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Polymer International 46 (1998) 289È297
Kinetics of Thermal Degradation of
Thermotropic Poly(p -oxybenzoate-co ethylene terephthalate) by Single Heating
Rate Methods
Xin-Gui Li,1* Mei-Rong Huang,1 Gui-He Guan2 & Tong Sun2
1 Department of Polymer Materials Science Engineering, College of Materials Science Engineering, Tongji University, 1239 Siping
Road, Shanghai 200092, PeopleÏs Republic of China
2 Department of Polymer Materials Science Engineering, China Textile University, Shanghai 200051, PeopleÏs Republic of China
(Received 24 July 1997 ; revised version received 4 November 1997 ; accepted 20 January 1998)
Abstract : The kinetics of decomposition of thermotropic liquid crystalline
poly(p-oxybenzoate-co-ethylene terephthalate) (poly(B-co-E), BE polymer) with
di†erent monomer ratios in both nitrogen and air were studied by dynamic thermogravimetry (TG) from ambient temperature to 800¡C. The kinetic parameters,
including the activation energy E@, the reaction order n, and the pre-exponential
factor Z, of the degradation of the BE polymers were evaluated by the single
heating rate methods of Friedman, FreemanÈCarroll and Chang. The BE polymers which degraded in two distinct stages in nitrogen and air, were stable under
nitrogen, while almost completely burned in air. The weight losses in the Ðrst
stage in nitrogen and air were dominated by the thermal degradation of both B
and E segments, but the weight losses in the second stage were governed by the
thermal degradation of B segment in nitrogen and by the oxidative degradation
of both B and E segments in air. The maximum rate of weight loss increased
linearly with the increase of E content and heating rate, but as E content
increased, the char yield at 800¡C in nitrogen decreased linearly. The E@, n and
ln Z values of the BE polymers in the Ðrst stage of thermal pyrolysis are higher
in nitrogen than in air, indicating that the degradation rate is slower in an inert
atmosphere. The E@ value increased with increasing heating rate but varied
irregularly with the variations of B/E ratios and molecular weight. The n and
ln Z values for the BE polymers in nitrogen were found to be in the wide ranges
1É5È5É6 and 12È48 min~1, respectively, suggesting a complex degradation
process. The estimated lifetimes of the BE polymers at 250¡C were calculated to
be at least 33 days in nitrogen and 3 h in air. ( 1998 SCI.
Polym. Int. 46, 289È297 (1998)
Key words : thermotropic polymer ; liquid crystalline polyester ; thermal degradation ; thermogravimetry ; decomposition kinetics ; thermostability
have been studied extensively,1h3 but only very few
investigators have attempted to study the kinetics of the
thermal degradation of BE polymer, which is mostly
used as high-tensile Ðbre, self-reinforced plastics and
advanced Ðbre-reinforced composite matrix.3h6
Thermogravimetry (TG) is a technique widely used
because of its simplicity and the information a†orded
INTRODUCTION
The thermal properties of thermotropic liquid crystalline poly(p-oxybenzoate-co-ethylene terephthalate)
(poly(B-co-E) designated as BE polymer for brevity)
* To whom all correspondence should be addressed.
289
( 1998 SCI. Polymer International 0959È8103/98/$17.50
Printed in Great Britain
X. G. L i et al.
290
from a simple TG thermogram. In this paper, TG and
di†erential thermogravimetry (DTG) measurements of
BE polymer are reported ; the kinetics of the thermal
degradation of BE polymer in nitrogen and air from
ambient temperature to 800¡C are investigated using
Friedman, FreemanÈCarroll and Chang techniques
which allow us to calculate every point in a TG curve
and a DTG curve, and the variations with temperature
are discussed.
Friedman method8
ln(da/dt) \ ln Z ] n ln(1 [ a) [ E@/RT
(1)
where a is the weight loss of the polymer undergoing
degradation at time t, R is the gas constant
(8É3136 J mol~1 K~1) and T is the absolute temperature
(K). The plot of ln da/dt as a function of 1/T should be
linear with a slope equal to E@/R. Additionally, the
E@/(nR) value could be determined from the slope of a
linear plot of ln (1 [ a) versus 1/T .
FreemanÈCarroll method9
EXPERIMENTAL
BE polymer with the structural formula shown in
Scheme 1 was synthesized following the procedure
described elsewhere.7 The intrinsic viscosity of the BE
polymers was measured at 0É5% concentration in
phenol/sym-tetrachloroethane 50/50 (vol) at 30¡C.
The TG and DTG thermograms were obtained using
a Du Pont Instrument 9900 computer/thermal analyzer
under a dynamic nitrogen atmosphere Ñowing at
50 ml min~1 and static air. Degradation experiments
under nitrogen were conducted at the heating rates of 1,
2, 5, and 10¡C min~1 using 21È32 mg samples, and in
every case a thermally stable residue was formed. The
char residues were obtained from the TG curve by
dividing the weight of the sample at 500È800¡C by the
initial weight of the sample. The degradation in air was
performed at the heating rates of 5 and 20¡C min~1
using a sample of 1È8 mg. The TG and DTG curves
were then printed.
*ln(da/dt)/*ln(1 [ a)
\ n [ (E@/R)*(1/T )/*ln(1 [ a)
The *ln(da/dt) and *ln(1 [ a) values are taken at
regular intervals of 1/T , in this case 1/T \ 0É000 01. By
plotting *ln(da/dt)/*ln(1 [ a) against *(1/T )/*ln(1 [ a),
a straight line was obtained with the slope and intercept
equal to [(E@/R) and n, respectively.
Chang method.10 Equation (1) can be rewritten in the
following form :
ln[(da/dt)/(1 [ a)n] \ ln Z [ E@/RT
(3)
A plot of ln[(da/dt)/(1 [ a)n] against 1/T will yield a
straight line if the decomposition order n is correctly
selected. The slope and intercept of this line will provide
the E@ and ln Z values, respectively.
Determination of half-life time t .10 The following
1@2
equation is integrated :
Determination of kinetic parameters of thermal
degradation
da/dt \ Z(1 [ a)n exp([E@/RT )
There are several methods (proposed by Friedman,8
Freeman and Carroll,9 Chang,10 Flynn and Wall,11
Chaterjee and Conrad,12 Horowitz and Metzger,13
Kissinger,14 Coats and Redfern,15 Van Krevelen,16
Reich,17 and Ozawa18) for calculating kinetic parameters which depend not only on the experimental conditions but also on the mathematical treatment of the
data. We have used the Friedman, FreemanÈCarroll
and Chang methods to evaluate the activation energy
E@, reaction order n, and frequency factor Z based on a
single heating rate measurement without making any
assumptions. Detailed descriptions of the three methods
are not given because the techniques for evaluating the
kinetic parameters from TG/DTG traces are easily
available from the literature.8h10 The equations
employed in the methods are listed below.
(2)
(4)
The half-life time t
may be calculated from eqns (4)
1@2
and (5) when n is di†erent from unity :
t
1@2
\ M(1 [ 0É51~n)/[Z(1 [ n)]N exp(E@/RT )
\ (1 [ 0É51~n)/[k(1 [ n)]
(5)
In this expression k is the rate constant of thermal
decomposition (min~1). When n equals unity :
t \ (0É693/Z)exp(E@/RT ) \ 0É693/k
1@2
(6)
Determination of lifetime t .10 The lifetime t of polymer
f
f
to failure is generally deÐned to be when the weight loss
reaches 5%. Therefore :
t \ M(1 [ 0É951~n)/[Z(1 [ n)]Nexp(E@/RT )
f
\ (1 [ 0É951~n)/[k(1 [ n)]
(n D 1)
t \ (0É0513/Z)exp(E@/RT ) \ 0É0513/k
f
(7)
(n \ 1) (8)
Scheme 1
POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998
T hermal degradation kinetics of thermotropic poly(B-co-E)
291
RESULTS AND DISCUSSION
Effect of B /E ratio on thermal decomposition in
nitrogen
The typical dynamic TG and DTG thermograms of B/E
(60/40) and (75/25) polymers in a dynamic nitrogen
atmosphere are shown in Figs 1 and 2. Figures 3 and 4
Fig. 3. Friedman plots of Ln (da/dt) or Ln (1 [ a) vs. reciprocal temperature for the direct calculation of E@ or n value of
thermal degradation of B/E (60/40) polymer in nitrogen at
heating rates of (L) 1¡C/min ; ( x ) 2¡C/min ; (…)
5¡C/min ; (|) 10¡C/min.
present their plots of ln(da/dt) versus 1/T and ln (1 [ a)
versus 1/T . The decomposition temperature T , the
d
temperature at the maximum rate of decomposition
T , E@, n, ln Z, and char yield are presented in Table 1
dm
and Fig. 5. It is evident that :19,20
Fig. 1. Dynamic TG thermograms of thermotropic liquidcrystalline polymer with B/E monomer ratios of (a) 60/40, and
(b) 75/25 in nitrogen at the heating rates of (ÈÈ) 1¡C/min ; (- - - -) 2¡C/min ; (È I È I) 5¡C/min ; (I I I) 10¡C/min.
Fig. 2. Dynamic DTG thermograms of thermotropic liquidcrystalline polymer with B/E monomer ratios of (a) 60/40, and
(b) 75/25 in nitrogen at the heating rates of (ÈÈ) 1¡C/min ; (- - - -) 2¡C/min ; (È I È I) 5¡C/min ; (I I I) 10¡C/min.
POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998
(a) There is deÐnitely an increase in T and T as the B
d
dm
unit content increases in the BE polymer. The B
unit content in the BE polymer plays an important
role in enhancing the thermostability of the
polymer.
Fig. 4. Friedman plots of Ln (da/dt) or Ln (1 [ a) vs. reciprocal temperature for the direct calculatin of E@ or n value of
thermal degradation of B/E (75/25) polymer in nitrogen at
heating rates of (L) 1¡C/min ; ( x ) 2¡C/min ; (…)
5¡C/min.
X. G. L i et al.
292
TABLE 1. Kinetic parameters of thermal degradation of thermotropic BE polymer
under nitrogen calculated by the Friedman method at 10ÄC min—1
B/E
(mol/mol)
100/0a
100/0b
80/20a
80/20b
75/25c
60/40
60/40a
30/70a
0/100a
0/100b
ÍiË
(dl gÉ1)
HMW
LMW
HMW
LMW
0·87
0·60
0·57
0·72
¿0·62
0·62
T /T
/T
d dm1 dm2
(¡C)
512/530/–
499/515/–
438/490/–
414/460/492
439/477/–
425/453/–
423/460/–
412/450/–
406/438/–
398/423/–
E¾
(kJ molÉ1)
253
282
185
201
257
211
204
102
83
172
n
3·0
2·6
4·7
3·7
5·1
3·1
2·8
1·1
0·5
0·9
ln Z
(minÉ1)
37
40
29
30
42
34
33
16
13
26
Char yield
(%)
(¡C)
36
40
28
30
28
25
22
16
6
12
800
600
800
600
600
600
800
800
800
600
a TG curves were obtained from ref. 19.
b TG and DTG curves were obtained from ref. 20.
c Kinetic parameters and char yield were obtained from the TG and DTG curves at 5¡C minÉ1.
(b) There is a linear increases in the char yields as the B
unit content increases because an increase of the B
content will decrease the number of hydrogen atoms
and retard the formation of more volatile degraded
products, thus resulting in an increase in char yield.
The formation of the char residues is probably due
to branch formation and crosslinking of the product
obtained mostly from B units during the thermal
degradation under nitrogen.
(c) There is a linear increase of the maximum rate of
decomposition as the E content increases from 20 to
100 mol%. This phenomenon can be explained as
being due to the heat instability of E units as compared with B units in the BE polymer. The degradation of a polymer containing more hydrogen atoms
might lead to the formation of more volatile pro-
Fig. 5. E†ect of E unit content in B/E polymer on the
maximum weight-loss rate and char yield in nitrogen at
10¡C/min heating rate.
ducts, thus resulting in a decreasing amount of carbonaceous mass and faster degradation.
(d) The liquid crystalline BE polymer containing 60È
80 mol% B unit exhibits E@ and ln Z values of 185È
257 KJ mol~1 (mean value 212 KJ mol~1) and
29È42 min~1 (mean value 34 min~1), respectively,
which are higher than those of pure E homopolymer
but lower than those of pure B homopolymer.
(e) The n values of 2É8È5É1 (mean value 3É9) for the BE
polymer exhibiting thermotropic liquid crystallinity
are higher than those of pure B homopolymer and
much higher than those of pure E homopolymer,21
polyvinyl chloride,8 polystyrene,22 polycarbonate23
and cellulose,24 which generally show a Ðrst-order
decomposition kinetics.
Generally, a decomposition order of zero characterizes the most rapid decomposition reaction. As the
order increases, the decomposition reaction
becomes slower. Therefore, the kinetic order of 5 for
the B/E (75/25) polymer indicates a much slower
degradation reaction, as shown in Fig. 2. It is seen
from Table 2 that the highest order of 5É6 was
observed for the B/E (75/25) polymer at the lowest
heating rates of 1 and 2¡C min~1, indicative of the
slowest degradation kinetics. Additionally, the
kinetic order of the BE polymers seems to be dependent on the sample weight because the B/E (75/25)
polymer exhibiting the highest order of thermal degradation has the greatest sample size, as shown in
Table 2. A detailed study of the dependence of degradation kinetics on the sample weight needs to be
done in the future.
(f ) The decomposition behavour of the BE polymer
under nitrogen is quite di†erent from that of the
respective B or E homopolymers. The TG results
obtained and discussed so far could be taken as
POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998
T hermal degradation kinetics of thermotropic poly(B-co-E)
293
TABLE 2. Effect of heating rate on kinetic parameters of thermal degradation of BE polymer under
nitrogen calculated by the Friedman method
B/E
(mol/mol)
ÍiË
(dl gÉ1)
Sample
size (mg)
Heating
rate (¡C minÉ1)
T /T
/T
d dm1 dm2
(¡C)
E¾
(kJ molÉ1)
n
ln Z
(minÉ1)
60/40
60/40
60/40
60/40
75/25
0·60
0·60
0·60
0·60
0·87
26·7
27·7
22·2
21·2
31·0
1
2
5
10
1
387/408/462
398/414/475
403/437/–
425/453/–
390/414/468
75/25
0·87
32·5
2
401/435/470
75/25
0·87
29·5
5
417/446/482
196
201
203
211
236
(171)a
248
(108)a
257
2·9
3·5
3·0
3·1
5·6
(2·6)a
5·6
(1·7)a
5·1
32
33
33
34
39
(24)a
41
(14)a
42
a The data in parentheses are the kinetic parameters of the second stage of thermal degradation.
proof of the presence of a random sequence distribution in the polymer backbone25 because no distinct peaks representative of thermal decomposition
of individual B and E homopolymers are observed
during the thermal degradation of the BE polymer.
In the case of random copolymer, generally stepwise
degradation of individual B and E homopolymer
segments merge into single steps located in between
the maximum degradation temperatures of the corresponding homopolymers.26
Effect of molecular weight on thermal degradation
Tables 1È4 list the kinetic constants of the thermal degradation of BE polymer with di†erent intrinsic viscosities or solid-state polymerization times, where a
longer solid-state polymerization time means higher
molecular weight. No clear relationship was found
between the kinetic parameters and molecular weight/
sequence distribution for the same B/E ratios of 80/20
and 60/40. The thermal decomposition temperature T ,
d
T and E@ depended slightly on the molecular weight of
dm
the BE polymer. T and T
increased with higher
d
dm
TABLE 3. Effect of intrinsic viscosity of B/E (60/40)
polymer on the activation energy of decomposition
in air at 20ÄC min—1a
ÍiË
(dl gÉ1)
0·67
0·71
0·74
0·79
0·82
0·88
Average
T /T
/T
d dm1 dm2
(¡C)
424/463/587
429/460/601
431/468/601
431/468/600
433/470/604
432/471/602
430/467/599
molecular weight, but the E@ value appeared to decrease
slightly in some cases. The dependence of T and T on
d
dm
the molecular weight indicates that these polymers may
involve end-group initiated thermal decomposition.
Kinetic parameters of thermal degradation in air
The TG and DTG curves of the BE polymers in air
indicated two degradation stages, corresponding to
thermal degradation and thermo-oxidative degradation
respectively. The T , T , E@, n and ln Z values reported
d dm
in Tables 3 and 5 are lower than those obtained in
nitrogen except that the T and T
values listed in
d
dm
Table 3 are slightly larger because of the higher heating
rate. These indicate faster kinetics in air than in nitrogen. Similar behaviour was observed for the thermal
degradation of wholly aromatic poly(ether ether ketone
ketone),28 chlorinated atactic polypropylene, and polyvinyl chloride.29 This may be due to the faster rate of
TABLE 4. Effect of solid-state polymerization and
sequence distribution of B/E (80/20) polymer on its
degradation under nitrogen at 10ÄC min—1 (ref. 20)
n
ln Z
(minÉ1)
BE polymer consisting of shorter B segment
and shorter E segment
0
387/428/–
134
2
412/459/499
141
4
396/446/480
126
8
413/448/474
150
3·5
4·2
2·8
2·7
19
20
15
22
BE polymer consisting of longer B segment
and longer E segment
0
384/420/–
142
2
410/452/487
111
4
400/448/486
101
8
418/455/486
146
2·4
2·5
2·6
3·5
21
14
12
21
Solid-state
polymerization
time (h)
T /T
/T
d dm1 dm2
(¡C)
E¾
(kJ molÉ1)
Activation energy (kJ molÉ1)
Thermal
pyrolysis
Thermo-oxidative
pyrolysis
177
143
168
155
131
144
153
202
164
143
145
134
184
162
a The sample size is about 1 mg.
POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998
X. G. L i et al.
294
TABLE 5. Kinetic parameters of thermal degradation of BE polymers under air calculated by the Friedman
method
B/E
(mol/mol)
100/0
100/0a
70/30b
65/35b
60/40b
54/46b
0/100c
0/100
PMBa
ÍiË
(dl gÉ1)
–
–
0·79
0·74
0·72
0·81
–
0·67
DP ¼ 200
Heating
rate
(¡C minÉ1)
T /T
/T
d dm1 dm2
(¡C)
20
10
5
5
5
5
5
20
10
530/570/597
475/–/525
362/404/495
350/378/487
345/390/490
347/405/510
385/428/520
390/453/538
412/430/575
Thermal pyrolysis
Oxidative pyrolysis
E¾
(kJ molÉ1)
n
ln Z
(minÉ1)
E¾
(kJ molÉ1)
n
ln Z
(minÉ1)
440
210
114
131
135
119
201
157
260
–
2·2
2·1
1·8
2·6
2·9
2·0
–
3·5
–
31
18
22
22
18
30
–
43
146
469
180
180
184
168
142
213
257
–
0·7
0·9
0·6
0·6
0·6
1·3
–
1·1
–
69
28
27
28
25
20
–
35
a TG curves were obtained from ref. 27 ; PMB is poly(p -mercaptobenzoate).
b Sample size for the TG measurements is about 8 mg.
c From ref. 21.
degradation of the Ñexible aliphatic moiety having a
higher thermooxidative instability in air.
In addition, the E@ and ln Z values of the thermooxidative degradation for BE polymer are larger than
those of the thermal degradation, but the n value of the
oxidative degradation is much smaller than that of the
thermal pyrolysis as listed in Table 5. One can note that
poly(p-mercaptobenzoate) shows di†erent kinetics of
degradation in air as compared with B homopolymer
because of the replacement of an oxygen atom in the
main chain by a sulphur atom.
Effect of heating rate on kinetic parameters of thermal
degradation
The dependence of the kinetic parameters of thermal
degradation on heating rate for BE polymer calculated
by the Friedman, FreemanÈCarroll and Chang methods
is shown in Figs 6È9 and Tables 2 and 6. The T , T ,
d dm
E@ and ln Z values obtained by Friedman method all
Fig. 6. E†ect of heating rate on the maximum decomposition
rate of the polymers with the B/E ratios of (…) 60/40 and (L)
75/25 in nitrogen.
Fig. 7. FreemanÈCarroll
plots
of
[[*Ln(da/dt)]/
[*Ln(1 [ a)] vs. [*(1/T )/*Ln(1 [ a)] for the B/E polymer in
nitrogen at heating rates of (L) 1¡C/min ; ( x ) 2¡C/min ;
(…) 5¡C/min ; (|) 10¡C/min.
Fig. 8. Chang plots of Ln[(da/dt)/(1 [ a)n] vs. reciprocal temperature for B/E (60/40) polymer in nitrogen at heating rates
of (L) 1¡C/min at n \ 3 ; (…) 2¡C/min at n \ 3.3 ; (|)
5¡C/min at n \ 2 ; (>) 10¡C/min at n \ 2.
POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998
T hermal degradation kinetics of thermotropic poly(B-co-E)
Fig. 9. Chang plots of Ln[(da/dt)/(1 [ a)n] vs. reciprocal temperature for B/E (75/25) polymer in nitrogen at heating rates
of (L) 1¡C/min at n \ 5.4 ; ( x ) 2¡C/min at n \ 5.4 ; (…)
5¡C/min at n \ 5.
295
BE polymer containing more than 70 mol% B unit,31 so
it might be mainly ascribed to the degradation of the B
unit.
Additionally, there are some di†erences in the kinetic
data calculated using the di†erent methods, as shown in
Tables 2 and 6. The Friedman method gave the lowest
E@ value but the highest n value of the three methods.
As shown in Fig. 8, only the Chang method actually
tends to form straight lines in the widest temperature
range, which means a smaller error in the calculation of
the kinetic parameters by this method. However, the
temperature range used for the determination of the
kinetic parameters by the Friedman and FreemanÈ
Carroll methods is wide enough to obtain reliable
results.
Thermal lifetime prediction
increase as the heating rate increases ; the n value
behaves di†erently. At lower heating rates the di†usion
of the degradation products apparently does not a†ect
the kinetics of the decomposition process, so these
values were found to be lower. On the contrary, the
degradation of the BE polymers in nitrogen is probably
faster than the di†usion of the degradation products
through the polymer melt, so that at higher heating rate
the kinetics of the degradation process are under di†usion control.28 This dependency of the kinetics on the
heating rate is in agreement with those of E homopolymer,21 highly thermally stable aromatic poly(ether
ether ketone), poly(ether ether ketone ketone),28 and
poly[3-dimethyl(methacryloyloxyethyl)ammonium propanesulphonate],30 but is contrary to that of polyvinyl
chloride.8 It should be noted that the kinetic data of the
second stage of thermal degradation in nitrogen for B/E
(75/25) polymer listed in Table 2 are much smaller than
those of the Ðrst stage. The second stage of thermal degradation in nitrogen appears in the thermograms of the
A major application of TG and DTG kinetic parameters is to predict the maximum useable temperature,
the optimum processing temperature regions, and the
estimated lifetime of the polymers. It o†ers a simple and
convenient approach to use as an accelerated ageing
process for quality control experiments. The decomposition kinetics at elevated temperature could be extrapolated back to the service conditions for which the
lifetime prediction is required. The rate-constant k, halflife time t
and estimated lifetime t for the BE poly1@2
f
mers generated from the weight losses a of 0É5 and 0É05
respectively, at di†erent temperatures in nitrogen and
air have been calculated and are listed in Tables 7 and
8. It can be predicted theoretically that the lifetimes of
the polymers with B/E ratios of 60/40, 75/25 and 80/20
heated at 250¡C in nitrogen could reach 33 days, 293
days and 69 days, respectively. When the polymers are
heated at 250¡C in air, however, the lifetimes for the
B/E 54/46, 60/40, 65/35 and 70/30 polymers are 11 h,
TABLE 6. Kinetic parameters of thermal degradation of the polymers with B/E ratios of
60/40 ( [ g ] = 0·6 dl g—1) and 75/25 ( [ g ] = 0·87 dl g—1) under nitrogen calculated by the
Freeman–Carroll and Chang methods
B/E
(mol/mol)
Heating
rate
(¡C minÉ1)
Freeman–Carroll
Chang
E¾
(kJ molÉ1)
n
ln Z
(minÉ1)
E¾
(kJ molÉ1)
n
ln Z
(minÉ1)
276
276
240
279
264
(233)a
269
(154)a
299
3·0
3·3
2·0
2·0
5·4
(2·0)a
5·4
(1·5)a
5·0
45
45
39
47
43
(34)a
43
(22)a
48
60/40
60/40
60/40
60/40
75/25
1
2
5
10
1
288
309
308
228
215
3·0
3·2
2·0
1·5
4·0
48
51
51
36
34
75/25
2
265
4·6
43
75/25
5
275
5·3
40
a Data in parentheses are the kinetic parameters of the second stage of thermal degradation.
POLYMER INTERNATIONAL VOL. 46, NO. 4, 1998
X. G. L i et al.
296
TABLE 7. Rate constant k of thermal degradation, half-life time t
and estimated lifetime t
1¿2
f
for BE polymers at different temperatures in nitrogen
Temp.
(¡C)
B/E (60/40)
200
250
300
350
400
B/E (75/25)
B/E (80/20)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
É8·56
É5·94
É4·51
É2·69
É1·64
8·75
6·33
4·69
2·88
1·84
7·29
4·67
3·23
1·42
0·38
É10·4
É6·87
É5·27
É2·91
É1·66
11·0
7·55
4·91
3·57
2·31
9·17
5·63
4·03
1·67
0·41
É9·26
É6·25
É5·38
É3·16
É1·95
9·57
6·66
5·69
3·56
2·36
8·00
5·00
4·12
1·90
0·70
and estimated lifetime t for BE polyTABLE 8. Rate-constant k of thermal degradation, half-life time t
1¿2
f
mers at different temperatures in air
Temp.
(¡C)
200
250
300
350
400
B/E (54/46)
B/E (60/40)
B/E (65/35)
B/E (70/30)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
lg k
(minÉ1)
lg t
1@2
(min)
lg t
f
(min)
É5·26
É4·07
É2·95
É2·16
É1·36
5·42
4·23
3·12
2·32
1·52
3·99
2·80
1·68
0·89
0·08
É5·15
É3·93
É2·55
É1·77
É0·73
5·25
4·21
2·66
1·87
0·83
3·88
2·66
1·28
0·49
É0·52
É4·95
É3·53
É2·44
É1·43
É0·67
4·92
3·50
2·41
1·40
0·63
3·67
2·25
1·16
0·15
É0·70
É4·83
É3·57
É2·62
É1·74
É1·08
4·84
3·59
2·64
1·76
1·11
3·55
2·29
1·34
0·46
É0·22
8 h, 3 h and 4 h, respectively. In reality, besides thermal
degradation, all sorts of other processes including
photodegradation, mechanical and chemical degradations, will shorten service lifetimes. Therefore, the real
lifetime of the polymers should be shorter than those
listed in Tables 7 and 8. The static processing time for
the BE polymers at 400¡C will last only for 2É4È5É0 min
in nitrogen and 0É2È1É2 min in air. The thermostability
of the BE polymer is certainly much higher in nitrogen
than in air. These results suggest that the possible meltprocessing temperature range for the BE polymer in an
inert atmosphere is about 200¡C (between 200 and
400¡C) in order to avoid any serious degradation and
maintain the molecular weight during processing.
However, the optimum processing temperature would
range from 250 to 350¡C because of the rapid change in
melt rheological properties at lower or higher temperature.32
Carroll and Chang methods. The kinetic parameters
exhibit a dependence on B/E ratio, molecular weight,
heating rate, testing atmosphere and method of calculation. The kinetic data in nitrogen suggest that BE
polymer exhibits good thermostability. In air the thermostability of BE polymer is greatly reduced because
the thermal degradation is accompanied by thermooxidative degradation. The degradation seems to be a
random scission process of the ester linkages. The
optimum melt-processing temperature for BE polymer
would range from 250 to 350¡C.
ACKNOWLEDGEMENTS
This project is supported by the National Natural
Science Foundation of China and by the Science Technology Development Foundation of College of
Materials Science Engineering of Tongji University of
China.
CONCLUSIONS
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have been easily calculated by the Friedman, FreemanÈ
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