.92 60–110 (6)a -0.9980 106.7 ± 3.4 5.361E?12

1.11 60–120 (7) -0.9968 105.0 ± 3.8 2.581E?12

0.47 60–120 (7) -0.9965 104.8 ± 4.0 1.703E?12

Munksjo TH70 in Nynas

Nytro 10CX under air

1.89 70–120 (6) -0.9984 102.2 ± 2.9 1.447E?12

1.15 70–110 (5) -0.9974 99.7 ± 4.2 5.448E?11

0.45 70–110 (5) -0.9970 96.6 ± 4.4 1.460E?11

Munksjo E.G. in Shell Diala

S under nitrogen

2.25 70–110 (5) -0.9959 106.9 ± 5.6 1.977E?12

0.62 90–130 (5) -0.9787 105.9 ± 12,8 5.148E?11

a n: number of data points

0.0025 0.0026 0.0027 0.0028 0.0029 0.0030-10

-9

-8

-7

-6

-5

-4

-3

ln k

1/T (K-1)

1.92

Clupak HD75 inNynas Nytro 10CX under air

1.11

% (w/w) H2O

0.47

Fig. 3 Typical examples of Arrhenius expressions of the

depolymerization rate constants

Cellulose (2009) 16:327–338 333

123

is obtained for Ea, which is in good agreement with

the results on the heterogeneous hydrolytic depoly-

merization of cellulose (Emsley and Stevens 1994;

Zou et al. 1996; Karst and Yang 2007). Moreover, the

severity of the ageing conditions has no significant

effect on this parametre, at least within the limits of a

linear approximation of an exponential function,

contrary to what is noted for Aa. The latter parametre,

when expressed in h-1 units rather than in

lmol g-1 h-1, shows the same order of magnitude

as obtained by Emsley and Stevens (1994) for kraft

papers in oil with a moisture ranging from \0.5 to

2% (w/w). Considering that the Munksjo TH70 is the

only paper tested under air that meets the 65 �C

winding temperature rise standard, it follows that the

frequency factors associated with this paper should be

of lower magnitude when compared to the Clupak

HD75. As seen in Table 4, this expected difference

appears over the full range of initial moisture covered

by the specimens. Moreover, slightly lower Aa values

are seen for the Munksjo E.G. when compared to the

Clupak HD75. Such a superiority in the thermal

resistence of the former paper could easily be lost if a

transformer designed to operate under N2 with non-

inhibited oil experiences air in-leakage. Indeed, an

additional test with the Munksjo E.G./Shell Diala S

system (168 h at 130 �C) showed that the rate of

scission is 3.0 times faster (1/DPv(168h) - 1/DPv(o))

when N2 is replaced with air (n = 5). For paper

specimens with a moisture content ranging from 0.3

to 5% (w/w), Fabre and Pichon (1960) reported a

factor of 2.5 that they attributed to the direct action of

dissolved O2 on the molecular chains rather than the

indirect action of the oil-oxidation products (given

additional amounts of water and hydrophilic acids in

the system that foster the acid-catalysed hydrolytic

degradation). However, to be able to predict trans-

former field conditions, the effectiveness of air as an

accelerator of ageing should be appreciated using

laboratory cells with the paper-volume ratio on the

air–oil exchange surface comparable to those of the

piece of equipment, which is likely not the case here.

Kinetics of methanol production

Typical examples of the methanol evolution with

time for the three paper/oil systems are illustrated in

Fig. 4. As pointed out in the experimental section,

these results were obtained by measuring the species

in oil samples collected from the same cells used for

the above study. A fairly good agreement is noted for

these plots with the trends shown for the scissions in

Fig. 1. There is also a levelling of the data with time,

as illustrated in Fig. 5 (isotherm plotted over a longer

time range than in Fig. 4), suggesting that the

reaction is shifting to a higher order. As in the case

of the scissions, some deviations from the linearity

are noticeable, especially for specimens with low

initial moisture (\1% (w/w)). Such deviations are

seen at low-ageing temperatures (case of 1/DPv(t)

- 1/DPv(o)) as well as at high temperatures. In addition,

0 4000 8000 12000 16000

0

2

4

6

8

10

12 120oC

110oC

100oC

90oC

80oC

70oC60oC

CH

3OH

(µm

ole/

g pa

per)

CH

3OH

(µm

ole/

g pa

per)

CH

3OH

(µm

ole/

g pa

per)

1.92% (w/w) H2OClupak HD75 in Nynas Nytro 10CX under air

0 4000 8000 12000 16000

0

2

4

6

8

10

12

14 0.45% (w/w) H2OMunksjö TH70 in Nynas Nytro 10CX under air

70oC

90oC

100oC

110oC

120oC

60oC

80oC

0 4000 8000 12000 16000

2.25% (w/w) H2OMunksjö E.G. in Shell Diala S under nitrogen

Ageing time (h)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

80oC

90oC

100oC

110oC

120oC

130oC

70oC

(a)

(b)

(c)

Fig. 4 Typical examples of the increase of the CH3OH

concentration in oil with time

334 Cellulose (2009) 16:327–338

123

some isotherms in the low temperature range show a

weak drop after 6,000 h of treatment (see T = 70 and

80 �C in Fig. 4b); in these particular cases, the k-values

were established based on 0–6,000 h period.

The rate constants obtained by assuming a zero-

order reaction at the early stage of the reaction (by

analogy with depolymerization) are compiled in

Table 5 (corresponding to the dashed lines in

Fig. 4). It is interesting to note that k increases

markedly with T and to a lesser extent with the initial

moisture of the paper specimens. Regardless of the

testing conditions, these rate constants are systemat-

ically lower than those expressed in identical units for

the scissions (see Tables 3 and 5). The ratio

kCH3OH=kscissions is approximately constant, but its

value under air is about one-third (0.27 ± 0.04 for

Clupak HD75 and 0.37 ± 0.14 for Munksjo TH70

(n = 21 in both cases)) of the original working

hypothesis of one scission = one molecule of

CH3OH (Jalbert et al. 2007) and even lower under

nitrogen (0.12 ± 0.03 for Munksjo E.G. (n = 14)). It

is unlikely that this disagreement is due to the

polydispersity index, since it is known that this

parametre for paper is generally higher than two,

rather than lower than one as the ratio of the constants

might suggest. The Arrhenius parametres obtained

from these data are listed in Table 6, with typical

examples shown in Fig. 6. As for the Arrhenius

related to scissions (see Fig. 3), good linearity is

noted over the entire range of T, confirming that the

CH3OH production is dominated by a given mech-

anism. An average activation energy of 106.9 ± 4.3

kJ mol-1 (n = 8) is calculated, which is in fairly

good agreement with the scission value (103.5 ± 3.7

kJ mol-1). This is certainly indicative of a strong

correlation between the rupturing of the glycosidic

bonds and the production of CH3OH from chopped

chains. As for the scissions, the severity of the ageing

conditions has no effect on Ea while Aa is seen to

increase with the initial moisture of the specimens

(Table 6). The frequency factors appear to be gener-

ally of lower value when compared to their

equivalents for the scissions (five cases over eight),

which is in line with the ratio calculated for the rate

constants. Moreover, any direct implication of O2 in

0 1000 2000 3000 4000 5000

0

2

4

6

8

10

12

pseudo zero-order

CH

3OH

(µ m

ole/

g pa

per)

Ageing time (h)

transition to a higher order

Munsksjo TH70in Nynas Nytro 10CX under air

1.15% H2O (w/w)

T = 100oC

Fig. 5 Transition of the methanol production from of a zero-

order to a higher order kinetic law

Table 5 Initial rate constants of methanol production as established from [CH3OH] versus time relationships

Clupak HD75 in Nynas

Nytro 10CX under air

Munksjo TH70 in Nynas

Nytro 10CX under air

Munksjo E.G. in Shell

Diala S under nitrogen

k (lmol g-1 h-1)

% (w/w) H2O % (w/w) H2O % (w/w) H2O

T (8C) 1.92 1.11 0.47 1.89 1.15 0.45 2.25 0.62

61.93 3.466E-05 3.173E-05 2.581E-05 1.110E-04 4.040E-05 3.412E-05 NA NA

71.2 9.364E-05 5.962E-05 4.160E-05 2.173E-04 1.143E-04 8.289E-05 1.579E-05 NA

79.95 2.492E-04 2.062E-04 1.230E-04 4.109E-04 2.711E-04 1.768E-04 2.993E-05 NA

92.04 9.636E-04 6.162E-04 3.924E-04 2.456E-03 9.983E-04 6.131E-04 1.087E-04 5.850E-05

103.21 2.387E-03 2.023E-03 1.582E-03 5.236E-03 3.024E-03 1.805E-03 2.772E-04 1.183E-04

113.23 5.437E-03 3.846E-03 2.806E-03 9.643E-03 6.316E-03 5.187E-03 8.470E-04 3.417E-04

120.26 1.104E-02 7.141E-03 6.694E-03 1.681E-02 8.866E-03 7.769E-03 1.850E-03 6.945E-04

130.98 NA NA NA NA NA NA 4.891E-03 2.062E-03

Cellulose (2009) 16:327–338 335

123

the overall production of CH3OH other than by

weakening the glycosidic bonds of the cellulose

would have led to substantially different frequency

factors, which is not evident here comparing the

values for the Clupak HD75 (tests under air) with the

ones for the Munsksjo E.G. (tests under nitrogen)

(Table 6).

In order to confirm the above yields for the

methanol production, additional tests under specific

time–temperature ageing conditions (168 h at

130 �C) were carried out, this time, with a larger

population of samples. Two types of ageing cells

were prepared, one with 0.5-g specimens of Clupak

HD75 in 9 ml of Nynas Nytro 10CX under air

(n = 5) and a second with 0.5-g specimens of

Munksjo E.G. in 15 ml of Shell Diala S under

nitrogen (n = 6). After treatment, the cells were

stabilized (3 h at 20 �C) and the CH3OH was then

determined in the oil as well as the DPv of the paper

specimens. The results compiled in Table 7 were

used to calculate the number of scissions per b-D-

glucopyranose unit (6,170 9 (2/DPv(t) - 2/DPv(o)))

as well as the ratio [CH3OH]/[scissions]. A forced

zero-order kinetics implies that this ratio corresponds

to that of the rate constants kCH3OH=kscissionsð Þ, within

the limits of the linear approximation of an exponen-

tial function. As seen in Table 7, these additional data

confirm the yields obtained for the methanol produc-

tion under air and N2. For the tests under air, it could

be attributed to the statistical production of one

molecule of CH3OH every three molecules of end

groups, while the remaining two could give rise to

other compounds. More than 30 molecules have

Table 6 Arrhenius parametres of the methanol production

System H2O %

(w/w)

Range of T covered by

the linear regression (�C)

r Activation energy

(kJ mol-1)

Frequency factor

(lmol g-1 h-1)

Clupak HD75 in Nynas

Nytro 10CX under air

1.92 60–120 (7)a -0.9995 107.8 ± 1.5 2.226E?12

1.11 60–120 (7) -0.9977 104.7 ± 3.1 5.973E?11

0.47 60–120 (6) -0.9960 104.7 ± 4.7 4.577E?11

Munksjo TH70 in Nynas

Nytro 10CX under air

1.89 70–120 (6) -0.9917 102.1 ± 6.6 6.771E?11

1.15 60–120 (7) -0.9984 104.1 ± 2.6 7.151E?11

0.45 60–120 (7) -0.9986 104.7 ± 2.5 6.398E?11

Munksjo E.G. in Shell

Diala S under nitrogen

2.25 70–130 (7) -0.9969 112.6 ± 4.0 1.514E?12

0.62 90–130 (5) -0.9928 114.1 ± 7.9 9.895E?11

a n = number of data points

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

ln k

1/T (K-1)

1.92

Rejected data

0.00280.0026 0.0027 0.0029 0.00300.0025

1.11

% H 2O (w/w)

0.47

Clupak HD75 inNynas Nytro 10CX under air

Fig. 6 Typical examples of Arrhenius expressions of the

CH3OH production rate constants

Table 7 Comparison

between methanol

production and scissions for

a system under specific

time–temperature ageing

conditions (168 h at

130 �C)

Type of sample DPv(t) [scissions]

lmol g-1[CH3OH]

lmol g-1[CH3OH]/

[scissions]

Tests under air

0.5-g specimen of Clupak HD75

in 9 ml of Nynas Nytro 10CX

oil (n = 5)

569.8 ± 10.3 11.58 3.91 0.34

Tests under nitrogen

0.5-g specimen of Munksjo E.G.

in 15 ml of Shell Diala S (n = 6)

801.3 ± 29 5.19 0.91 0.17

336 Cellulose (2009) 16:327–338

123

effectively been identified by GC-MS from the

headspace sampling of a Clupak HD75 specimen

after a treatment of 912 h at 120 �C (see Table 3 in

Jalbert et al. 2007). It is also important to consider

that contrary to the 1:1 relation reported by Whitmore

and Bogaard (1994) for the scissions and the

apparition of reducing end groups in which both

parametres were determined on the cellulose chains,

the present results were obtained from the assessment

of heterogeneous phases, paper and oil. In such case,

the kCH3OH=kscissions and [CH3OH]/[scissions] ratios

give apparent yields because of the large amount of

CH3OH formed at the chain break that could remain

inside the structure of the paper. It had been noted in

a previous work (see Fig. 4 in Jalbert et al. 2007) that

the Manning 220 Mannitherm D paper (thermally

upgraded with 3.9% (w/w) N2) yielded twice as

much CH3OH than the Clupak HD75 under identical

ageing conditions (168 h at 130 �C under air). It was

then argued that the partitioning of the species could

be modified by the presence of the paper-incorporated

nitrogenous substances. This appeared to be substan-

tiated by a decrease in the amount of CH3OH in oil as

a function of the nitrogen content in the paper.

Finally, to give an idea of the influence that the

partitioning could have in the present results, a total

of 3 g of unaged cellulose (made from a 0.3-g

specimen of Clupak HD75 and 2.7-g piece of Hi-Val

Transformerboard) was introduced into a series of

cells containing 50 ml of a fresh solution of methanol

in oil (cellulose/oil weight–volume ratio of about

1:18). After being sealed under air, the cells that were

placed in a forced-air oven at 40 �C were withdrawn

at varied lengths of time (1, 2, 4, 8, 16, 20 and 24 h)

in order to assess the methanol remaining in the

solution (specimens free of methanol at time zero).

The evolution of the CH3OH in the oil over time is

illustrated in Fig. 7. As seen in this figure, at least

58% of the initial amount of CH3OH in the solution

became inaccessible to the analysis after about 20 h

of equilibation, confirming that a large portion of the

CH3OH produced from the scissions during the

kinetic tests had remained in the paper.

Conclusions

This study on the kinetics of cellulose degradation

through thermal-ageing in the range of 60–130 �C of

standard wood kraft paper specimens in oil provides

further solid evidence to support that methanol occurs

from the cellulose chopped-end chains, thus confirm-

ing the possibility that the species could be used to

perform a non-intrusive estimate of the condition of

the paper insulation in electrical equipment (this

demonstration will be extended to thermally

upgraded papers in part two of this study). It also

reveals an apparent yield of one-third molecule of

methanol per scission for the ageing tests conducted

under air and even less for the ones under nitrogen.

The importance to be given in the establishment of

such yield to the amount of species not accessible to

the analysis because of a partitioning with the

cellulose was also shown.

Acknowledgments The authors sincerely acknowledge

P. Gervais, DESTT TransEnergie, Hydro-Quebec, for his

invaluable support during the project. Special mention is made

of the financial support of Electricite de France, without whom

the study of the system under nitrogen could not have been

completed. The authors would also like to acknowledge M.

Dahlund, from ABB Transformers, who kindly provided some

sheets of Munksjo Thermo 70 insulating paper.

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