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Thermooxidative degradation of polyolefins in the solid state: Part 1. Experimental kinetics of functional group formation
Po/wner
Degradatron
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Thermooxidative degradation of polyolefins in the solid state: Part 1. Experimental kinetics of functional group formation F. Gugumus Ciba-Geigy AG, CH-4002 Basel, Switzerland
(Received 21 September
1995; accepted 26 October
1995)
The knowledge accumulated so far on thermal oxidation of polyolefins does not yield a model of the processes that is sufficiently accurate to account for a majority of the experimental data accumulated with unstabilized as well as with stabilized polymers. The present paper is concerned with some aspects of thermal oxidation of polyolefins, especially polyethylene, in the solid state. In this respect, the kinetics of thermal oxidation of polyethylene reported in the literature are tested with numerous experimental data generated in our laboratory. It is found that the experimental and formal kinetics proposed so far do not account for our results. Thus, instead of quadratic or biquadratic formation of carbonyl groups with oxidation time as reported in the literature, the rate law seems more complicated. It is found that there is an exponential type increase in the early stages of the oxidation. This is followed by a linear increase in later stages. The discrepancy with the literature results is attributed on the one hand to the fact that the conclusions were based on a very limited amount of experimental results. It is attributed on the other hand to the use of fitting to power laws. The possible pitfalls associated with this procedure are discussed in detail. 0 1996 Elsevier Science Limited
1 INTRODUCTION
phenomena occurring on oxidative degradation just to improve the lifetime under use conditions. However, although stabilization against thermooxidative degradation has made considerable progress,‘.4.24.2’ the physico-chemical processes occurring on thermal oxidation of polyolefins in the solid state are far from being understood. Many oxidation products were detected, and numerous factors controlling thermal oxidation were identified (oxygen pressure, temperature, initiation rate, sample thickness, stretch ratio, supermolecular structure, etc.). There are only a few attempts to relate the experimental kinetics of oxidation of solid polyolefins to the chemical phenomena thought to occur in the solid polymer. Such attempts are reported by Emanuel5 and Petruj ef ~1.’ I,‘* Emanuel, sumliterature, reports marizing mainly Russian parabolic increase of oxygen absorption and
Thermal oxidation of polyolefins has been the object of numerous investigations.‘-l4 Although many features of the phenomena were uncovered through past work, they have not yet been completely elucidated and research is still very active in this field.‘5-23 However, many studies were relative to polymer melts. They can be considered to simulate more or less the conditions encountered on polymer processing. Other studies, mainly of academic interest, involved polymers in solution, in attempts to isolate the chemical reactions that may cause oxidative degradation. Some work was concerned with polyolefins in the solid state. This is, of course, more relevant to the conditions encountered in practical use of the polymers. Very often it was initiated for a better understanding of the 131
132
F. Gugumus
carbonyl group formation with aging time. Petruj et al. find also quadratic increase of the concentration of hydroperoxide groups with aging time of polyethylene films at 105°C. However, in contrast with the relationship reported by Emanuel, they found bi-quadratic increase of the concentration of carbonyl groups. In the following, these different results will be tested with our own experimental results.
2 EXPERIMENTAL The polymers used for sample preparation were mainly unstabilized commercial resins stored in a refrigerated room to minimize oxidation. PE-LD films up to a thickness of 250 pm were blown at 200°C. A 500 pm melt-cast film was prepared by extrusion of PE-LD at 200°C. Films with thicknesses between 60 pm and 1 mm were also prepared by compression molding at 170°C for 6 min. The films up to 0.5 mm thickness were immediately quenched in cold water (1O’C). The 1 mm thick plaques were not quenched in water but cooled in a cold press with circulating water. A 500 pm thick PP film was prepared by extrusion of third generation PP at 230°C. Oxidation of polyolefins in the solid state, in the form of films of various thickness, was performed in forced draft air ovens (HORO, Model 080 V/EL) mainly at a temperature of 80°C. Some experiments involved slightly higher temperatures, 90 and 100°C or lower temperatures, 60 and 40°C. Functional group formation was measured by IR spectrophotometry with an unexposed sample in the reference beam. The data are mostly expressed as the absorbance at the wavelength corresponding to the maximum of the absorption of the band, e.g. &,,, for the absorbance associated hydroperoxides whose maximum is usually between 3400 and 3420cm-‘. The spectrophotometers used were Perkin-Elmer models 781,782 and FT-IR 1710.
3 EXPERIMENTAL KINETICS OF THERMAL OXIDATION OF POLYETHYLENE Most studies on thermooxidation of unstabilized polyethylene up to now were performed with low
density polymer (PE-LD). This can be attributed, on the one hand, to historical reasons because this polymer has been available for more than 60 years since it was synthesized for the first time in 1933. It can be attributed, on the other hand, to the fact that PE-LD can be processed in the absence of any stabilization without too much deterioration. This is not so for the different high-density (PE-HD) and linear low density (PE-LLD) polyethylenes which need an adequate stabilization for storage and processing. The changes in the IR spectrum of a typical PE-LD blown film subject to thermal oxidation in a forced air oven are shown in Fig. 1. It can be seen, that there is considerable increase both in the hydroxyl and in the carbonyl absorbance regions. Simultaneously, there are also changes in the hydrogen deformation peaks of the carboncarbon double bonds as well as in the carbon-oxygen single bond absorption region. The hydroxyl and carbonyl groups usually account for most of the oxidation products on thermooxidative degradation of polyethylene. In this respect, carbonyl group concentrations are often used to monitor oxidative degradation of polyolefins. The correlation with the deterioration of the mechanical properties of the polymer is usually good, at least qualitatively. Correlation on a quantitative basis is not always excellent because only part of the carbonyl groups formed correspond effectively to chain scissions, mostly responsible for the degradation of the mechanical properties. In the following, as in previous work, the carbonyl concentration measured by IR spectroscopy will be used to monitor the thermal oxidation of polyolefins.
3.1 Kinetics of carbonyl group formation
Different samples of a 200 pm thick PE-LD film were aged in a forced draft air oven at 100°C. The carbonyl absorbance of the films, measured by IR spectroscopy, was plotted in Fig. 2 as a function of aging time at 100°C. It can be seen, that the plots of Fig. 2 show the typical auto-accelerated behavior reported in the literature. The same data were re-plotted in Fig. 3 in a double-logarithmic representation. For all the samples shown in Fig. 3, segments of straight lines are obtained. These straight lines indicate carbonyl group formation as a function of aging
Thermooxidative
degradation of polyolefins in the solid state-l
133
0.160 0.26 0.24
0.160
0.16 8 5
0.16
e g
0.14
$
0.12
0.00 3650
3200
3400
3100
1850
l&O
1750
17Oa
1650
1600
1000
950
900
650
600
CM-l
Fig. 1. Changes in the IR absorption
spectrum of a PE-LD film on thermal oxidation at 80°C. Film thickness: 200~m. times (h): 183,279, 377, 500, 614,781.
time according to power laws, as reported in the literature.5,“,‘2 They correspond to an equation of the type: (Carbonyl) = at’ where t is the aging time and a and b are constants characteristic of the sample and the aging conditions. It is expected that, for samples of the same polymer, taken from the same film and aged under the same conditions, the exponents b in the formula given above should be the same. However, this is not so for the samples shown in Fig. 3. The values of the exponents b found for 2.6 and 8.9, these samples vary between depending on the particular sample and possibly other, unknown, factors. Calculation of the exponents by means of a curve-fitting program yields, as expected, values close to those deduced from the double-logarithmic representation in Fig. 3. Hence, significant differences that might occur, cannot be attributed to the fact that, in older work, conclusions were often based on double-logarithmic representations. Some of the values of the exponent b deduced from the data in Fig. 3 support to some extent formation of carbonyl groups according to a
Aging
quadratic increase with aging time as reported Emanuel:5 (Carbonyl) = at*
by
Other values of the exponent b, even more numerous with the samples in Fig. 3, support rather a bi-quadratic equation such as that found by Petruj et ~l.:“,‘~ (Carbonyl) = at” Most values of b deduced from the data in Fig. 3 point to equations of an order between two and four and of even higher than four, up to six and more. Hence, it is quite clear, that neither of the equations reported so far to account for carbonyl formation as a function of thermal aging time, can account even for a very restricted amount of experimental data such as that shown in Fig. 3. An additional plot of the same data yields a possible explanation for the discrepancy between various data sets of thermal oxidation of polyethylene films. It can be seen in Fig. 4, that the semi-logarithmic plot yields straight lines up to relatively high values of the carbonyl group concentration. The useful mechanical properties of the polymer samples will usually be lost long before such carbonyl values are reached on
F. Gugumus
134
10.00
1.25
0 Sample
1 (8 = 4.8)
Sample
2 (s = 8.9)
q
1 .oo
0 Sample
1
0 Sample
2
A Sample
3
Q Sample
4
l
Sample
5
n
Sample
6
A Sample
7
A Sernple 3 (s = 3.8) 0 Sample
4 (s = 2.6)
Sample
5 (8 = 3.9)
. Sample
6 (s = 6.2)
A Sample
7 (8 = 3.1)
l
0.25
0.00 0
250
500
Time at 100X(h)
10
100 Time at 100%
1000 (h)
Fig. 2. Carbonyl formation in 200km thick PE-LD blown films on thermal oxidative aging at 100°C (normal plot).
Fig. 3. Carbonyl formation in 200/1m thick PE-LD blown films on thermal oxidative aging at 100°C (double-log plot; s = slope).
thermal oxidation. The plot of Fig. 4 is quite puzzling, at least at first sight. As a matter of fact, it suggests an exponential increase of the carbonyl group concentration with aging time. The fundamental problems involved with such an interpretation led to a broad investigation and re-examination of thermal oxidation of polyolefins, especially polyethylene. First of all, it was checked that the results obtained with the low density polyethylene used for our experiments were not specific to that particular PE-LD, nor to the aging temperature of 100°C. Films were prepared by compression molding, starting from six different high pressure polyethylenes which had been polymerized in tubular reactors as well as in autoclaves. The carbonyl absorbance measured on simultaneous aging of the films in a draft air oven at 90°C is shown in Fig. 5 as a double-logarithmic representation. It can be seen in Fig. 5 that, in these experiments also, the exponent of the power law varies considerably. This is much less so if the same data are presented in a semi-logarithmic plot (Fig. 6). It can be seen in Fig. 6 that the different PE-LD samples show
approximately parallel increase of carbonyl absorbance with aging time at 90°C. It can also be seen in Fig. 6 that, if carbonyl group absorbance is measured up to high values, there is strong deviation from linearity. The data generated at 80°C with film samples from the same polymers, also show varying behavior in the double-logarithmic plot (Fig. 7). However, this time the semi-logarithmic plot of the data also shows pronounced differences between the polyethylenes tested (Fig. 8). It can be seen in Fig. 8 that the segments of straight lines are not all approximately parallel as at 90°C. Furthermore, the deviation from linearity occurs already for lower values of the carbonyl absorbance than at 90°C. These results show quite clearly that the increase of carbonyl groups on thermal oxidation does not follow a single, purely exponential curve. Figure 9 shows a special aspect of the increase of carbonyl absorbance on thermal oxidation. It can be seen that, after an initial, autoaccelerated increase, the rate of formation of carbonyl seems to become constant. This is even more visible in Fig. 10 corresponding to the aging at 80°C because more
Thermooxidative
135
degradation of polyolefins in the solid state-l 1O.W
10.00 0 Sample 1
E 0 Sample 2
4 Sample 3 0 Sample 4
. Sample 5 n
Sample 6
. Sample 7
1.oo yg t $ 5 I 5 c B 9 0.10
0.01
0.01 100
0
150
200
250
300
350
loo
0
200
Time al 1W”C (h)
Fig. 4. Carbonyl formation in 2OOpm thick PE-LD blown films on thermal oxidative aging at 100°C (single-log plot).
300
400
500
600
Time al 90°C (h)
Fig. 6. Carbonyl formation on thermal oxidation at 90°C of 500 pm films (semi-log plot).
O PE-LD-A (s = 4.3) q
PE-LDB
(a = 2.5)
q
stPE-LD-C (s = 3.7)
PE-LDB
(a = 2.4)
A PE-LD-C (s = 5.0) o PE-LD-D (s = 4.6)
o PE-LD-D (s = 4.5)
. PE-LD-E (s = 3.1)
. PE-LD-E (s = 3.6)
. PE-LD-F (s = 4.9)
. PE-LD-F (8 = 4.8)
loo Time a SO’C (h)
1000
loo Time at 8O’C (h)
Fig. 5. Carbonyl formation on thermal oxidation at 90°C of 500 pm films prepared by compression molding of different unstabilized PE-LD samples (double-log plot; s = slope).
Fig. 7. Carbonyl formation on thermal oxidation at 80°C of 500 pm films (double-log plot: s = slope).
F. Gugumus
136
1
2.00 I
10.00 i
I-
J
o PE-LD-A
O PE-LD-A
PE-LD-B
II PE-LD-B
A PE-LD-C
~bPE-LD-C
o PE-LD-D I-----l
o PE-LD-D
q
. PE-LD-E 1.50 -
. PE-LDF
‘g z $ 1
1.00 -
t B i 9 0.10
0.50 -
0.01 0
500
1000
1500
0.00
4-e 0
Time at 80°C (h)
500
1000 Time at 80%
Fig. 8. Carbonyl formation on thermal oxidation at 80°C of -500 jkrn films (semi-log plot).
2.00
PE-LD-B
& PE-LD-C D PE-LD-D 1.50
. PE-LD-E
3.2 Kinetics of hydroperoxide
. PE-LD-F
‘E 0 f $ 1 z
Fig. 10. Carbonyl formation on thermal oxidation at 80°C of 500 pm films (normal plot).
data points are available. There is another point to be deduced from the plots of Figs 9 and 10: the constant rates of carbonyl formation are comparable for the different PE-LDs tested, although they are not identical.
o PE-LD-A q
1500
(h)
1.00
f c 0 s
0.50
0.00 0
250
500
750
Time at 5W’C (h)
Fig. 9. Carbonyl
formation on thermal oxidation 500 pm films (normal plot).
at 90°C of
formation
As already mentioned, hydroperoxides were reported to be formed according to a quadratic ~aw~s.llJ2 These findings must be considered to be as fortuitous as the corresponding findings for the carbonyl groups. It can be seen in the double-logarithmic plot of Fig. 11 for the associated hydroperoxides that the exponent of the power law varies between 1.9 and 4.5. These values give an idea of the spreading; they should not be seen as extreme values. The corresponding plot for the free hydroperoxides in Fig. 12 shows comparable spreading of the exponent values. However, the absolute values are usually lower than those for the associated hydroperoxides. The plot of Fig. 11 shows deviations from linearity for high values of the concentrations of the associated hydroperoxides. In the plot for the free hydroperoxides in Fig. 12, there is also
Thermooxidative
o PE-LD-A
(s = 3.2)
q PE-LD-B
(8 = 1.9)
A PE-LD-C
(s = 2.7)
o PE-LD-D
(8 = 3.5)
. PE-LD-E
(s = 2.6)
. PE-LD-F
(s = 4.5)
100
10
137
degradation of polyolefins in the solid state-l
1000
Time at WC(h)
Fig. 11. Formation of associated hydroperoxides on thermal oxidation at 90°C of 500pm films (double-log plot; s = slope).
deviation from linearity if the concentration increases. It can be seen that the concentration passes through a maximum and starts to decrease. This phenomenon has already been discussed previously (see Ref. 23 and references cited therein) and will be treated in more detail in the following work. The conclusions regarding the spreading of the values of the exponents of the power law remain the same if instead of free or associated hydroperoxides, the sum of both hydroperoxides is plotted as shown in Fig. 13. The conclusions are again similar with the corresponding plots of the data generated at 80°C (Figs 14-16). The semi-logarithmic representation of the absorbance of the associated hydroperoxides as a function of aging time at 90 and 80°C in Figs 17 and 18, respectively, shows different aspects of the kinetics of hydroperoxide formation. It can be seen in Figs 17 and 18 that the logarithm of the absorbance increases linearly with aging time in the first stages of oxidation. However, the plots show increasing downward curvature with advancing oxidation. This change is explained, at least in part, by the representation of the same
1.00 Io PE-LD-A
(s = 2.6)
q PE-LD-B
(s = 1 .O)
A PE-LD-C
(s = 2.0)
o PE-LD-D
(s = 2.1)
. PE-LD-E . PE-LO-F
(s = 2.0) (s = 3.3)
o PE-LD-A
(s = 2.5)
o PE-LDB
(s = 1.6)
d PE-LD-C
(s = 2.4)
o PE-LO-D
(s = 3.0)
. PE-LD-E
(s = 2.4)
. PE-LD-F
(s = 3.5)
1
0.01
L
10
100
1000
Time at 60°C (h)
Fig. 12. Formation of free hydroperoxides on thermal oxidation at 90°C of 500 pm films (double-log plot; s = slope).
100 Time al 90°C (h)
Fig. 13. Formation of hydroperoxides on thermal oxidation at 90°C of 500 pm films (double-log plot: s = slope).
138
F. Gugumus 1 .oaI-
1.00
h PE-LD-C
(S = 3.2)
o PE-LD-D
(8 I 3.6)
o PCLDD
(s = 3.4)
. PE-LDE
(8 e 3.1)
. PE-LD-E
(S = 3.2)
. PE-LD-F
(5 = 4.2)
I-
I
+
100
1000
10000
Time at 80°C (h)
Time at 80°C (h)
Fig. 14. Formation
oxidation
at
of associated hydroperoxides on thermal 80°C of SOOpm films (double-log plot: s = slope).
Fig. 16. Formation
of hydroperoxides on thermal oxidation at 80°C of 500 Km films (double-log plot: s = slope).
0.11D-
I
qf=
0 PE-LD-A
(S = 2.3)
[1 PE-LDB
(s = 1.2)
b PE-LD-C
(8 = 2.0)
o PE-LD-D
(8 = 1.8)
. PE-LD-E
(S = 2.8)
. PE-LD-F
(I = 2.3)
r_
f
i
1000 250
Time at 30°C (h)
Fig.
15. Formation
oxidation
at
80°C
of free hydroperoxides on thermal of 500pm films (double-log plot: s = slope).
500 Time at 90%
Fig. 17. Formation
(h)
of associated hydroperoxides on thermal oxidation at 90°C of 500 pm films (semi-log plot).
Thermooxidative
degradation of polyolefins in the solid state-l 0.75 O PE-LD-A q
PE-LD-B
& PE-LD-C o PE-LD-D . PE-LD-E . PE-LD-F
0.25
4 250
1000
of associated hydroperoxides on thermal oxidation at 80°C of 500 pm films (semi-log plot).
data in normal coordinates as shown in Figs 19 and 20, respectively. It can be seen that, after the initial, autoaccelerated stages, the concentration of associated hydroperoxides increases linearly with oxidation time for a long time period. This parallels the corresponding behavior found for carbonyl groups as shown above. Representation of the absorbance of the free hydroperoxides in semi-logarithmic coordinates yields segments of straight lines in the initial stages of oxidation, followed by progressive leveling off maintained for a more or less pronounced time period. Afterwards, the absorbance decreases until it can no longer be distinguished as a separate peak in the broad hydroxyl peak accounting mainly for associated hydroperoxides. This is shown with the data generated with samples of six different low density polyethylenes at 90 and 80°C in Figs 21 and 22, respectively. It is clear that the exponential increase, if it is exponential at all, is observed in the initial stages of oxidation only. The representation of the absorbance of the free hydroperoxides in normal coordinates completes the view on the kinetics of free hydroperoxide formation. It can be seen in
750
Time at 90°C (h)
TimeatEO”C(h)
Fig. 18. Formation
500
Fig. 19. Formation
of associated hydroperoxides on thermal oxidation at 90°C of 500 pm films (normal plot).
0.75
0.50 ‘E 0 z f r z $ ii B a 0.25
0.00 0
1000 Time at 80°C (h)
Fig. 20. Formation
of associated hydroperoxides on thermal oxidation at 80°C of 500 Km films (normal plot).
F. Gugumus
q
PE-LD-8
A PE-LD-C o PE-LD-D . PE-LD-E . PE-LD-F
250
0
500
750
Time at 90°C (h)
Fig.
21. Formation of free hydroperoxides on thermal oxidation at 90°C of 500 pm films (semi-log plot).
Fig. 23 for the data obtained at 90°C and, even more so, in Fig. 24 for the data generated at 80°C that the increase of the concentration of the free hydroperoxides with aging time proceeds according to S-shaped curves. It is only some time after reaching its maximum value that the concentration decreases more or less rapidly, depending on the specific sample and aging temperature. In summary, the data presented show that the experimental kinetics for some functional groups on thermal oxidation of polyethylene do not correspond to the equations proposed so far in the literature. The rate law describing the formation of carbonyl groups is neither quadratic in aging time nor biquadratic. It seems rather close to an exponential-type increase in the early stages of oxidation. With ongoing oxidation, the increase of carbonyl’ groups becomes linear with at a constant rate. time, i.e. it proceeds Formation of associated hydroperoxides also does not correspond to a quadratic law. It seems to follow an exponential-type increase in the early stages of oxidation and change to a linear increase later on, as observed for the carbonyl groups. The kinetic law is again different for the
- -
0.15
0.10
d PE-LD-A D PE-LD-6 A PE-LD-C o PE-LD-D . PE-LD-E . PE-LD-F
z 8 f E
B a
0.05 -
0.01 0
500
1000
1500
2000
250
Fig.
22. Formation of free hydroperoxides on thermal oxidation at 80°C of 500 pm films (semi-log plot).
500
750
Time at 90% (h)
Time at 90°C (h)
Fig.
23. Formation
of free hydroperoxides on thermal oxidation at 90°C of 500 pm films (normal plot).
Thermooxidative 0.10
!
0.01
--I
0
500
1000
1500
141
degradation of polyolejins in the solid state-l
2000
Time al 80°C (h)
Fig. 24. Formation of free hydroperoxides on thermal oxidation at 80°C of 500 /*rn films (normal plot).
The statement will be illustrated by a constructed example as shown in Fig. 25 with the data presented in Table 1. The plot in Fig. 25 represents, e.g., carbonyl formation as a function of oxidation time. The starting point of the construction is carbonyl group concentration increasing linearly with time from the start of the experiment. This is represented by the straight line passing through the origin of the coordinates in Fig. 25. The other straight lines shown in Fig. 25 are simply deduced from the first line by parallel translation to increasing times. This is to simulate processes showing the same rate of formation of oxidation products after various induction times. The data in Fig. 25 have been replotted in a double logarithmic representation in Fig. 26. Of course, the data pertaining to the original straight line also yield a perfect straight line in the double logarithmic representation. In addition, the data deduced by translation also yield reasonably straight segments of straight lines. With increasing ‘induction periods’, the slope of the lines in Fig. 26 increases markedly. The results of fitting the data to the power law are given in Table 1. It can be seen that, from the
free hydroperoxides. After an initial autoaccelerated stage, the concentration passes through a more or less flat maximum before it starts to decrease. 4 POSSIBLE REASONS FOR CONFLICTING EXPERIMENTAL KINETICS In the preceding section, we have seen that conclusions based on a very restricted amount of experimental data are certainly one major reason for conflicting interpretations of thermal oxidation of polyolefins. Nevertheless, this point alone does not explain all the contradictions. There is another reason for the discrepancies between experimental kinetic laws for carbonyl formation on oxidation of polyolefins. It consists in the use of double logarithmic representations or its equivalent nowadays, data fitting to a power law. This is not meant to discourage use of such representations or fittings but to encourage their use with great care. This care is especially important in the area under investigation since oxidation phenomena very often show pronounced induction periods.
0
500
1000
1500
2OKl
2500
3000
3500
Time (au.)
Fig. 25. Formation of oxidation products according to linear increase with aging time (normal plot: a.u. = arbitrary units).
F. Gugumus
142
Table 1. Formation of oxidation products according to a linear law Concentration
Oxidation
C
fA 0.05
150
0.10 0.15 0.20 0.25 0.30 0.35 0.40
300 450 600 750 900 1050 1200
Parameters
650 800 950 1100 1250 1400 1550 1700
1150 1300 1450 1600 1750 1900 2050 2200
2150 2300 2450 2600 2750 2900 3050 3200
of the fitting to the power law c = at”
0.333.10-’ a Power b 1.ooo Correlation coefficient r* 1.ooo Concentration
products formed after aging times tcl k ts
0.83.10-’ 2.084 0.976
0.628.10~‘* 2.614.W” 3.049 4.924 0.960
0.946
and aging time in arbitrary units.
original to the fourth data series, the exponent of the power law increases from the original value 1 to almost 5! Even in the last instance, the value of the correlation coefficient, r2 = 0.94, still indicates a fair correlation. Hence, it is quite obvious that more or less pronounced induction times with different samples of the same origin 1.O()--
can lead to considerable differences in the exponent of the power law. This can explain, at least in part, differing values reported in the literature for the same phenomena examined under comparable conditions. If the data used so far are plotted according to a single logarithmic representation, as in Fig. 27, there is no linearity and the data deduced by translation of the original data yield curves similar to the curve obtained with the original data. There is another advantage of single logarithmic plots which yield straight lines if the concentration of oxidation products increases exponentially with aging time. It is the fact that, if the induction period varies considerably from one experiment to the other, the representation remains a straight line with the same slope. There is just a parallel shift corresponding to the induction time. This is illustrated in Fig. 28 for the same increments of 500, 500 and 1000 arbitrary units in induction time as previously but, of course, different values of the concentration of oxidation products to fit the exponential increase. The corresponding data are shown in Table 2. 1.00
l-
Ii10
100
loo0
wooo
Time (a.~.)
Fig. 26. Formation of oxidation products according to linear increase with aging time (double-log plot; a.u. = arbitrary units).
1000
zoo0
3ooa
Time (a.~.)
Fig. 27. Formation of oxidation products according to linear increase with aging time (single-log plot; a.u. = arbitrary units).
Thermooxidative
degradation of polyolejins in the solid state-l
measure of the rate of formation products.
143
of oxidation
5 CONCLUSIONS 1.00
F d
5 t L
z
e
s
0.10
I, /
0.01 0
,,, , 3ooo
2ooo
1000
( 4ooo
Time (a.~.)
Fig. 28. Formation
exponential
of oxidation products according to law (single-log plot; a.u. = arbitrary units).
Thus, the existence of induction times does not introduce any artifacts into rate laws corresponding to exponential increase. This is, of course, a consequence of the properties of the exponential function. In fact, if t,, is the induction time of a process, the following relation is valid: a exp b(t - to) = u exp(-bt,)
exp(bt) = a, exp(bt)
This equation shows that the effect of the induction time is restricted to the pre-exponential factor. It does not affect the exponent which is a
Table 2. Formation
Concentration
of oxidation exponential
Oxidation
C
._~ 0.04 0.10 0.15 0.20 0.40 0.70 1.oo
fA
300 500 580 650 800 920 1000 ~~_._ ._ -- ._~ Concentration and aging time The original data correspond 0.01 exp(O.O0460517t,).
products law
according
to an
products formed after aging times tl3 tc tn 800 1000 1080 1150 1300 1420 1500
1300 1500 1580 1650 1800 1920 2000
2300 2500 2580 2650 2800 2920 3000
in arbitrary units. approximately to the function
The kinetics reported in the literature for carbonyl and hydroperoxide group formation on thermal oxidation of low density polyethylene in the solid state are not generally valid. As a rule, they account only for the samples with which they have been generated. Besides the limited amount of data from which the conclusions were drawn, there is another main reason for conflicting interpretations. It resides in almost exclusive or at least preferential fitting of experimental results to power laws. The latter can lead to considerable misinterpretations, especially in the presence of more or less pronounced induction times which are the rule rather than the exception with phenomena such as thermal oxidation of polyolefins. The result may be a good fit to some power law, although there is no causal relationship between that power law and the phenomenon studied. The comparison of different kinds of plots of the data shows that carbonyl groups and associated hydroperoxides are formed in the early stages of oxidation according to autoaccelerated kinetics presenting some characteristics of the exponential type. However, in later stages of oxidation, carbonyl groups and free hydroperoxides increase linearly with oxidation time. The concentration of free hydroperoxide groups increases according to a S-shaped curve before it begins to decrease in an advanced oxidation stage. In forthcoming work we attempt to relate the experimental aspects studied here to the physico-chemical processes thought to occur on thermal oxidation of polyolefins (see Parts 2 and 3 of this paper).
REFERENCES Reich, L. & Stivala, S. S., Autoxidation of Hydrocarbons and Polyolejins. Marcel Dekker, New York, 1969. Holmstriim, A. & Sorvik, E. M.. J. Poiym. Sci., Polym. Chem. Ed., 16 (1978) 2555, Gugumus, F., In Oxidation Inhibition in Organic Materials, Vol. 1, ed. P. P. Klemchuk & J. Pospisil. CRC Press Inc., Boca Raton, 1990. p. ab. Gugumus, F., In Plastics Additives. 3rd edn, ed. R.
F. Gugumus
5. 6. 7.
8.
Gaechter & H. Mueller. Hanser Publishers, Munich. 1990, p. 1. Emanuel, N. N., Polym. Sci. USSR, 27 (1985) 1505. Ttidiis, F. & Iring, M., Acta Polym., 39 (1988) 19. Iring, M. & Tiidiis, F., Prog. Polym. Sci., 15 (1990) 217. Piroja, E. K. & Lippmaa, H.V., Acta Polym., 36 (1985)
196. 9. Denisov, Ye. T., Polym. Sci. USSR, 19 (1978) 2893.
10. Kirgin, A. V. & Shilov, Yu. B. Denisov, Ye. T. & Yefimov, A. A., Polym. Sci. USSR, 28 (1986) 2486. 11. Petruj, J. & Marchal, J., 19th Prague Microsymp. on Macromolecules, Mechanism of Degradation and Stabilization of Hydrocarbon Polymers, Preprint M 46,
Prague, 1979. 12. Petruj, J. & Marchal, J., Radiat. Phys. Chem., 16 (1980) 27. 13. Arnaud, J., Moisan, J. Y. & Lemaire, J., Macromolecules,
17 (1984) 332.
14. Fanton, E. Pellereau, Arnaud R. & Lemaire. J., J. Polym. Photochem., 6 (1985) 437. 15. Fanton, E., Tidjani, A. & Arnaud, R., Polymer, 35 (1994) 433.
P., Hennekens, J. & Vincent, J., Preprints of the Int. Con& on Advances in the Stabilization and Controlled Degradation of Polymers, Luzern, 24-26
16. Gijsman,
May, 1989, pp. 33-48. 17. Gijsman, P., Hennekens,
J. & Vincent,
J., Polym.
Degrad. Stab., 42 (1993) 95. 18. Langlois, V., Audouin, L., Courtois, P. & Verdu, J., Angew. Makromol. Chem., 208 (1993) 47, 19. Celina, M. & George, G. A., Polym. Degrad. Stab., 40 (1993) 323. 20. Gugumus, F., Polym. Degrad. Stab., 44 (1994) 299. 21. Gugumus, F., Angew. Makromol. Chem., 137 (1985)
189. 22. Gugumus, F., Preprints of the Int. Conf: on Advances in the Stabilization and Controlled Degradation Polymers, Luzern, Switzerland, 21-23 May, 1986.
23. Gugumus, F., Paper presented Microsymposium, Degradation and Switzerland, 11-12 October, 1994. 24. Gugumus, F., Polym. Degrad. Stab., 25. Gugumus, F., Polym. Degrad. Stab.,
of
at the EPF 94 Stabilisation, Basel, 24 (1989) 289. 44 (1994) 273.
Degradatron
Printed
and Stuhility 52 (lYY6) 131-144
0 1YY6 Elsevier Science Limited m Northern Ireland. All rights reserved
0141.3910(95)00229-4
0141-3910/96/$15.00
Thermooxidative degradation of polyolefins in the solid state: Part 1. Experimental kinetics of functional group formation F. Gugumus Ciba-Geigy AG, CH-4002 Basel, Switzerland
(Received 21 September
1995; accepted 26 October
1995)
The knowledge accumulated so far on thermal oxidation of polyolefins does not yield a model of the processes that is sufficiently accurate to account for a majority of the experimental data accumulated with unstabilized as well as with stabilized polymers. The present paper is concerned with some aspects of thermal oxidation of polyolefins, especially polyethylene, in the solid state. In this respect, the kinetics of thermal oxidation of polyethylene reported in the literature are tested with numerous experimental data generated in our laboratory. It is found that the experimental and formal kinetics proposed so far do not account for our results. Thus, instead of quadratic or biquadratic formation of carbonyl groups with oxidation time as reported in the literature, the rate law seems more complicated. It is found that there is an exponential type increase in the early stages of the oxidation. This is followed by a linear increase in later stages. The discrepancy with the literature results is attributed on the one hand to the fact that the conclusions were based on a very limited amount of experimental results. It is attributed on the other hand to the use of fitting to power laws. The possible pitfalls associated with this procedure are discussed in detail. 0 1996 Elsevier Science Limited
1 INTRODUCTION
phenomena occurring on oxidative degradation just to improve the lifetime under use conditions. However, although stabilization against thermooxidative degradation has made considerable progress,‘.4.24.2’ the physico-chemical processes occurring on thermal oxidation of polyolefins in the solid state are far from being understood. Many oxidation products were detected, and numerous factors controlling thermal oxidation were identified (oxygen pressure, temperature, initiation rate, sample thickness, stretch ratio, supermolecular structure, etc.). There are only a few attempts to relate the experimental kinetics of oxidation of solid polyolefins to the chemical phenomena thought to occur in the solid polymer. Such attempts are reported by Emanuel5 and Petruj ef ~1.’ I,‘* Emanuel, sumliterature, reports marizing mainly Russian parabolic increase of oxygen absorption and
Thermal oxidation of polyolefins has been the object of numerous investigations.‘-l4 Although many features of the phenomena were uncovered through past work, they have not yet been completely elucidated and research is still very active in this field.‘5-23 However, many studies were relative to polymer melts. They can be considered to simulate more or less the conditions encountered on polymer processing. Other studies, mainly of academic interest, involved polymers in solution, in attempts to isolate the chemical reactions that may cause oxidative degradation. Some work was concerned with polyolefins in the solid state. This is, of course, more relevant to the conditions encountered in practical use of the polymers. Very often it was initiated for a better understanding of the 131
132
F. Gugumus
carbonyl group formation with aging time. Petruj et al. find also quadratic increase of the concentration of hydroperoxide groups with aging time of polyethylene films at 105°C. However, in contrast with the relationship reported by Emanuel, they found bi-quadratic increase of the concentration of carbonyl groups. In the following, these different results will be tested with our own experimental results.
2 EXPERIMENTAL The polymers used for sample preparation were mainly unstabilized commercial resins stored in a refrigerated room to minimize oxidation. PE-LD films up to a thickness of 250 pm were blown at 200°C. A 500 pm melt-cast film was prepared by extrusion of PE-LD at 200°C. Films with thicknesses between 60 pm and 1 mm were also prepared by compression molding at 170°C for 6 min. The films up to 0.5 mm thickness were immediately quenched in cold water (1O’C). The 1 mm thick plaques were not quenched in water but cooled in a cold press with circulating water. A 500 pm thick PP film was prepared by extrusion of third generation PP at 230°C. Oxidation of polyolefins in the solid state, in the form of films of various thickness, was performed in forced draft air ovens (HORO, Model 080 V/EL) mainly at a temperature of 80°C. Some experiments involved slightly higher temperatures, 90 and 100°C or lower temperatures, 60 and 40°C. Functional group formation was measured by IR spectrophotometry with an unexposed sample in the reference beam. The data are mostly expressed as the absorbance at the wavelength corresponding to the maximum of the absorption of the band, e.g. &,,, for the absorbance associated hydroperoxides whose maximum is usually between 3400 and 3420cm-‘. The spectrophotometers used were Perkin-Elmer models 781,782 and FT-IR 1710.
3 EXPERIMENTAL KINETICS OF THERMAL OXIDATION OF POLYETHYLENE Most studies on thermooxidation of unstabilized polyethylene up to now were performed with low
density polymer (PE-LD). This can be attributed, on the one hand, to historical reasons because this polymer has been available for more than 60 years since it was synthesized for the first time in 1933. It can be attributed, on the other hand, to the fact that PE-LD can be processed in the absence of any stabilization without too much deterioration. This is not so for the different high-density (PE-HD) and linear low density (PE-LLD) polyethylenes which need an adequate stabilization for storage and processing. The changes in the IR spectrum of a typical PE-LD blown film subject to thermal oxidation in a forced air oven are shown in Fig. 1. It can be seen, that there is considerable increase both in the hydroxyl and in the carbonyl absorbance regions. Simultaneously, there are also changes in the hydrogen deformation peaks of the carboncarbon double bonds as well as in the carbon-oxygen single bond absorption region. The hydroxyl and carbonyl groups usually account for most of the oxidation products on thermooxidative degradation of polyethylene. In this respect, carbonyl group concentrations are often used to monitor oxidative degradation of polyolefins. The correlation with the deterioration of the mechanical properties of the polymer is usually good, at least qualitatively. Correlation on a quantitative basis is not always excellent because only part of the carbonyl groups formed correspond effectively to chain scissions, mostly responsible for the degradation of the mechanical properties. In the following, as in previous work, the carbonyl concentration measured by IR spectroscopy will be used to monitor the thermal oxidation of polyolefins.
3.1 Kinetics of carbonyl group formation
Different samples of a 200 pm thick PE-LD film were aged in a forced draft air oven at 100°C. The carbonyl absorbance of the films, measured by IR spectroscopy, was plotted in Fig. 2 as a function of aging time at 100°C. It can be seen, that the plots of Fig. 2 show the typical auto-accelerated behavior reported in the literature. The same data were re-plotted in Fig. 3 in a double-logarithmic representation. For all the samples shown in Fig. 3, segments of straight lines are obtained. These straight lines indicate carbonyl group formation as a function of aging
Thermooxidative
degradation of polyolefins in the solid state-l
133
0.160 0.26 0.24
0.160
0.16 8 5
0.16
e g
0.14
$
0.12
0.00 3650
3200
3400
3100
1850
l&O
1750
17Oa
1650
1600
1000
950
900
650
600
CM-l
Fig. 1. Changes in the IR absorption
spectrum of a PE-LD film on thermal oxidation at 80°C. Film thickness: 200~m. times (h): 183,279, 377, 500, 614,781.
time according to power laws, as reported in the literature.5,“,‘2 They correspond to an equation of the type: (Carbonyl) = at’ where t is the aging time and a and b are constants characteristic of the sample and the aging conditions. It is expected that, for samples of the same polymer, taken from the same film and aged under the same conditions, the exponents b in the formula given above should be the same. However, this is not so for the samples shown in Fig. 3. The values of the exponents b found for 2.6 and 8.9, these samples vary between depending on the particular sample and possibly other, unknown, factors. Calculation of the exponents by means of a curve-fitting program yields, as expected, values close to those deduced from the double-logarithmic representation in Fig. 3. Hence, significant differences that might occur, cannot be attributed to the fact that, in older work, conclusions were often based on double-logarithmic representations. Some of the values of the exponent b deduced from the data in Fig. 3 support to some extent formation of carbonyl groups according to a
Aging
quadratic increase with aging time as reported Emanuel:5 (Carbonyl) = at*
by
Other values of the exponent b, even more numerous with the samples in Fig. 3, support rather a bi-quadratic equation such as that found by Petruj et ~l.:“,‘~ (Carbonyl) = at” Most values of b deduced from the data in Fig. 3 point to equations of an order between two and four and of even higher than four, up to six and more. Hence, it is quite clear, that neither of the equations reported so far to account for carbonyl formation as a function of thermal aging time, can account even for a very restricted amount of experimental data such as that shown in Fig. 3. An additional plot of the same data yields a possible explanation for the discrepancy between various data sets of thermal oxidation of polyethylene films. It can be seen in Fig. 4, that the semi-logarithmic plot yields straight lines up to relatively high values of the carbonyl group concentration. The useful mechanical properties of the polymer samples will usually be lost long before such carbonyl values are reached on
F. Gugumus
134
10.00
1.25
0 Sample
1 (8 = 4.8)
Sample
2 (s = 8.9)
q
1 .oo
0 Sample
1
0 Sample
2
A Sample
3
Q Sample
4
l
Sample
5
n
Sample
6
A Sample
7
A Sernple 3 (s = 3.8) 0 Sample
4 (s = 2.6)
Sample
5 (8 = 3.9)
. Sample
6 (s = 6.2)
A Sample
7 (8 = 3.1)
l
0.25
0.00 0
250
500
Time at 100X(h)
10
100 Time at 100%
1000 (h)
Fig. 2. Carbonyl formation in 200km thick PE-LD blown films on thermal oxidative aging at 100°C (normal plot).
Fig. 3. Carbonyl formation in 200/1m thick PE-LD blown films on thermal oxidative aging at 100°C (double-log plot; s = slope).
thermal oxidation. The plot of Fig. 4 is quite puzzling, at least at first sight. As a matter of fact, it suggests an exponential increase of the carbonyl group concentration with aging time. The fundamental problems involved with such an interpretation led to a broad investigation and re-examination of thermal oxidation of polyolefins, especially polyethylene. First of all, it was checked that the results obtained with the low density polyethylene used for our experiments were not specific to that particular PE-LD, nor to the aging temperature of 100°C. Films were prepared by compression molding, starting from six different high pressure polyethylenes which had been polymerized in tubular reactors as well as in autoclaves. The carbonyl absorbance measured on simultaneous aging of the films in a draft air oven at 90°C is shown in Fig. 5 as a double-logarithmic representation. It can be seen in Fig. 5 that, in these experiments also, the exponent of the power law varies considerably. This is much less so if the same data are presented in a semi-logarithmic plot (Fig. 6). It can be seen in Fig. 6 that the different PE-LD samples show
approximately parallel increase of carbonyl absorbance with aging time at 90°C. It can also be seen in Fig. 6 that, if carbonyl group absorbance is measured up to high values, there is strong deviation from linearity. The data generated at 80°C with film samples from the same polymers, also show varying behavior in the double-logarithmic plot (Fig. 7). However, this time the semi-logarithmic plot of the data also shows pronounced differences between the polyethylenes tested (Fig. 8). It can be seen in Fig. 8 that the segments of straight lines are not all approximately parallel as at 90°C. Furthermore, the deviation from linearity occurs already for lower values of the carbonyl absorbance than at 90°C. These results show quite clearly that the increase of carbonyl groups on thermal oxidation does not follow a single, purely exponential curve. Figure 9 shows a special aspect of the increase of carbonyl absorbance on thermal oxidation. It can be seen that, after an initial, autoaccelerated increase, the rate of formation of carbonyl seems to become constant. This is even more visible in Fig. 10 corresponding to the aging at 80°C because more
Thermooxidative
135
degradation of polyolefins in the solid state-l 1O.W
10.00 0 Sample 1
E 0 Sample 2
4 Sample 3 0 Sample 4
. Sample 5 n
Sample 6
. Sample 7
1.oo yg t $ 5 I 5 c B 9 0.10
0.01
0.01 100
0
150
200
250
300
350
loo
0
200
Time al 1W”C (h)
Fig. 4. Carbonyl formation in 2OOpm thick PE-LD blown films on thermal oxidative aging at 100°C (single-log plot).
300
400
500
600
Time al 90°C (h)
Fig. 6. Carbonyl formation on thermal oxidation at 90°C of 500 pm films (semi-log plot).
O PE-LD-A (s = 4.3) q
PE-LDB
(a = 2.5)
q
stPE-LD-C (s = 3.7)
PE-LDB
(a = 2.4)
A PE-LD-C (s = 5.0) o PE-LD-D (s = 4.6)
o PE-LD-D (s = 4.5)
. PE-LD-E (s = 3.1)
. PE-LD-E (s = 3.6)
. PE-LD-F (s = 4.9)
. PE-LD-F (8 = 4.8)
loo Time a SO’C (h)
1000
loo Time at 8O’C (h)
Fig. 5. Carbonyl formation on thermal oxidation at 90°C of 500 pm films prepared by compression molding of different unstabilized PE-LD samples (double-log plot; s = slope).
Fig. 7. Carbonyl formation on thermal oxidation at 80°C of 500 pm films (double-log plot: s = slope).
F. Gugumus
136
1
2.00 I
10.00 i
I-
J
o PE-LD-A
O PE-LD-A
PE-LD-B
II PE-LD-B
A PE-LD-C
~bPE-LD-C
o PE-LD-D I-----l
o PE-LD-D
q
. PE-LD-E 1.50 -
. PE-LDF
‘g z $ 1
1.00 -
t B i 9 0.10
0.50 -
0.01 0
500
1000
1500
0.00
4-e 0
Time at 80°C (h)
500
1000 Time at 80%
Fig. 8. Carbonyl formation on thermal oxidation at 80°C of -500 jkrn films (semi-log plot).
2.00
PE-LD-B
& PE-LD-C D PE-LD-D 1.50
. PE-LD-E
3.2 Kinetics of hydroperoxide
. PE-LD-F
‘E 0 f $ 1 z
Fig. 10. Carbonyl formation on thermal oxidation at 80°C of 500 pm films (normal plot).
data points are available. There is another point to be deduced from the plots of Figs 9 and 10: the constant rates of carbonyl formation are comparable for the different PE-LDs tested, although they are not identical.
o PE-LD-A q
1500
(h)
1.00
f c 0 s
0.50
0.00 0
250
500
750
Time at 5W’C (h)
Fig. 9. Carbonyl
formation on thermal oxidation 500 pm films (normal plot).
at 90°C of
formation
As already mentioned, hydroperoxides were reported to be formed according to a quadratic ~aw~s.llJ2 These findings must be considered to be as fortuitous as the corresponding findings for the carbonyl groups. It can be seen in the double-logarithmic plot of Fig. 11 for the associated hydroperoxides that the exponent of the power law varies between 1.9 and 4.5. These values give an idea of the spreading; they should not be seen as extreme values. The corresponding plot for the free hydroperoxides in Fig. 12 shows comparable spreading of the exponent values. However, the absolute values are usually lower than those for the associated hydroperoxides. The plot of Fig. 11 shows deviations from linearity for high values of the concentrations of the associated hydroperoxides. In the plot for the free hydroperoxides in Fig. 12, there is also
Thermooxidative
o PE-LD-A
(s = 3.2)
q PE-LD-B
(8 = 1.9)
A PE-LD-C
(s = 2.7)
o PE-LD-D
(8 = 3.5)
. PE-LD-E
(s = 2.6)
. PE-LD-F
(s = 4.5)
100
10
137
degradation of polyolefins in the solid state-l
1000
Time at WC(h)
Fig. 11. Formation of associated hydroperoxides on thermal oxidation at 90°C of 500pm films (double-log plot; s = slope).
deviation from linearity if the concentration increases. It can be seen that the concentration passes through a maximum and starts to decrease. This phenomenon has already been discussed previously (see Ref. 23 and references cited therein) and will be treated in more detail in the following work. The conclusions regarding the spreading of the values of the exponents of the power law remain the same if instead of free or associated hydroperoxides, the sum of both hydroperoxides is plotted as shown in Fig. 13. The conclusions are again similar with the corresponding plots of the data generated at 80°C (Figs 14-16). The semi-logarithmic representation of the absorbance of the associated hydroperoxides as a function of aging time at 90 and 80°C in Figs 17 and 18, respectively, shows different aspects of the kinetics of hydroperoxide formation. It can be seen in Figs 17 and 18 that the logarithm of the absorbance increases linearly with aging time in the first stages of oxidation. However, the plots show increasing downward curvature with advancing oxidation. This change is explained, at least in part, by the representation of the same
1.00 Io PE-LD-A
(s = 2.6)
q PE-LD-B
(s = 1 .O)
A PE-LD-C
(s = 2.0)
o PE-LD-D
(s = 2.1)
. PE-LD-E . PE-LO-F
(s = 2.0) (s = 3.3)
o PE-LD-A
(s = 2.5)
o PE-LDB
(s = 1.6)
d PE-LD-C
(s = 2.4)
o PE-LO-D
(s = 3.0)
. PE-LD-E
(s = 2.4)
. PE-LD-F
(s = 3.5)
1
0.01
L
10
100
1000
Time at 60°C (h)
Fig. 12. Formation of free hydroperoxides on thermal oxidation at 90°C of 500 pm films (double-log plot; s = slope).
100 Time al 90°C (h)
Fig. 13. Formation of hydroperoxides on thermal oxidation at 90°C of 500 pm films (double-log plot: s = slope).
138
F. Gugumus 1 .oaI-
1.00
h PE-LD-C
(S = 3.2)
o PE-LD-D
(8 I 3.6)
o PCLDD
(s = 3.4)
. PE-LDE
(8 e 3.1)
. PE-LD-E
(S = 3.2)
. PE-LD-F
(5 = 4.2)
I-
I
+
100
1000
10000
Time at 80°C (h)
Time at 80°C (h)
Fig. 14. Formation
oxidation
at
of associated hydroperoxides on thermal 80°C of SOOpm films (double-log plot: s = slope).
Fig. 16. Formation
of hydroperoxides on thermal oxidation at 80°C of 500 Km films (double-log plot: s = slope).
0.11D-
I
qf=
0 PE-LD-A
(S = 2.3)
[1 PE-LDB
(s = 1.2)
b PE-LD-C
(8 = 2.0)
o PE-LD-D
(8 = 1.8)
. PE-LD-E
(S = 2.8)
. PE-LD-F
(I = 2.3)
r_
f
i
1000 250
Time at 30°C (h)
Fig.
15. Formation
oxidation
at
80°C
of free hydroperoxides on thermal of 500pm films (double-log plot: s = slope).
500 Time at 90%
Fig. 17. Formation
(h)
of associated hydroperoxides on thermal oxidation at 90°C of 500 pm films (semi-log plot).
Thermooxidative
degradation of polyolefins in the solid state-l 0.75 O PE-LD-A q
PE-LD-B
& PE-LD-C o PE-LD-D . PE-LD-E . PE-LD-F
0.25
4 250
1000
of associated hydroperoxides on thermal oxidation at 80°C of 500 pm films (semi-log plot).
data in normal coordinates as shown in Figs 19 and 20, respectively. It can be seen that, after the initial, autoaccelerated stages, the concentration of associated hydroperoxides increases linearly with oxidation time for a long time period. This parallels the corresponding behavior found for carbonyl groups as shown above. Representation of the absorbance of the free hydroperoxides in semi-logarithmic coordinates yields segments of straight lines in the initial stages of oxidation, followed by progressive leveling off maintained for a more or less pronounced time period. Afterwards, the absorbance decreases until it can no longer be distinguished as a separate peak in the broad hydroxyl peak accounting mainly for associated hydroperoxides. This is shown with the data generated with samples of six different low density polyethylenes at 90 and 80°C in Figs 21 and 22, respectively. It is clear that the exponential increase, if it is exponential at all, is observed in the initial stages of oxidation only. The representation of the absorbance of the free hydroperoxides in normal coordinates completes the view on the kinetics of free hydroperoxide formation. It can be seen in
750
Time at 90°C (h)
TimeatEO”C(h)
Fig. 18. Formation
500
Fig. 19. Formation
of associated hydroperoxides on thermal oxidation at 90°C of 500 pm films (normal plot).
0.75
0.50 ‘E 0 z f r z $ ii B a 0.25
0.00 0
1000 Time at 80°C (h)
Fig. 20. Formation
of associated hydroperoxides on thermal oxidation at 80°C of 500 Km films (normal plot).
F. Gugumus
q
PE-LD-8
A PE-LD-C o PE-LD-D . PE-LD-E . PE-LD-F
250
0
500
750
Time at 90°C (h)
Fig.
21. Formation of free hydroperoxides on thermal oxidation at 90°C of 500 pm films (semi-log plot).
Fig. 23 for the data obtained at 90°C and, even more so, in Fig. 24 for the data generated at 80°C that the increase of the concentration of the free hydroperoxides with aging time proceeds according to S-shaped curves. It is only some time after reaching its maximum value that the concentration decreases more or less rapidly, depending on the specific sample and aging temperature. In summary, the data presented show that the experimental kinetics for some functional groups on thermal oxidation of polyethylene do not correspond to the equations proposed so far in the literature. The rate law describing the formation of carbonyl groups is neither quadratic in aging time nor biquadratic. It seems rather close to an exponential-type increase in the early stages of oxidation. With ongoing oxidation, the increase of carbonyl’ groups becomes linear with at a constant rate. time, i.e. it proceeds Formation of associated hydroperoxides also does not correspond to a quadratic law. It seems to follow an exponential-type increase in the early stages of oxidation and change to a linear increase later on, as observed for the carbonyl groups. The kinetic law is again different for the
- -
0.15
0.10
d PE-LD-A D PE-LD-6 A PE-LD-C o PE-LD-D . PE-LD-E . PE-LD-F
z 8 f E
B a
0.05 -
0.01 0
500
1000
1500
2000
250
Fig.
22. Formation of free hydroperoxides on thermal oxidation at 80°C of 500 pm films (semi-log plot).
500
750
Time at 90% (h)
Time at 90°C (h)
Fig.
23. Formation
of free hydroperoxides on thermal oxidation at 90°C of 500 pm films (normal plot).
Thermooxidative 0.10
!
0.01
--I
0
500
1000
1500
141
degradation of polyolejins in the solid state-l
2000
Time al 80°C (h)
Fig. 24. Formation of free hydroperoxides on thermal oxidation at 80°C of 500 /*rn films (normal plot).
The statement will be illustrated by a constructed example as shown in Fig. 25 with the data presented in Table 1. The plot in Fig. 25 represents, e.g., carbonyl formation as a function of oxidation time. The starting point of the construction is carbonyl group concentration increasing linearly with time from the start of the experiment. This is represented by the straight line passing through the origin of the coordinates in Fig. 25. The other straight lines shown in Fig. 25 are simply deduced from the first line by parallel translation to increasing times. This is to simulate processes showing the same rate of formation of oxidation products after various induction times. The data in Fig. 25 have been replotted in a double logarithmic representation in Fig. 26. Of course, the data pertaining to the original straight line also yield a perfect straight line in the double logarithmic representation. In addition, the data deduced by translation also yield reasonably straight segments of straight lines. With increasing ‘induction periods’, the slope of the lines in Fig. 26 increases markedly. The results of fitting the data to the power law are given in Table 1. It can be seen that, from the
free hydroperoxides. After an initial autoaccelerated stage, the concentration passes through a more or less flat maximum before it starts to decrease. 4 POSSIBLE REASONS FOR CONFLICTING EXPERIMENTAL KINETICS In the preceding section, we have seen that conclusions based on a very restricted amount of experimental data are certainly one major reason for conflicting interpretations of thermal oxidation of polyolefins. Nevertheless, this point alone does not explain all the contradictions. There is another reason for the discrepancies between experimental kinetic laws for carbonyl formation on oxidation of polyolefins. It consists in the use of double logarithmic representations or its equivalent nowadays, data fitting to a power law. This is not meant to discourage use of such representations or fittings but to encourage their use with great care. This care is especially important in the area under investigation since oxidation phenomena very often show pronounced induction periods.
0
500
1000
1500
2OKl
2500
3000
3500
Time (au.)
Fig. 25. Formation of oxidation products according to linear increase with aging time (normal plot: a.u. = arbitrary units).
F. Gugumus
142
Table 1. Formation of oxidation products according to a linear law Concentration
Oxidation
C
fA 0.05
150
0.10 0.15 0.20 0.25 0.30 0.35 0.40
300 450 600 750 900 1050 1200
Parameters
650 800 950 1100 1250 1400 1550 1700
1150 1300 1450 1600 1750 1900 2050 2200
2150 2300 2450 2600 2750 2900 3050 3200
of the fitting to the power law c = at”
0.333.10-’ a Power b 1.ooo Correlation coefficient r* 1.ooo Concentration
products formed after aging times tcl k ts
0.83.10-’ 2.084 0.976
0.628.10~‘* 2.614.W” 3.049 4.924 0.960
0.946
and aging time in arbitrary units.
original to the fourth data series, the exponent of the power law increases from the original value 1 to almost 5! Even in the last instance, the value of the correlation coefficient, r2 = 0.94, still indicates a fair correlation. Hence, it is quite obvious that more or less pronounced induction times with different samples of the same origin 1.O()--
can lead to considerable differences in the exponent of the power law. This can explain, at least in part, differing values reported in the literature for the same phenomena examined under comparable conditions. If the data used so far are plotted according to a single logarithmic representation, as in Fig. 27, there is no linearity and the data deduced by translation of the original data yield curves similar to the curve obtained with the original data. There is another advantage of single logarithmic plots which yield straight lines if the concentration of oxidation products increases exponentially with aging time. It is the fact that, if the induction period varies considerably from one experiment to the other, the representation remains a straight line with the same slope. There is just a parallel shift corresponding to the induction time. This is illustrated in Fig. 28 for the same increments of 500, 500 and 1000 arbitrary units in induction time as previously but, of course, different values of the concentration of oxidation products to fit the exponential increase. The corresponding data are shown in Table 2. 1.00
l-
Ii10
100
loo0
wooo
Time (a.~.)
Fig. 26. Formation of oxidation products according to linear increase with aging time (double-log plot; a.u. = arbitrary units).
1000
zoo0
3ooa
Time (a.~.)
Fig. 27. Formation of oxidation products according to linear increase with aging time (single-log plot; a.u. = arbitrary units).
Thermooxidative
degradation of polyolejins in the solid state-l
measure of the rate of formation products.
143
of oxidation
5 CONCLUSIONS 1.00
F d
5 t L
z
e
s
0.10
I, /
0.01 0
,,, , 3ooo
2ooo
1000
( 4ooo
Time (a.~.)
Fig. 28. Formation
exponential
of oxidation products according to law (single-log plot; a.u. = arbitrary units).
Thus, the existence of induction times does not introduce any artifacts into rate laws corresponding to exponential increase. This is, of course, a consequence of the properties of the exponential function. In fact, if t,, is the induction time of a process, the following relation is valid: a exp b(t - to) = u exp(-bt,)
exp(bt) = a, exp(bt)
This equation shows that the effect of the induction time is restricted to the pre-exponential factor. It does not affect the exponent which is a
Table 2. Formation
Concentration
of oxidation exponential
Oxidation
C
._~ 0.04 0.10 0.15 0.20 0.40 0.70 1.oo
fA
300 500 580 650 800 920 1000 ~~_._ ._ -- ._~ Concentration and aging time The original data correspond 0.01 exp(O.O0460517t,).
products law
according
to an
products formed after aging times tl3 tc tn 800 1000 1080 1150 1300 1420 1500
1300 1500 1580 1650 1800 1920 2000
2300 2500 2580 2650 2800 2920 3000
in arbitrary units. approximately to the function
The kinetics reported in the literature for carbonyl and hydroperoxide group formation on thermal oxidation of low density polyethylene in the solid state are not generally valid. As a rule, they account only for the samples with which they have been generated. Besides the limited amount of data from which the conclusions were drawn, there is another main reason for conflicting interpretations. It resides in almost exclusive or at least preferential fitting of experimental results to power laws. The latter can lead to considerable misinterpretations, especially in the presence of more or less pronounced induction times which are the rule rather than the exception with phenomena such as thermal oxidation of polyolefins. The result may be a good fit to some power law, although there is no causal relationship between that power law and the phenomenon studied. The comparison of different kinds of plots of the data shows that carbonyl groups and associated hydroperoxides are formed in the early stages of oxidation according to autoaccelerated kinetics presenting some characteristics of the exponential type. However, in later stages of oxidation, carbonyl groups and free hydroperoxides increase linearly with oxidation time. The concentration of free hydroperoxide groups increases according to a S-shaped curve before it begins to decrease in an advanced oxidation stage. In forthcoming work we attempt to relate the experimental aspects studied here to the physico-chemical processes thought to occur on thermal oxidation of polyolefins (see Parts 2 and 3 of this paper).
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