Cellulose (2007) 14:295–309
DOI 10.1007/s10570-007-9124-1
Identification of a chemical indicator of the rupture of
1,4-b-glycosidic bonds of cellulose in an oil-impregnated
insulating paper system
Jocelyn Jalbert Æ Roland Gilbert Æ
Pierre Tétreault Æ Brigitte Morin Æ
Denise Lessard-Déziel
Received: 20 February 2007 / Accepted: 24 April 2007 / Published online: 21 June 2007
Springer Science+Business Media B.V. 2007
Abstract In this study, headspace gas chromatography/mass spectrometry has been used to assess the
volatile by-products generated by the ageing of oilimpregnated paper insulation of power transformers.
Sealed-glass ampoules were used to age under
oxidative conditions 0.5-g specimens of insulating
paper in 9 mL of inhibited mineral oil in a
temperature range of 60–120 8C and moisture of
0.5, 1 and 2% (w/w). A linear relationship between
one of the oil-soluble degradation by-products, i.e.
methanol, and the number of ruptured 1,4-b-glycosidic bonds of cellulose, regardless of the type of
paper (ordinary Kraft or thermally-upgraded (TU)
Kraft paper), was established for the first time in
this field. Ageing at 130 8C of model compounds of
the Kraft paper constituents (a-cellulose, hemicellulose and lignin) and two cellulosic breakdown byproducts (D-(+)-glucose and 1,6-anhydro-b-D-glucopyranose) confirmed that the a-cellulose degradation
was mostly responsible for the presence of this
molecule in the system. Furthermore, additional
130 8C-tests with six different papers and pressboard
samples under a tight control of initial moisture
indicated that at least one molecule of methanol
is formed for each rupture of 1,4-b-glucosidic bond
J. Jalbert R. Gilbert (&) P. Tétreault
B. Morin D. Lessard-Déziel
Institut de recherche d’Hydro-Québec, 1800, boulevard
Lionel-Boulet, Varennes, QC, Canada J3X 1S1
e-mail: gilbert.roland@ireq.ca
of the molecular chains. Stability tests showed that the
ageing indicator is stable under the oxygen and
temperature conditions of open-breathing transformers. The presence of methanol was detected in 94% of
oil samples collected from over than 900 in-service
pieces of equipment, confirming the potential for this
application. Lastly, the tests have shown that oiloxidation by-products and TU-nitrogenous agents
modify the methanol partitioning coefficients in the
paper/oil/air system, which makes their study essential over a range of field conditions encountered by
power transformers. Results are presented and discussed in comparison with 2-furfuraldehyde, which is
the current reference in the domain.
Keywords Ageing indicator Cellulose insulation
Damage monitoring Degree of polymerization
Glycosidic bond scissions Kraft paper Methanol
Mineral oil Remaining life Thermally upgraded
paper Transformer Volatile degradation
by-products 2-Furfuraldehyde
Introduction
Cellulose, an unbranched homopolysaccharide composed of b-D-glucopyranose rings joined together by
1,4-b-glucosidic bonds, has been used for over
100 years as electrical insulation in oil-filled transformers (a typical power transformer contains about
12000 kg of cellulose and 40000 kg of oil). Its
123
296
inherent good mechanical and electrical properties,
its ease of use in the manufacturing process, and its
abundance (obtained from the delignification of wood
pulp by the Kraft process) have made it a virtually
universal choice. Moreover, this material is generally
recognized as the most significant limiting factor in
the operating temperature and thermal life of transformers. The combined action of temperature, oxygen
and moisture cause the insulation to lose mechanical
strength and become weak and brittle. The transformer is then at the mercy of the first short-circuit
whose longitudinal electromagnetic stresses crush the
paper, in spite of all precautions that may have been
taken. Owing to the worldwide growth of large-scale
utility systems in the 1950s and 1960s, a large
number of transformers now operate at an age beyond
the nominal design life. Then, some means of
condition monitoring has become essential to promote satisfactory programs of planned maintenance
or replacement.
From the standpoint of molecular structure, the
ageing of the cellulose fibers is related to the crosswise
break of the inter- and intra-molecular hydrogen bonds
and the lengthwise break of the molecular chains
through the rupture of the 1,4-b-glucosidic bonds.
Specifically, the average length of the cellulose chains
is the parameter that governs the mechanical strength
of the insulating papers. Such a relationship makes the
viscometric degree of polymerization (DPv) an appropriate measurement for directly assessing the progress
of paper ageing (20% of tensile strength corresponds to
DPv & 200). This widely used parameter by electric
power utilities gives information on the average
number of b-D-glucopyranose rings per cellulose
molecule. However, the DPv measurement is impractical in the field due to the need to de-energize the
transformer to extract representative paper specimens.
To overcome the difficulty, it would be especially
useful to identify a relationship between the cellulose
DPv and a specific product dissolved in the insulating
oil that is resulting from ageing. Once established, such
a relationship would permit oil analyses to be used to
assess the insulating paper condition of transformers in
operation.
As early as 1981, Tamura et al. reported a
relationship between the amount of carbon oxides
(CO and CO2) in the oil and the degree of polymerization of insulation papers. However, its applicability was found to be limited considering that these
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Cellulose (2007) 14:295–309
indicators could arise not only from the degradation
of paper but also the decomposition of oil during
long-term oxidation. Three years later, Burton et al.
(1984) suggested the use of a family of furan
compounds that could be directly extracted from the
oil to characterize the thermal decomposition of
insulation papers. The advantage of such compounds
over carbon oxides is that they arise more specifically
from the breakdown of the paper insulation. The
cumulative amount of the most abundant furan found
in transformer oil, 2-furfuraldehyde (2-FAL), was
very early related directly to the reduction of the
degree of polymerization of the cellulose, both in the
laboratory and from in-service transformers (Schroff
and Stannett 1985; Burton et al. 1988). However,
considering that 2-FAL could be generated not only
from the cellulose degradation but also from hemicellulose (5-membered ring polysaccharides known
to possess the lowest stability of all wood-pulp
materials), this decreases its value as a chemical
indicator (Emsley and Stevens 1994). Moreover, the
recent literature is conclusive on the existence of
significant differences in the relationship between 2FAL and DPv during ageing with the type of paper
(ordinary Kraft versus thermally upgraded (TU) Kraft
paper), and varying contents of water (Oommen et al.
1993; Soares et al. 2001; Lundgaard et al. 2004).
Despite the numerous attempts to correlate 2-FAL
with cellulose damage, no satisfactory relationship
has yet been established to cover the various inservice transformer conditions.
Since the first use of high-performance liquid
chromatography (HPLC) by the electric power
industry (Burton et al. 1984), the research that
followed on ageing was for the most part subject to
HPLC standard test methods (ASTM D5837 or IEC
61198) for assessing oil samples (giving access to a
limited number of molecules of a specific class). This
use of an unidirectional analytical approach is at the
very least surprising considering that studies in the
1960s in connection with the thermal degradation of
pure cellulose indicated for the by-products a large
spectrum of low molecular weight compounds starting from small molecules (H2, CH4, CO2) through
intermediate-size hydrocarbons, alcohols and carbonyl compounds to 1,6-anhydro-b-D-glucopyranose
and other anhydrides equal in weight to the cellulose
monomer (Schwenker and Beck 1963; Glassner and
Pierce 1965; Shafizadeh 1968; Kilzer 1971). During
Cellulose (2007) 14:295–309
this period, the total number of volatile by-products
isolated and identified by gas chromatography was
well over 50 and included formaldehyde, acetaldehyde, methanol, acetone, ethanol, glyoxal, 2-butanone and furan. Very little attention in the field has
been paid to these molecules over the last 20 years.
To our knowledge, only two papers reported the use
of such by-products for the assessment of paper
insulation, one based on the use of a semiconductortype sensor in the oil headspace and the other on the
determination of acetone in oil (Abe et al. 1994;
Awata et al. 1997).
In this investigation, the limits of the HPLC test
methods are overcome by using headspace gas
chromatography/mass spectrometry (HSGC/MS) to
assess the volatile by-products generated by the
ageing of oil-impregnated cellulosic materials. The
study was carried out with the aim of identifying an
oil-soluble chemical indicator that is specific to the
rupture of the 1,4-b-glucosidic bonds of cellulose
and present in the oil regardless of the type of paper
(ordinary Kraft or TU Kraft). The origin of the
identified indicator was confirmed by studying model
compounds of the different components of paper
insulation (cellulose, hemicellulose and lignin). The
viscometric degree of polymerization of the paper
specimens was used to establish the existence of a
relationship between the amount of indicator in the
oil and the fraction of glycosidic bonds ruptured, and
to demonstrate the no significance of the amount of
moisture on the number of molecules formed by
ruptured bond. Tests were also carried out to
evaluate the stability of the ageing indicator under
the operational conditions of transformers. Finally,
our capacity to detect the indicator in the field was
measured by assessing oil samples collected from
in-service electrical equipment. The results are
presented and discussed in this paper in comparison
with 2-FAL, which is the current reference in the
domain.
Experimental section
Materials
The test specimens were obtained from five insulating
papers of different manufacture: one ordinary Kraft
297
paper known as Clupak HD75 (wood cellulose
containing up to 7% (w/w) of lignin and hemicellulose), one ordinary Kraft paper that underwent
additional purification at the pulp stage (elimination
of the residual lignin and hemicellulose) known as
Munksjö Thermo-70, and three thermally upgraded
papers (obtained by incorporating variable amounts
of stabilizing nitrogenous agents, particularly dicyandiamide), known as CE Rotherm and Manning 220
Mannitherm D. Specimens were also obtained from a
low density calendered transformerboard. Some
characteristics of the materials are given in Table 1.
All the tests were carried out with specimens
immersed in Naphthenic Nynas 10 CX insulating
oil (Nynas Naphthenics) that contains about 3000 mg/
kg of 2,6-di-tert-butyl-p-cresol (oil antioxidant). To
trace the origin of the indicators, the following model
compounds were studied: microcrystalline a-cellulose (Aldrich #31,069-7), Whatman No. 41 paper
(considered to contain more than 98% of a-cellulose,
Fisher #1441-866), xylan isolated from birch (Fluka
#95588), mannan isolated from Saccharomyces cerevisiae (Sigma #M7504), and alkali Kraft lignin
(Aldrich #37,095-9). Two intermediate by-products
of the degradation of a-cellulose were also tested: D(+)-glucose (Sigma #G7528) and 1,6-anhydro-b-Dglucopyranose (levoglucosan) (Sigma #G7528).
Moisture conditioning
A glove box Model DL-001-SP equipped with an
automatic pressure control, Model HE-63P from
Vacuum/Atmosphere Co. (Hawthorne, CA), and
two internal fans for providing uniform humidity
was use for moisture conditioning. The glove box is
supplied with air freed from water vapor by passing
compressed air through a Balston unit Model 75–60,
Lexington, MA (dew point at 73 8C). The atmosphere of the glove box is monitored by using a
microprocessor humidity controller (0–100% RH
with 0.1% RH resolution, Model 37700-03 from
ETS Electro-Tech Systems, PA) equipped with a
capacitive film sensor and an RTD temp sensor. The
signal of the humidity sensor is used to meet a setpoint value by activating a dehumidifier, Drierite
desiccant Model 37700-50 or an ultrasonic humidifier
distillation system, Model 37700-60 (also from ETS
Electro-Tech Systems).
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Cellulose (2007) 14:295–309
Table 1 Some characteristics of the insulation papers and pressboard studied
Munksjö Thermo-70
Munksjö Paper AB Sheets
Sweden
63 ± 3 61–67
0.75–0.85
0.013
Clupak HD75
Weidman Whitely
Limited
Tullis Russel Co.,
Scotland, UK
Sheets
80 ± 2 N.A.b
1.00–1.15
0.073
Wrapped on
copper
conductor
83 ± 3 N.A.
N.A.
0.91
Roll
75 ± 2 58
0.90 ± 0.05
1.15
Manning 220 Mannitherm D
Lydall Inc., USA
(mixture of manilla hemp and
Manning Div.
Kraft wood pulp)c
Sheets
84 ± 1 48
0.63
3.9
Hi-Val Kraft transformerboard
Sheets
1798 ± 9 N.A.
Tullis Russel Co.,
Scotland, UK
a
EHV-Weidmann
Thickness Nominal
grammage
(mm)
(g/m2)
Nitrogen content
(%)a(w/w)
Manufacturer
CE Rotherm (Insuldur type)
Format
Apparent
density
(g/cm3)
Type of paper and pressboard
0.90–1.05
< 0.03
Total nitrogen by the Kjeldahl method
b
N.A.: Non Available
c
The wood pulp does not exceed 50%
Ageing cells for stability tests
The stability of the chemical indicators was assessed
using a solution prepared by adding 5 mL of methanol
(Fisher #AC32695) and 2.5 mL of acetone (Aldrich
#27,072-5), ethanol (Les Alcools de Commerce Inc.)
and 1-butanol (Fisher #A399) in 0.5 L of insulating
oil. Seventy-five 20-mL glass ampoules were volumetrically filled with 3 mL of this solution and 6 mL
of oil for a final concentration of about 1500 mg/kg
for each compound, except for methanol with
3000 mg/kg. These ampoules were then sealed in
open air and introduced in forced-air ovens (Salvis,
Sweeden) thermostatically maintained at 70, 90, 110
and 130 8C. Three ampoules were analyzed at the
time of preparation and then at 41, 113, 185, 449, 785
and 1505 h of ageing. To verify a possible contribution of oil ageing in the formation of these compounds, an equal number of ampoules containing
9 mL of insulating oil (blank samples) were aged
under identical conditions.
Ageing cells for the 168-h tests at 130 8C
Five strips of 20 · 2.5 cm (depending on the paper
grammage) weighing 0.5 g were cut from each sheet
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of insulating paper and rolled loosely. The same
number of strips with equivalent weight was also
prepared with the transformerboard. The specimens
were first vacuum-preconditioned in the glove-box
hatch to attenuate the humidity disparity noted
through the various materials. They were then
introduced in the glove box along with an equal
number of empty 20-mL glass ampoules (VWR
#12010L-20) and a 1-L bottle of oil. Prior to this, the
ampoules were serially cleaned using an ultrasonic
bath, rinsed three times with demineralized water and
dried at 130 8C for 24 h in a vacuum oven. When the
target humidity content was achieved in the specimens (1.2–1.6% H2O (w/w)), each strip was inserted
into a 20-mL pre-weighed glass ampoule. The
ampoules were filled volumetrically with 9 mL of
oil (giving an oil-paper volume-weight ratio of 18:1),
temporarily closed and withdrawn from the glove box
to be sealed in open air. The sealed ampoules were
placed in a forced-air oven maintained at 130 8C for
7 days. After the thermal treatment, the ampoules
were allowed to cool for 3 h in a low temperature
incubator (Fisher Scientific, model 146D) maintained
at 20 8C, after which oil aliquots were transferred into
a 10-mL headspace vial (Supelco #27295) and 2-mL
amber screw-top glass vial (Agilent #5182-0716) for
Cellulose (2007) 14:295–309
299
analysis. Each paper specimen was then kept in the
dark until it could be analyzed for the degree of
polymerization. To ensure that the degradation products observed at the end of the tests mostly resulted
from the papers and not the oil, an equal number of
cells containing only the equilibrated oil were aged
under identical conditions. Some ageing cells were
also prepared by introducing 0.1 g of each model
compound and cellulose degradation by-products in
five ampoules with 9 mL of oil. Contrary to the
previous cells, the humidity of the starting materials
(constituents and oil) was not equilibrated prior to
testing.
Ageing cells for tests with varying temperature
and moisture conditions
The conditioning of the cell components (paper and
oil) was done as for the tests at 130 8C, except that
the specimens were not subjected to vacuum preconditioning and the RH % of the glove box was
successively changed to obtain specimens at three
moisture levels: 0.5%, 1.0% and 2.0% (w/w). After
conditioning, the sealed-test cells were placed in
forced-air ovens maintained at 60, 70, 80, 90, 100,
110 and 120 8C. They were withdrawn from the
ovens after varied lengths of time depending upon the
ageing temperature (maximum time of 14 000 h for
T = 60 8C). As in the case of the previous tests, the
withdrawn cells were allowed to cool for 3 h in the
low temperature incubator, after which the ampoules
were broken and oil aliquots collected for the
analysis. The paper specimens were set apart for the
determination of the degree of polymerization.
Apparatus and methods
A G1888 static headspace sampler coupled with a
6890N gas chromatograph equipped with a 5973N
mass selective detector at 70 eV ionization energy in
the electron impact mode (all from Agilent Technologies) was used to assess the volatile degradation byproducts of the cellulose. The instrument interface
was maintained at 250 8C and a mass range, m/
z = 10–300 amu, in a 0.21-s cycle, was scanned in
total ion count mode (TIC). The separation was
performed with a 60-m-long Stabilwax (Restek) polar
column, 0.25 mm in diameter and with 0.5 mm film
thickness, under the instrumental conditions given in
Table 2. The signal was calibrated by injecting a
series of dilutions prepared from a mother solution of
methanol, acetone, ethanol and 1-butanol in oil, each
at a concentration of 5 ppm (v/v), except for
methanol with 10 ppm (v/v) (6-point calibration
Table 2 Instrumental conditions for the HSGC/MS analysis of the volatile degradation by-products of cellulosic insulating material
Headspace sampler parameters
Temperature:
Sample
60 8C
Transfer line
120 8C
Gas sampling valve and injection loop
150 8C
Pressure:
Vial over-pressure
70 kPa
Times:
Equilibration at 60 8C with shaking
60 min
Shaking:
Pressurization
0.2 min
Loop fill
0.2 min
Loop equilibration
0.2 min
Injection
0.9 min
Power
Maximum level
Gas chromatograph parameters
He carrier gas flow:
Oven:
2 mL min-1
20 8C for 12 min
20–110 8C at 5 8C min-1
110–240 8C at 24 8C min-1
240 8C for 12 min
123
300
(a)
Clupak HD75 specimen
30
15
22
10
20
23
24
20
5
0
33
34
35
10
2
6
(b)
(c)
3 4
1
0
56
7
9
8
21
25
26
15
19
17
29
30
13 14
10 12
Clupak HD75 specimen + oil
30
27
28
18
16
11
Abundance of the TIC signal x 10
curves). Quantification was done in selected ion
monitoring (SIM) mode. 2-FAL was assessed using a
high-performance liquid chromatograph (Agilent
Technologies, 1100 Series) based on a method
adapted from Lessard et al. (1995). The signal was
calibrated (6-point calibration curve) by injecting a
series of dilutions prepared from a mother solution of
2-furfuraldehyde (Aldrich #31,991-0) in oil at a
concentration of 2500 ppm (w/w). The determination
of the average viscometric degree of polymerization
was performed after shredding the samples using a
water-cooled shredder (Janke&Kunkel, IKA-Werk)
and achieving dissolution in an aqueous solution of
bis(ethylenediamine)copper(II) hydroxide (Anachemia #29072-360). The procedure used was based
on ASTM D4243. Prior to measurement, the oilimpregnated samples were degreased in a Soxhlet
(Soxtec Avanti 2050) using fresh distilled hexane.
The moisture content of the specimens was determined by titration with a Karl Fischer 756 KF
Coulometer (Brinkmann) according to Method C of
ASTM D1348.
Cellulose (2007) 14:295–309
28
22
20
21
19
16
10
2
1
0
9
3
5
A 13
14
17
7
Oil
30
28
Results and discussion
Identification of molecules with potential
diagnostic significance
Figure 1a shows a typical TIC chromatogram of the
volatile degradation by-products obtained from the
injection of the headspace of a paper specimen heated
at 120 8C for 912 h (*0.5 g of Clupak HD75 in
presence of air in a 20-mL headspace vial). The
components identified by comparing the mass spectrum recorded at the maximum of each peak with
those from the NIST MS 2002 library are listed in
Table 3. As seen in this table, the peaks eluted in the
time range of 5–25 min are associated with lowmolecular-weight volatile carbonyl compounds, with
the exception of peak 2, which is identified as carbon
disulfide. Most of these molecules have already been
reported in the pyrolysates of pure cellulose (Schwenker and Beck 1963; Glassner and Pierce 1965;
Shafizadeh 1968; Kilzer 1971). The proposed mechanism to account for their presence involves the
formation by chain scission of carbonium ions that
may decompose irreversibly to form unsaturated
products containing aldehyde and enol groups (Byrne
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20
29
10
2
Unaged oil
12
10
5
0
5
10
7
15
15
16
17
A
20
25
30
35
40
45
Retention time (min)
Fig. 1 Typical TIC HSGC/MS chromatograms of volatile
degradation by-products recorded after 912 h of ageing at
120 8C in presence of air
et al. 1966). These products may yield volatile
carbonyl compounds such as those compiled in
Table 3, or alternately, they may undergo aldol-type
condensation with the elimination of water to form
ethylenic crosslinks between carbon chains and thus
ultimately form a carbon-rich char (paper with a
darkened aspect). Over 25–35 min, the comparison of
the mass spectra is indicative of higher molecular
weight carbonyl compounds (including 2-FAL), with
the exception of peak 18, which is matched with
Cellulose (2007) 14:295–309
301
Table 3 Identification of the volatile degradation by-products in the TIC-chromatogram recorded from the headspace sampling of a
Clupak HD75 specimen after 912 h of ageing at 120 8Ca
Peak numbera
Retention time (min)
Compounds identified
MW
% match
1
6.34
Acetaldehyde
44
2
6.90
Carbon disulfide
76
9
3
8.08
Methyl formate
60
78
4
9.59
Furan
68
91
5
10.43
Acetone
58
86
6
11.31
Acetic acid, methyl ester
74
91
7
14.34
Butanal
72
94
8
15.84
2-Butanone
72
91
9
16.28
Methanol
32
43
10
18.58
Ethanol
46
83
11
19.01
Furan, 2,5-dimethyl
96
94
12
19.69
2-ethylacrolein
84
95
13
20.35
2,3-butanedione
86
90
14
24.01
2,3-pentadione
100
83
15
16
24.24
26.75
1,4-dioxane
Water
88
18
94
1
17
27.97
1-butanol
74
91
18
30.41
Cyclohexene, 3,5,5-trimethyl
19
31.87
2-propanone, 1-hydroxy
20
33.11
Ethanol, 2-butoxy
118
91
21
33.40
Acetic acid
60
94
22
33.37
2-furfuraldehyde
96
95
23
33.98
Formic acid
46
90
24
34.26
Propanoic acid
74
95
25
35.75
2-Butyl-3,4,5,6-tetrahydropyridine
139
47
26
36.63
Formamide, N, N-dibutyl-
157
97
27
37.07
1-piperidinecarboxyaldehyde
113
97
28
37.65
2,6-di-tert-butyl-p-cresol
220
95
29
38.46
Ethanol, 2,20 -oxybis-
106
90
30
39.09
Benzothiazole
135
94
a
91
124
38
74
91
The numbers in the table show the peak numbers in Figure 1
3,5,5-trimethyl-cyclohexene. The peak detected at
26.75 min shows that a great amount of water was
present in the vial at the end of the test. Over the 35–
45 min, the peak identification becomes more uncertain because of the gradual loss of resolution; no
attempt was made to improve this resolution. Nevertheless, the mass spectra comparison pointed the
presence of 2,6-di-tert-butyl-p-cresol (peak 28) and a
compound containing a sulfur atom (peak 30).
Figure 1b shows the TIC chromatogram of the
volatile by-products found in the headspace of a
similar aged specimen except that 9 mL of insulating
oil was present in the vial. It is interesting to see how
the compositional profile of the oil components may
interfere with the elution of the paper degradation byproducts. The 2,6-di-tert-butyl-p-cresol, which is
present in substantial amounts in unaged oil,
appeared as a well-shaped peak in the overlap on
the right side of the profile. Under the present HSGC/
MS conditions, it is evident that the degradation byproducts eluted after 30 min could hardly be retained
as indicators, thereby eliminating 2-FAL. On the
other hand, Fig. 1c shows a TIC chromatogram of a
sample collected in the headspace of an oil sample
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302
Cellulose (2007) 14:295–309
Fig. 2 Stability with time
of the molecules of
potential diagnostic
significance under
conditions prevailing for
open-breathing equipment
T = 70 °C
Concentration (mg/kg)
12
Methanol
Acetone
Ethanol
1-butanol
(a)
T = 90 °C
12
10
10
8
8
6
6
4
4
2
2
0
(b)
0
0
400
800
1200
1600
0
T = 110 °C
Concentration (mg/kg)
12
800
1200
1600
800
1200
1600
T = 130 °C
12
(c)
10
10
8
8
6
6
4
4
2
2
0
(d)
0
0
400
800
Time (h)
aged under identical conditions with in overlay the
signal recorded for unaged oil. It is evident from
these chromatograms that oil oxidation may contribute to a certain extent to the signal of some of the
paper degradation by-products detected at retention
times under 30 min (peaks 5, 7, 10, 12 and 17). A
peak showing a 90% match with 2-methyl-2-propanol
was seen to be associated only with the ageing of oil
(totally absent in Fig. 1a and identified as A in
Figs. 1b and 1c). After examining the mass spectrum
for co-elution and confirming peak identity by
injecting pure compounds, acetone (peak 5), methanol (peak 9), ethanol (peak 10) and 1-butanol (peak
17) were retained for further testing (all detected in
preliminary oil samples collected from in-service
transformers). The interference of oil ageing over the
123
400
1200
1600
0
400
Time (h)
use of these molecules for estimating paper damage
would therefore have to be evaluated.
Stability of the indicators under some operating
conditions
The stability of the molecules with time was assessed
in a range of temperatures that could be found in
open-breathing transformers. The oxidative conditions prevailing in such units (oil in contact to the
atmosphere through a conservator), were modeled by
filling with air the headspace over the 9-mL oil
solution of the four molecules. The results are
depicted in Fig. 2. As seen in this figure, within the
experimental variability measured with triplicate
cells, the concentration of the four molecules
Cellulose (2007) 14:295–309
remained reasonably stable over the 1600-h test
duration when subjected to 70 and 90 8C. However, at
110 8C, acetone is dramatically unstable after an
induction period of about 450 h, showing an increase
of 2.1 mg/kg of oil/h. It is even worse at 130 8C where
an increase of 11.6 mg/kg of oil/h is measured after a
very short induction period (<100 h), which is
indicative of a rapid exhaustion of the 2,6-di-tertbutyl-p-cresol, followed by a drop after about 800 h
of testing. As it could be deduced from tests carried
out with the blank samples, the appearance of acetone
could essentially be attributed to the oxidizing
deterioration of the oil components. Indeed, an
increase rate of 2.2 and 11.4 mg/kg of oil/h were
measured at 110 and 130 8C, respectively, which
totally accounts for the values noted for the solution.
Contrary to what was reported by Awata et al. (1997)
for breathing transformers, oil oxidation contributes
significantly under our conditions to the uptake of
acetone, making this molecule inappropriate for
indicating paper damage. On the other hand, a
measurable decrease of CH3OH is noted when the
solution is tested at 110 and 130 8C (note that the
latter temperature is about ten degrees over what is
normally experienced in the hotter parts of a fully
loaded transformer). Such a disappearance could be
attributed to a modification of the matrix polarity by
the presence of oil-oxidation by-products. A reduction of the gas-oil partitioning coefficient of methanol
with time would have the effect of reducing the
amount of molecules accessible to the HSGC/MS
analysis as reported for CO (polar compound) in a
similar case (Jalbert et al. 2003). As for the two other
candidates (CH3CH2OH and CH3CH2CH2CH2OH),
this apparent loss could easily be corrected by using
the appropriate gas-oil partitioning coefficients. In
comparison, Unsworth and Mitchell (1990) reported
an induction period of 800–1000 h at 110 8C for the
2-FAL in solution in uninhibited oil under oxidative
conditions, after which significant losses were
observed.
Origin of the indicators
For this application, it is essential to verify that the
indicator is being generated by the damage of acellulose rather than by secondary paper constituents.
A typical Kraft paper, which is found in most of the
equipment still in operation, is composed of about
303
95–97% of a-cellulose, up to 4% of hemicellulose
and up to 3% of lignin, along with varying trace
amounts of inorganic salts (Clark 1962). Moreover,
there is since the mid 1960s a growing trend toward
using TU Kraft papers for oil-filled transformer
applications. In the case of the TU products studied
in this paper (Table 1), variable amounts of stabilizing nitrogenous agents were added to the Kraft
components, and in addition for the Manning 220
Mannitherm D, medium hemp fibers were mixed with
the modified Kraft fibers for improving the mechanical strength. Note that these two types of papers are
often combined in the same piece of equipment.
While a-cellulose is known to possess a crystalline/
amorphous character, the hemicelluloses are for their
part amorphous polymers that are preferentially
hydrolyzed (Shafizadeh 1982). Acetyl-4-O-methylglucuronoxylans (xylan) constitute the main hemicelluloses of hardwoods, while glucomannans (mannan)
are found in softwoods. Lignin is a randomly linked,
amorphous, high-molecular-weight phenolic compound that is more abundant and polymeric in
softwoods than in hardwoods. It is generally considered to possess the greatest stability of all wood
constituents in thermal treatment below 200 8C
(Kollmann and Fengel 1965). To establish the
contribution of the above constituents, model compounds were then aged in oil at 130 8C for 168 h
under oxidative conditions. The results are presented
in Table 4 together with those of a major by-product
of the cellulose hydrolysis, D-(+)-glucose, and of
cellulose pyrolysis, 1,6-anhydro-b-D-glucopyranose.
It is interesting to note that all the indicators are
generated by the model a-cellulose compounds
tested, though the ethanol generation would significantly be masked by a concurrent formation from the
oxidation of the oil components (last line in Table 4).
The microcrystals of a-cellulose were obtained from
cotton linters from which the amorphous regions
linking the naturally occurring crystals were removed
by acid hydrolysis (DPv of 157 ± 4 in this case). This
well-structured cellulose yielded a higher amount of
CH3OH when compared to Whatman paper, which
suggests that the surface area of the tested material
(powder of 20 mm particle size) was largely compensating for the lesser penetrability of the chemical
reagents into the structure. In contrast, the microcrystals of the Whatman paper (also from cotton
linters with a DPv of 281 ± 4) are still embedded by
123
304
Cellulose (2007) 14:295–309
Table 4 Contribution of the Kraft paper components to the formation of the molecules of potential diagnostic significance as
established with model compoundsa
Model compounds studied
CH3OH
CH3CH2 OH
CH3CH2CH2CH2OH
mg/kg of oil/g of component
2-FAL
Components of Kraft paper
a-cellulose
Microcrystals from cotton linters
8940
640
430
29330
Whatman paper No. 41
1730
1080
590
14470
lignin
Alkali Kraft lignin
153200
<D.L.
<D.L.
1000
hemicelluloses from
softwoods
Mannan isolated from Saccharomyces
cerevisiae
<D.L.b
5390
12630
560
Hemicelluloses from
hardwoods
Xylan isolated from birch
227500
14770
790
674350
Major by-product from
a-cellulose hydrolysis
D-(+)-glucose
<D.L.
620
460
29990
Major by-product from
a-cellulose pyrolysis
1,6-anhydro-b-D-glucopyranose
1750
38630
<D.L.
2900
440
430
48
<D.L.
Blank oil
a
Based on five replicates (each data = amount from model compound in oil heated at 130 8C––amount from oil heated at 130 8C )
b
D.L.: Detection Limit
less stable amorphous regions (7.4 ± 0.4% amorphous
(Nelson 1960)), which means that a reduction of the
stability of the material was expected. For the alkali
lignin, the results showed a major yield of CH3OH
with no detectable amount for the two other candidates. The fact that lignin constitutes less than 3% of
the paper weight and has the greatest stability of all
paper constituents could largely attenuate the impact
of this finding for the present application. In regard to
hemicelluloses, no detectable amount of CH3OH is
found for mannan while a large contribution is
associated to xylan. This observation is of interest
considering that the insulating papers are generally
manufactured from softwood species (mostly Black
Spruce). The higher content of acetyl and methoxy
groups in xylan may explain why this material has
generated such a large amount of methanol. On the
other hand, the test performed with D-(+)-glucose
showed no detectable amount of CH3OH, although
moderate yields are noted for the two other candidates. In the case of 1,6-anhydro-b-D-glucopyranose,
a moderate contribution to methanol was measured
together with a major production of ethanol. In
addition, the results in Table 4 indicate that 1-butanol
is mostly associated with the softwood hemicelluloses and not with a-cellulose, which makes this
molecule of lesser interest as an indicator of paper
damage. Finally, it is interesting to note that the
123
relative yields measured for 2-FAL (last column in
Table 4) are in conformity with data reported by
Scheirs et al. (1998), showing the highest amount
from the hardwood hemicelluloses (xylan), followed
by a moderate contribution from a-cellulose and a
weak contribution from lignin. It is also significant
that 2-FAL is in large part produced from D-(+)glucose whereas this is not the case for CH3OH,
though both appear to be formed in relatively small
amounts from 1,6-anhydro-b-D-glucopyranose.
To further demonstrate that methanol originates
from a-cellulose and more specifically from the
rupture of its 1,4-b-glucosidic bonds, additional
ageing tests were carried out at 130 8C for 168 h
with five strips of each insulating paper and pressboard. Contrary to the above test with model
compounds, the moisture of the starting material
was conditioned at about the same level (1.2–1.6%
H2O (w/w)). The results are presented in Table 5.
From the initial DPv (DPv, 0 h), an average weight for
the molecular cellulose chains was obtained by
dividing the gram-formula weight of the average
chain length (using the formula suggested by Klemm
et al. 1998) by the Avogadro number. From the final
DPv (DPv, 168 h), the average statistical number of
ruptured 1,4-b-glucosidic bonds (NS) experienced by
the molecular chains was obtained by applying the
relation NS = DPv, initial/DPv, final
1 (Lundgaard
Cellulose (2007) 14:295–309
305
Table 5 Confirmation of the existence of a relationship between methanol and the number of ruptured 1,4-b-glucosidic bonds of the
cellulose (data from tests at 130 8C for 168 h with 0.5-g specimen immersed in 9 ml of oil under air atmosphere)a
DPv,
DPv,
CE Rotherm
(1.15% N2)
Manning 220
Mannitherm D
Munksjö
Thermo-70
Clupak HD75 Hi-Val
CE Rotherm
transformerboard (0.91% N2)
0h
1144 ± 9
1168 ± 17
1170 ± 21
1140 ± 18
1099 ± 9
1300 ± 13
168 h
568 ± 11
570 ± 10
523 ± 9
618 ± 20
640 ± 8
690 ± 4
Average weight of a single
molecular chain (g)
3.081 · 10
NS
1.016
19
Weight of specimen in ampoule 0.4666
(g)
3.146 · 10
19
3.152 · 10
19
3.071 · 10
19
2.960 · 10
19
3.502 · 10
1.050
1.237
0.844
0.718
0.886
0.5134
0.4516
0.4718
0.5182
0.4776
19
# of molecular chains in ampoule 1.514 · 1018
1.518 · 1018
1.333 · 1018
1.428 · 1018
1.628 · 1018
1.268 · 1018
18
18
18
18
18
1.123 · 1018
# of molecules of indicator
according to 1 scission = 1
molecule
1.539 · 10
1.593 · 10
1.649 · 10
1.206 · 10
1.168 · 10
81.9
245.6
84.8
254.3
87.8
263.2
64.2
192.6
62.2
186.5
59.8
179.3
CH3OH
59.1
63.5
73.8
72.0
80.1
100.6
2-FAL
3.7
4.9
7.4
1.6
1.0
0.4
Expected amount in ampoule (mg)
CH3OH
2-FAL
Measured amount in oil (mg)
a
Based on five replicates
et al. 2004). The number of molecular chains present
in each ampoule at the beginning of the test was
estimated knowing the weight of both, the paper
specimens (after subtracting a contribution of 7% for
the lignin and hemicellulose components except for
Munksjö Thermo-70) and the average molecular
chains (see Table 5). By multiplying the number of
molecular chains by the number of ruptured bonds
and assuming that cleavage of a 1,4-b-glucosidic
bond leads to one molecule of indicator, it is then
possible to calculate a microgram amount of CH3OH
that should be found in the ampoule at the end of the
test. As seen in Table 5, the concept that at least one
molecule of CH3OH is generated each time a 1,4-bglucosidic bond is ruptured is well supported by the
measured data considering that a certain amount of
methanol was subtracted from the analysis due to
absorption in the paper. For these tests, the effect of
oil ageing on the partitioning of methanol is assumed
to be at the same level for all the materials. It is also
interesting to see that the nitrogenous agents incorporated in the Kraft papers (expressed in % N2 by dry
weight in Table 1) reduce the material’s capacity to
retain methanol. Finally, our data are conclusive on
the difficulty for 2-FAL to reach a good level of
agreement between the expected and measured
values. Contrary to what was obtained for the
ordinary Kraft specimens, very low amounts of 2FAL were detected in the ampoules of the TU papers.
This behavior has also been reported by other authors
who formulated the possibility of the 2-FAL destruction through a reaction with dicyandiamide, one of
the agents used for TU papers (Morais et al. 1999).
Dependence of methanol on temperature
and moisture
To verify the dependence of methanol on temperature
and moisture, ageing tests were carried out with
specimens of the five insulating papers for which the
moisture content was set at 0.5%, 1% or 2% (w/w)
(corresponding to a range of values found for
transformer papers). The temperatures in the range
60–120 8C were chosen in order to maintain the
system in a single mode of deterioration to which the
Arrhenius equations could be applied. The sealed
ampoules used for these tests all contained a 0.5 gpaper strip immersed in 9 mL of oil with the
headspace filled with air (oxidative conditions).
Typical results of the DP change (expressed as
123
306
Cellulose (2007) 14:295–309
(1986), it could be incorrect to classify the fine
structure in cellulose into only two straight fractions—amorphous and crystalline—but rather into
several fractions, which vary in degree of perfection
of lateral arrangement of the chain molecules. On the
other hand, it is very revealing to see that the CH3OH
versus Time plots displayed in Fig. 3b show patterns
very similar to those observed for the depolymerization (Fig. 3a). To the authors’ knowledge, such a
good agreement with the DPv of the aged specimens
has never been reported in the past (including for 2FAL). By obtaining the rates of the CH3OH formation in the second section of the degradation for the
temperatures experienced for in-service transformers,
it would then be possible to evaluate the thermal
deterioration and the thermal life of the insulating
papers over long periods (the details on the kinetics
are beyond the scope of this paper).
Typical overall results of the dependence of
CH3OH in oil on the number of chain scissions are
shown in Fig. 4 together with the equivalent relation
for 2-FAL. These relations were built up by integrating the data obtained at the seven temperatures and
three moisture levels tested (each symbol corresponds
to a different initial moisture content). It is very
interesting to note that the methanol production is
virtually linear with NS regardless of the type of
paper assessed (ordinary Kraft or TU Kraft papers). It
is obvious that a greater dispersion of the data points
is seen when going to NS > 3, corresponding to a DPv
fall below 300 (upper value adopted by the electric
power utilities as a criterion of the end of service
life). The slopes of the linear relations for the TU
1/DPv, t) for all temperatures at one moisture level as
a function of ageing times are shown in Fig. 3 for CE
Rotherm (case of 1.15% N2) along with the evolution
of the CH3OH content of the oil. As usually reported
for the heterogeneous hydrolysis of cellulose (Feller
et al. 1986), three sections are expected for the curves
in Fig. 3a: a first section corresponding to a fast initial
rate followed by one of moderate rate, and then by
one with an extremely slow rate. In this figure, it is
possible to distinguish the fast rate from the moderate
rate only for T < 100 8C. Moreover, this section is
extended over a wide timescale (0–1000 h), contrary
to what is seen for pure hydrolysis, probably due to
the presence of oil in the system. The fast rate section
is considered to represent an attack on a small
fraction of 1,4-b-glucosidic bonds of the cellulosic
fibrils that are particularly sensitive to rupture. These
bonds, which are designated as weak links in the
literature, could be of native origin (Kilzer 1971) or
could have been introduced during the delignification
of wood pulp. The second section of the curves
implies the rupture of the 1,4-b-glucosidic bonds
between the normal b-D-glucopyranose units located
in the amorphous regions of the cellulosic fibrils. In
the case of the curves at T > 100 8C, this latter section
appears to be analyzable into more than one straight
line if sufficient data at very long time intervals are
available. This is indicative of a transition through the
third section corresponding to an attack of the
microcrystalline regions of the cellulosic fibrils. The
patterns of the curves in Fig. 3a do not allow a clear
transition to be identified between the moderate and
slow rate sections. As discussed by Feller et al.
(b)
(a)
Formation of ageing indicator
Paper depolymerization
450
6
400
DP = 200
5
Temperature (°C)
120
110
100
90
80
70
60
350
-3
(µg CH 3OH/g paper)
Criterion of end
of service life
4
1/DPv, t x 10
Fig. 3 Typical overall
results for the relationship
between paper
depolymerization and
formation of indicator with
time—case of CE Rotherm
1.15% N2 specimens
equilibrated at 2% (w/w)
H2O
DP = 300
DP = 437
3
DP = 446
DP = 445
2
300
250
200
150
100
1
50
0
0
0
4000
8000
Time (h)
123
12000
16000
0
4000
8000
12000
Time (h)
16000
Cellulose (2007) 14:295–309
Clupak HD75 specimens
25
DPv = 200
initial % (w/w) H2O
1.92
(a)
100
1.11
DPv = 250
DPv, initial = 1225
3
3
80
DPv = 300
60
15
DPv = 200
10
40
DPv, initial = 1225
DPv = 250
5
20
(µ g 2-FAL/kg of oil) x 10
0.47
20
(µ g CH3OH /kg of oil) x 10
Fig. 4 Typical overall
results for the dependence
of methanol and 2furfuraldehyde with the
number of ruptured 1,4b-glucosidic bonds of the
cellulose
307
DPv = 300
0
0
0
1
2
3
4
5
6
0
1
2
NS
3
4
5
6
NS
Manning 220 Mannitherm D specimens
100
initial % (w/w) H2O
1.99
1.23
0.49
DPv, initial = 1275
80
3
25
(µg CH3OH/kg of oil) x 10
3
30
(b)
DPv, initial = 1275
20
60
DPv = 300
15
40
DPv = 450
10
20
(µg 2-FAL/kg of oil) x 10
35
5
DPv = 300
0
0
0
1
2
3
NS
Kraft papers are somewhat more pronounced than for
the ordinary Kraft, as can be seen in Fig. 4 (also as
expected based on the data in Table 5). These
variations in the slopes could be attributed to the
presence of nitrogenous agents, which modify the
partitioning of methanol of this paper/oil system.
Furthermore, the moisture content of the starting
material seems to have no significance on the number
of CH3OH molecules formed for each ruptured 1,4-bglucosidic bond. In comparison, the production of 2FAL with the number of chain scissions differs
markedly from CH3OH and also between the two
paper types investigated. In the case of Clupak HD75,
the concentration is seen to increase very slowly up to
a NS of 2, corresponding to a DPv of about 400, and
4
5
6
0
1
2
3
4
5
6
NS
then to increase exponentially to a maximum of
100 mg/kg of oil. A turning point at about 400 b-Dglucopyranose rings has already been reported by
others under similar conditions (Heywood 1997).
This implies that any life-predicting model based on
2-FAL would suffer from a lack of sensitivity in the
1200–400 DPv range. The situation is even worse for
the TU papers, as shown for Manning 220 Mannitherm D in Fig. 4b, where 2-FAL is found to increase
and then to decrease with all the values below
0.1 mg/kg of oil. Such a discrepancy between Kraft
and TU Kraft papers for 2-FAL with the number of
chain scissions was recently reported for tests
performed under similar conditions (Lundgaard
et al. 2004). The data for the Clupak HD75 specimens
123
308
Cellulose (2007) 14:295–309
(see Fig. 4) did not allowed a distinction to be made
between the three-moisture levels on the production
of 2-FAL. For this specific case, about half of the
HPLC analyses were performed on oil samples that
were in contact with the paper strip after the test for a
longer period than the mandatory 3 h, which is seen
as a source of additional dispersion.
Occurrence of methanol in field equipment
For a final view of the possibility of using methanol
as an indicator of paper damage, its occurrence in the
field was measured in over than 900 pieces of
equipment of the actual transmission network of
Hydro-Québec (including power transformers, autotransformers, shunt reactors and grounding reactors
with the oldest units operating for over 80 years). The
analytical results are presented in Fig. 5 together with
the situation experienced for 2-FAL. Under a detection limit of about 10 mg/kg for CH3OH and 2 mg/kg
for 2-FAL, the results showed that the former is seen
in 94% of the cases with a reduction to 56% for the
latter. Moreover, the 2-FAL concentrations were seen
to decrease markedly in the 1980s corresponding to
the introduction at Hydro-Québec of the TU-Kraft
papers.
CH 3OH
22
20
% of detection = 94 % (n = 943)
(a)
3
Concentration (mg/kg of oil)
2
1
0
2-FAL
12
% of detection = 56% (n = 943)
(b)
11
6
4
2
0
0
20
40
60
80
100
Age (years)
Fig. 5 Occurrence of methanol and 2-furfuraldehyde in oil
samples collected from equipment of the Hydro-Québec’s
transmission network
123
Conclusion
Notwithstanding the fact that this study was carried
out without a precise understanding of the chemical
reactions involved in the system, sufficient evidence
was gained to link the evolution of methanol in oil to
the depolymerization of a-cellulose (rupture of 1,4-bglucosidic bonds). In this respect, the molecule was
found to possess the best potential among all the
HSGC/MS investigated cellulose degradation byproducts. This finding introduces the possibility of
developing a sensitive ageing model for assessing the
condition of both ordinary Kraft and TU-Kraft papers
in power transformers. However, to go further with
this application, the effect of oil ageing and nitrogenous agents on the partitioning of methanol would
have to be clarified (under study in our laboratory).
Acknowledgements The authors would like to express their
gratitude to M. C. Lessard and B. Noirhomme from IREQ for
valuable discussions on this concept. They would also like to
thank A. Jolicoeur and P. Gervais from TransÉnergie for
supporting the project from the beginning. Finally, a note of
appreciation is extended to E. Grenier and M. Dontigny for the
initial technical support.
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