Polymer Degradation and Stability 92 (2007) 189e196
www.elsevier.com/locate/polydegstab
Thermogravimetric analysis of PVC/ELNR blends
M.N. Radhakrishnan Nair a, George V. Thomas b, M.R. Gopinathan Nair b,*
b
a
Department of Chemistry, D.B. College, Thalayolaparambu P.O., Kottayam, Kerala 686 605, India
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam, Kerala 686 560, India
Received 21 September 2006; received in revised form 24 November 2006; accepted 27 November 2006
Available online 17 January 2007
Abstract
The thermal behaviour of a series of solution-cast blends of polyvinyl chloride/epoxidised liquid natural rubber (ELNR) of different mole
percentage of epoxidation has been examined using thermogravimetric analysis. Thermal degradation is found to occur through a two-step route
in which the first step corresponds to the dehydrochlorination of PVC to form a polyene and the second step is attributed to the decomposition of
the ELNR and the polyene. Introduction of 20 and 50 mol% of epoxy group into the liquid NR is found to enhance the thermal stability of PVC.
Probable mechanisms of degradation have been suggested on the basis of the kinetic analysis of the degradation studies. It is found that the
mechanism is influenced by the epoxy content of the blend system. Activation energy for the degradation and the entropy change have also
been reported.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Polymer blends; Thermogravimetry; Degradation kinetics; Activation energy; PVC; ELNR
1. Introduction
It is well known that PVC being a proton-donating polymer
interacts with oxirane rings of epoxidised oils and hydrocarbons resulting in the plasticization of the resin. Thus epoxidised oils have been used as partial replacement of
plasticizers, which also act as thermal stabilizers for PVC
[1]. Polymeric additives with epoxy groups are found to be
better choices compared to the low molecular weight epoxy
compounds for improving thermal characteristics of chlorinated polymers in general [2e5]. The oxirane rings lead to
exothermic interaction with the chlorinated structural units
in the polymer. Thus, epoxidised natural rubber has been used
as a compatibiliser in blends consisting of chlorinated polyethylene and polyvinyl chloride [6,7]. This was found to be highly
effective in getting a wide spectrum of mechanical properties
for the blend system. The compatibility behaviour of epoxidised natural rubber with hydroxyl containing polymeric resins
* Corresponding author. Tel.: þ91 481 2731036; fax: þ91 481 2731009.
E-mail address: mrg.nair@rediffmail.com (M.R. Gopinathan Nair).
0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2006.11.014
such as novolac, resole and epoxy resins was also studied and
found to be much effective in diversifying their properties [8].
Epoxidised polybutadiene with a higher mole percent of epoxidation was found to be miscible with polyvinyl chloride. This
was established by applying a scheme based on copolymere
copolymer miscibility theory [9].
In the present work it is proposed to study the thermal behaviour of a few series of blends consisting of polyvinyl chloride and epoxidised liquid natural rubber (ELNR). ELNR is
expected to act as a polymeric additive to PVC. Its low molecular weight will enable greater diffusion into the PVC matrix
leading to molecular level interaction between the blend
components.
2. Materials
PVC with K value 65, M W ¼ 1:4 105 , and density 1.37 (M/
s Chemicals and Plastics, Chennai, India) was used for blending. Natural crumb rubber (ISNR-5) with M W ¼ 8:2 105 ,
and with intrinsic viscosity in benzene at 30 C ¼ 4.45 dL/g,
was received from Rubber Research Institute of India (RRII),
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M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
Kottayam, India. Liquid natural rubber with M V ¼ 4900 (LNR)
is obtained by depolymerisation of NR [10]. Epoxidised LNR
(ELNR) of 20 mol% epoxidation (ELNR-20) and 50 mol%
epoxidation (ELNR-50) were prepared by the epoxidation
of LNR in our laboratory [11]. 2-Butanone, was supplied by
E. Merck (India) Ltd, was dried over anhydrous calcium chloride and distilled before use. Glacial acetic acid, toluene, methanol and hydrogen peroxide (30 w/v) were of reagent grade
supplied by BDH, Mumbai. Toluene and methanol were dried
and distilled before use.
3. Experimental
3.1. Preparation of liquid natural rubber (LNR)
It was prepared by a reported procedure [10] given as follows. Natural crumb rubber was masticated for 8 min at
40 C. About 100 g of it was dissolved in 1 l toluene and the
solution was charged into a flat-bottomed borosilicate glass
flask of 2 l capacity. The flask was fitted with a water condenser and a mechanical stirrer. Hydrogen peroxide solution
(100 ml) was added and thoroughly mixed with the solution.
The mixture was then homogenised to a certain extent by adding 150 ml of methanol. The whole assembly was placed in
sunlight. After about 50 h of irradiation, the clear supernatant
toluene layer was decanted. Toluene was removed by distillation under reduced pressure. The residual viscous liquid rubber containing some toluene was kept tightly corked in the
absence of light.
3.2. Preparation of epoxidised liquid natural rubber
3.2.1. ELNR-50
LNR (25 g) dissolved in 166 ml of toluene to obtain a 15%
solution was stirred at 50 C for 10 h with 1.1 mol of H2O2
and 0.35 mol of glacial acetic acid. At the end of the reaction,
the product was isolated by precipitating from methanol and
dried in vacuum. It was then characterized by titrimetric, IR,
and NMR techniques.
3.2.2. ELNR-20
LNR (25 g) dissolved in 166 ml of toluene to obtain a 15%
solution was stirred at 50 C for 3 h with 0.55 mol of H2O2
and 0.35 mol of glacial acetic acid. The product was isolated
as before.
3.3. Preparation of blends
A series of blends of PVC and liquid natural rubber/epoxidised liquid natural rubber were prepared with the help of
a common solvent, viz., 2-butanone as follows. A 3% w/v solution of PVC in 2-butanone was prepared and added to the
rubber solution made in the same solvent (30% w/v) at compositions such as 70/30, 50/50 and 30/70 of PVC and the rubber.
It was thoroughly mixed by a magnetic stirrer for 5 h and cast
on a glass plate. The samples were then dried in vacuum at
70 C for two days to remove the traces of residual solvent.
3.4. Thermogravimetric analysis
The thermogravimetric analysis (TGA) was carried out on
a Dupont 2000 TGA in N2 atmosphere at a heating rate of
10 C/min. The samples were scanned from 30 C to 600 C.
3.5. Designation of blends
The blends were designated as follows. PVC/LNR (70/30)
means a blend of 70 parts PVC and 30 parts LNR; PVC/
ELNR-20 (70/30) means a blend of 70 parts PVC and 30 parts
epoxidised liquid natural rubber of 20 mol% epoxidation;
PVC/ELNR-50 (70/30) means a blend of 70 parts PVC and
30 parts epoxidised liquid natural rubber of 50 mol% epoxidation, etc.
4. Results and discussion
The blends were prepared in three series, viz., PVC/LNR,
PVC/ELNR-20 and PVC/ELNR-50. Samples were made in
different compositions such as 70/30, 50/50 and 30/70 of
PVC and the rubber. All the samples were subjected to thermogravimetric analysis and the data were used to assess the
effect of epoxy content of ELNR on the degradation pattern
of PVC. For the sake of comparison thermogravimetric traces
of PVC, LNR, ELNR-20, and ELNR-50 are given in
Fig. 1(a)e(d). The corresponding data are tabulated in Table
1. A two-stage degradation pattern is seen in the case of
PVC. The first stage begins at 180 C (T0) and ends at
375 C with a peak temperature at 300 C (T1max). This corresponds to a weight loss of 59.5%, which is attributed to the
elimination of HCl molecules leaving behind longer polyene
chains. The second stage of degradation begins at 375 C
(T20) and ends at 490 C with a peak temperature of 466 C
(T2max). Thermal degradation of the polyene sequences occurs
during this stage yielding volatile aromatic and aliphatic compounds by the intramolecular cyclisation of the conjugated
sequences [12]. The total weight loss at this stage is found
to be 90%.
Liquid natural rubber exhibits only a single stage degradation that occurs between 240 and 450 C. The peak temperature is observed at 384 C and the weight loss at the end of
this stage is found to be 98%. Fragmentation of polyisoprene
chains must occur during the degradation yielding volatile
fragments such as isoprene and dipentene [13,14]. The degradation of ELNR-20 and ELNR-50 is found to follow similar
pattern and shows only single stage degradation. The onset
temperature rose from 240 C to 275 C and to 295 C, respectively, as the epoxy content is increased from 0 to 20 and to
50 mol%. Similarly the peak temperatures increased from
384 C to 396 C and to 415 C, respectively, with the corresponding increase in epoxy content. Weight loss for ELNR20 and ELNR-50 was 96% and 93%, respectively. The data
clearly indicate that the breakdown of the rubber molecules
is delayed by the presence of epoxy group, which could be
due to the higher extent of interaction among the rubber molecules through the polar epoxy groups.
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M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
Fig. 1. Thermogravimetric curves of (a) PVC, (b) LNR, (c) ELNR-20 and (d) ELNR-50.
Thermogravimetric curves of PVC/LNR blends at 70/30,
50/50 and 30/70 compositions are given in Fig. 2(a)e(c).
The data obtained are noted in Table 2. All the samples undergo a two-stage degradation pattern. The first stage degradation with a peak temperature around 260 C for the different
compositions corresponds to the dehydrochlorination of
PVC. Weight loss at this stage appropriated to the PVC content in the blend and was found to be 43.4, 30, and 17.7%, respectively, which is consistent with that of PVC as described
above. The table also shows that the peak temperature for
the second stage degradation occurs around 450 C which is
again the degradation temperature corresponding to that of
the NR and the residual polyene formed after the dehydrochlorination of PVC. Over 90% weight loss occurs at this stage indicating the completion of the degradation process. All the
above observations correspond to those of the constituent
polymers irrespective of the sample composition suggesting
that the degradation pattern of one constituent is not
Table 1
Thermogravimetric data of PVC, LNR, ELNR-20 and ELNR-50
Sample
PVC
LNR
ELNR-20
ELNR-50
First stage
Second stage
Onset temperature, C (T10)
Temperature range, C
Peak temperature, C (T1max)
Wt. loss (%)
Peak temperature, C (T2max)
Wt. loss (%)
180
240
275
295
180e375
240e450
275e500
295e500
300
384
396
415
59.5
98
96
93
466
e
e
e
90
e
e
e
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M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
Fig. 2. Thermograms of (a) PVC/LNR (70/30), (b) PVC/LNR (50/50) and (c) PVC/LNR (30/70).
influenced by the presence of the other. This is due to the lack
of interaction between them.
The TG and DTG curves of the PVC/ELNR-20 blends are
given in Fig. 3(a)e(c). Data collected from them are tabulated
in Table 2. Compared to PVC/LNR samples described above,
peak temperature of the first stage degradation of this series
occurs at higher temperatures, which seem to be increasing
with the ELNR content of the sample. An increase from
Table 2
Thermogravimetric data of PVC/LNR, PVC/ELNR-20 and PVC/ELNR-50 blends
Blend
PVC/LNR (70/30)
PVC/LNR (50/50)
PVC/LNR (30/70)
PVC/ELNR-20 (70/30)
PVC/ELNR-20 (50/50)
PVC/ELNR-20 (30/70)
PVC/ELNR-50 (70/30)
PVC/ELNR-50 (50/50)
PVC/ELNR-50 (30/70)
First stage
T(50)( C)
Second stage
Onset
temperature,
C (T10)
Temperature range,
C (T1max)
Peak temperature,
C
Wt. loss
(%)
Peak
temperature,
C (T2max)
Wt.
loss (%)
125
140
148
150
185
186
189
216
217
125e380
140e380
148e390
150e350
185e375
186e350
189e385
216e400
217e400
256
261
266
250
281
277
276
283
284
62
60
59
59
55
52
55
49
48
440
452
440
433
474
462
450
480
471
92
93
94
89
85
83
86
82
81
327
313
326
296
341
415
351
365
385
M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
193
Fig. 3. Thermograms of (a) PVC/ELNR-20 (70/30), (b) PVC/ELNR-20 (50/50) and (c) PVC/ELNR-20 (30/70).
250 C to 281 C is observed as the ELNR-20 content is increased from 30 to 50 parts and remains around this level
even at 70 parts.
The weight loss after the first stage degradation was found
to be 59, 55, and 52%, respectively, for the samples with 30,
50 and 70 parts of the rubber and the values appropriated to
the PVC content are 41.3, 27.5, and 15.6% for each sample.
It is to be noted that the mass loss undergoes a decrease as
the ELNR-20 content increases in the blend, which could be
an indication of its stabilising effect on PVC. However, the
second stage degradation occurs as before without any marked
variation in the temperature range.
Similar results are obtained in the case of the other series of
samples, viz., PVC/ELNR-50. Thermogravimetric curves are
given in Fig. 4(a)e(c) and the data are tabulated in Table 2.
As shown in the table, peak temperature of the first stage degradation is higher at around 280 C and the weight loss appropriated to PVC content is decreasing from 38.5 to 14.4% as the
ELNR-50 content is increased from 30 to 70%. Here again the
second stage degradation proceeds as usual.
It was well established that epoxy group absorbs hydrogen
chloride (Scheme 1), there by stalling its catalytic activity for
further dehydrochlorination of the PVC [1]. The temperature
at which 50% weight loss happens is tabulated in Table 2. It
is noted that the values are highest for the ELNR-50 blend reflecting maximum thermal stability. Percentage mass loss at
three different temperatures selected at random, viz., 250,
350 and 450 C (Dm1, Dm2, Dm3) for all the three series of
blends are shown in Table 3.These data are intended to follow
the stability of the blends in terms of the mass loss. Values are
lowest for the different compositions involving ELNR-50 indicating its role in the thermal stabilisation of PVC.
A comparison of (T10) and (T1max) values for the blends in
each series shows that 70/30 blends possess the lowest value,
and 50/50 blends possess the highest value which tends to stabilise with the 30/70 composition. Hence it is found that 50
parts of ELNR with 50 mol% of epoxidation is a suitable composition which brings about maximum stabilisation to the
PVC.
4.1. Degradation kinetics
Solid-state thermal decomposition reactions were generally
homogenous in nature. The most prominent rate controlling
194
M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
Fig. 4. Thermograms of (a) PVC/ELNR-50 (70/30), (b) PVC/ELNR-50 (50/50) and (c) PVC/ELNR-50 (30/70).
process operative in a particular case is chosen and used for
deriving the rate equation.
The decomposition kinetics of the blends was derived from
the TG curves by applying an analytical method proposed by
Coats and Redfern [15]. The integral equation used has the
form
gðaÞ
AR
2RT
E
ln
¼ ln
1
T2
FE
E
RT
where g(a) is the kinetic model function, a is the fraction decomposed at any temperature, and F is the heating rate. A is
the pre-exponential factor (Arrhenius parameter) which is calculated from the intercept by the relation
intercept ¼ ln ðAR=FEÞ
The slope of the plot of left hand side against 1/T is
a straight line, from which the energy of activation, E, is
calculated.
Different authors, based on the integral method have derived several mechanistic equations to determine the decomposition kinetics. Satava [16] has chosen the nine equations
Scheme 1.
based on nine possible mechanisms [i.e., nine different forms
of g(a)]. The form of g(a) that best represents the experimental data gives the proper mechanism.
In the present study all TG data were analysed using the
nine mechanistic equations proposed by Satava. Results of
the analysis were used to derive the best fit curves with the
highest correlation coefficients. The data are summarised in
Table 4.
The observed mechanisms of decomposition of the homopolymers are as follows. First stage decomposition of PVC involves the mechanism, viz., one-dimensional diffusion
according to the mechanistic equation obtained in the form
(a)2 ¼ kt. This step is due to the dehydrochlorination of
PVC which is an autocatalytic process. Second stage involved
the scission of the polyene chains and the Mampel equation
(ln(1 a) ¼ kt) best represents the experimental data which
give the mechanism, viz., random nucleation with one nucleus
on each particle. LNR decomposition also followed the onedimensional diffusion mechanism. ELNR-20 and ELNR-50
decomposed by the three-dimensional diffusion pathways,
which may be caused by the introduction of the oxirane rings
into the chain.
PVC/LNR blends showed the same decomposition mechanism as that of PVC, viz., one-dimensional diffusion for the
first stage and random nucleation for the second stage indicating little interaction between the components. PVC/ELNR-20
(70/30) blend showed two mechanisms, viz., random nucleation for the first stage and three-dimensional diffusion for
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M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
Table 3
Mass loss due to the blends at different temperatures
Blend
Mass loss
Dm1(%) (250 C) Dm2(%) (350 C) Dm3(%) (450 C)
PVC/LNR (70/30)
PVC/LNR (50/50)
PVC/LNR (30/70)
PVC/ELNR-20 (70/30)
PVC/ELNR-20 (50/50)
PVC/ELNR-20 (30/70)
PVC/ELNR-50 (70/30)
PVC/ELNR-50 (50/50)
PVC/ELNR-50 (30/70)
32.7
15.6
17.5
12.9
11.0
9.8
11.9
9.7
9.3
69.7
55.6
52.5
67.4
54.8
51.2
48.8
47.6
46.3
77.8
76.3
67.5
74.2
67.4
58.1
66.7
65.1
56.1
the second stage which are due to PVC and ELNR-20, respectively. Existence of two different mechanisms indicates insufficient interaction between the blend components. For all other
blends, viz., PVC/ELNR-20 (50/50), PVC/ELNR-20 (30/70),
PVC/ELNR-50 (70/30), PVC/ELNR-50 (50/50) and PVC/
ELNR-50 (30/70), the Mampel equation best represents the
experimental data. This corresponds to the random nucleation
mechanism. It is noteworthy that the above data show a change
in mechanism with an increase in the mol% of epoxidation and
also with an increase in the quantity of epoxidised rubber present in the blends. From this it is obvious that the blend characteristics undergo a change with respect to the epoxy
content which could be due to the greater interaction between
the blend components enabled by the higher epoxy content.
Activation energy for the first stage degradation of PVC,
LNR, ELNR-20 and ELNR-50 was 72.94, 42.7, 78.56, and
83.94 kJ/mol, respectively. It could be noted that the activation
energy of liquid rubber sharply increases from 42.7 to 78.56,
and to 83.94 kJ/mol with an increase in mol% of epoxidation
from 0 to 20% and then to 50%. This is a case of the influence
of epoxy group on the degradation of the rubber molecules.
Regarding the blends, the activation energy for the first stage
degradation of PVC/LNR is in the order 46.1, 42.8,
35.18 kJ/mol for the 70/30, 50/50 and 30/70 blends, respectively. For PVC/ELNR-20 blends the values are 40.9, 72.84,
77.25 kJ/mol for the three compositions. Higher values of
activation energy at higher ELNR content indicate greater
degree of interaction between the blend components which
slows down the dehydrochlorination process. PVC/ELNR-50
blends showed still higher values, viz., 64, 123 and 109 kJ/
mol, respectively. The marked increase in activation energy
values in this case is also in agreement with the observation
that the liquid rubber with 50 mol% of epoxidation leads to
greater stabilisation of PVC against the dehydrochlorination.
Activation energy for the second stage degradation of PVC
is 102.9 kJ/mol. The values remain high for all the blend systems (Table 4). Higher activation energy is construed by the
fact that this stage involves chain breaking.
The entropy of activation (DS ) was also calculated for each
stage of thermal decomposition for the various blends using
the relationship,
A ¼ kTs =h eDS=R
where A is the Arrhenius parameter, k is the Boltzmann constant, Ts is the peak temperature, DS is the entropy of activation and R is the gas constant. Table 4 shows that DS has
negative values. The negative values suggest that the segment
undergoes some chain alignment at elevated temperatures
forming the activated complex. Entropy of activation in the
first stage ranges from 238 to 272 J/degree/mol. Second
stage values are little lower and ranges from 186 to
263 J/degree/mol.
Arrhenius parameter, A, exhibits a regular trend. Values for
second stage degradations are much higher than the first stage
values as shown in Table 4. Higher values for the second stage
could be accounted by the higher collision frequencies at the
degradation temperatures.
5. Conclusion
Thermogravimetric data showed that (T10) and (T1max) values
of liquid natural rubber increased with an increase in the epoxy
content, indicating that the presence of epoxy group delayed
the breakdown of the liquid rubber molecules. For the PVC/
LNR blends, the thermal data correspond to those of the
Table 4
Kinetic parameters of the blends and the blend components
Sample
PVC
LNR
ELNR-20
ELNR-50
PVC/LNR (70/30)
PVC/LNR (50/50)
PVC/LNR (30/70)
PVC/L-ELNR-20 (70/30)
PVC/ELNR-20 (50/50)
PVC/ELNR-20 (30/70)
PVC/L- ELNR-50 (70/30)
PVC/ELNR-50 (50/50)
PVC/ELNR-50 (30/70)
Activation energy E (kJ/mol)
Arrhenius parameter A (s1)
Entropy of activation DS (J/degree/mol)
Stage 1
Stage 2
Stage 1
Stage 1
Stage 2
73
43
79
84
46
43
35
41
73
77.25
64
122.6
109
103
e
e
e
76
154
117
125
160
183
204
222
200
2
530
649
943
1
0.2
1
714
832
196
609
326
439
246
243
248
271
265
265
270
271.8
238
241
247
247
218
236
e
e
e
263
199
228
235
186
201
228
218
183
Stage 2
7
e
e
e
916
574
451
858
2629
553
2500
46 058
4109
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M.N. Radhakrishnan Nair et al. / Polymer Degradation and Stability 92 (2007) 189e196
constituent polymers irrespective of the sample composition
suggesting that the degradation pattern of one constituent is
not influenced by the presence of the other. This could be due
to the lack of interaction between the blend components. Compared to PVC/LNR samples, the peak temperatures of the first
stage degradation of PVC/ELNR-20 and PVC/ELNR-50 series
occur at higher temperatures, which seem to be increasing with
the ELNR content of the respective blend. Mass loss undergoes
a decrease as the ELNR content increases in the blend. This is
maximum for ELNR-50, which could be an indication for its
stabilising effect on PVC. The second stage degradation of
all samples, however, occurs without any marked variation.
Higher activation energy indicated from kinetic analysis also
supports this observation. From kinetic studies it is seen that
PVC/LNR blends exhibited the same decomposition mechanism as that of PVC, viz., one-dimensional diffusion for the
first stage and random nucleation for the second stage indicating little interaction between the components. PVC/ELNR-20
(70/30) blend showed two mechanisms; random nucleation
for the first stage and three-dimensional diffusion for the second stage, which are due to PVC and ELNR-20, respectively,
indicating insufficient interaction between the blend components. For all other blends, the Mampel equation best represents the experimental data. This corresponds to the random
nucleation mechanism. The change in mechanism is obviously
caused by an increase in the epoxy content of the blend system
which offers greater interaction between the blend components.
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