ELSEVIER

Polymer Degrodntion and Stability 61 (1998) 289-295 0 1998 Elsevier Science Limited. All rights resewed

Printed in Great Britain PII: SO141-3910(97)00211-5 0141-3910/98/S--see front matter

Thermogravimetric evaluation of the kinetics of decomposition of polyolefin blends

Carmen Albano* & Eunice de Freitas Universidad Central de Venezuela, Fact&ad a’e Ingenieria, Escuela de Ingenieria Quimica, PO Box 50656, Caracas 1040, Venezuela

(Received 4 December 1996; revised version received 29 July 1997; accepted 18 August 1997)

The thermodegradative behavior of polypropylene (PP), of non-degraded and degraded high density polyethylene (HDPE) and their blends was studied, in order to analyze their thermal stability. Van-Krevelen (V-K), Coats-Redfem (C-R) and Horowitz-Metzger (H-M) integral methods as well as the Freeman-Carroll (F-C) differential approach were used to determine kinetic parameters. The acti- vation energy (E,) obtained for PP mixed with non-degraded HDPE (5 to 50%) is lower than the E, corresponding to pure polymers and does not depend on the HDPE concentration. Blends of degraded materials show an approximate value of E, of 200 kJ/mol for H-M, 160 KJ/mol for V-K and 150 KJ/mol for C-R, for mixtures with concentrations by weight of HDPE up to 20%, but this value decreases drastically with higher concentrations. 0 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

Almost all polymers have decomposition reactions in any stage of their life cycle, i.e. during manu- facturing, storing, processing, or final usage of the product. Factors related to polymers, to produc- tion and manufacturing processes, and the envir- onment are among the possible causes of decomposition. It can therefore be said that the strongest degrading agents are light, heat and the atmosphere. The action of these factors depends on the characteristics of the polymers in ques- tion.

In the case of polyblends, there are additional factors, such as those relating to the compatibility and structure of the composites, that affect the process of decomposition. In their studies of PEO/ PMMA blends, Calahorra and Cortazar’ found that the behavior of polymer degradation can be dramatically modified by the presence of a second polymer. They concluded that there may be interactive reactions competing with the processes of decomposition during the degradation of a binary blend.

*To whom correspondence should be addressed. E-mail: cal- bano@serverl.ucv.edu.ve

289

Most recycled polymer is degraded due to the action of environmental agents on the finished product. Therefore, it is necessary to fully examine the thermal degradation of the blends in the pre- sence of a recycled polymer in order to determine the thermal stability of the polymers and their blends.

The main goal of the present research is to study the thermal stability of PP blends with degraded and non-degraded HDPE in order to analyze the behavior of this recycled polymer.

2 THEORETICAL BACKGROUND

In the kinetic studies of these thermal systems, the mass-temperature relation, which can predict the behavior of the degradation reactions, must be examined. The purpose of these studies is to eval- uate the kinetic parameters through the course of the degradation, also allowing an understanding of the decomposition mechanisms and of the rela- tionship of the data obtained in the process to other interesting properties.

In kinetic studies of decomposition, the con- centration of the reactants and the products is one of the main factors. Reactions are considered

290 C. Albano, E. de Freitas

irreversible, and the speed depends on the activa- tion energy and order of the reaction. The para- meters can be calculated from a dynamic experimental curve.

When we combine the reaction rate and the Arrehenius expression, we have the following basic expression:

da dt =f(a)A exp

where T is the temperature (K), dar/dt is the rate of reaction (t-‘),S( a is a function of 01 depending on ) the reaction mechanism, a is the reacted fraction at the time t, E is the activation energy (J/mol), A is the pre-exponential factor and R is the gas con- stant.

With this basic expression, we can develop dif- ferent methods, in order to evaluate the reaction order, the activation energy and a possible decomposition mechanism. The differential method of Freeman and Carroll (F-C)* is fre- quently used for the determination of kinetic parameters:

AlnR, -=- EA (8 AlnW, RAlnW, +’ (2)

where Rt = da/dt is the thermogravimetric velocity (%/K), W, = (1 - a), and n is the reaction order.

The integral method of Coats and Redfern is also used. When n has a value different from 1, the expression is:

(3)

where B is the rate of heating (degree/time). When n = 1, the expression becomes:

log(-l~yy)) = log$(, -3 -& (4)

Van-Krevelen4 using the same methodology as Coats and Redfern, obtained, for n # 1:

where T,, is the temperature at the maximum rate

(Q When n = 1, the expression becomes:

ln(ln(1 - a)) =

(6)

The Horowitz and Metzger method5 is a simplifi- cation of that of Coats and Redfern. Based on eqns (3) and (4), a deviation of characteristic tempera- ture is defined as 8 = T - Ts. where T is tempera- ture at any time (K), and Ts = the TGA differential curve peak temperature.

For n # 1 the following expression is obtained:

ln(l(1 - (r)lTn) = -$$+ ln(1 -n) (7) s

When n = 1, eqn (7) becomes:

ln(-ln(1 -a)) = $ s

(8)

In studying the thermal degradation of syndiotac- tic polystyrene, Ravanetti and Zini6 obtained an activation energy of 209-251 kJ/mol using both integral (Van-Krevelen, Coats-Redfern) and dif- ferential (Freeman-Carroll) methods.

Heide et al.’ developed a method similar to Freeman-Carroll’s to analyze the reaction mechanism. Consider that f(a) is a function dependent on the real reaction mechanism.

The expression is then the following:

A(ln($)) - NW@)) E A$

A(ln(1 -a)) = z RA(ln(1 - a))

The possible reaction mechanisms (Criado et are presented in Table 1.

(9)

a1.8)

Thermogravimetric evaluation of the kinetics of &composition of polyolejin blencis

Table 1. Kinetkqh d for the aWru&ion of the simulated decomposition reactibas(8)

291

Mechanisms f(a) Rate controlling process

Rl 1 Zero-order mechanism

R2 (1 - a)1’2 Phase-boundary reaction (cylindrical symmetry)

R3 (1 - a)2’3 Phase-boundary reaction (spherical symmetry)

Fl (1 - 4 Random nucleation

An n(1 -a)[-ln(1 -a)]‘-“n Random nucleation and growth of nuclei (Avrami-Erefeev equation)

Dl 1% One-dimensional diffusion

D2 I [-ln(l-;;ll Two-dimensional diffusion

D3 &!$& Three dimensional diffusion (Jander equation)

D4 1

2[(1-4’3-I] Three dimensional diffusion (Ginstein equation)

The degradation of polymers is a complex phenomenon, involving many simple reactions that are difficult to analyze separately. The quantitative contributions of these reactions to the degradation process are impossible to evaluate. For this reason, though the global mechanism does not agree with the real mechanisms, it is useful as an indicator in quantifying the global rate of the reaction, in acquiring further knowledge of the structure of polymers, and in analyzing the behavior of the fin- ished product.

3 EXPERIMENTAL

PP (Pro-Fax 6431) and degraded and non-degra- ded HDPE (Altaven 6200B) polymers were mixed in an extruder at 5, 10, 20, 30, 40 and 50% con- centrations by weight of PP. Later, thermograms were obtained in the TGA under the following conditions: the samples were heated up to a tem- perature of 773K at a rate of lO”/min, in a nitrogen amosphere. Subsequently, the blend was kinetically examined using the different integral and differ- ential methods in order to determine the global energy of activation and the global order of reac- tion.

4 RESULTS AND DISCUSSION

The analysis of the kinetics of the degradation of PP, degraded and non-degraded HDPE, and their mixtures, is based on thermograms obtained in the TGA (Figs 1 and 2).

For all mixtures studied, residues of less than 0.3 mg were obtained, based on a sample weight of between 9 and 10 mg. Therefore, it can be concluded

that nearly all of the material subjected to degra- dation was transformed into volatile material.

The results obtained regarding the initial tem- perature of the decomposition process (Figs 1 and 2), show that the introduction of HDPE results in the degradation beginning at slightly lower tem- peratures. This may indicate that in PP blends with degraded and non-degraded HDPE, there could be

Cj)t -_)~%HDPE,~C--~IO%H~PE,~C-~~~%~~~E

@t--i 3O%HDPE,@ k---+W%HDPE,@ +-]pP,~( -+HDPE

Fig. 1. Non isothermal TG curves of PP with HDPE non degraded blends.

292 C. Albano, E. de Freitas

10 am 4m t.&lm PC L uo 700 v

~(---~~%HDPE,@&--~~~%HOPE,@H~~%~D~~

@ (---) ZO%tWE,@ t--W% HDPE ,@ t---i 50% HDPE

@e---I pp, @H HDPE

Fig. 2. Non isothermal TG curves of PP with HDPE degraded blends.

an initial induction period which is slightly lower than if the polymers were not mixtures.

The values of the initial temperature of decom- position are very important, since they could indicate which would be the processing and manufacturing temperatures without continuing or initiating a process of decomposition, especially in blends of recycled material.

The different values of activation energy and overall order of reaction obtained through the sev- eral methods used in pure polymers and in PP and non-degraded and degraded HDPE are shown in Tables 2 and 3. The methods used in the experi- ments were those of Horowitz-Metzger(H-M), Van-Krevelen(V-K), Coats-Redfern(C-R) and Freeman-Carroll(FC).

As far as the activation energy of the pure poly- mers is concerned (Table 2), PP shows values that, in the case of the integral method, vary between 196 and 259kJ/mol and, in the case of F-C, of 121 kJ/mol. Through the F-C method Gambiroza and Cunko9 obtained a result of 96 kJ/mol for that

Table 2. Global activation enemy (&) for the degradation of PP, degraded (d) and non-degraded (ml) HDPE and their blends

(values between parentheses are theoretical)

HDPE (%)

Ond 5nd 1Ond 20nd 30nd 50nd 1OOnd 5d 10d 20d 30d 40d 5Od 1OOd

H-M V-K C-R F-c (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol)

259 221 196 121 188(264) 1 SO(226) 142(201) 113(121) 196(264) 175(230) 159(205) 150(125) 188(268) 150(238) 133(217) 104(125) 201(276) 180(247) 163(226) 129(129) 201(284) 170(263) 154(242) 163(133)

309 301 288 146 196(264) 163(226) 142(201) 108(121) 196(264) 167(230) 146(205) 108(129) 188(272) 159(238) 142(217) 121(133) 113(276) 92(247) 66(226) 121(142) 117(280) 92(255) 71(234) 83(150) 121(288) lOO(263) 75(242) 87(154)

318 305 288 188

polymer. Mucha and Kryszewskil” also determined the activation energy for PP using other methods, as used by Flynn” and Ozawa and Kate,‘* and the values varied between 213 and 255 kJ/mol. As can be seen, this range is very similar to that obtained in the present paper through integral methods.

The thermal decomposition of PP takes place via the action of free radicals, and the final product depends on the predominant radical present in the first stage of decomposition. The presence of ter- tiary carbons in the polymer is a major factor contributing to the rupture of chains through the /I-scission mechanism.

Table 2 also shows that the values of activation energy identified by the integral method for the decomposition of non-degraded and degraded HDPE are very similar: 288-3 18 kJ/mol. On the

Table 3. Global reaction order (m) for the degradation of PP, degraded (d) and ma-degraded (INI) HDPE and their blends

HDPE H-M V-K C-R F-C W) (adm) (adm) (adm) (adm)

0 1.3 1.1 1.0 0.2 5nd 1.1 0.8 1.0 0.2 1Ond 1.0 1.0 1.0 0.5 20nd 1.4 1.0 1.0 0.7 30nd 1.0 1.0 1.0 0.5 50nd 1.8 1.0 1.0 0.7 1OOnd 2.2 2.2 2.2 0.7 5d 1.3 1.0 1.0 0.4 10d 1.1 1.0 1.0 0.3 20d 1.0 0.9 0.9 0.2 30d 0.9 0.8 0.7 0.7 40d 0.7 0.5 0.5 0.9 50d 0.0 0.0 0.0 0.2 1OOd 2.5 2.5 2.5 1.1

Thermogravimetric evaluation of the kinetics of &composition of polyolejin blends 293

other hand, when the differential method was applied, the values were 146-188 kJ/mol, respec- tively. From the similarity obtained through the integral methods, it can be deduced that prior degradation has not significantly affected the ther- mal stability of the material. Mucha and Krys- zewski’O determined the activation energy for HDPE, and obtained values fluctuating between 192-301 kJ/mol. These results agree with the find- ings obtained in our research. The small differences found can be attributed to the fact that the activa- tion value depends on the degree of branching and on the molecular weight of the HDPE, as was shown by Zamora13 in his studies of polyethylenes with different degrees of branching.

As far as the order of reaction is concerned (Table 3), the results show that the order for PP has lower values than the values of degraded and non-degraded HDPE. This could be the result of the different decomposition mechanisms of the polymers studied.

It is worthwhile to point out that polyolefins show two forms of thermal decomposition: one having the typical characteristics of solid-gas reactions that occur via free radicals, and the other a chain reaction. The former reactions are affected by the diffusivity of the volatile reaction products, and the latter by the mechanism of chain reaction. This leads to the conclusion that the decomposition takes place via a very complex mechanism, since each reaction has its own activa- tion energy. Therefore, the results obtained show the effective activation energy of the real mechan- ism of decomposition.

Table 2 presents the values of the activation energies of PP with non-degraded and degraded HDPE polyblends. The activation energies (Ea) obtained through the integral methods are lower, in the case of the blends, than the Ea correspond- ing to non-degraded polymers and, in the case of no&degraded HDPE, do not depend on HDPE concentration. In PP blends with degraded HDPE, the activation energy of degraded HDPE, in a 5 to 20% concentration, is in a range of 167 to 196 kJ/ mol, and falls abruptly with an increase in the concentration of the recycled material. This beha- vior is not observed with the Freeman-Carroll method, which could be due to the assumptions made by that method.

Analysis of the parameter relating to &, (theore- tical E,) additivity obtained from the activation energy of corresponding homopolymers is useful. The equation used is Donatelli’s:14 Eat., = c EaiXi

where Eai is the activation energy corresponding to the homopolymers and Xi the weight fraction of the homopolymers.

When the activation energy of a blend is higher than the additivity behavior (theoretical E,) of the E, corresponding to the homopolymers (Table 2), there has been a certain stabilization of the mate- rial. In the studied blends, the activation energy values are lower than the E, obtained by additivity and than the E, of the pure components. This confirms that one of the polymers speeds up the process of decomposition of the other. This accel- erating action could even have a catalytic char- acter. This characteristic is far more noticeable in the degraded PP/HDPE blends, when the con- centration of this recycled material is greater than 20%.

Although non-degraded and degraded HDPE show an activation energy higher than the E, of PP, it speeds up the process of decomposition in the blends. The effect is stronger in the degraded PP/HDPE blends.

In the case of blends with non-degraded HDPE, the order of reaction obtained by the V-K and C-R methods (Table 3), is maintained at an almost constant level in all of the compositions. In addi- tion, the order of reaction obtained in the different compositions is always lower than the order of the non-degraded HDPE. Therefore, this parameter is affected by the addition of the polymer.

In contrast, in PP/degraded HDPE blends (Table 3), the order of reaction, except for the values obtained by the F-C method, is maintained at almost one up to an 80% PP concentration, and subsequently drops suddenly. This is probably due to abrupt changes in the mechanisms of decom- position.

Correlation factors and the intercepts are pre- sented in Table 4, applying the mechanisms pro- posed by the Criado method. Table 5, which reports the different global mechanisms that could be adapted to the experimental results, is based on these results. PP and unused HDPE were found to have the best correlation with a controlled reaction mechanism in the interface with spherical sym- metry (R3). This leads one to think that commer- cial polyolefins, which have additives, induce a similar mechanism in both PP and HDPE. Given the result obtained, the degradation could be in the interface between the amorphous part and the crystalline part.

Table 5 shows that the degraded HDPE adapted better to the mechanism of random nucleation, the

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Thermogravimetric evaluation of the kinetics of decomposition of polyolejin blends 295

Table 5. Reaction medmisms with bigbest correlations for PP, degraded (d) and non-degraded (nd) HDPE, and their Meads

% HDPE Mechanisms

Ond Snd 1Ond 20nd 30nd 50nd 1OOnd 5d 10d 20d 30d 40d 50d 1OOd

Phase-boundary reaction (spherical symmetry) Zero orden mechanism

One dimensional diffusion Phase-boundary reaction (cylindrical symmetry)

Random nucleation and growth of nuclei (Avrami-Erofeev equation) Zero-order mechanism, phase-boundary reaction (cylindrical and spherical symmetry)

Phase-boundary reaction (spherical symmetry) Random nucleation, phase-boundary reaction (spherical and cylindrical symmetry) Random nucleation, phase-boundary reaction (spherical and cylindrical symmetry) Random nucleation, phase-boundary reaction (spherical and cylindrical symmetry)

phase-boundary reaction (cylindrical symmetry) Zero-order mechanism

Phase-boundary reaction (spherical and cylindrical symmetry) Random nucleation and growth of nuclei (Avrami-Erofeev equation)

order n (A,), which is due to the initial process of Based upon these facts, it can be said that it is decomposition in which nuclei that control the possible to recycle recycled HDPE at up to 20% degradation process may be formed. concentrations, in PP blends.

The different mechanisms identified in explaining the process of decomposition of blends of PP with degraded and non-degraded HDPE do not permit the selection of one particular probable mechanism for all compositions. This is due to the numerous reactions involved in the degradation of these blends. It is probable that the process was effected simultaneously by different mechanisms or that when temperature increased, the decomposition mechanism changed. For this last reason, it would be useful to carry out analyses at different tem- peratures in order to find the most probable mechanism for each temperature range.

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The findings lead to the following conclusions:

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In PP blends with non-degraded HDPE, the activation energy as well as the order of reac- tion do not depend on HDPE concentration. In PP blends with degraded HDPE, one of the components acts as an accelerator of decom- position process at HDPE concentrations higher than 20%.

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