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Cellulose ISSN 0969-0239Volume 21Number 1 Cellulose (2014) 21:823-833DOI 10.1007/s10570-013-0087-0

Thermal lifetime of cellulose insulationmaterial evaluated by an activation energybased method

R. Setnescu, L. V. Badicu,L. M. Dumitran, P. V. Notingher &T. Setnescu

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ORIGINAL PAPER

Thermal lifetime of cellulose insulation material evaluatedby an activation energy based method

R. Setnescu • L. V. Badicu • L. M. Dumitran •

P. V. Notingher • T. Setnescu

Received: 10 December 2012 / Accepted: 14 October 2013 / Published online: 22 October 2013

� Springer Science+Business Media Dordrecht 2013

Abstract A rapid method, based on a logarithmic

degradation model of insulation material, is proposed

to reduce the test duration in lifetime assessment of

cellulose paper insulating materials. This method

proposes the determination of the activation energy

from a non-isothermal measurement made by differ-

ential scanning calorimetry or another thermal ana-

lysis technique and an aging test at a single elevated

temperature. The use of the onset temperature of the

exothermal peak at ca. 300 �C is proposed for

evaluation of the activation energy of degradation.

For comparison, the thermal aging of Kraft cellulose

paper for power transformer insulation was performed

according to the general standard IEC 60216-1/2001 at

three different temperatures: 155, 135 and 115 �C, and

subsequently, the lifetimes at different service tem-

peratures were estimated. The experimental data

proved to have good agreement between the applied

methods, the differences being\10 % in terms of the

estimated lifetime across the range of service temper-

atures. The novel proposed method is effective in

terms of both energy and manpower costs as compared

to the current method: a factor of around 10 in the case

of reducing the aging time, a factor of 3 for the time

needed for measurements, and a factor of 10 for the

reduction of power intake.

Keywords Lifetime (durability) estimation �Insulation materials � Differential scanning

calorimetry � Cellulose paper

Introduction

It is well known that under the service conditions in

power transformers, cellulose paper is subjected to

thermal and electrical stress in the presence of traces

of air and water, resulting in different types of

degradation (depolymerization, oxidation, decompo-

sition) of the macromolecular chains. As a result, the

mechanical properties, especially the mechanical

strength, of the cellulose material are worsened,

leading to both local mechanical failures and contam-

ination of oil by paper fragments. The mentioned

mechanical failures will consequently lead both to

electrical failures and to clogging of the oil ducts. The

latter process is a main cause of transformers’ thermal

failures (Emsley and Stevens 1994). Therefore, the

R. Setnescu � T. Setnescu

Valahia University of Targoviste, 2 Bld. Carol,

Targoviste 130082, Romania

R. Setnescu (&) � T. Setnescu

National R&D Institute for Electrical Engineering (ICPE-CA),

313 Splaiul Unirii, Bucharest 030138, Romania

e-mail: Radu.Setnescu@yahoo.com; rsetnescu@yahoo.com

L. V. Badicu � L. M. Dumitran � P. V. Notingher (&)

University Politehnica of Bucharest, 313 Splaiul

Independentei Str., Bucharest 060042, Romania

e-mail: petrunot@elmat.pub.ro

123

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DOI 10.1007/s10570-013-0087-0

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service life of a transformer is critically limited by the

lifetime of its solid insulation (Prevost 2005).

Basically, the evaluation of a material’s lifetime

requires three elements (Ding and Wang 2011),

namely: a critical degradation parameter, a degrada-

tion variable or indicator, i.e. a parameter unequivo-

cally related to the degradation state of material; a

kinetic model, an equation describing the relationship

between the degradation parameter and the aging time;

and a method for accelerated testing, based on the

kinetic model, that will provide data for lifetime

evaluation.

Both cellulose degradation and durability have

largely been studied in relation either to conservation

of various historical documents (Whitmore and Bog-

aard 1994; Rychly et al. 2002; Blumich et al. 2003;

Łojewski et al. 2011) or to application as electrical

insulating material in high voltage cables and trans-

formers (Emsley and Stevens 1994; Liland et al. 2008;

Gilbert et al. 2009; Ding and Wang 2011). The

pyrolysis of cellulose has been intensively studied in

relation to the use of vegetal materials in different

processes of bio-fuels and bio-chemicals production

(Emsley 2008; Sanchez-Jimenez et al. 2011). At

ambient temperature, hydrolysis is the main process

that occurs (if no light or weathering are present),

while at elevated temperatures, oxidation and pyroly-

sis become dominant, leading to both carbonyl groups

and glycosidic bond scission (Lojewski et al. 2011).

The cellulose paper used for common electrical

insulation in transformers is obtained by the Kraft

process and contains ca. 90 % cellulose, 6–7 % lignin

and the rest, up to 100 %, consists mainly of pentosans

(Emsley and Stevens 1994).

The normal value of the average polymerization

degree (DP) for natural wood cellulose ranges between

3,000 and 10,000 anhydroglucose units (O’Sullivan

1997; Wang 2009), being strongly dependent on

source, acquiring processes and further treatments

(Wang 2009). The degradation of cellulose is an

irreversible process occurring by chain scission. The

variation of DP is a direct measure of the degradation

level, DP value hence being considered a degradation

parameter. The DP values of freshly produced paper

for insulation purposes usually range between 1,000

and 1,500 (Saha 2003). The initial drying process in

the transformer results in a decrease of DP up to ca.

950. Subsequently, service conditions progressively

reduce the DP value. A tensile strength decrease

accompanies the decrease of DP, in the range 850–250

(anhydroglucose units), then an abrupt drop in tensile

strength values was reported (Shroff and Stannett

1985). The quality of insulation paper with DP values

of around 200 is considered poor enough to take this as

the end of life criterion (Shroff and Stannett 1985;

Saha 2003; Glomm Ese et al. 2010), but it was noted

that many transformers continued to operate at lower

DP levels (McNutt 1992).

The values of activation energies for the loss of

mechanical strength were found to be similar to those

of the depolymerization process (Emsley and Stevens

1994). The correlation between the average polymer-

ization degree and the tensile strength of insulation

paper was discussed in earlier papers by Emsley et al.

(1997, 2000).

The mass loss rate as measured from thermal analysis

(TG, DTG) has been widely used for kinetic character-

ization of the degradation processes under either inert or

oxidative atmosphere (Kashiwagi and Nambu 1992;

Capart et al. 2004). Other thermal analysis techniques,

such as chemiluminescence (CL) (Strlic et al. 2000;

Rychly et al. 2002) or differential scanning calorimetry

(DSC) (Soares et al. 1995) have been used as well to

characterize the mechanisms and kinetics of cellulose

paper degradation. In some cases, the thermal analysis

techniques were coupled to MS, GC, FTIR or other

analysis techniques (Price et al. 2000; Statheropoulos

and Kyriakou 2000) in order to analyze the gases

resulting from the degradation processes.

It must also be noted that the normal humidity of the

paper used as transformer insulation is 4–5 %, but

after winding, the paper insulation dries to a humidity

of\0.5 % (Emsley and Stevens 1994). Afterward, the

dried paper is submitted to an oil-impregnation

process, which imparts increased dielectric strength

and enables a better cooling of the windings (Emsley

and Stevens 1994). Therefore, the water amounts

detected in aged transformers originate mainly from

cellulose degradation rather than from the initial

humidity of the insulating paper. The water from

cellulose degradation during the thermal aging will

induce increased electrical conductivity and bubble

gas formation. Hence, the presence of degradation

products of cellulose will damage the electrical

properties of the insulation system, especially under

overload conditions (Emsley and Stevens 1994).

A study of different types of cellulose papers for

transformer insulation subjected to thermal aging in

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the range of 110–140 �C revealed that aging is

strongly dependent on temperature and on the pre-

sence of water and oxygen (Shroff and Stannett 1985).

However, in the case of thermally upgraded paper, no

significant influence of water and oxygen on the aging

process was observed.

Methanol production during the aging process has

been proposed as a universal indicator for cellulose

degradation in power transformers (Gilbert et al. 2009,

2010). As the initial rate constants for depolymeriza-

tion and methanol formation in thermally upgraded

paper were found to be near those for standard

cellulose, it was concluded that they result from the

aging patterns just before the retardant action occurred

(Gilbert et al. 2010). Since the values of the frequency

factors for methanol and chain-end groups are similar,

it was concluded that the rate of CH3OH production

from chain scissions is much higher than the depoly-

merization rate. The latter feature becomes the rate-

determining step of the general degradation process

(Gilbert et al. 2010).

For thermally upgraded paper, it was found that the

amount of stabilizer in the fibrous structure is enough

to suppress the self-catalyzing and external catalyst

supply effects (Gilbert et al. 2010). Therefore, the

extended lifetime of the thermally upgraded papers

was assumed to decrease the frequency of chain

scissions with time.

Different accelerated aging tests have been pro-

posed and some have been standardized for durability

estimation of materials in different conditions (Brown

1991; Song et al. 1998). The actual standardized

method used for the evaluation of the paper insulation

lifetime in power transformers is based on the general

standard IEC 60216-1/2001 (IEC 60216 2001).

According to this standard, the insulation material

shall be submitted to aging at three different temper-

atures. Electrical (e.g. resistivity or conductivity) or

mechanical (tensile strength) measurements are to be

performed on samples taken out at established aging

periods. Plots of the characteristic electrical or

mechanical parameter against the aging time shall

be obtained for each aging temperature. For each

aging temperature chosen, the maximum aging time

shall be extended beyond the time required to reach

the end of life criterion, for the tensile strength, for

example, beyond a decrease to 50 % of the initial

value. The activation energy of material degradation,

calculated for the end of life criterion, is used for

subsequent estimation of the lifetime at lower service

temperatures. It is clear that such a procedure is time-

and materials-consumptive, and a more economical

method is desirable, at least for the primary evalu-

ation of different materials subjected to qualification

tests of cellulose insulation materials for power

transformers.

Therefore, the aim of this paper is the elaboration of

a simpler, more rapid and hence more cost-effective

method for lifetime evaluation based on thermal

analysis and electrical measurements. Roughly, the

proposed procedure is based on a simple exponential

equation [ln (D(T) = a ? b/T)] (Notingher 2005),

where D(T) is the lifetime at a given temperature T.

The coefficients a and b are determined from DSC

measurements (which provide the activation energy of

the degradation process) and an accelerated aging test

at a single elevated temperature (as for example

155 �C). This simplification is based on the assump-

tion that the overall degradation processes involved in

long term aging and in short term thermal degradation

tests would be characterized by similar values of the

activation energies: the alteration of electrical prop-

erties during the long-term aging test is the result of

different simultaneous mechanisms (oxidation, depo-

lymerization, hydrolysis) the occur, as well the

accelerated thermo-oxidative conditions of DSC mea-

surement. In this work, we checked the applicability of

the apparent activation energy calculated from DSC as

a substitute for the corresponding value determined

from standardized long-term aging tests for Kraft

electro-insulating paper.

Experimental

Materials

The insulation paper samples subjected to accelerated

aging experiments at elevated temperatures were

square sheets of Kraft type paper (Weidmann AG)

with dimensions of 100 9 100 9 0.24 mm3. For

thermal analysis measurements (DSC, STA, CL), as

well as for the ATR-FTIR records before and after

oven thermal stress at 185 �C, samples of Kraft paper

of 0.08 mm thickness were used.

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Instruments and methods

Differential scanning calorimetry (DSC)

DSC measurements were performed in non-isothermal

mode on a Setaram 131 EVO (Setaram Instrumenta-

tion, France) instrument using square sheet paper

samples of about 3 9 3 mm2. The measurements were

carried out in the presence of air (air flow 50 ml/min),

in the temperature range 30–380 �C at 4 different

heating rates, namely 2, 4, 6 and 10 �C/min. The

exothermic peak at ca. 300 �C, roughly related to

oxidation, was used for the activation energy. The

oxidation onset temperature (OOT) was determined,

according to (ASTM E2009-08 2008) as the crossing

point of the recorded baseline and the slope of the

oxidation exotherm (Fig. 1), using the specific func-

tion (Temperature determination) of Calisto Data

Processing (CDP) software (Setaram/AKTS).

Simultaneous thermal analysis: DSC, TG, DTG

Simultaneous thermal analysis (STA), i.e. DSC,

thermogravimetry (TG) and derivate thermogravime-

try (DTG), measurements were performed in a mixture

of oxygen and nitrogen atmosphere (oxygen flow

20 mL/min, nitrogen flow 80 mL/min), on samples of

about 2 mg, using Netzsch STA 449 (Netzsch, Ger-

many) equipment. The temperature range was

20–500 �C and the heating rates were 2, 4, 6 and

10 K/min. The temperature of the decomposition start

was evaluated from TG and DTG measurements.

FTIR spectroscopy

The Fourier transform infra-red (FTIR) spectra were

recorded directly on the paper sample using the

attenuated total reflectance (ATR) technique, before

and after thermal stress at 185 �C. A Jasco 4200 FTIR

spectrometer (Jasco Inc., Japon) coupled to an ATR

accessory (with a diamond crystal) was used. The

following measuring conditions were applied for all

spectra: the spectral range, 4,000–400 cm-1; scans

number, 48/spectrum; resolution, 4 cm-1. The exper-

imental data were processed with the Spectra Pro-

cessing software (Jasco).

Oven thermal stress at 185 �C

This treatment was carried out in an oven with air

circulation of Memmert UNE 400 type (Memmert,

Schwabach, Germany). The cellulose paper samples,

in the form of squares of 2 9 2 cm were accurately

weighed (Mettler-Toledo XS 105 type (Mettler-

Toledo, Switzerland) before the treatment and after

1, 2, 3 and 4 h of thermal treatment at 185 �C. After

the treatment, the samples were kept in a desiccator

with solid CaCl2, at room temperature.

Fig. 1 DSC curve from

Kraft paper recorded at

10 K/min. Under air

atmosphere (air flow:

50 mL/min)

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Long-term aging at elevated temperature

Long-term aging tests were carried out at three

different temperatures chosen in concordance with

the recommendations of IEC 60216-1 (IEC 60216-1

2001). Considering the temperature index for cellulose

as IT = 90 �C, (i.e. the operating time at 90 �C should

be at least 20,000 h) resulted in values of the testing

temperatures of T1 = 155 �C, T2 = 135 �C, and

T3 = 115 �C, with a test duration for the highest

temperature (155 �C) of more than 100 h but\500 h,

as required by the mentioned standard. For the lowest

temperature, the recommended conditions concerning

the aging time of at least 5,000 h and the difference

between the test temperature and the IT being no more

than 25 K were met as well.

Aging at elevated temperature was performed in a

forced air circulation oven of Raypa (Raypa, Spain).

The relative humidity of air entering the oven was

around 45 % at room temperature. All samples were

thermally conditioned before aging at 90 �C during

48 h. The aged samples were taken out after different

established aging periods, in order to measure their

electrical properties.

Electrical properties measurements

The volume resistivity qv(t) was calculated, according

to Eq. (1) (Notingher et al. 2008), from the absorption/

resorption currents measured with a Keithley 6517

Electrometer (Keitley, USA) coupled to a special test

cell equipped with a guard ring; a DC voltage of 300 V

was applied during 3,600 s:

qvðtÞ ¼S

l� U0

iaðtÞ � irðtÞð1Þ

where ia(t) and ir(t) are respectively the absorption and

the resorption currents (measured at a certain moment

t), U0 is the applied DC voltage, S is the active

electrodes surface area and l is the thickness of the

tested sample.

Relative humidity conditions

As it was already mentioned (see the Sect. 2.2.5), the

relative humidity in the laboratory was 45 % at 24 �C.

It didn’t presented important variations during the

experiments due to relatively stable weather and

stabilizing influence of an air conditioning apparatus.

Results and discussion

Life-time evaluation of Kraft paper following IEC

60811

The lifetime of an insulation system is the time

elapsed to failure. For an electrical insulation system,

it is the time necessary to attain the electrical failure

of a critical element (material) in that system. The

lifetime prediction requires the definition of a

(critical) diagnostic parameter describing the state

of the material and the limit of the critical value of

this parameter (end of life criterion), related to the

safe function of the material and consequently of the

equipment as a whole. Either the electrical resistivity

or the conductivity is usually chosen as the critical

parameter for insulation materials. The lifetime can

be defined as the necessary time to produce a change

in the value of the selected parameter beyond an

acceptable limit (end of life criterion). The correct

prediction of material lifetime requires knowing the

deterioration kinetics corresponding to a certain

degradation model of the established parameter (the

degradation parameter).

For practical use, in the case of the studied Kraft

paper, it can be assumed that the material deterioration

rate (vR) depends on temperature according to an

Arrhenius type equation, as given in Eq. (2):

vR ¼ v0 exp � Wa

RgT

� �; ð2Þ

where Wa, is the apparent activation energy of the

degradation reaction, T is the test temperature and Rg

is the general gas constant (8.314 J/mol K).

In Fig. 2 the degradation curves are presented for

the studied cellulose paper at three different temper-

atures: T1 = 155 �C, T2 = 135 �C, and T3 = 115 �C.

Assuming a value of 80 TX m for the volume

resistivity (qv) as the end of life criterion, a straight

line of the lifetime as a function of temperature in

coordinates ln D(T) = f(1/T) is obtained (see Fig. 3).

The equation of this straight line was found to be ln

D(T) = -25.49 ? 13,114.16/T; the corresponding

value of the activation energy is Wa1 = 109 kJ/mol.

This figure concurs with similar data obtained on

cellulose paper degradation, for example by Lundg-

aard et al. (2004).

The lifetime values at different temperatures were

calculated applying the linear regression for

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temperature in the range of transformer service

conditions (Table 1). The lifetime values of 43.73

and 5.58 years were found respectively for 70 and

90 �C, which are in reasonable agreement to the

durability required for power transformers as well as

to the IT value for Kraft cellulose paper.

From the standpoint of materials, energy and

manpower consumption, the above method, based on

the IEC 60216 standard, is economically ineffective,

especially when many materials are to be tested and

selected. A rough evaluation indicates that applying

this method involved 200 samples (100 9 100 9

0.24 mm3), more than 6,000 h of aging and 90 h of

testing for only one material. In terms of electrical

power, around 6,000 kWh were consumed for this kind

of testing. Although the standardized method provides

reliable data, as generally recognized, the above

evaluation illustrates the practical need for a more

rapid and more cost-effective.

Life-time evaluation of Kraft paper following

the proposed rapid procedure

It has been shown that the lifetime of an insulation

material exposed to a temperature stress T can be

described by an exponential equation as given in Eq. 3

(Notingher 2005):

ln DðTÞ ¼ aþ b

T; ð3Þ

where a is a material constant depending on both the

limit value and the rate constant of the degradation

process, b = Wa/Rg and D(T) is the aging time up to

reaching the limit value (end of life) of the diagnostic

parameter, and Wa and Rg stand for the activation

energy and general gas constant respectively.

As can be readily observed, plotting ln D(T) versus

1/T yields a straight line with the slope equal to b (i.e.

Wa/Rg), enabling the calculation of Wa.

With the new proposed simpler method, the acti-

vation energy can be calculated from a rapid thermal

analysis test using the high-temperature exothermic

peak from DSC measurements. Only a single aging

test carried out at an elevated temperature is required

to provide the data for lifetime evaluation using the

Eq. (3).

The most evident effect observed in thermal

analysis of cellulose paper is the exothermal peak at

around 300 �C (see the kinetic parameters derived

from this peak in Table 2). It was assumed that the

lifetime of the material under the DSC test is related to

this exothermic peak, which is due mainly to thermally

induced oxidation. Depolymerization, hydrolysis and

gas evolution processes occur simultaneously in long-

term aging tests as well as in thermal analysis

Fig. 2 Accelerated thermal aging curves of the Kraft paper

tested according IEC 60216-1/2001 [11]

Fig. 3 The lnD(T) versus 1/T plotting for activation energy

determination (ln D(T) = -25.49 ? 13,114.16/T)

Table 1 The values of the lifetime (in years) of Kraft paper

calculated for different transformer operation temperatures

Method Temperature (�C)

70 80 90 100 110 155

IEC 60216 43.73 15.17 5.58 2.16 0.88 0.013

Proposed in the

present paper

48.24 15.86 5.54 2.05 0.80 0.02

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measurement and accompany or overlap the oxidation

mechanism. The onset of this peak, called below the

oxidation onset temperature (OOT) even though other

processes than oxidation are involved, was chosen for

activation energy calculations. The low degradation

level in the temperature region of OOT has been

shown by TGA measurements (Fig. 4), which indi-

cated a weight-loss of\5 % in the temperature range

30 �C—OOT.

Besides the main exothermic peak, the DSC curves

from the studied material also presented some small

peaks at temperatures under 200 �C (see Fig. 1, the

encircled region), but the kinetic parameters of these

peaks could be not fitted to the heating rate in air. After

a thermal treatment of up to 4 h at 185 �C, a

temperature within the range of these small peaks,

no changes were observed in the position or absorption

of the characteristic cellulose peaks (Ali et al. 2001;

Lojewska et al. 2005), suggesting that no significant

degradation is related to these peaks (Fig. 5). Only a

mass decrease as the duration of the thermal treatment

increased was observed. This process can be assigned

to moisture desorption (Scheirs et al. 2001). It is

known that the elimination of chemically bound water

starts at higher temperatures (220 �C) and occurs up to

500 �C, its thermal effect (endotherm) being over-

lapped with other degradation processes in the main

peak, which starts at ca. 300 �C (Ramiah 1970;

Scheirs et al. 2001). In these conditions, the use of

the main DSC peak for calculation the activation

energy of the aging processes looks more reasonable.

The straight lines obtained for each method,

presented in Table 2, enable the calculate of activation

energy Wa from their slopes, which are equal or

proportional to—Wa/Rg. Figure 6 is presented an

example for the determination of Wa according to

the Kissinger method. The onset temperature of the

main exothermic peak has been used in these calcu-

lations. The parameters of the straight lines (a0, b0 and

r) for the different methods applied are presented in

Table 3, with the values of the calculated activation

energies. It can be seen that there is a good fit between

the experimental data for the different equations used

for activation energy calculation, and the resulting Wa

values are close to each other.

The average value of the activation energies

summarized in Table 3, noted as Wa2 (112 kJ/mol),

was subsequently used in lifetime estimation by the

proposed method.

The value of Wa2 calculated from OOT is in good

agreement with previously reported values for differ-

ent processes related to cellulose degradation and

Table 2 Kinetic parameters for Kraft paper oxidation calcu-

lated from the non-isothermal DSC curves (the peak at ca.

300 �C)

b (�C/min) OOT (�C) Tm (�C) DHox (J/g)

2 269.95 310.8 -1,616

4 275.60 306.1 -1,444

6 288.96 330.9 -1,722

10 302.05 329.2 -764

Fig. 4 STA

(DSC ? TG ? DTG)

curves of Kraft paper

recorded in presence of a

mixture O2 ? N2 (20 and

80 % respectively) at a

heating rate of 4 K/min

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subsequent failure of cellulose based materials,

namely:

• hydrolytic depolymerization of cellulose, 110 kJ/mol

(Emsley and Stevens 1994; Liland et al. 2008);

• CO and CO2 evolution, 105–150 kJ/mol (Hino and

Suganuma 1972);

• aging (air and humidity presence), 110–114 kJ/mol

(Lundgaard et al. 2008) or 111 kJ/mol (Emsley

and Stevens 1994).

However, it should be mentioned that the reported

activation energy values found for pure oxidation

processes in cellulose were considerably lower (Liland

et al. 2008). This disagreement can be explained by a

more complex mechanism of the processes related to

the exothermic peak, which involve not only oxida-

tion, but also depolymerization and gas formation.

Using the average value of the activation energies

calculated from DSC measurements (Wa2), and the

lifetime value for Kraft paper obtained for T = T1 =

155 �C (Fig. 2), the parameters a and b for Eq. (3)

were calculated, resulting in the following values:

a = -26.21 and b = 13,471.72 K-1. The calculated

lifetime values for different operating temperatures of

the Kraft paper are summarized in Table 1.

According to the present data, in the case of

operation at a constant and moderate temperature

(T = 80 �C), the lifetime of the insulation material is

D = 15–16 years, but when the temperature increases

to 90 �C, the lifetime of the studied Kraft paper

becomes three times lower. When hot spots occur, the

Kraft paper lifetime can be reduced in those areas down

to a few 100 h (as for example, D = 0.0463 years for

T = 155 �C). On the other hand, good concordance

can be observed between the lifetime values estimated

by the applied methods (standardized and proposed).

The experiment related to the proposed method

required roughly 600 h of laboratory aging, 30 h of

DSC testing, data processing, and interpretation. In terms

of electrical power, around 600 kWh were consumed.

Even though the above experimental data provide a

rather low degradation level at the onset temperature

of the exothermic peak, the use of the activation

energy value calculated on this basis appears some-

what strange. The considerably higher temperatures

relative to the aging process could result in different

reaction pathways. However, the reasonably close

values of Wa1 found by the standardized method and

Wa2 from DSC (Table 2), as well as the similar

lifetime values determined by both methods (Table 3),

support the applicability of the proposed method. In

actuality, the Wa2 value includes the reactions that

occur during heating up to the main peak rather those

responsible for the main peak itself. The isothermal

onset time of the main peak can be calculated using the

equation proposed by Gimzewski (1992) for hydro-

carbon oxidation, as given in Eq. (4):

e� Wa

RTiso

Ztind

0

dt ¼ 1

b

ZTind

0

e�WaRT dT ð4Þ

Fig. 5 ATR-FTIR spectra of the Kraft paper submitted to

thermal treatment at 185 �C for different time periods: 1 initial,

untreated; 2 1 h; 3 3 h

Fig. 6 Activation energy Wa determination from OOT by

Kissinger method for the studied Kraft paper

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where Wa is the activation energy, T is the thermody-

namic temperature; Tiso is the temperature of an

isothermal experiment; Tind is the oxidation induction

temperature (OOT); R is the gas constant; t is the time;

b is the heating rate; and tind is the induction time. The

definite integralR tind

0dt is the oxidation induction time

(OIT) in isothermal conditions. In our case, the

process is more complicated than in the case of

hydrocarbon oxidation because of other additional

reactions different than oxidation: depolymerization,

hydrolysis, and decomposition. Hence, the term oxi-

dation used here has a more complex meaning,

including all other reactions that lead to the onset of

the exothermic peak. In the same manner, the oxida-

tion induction time is, in fact, the time up to the

starting of exothermal degradation (called below

induction time). There are some data in the literature

indicating that the degradation kinetics of cellulose

(described by mass loss in TGA) has a sigmoidal

profile specific to an auto-accelerated process (Capart

et al. 2004), similar to the polyolefins oxidation

process. Generally, the oxidation induction time from

isothermal measurements has the meaning of the

material’s lifetime under the experimental conditions.

Assuming the same meaning in our case, a value of

around 10.8 min was calculated for 265 �C (near to

the onset temperature at the lowest heating rate) and

336 min at 200 �C. However, it is interesting to

observe that the value of the time to onset (112 h)

calculated for 155 �C was practically the same as in

the case of the lifetime determined from electrical

measurements at the same temperature following the

IEC method (114 h). At lower temperatures, the

values of the induction time calculated with Eq. (4)

were lower than those found by electrical measure-

ments, but nonetheless comparable. Fig. 7 presents the

values of the induction times calculated with Eq. (4),

i.e. using the activation energy Wa2 from the onset of

the main exothermic peak of cellulose degradation. As

Table 3 Activation energy values for oxidation process and fitting parameters of the linear relationships (y = a0 ? b0x) corre-

sponding to different methods applied for calculations (Tf = OOT)

Method Equation Plot Parameters Wa (kJ/mol) Wa2 (kJ/mol)

Kissinger (1957), Starink (1996) ln bT2

f

¼ � Wa

RgTfþ C1 ln

T2f

b ¼ f ð1=Tf Þ r = -0.96109 109.4 112

a0 = 12.54

b0 = -13,170.6

Ozawa (1992) ln b ¼ �1; 0518 � Wa

RgTfþ C2 lnb ¼ f ð1=Tf Þ r = 0.96622 113.4

a0 = 27.31

b0 = -14,352.81

Starink (1996), Boswell (1980) ln bTf¼ � Wa

RgTfþ C3 ln b

Tf¼ f ð1=Tf Þ r = -0.9635 114.6

a0 = 19.98

b0 = -13,791.58

Starink (1996) ln bT1:8

f

¼ � Wa

RgTfþ C4 ln b

T1:8f

¼ f ð1=Tf Þ r = -0.96082 110.4

a0 = 14.01

b0 = -13,212.10

Fig. 7 Lifetime values versus 1/T for the Kraft cellulose paper

submitted to thermal degradation in presence of air in different

conditions: square box induction time of the exothermic peak

from DSC measurements (DSC lifetime); circle lifetime

evaluated from thermal aging and electrical measurements

according to IEC-60216-1/2001; triangle lifetime evaluated

according to the proposed method

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the lifetime values evaluated by standardized method

and those estimated by the proposed method fall

acceptably close to the straight line of the calculated

induction time values (DSC lifetimes), it can be

concluded that Wa2 calculated from the main exother-

mic peak can be reasonably used for aging evaluation

following the proposed method.

Conclusions

Considering that the lifetime of the Kraft paper

insulation material D(T) presents an exponential

(Arrhenius type) dependence on temperature T, the

use of the proposed method involves the achievement

of two important steps: first is the determination of the

activation energy (Wa) by DSC or other thermal

analysis method, and then, the value of the parameter

b in Eq. (1) is calculated. The second step is the

conduct of an accelerated aging test of the Kraft paper

material at an elevated temperature, as for example

T = 155 �C, and finding the lifetime D under these

conditions. Subsequently, using the calculated param-

eter b, the parameter a in Eq. (3) for the lifetime curve

is calculated. With parameters a and b, the evaluation

of the Kraft paper lifetime for different operating

temperature is possible, using the simple linear

equation, Eq. (3).

Our results are in good agreement with the data

obtained by the classical method (IEC 60216) per-

formed in a parallel experiment, the differences in the

estimated lifetime being \10 % over the whole the

range of service temperatures. The proposed method

appears convenient in terms of manpower, materials

and energy costs.

The use of the exothermal peak for OOT and

subsequent Wa calculation hold in the case of the

studied material because no significant changes were

observed during the sample heating at lower temper-

atures. Moreover, the processes involved in the exo-

thermic peak are more or less similar to those occurring

in the case of oven aging at elevated temperature and

hence in the real case of aging under service conditions.

Further work is planned to check the applicability of

the proposed method in the case of other cellulose

materials as well as for different conditions, including

the influence of specific impurities.

The novel method proposed in this work requires

shorter periods of both aging and testing as compared

to the IEC 60216 standard: a factor of around 10 in the

case of the aging experiments, while, in the case of

testing, the measurement time is reduced by a factor of

around 3; the energy consumption was 10 times lower.

Acknowledgments This work has been supported by the

Romanian Ministry of Labor, Family and Social Protection

through the Financial Agreements, POSDRU/6/1.5/S/16 5159

and POSDRU/89/1.5/S/62557, as well as by the Romanian

Ministry of Education, Research and Youth, Projects MIDMIT

22 080/2008 and Grant 10EU/2011. R. S. acknowledges

Dr. Traian Zaharescu, senior researcher at R&D Institute of

Electrical Engineering in Bucharest (ICPE-CA), for his kind

help and very useful discussions.

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