lable at ScienceDirect

Polymer Degradation and Stability 94 (2009) 246–252

Contents lists avai

Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Influence of iron content on thermal stability of magnetic polyurethane foams

Jingjing Zhang a, Lin Li a,*, Guang Chen a, Paul Wee b

a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporeb Kinetics Design and Development, Singapore Technologies Kinetics Ltd, 249, Jalan Boon Lay, Singapore 619523, Singapore

a r t i c l e i n f o

Article history:Received 18 June 2008Received in revised form17 October 2008Accepted 21 October 2008Available online 6 November 2008

Keywords:Magnetic foamTGAThermal degradationActivation energy

* Corresponding author. Tel.: þ65 6790 6285; fax:E-mail address: mlli@ntu.edu.sg (L. Li).

0141-3910/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2008.10.020

a b s t r a c t

Magnetorheological (MR) materials are a group of smart materials which have the controllable magneticproperties with an external magnetic field. Magnetic foams, a specific type of MR solids, were synthe-sized from flexible polyurethane (PU) foams and carbonyl iron particles. Effects of the carbonyl ironparticles on the thermal stability of the magnetic foams have been studied. Thermogravimetric analysis(TGA) was applied to characterize the thermal degradation process of the magnetic foams and then theapparent activation energy of degradation was calculated by using Ozawa’s method [Ozawa T. A newmethod of analyzing thermogravimetric data. Bulletin of the Chemical Society of Japan 1965; 38: 1881–1886.]. The carbonyl iron particles were found to improve the thermal stability of magnetic foams innitrogen by showing higher 10 wt% loss temperature, slower weight loss rate and higher apparentactivation energy than pure PU foams. But the magnetic foams were observed to have slightly worsethermal stability in air than pure PU foams at the earlier degradation stage. At the later degradationstage, the magnetic foams exhibited the higher activation energy than pure PU foams in air.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Magnetorheological (MR) materials are known as a group ofsmart materials that consist of magnetically polarisable particles ina non-polarisable matrix [2]. The rheological properties of MRmaterials can be controlled by an external magnetic field. MRmaterials can be generally classified into two types, by the physicalstate of matrix materials: MR fluids and MR solids. MR fluids, firstreported by Rabinow in 1948 [3], show smart rheological propertiesthrough the reversible formation of chains from the magneticparticles, which are controlled by a magnetic field. As compared toMR fluids, MR solids can overcome the problems caused by leakagewhen MR fluids are used in applications such as dampers. More-over, MR solids have regular and controllable shapes for variousapplications. MR elastomers have been recently reported to possessthe controllable field-dependent modulus [2,4–6].

Magnetic foams, as a new type of MR materials, use flexiblepolyurethane (PU) foams as the non-polarisable matrix andcarbonyl iron particles as the polarisable fillers. Carbonyl ironparticles are chosen because of the high permeability, low rema-nent magnetization and high saturation magnetization. They aretherefore expected to provide high interparticle attraction andreversible MR effect [2,7]. Flexible foams have been widely used inour daily life applications. With the controllable MR effect,

þ65 6791 1859.

All rights reserved.

magnetic foams are considered to be useful in wide industrial andcommercial applications, such as cushions, vibration absorbers, anddampers, where thermal environments are very possible to beinvolved.

Thermal degradation of pure PU foams was intensively studiedin the past five decades. According to the literature, polyurethanebonds start to dissociate within the temperature range 200–300 �C[8–10]. The thermal degradation mechanisms of pure PU foams inan inert environment are proposed to follow three steps [11]:

1. Cleavage of the urethane bonds into the starting componentspolyol and isocyanate:

2. Dissociation of an intermediate six-membered ring to amine,carbon dioxide, and olefin:

J. Zhang et al. / Polymer Degradation and Stability 94 (2009) 246–252 247

3. Dissociation of an intermediate four-membered ring tosecondary amine and carbon dioxide:

On the other hand, the thermal degradation of pure PU foamsunder oxidative conditions is much more complicated. Jellinek andDunkle [12] suggested that the reaction mechanisms are: chainscission, crosslinking, oxidation and gas evolution. More detaileddegradation kinetics and the radical reactions are available aswell [13].

Since magnetic foams are a type of new materials, the effects ofthe carbonyl iron particles on the thermal stability have not beenreported in the literature. This work aims to study the influence ofiron content in the magnetic foams on the thermal degradationprocess and the thermal stability using thermogravimetric analysis(TGA). The heating rate is varied from 5 to 40 �C/min and theimportant degradation kinetic parameter, apparent activationenergy, at different degrees of weight loss, will be obtained usingOzawa’s method [1].

2. Experimental

2.1. Materials

The materials for the synthesis of a high density (55 kg/m3)flexible PU foam were polypropylene glycol (PPG, Seraya Chemicals,Singapore), toluene-2,4-diisocyanate (TDI, Sigma Aldrich), siliconesurfactant (Niax� L600), tin catalyst (stannous octotate) (Niax� D-19) and amine catalysts (Niax� A-1 and A33). All of the surfactantsand catalysts used in this study were kindly provided by Momen-tive Performance Materials, Singapore. The carbonyl iron particleswith an average particle size of 4.5–5.2 mm were purchased fromSigma Aldrich.

Synthesis of magnetic foams has been reported elsewhere [14].Table 1 gives the naming of the magnetic foams samples preparedin this work. The number 55 in MFXX-55 refers to the nominaldensity of 55 kg/m3 for the high density of magnetic foams and theXX refers to the weight percentage content of iron particles basedon the polyol weight. For instance, if 100 g of polyol is required ina formulation for a 55 kg/m3 PU foam and 50 g of iron particles isadded, the sample name is denoted as MF50-55.

2.2. Measurements

The samples were cut into an average weight of 10 mg, andtested on TGA 2950 (TA Instruments). TGA curves were obtained atfour different heating rates (b), namely 5, 10, 20, and 40 �C/min, innitrogen and air environments, respectively. Ozawa’s method wasapplied to calculate apparent activation energy of degradation fromthe TGA experimental data.

Table 1Sample codes and compositions of PU foams and magnetic foams.

Sample code Composition

PU-55 Pure PU with 55 kg/m3 (nominal density)MFXX-55s MF25-55 PU-55þ 25 wt% iron particles

MF50-55 PU-55þ 50 wt% iron particlesMF75-55 PU-55þ 75 wt% iron particles

3. Results and discussion

3.1. Thermogravimetric analysis

Figs. 1 and 2 illustrate the thermogravimetric (TG) and differ-ential TG (DTG) curves for the PU-55 and the magnetic foams innitrogen and air at a heating rate of 5 �C/min, respectively. Theonset temperature (Ton) is defined as the temperature when thestart of weight loss, which can be determined from the onset pointof the DTG curves, as shown in Fig. 1a. The TG characteristic datasuch as Ton, the peak temperature (Tmax) at each degradation stagefrom the DTG curves and the residual percentage at 600 �C from theTG curves are given in Table 2.

The TG and DTG curves of PU-55 and the MFs obtained innitrogen display two distinct degradation stages (Fig. 1). Themagnitudes of the two peaks in DTG were obtained, representingthe maximum weight loss rates at the two stages. The two-stagedegradation behavior was consistent with the results obtained bySong et al. [15]. According to the literature [6,16], the weight loss atthe initial stage was due to the degradation of hard segments,isocyanate (TDI) and the second-stage degradation was associatedto the soft segments, polyol (PPG). Grassie and Zulfiqar proposedthe formation of carbodiimide from the thermally decomposedisocyanates during the degradation process [11]:

2R—N]C]O / R—N]C]N]R D CO2 (4)

In addition, volatile polyureas may be formed by reactionbetween isocyanates and amines. As a result, isocyanates areevolved in the relatively low temperature range and released asa product termed ‘‘yellow smoke’’. After the decomposition of thehard-segment isocyanates, the left liquid state polyol is responsiblefor the molten burning characteristics of flexible PU foam [8]. Thisexplained why the hard-segment isocyanate degraded at the firststage and the soft-segments polyol degraded at the second-stage.

Fig. 1 demonstrates the significant shift of the TG curves tohigher temperature with increasing the iron content in the MFs.The weight loss at a given temperature from the TG curves and theweight loss rates at the two degradation stages from the DTGcurves decreased in the order: PU-55>MF25-55>MF50-55>MF75-55. The results indicate that the iron particles delayedor suppressed the thermal degradation of the magnetic foams innitrogen. The iron particles are inert in nitrogen. During thedegradation of PU, the iron particles may act as fillers to restrict thePU molecular chain motion upon heating and thus delay thethermal degradation of magnetic foams.

The degradation behaviors of the magnetic foams in air aremuch different from those in nitrogen. As shown in Fig. 2, a three-stage degradation behavior was shown for PU-55 and MF25-55,while a weak two-stage degradation behavior was observed forMF50-55 and MF75-55. The DTG curves revealed the complexity ofthe oxygen-involved degradation, which included not only theoxidation of polyurethane but also the oxidation of the iron parti-cles and even the metal–polymer interaction during the oxidativedecomposition [17].

The extracted initial 10 wt% and 50 wt% loss temperatures of thefoams in nitrogen and air are plotted in Fig. 3. The 10 wt% loss and50 wt% loss temperature in nitrogen increased slightly withincreasing the iron content, suggesting a stabilizing effect by theiron particles in the magnetic foams.

In Fig. 3, the lowest temperatures were observed at 10 wt% lossfor the magnetic foams in air. The more the iron content, the lowerthe 10 wt% loss temperature. However, the 50 wt% loss temperaturein air did not show such a trend, but varied with the iron content,indicating that the oxygen-involved degradation was much morecomplicated than that in nitrogen. Generally, the 10 wt% loss and50 wt% loss temperatures were higher in nitrogen than the

Fig. 1. TG and DTG curves of PU-55 and MFs in nitrogen, at a heating rate of 5 �C/min.

J. Zhang et al. / Polymer Degradation and Stability 94 (2009) 246–252248

corresponding temperatures in air for all the foams. The differencein temperature between nitrogen and air was not large at the initial10 wt% loss, but became very large at the 50 wt% loss, indicating thedifferent degradation mechanisms involved in the twoenvironments.

Upon heating, the degradation of magnetic foams started fromthe PU strut where the iron particles were embedded or wrapped.The oxidative degradation of the PU was dominating at the initialstage. Thus the degradations PU-55 and MF25-55 were fast at theinitial stage with high weight loss rates and gradually becameslower. With more iron particles, the well-dispersed iron particlesin MF50-55 and MF75-55 separated the polymer struts into smallerregions and increased the surface area for the oxidation reactions.The interaction between the metal and the polymer might occurafter the formation of the metal oxide clusters, according to Tan-nenbaum et al. [17]. Metal oxide clusters, as the main product of themetal carbony1 decomposition, could provide large surface areas

for their reactions with the matrix polymer. In addition, Kalnin et al.and Day et al. also suggested that metal (copper, steel, iron oxide)could have a catalytic effect on the degradation process of polymerssuch as polyethylene, polypropylene, polyurethane, poly(vinylchloride) and acrylonitrile butadiene styrene (ABS) [18,19]. Eventhough the mechanism for the oxygen-involved thermal degrada-tion of the MFs has not been fully understood in this study, it isimportant to know the fact that the iron particles lowered the10 wt% loss degradation temperature and quickened the thermaldegradation of the MFs in air.

The actual iron content percentages in the magnetic foams,which were calculated by the formula of iron weight/total weight ofall the components, are given in Table 2. The iron residuepercentages at 600 �C are also shown in Table 2. The iron residuepercentages of the magnetic foams in nitrogen were verifiable withthe actual ones, while the residue percentages in air were signifi-cantly larger than the actual ones. Fig. 2 shows that the residue

Fig. 2. TG and DTG curves of PU-55 and magnetic foams in air at a heating rate of 5 �C/min.

J. Zhang et al. / Polymer Degradation and Stability 94 (2009) 246–252 249

weights of the magnetic foams slightly rise up after 400 �C. It isbelieved that the iron oxides formed in the oxidative reactionscaused the increase in the residue weights during and after thedegradation process [17].

Table 2TGA parameters for PU-55 and magnetic foams in nitrogen and air at a heating rate of 5

Sample Stage 1 Stage 2 Stage 3

T1on (�C) T1max (�C) T2on (�C) T2max (�C) T3on (�C) T3m

N2 PU-55 231.2 274.2 310.7 355.8 – –MF25-55 225.8 273.1 313.2 356.9 – –MF50-55 220.5 267.4 311.4 349.5 – –MF75-55 217.7 269.3 312.1 353.9 – –

Air PU-55 237.9 263.5 270.6 299.7 313.5 36MF25-55 232.4 258.7 268.2 295.2 305.3 24MF50-55 223.8 257.1 – – – –MF75-55 223.7 240.7 – – – –

a Calculated from the formula (iron weight)/(weight of all components in foams).

3.2. Degradation kinetic analysis

Activation energy is one of the important parameters to char-acterize the thermal degradation kinetics. There were a few popular

�C/min.

10 wt% losstemperature (�C)

50 wt% losstemperature (�C)

Residue at600 �C (%)

Actual ironpercentage (%)a

ax (�C)

256.9 344.6 0 0257.3 350.0 15.0 15.25257.5 349.6 27.9 26.5261.8 358.0 36.8 35.1

0.5 250.8 287.5 0 06.1 248.0 296.4 19.9 15.3

242.0 280.4 38.2 26.5237.4 287.3 51.0 35.1

230

270

310

350

0 25 50 75Iron content, wt%

Te

mp

era

tu

re

,°C

10wt%loss in nitrogen10wt% loss in air50wt% loss in nitrogen50wt% loss in air

Fig. 3. Temperature at 10 wt% loss and 50 wt% loss of PU-55 and magnetic foams innitrogen and air, at a heating rate of 5 �C/min.

J. Zhang et al. / Polymer Degradation and Stability 94 (2009) 246–252250

analytical methods for calculating the apparent activation energy ofdegradation from the TG experimental data, e.g. Ozawa, Flynn,Kissinger, Van Krevelen, etc. [20–22]. The apparent activationenergy determined from the experimental TG data is stronglydependent on the applied method, and the degree of conversion [6].

Fig. 4 demonstrates the TG curves for one sample (MF75-55) atfour different heating rates (5, 10, 20 and 40 �C/min) in nitrogen

Fig. 4. TG curves of MF75-55 in nitrogen and air, at four different heating rates, 0.5–40 �C/min.

and air. The TG curves shift to the left (i.e. higher temperature) withincreasing heating rate in a good order. This phenomenon is knownto be the time-dependent properties of polymers. A higher heatingrate would give a shorter time to the polymer to ‘‘react’’ with thetemperature change, so leading to a resisting and delaying effect onthe degradation.

In this study, Ozawa’s method was applied to calculate theapparent activation energy [1,21]. The TG curves for one sample atfour different heating rates (b) were required (Fig. 4) and temper-atures at various degrees of conversion (a) were extracted from theTG curves .The degree of conversion is defined as:

a ¼ 1�wðtÞ=w0 (5)

where w0 and w(t) are the initial weight and the weight at time t[22]. It also refers to the reaction rate in the thermogravimetricanalysis.

By plotting ln b against 1000/T, the apparent activation energy(Ea) could be calculated from the slope of the linear relationship ateach degree of conversion, a:

Ea ¼ �R,slope1:052

ðkJ=molÞ (6)

where R is the universal gas constant.Fig. 5 illustrates the relationships between ln b and 1000/T for

MF75-55 at 10 different degrees of conversion from 5% to 50% innitrogen and air. According to Eq. (6), the slope of each straight linecould give an apparent activation energy at the correspondingdegree of conversion. Table 3 lists the calculated apparent activa-tion energy and correlation coefficient (r2) for MF75-55. Theapparent activation energies for the pure PU foams and MFs in bothnitrogen and air are plotted against the degree of conversion inFig. 6.

MF75-55 (Nitrogen)

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.201000/T, K

-1

1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.201000/T, K

-1

ln

β,

°C

/m

in

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

ln

β,

°C

/m

in

5.0%10.0%15.0%20.0%25.0%30.0%35.0%40.0%45.0%50.0%

5.0%10.0%15.0%20.0%25.0%30.0%35.0%40.0%45.0%50.0%

MF75-55 (Air)b

a

Fig. 5. ln b versus 1000/T for MF75-55: (a) in nitrogen and (b) air.

Table 3Activation energy and correlation coefficient for MF75-55.

a, % Nitrogen Air

Ea (kJ/mol) r2 Ea (kJ/mol) r2

5 191.6 0.9947 186.5 0.95610 178.4 0.997 163.4 0.96615 167.8 0.998 152.0 0.96920 156.0 0.988 150.0 0.96925 181.7 0.967 153.7 0.97430 190.9 0.966 159.9 0.98435 195.8 0.969 169.6 0.99540 199.3 0.972 177.7 0.99945 202.6 0.975 184.9417 0.999150 205.5 0.978 214.9 0.995

J. Zhang et al. / Polymer Degradation and Stability 94 (2009) 246–252 251

As observed in Fig. 6a, the apparent activation energies atdifferent degrees of conversion (5–50%) in nitrogen were relativelyhigh and stable except a minimum value in the a range around 20–25%. The activation energy varied in the range from 137 to 206 kJ/mol. At the initial degradation stage, the magnetic foams showedhigher activation energies than that of PU-55. With increasing thedegree of conversion or weight loss percentage, the activationenergy of the magnetic foams became lower than that of PU-55,except MF75-55 that exhibited the highest activation energyoutstandingly.

The apparent activation energies in air shown in Fig. 6b varied inthe range from 110 to 217 kJ/mol, which was larger than that innitrogen (Fig. 6a). The effects of iron content on the activationenergy in air were also more significant than those in nitrogen. Itwas interesting to note that the effects of iron content on theactivation energy were different at low and high degrees ofconversion or weight loss percentage. At low degrees of conversion

Nitrogen

60

100

140

180

220

260

0 10 20 30 40 50 60

Degree of Conversion, %

0 10 20 30 40 50 60

Degree of Conversion, %

Ea,K

J/m

ol

60

100

140

180

220

260

Ea,K

J/m

ol

PU-55M F25-55M F50-55M F75-55

PU-55M F25-55M F50-55M F75-55

Air

a

b

Fig. 6. Apparent activation energy of PU-55 and magnetic foams at different degrees ofconversion: (a) in nitrogen and (b) in air.

(approximately <27%), except one point at 5% weight loss forMF25-55, the pure PU showed the highest Ea, followed by MF25-55,MF50-55 and MF75-55 in a good order, implying that Ea decreasedwith increasing the iron content. After undergoing a transitionregion in the vicinity of 30% weight loss, the effects of iron contenton Ea turned in an opposite way at high weigh loss percentages(approximately >33%): Ea increased with increasing the ironcontent. In Fig. 6b, all the Ea curves did not show an apparent valleyat the mediate weigh loss percentages as observed in Fig. 6a. Thecommon point in Fig. 6a and b was that MF75-55 always showedthe highest Ea at high weigh loss percentages.

When degraded in air, the general pattern for the effect of ironparticles was that the activation energy decreased as the ironcontent increased at the initial degradation stage, indicatinga reduced thermal stability by the presence of iron particles in themagnetic foams. However, the activation energy of PU-55decreased monotonously with increasing the degree of conversionwhile MF25-55 and MF50-55 exhibited a weaker dependence of Ea

on the degree of conversion than PU-55. This result might suggestthat the iron particles weakened the PU matrix at the early stage ofdegradation but tended to stabilize the partially degraded matrix atthe later stage of degradation by forming the metal–polymercomplexes in the presence of oxygen.

4. Conclusions

The thermal degradation properties of the magnetic PU foams,which contained 0–75 wt% iron particles, were investigated usingthe thermogravimetric method with nitrogen and air as the furnaceatmosphere, respectively. The differential thermogravimetriccurves proved that the weight loss rates of the magnetic foamswere slower than the pure PU foam. At the early stage of degra-dation (for example, �10% weight loss), the carbonyl iron particleswere found to be able to improve the thermal stability of themagnetic foams in nitrogen, but to deteriorate the thermal stabilityin air. The effects of heating rate on the thermal degradation wereexamined. The apparent activation energy of degradation wascalculated using Ozawa’s method. The calculated apparent activa-tion energy varied with the degree of conversion or weight loss inboth nitrogen and air. At the initial degradation stage, the magneticfoams showed the higher activation energy in nitrogen but thelower activation energy in air. At the later degradation stage, themagnetic foams exhibited the higher activation energy in air.

Acknowledgements

This work was supported by a joint project between NanyangTechnological University and Singapore Technologies Kinetics. Theauthors thank Momentive Performance Materials, Singapore, forkindly providing the surfactants and catalysts.

References

[1] Ozawa T. A new method of analyzing thermogravimetric data. Bulletin of theChemical Society of Japan 1965;38:1881–6.

[2] Lokander M, Stenberg B. Performance of isotropic magnetorheological rubbermaterials. Polymer Testing 2003;22:245–51.

[3] Rabinow J. The magnetic fluid clutch. AIEE Transactions 1948;67:1308–15.[4] Hua Y, Wang YL, Gong XL, Gong XQ, Zhang XZ, Jiang WQ, et al. New magne-

torheological elastomers based on polyurethane/Si–rubber hybrid. PolymerTesting 2005;24:324–9.

[5] Lokander M, Stenberg B. Improving the magnetorheological effect in isotropicmagnetorheological rubber materials. Polymer Testing 2003;22:677–80.

[6] Petrovic ZS, Zavarge SZ, Flynn JH, Macknight WJ. Thermal degradation ofsegmented polyurethanes. Journal of Applied Polymer Science 1994;51:1087–95.

[7] Munoz BC, Jolly MR. Composites with field responsive rheology. In: Brostow W,editor. Performance of plastics. Munich: Carl Hanser Verlag; 2001. p. 553–74.

[8] Woolley WD, Fardell P. Basic aspects of combustion toxicology. Fire SafetyJournal 1982;3(2):29–48.

[9] Herpol C, Vandevelde P. Journal of Combinatorial Toxicology 1982;9:165.

J. Zhang et al. / Polymer Degradation and Stability 94 (2009) 246–252252

[10] Ballistreri A, Foti S, Maravigna P, Montaudo G, Scamporrino E. Mechanism ofthermal degradation of polyurethanes investigated by direct pyrolysis in themass spectrometer. Journal of Polymer Science: Polymer Chemistry Edition1980;18(6):1923–31.

[11] Grassie N, Zulfiqar M. Thermal degradation of the polyurethane from 1,4-butanediol and methylene bis(4-phenyl isocyanate). Journal of PolymerScience: Polymer Chemistry Edition 1978;16(7):1563–74.

[12] Jellinek HHG, Dunkle SR. Kinetics and mechanism of HCN-evolution from theoxidation of polyurethanes. Journal of Polymer Science: Polymer ChemistryEdition 1983;21:487–511.

[13] Wlodarczak Daniela. Studies of temperature and atmosphere compositioninfluence on thermal degradation products of polyurethane foam. Journal ofApplied Polymer Science 1988;36:377–86.

[14] Zhang J, Li L, Chen G, Jiang SP. Synthesis of a new type of magnetic foams frompolyurethane and carbonyl iron, submitted for publication.

[15] Song YM, Chen WC, Yu TL, Linliu K, Tseng YH. Effect of isocyanate on thecrystallinity and thermal stability of polyurethanes. Journal of Applied Poly-mer Science 1996;62(5):827–34.

[16] Wang TL, Hsieh TH. Effect of polyol structure and molecular weight on thethermal stability of segmented poly(urethaneureas). Polymer Degradation andStability 1997;55:95–102.

[17] Tannenbaum R, Flenniken CL, Goldberg EP. Magnetic metal–polymercomposites: thermal and oxidative decomposition of Fe(CO)5 and Co2 (CO)8 ina poly (vinylidene fluoride) matrix. Journal of Polymer Science: PolymerPhysics 1990;28:2421–33.

[18] Kalnin MM, Malers YY. Relationship between kinetic parameters of ther-mooxidation and the variation of adhesion in the polymeric boundary layerduring high-temperature adhesive interaction of polyethylene with steel.Polymer Science U.S.S.R. 1985;27(4):890–7.

[19] Day M, Cooney JD, MacKinnon M. Degradation of contaminated plastics:a kinetic study. Polymer Degradation and Stability 1995;48:341–9.

[20] Regnier N, Guibe C. Methodology for multistage degradation of polyimidepolymer. Polymer Degradation and Stability 1997;55(2):165–72.

[21] Flynn JH, Wall LA. Polymer Letters 1966;4:323.[22] Flynn JH. In: Shalaby SW, editor. Thermal methods in polymer analysis. Phil-

adelphia: The Franklin Institute Press; 1978. p. 163–86.