Study on thermal decomposition of calix[4]arene and its application in thermal stability of polypropylene

Article abstract of DOI:10.1016/j.tca.2010.12.012

Thermal decomposition kinetics of calix[4]arene (C4) was investigated using thermogravimetric analysis (TGA) and derivative of TG curve (DTG). TG experiments were carried out under static air atmosphere with nominal heating rates of 1.0, 2.5, 5.0 and 10.0 K/min. Model-fitting methods and model-free methods such as Friedman and Ozawa-Flynn-Wall methods were employed to evaluate the kinetic parameters such as activation energy (E_a), exponential factor (ln A) and reaction order (n). To determine the antioxidant property of C4 the non-isothermal kinetics of polypropylene (PP) with C4 as additive was studied. The FTIR, ESR and ¹³C NMR CP-MAS techniques were used to propose the decomposition mechanism of C4 in the presence of PP.

Full text of DOI:10.1016/j.tca.2010.12.012

Thermochimica Acta 515 (2011) 24–31
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Study on thermal decomposition of calix[4]arene and its application in thermal
stability of polypropylene
K. Chennakesavulu∗, M. Raviathul Basariya, P. Sreedevi, G. Bhaskar Raju,
S. Prabhakar, S. Subba Rao
National Metallurgical Laboratory Madras Centre, CSIR Complex, Taramani, Chennai 600113, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermal decomposition kinetics of calix[4]arene (C4) was investigated using thermogravimetric analysis
(TGA) and derivative of TG curve (DTG). TG experiments were carried out under static air atmosphere with
nominal heating rates of 1.0, 2.5, 5.0 and 10.0 K/min. Model-fitting methods and model-free methods such
as Friedman and Ozawa–Flynn–Wall methods were employed to evaluate the kinetic parameters such
as activation energy (Ea), exponential factor (ln A) and reaction order (n). To determine the antioxidant
property of C4 the non-isothermal kinetics of polypropylene (PP) with C4 as additive was studied. The
FTIR, ESR and 13C NMR CP-MAS techniques were used to propose the decomposition mechanism of C4 in
the presence of PP.
Received 25 June 2010
Received in revised form
10 November 2010
Accepted 16 December 2010
Available online 29 December 2010
Keywords:
Calix[4]arene
Thermal decomposition
Activation energy
Friedman analysis
Model-fitting methods
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
due to the higher melting points of calix[n]arene and lack of exper-
imental data concerning polyolefins-calixarene additive systems.
Calixarenes are cyclic oligomers (general formulae: C7nH6nOn)
which are derived by the condensation of phenol–formaldehyde
in alkali media and are capable of forming complexes with metal
ions, anions and neutral molecules [1]. It is reported that in USA
alone, nearly 20,000 tonnes of antioxidants were consumed dur-
ing 1983 for the stabilization of plastics, it clearly reveals the
importance of antioxidants in polymer industry [2]. In last two
decades, high molecular weight calix[n]arenes have been of major
industrial interest because of their low mobility in polymeric mate-
rials like polyolefins. It is also reported that calix[n]arenes are
inhibitors of polyolefins auto-oxidation [3]. The antioxidant activ-
ity of the calixarenes was studied by chemiluminescence methods
in thermal stabililization of polyethylenes and this proves that
the calixarenes are good antioxidant additives for polyethylenes
[4]. Feng et al. reported that the thermal stability effect of p-tert-
butylcalix[n]arene on ␥-radiation degradation of polypropylene
was due to the suppression of PP chain scission by calix[n]arene
[5]. The thermal stability of the calixarenes needs to be established
Though the inclusion behavior of calixarenes with different sol-
vents [6,7] was studied with TGA at different atmospheres, to the
best of our knowledge this is the first thermo-kinetic study reported
on thermal degradation of C4. In the present study the evaluated
kinetic parameters of C4 suggests its applicability as an antioxidant
for PP.
2. Experimental
2.1. Materials
The analytical grade chemicals p-tert-butyl phenol, polypropy-
lene, formaldehyde, para-formaldehyde, AlCl3 and HPLC grade
solvents such as chloroform, toluene, xylene and methanol were
purchased from Merk speciality chemicals. HPLC grade solvents are
used as received without further purification.
2.2. Synthesis of calix[4]arene
5, 11, 17, 23-Tetra-t-butyl-35, 36, 37, 38-tetra hydroxy
calix[4]arene was synthesized as earlier reported [8]. The obtained
p-tert-butyl calix[4]arene was further de-tert-butylated with
AlCl3/toluene as described in the literature to obtain 25, 26, 27,
28-tetrahydroxycalix[4]arene [9].
∗
Corresponding author at: National Metallurgical Laboratory Madras Centre, CSIR
Complex, Taramani, Chennai, Tamil Nadu 600113, India. Tel.: +91 44 2254 2077;
fax: +91 44 2254 1027.
E-mail addresses: chennanml@yahoo.com, chenna velpumadugu@yahoo.com
(K. Chennakesavulu).
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.12.012

K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
25
2.3. Characterization of 25, 26, 27, 28-Tetrahydroxycalix[4]arene
(C4)
2.7. Theoretical background
The kinetic analysis of solid state thermal decomposition [12,13]
was governed by a single step kinetic equation (1).
Melting point: 587–589 K; LCMS-ESI (M−1)+: 423; elemental
analysis: molecular formulae C28H24O4, calculated (C, 79.22%), (H,
5.70%), found (C, 78.87%), (H, 5.12%); 1H NMR (500 MHz, pyridine-
d5) ı (ppm): 9.7 (s, 4H, –OH), 7.1 (d, 8H, Ar–H), 6.6 (t, 4H, Ar–H), 4.1
(bs, 8H, Ar–CH2–); 13C NMR (500 MHz, pyridine-d5) ı (ppm): 150.3,
129.1, 121.4, 116, 31.7; DEPT-NMR (500 MHz, pyridine-d5) ı (ppm):
149.9, 129.1, 121.4, 116 (aromatic ring –CH), 31.7 (bridged methy-
lene –CH2–); 13C NMR CP-MAS (300 MHz, pyridine-d5) ı (ppm):
151.6, 129, 122, 120, 31.6.
d˛
= k(T)f (˛)
(1)
dt
where ˛ is the conversion of the reaction, d˛/dt is the Rate of
conversion, f(˛) is the reaction model and k(T) is the temperature
dependent rate constant
The fractional conversion ˛ is expressed as (m0 − mt)/(m0 − mf),
where m0 and mf are the initial and final weights of the sample,
respectively, and mt is the weight of the sample at time t. The
explicit temperature dependence of the rate constant is described
according to the Arrhenius equation (2).
ꢀ
ꢁ
Ea
2.4. Characterization
k(T) = A exp
−
(2)
RT
1H NMR, 13C NMR and DEPT-NMR were obtained by using
Bruker Avance-500 MHz NMR spectrometer. Solid state 13C NMR
cross polarisation magic angle spinning spectrum (13C NMR
CP-MAS) was obtained by using Bruker Avance 300 MHz NMR spec-
trometer. The samples were taken in 2.5 mm diameter of zircon
rotors and spun at 7 kHz. The elemental analysis was done with
Euro Vectors Elemental analyzer. HPLC and LCMS-ESI analysis were
done with Schimadzu and Agilent instruments respectively. X-
band ESR spectra was recorded on a Bruker-EMX spectrometer, at
room temperature with a modulation frequency of 100 kHz and
microwave power set at 3.2 mW. The g values were measured by
taking DPPH as an external reference. The error in the reported
g values is of the order of 0.0002. The FTIR spectrum of C4 and
thermally treated C4 samples are recorded at room temperature
by Perkin Elmer Instrument.
By combining Eqs. (1) and (2) the following is obtained.
ꢀ
ꢁ
d˛
dt
−Ea
= A exp
f (˛)
(3)
RT
where A is the pre-exponential factor (rate constant at infinite tem-
perature), R is the gas constant, Ea is the apparent activation energy
and T is the absolute temperature.
According to non-isothermal kinetic theory, the fractional con-
version ˛ can be expressed as a function of temperature, which is
dependent on the time of heating. The explicit time dependence of
Eq. (3) can be eliminated through the trivial transformation
ꢀ
ꢁ
d˛
dT
1
ˇ
−Ea
=
A exp
f (˛)
(4)
RT
where ˇ = dT/dt is the heating rate
Kinetic parameter calculations based on TGA data can be
obtained from Eqs. (3) and (4). In the present study, two different
approaches namely model-free methods [14–16] and model-fitting
methods [17] were used to calculate the kinetic parameters.
The kinetic parameters obtained using non-isothermal data was
observed to be advantageous over conventional isothermal studies
[18].
2.5. Sample preparation
Generally calix[n]arene forms stable inclusion complexes with
volatile organic guest molecules. In order to remove the traces
of solvents in C4 cavity, prior to the TGA experiments the sam-
ple was degassed at 473 K under vacuum (10−5 Torr) for 48 h. The
mixture of polypropylene and C4 are taken in an agate bowl and
ground well with agate pestle results. High specific surface area of
the calixarenes will facilitate to contact with maximum amount of
polypropylene [10].
2.7.1. Model-free or isoconversional method
To gain more information regarding the decomposition mech-
anism, model-free methods which are recommended as
a
trustworthy were adopted for kinetic analysis. The basic assump-
tion of these methods is that the reaction model is independent of
the heating rate [19].
The Ea values calculated by this method are independent of any
implicit reaction model, which allows one to explore multi step
kinetics [20,21]. The most common model-free methods such as
Friedman [14] and Ozawa–Flynn–Wall [15,16] were employed for
the thermo-kinetic analysis of C4 and PP.
2.6. Thermogravimetric analysis
TGA/DTG experiments were performed with Versa Therm Cahn
thermo balance TG-151 with sensitivity of 10 ␮g. Thermal exper-
iments were conducted in the temperature range of 300–1100 K
with samples weighing around 20 0.01 mg. The analyses were
carried out at four nominal heating rates (1.0, 2.5, 5.0 and
10.0 K/min) under static air atmosphere. Generally, the efficiency
of antioxidants in polymers is determined by three processes: the
chemical reactivity, physical loss and a minimal effective concen-
tration [11]. The physical loss of antioxidants may enhance under
flowing nitrogen or any other atmosphere. In order to reduce the
physical loss (evaporation, leaching and blooming) of antioxidant
the static air atmosphere was preferred in the present study. DTA
was carried out using Netzsch 409C model with 5.0 K/min heating
rate. The Netzsch thermo kinetics software was used for the kinetic
analysis of experimental data.
2.7.1.1. Friedman method. The method [22] based on the inter
comparison of the rate of conversion d˛/dt for a given degree of con-
version ˛ determined using dynamic heating rate ˇ is considered
as the most general derivative technique.
After taking logarithms of Eq. (4) and rearranging, we obtain
ꢀ
ꢁ
d˛
dT
Ea
ln
ˇ
= ln[Af (˛)] −
(5)
RT
The Ea values over a wide range of conversions were obtained
by plotting ln(ˇ d˛/dT) versus 1/T for a constant ˛ value.
2.7.1.2. Ozawa–Flynn–Wall method. This method [15,16] is an inte-
gral method which can determine the Ea without the knowledge of
reaction order for given values of conversion.

26
K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
Table 1
Algebraic expression for f(˛) and g(˛) for the various kinetic reaction models.
S.No.
Reaction model
Code
f(˛)
g(˛)
1
2
3
4
5
6
7
8
9
10
11
12
13
First order
Second order
Third order
Avrami Erofeev equation
Avrami Erofeev equation
Avrami Erofeev equation
Prout–Tomkins equation
1D diffusion
F1
F2
F3
1 − ˛
−ln(1 − ˛)
(1 − ˛)2
(1 − ˛)−1 − 1
0.5(1 − ˛)−2 − 1
[−ln(1 − ˛)]1/2
[−ln(1 − ˛)]1/3
[−ln(1 − ˛)]1/n
ln[˛/(1 − ˛)]
(1 − ˛)3
A2
A3
An
B1
D1
D2
D3
D4
R2
R3
2(1 − ˛)[−ln(1 − ˛)]1/2
3(1 − ˛)[−ln(1 − ˛)]2/3
n(1 − ˛)[−ln(1 − ˛)](1−1/n)
˛(1 − ˛)
2
1/2˛
˛
2D diffusion
[−ln(1 − ˛)]−1
(3/2)(1 − ˛)2/3[1 − (1 − ˛)1/3
(3/2)[(1 − ˛)−1/3−1]−1
2(1 − ˛)1/2
˛ + (1 − ˛) ln(1 − ˛)
−1
2
Jander’s equation
Ginstling–Brounstein
2D phase boundary
3D phase boundary
]
[1 − (1 − ˛)1/3
]
1 − 2˛/3 − (1 − ˛)2/3
1 − (1 − ˛)1/2
3(1 − ˛)2/3
1 − (1 − ˛)1/3
Integrating Eq. (4) and rearranging, we obtain
as independent, competing and parallel reactions. Each model with
selected reaction types gives quite reliable and consistent kinetic
parameters. The reaction model may take various forms based on
nucleation and nucleus growth, phase boundary reaction, diffu-
sion and chemical reaction [22–24]. The unknown parameters were
found from the fitting of measured data with the simulated curves
for the given model and reaction types. The different kinetic reac-
tion models [12,13] used for the analysis to identify the best one
are represented in Table 1.
AEa
0.4567Ea
log ˇ = log
− 2.314 −
(6)
(7)
g(˛)R
RT
where
ꢂ
˛
d˛
g(˛) =
f (˛)
0
By plotting log ˇ against 1/T at certain conversion rate, the slope
−Ea/R leads to Ea.
3. Results and discussion
2.7.2. Model-fitting method
3.1. Thermal decomposition of C4
The model-fitting kinetic analysis depends on the reaction type
and reaction model. The reaction models used for analysis include
one to six step processes, in which the individual steps are linked
The TGA/DTG curve of C4 is shown in Fig. 1a. The thermal decom-
position of C4 occurred in the range of temperature 600–950 K with
a total mass loss of 98%. It is interesting to note that C4 follows the
two stage decomposition. The first stage decomposition occurred
in the range of 600–750 K with mass loss of 40%. At this stage (i.e.
750 K) the samples are quenched and subjected for various instru-
mental techniques to determine the degradation of pathway for C4.
Fig. 1. (a) TGA/DTG curves C4 (heating rate: 10.0 K/min) and (b) DTA curve of C4
(heating rate: 5.0 K/min, here upward peaks corresponds to endothermic peaks).
Fig. 2. (a) Friedman analysis of C4 (heating rates: 1.0, 2.5, 5.0 and 10.0 K/min). (b)
Ea vs. ˛ plot (here the ln A values are evaluated by using F1 kinetic model).

K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
27
Table 2
Kinetic triplets of C4 for A → B → C reaction model with non-linear regression.
Model A → B
Ea1 (kJ/mol)
ln A1 (1/s)
Model B → C
Ea2 (kJ/mol)
ln A2 (1/s)
r2
F1
F1
F2
Fn
67
62
106
97
41
25
2.2
1.7
6.1
5.2
0.3
1.1
0.6
23
5.8
6.9
8.3
7.0
3.2
3.3
F1
D1
F2
119
108
197
112
54
3.9
4.8
10.2
4.1
0.2
1.5
1.1
9.0
3.4
3.7
4.6
3.5
2.6
2.6
0.9954
0.9999
0.9916
0.9906
0.9012
0.8965
0.9013
0.9236
0.9998
0.9975
0.9983
0.9983
0.9983
0.9984
Fn
A2
A3
An
Bna
D1
D2
D3
D4
R2
R3
A2
A3
An
Bna
D1
D2
D3
D4
R2
R3
29
23
30
288
105
119
141
127
78
187
105
113
135
119
94
80
97
The second stage decomposition occurred in the range of 750–950 K
with mass loss of 45%. The loss of inclusioned methanol molecules
in C4 [25] is not seen in the present case as the sample was
degassed prior to TGA. The DTA curve of C4 is shown in Fig. 1b. An
endothermic peak was observed around 590 K indicating the melt-
ing of the C4 and an endothermic peak between 700 K and 850 K
indicates the cleavage of methylene group from phenolic arene
groups.
3.2. The kinetics of decomposition of C4
The Friedman analysis for the thermal decomposition reaction
of C4 and its corresponding energy plots is shown in Fig. 2. These
plots show good separation of the two consecutive reactions i.e.
A → B → C with some scatter of points at the beginning and end of
the plots. As the thermal decomposition proceeds, the Ea remains
constant and reaches a maximum value for at ˛ 0.3–0.6. This
increase in Ea implies the depolymerisation reaction and results the
open chain polyphenols. Usually, the methylene bridge between
two phenolic rings is very stable. The phenolic –OH activates the
breakage of methylene linkage (depolymerisation reaction) due to
keto-enol tautomerism [26]. On further decomposition Ea remains
constant. Thus it can be concluded that the Ea1 of the first reac-
tion step is lower than that of the Ea2. The differential Friedman Ea
plot leads to apparent Ea1 of 60 kJ/mol, Ea2 of 110 kJ/mol and ln A1
of 1.5/s, ln A1 of 5/s respectively. In case of model-fitting approach
the non-linear regression models were used for the calculation of
kinetic parameters. The most probable kinetic model was analyzed
by choosing best fit based on the value of the correlation coeffi-
cient (r2) which was close to one. The Ea, ln A and r2 for non-linear
regression methods of C4 are shown in Table 2. Among the tested
F1
D1
Fig. 3. Fit of TG measurements of C4, simulated with reaction model. A−→ B−→ C
(here r2 = 0.9999).
F1
D1
models, the C4 obeyed the reaction model A−→ B−→ C. Fig. 3 shows
the fitted curves for non-linear regression of the TGA kinetic data
for a two step degradation of C4. It can be concluded that the calcu-
lated Ea values from model-free and model-fitting methods were
close to each other. Therefore, the determined kinetic triplets are
reasonable and can be applied to arrive at the best kinetic models
for C4.
3.3. ESR analysis
The ESR spectra of TGA quenched solid calix[4]arene at 745 K
are shown in Fig. 4. The ESR spectrum shows a symmetrical singlet,
this might probably due to the formation of isotropic free radicals.
The g factor was measured to be 2.0045. The g-value is depends
on the orientation of crystal and radical situated in a crystal with
respect to applied magnetic field. The free radicals of water solu-
ble calix[n]arenes do not show their hyperfine interaction, as the
Fig. 4. ESR spectrum of TC4 at 745 K.
formed radical is in the centre of –OH groups of calix[n]arenes cav-
ity [27]. The quenched sample was kept unanalyzed for one week,
but the ESR signal still appeared, proving the stability of the formed
free radicals.

28
K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
Fig. 5. 13C NMR CP-MASS of TC4 at various temperatures.
3.4. 13C NMR CP-MAS analysis
saturated carbon was modified with the temperature and thus the
formation of new species from C4.
The 13C NMR CP-MAS of C4 at room temperature and quenched
samples at various temperatures are shown in Fig. 5. It appears that
at room temperature, the aromatic carbons (1–4, which are num-
bered in Fig. 5b) are observed from 151 ppm to 120 ppm whereas
signals corresponding to bridged methylene carbons (5 in Fig. 5b)
are observed at 31 ppm. The signals corresponding to aromatic car-
bons at various temperatures was found to broaden drastically in
the case of quenched calix[4]arene. The broadened aromatic car-
bon signal obtained may be due to the formation of stable phenoxy
free radical and delocalization of free electron throughout the arene
ring. Similar broader signals were observed by Qin et al. due to the
formation of free radicals within the apocynum fiber in its 13C NMR
CP-MAS [28]. Finally the bridged methylene signal was shifted and
almost disappeared at 860 K. This indicates the environment of the
3.5. FTIR analysis
The FTIR of C4, thermally treated C4 (TC4) quenched at 750 K
during TG measurement are recorded at room temperature and
shown in Fig. 6. All the observed bands of C4 are well matches
with the literature elsewhere [29–31]. The bands at 3541, 3232
and 3152 cm−1 refers to –OH stretching vibration of C4. This indi-
cates the strong hydrogen bond of rigid C4. In the case of TC4, the
phenolic broad band shifts to 3434 cm−1 indicating the presence
of free phenolic groups (i.e. open chain poly phenols formed from
C4 which is free from strong hydrogen bonding). The C4 bands at
2933 and 2868 cm−1 corresponds to asymmetric and symmetric
–CH stretching of methylene groups respectively. But in TC4 the
corresponding –CH stretching vibration bands are shifted to 2921
and 2853 cm−1 respectively. This clearly suggests the breakage of
C4 cavity at bridged methylene with phenolic groups. The bands
of C4 at 1607 and 1593 cm−1 corresponds to aromatic ring –C C–
stretching vibrations. In the case of TC4 these bands appears as a
high intensity broad singlet 1580 cm−1. This indicates formation
of C–O. in C4 cavity [32]. The band of C4 at 1440–1470 cm−1 cor-
responds to vibrations of aromatic –C C–H and –CH2– bending
vibrations. In the case of TC4, the corresponding bands are appeared
Table 3
Kinetic triplets of PP for various reaction models.
Model
Ea (kJ/mol)
ln A (1/s)
r2
F1
F2
Fn
175
157
158
135
137
184
174
182
194
213
200
145
147
12.2
10.2
10.4
7.8
0.9980
0.9999
0.9999 (n = 2.0)
0.9920
0.9847
0.9913 (n = 0.7)
0.9823 (n = 1.0)
0.9951
0.9960
0.9968
A2
A3
An
Bna
D1
D2
D3
D4
R2
R3
8.0
11.6
11.0
11.1
11.9
12.9
11.7
8.2
0.9908
0.9867
0.9874
8.2
Fig. 6. FTIR analysis of C4.

K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
29
Fig. 7. (a) TGA/DTG of polypropylene (heating rate: 1.0 K/min). (b) Friedman Ea plot of PP. (c) TGA of PP + C4. (d) Friedman Ea plot of PP + 2% C4.
as a broad band around 1450 cm−1. This indicates the aromatic ring
vibration was obscured by –CH2– vibration [33]. The band of C4 at
1250 cm−1 refers asymmetric stretch of C–C–OH stretching vibra-
tion. In the case of TC4 the corresponding band was observed at
1190 cm−1 indicates the formation of C–O species. At RT the high
intensity bands of C4 at 838 and 750 cm−1 corresponds to –C–H out
of plane bending vibrations. In the case of TC4 the corresponding
bands are shifted to 812 and 736 cm−1 respectively. This confirms
the formation of ortho and para substituted benzene products.
≈90 kJ/mol for nitrogen atmosphere and air atmosphere respec-
tively [35]. The effect of atmospheric oxygen was the reason for
variation of Ea. The free radicals generated during thermal decom-
position can react with oxygen and results propagation reaction
with newlyoxidizedfreeradicals formation maycause thelowering
of Ea values. In present study, based on the correlation coefficient
the decomposition of PP obeys the F2 reaction model. TGA of PP
was done in the presence of various weight percentages of C4, the
results are presented in Fig. 7c. Thermo oxidative degradation of
PP was reduced with increase in C4 weight percentage. The crit-
ical stabilizer concentration of C4 was yet to be determined for
thermo oxidative degradation of PP. In the presence of C4, the sin-
gle step TG curve of PP becomes multi step decomposition. It is
evident that during decomposition of PP, the C4 is exhibiting its
antioxidant activity. Generally, the efficiency of antioxidant capac-
ity depends on the effect induced by the phenolic groups, the spatial
protections of double bond by delocalization of ␲-electrons and the
interaction of oxygen atoms of OH units and the trapped radicals
[5]. The non-isothermal kinetic analysis of PP + 2.0% of C4 mixture
was done based on the TGA data collected at heating rates of 1.0,
2.5, 5.0 and 10.0 K/min. The high density polyethylene and low den-
sity polyethylenes can be thermally stabilized by the addition of
0.2–1.0% of calix[n]arenes [4]. Friedman Ea plots of PP and PP + 2.0%
C4 mixture are shown in Fig. 7b and d. The Ea plot of PP + 2.0%
3.6. Thermal decomposition kinetics of PP with C4
The TGA/DTG of PP at 1.0 K/min is shown in Fig. 7a. It shows
that the degradation of polypropylene occurs in a single step in
the temperature range of 600–900 K. In order to obtain the kinetic
triplets of PP and PP + 2% C4 the non-isothermal TGA measurements
were done at 1.0, 2.5, 5.0 and 10.0 K/min. From Fig. 7b the calcu-
lated Ea and ln A of PP from Friedman analysis were 158 kJ/mol and
10.3/s respectively, whereas Ozawa–Flynn–Wall analysis results
in 160 kJ/mol of Ea and 10.5/s of ln A. The corresponding kinetic
parameters of PP through model-fitting method are shown in
Table 3. Aboulkas et al. reported that the Ea of 207 kJ/mol, ln A of
31/s and first order for decomposition of PP under nitrogen environ-
ment [34]. Peterson et al. reported that the Ea of ≈200 kJ/mol and

30
K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
Fig. 8. Antioxidant activity of C4 during thermal decomposition of PP.
C4 reveals the decomposition follows complex reactions. The Ea
values were enhanced drastically due to the formation of stable
intermediate during decomposition. From ESR, 13C NMR CP-MAS,
FTIR characterizations and the kinetic studies the decomposition
mechanism of C4 in the presence of PP was proposed and is shown
in Fig. 8. The pure PP was subjected to thermal decomposition,
the initiation step involves the random C–C scission and results
the formation of less stable primary and secondary radicals. In
the presence of air atmosphere the propagation step involves the
formation of peroxyradicals. In termination step the radicals may
gain hydrogen atom from phenolic groups of C4 and form paraf-
fins or olefins. At higher temperatures the formed C4 free radicals
result the open chain polyphenolic free radicals. Thus the presence
of C4 prevents the decomposition of PP. The FTIR and ESR analy-
sis confirms the formation of stable open chain polyphenolic free
radicals.
4. Conclusions
Thermal decomposition of C4 obeys the two step decomposi-
F1
D1
tion mechanism A−→ B−→ C. During the decomposition of C4 the
formation of stable open chain polyphenolic free radicals were con-
firmed by FTIR, 13C NMR CP-MAS and ESR techniques. The C4 was
examined as an additive for thermal decomposition of PP and its
kinetic parameters are also evaluated. The decomposition of PP fol-
lows the single step decomposition (F2 reaction model), but in the
presence of a 0.2–2.0% of C4 the PP obeys the multi step decompo-

K. Chennakesavulu et al. / Thermochimica Acta 515 (2011) 24–31
31
sition. The decomposition mechanism of C4 in the presence of PP
was proposed. The TGA experiments and Friedman Ea plots of PP
and PP + 2.0% C4 clearly shows the antioxidant property of C4.
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Acknowledgements
The authors K. Chennakesavulu and M. Raviathul Basariya Senior
Research Fellows are grateful to Council of Scientific and Indus-
trial Research (CSIR), New Delhi (India) for the financial support.
The authors are grateful to NMR Facility, SAIF-Indian Institute of
Technology, Madras (India) and NMR Centre-Indian Institute of Sci-
ence, Bangalore (India) for providing the necessary spectral and
analytical data.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
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