POLYMERTESTING

Polymer Testing 26 (2007) 526–536

ARTICLE IN PRESS

0142-9418/$ - se

doi:10.1016/j.po

�CorrespondiE-mail addre

www.elsevier.com/locate/polytest

Material Behaviour

Kinetics of non-isothermal degradation of nanocompositesbased on functionalized polyethylenes

C. Marazzatoa, Y. Penevab, E. Lefterovac, S. Filippia, L. Minkovab,�

aDipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universita degli Studi di Pisa,

Via Diotisavi. 2, 56126, Pisa, ItalybInstitute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bontchev Street bl.103A Sofia 1113, Bulgaria

cInstitute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G. Bontchev Street bl. 10, Sofia 1113, Bulgaria

Received 1 December 2006; accepted 12 January 2007

Abstract

Polymer/organoclay nanocomposites based on an ethylene–acrylic acid copolymer (EAA), a high-density polyethylene

(HDPE) grafted with maleic anhydride (HDMA) and HDPE/HDMA blends with 80/20 and 50/50wt/wt composition have

been studied by thermogravimetry in nitrogen and air atmospheres. The following parameters have been determined from

the thermogravimetric (TG) and differential thermogravimetric (DTG) curves of the samples: T10, T50, Tmax, the degree of

conversion at 450 1C and the residue. The study of the non-isothermal degradation kinetics of the neat polymers and their

nanocomposites has allowed the determination of the activation energy of degradation, the rate of degradation and the

reaction order. The analysis of all parameters has led to the conclusion that in the presence of clay, the thermal stability in

nitrogen atmosphere decreases, while the thermal stability in air increases. Moreover, the effect of clay on the degradation

behavior of the polymer matrix is more pronounced for the nanocomposites having fully exfoliated structure than that of

the nanocomposites containing intercalated clay stacks. The results have been interpreted by the better dispersion of the

clay layers in the exfoliated nanocomposites, leading to a better barrier effect for the oxygen flow in air and to an increase

in the number of clay catalytic sites in nitrogen.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Functionalized polyethylenes/organoclay nanocomposites; Thermogravimetry; Kinetics of non-isothermal degradation;

Thermal stability

1. Introduction

Two of the most attractive features of polymer/clay nanocomposites is their good thermal stabilityin air and their fire retardancy. Recently, severalpapers have been published dealing with the thermal

e front matter r 2007 Elsevier Ltd. All rights reserved

lymertesting.2007.01.006

ng author. Tel.: +359 2 9793668.

ss: lminkova@gmx.net (L. Minkova).

and thermo-oxidative degradation of differentpolymer/clay nanocomposites.

It has been shown that an organically modifiedmontmorillonite does not improve the thermalstability of low-density polyethylene (LDPE) in aninert atmosphere, while in air the clay inhibitspolymer degradation by improving the barrier tothe diffusion of oxygen through the material [1].Similar results have been obtained for other poly-olefins/clay nanocomposites [2–4], demonstrating

.

ARTICLE IN PRESSC. Marazzato et al. / Polymer Testing 26 (2007) 526–536 527

that the nanocomposite formation has almost noeffect on the thermal degradation of the materialunder nitrogen. However, an increase in the thermaldecomposition temperature in nitrogen for maleicanhydride grafted PE in the presence of an organi-cally modified montmorillonite has been reported [5].Conversely, a reduction in degradation temperaturewith the addition of the organoclay has beenobserved in nitrogen for PE copolymers/clay nano-composites [6]. An increase in peak degradationtemperature has been detected under oxidativeconditions for the same materials [6].

The thermal degradation in nitrogen has beenstudied for nanocomposites based on polystyrene(PS), acrylonitrile–butadiene–styrene terpolymer,polypropylene and PE, and an enhancement of theonset temperatures relative to that of the virginpolymers has been established [7]. Conversely, theonset temperature of thermal degradation in nitro-gen was not found to increase for PS/clay andpoly(methyl methacrylate)/clay nanocomposites [8].The investigation of the thermal degradation ofethylene–vinyl acetate copolymer (EVA)/clay nano-composites has shown that the process is acceleratedin nitrogen, while in air a significant delay of weightloss could be observed [9–11]. Nanocomposites ofpoly(o-caprolactone)/clay were shown to degrade inair at a higher temperature compared to the neatpolymer [12]. This beneficial effect has beenexplained by a decrease in the diffusion of oxygenand volatile products throughout the compositematerial [12].

The studies of the degradation kinetics ofnanocomposites are relatively scarce. The kineticanalysis of the thermo-oxidative degradation ofnanocomposites based on linear low-density poly-ethylene (LLDPE) has been performed using theOzawa–Flynn–Wall method [13]. The obtainedkinetic parameters have confirmed that the thermalstability of the nanocomposites is enhanced incomparison to that of the neat LLDPE [13]. Thestudy of the degradation kinetics according to theOpfermann method of PS/clay nanocomposites hasshown that the thermal stability of PS is signifi-cantly increased in the presence of the clay underboth pyrolytic and thermo-oxidative conditions[14]. The invariant kinetic parameters method hasbeen used to model the thermal degradation innitrogen and air atmospheres of polyamide-6/claynanocomposites [15]. The activation energy ofdegradation barely changes in nitrogen, while inair, the activation energy of nanocomposites is

higher than that of the neat polyamide-6 [15]. Thedynamic heating rate mode of high-resolutionthermogravimetry has been applied for the studyof the degradation kinetics of epoxy/montmorillo-nite nanocomposites [16].

In previous papers, the preparation and char-acterization of novel nanocomposites based onfunctionalized polyethylenes has been reported[17–19]. Ethylene–acrylic acid copolymers (EAAs)(Escor 5000, 5001 and 5100, by Exxon) havingdifferent concentration of acrylic acid (from 6 to11wt%) and different molar mass, as well as Znionomer of Escor 5100, have been used as matricesfor the nanocomposites preparation [17,18]. High-density maleic acid (HDMA) (Polybond 3009) andits blends with high-density polyethylene (HDPE)have also been proved for nanocomposites prepara-tion [19]. The results from X-ray diffraction (XRD)measurements and transmission electron micro-scopy (TEM) observations have shown that EAAsform mixed intercalated and exfoliated structureswith the organoclay [17,18], while in the nanocom-posites based on HDMA fully exfoliated structuresmay be observed [19]. In the nanocomposites basedon blends of HDPE/HDMA, the degree of exfolia-tion/intercalation depends strongly on blend com-position [19].

In the present paper, the nanocomposites basedon the EAA and HDMA functionalized polyethy-lenes, as well as on HDPE/HDMA blends with 80/20 and 50/50wt/wt composition, have been studiedby thermogravimetry. The aim was to evaluate theinfluence of the degree of intercalation/exfoliationon degradation kinetics and on the thermal stabilityof the materials under nitrogen and air.

2. Experimental

2.1. Materials

The functionalized polyethylenes used as matriceswere: (i) an ethylene–acrylic acid copolymer with11wt% acrylic acid and MFI of 8 g/10min (Escor5100, produced by Exxon) (EAA), (ii) a maleicanhydride grafted HDPE with 1wt% MA and MFIof 5 g/10min (Polybond 3009, produced by Cromp-ton Corporation) (HDMA). The HDPE (Eltex 404,produced by Solvay) had MFI ¼ 3.5 g/10min andr ¼ 944 kg/m3.

The organically modified montmorillonite wasCloisite15A (Cl15A) provided by Southern ClayProducts. The organic modifier in Cl15A was

ARTICLE IN PRESS

1 2 3 4 5 6 7 8 9 10 11

7

6

5

4

3

2

1

Inte

nsity, a.u

.

2Θ, degree

Fig. 1. X-ray patterns of the clay and the polymer nanocompo-

sites. Curve 1—Cloisite 15A; curve 2—EAA+5% clay; curve 3—

EAA+10% clay; curve 4—HDMA+5% clay; curve 5—

HDMA+10% clay; curve 6—HDPE/HDMA 50/50+5% clay;

curve 7—HDPE/HDMA 80/20+5% clay.

0 100 200 300 400 500 600 700 800 900 100050

60

70

80

90

100

110

120

Weig

ht lo

ss, %

Temperature,°C

nitrogenair

0 100 200 300 400 500 600 700 800 900 1000

0.0

0.1

0.2

0.3

0.4

Derivative w

eig

ht, %

/°C

Temperature,°C

nitrogenair

Fig. 2. TG and DTG curves of the clay in nitrogen and air

atmosphere.

C. Marazzato et al. / Polymer Testing 26 (2007) 526–536528

M2(HT)2 (dimethyl, dihydrogenatedtallow, quater-nary ammonium) and its concentration was 125meq/100 g clay.

All polymers were dried under vacuum for 12 h ata temperature below their melting point.

2.2. Composite preparation

The nanocomposites were prepared by meltcompounding in a Brabender Plasticorder staticmixer. The components were placed in the 50mlmixing bowl and blended for 2.5min at 30 rpm andabout 7.5min at 60 rpm in a nitrogen atmosphere at150 1C. The following nanocomposites were pre-pared: EAA+5 and 10wt% Cl15A, HDMA+5and 10wt% Cl15A, HDPE/HDMA 80/20+5wt%Cl15A, HDPE/HDMA 50/50+5wt% Cl15A. Theneat polymers were subjected to the same treatment.

2.3. Thermogravimetric measurements

The thermogravimetric measurements of thenanocomposites and neat polymers were made ona TGA Q500 apparatus in nitrogen and air. Theheating rate was 5 1C/min. The weights of thesamples were about 5mg. The activation energyand the order of the reaction were determined by theinflection point method [20].

3. Results and discussion

The XRD pattern of Cl15A displays a reflectionat 2yffi2.81, which corresponds to a d001 spacing ofabout 3.10 nm [17–19]. After melt blending withfunctionalized polyethylenes, the position of the(0 0 1) peak shifts to a lower angles for EAA andHDPE/HDMA 80/20, while for HDMA andHDPE/HDMA 50/50 the peak disappears (Fig. 1)[17–19]. The XRD results, together with the TEMmicrographs (not shown here), demonstrated thatthe EAA-based nanocomposites possess mixedintercalated and exfoliated structure, those with anHDPE/HDMA 80/20 matrix are mainly interca-lated, while the ones with HDMA and HDPE/HDMA 50/50 matrixes are predominantly exfo-liated [17–19].

The thermogravimetric (TG) and differentialthermogravimetric (DTG) curves under nitrogenand air of neat Cl15A are presented in Fig. 2. It maybe seen that the temperatures of maximal degrada-tion rate (Tmax) are about 300 1C in both atmo-spheres. At 400 1C, the thermal degradation process

ARTICLE IN PRESS

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100W

eig

ht,

%

Temperature, °C

EAAEAA + 5% clay

EAA + 10% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Derivative w

eig

ht, %

.min

-1

Temperature, °C

EAAEAA + 5% clay

EAA + 10% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100

Weig

ht,

%

Temperature, °C

HDMA

HDMA + 5% clay

HDMA + 10%clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

1

2

3

4

5

Derivative w

eig

ht, %

.min

-1

Temperature, °C

HDMA

HDMA + 5%clay

HDMA + 10% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

HDPE/HDMA 50/50

HDPE/HDMA 50/50 + 5% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

1

2

3

4

5

6

Derivative w

eig

ht, %

.min

-1

Temperature, °C

HDPE/HDMA 50/50

HDPE/HDMA 50/50 + 5% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

HDPE/HDMA 80/20

HDPE/HDMA 80/20 + 5% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

1

2

3

4

5

Derivative w

eig

ht, %

.min

-1

Temperature, °C

HDPE/HDMA 80/20HDPE/HDMA80/20 + 5% clay

Fig. 3. TG and DTG curves of the neat polymers and blends and their nanocomposites in nitrogen atmosphere.

C. Marazzato et al. / Polymer Testing 26 (2007) 526–536 529

ARTICLE IN PRESSC. Marazzato et al. / Polymer Testing 26 (2007) 526–536530

is almost finished. This result is in agreement withthe study of Vaia et al. [21], showing that thedegradation of organic-modified montmorillonitesin nitrogen proceeds between 200 and 400 1C. Theresidue is about 60wt% in nitrogen and 70% in airup to about 550 1C, while at higher temperatures(up to about 950 1C) thermo-oxidative degradationtakes place in air and leads to further reduction ofthe residue to about 57wt%.

3.1. Thermal degradation of the nanocomposites in

nitrogen

The TG and DTG curves of all samples arepresented in Fig. 3. It may be observed that thedegradation process of the neat polymer matrixesEAA, HDMA and HDPE/HDMA blends, as wellas that of their nanocomposites, proceeds as a one-step process. These results are at variance with thosein Ref. [7], where the TG curves of PE nanocompo-sites showed two degradation steps, while the neatpolymer showed only one step.

The TG and DTG curves have been used torecord (Table 1) the temperatures corresponding to10% and 50% weight loss (T10 and T50, respec-tively), the temperature of the maximal degradationrate determined from the DTG peak (Tmax), thedegree of conversion at 450 1C (a450), the weight%of the residue. All these parameters have been takenas a relative measure for the thermal stability of thematerials [22]. T10 of the samples varies between 385and 440 1C for the neat polymers and theirnanocomposites. This means that the degradationprocess of the nanocomposites practically starts(and proceeds) after the organoclay degradation isover. The T10 values of the samples are plotted inFig. 4. As can be seen, these values decrease in the

Table 1

Thermogravimetric parameters of the samples in nitrogen atmosphere

Sample T10 (1C) T50 (1C) Tmax (1C)

EAA 419.3 449.6 453.7

EAA+5% clay 393.5 441.5 449.5

EAA+10% clay 385.4 432.1 443.2

HDMA 439.1 463.5 469.5

HDMA+5%clay 422.8 440.2 442.5

HDMA+10%clay 418.0 436.7 438.6

HDPE/HDMA 50/50 440.2 464.8 470.6

HDPE/HDMA 50/50+5% clay 419.3 438.0 441.5

HDPE/HDMA 80/20 433.2 463.6 468.3

HDPE/HDMA 80/20+5% clay 440.2 460.0 462.4

presence of clay, with the only exception of theHDPE/HDMA 80/20 blend. Concerning the valuesof T50 and Tmax, they also decrease in the presenceof clay, even for the nanocomposite with HDPE/HDMA 80/20 (Fig. 4). Moreover, it can be notedthat the decrease of T10, T50 and Tmax in thepresence of clay is slightly more pronounced for thesamples with higher exfoliation level (HDMA andHDPE/HDMA 50/50). The degree of conversion at450 1C (a450) increases strongly upon addition of5wt% clay for the samples with high exfoliationlevel, and moderately when the clay content isincreased to 10wt%. For the nanocomposites withmixed intercalated and exfoliated morphology theincrease of a450 is more gradual, whereas it ispractically zero for the composite with HDPE/HDMA 80/20 (Fig. 4, Table 1). These results showthat the thermal stability of the materials decreasesin the presence of the clay and that the level of claydispersion influences the thermal degradation pro-cess. It should be mentioned that the residue of theneat polymers is negligible (0.2–1.1wt%). Theresidue of the nanocomposites almost correspondsto the inorganic content of the clay in the sample.

The activation energy E (kJ/mol) was determinedaccording to the inflection point method. Themethod is based on the Arrhenius equation:

da=dT ¼ Ab�1f ðaÞ exp �E=RT� �

,

where a is the degree of conversion, A the pre-exponential factor, b the heating rate, E the activa-tion energy, R the gas constant, T the temperaturein K, and f(a) ¼ (1�a)n, where n is the order of thereaction.

According to the inflection point method, theactivation energy is

E ¼ nRT2m da=dT� �

m1� amð Þ

�1,

a450 (wt%) E (kJ/mol) Kd (%/min) Residue (wt%)

54 271.7 3.7 0.2

70 259.8 3.9 3.7

88 256.7 3.9 6.0

24 267.0 4.2 1.0

86 234.1 5.3 3.4

93 233.0 5.4 6.2

21 292.6 3.5 1.1

90 259.2 4.2 5.4

24 283.4 4.9 0.8

23 271.8 4.8 3.4

ARTICLE IN PRESS

Fig. 4. The dependence of T10, T50, Tmax, degree of conversion at 450 1C, activation energy and the rate of degradation on the content of

the clay in nitrogen atmosphere.

C. Marazzato et al. / Polymer Testing 26 (2007) 526–536 531

ARTICLE IN PRESS

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100W

eig

ht,

wt.

%

Temperatute, °C

EAA

EAA + 5% clay

EAA + 10% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Derivative w

eig

ht,

%.m

in-1

Temperature, °C

EAA

EAA + 5% clay

EAA + 10% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100

Weig

ht, w

t. %

Temperature, °C

HDMA

HDMA + 5%clay

HDMA + 10%clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Derivative w

eig

ht, %

.min

-1

Temperature, °C

HDMA

HDMA + 5% clay

HDMA + 10% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

HDPE/HDMA 50/50

HDPE/HDMA 50/50 + 5%clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

1.0

Derivative w

eig

ht, %

.min

-1

Temperature, °C

HDPE/HDMA 50/50

HDPE/HDMA 50/50 + 5% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0

20

40

60

80

100

Weig

ht, %

Temperature, °C

HDPE/HDMA 80/20

HDPE/HDMA 80/20 + 5% clay

0 50 100 150 200 250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

1.0

Derivative w

eig

ht, %

.min

-1

Temperature, °C

HDPE/HDMA 80/20

HDPE/HDMA 80/20 + 5% clay

Fig. 5. TG and DTG curves of the neat polymers and blends and their nanocomposites in air atmosphere.

C. Marazzato et al. / Polymer Testing 26 (2007) 526–536532

ARTICLE IN PRESSC. Marazzato et al. / Polymer Testing 26 (2007) 526–536 533

where m is the index characterizing the temperaturevalue at which a da/dT maximum exists, i.e. d2a/dT2¼ 0.

n is determined according to the equation ofDoyle [20]:

n1=1�n ¼ 1� amð Þ.

The values for the activation energy of thesamples are presented in Fig. 4 and Table 1. Theresultant activation energy of the neat polymers is ofthe same order as those cited in Ref. [23]. Theactivation energy decreases in the presence of clay.The decrease is more pronounced for HDMA andthe blend HDPE/HDMA 50/50, while for EAA andthe blend HDPE/HDMA 80/20 this decrease issmaller. The rate of degradation Kd, determined asthe percent mass loss per minute from the linearpart of the TG curves, shows the opposite depen-dence (Fig. 4, Table 1). The unfilled samplesHDMA and HDPE/HDMA 50/50 undergo degra-dation at a lower rate than the filled samples. Thesamples EAA and HDPE/HDMA 80/20 degrade atan almost equal rate in the absence and the presenceof the filler. The order of reaction varies between 1.0and 1.3. More precise determination of the mechan-ism of the degradation reaction will be performedin the future in our laboratories using differentcooling rates.

The analysis of the kinetic parameters of the non-isothermal degradation of the samples in nitrogenconfirms that the unfilled samples are more stablethermally than the filled ones. The decrease of thethermal stability is more pronounced for thesamples with higher exfoliation level.

The results lead to the assumption that theresidue remaining after the thermal degradation ofthe clay influences the degradation rate and thethermal stability of the polymer matrix. It has been

Table 2

Thermogravimetric parameters of the samples in air atmosphere

Sample T10 (1C) T50 (1C) Tmax (1C)

EAA 379.5 431.4 429.7

EAA+5% clay 374.3 439.9 439.9

EAA+10% clay 322.4 447.0 447.0

HDMA 386.4 439.9 441.8

HDMA+5%clay 395.1 467.6 476.4

HDMA+10%clay 381.2 468.5 476.4

HDPE/HDMA 50/50 336.2 407.2 408.9

HDPE/HDMA 50/50+5% clay 381.2 448.6 448.6

HDPE/HDMA 80/20 310.3 412.4 429.7

HDPE/HDMA 80/20+5% clay 334.5 407.2 407.2

shown that by the degradation of the clay aprotonated silicate has been formed, which in turncatalyses the degradation of the polymer [6,9].Evidently, the dispersion of the residue, in respectof the active catalytic sites in the polymer matrix,influences the polymer degradation. The betterdispersion of the residue in the exfoliated nano-composites leads to an increase in the number of thecatalytic sites, which in turn results in a greaterdecrease of the thermal stability of these materialsthan for the intercalated nanocomposites.

3.2. Thermo-oxidative degradation of the

nanocomposites in air

The TG and DTG curves of the samples arepresented in Fig. 5. The values of T10, T50, Tmax anda450 are collected in Table 2 and shown graphicallyin Fig. 6. The degradation process proceeds in onestep, as demonstrated by the monomodal DTGpeaks. It may be seen that the T10 values of thesamples are between 310 and 395 1C, while the T50

values fall in the range 407–468 1C. This means thatthe thermo-oxidative degradation of the samplesbegins and proceeds at lower temperatures, com-pared with the thermal degradation in nitrogen. Itshould be noted that, despite this, the values of T10

of all samples are still higher that Tmax of the neatclay. Thus, the thermo-oxidative degradation of thenanocomposites proceeds mainly in the presence offully degraded clay.

All the nanocomposites, except the one based onHDPE/HDMA 80/20, have T50 and Tmax higherthan the neat polymers (Table 2, Fig. 6). This meansthat the thermostability of the materials increases inthe presence of clay for the samples havingexfoliated structure. For the samples possessingintercalated structure, the thermostability remains

a450 (wt%) E (kJ/mol) Kd (%/min) Residue (wt%)

66 167.2 0.73 1.1

60 170.5 0.72 4.5

53 175.4 0.51 14.5

60 170.4 1.30 0.2

36 210.5 0.73 3.3

40 215.3 0.51 7.6

80 188.1 1.20 0.7

51 210.5 0.98 3.7

77 190.5 1.60 0.1

79 196.7 0.92 3.4

ARTICLE IN PRESS

EAA

EAA + 5% clay

EAA + 10% clay --HDMA

HDMA + 5% clay

HDMA + 10% clay --

HDPE/HDMA 50/50

HDPE/HDMA 50/50+5%clay --

HDPE/HDMA 80/20

HDPE/HDMA 80/20+5%clay

200

250

300

350

400

450

500

T10

, °C

EAA

EAA+5%clay

EAA+10%clay --HDMA

HDMA+5%clay

HDMA+10%clay --

HDPE/HDMA 50/50

HDPE/HDMA 50/50 +5%clay --

HDPE/HDMA 80/20

HDPE/HDMA 80/20+5%clay

200

250

300

350

400

450

500

T50,

°C

EAA

EAA + 5% clay

EAA + 10% clay --HDMA

HDMA + 5% clay

HDMA + 10% clay --

HDPE/HDMA 50/50

HDPE/HDMA 50/50+5%clay --

HDPE/HDMA 80/20

HDPE/HDMA 80/20+5%clay

200

250

300

350

400

450

500

Tm

ax,

°C

EAA

EAA + 5% clay

EAA + 10% clay --HDMA

HDMA + 5% clay

HDMA + 10% clay --

HDPE/HDMA 50/50

HDPE/HDMA 50/50+5%clay --

HDPE/HDMA 80/20

HDPE/HDMA 80/20+5%clay

0

10

20

30

40

50

60

70

80

90

100

De

gre

e o

f co

nve

rsio

n a

t 4

50

EAA

EAA+5%clay

EAA+10%clay --HDMA

HAMA+5%clay

HDMA+10%clay --

HDPE/HDMA 50/50

HDPE/HDMA 50/50+5%clay --

HDPE/HDMA 80/20

HDPE/HDMA 80/20+5%clay

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Kd,

%/m

in

EAA

EAA+5%clay

EAA+10%clay --HDMA

HDMA+5%clay

HDMA+10%clay --

HDPE/HDMA 50/50

HDPE/HDMA 50/50 +5%clay --

HDPE/HDMA 80/20

HDPE/HDMA 80/20+5%clay

0

50

100

150

200

250

300

E,

kJ/m

ol

Fig. 6. The dependence of T10, T50, Tmax, degree of conversion at 450 1C, activation energy and the rate of degradation on the content of

the clay in air atmosphere.

C. Marazzato et al. / Polymer Testing 26 (2007) 526–536534

ARTICLE IN PRESSC. Marazzato et al. / Polymer Testing 26 (2007) 526–536 535

almost the same or increases slightly. This has beenconfirmed by the dependence of the degree ofconversion at 450 1C on the clay content. It maybe seen that, for the samples with HDMA andHDPE/HDMA 50/50, a450 decreases more stronglycompared to those with EAA; for the sample withHDPE/HDMA 80/20, this value remains almost thesame (Table 2, Fig. 6). It should be noted that allparameters reveal that the thermostability ofHDPE/HDMA 80/20 barely changes in the pre-sence of clay. This sample has intercalated structurebut probably it comprises some non-intercalatedsilicate layers stacks as well. Concerning T10, itincreases upon clay addition for HDPE/HDMA50/50 and 80/20, while for HDMA it remains almostthe same, and for EAA even decreases slightly. Thelatter could be due to the degradation process ofthe clay itself. It should be noted that, for most ofthe investigated materials, the amount of residuealmost corresponds to the inorganic content of theclay in the sample. The only exception is the sampleEAA+10% clay, whose residue was higher thanexpected.

The activation energy E was determined by theinflection point method. The obtained values for theneat polymers (Table 2, Fig. 6) are in agreementwith those published in the literature for thethermo-oxidative degradation of polyethylene inair [24]. The activation energy for the filled samplesincreases with an increase of the clay loading(Fig. 6). The rate of degradation shows the oppositetrend. It should be mentioned that the effects aremore pronounced for the nanocomposites withhigher exfoliation level. The order of the reactionvaries between 0.8 and 1.

The results show that the thermal stability in airatmosphere increases in the presence of the clay;moreover, the increase is more pronounced for thenanocomposites with higher level of exfoliation.The increase of the thermal stability can beexplained by the improved barrier to diffusion ofoxygen into the material [1,6]. As the degradation ofthe clay proceeds in the solid polymer matrixes, itcould be assumed that the dispersion of the resultingresidue reproduces the dispersion of the clay itself.The better dispersion of the residue in the exfoliatednanocomposites leads to improved barrier proper-ties; on the contrary, the residue of clay degradationin the intercalated stacks cannot provide sufficientbarrier to hinder the diffusion of oxygen. The resultsare in agreement with those for epoxy/montmor-illonite nanocomposites [16].

4. Conclusions

The investigation of the non-isothermal degrada-tion kinetics of nanocomposites based on functio-nalized polyethylenes in nitrogen and airatmosphere leads to the following conclusions: theaddition of the clay to the matrix polymers results ina decrease of the thermal stability in nitrogenatmosphere. This is explained by the creation ofactive catalytic sites by the degradation of the clayitself. The decrease in the thermal stability for thenanocomposites having higher exfoliation level ismore pronounced than that for nanocompositescontaining intercalated clay stacks. On the contrary,the thermal stability of the nanocomposites in-creases in comparison to that of the neat matrixes inair. This result has been interpreted by the barriereffect of the clay residue on the oxygen flow into thesample. The increase of thermal stability is moresignificant for the nanocomposites with exfoliatedstructure. In nitrogen, the exfoliated nanocompo-sites have more active catalytic sites than theintercalated ones, which is due to the betterdispersion of the former. In air, the exfoliated claylayers ensure a better barrier effect.

Acknowledgments

The research was carried out within the PRIN2004 098403 project funded by the Italian Ministryof Education, University and Research (MIUR).The cooperation among the Institutions was alsosupported by NATO (Grant CBP.EAP.CLG981257). The authors thank Prof. P.L. Magagninifrom the University of Pisa, Italy for the usefuldiscussion.

References

[1] J. Morawiec, A. Pawlak, M. Slouf, A. Galeski, E.

Piorkowska, N. Krasnikowa, Eur. Polym. J. 41 (2005) 1115.

[2] J. Zhang, C.A. Wilkie, Polym. Degrad. Stabil. 80 (2003) 163.

[3] S. Su, D. Jiang, C.A. Wilkie, Polym. Degrad. Stabil. 83

(2004) 321.

[4] M. Zanetti, P. Bracco, L. Costa, Polym. Degrad. Stabil. 85

(2004) 657.

[5] H. Zhai, W. Xu, H. Guo, Z. Zhou, S. Shen, Q. Song, Eur.

Polym. J. 40 (2004) 2539.

[6] C.M.L. Preston, G. Amarasinghe, J.L. Hopewell, R.A.

Shanks, Z. Mathys, Polym. Degrad. Stabil. 84 (2004) 533.

[7] X. Zheng, C.A. Wilkie, Polym. Degrad. Stabil. 82 (2003)

441.

[8] D. Wang, J. Zhu, Q. Yao, C.A. Wilkie, Chem. Mater. 14

(2002) 3837.

ARTICLE IN PRESSC. Marazzato et al. / Polymer Testing 26 (2007) 526–536536

[9] M. Zanetti, G. Camino, R. Thomann, R. Mulhaupt,

Polymer 42 (2001) 4501.

[10] T.R. Hull, D. Price, Y. Liu, C.L. Wills, J. Brady, Polym.

Degrad. Stabil. 82 (2003) 365.

[11] W. Zhang, D. Chen, Q. Zhao, Y. Fang, Polymer 44 (2003)

7953.

[12] B. Lepoittevin, M. Devalckenaere, N. Pantoustier, M.

Alexandre, D. Kubies, C. Calberg, R. Jerome, P. Dubois,

Polymer 43 (2002) 4017.

[13] L. Qiu, W. Chen, B. Qu, Polymer 46 (2005) 1.

[14] S. Bourbigot, J.W. Gilman, C.A. Wilkie, Polym. Degrad.

Stabil. 84 (2004) 483.

[15] F. Dabrowski, S. Bourbigot, R. Delobel, M. Le Brais, Eur.

Polym. J. 36 (2000) 273.

[16] A. Gu, G. Liang, Polym. Degrad. Stabil. 80 (2003) 383.

[17] P. Magagnini, S. Filippi, C. Marazzato, F.P. La Mantia, L.I.

Minkova, e-Polymers 2005, no. 086.

[18] S. Filippi, C. Marazzato, P. Magagnini, L. Minkova, N.

Tzankova Dintcheva, F.P. La Mantia, Macromol. Mater.

Eng. 291 (2006) 1208.

[19] C. Marazzato, S. Filippi, P. Magagnini, Y. Peneva, N.

Dintcheva, F.P. La Mantia XVII Convegno Italiano di

Scienza e Tecnologia delle Macromolecole, Napoli, Settem-

bre 2005.

[20] H.H. Horowitz, G. Metzger, Analyt. Chem. 35 (1963)

1464.

[21] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia

Chem. Mater. 13 (2001) 2979.

[22] W. Wendlandt (Ed.), Thermal Methods of Analysis, Wiley,

New York, London, Sydney, Toronto, 1978.

[23] D.V. Van Krevelen (Ed.), Properties of Polymers Correla-

tions with Chemical Structure, Khimia, Moscow, 1976.

[24] L. Novakovic, O. Gal, V. Markovic, T. Stanett, Radiat.

Phys. Chem. 26 (1985) 331.