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Chemiluminescence from oxidation of polypropylene: Correlation with peroxide concentration
ELSEVIER
PII:
SOlSl-3910(96)00079-l
Polymer Degradation and Stability 53 (19%) 119-127 0 1996 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0141-3910/%/$15.00
Chemiluminescence from oxidation of polypropylene: Correlation with peroxide concentration A. Kron,O B. Stenberg,” T. Reitbergeti & N. C. Billingham’ “Department of Polymer Technology, bDepartment of Nuclear Chemistry, KTH, S-100 44 Stockholm, Sweden “School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNI 9QJ, UK
(Received
15 January
1996; accepted 17 February
1996)
The chemiluminescence (CL) on heating in an inert atmosphere has been measured for samples taken at various times during oxidation of polypropylene (PP). Proportional relationships were obtained when the integrated emission (TLI) was plotted against peroxide concentration for all studied sets of parameters. Changes in melting temperature and molecular weight with ageing time were found to correlate with changes in the TLI. Oxidised PP was exposed to dimethylsulfide (DMS) for different periods with subsequent measurements of the CL and the peroxide concentration. The peroxide concentration has earlier been found to exhibit a two-step decay with DMS exposure time, indicating two fractions of peroxides with different reactivity. The CL was found to follow the same two-step decay, indicating that both peroxide fractions contribute to CL. The oxidative stability of PP, measured by CL induction times in oxygen, increases when the polymer is exposed to DMS. 0 1996 Elsevier Science Limited
Oxidation progresses through reactions of oxygen with alkyl radicals leading to the formation of hydroperoxides on the polymer backbone. Subsequent decomposition of the hydroperoxides to new radicals leads to an autoaccelerating cycle of reactions, continuing as long as oxygen is present. In a low-molecularweight hydrocarbon an alkoxy or peroxy radical abstracts hydrogen atoms from adjacent molecules. This happens in polymers as well. In a solid material hindered reactant mobility increases the probability of p-scissions of alkoxy radicals to form ketones/aldehydes and alkyl radicals.4 Polymers containing tertiary carbons (such as PP) are particularly vulnerable to chain scission. Due to the incorporation of oxidation products and to scissions of the polymer chains the mechanical properties of the polymer deteriorate so that its useful life is shortened. The formation and decomposition of the hydroperoxides are key reactions in oxidation, and therefore of great importance in studies of
1 INTRODUCTION Polyolefins are oxidatively degraded in UV-light or at elevated temperatures, leading to unwanted physical effects like embrittlement and discolouration. Due to the commercial importance and large volume production of polyolefins like polyethylene (PE) and polypropylene (PP) a great deal of research has been performed on their oxidative degradation, with better materials and stabilising systems as the result” However there are still many unanswered questions about oxidative degradation, especially its early stages. Oxidation of polymers is often described by using a simplified free-radical reaction scheme.1’l-4 It can be used for polymers only as long as one bears in mind that the restricted mobility of the polymeric radicals and the peroxides in the solid phase makes the oxidation heterogeneous. Heterogeneous oxidation also occurs because no oxidation takes place in the crystalline phase due to its impermeability to oxygen. 119
A. Kron et al.
120
yields a carbonyl together with an alcohol group and a molecule of oxygen.“,13 The mechanism is shown in Fig. 1; it requires at least one of the peroxyl radicals to be primary or secondary. One of the objections to the Russell mechanism is that PP should not give any CL since its peroxy radicals are tertiary. PP actually emits relatively intense CL. The explanation could be that the PP alkoxy radicals lead to chain cleavages through p-scissions to give a chain-end ketone and a chain-end alkyl radical. The alkyl radical will react with oxygen to give a primary or secondary peroxy radical which can terminate with a tertiary peroxy radical to give CL.14 Whatever the process for the formation of CL, the intensity, the frequency of arrival of photons at the detector, is a measure of reaction rate and can be expressed in the form:
oxidative degradation. However, the detection and quantitative measurement of hydroperoxides is not trivial. Common detection methods are FTIR and redox reactions, the former often giving ambiguous results due to superpositioned peaks. Among the redox reactions used in studies of solid samples iodometry is reported to be reliable but tedious while colourimetric Fe(II1) determination has been shown to be less precise.’ Derivatisation of oxidation products with NO in combination with PTIR measurements has been reported to give good results.5’6 In recent years the chemiluminescence (CL) technique has been suggested as a method with potential to answer some of the queries about oxidation. The CL technique measures the weak luminescence that is emitted as a result of oxidative reactions. Chemiluminescence from polymers was first reported by Ashby and by Schard and Russell8 in the early 1960s. Since then instrumental development has increased the sensitivity of the method, making it possible to perform CL measurements at temperatures down to ambient conditions.4 Chemiluminescence from polymers is believed to result from deactivation of an excited carbonyl group formed in the oxidation.g-ll However, the reactions responsible for the formation of these carbonyls are still debated. To give rise to an excited carbonyl group the reaction must afford sufficient energy, and several schemes have been proposed which fulfil the energy requirement. Unimolecular decomposition of hydroperoxides to radicals which then disproportionate in a cage reaction” is suggested, as well as metathesis of alkoxy or peroxy radica1s.l’ The most widely accepted mechanism is the Russell mechanism; a termination reaction between two peroxyl radicals which, through an intermediate tetroxide,
2RW
-
-->
R-O
I = G-@-R where I is the intensity, R is the reaction rate, G is a geometrical term, which must be kept constant for comparative measurements, and @ is the CL quantum efficiency.15 In principle, CL is a very attractive method for studying peroxidation in PP because the measurement is very simple and needs little sample preparation, in marked contrast with the conventional iodometric approach. There is thus considerable potential benefit from looking at the relationship of CL and peroxide content. The previous discussion shows that the identity and reactivity of the different types of peroxides formed during oxidation processes are of interest in studies of oxidative degradation. Heating of a sample in inert atmosphere and following the decay of the peroxide concentration is a way to investigate the peroxide reactivity.
/O-O, H-C-R”
/
ka
II
II
->
C R’
1 ->
0* ROH+Ozz+
0
0’
/
0
\
R”
/
R’
C
+ hv \
R’
Fig. 1. The Russell mechanism for CL emission.
; / R
\
R”
Chemiluminescence from oxidation of PP
Gijsman et ~1.‘~have recently studied the decay of peroxides in oxidised, unstabilised PP in inert atmospheres and found it to consist of two stages, a rapid initial step and a subsequent slow one. They fitted the decay with two consecutive first-order reactions and assumed them to originate from fractions of peroxides with different stability. Similar first-order fits for PP peroxide decay were earlier reported by Chien and Jablonerl’ and by Zolotova and Denisov.” The peroxide concentration decay when heating PP in inert atmosphere and the decay of the CL emitted from the PP sample have been shown to have different rates.15 The CL was found to decay faster than the peroxide concentration, approaching a zero value while there was still a considerable amount of titratable led to the peroxides left. This observation suggestion that among the peroxides there is a fast-decomposing fraction that is CL yielding and a fraction with slower decomposition rate that does not give CL. effects of exposing peroxidised The polypropylene to dimethylsulfide (DMS) have been reported by Zahradnickova et aZ.19Destruction of the peroxides is accompanied by oxidation of DMS to dimethylsulfoxide. The decline in peroxide concentration when oxidised PP was exposed to gaseous DMS was shown to consist of two stages, with an initial high reaction rate and a subsequent low reaction rate. From studies of model compounds it was suggested that the fast-reacting peroxides were peracids. The decay of peroxides with exposure time when exposed to DMS has also been shown to be a two-stage process by Carlsson et ~1.” Their studies of the reaction between DMS and PP peroxides (through derivatisation combined with FTIR) showed that the decrease in set- and tert-hydroperoxide concentrations accounted for most of the decrease in peroxide concentration and that the decay of both those species had a two-stage behaviour. They proposed that the fast-decomposing process could be an effect of carboxylic acids acting as catalysts. This observation has been questioned by Gijsman et uE.,2l and the nature of the fast-decomposing fraction of titratable peroxides remains contentious. The aim of the present work was to study correlations between peroxide concentration and chemiluminescence during thermo-oxidative degradation, and to investigate the effects of DMS exposure on CL from peroxidised PP.
121
2 EXPERIMENTAL The studied polypropylene, PP41-WAX, was supplied by BASF Ltd and used as supplied. The polymer was isotactic and has a melting point of 158°C. It was in the form of a powder which has been storage stabilised with BHT. For accelerated ageing the powder was oxidised in an air-circulating oven at 70 or 120°C. Reactions with DMS were performed by adding 2 ml of DMS to 100 mg of PP powder in a nitrogen atmosphere and allowing it to react for lo-300 min. The samples were then washed, first with hexane and then water, and evacuated for at least 96 h at room temperature in order to remove both remaining DMS and the DMSO formed. This was done because of evidence that DMSO acts as an oxidising agent for 13- and interferes with the iodometric measurements of the peroxide concentrations.22 For iodometric measurements a PP sample (3-10mg depending on peroxide content) was added to a mix of 7 ml of (10 parts of isopropanol:l part acetic acid) solution and 2 ml of a saturated solution of NaI in isopropanol (10 g NaI in 50 ml of isopropanol), and refluxed for 20 min under a flow of nitrogen and protected from light. After the reflux 1 ml of water was added. The concentration of 13- was measured spectroscopically as the absorption at 360 nm (using either a Hewlett Packard 845114 or a Philips PU 8720 spectrophotometer). A reference solution containing no PP was treated the same way and used as a blank. Chemiluminescence measurements were made with two CL-detectors which have been described in earlier papers.15,23 The instrument mainly used (denoted CLDl) was a CLDlOO chemiluminescence detector from Tohuko Ltd. The second apparatus (denoted CLD2) was constructed at the University of Sussex. The CL measurements were made on a single layer of PP particles spread onto a circular aluminium pan (with a diameter of 2cm). Before the measurement was started the sample chamber was purged with nitrogen at room temperature for about 30 min. In the CL measurements in nitrogen atmosphere of total luminous intensity (TLI) the temperature was set to 120 or 150°C at the beginning of the measurement and the emitted CL was recorded during the temperature increase. Due to the construction of CLDl, CL
122
A. Kron et al.
measurements with this detector were of the temperature-ramping type, as opposed to the isothermal measurements previously reported.” However it has been shown that the TLI measured during a ramp experiment in nitrogen is proportional to TLI from an isothermal experiment,l’ so either of the two methods was expected to be acceptable. In CL measurements of OIT (oxygen induction time) the sample was first heated to 100°C in nitrogen and left until isothermal conditions were reached. Oxygen was then admitted to the sample chamber and the measurement was started. The length of the OIT was defined as the time from oxygen admittance to the time when the signal had increased by 25% of its initial value. Gel permeation chromatography (GPC) studies were made with a Waters 150C high-temperature instrument with a refractive index detector. 1,2,4-Trichlorobenzene was used as the mobile phase at 135°C. The flow rate was 1 ml/min. Calibration was performed with polystyrene standards. The values obtained are therefore not absolute values for PP, but are valid for calculation of chain scissions. PP melting points were measured with a Mettler-Toledo TA8000 thermal analysis instrument. Heating rate was lO”C/min. The temperatures presented are the peak temperatures.
3 RESULTS AND DISCUSSION 3.1 CL and peroxide formation When PP oxidises at an elevated temperature, concentration stays at a low the peroxide constant value for a certain period of time. The length of this induction time depends on the oxidation temperature and on the stability of the sample. At the end of the induction time autoacceleration leads to the formation of more and more peroxides. It is during this stage that the material loses most of its mechanical properties. By heating the sample in an inert atmosphere to temperatures at which peroxide decomposition is rapid, the peroxides can be destroyed. If this is done in a chemiluminescence (CL) measurement a CL peak is 0btained.l’ The total luminous intensity (TLI) is defined as the area under the
peak (total number of photons emitted) per unit weight of sample. Figure 2 shows the peroxide concentration in PP as a function of ageing time at 70°C. There is an induction period of around 900 h, due to the antioxidant. For every sample taken for peroxide measurement, a CL measurement in nitrogen was performed and TLI calculated, the results also being shown in Fig. 2. Both the peroxide concentration and the TLI show a maximum after about 1500 h of ageing. The curves in Fig. 2 follow each other, indicating a proportional relationship. In Fig. 3 the TLI is plotted against the peroxide concentration for both the autoacelerating phase and the later, decelerating stage of oxidation. In the accelerating phase, the relationship of TLI and peroxide concentration is linear over the whole range up to the maximum peroxide concentration of over 330 mmol kg-‘. This is in contrast to the results reported by Billingham et a/.,” where there was marked curvature above 150 mmol kg-’ for samples oxidised at 70°C. The slope of Fig. 2 corresponds to around 10’ peroxide decompositions for each photon detected, in agreement with earlier data. In the decline phase the plot is steeper and has an intercept. This is consistent with the earlier report that TLI decays faster than peroxide concentration when peroxidised PP is decomposed.15 However, the decline phase represents oxidation of very heavily oxidised PP and results obtained in this region need to be interpreted cautiously. It has been pointed out by Billingham et al.” that earlier reports on the relation between CL and peroxide concentration in PP have not been made on well characterised samples or at isothermal conditions24’25 so the evidence for proportional relationships might only be circumstantial. We have therefore made a series of measurements to check if the proportionality shown in Fig. 3 was valid under other conditions. The relationship between TLI and concentration of peroxides was investigated for two different ageing temperatures (70 and 120°C) and for two different temperatures of CL measurement (120 and 15O“C), using CLDl. All the series of measurements gave proportional relationships between the TLI and the peroxide concentration throughout the accelerating phase (Fig. 4), though not with the same slopes. For oxidation at 70°C the slope is slightly larger if detection is at
Chemiluminescence from oxidation of PP
0
500
1000
1500
2000
2500
123
3000
:
Ageing time at 70’Clhr of peroxides (0) and total luminous intensity, TLI, (0) of PP powder aged at 70°C in air for different times.
Fig. 2. Concentration
150°C than for detection at 120°C but the effect is not large. In contrast, for oxidation at 120°C we see five times fewer photons per unit of peroxide decomposed. This is in agreement with previous workI in which we proposed that the fastdecomposing fractions of peroxides are more CL emitting than the slow-decomposing ones. Since more of the fast-decomposing peroxides are produced in oxidation at 70°C than at 12O”C, because of their higher stability at lower temperature, the greater sensitivity of CL at low oxidation temperature is consistent. To test whether the discrepancy with earlier results is a function of the sample or of the detector, we also performed the CL measurements on samples aged at 70°C using the
instrument (CLD2) used in previous work. The results again show proportionality over the whole concentration range. The slopes are not comparable because of different collection efficiencies of the two detectors. The changes in molecular weight and melting temperature during ageing at 70°C have been studied by gel permeation chromatography (GPC) and differential thermal analysis (DTA), respectively. The changes in the molecular weight, expressed as chain scissions are shown in Fig. 5(a) and the changes in the melting point in Fig. 5(b). Both curves show an abrupt decrease
1
0 0
1
2
1O4 [ROOH]/mol 0
1
2
1 O4 [ROOH]/mol
3
g-’
Fig. 3. TLI (measured at 120°C over 5000 s in CLDl) versus concentration of peroxides for stabilised PP powder aged at 70°C; (0) phase before maximum, O-1400 h, (+) phase after maximum, 1400-3400 h.
3
g-’
Fig. 4. TLI versus concentration of peroxides for four series of measurements on PP powder; (V) PP aged at 7O”C, TLI measured at 120°C over 5000s in CLDl; (V) PP aged at 7O”C, TLI measured at 150°C over 2500s in CLDl; (0) PP aged at 7O”C, TLI measured at 150°C over 2000s in CLD2; (0) PP aged at 12O”C, TLI measured at 120°C over 5000 s in CLDl.
A. Kron et al.
124
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8-
F .o ln 22 ::
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4
5
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0g 0
1000
500
1500
2000
2500
3000
Ageing time at 70Whr 160
(b)
158
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0
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156 0
$ .-
152. 154
g
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i? 'Z
148 -
s
0
146
0 0
144 0
142 t
Ageing time at 70’Clhr Fig. 5. (a) Chain scission (l/Mn
- l/Ma”) for PP powder during ageing at 70°C in air. (b) Changes in melting temperatures PP powder during ageing at 70°C in air.
after about 1000 h, which indicates that degradation becomes significant at this point. The decrease takes place at the same time as the peroxide concentration and the TLI start to increase. 3.2 Effects of dimethylsulfide on peroxides and CL The effect of dimethylsulfide (DMS) exposure on has been reported concentration peroxide earlier.‘9~20 We have investigated the effects of exposure of oxidised PP to DMS on the CL in nitrogen (TLI) and on the peroxide concentration. Oxidised PP samples were exposed to DMS for different periods of time with subsequent measurements of TLI and peroxide concentration. The two-stage decay behaviour found for the peroxide concentration by Zahradnickoval’
for
and by Carlssonzo was found for both the CL and the peroxide concentration. Figure 6 shows typical data, from a sample aged for 1420 h at 70°C to a peroxide concentration of 330 mmol kg-‘. Initial exposure to DMS causes a rapid decay of the peroxide concentration to about 15% of that at zero exposure time. After the initial rapid reaction, the remaining peroxides are destroyed extremely slowly and the concentration is almost constant. According to Gijsman et a1.,16 about 40% of peroxides formed under these oxidation conditions are of the fast-decomposing type. Thus it seems that both fast- and slow-decomposing peroxides are destroyed under these conditions of DMS exposure, but a significant fraction of titratable peroxide is much more resistant to DMS reaction. The interesting observation is that the TLI and
Chemiluminescence
f
0 g
0.4.: i
. . . . . . . . . . . . . . . . . ..O...................~ 0.0 0
50
1M)
150
200
250
l
300
Time of exposure to DMSl min Fig. 6. Relative concentration
of peroxides (0) and relative TLI (0) of PP powder aged for 1420 h at 70°C in air for different exposure times to DMS.
the peroxide concentration follow each other well. The speculation that CL is directly associated with the more reactive of the two peroxide fractions with different decay rates is not confirmed if we assume that the fastdecomposing peroxides are all destroyed in the early stages of DMS treatment. The fraction of the peroxides that is left unreacted after the first stage of DMS reaction is as capable of giving CL as that destroyed early in the treatment. A possible explanation for the residual slow-reacting fraction is that some peroxides are physically protected from the DMS by being so far into the bulk of the sample that the DMS can not diffuse to them. This was tested by a particle size fractionation experiment. A typical sample of PP powder had a relatively wide particle size distribution, from diameters less than 250 pm up to 1 mm. To investigate the influence of the particle size on the oxidation a series of samples aged for different periods at 70°C were fractionated by sieving. Two fractions were taken; one with diameters smaller than 250 pm and one with diameters bigger than 500 pm. The peroxide concentrations were measured iodometrically for duplicate samples, with and without exposure to DMS. The results are shown in Table 1. Before DMS treatment, the concentration of peroxides is higher in small particles than in large ones for all ageing times, with the effect most marked at the highest peroxide concentrations. This suggests that the peroxides are mostly located near the particle surface. We also found that the two groups of particles do not have the
from oxidation of PP
125
same behaviour with ageing time. The bigger particles have a later maximum in peroxide concentration than the smaller ones. The maximum is thought to appear when the peroxide concentration has reached a critical value where the decomposition rate of the peroxides is bigger than their formation rate. If the oxidation is surface limited then this critical peroxide concentration is equivalent to a certain percentage of the particle surface being oxidised. The later maximum for the larger particles may be due to the fact that there is more oxidisable surface on a large particle when the smaller particle reaches its critical value. This effect probably reflects the highly crystalline morphology of the as-synthesised particles, which restricts oxygen access. The results in Table 1 also show the effect of exposure to DMS for different particle sizes. The fractional decrease in peroxide concentration is found to be about the same for the small and the big particles for all ageing times, though the fraction of peroxides remaining after treatment increases with the ageing time. Since the effect of exposure to DMS is independent of particle size we conclude that physical hindrance of the reaction with DMS is unlikely. Another possible explanation for the DMSunreactive fraction is that oxidation products other than peroxides may contribute to the oxidation of iodide ions at these high levels of oxidation. We cannot exclude this, particularly as the fraction of ‘inert’ material increases significantly with extent of oxidation. It is significant that the same behaviour was
Table 1. Effect of partiele size on peroxide concentration in PP aged at 7O”C,with or without subsequent exposure to DMS Ageing time at Particle Peroxide 70°C (h) diameter (pm) concentration after ageing (mmol kg-‘)
500 500 1420 1420 1710 1710 2750 2750
< 250 >.500 < 250 >500 < 250 >500 < 250 >500
3 2.5 410 157 355 257 146 139
Peroxide concentration after ageing and DMS exposure (mmol kg-‘) 77 32 101 79 92 80
A. Kron
126
found for TLI versus DMS exposure time, so that even samples in which all of the fastdecomposing peroxides should have been removed by DMS treatment are luminescent. This the fastwith where contrasts samples decomposing peroxides have been removed by annealing, when the CL is reduced to near zero.15 3.3 Effect of DMS on oxidative stability Increased oxidative stability of DMS-exposed PP has been shown by Gijsman et a1.,16 using oxygen absorption measurements. We have studied oxidation of DMS-exposed PP by measuring CL in oxygen. The results are presented in Table 2 as oxidation induction times (OIT) at 100°C. Experiments were carried out on samples which had been aged for different periods at 70°C. Dual samples were measured, with and without exposure to DMS. The OIT was found to be prolonged by DMS exposure in all but one case. For the sample aged for only 500 h the effect was the opposite. This is thought to be an effect of stabiliser that remains in the sample after this relatively short ageing time. The stabiliser is thought to evaporate during the storage in vacuum which is done after every DMS exposure experiment, thus leaving the DMS exposed sample more unstabilised than the unexposed. This view was supported by examining samples washed and vacuum-treated without exposure to DMS. Without treatment the unaged powder had an OIT of 1055 min and this was reduced to 670min by washing and drying. Similarly, a sample, aged for 500 h but not exposed to DMS had an OIT of only 209min after vacuum treatment, as compared with 500min with no treatment, thus confirming the proposed explanation. The stabiliser is thought to give minor effects when the samples have reached the phase of accelerating oxidation (shown in Fig. 2 to start at about 1000 h). For ageing times of 1420 h or Table 2. Oxygen induction times (OIT) at 100°C for PP aged at 70°C with or without subsequent exposure to dimethylsulfide _._~
Ageing time at 70°C (h)
OIT for unexposed PP (min)
OIT for DMS exposed PP (min)
500 1020 1420 1710
500 180 2 1
290 205 185 130
et al. more the DMS exposure leads to a higher oxidative stability. Measurements of CL in oxygen thus confirm the results of oxygen absorption measurements.‘6 Comparison of Tables 1 and 2 shows that the effect is profound for moderately oxidised samples. For example, the OIT at 100°C for PP oxidised at 70°C for 1420 and 1710 h is extended by almost 100 times, despite the fact that between 20 and 30% of the original peroxides remain after DMS treatment. It is not clear whether the CL-active, DMS-resistant material is indeed rather stable peroxide or some other oxidation product.
4 CONCLUSIONS CL in inert atmosphere (TLI) has been found to be proportional to the concentration of iodometrically-titratable peroxide accumulated during accelerated ageing of the studied PP, for all levels of oxidation up to the maximum observed peroxide concentrations. This proportionality was observed for two different ageing temperatures, for two oven temperatures during CL measurements and for two different CL instruments. It is in contrast to earlier studies, where the proportionality was seen only for low ( < 150 mmol kg-‘) peroxide concentrations. The reason for this discrepancy is not clear, though it appears to originate in the PP samples rather than in experimental differences, since experiments on the present samples using the apparatus and techniques of the previous work show the same proportionality. DMS treatment shows a two-step decay of peroxide concentration with exposure time. Comparison with the work of Gijsman”j suggests that both fast- and slow-decomposing peroxides react with DMS under our conditions. Nevertheless, samples can be produced by DMS treatment which have significant titratable peroxides which are virtually unreactive with DMS. Particle size experiments suggest that the low reactivity of a fraction of peroxides is not due to diffusion limitations. We cannot exclude the possibility that oxidation products other than peroxides may contribute to the oxidation of iodide ions at these high levels of oxidation, particularly as the fraction of ‘inert’ material increases significantly with extent of oxidation. CL measurements in oxygen show that
Chemiluminescence from oxidation of PP
exposure to DMS has a profound stabilising effect on oxidised PP, despite the presence of significant concentrations of peroxide as measured by CL or titration. The present and previous studies agree that the CL monitors peroxide formation very well for the lower concentrations of peroxide which are relevant to all practical ageing conditions. The amount of CL measured is a function of the geometric collection efficiency of the apparatus, so that the proportionality ‘constant’ depends upon the apparatus used and the sample form. In addition, both here and previously, we find that the temperature of oxidation affects the proportionality. Thus, CL is a very simple and rapid method for monitoring peroxidation in PP, although it must be used with care, particularly in comparing samples with different forms (film, powder, plaque, etc.) or from different ageing temperatures.
2. Jellinek,
H.
Stabilization
127
H.
G.,
Aspects
of
Degradation
of Polymers. Elsevier, Amsterdam,
and
1978.
N. S., Degradation and Stabilization of Polyolefins. Applied Science Publishers, London, 1983. 4. George, G. A., Luminescence Techniques in Solid State Polymer Research, Chap. 3, ed. L. Zlatkevich. Dekker, 3. Allen,
New York, 1989. 5. Carlsson, D. J. & Lacoste, J., Polym. Degrad. Stab., 32 (1991) 377.
6. Scheirs,
J.,
Carlsson,
D.
J.
&
Bigger,
S.
W.,
Polym.-Plast. Technol. Eng., 34 (1995) 97. 7. Ashby, G. E., J. Polym. Sci., 50 (1961) 99. 8. Schard, M. P. & Russell, C. A., J. Appl, Polym. Sci., 8 (1964) 985. 9. George, G. A., Developments in Polymer Degradation,
10. 11. 12. 13. 14. 15.
Vol. 3, Chap. 6, ed. N. G. Grassie. Applied Science Publishers, London, 1981. Reich, L. & Stivala, S. S., Die Macromol. Chem., 103 (1967) 74. Vasiliev, R. F., Die Macromol. Chem., 126 (1969) 231. Quinga, E. M. Y. & Mendenhall, G. D., J. Am. Chem. Sot., 105 (1983) 6520. Russell, G. A., J. Am. Chem. Sot., 79 (1957) 3871. Mayo, F. R., Macromolecules, 11(1978) 942. Billingham, N. C., Then, E. T. H. & Gijsman, P. J., Polym. Degrad. Stab., 34 (1991) 263.
16. Gijsman,
P., Hennekens,
J. & Vincent,
J., Polym.
Degrad. Stab., 42 (1993) 95.
ACKNOWLEDGEMENTS NUTEK is thanked for financial support to this project. The Ragnar and Astrid Signeuls Foundation and the KTH Fund for International Exchange are thanked for travelling awards to allow AK to carry out some of this work at University of Sussex, Brighton, UK.
REFERENCES 1. Scott, G., Atmospheric Oxidation and Antioxidants. Elsevier, London and New York, 1965.
17. Chien, J. C. W. & Jabloner, H., J. Polym. Sci., Al, 6 (1968) 393. 18. Zolotova, N. V. & Denisov, E. T., J. Polym. Sci., 9 (1971) 3311.
19. Zahradnickova,
A., Sedlar, J. & Dastych, D., Polym.
Degrad. Stab., 32 (1991) 155.
20. Falicki, S., Carlsson, D. J., Gosniak, D. J. & Cooke, J. M., Polym. Degrad. Stab., 41 (1992) 205. 21. Gijsman, P., Kroon, M. & van Oorschot, M., Polym. Degrad. Stab., in press (1996). 22. Patai, S., Rapoport, Z. & Stirling, C. J. M., The Chemistry of S&phones and Sulphoxides, Chap. 20. Wiley, Chichester, 1988. 23. Forsstrom, D., Kron, A., Reitberger, T., Stenberg, B. & Terselius, B., Polym. Degrad. Stab., 43 (1994) 277. 24. Matisova-Rychla, L., Fodor, Zs., Rychly, J. & Iring, M., Polym. Degrad. Stab., 3 (1980) 371. L., Polym. Degrad. Stab., 19 (1987) 51.
25. Zlatkevich,
PII:
SOlSl-3910(96)00079-l
Polymer Degradation and Stability 53 (19%) 119-127 0 1996 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0141-3910/%/$15.00
Chemiluminescence from oxidation of polypropylene: Correlation with peroxide concentration A. Kron,O B. Stenberg,” T. Reitbergeti & N. C. Billingham’ “Department of Polymer Technology, bDepartment of Nuclear Chemistry, KTH, S-100 44 Stockholm, Sweden “School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNI 9QJ, UK
(Received
15 January
1996; accepted 17 February
1996)
The chemiluminescence (CL) on heating in an inert atmosphere has been measured for samples taken at various times during oxidation of polypropylene (PP). Proportional relationships were obtained when the integrated emission (TLI) was plotted against peroxide concentration for all studied sets of parameters. Changes in melting temperature and molecular weight with ageing time were found to correlate with changes in the TLI. Oxidised PP was exposed to dimethylsulfide (DMS) for different periods with subsequent measurements of the CL and the peroxide concentration. The peroxide concentration has earlier been found to exhibit a two-step decay with DMS exposure time, indicating two fractions of peroxides with different reactivity. The CL was found to follow the same two-step decay, indicating that both peroxide fractions contribute to CL. The oxidative stability of PP, measured by CL induction times in oxygen, increases when the polymer is exposed to DMS. 0 1996 Elsevier Science Limited
Oxidation progresses through reactions of oxygen with alkyl radicals leading to the formation of hydroperoxides on the polymer backbone. Subsequent decomposition of the hydroperoxides to new radicals leads to an autoaccelerating cycle of reactions, continuing as long as oxygen is present. In a low-molecularweight hydrocarbon an alkoxy or peroxy radical abstracts hydrogen atoms from adjacent molecules. This happens in polymers as well. In a solid material hindered reactant mobility increases the probability of p-scissions of alkoxy radicals to form ketones/aldehydes and alkyl radicals.4 Polymers containing tertiary carbons (such as PP) are particularly vulnerable to chain scission. Due to the incorporation of oxidation products and to scissions of the polymer chains the mechanical properties of the polymer deteriorate so that its useful life is shortened. The formation and decomposition of the hydroperoxides are key reactions in oxidation, and therefore of great importance in studies of
1 INTRODUCTION Polyolefins are oxidatively degraded in UV-light or at elevated temperatures, leading to unwanted physical effects like embrittlement and discolouration. Due to the commercial importance and large volume production of polyolefins like polyethylene (PE) and polypropylene (PP) a great deal of research has been performed on their oxidative degradation, with better materials and stabilising systems as the result” However there are still many unanswered questions about oxidative degradation, especially its early stages. Oxidation of polymers is often described by using a simplified free-radical reaction scheme.1’l-4 It can be used for polymers only as long as one bears in mind that the restricted mobility of the polymeric radicals and the peroxides in the solid phase makes the oxidation heterogeneous. Heterogeneous oxidation also occurs because no oxidation takes place in the crystalline phase due to its impermeability to oxygen. 119
A. Kron et al.
120
yields a carbonyl together with an alcohol group and a molecule of oxygen.“,13 The mechanism is shown in Fig. 1; it requires at least one of the peroxyl radicals to be primary or secondary. One of the objections to the Russell mechanism is that PP should not give any CL since its peroxy radicals are tertiary. PP actually emits relatively intense CL. The explanation could be that the PP alkoxy radicals lead to chain cleavages through p-scissions to give a chain-end ketone and a chain-end alkyl radical. The alkyl radical will react with oxygen to give a primary or secondary peroxy radical which can terminate with a tertiary peroxy radical to give CL.14 Whatever the process for the formation of CL, the intensity, the frequency of arrival of photons at the detector, is a measure of reaction rate and can be expressed in the form:
oxidative degradation. However, the detection and quantitative measurement of hydroperoxides is not trivial. Common detection methods are FTIR and redox reactions, the former often giving ambiguous results due to superpositioned peaks. Among the redox reactions used in studies of solid samples iodometry is reported to be reliable but tedious while colourimetric Fe(II1) determination has been shown to be less precise.’ Derivatisation of oxidation products with NO in combination with PTIR measurements has been reported to give good results.5’6 In recent years the chemiluminescence (CL) technique has been suggested as a method with potential to answer some of the queries about oxidation. The CL technique measures the weak luminescence that is emitted as a result of oxidative reactions. Chemiluminescence from polymers was first reported by Ashby and by Schard and Russell8 in the early 1960s. Since then instrumental development has increased the sensitivity of the method, making it possible to perform CL measurements at temperatures down to ambient conditions.4 Chemiluminescence from polymers is believed to result from deactivation of an excited carbonyl group formed in the oxidation.g-ll However, the reactions responsible for the formation of these carbonyls are still debated. To give rise to an excited carbonyl group the reaction must afford sufficient energy, and several schemes have been proposed which fulfil the energy requirement. Unimolecular decomposition of hydroperoxides to radicals which then disproportionate in a cage reaction” is suggested, as well as metathesis of alkoxy or peroxy radica1s.l’ The most widely accepted mechanism is the Russell mechanism; a termination reaction between two peroxyl radicals which, through an intermediate tetroxide,
2RW
-
-->
R-O
I = G-@-R where I is the intensity, R is the reaction rate, G is a geometrical term, which must be kept constant for comparative measurements, and @ is the CL quantum efficiency.15 In principle, CL is a very attractive method for studying peroxidation in PP because the measurement is very simple and needs little sample preparation, in marked contrast with the conventional iodometric approach. There is thus considerable potential benefit from looking at the relationship of CL and peroxide content. The previous discussion shows that the identity and reactivity of the different types of peroxides formed during oxidation processes are of interest in studies of oxidative degradation. Heating of a sample in inert atmosphere and following the decay of the peroxide concentration is a way to investigate the peroxide reactivity.
/O-O, H-C-R”
/
ka
II
II
->
C R’
1 ->
0* ROH+Ozz+
0
0’
/
0
\
R”
/
R’
C
+ hv \
R’
Fig. 1. The Russell mechanism for CL emission.
; / R
\
R”
Chemiluminescence from oxidation of PP
Gijsman et ~1.‘~have recently studied the decay of peroxides in oxidised, unstabilised PP in inert atmospheres and found it to consist of two stages, a rapid initial step and a subsequent slow one. They fitted the decay with two consecutive first-order reactions and assumed them to originate from fractions of peroxides with different stability. Similar first-order fits for PP peroxide decay were earlier reported by Chien and Jablonerl’ and by Zolotova and Denisov.” The peroxide concentration decay when heating PP in inert atmosphere and the decay of the CL emitted from the PP sample have been shown to have different rates.15 The CL was found to decay faster than the peroxide concentration, approaching a zero value while there was still a considerable amount of titratable led to the peroxides left. This observation suggestion that among the peroxides there is a fast-decomposing fraction that is CL yielding and a fraction with slower decomposition rate that does not give CL. effects of exposing peroxidised The polypropylene to dimethylsulfide (DMS) have been reported by Zahradnickova et aZ.19Destruction of the peroxides is accompanied by oxidation of DMS to dimethylsulfoxide. The decline in peroxide concentration when oxidised PP was exposed to gaseous DMS was shown to consist of two stages, with an initial high reaction rate and a subsequent low reaction rate. From studies of model compounds it was suggested that the fast-reacting peroxides were peracids. The decay of peroxides with exposure time when exposed to DMS has also been shown to be a two-stage process by Carlsson et ~1.” Their studies of the reaction between DMS and PP peroxides (through derivatisation combined with FTIR) showed that the decrease in set- and tert-hydroperoxide concentrations accounted for most of the decrease in peroxide concentration and that the decay of both those species had a two-stage behaviour. They proposed that the fast-decomposing process could be an effect of carboxylic acids acting as catalysts. This observation has been questioned by Gijsman et uE.,2l and the nature of the fast-decomposing fraction of titratable peroxides remains contentious. The aim of the present work was to study correlations between peroxide concentration and chemiluminescence during thermo-oxidative degradation, and to investigate the effects of DMS exposure on CL from peroxidised PP.
121
2 EXPERIMENTAL The studied polypropylene, PP41-WAX, was supplied by BASF Ltd and used as supplied. The polymer was isotactic and has a melting point of 158°C. It was in the form of a powder which has been storage stabilised with BHT. For accelerated ageing the powder was oxidised in an air-circulating oven at 70 or 120°C. Reactions with DMS were performed by adding 2 ml of DMS to 100 mg of PP powder in a nitrogen atmosphere and allowing it to react for lo-300 min. The samples were then washed, first with hexane and then water, and evacuated for at least 96 h at room temperature in order to remove both remaining DMS and the DMSO formed. This was done because of evidence that DMSO acts as an oxidising agent for 13- and interferes with the iodometric measurements of the peroxide concentrations.22 For iodometric measurements a PP sample (3-10mg depending on peroxide content) was added to a mix of 7 ml of (10 parts of isopropanol:l part acetic acid) solution and 2 ml of a saturated solution of NaI in isopropanol (10 g NaI in 50 ml of isopropanol), and refluxed for 20 min under a flow of nitrogen and protected from light. After the reflux 1 ml of water was added. The concentration of 13- was measured spectroscopically as the absorption at 360 nm (using either a Hewlett Packard 845114 or a Philips PU 8720 spectrophotometer). A reference solution containing no PP was treated the same way and used as a blank. Chemiluminescence measurements were made with two CL-detectors which have been described in earlier papers.15,23 The instrument mainly used (denoted CLDl) was a CLDlOO chemiluminescence detector from Tohuko Ltd. The second apparatus (denoted CLD2) was constructed at the University of Sussex. The CL measurements were made on a single layer of PP particles spread onto a circular aluminium pan (with a diameter of 2cm). Before the measurement was started the sample chamber was purged with nitrogen at room temperature for about 30 min. In the CL measurements in nitrogen atmosphere of total luminous intensity (TLI) the temperature was set to 120 or 150°C at the beginning of the measurement and the emitted CL was recorded during the temperature increase. Due to the construction of CLDl, CL
122
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measurements with this detector were of the temperature-ramping type, as opposed to the isothermal measurements previously reported.” However it has been shown that the TLI measured during a ramp experiment in nitrogen is proportional to TLI from an isothermal experiment,l’ so either of the two methods was expected to be acceptable. In CL measurements of OIT (oxygen induction time) the sample was first heated to 100°C in nitrogen and left until isothermal conditions were reached. Oxygen was then admitted to the sample chamber and the measurement was started. The length of the OIT was defined as the time from oxygen admittance to the time when the signal had increased by 25% of its initial value. Gel permeation chromatography (GPC) studies were made with a Waters 150C high-temperature instrument with a refractive index detector. 1,2,4-Trichlorobenzene was used as the mobile phase at 135°C. The flow rate was 1 ml/min. Calibration was performed with polystyrene standards. The values obtained are therefore not absolute values for PP, but are valid for calculation of chain scissions. PP melting points were measured with a Mettler-Toledo TA8000 thermal analysis instrument. Heating rate was lO”C/min. The temperatures presented are the peak temperatures.
3 RESULTS AND DISCUSSION 3.1 CL and peroxide formation When PP oxidises at an elevated temperature, concentration stays at a low the peroxide constant value for a certain period of time. The length of this induction time depends on the oxidation temperature and on the stability of the sample. At the end of the induction time autoacceleration leads to the formation of more and more peroxides. It is during this stage that the material loses most of its mechanical properties. By heating the sample in an inert atmosphere to temperatures at which peroxide decomposition is rapid, the peroxides can be destroyed. If this is done in a chemiluminescence (CL) measurement a CL peak is 0btained.l’ The total luminous intensity (TLI) is defined as the area under the
peak (total number of photons emitted) per unit weight of sample. Figure 2 shows the peroxide concentration in PP as a function of ageing time at 70°C. There is an induction period of around 900 h, due to the antioxidant. For every sample taken for peroxide measurement, a CL measurement in nitrogen was performed and TLI calculated, the results also being shown in Fig. 2. Both the peroxide concentration and the TLI show a maximum after about 1500 h of ageing. The curves in Fig. 2 follow each other, indicating a proportional relationship. In Fig. 3 the TLI is plotted against the peroxide concentration for both the autoacelerating phase and the later, decelerating stage of oxidation. In the accelerating phase, the relationship of TLI and peroxide concentration is linear over the whole range up to the maximum peroxide concentration of over 330 mmol kg-‘. This is in contrast to the results reported by Billingham et a/.,” where there was marked curvature above 150 mmol kg-’ for samples oxidised at 70°C. The slope of Fig. 2 corresponds to around 10’ peroxide decompositions for each photon detected, in agreement with earlier data. In the decline phase the plot is steeper and has an intercept. This is consistent with the earlier report that TLI decays faster than peroxide concentration when peroxidised PP is decomposed.15 However, the decline phase represents oxidation of very heavily oxidised PP and results obtained in this region need to be interpreted cautiously. It has been pointed out by Billingham et al.” that earlier reports on the relation between CL and peroxide concentration in PP have not been made on well characterised samples or at isothermal conditions24’25 so the evidence for proportional relationships might only be circumstantial. We have therefore made a series of measurements to check if the proportionality shown in Fig. 3 was valid under other conditions. The relationship between TLI and concentration of peroxides was investigated for two different ageing temperatures (70 and 120°C) and for two different temperatures of CL measurement (120 and 15O“C), using CLDl. All the series of measurements gave proportional relationships between the TLI and the peroxide concentration throughout the accelerating phase (Fig. 4), though not with the same slopes. For oxidation at 70°C the slope is slightly larger if detection is at
Chemiluminescence from oxidation of PP
0
500
1000
1500
2000
2500
123
3000
:
Ageing time at 70’Clhr of peroxides (0) and total luminous intensity, TLI, (0) of PP powder aged at 70°C in air for different times.
Fig. 2. Concentration
150°C than for detection at 120°C but the effect is not large. In contrast, for oxidation at 120°C we see five times fewer photons per unit of peroxide decomposed. This is in agreement with previous workI in which we proposed that the fastdecomposing fractions of peroxides are more CL emitting than the slow-decomposing ones. Since more of the fast-decomposing peroxides are produced in oxidation at 70°C than at 12O”C, because of their higher stability at lower temperature, the greater sensitivity of CL at low oxidation temperature is consistent. To test whether the discrepancy with earlier results is a function of the sample or of the detector, we also performed the CL measurements on samples aged at 70°C using the
instrument (CLD2) used in previous work. The results again show proportionality over the whole concentration range. The slopes are not comparable because of different collection efficiencies of the two detectors. The changes in molecular weight and melting temperature during ageing at 70°C have been studied by gel permeation chromatography (GPC) and differential thermal analysis (DTA), respectively. The changes in the molecular weight, expressed as chain scissions are shown in Fig. 5(a) and the changes in the melting point in Fig. 5(b). Both curves show an abrupt decrease
1
0 0
1
2
1O4 [ROOH]/mol 0
1
2
1 O4 [ROOH]/mol
3
g-’
Fig. 3. TLI (measured at 120°C over 5000 s in CLDl) versus concentration of peroxides for stabilised PP powder aged at 70°C; (0) phase before maximum, O-1400 h, (+) phase after maximum, 1400-3400 h.
3
g-’
Fig. 4. TLI versus concentration of peroxides for four series of measurements on PP powder; (V) PP aged at 7O”C, TLI measured at 120°C over 5000s in CLDl; (V) PP aged at 7O”C, TLI measured at 150°C over 2500s in CLDl; (0) PP aged at 7O”C, TLI measured at 150°C over 2000s in CLD2; (0) PP aged at 12O”C, TLI measured at 120°C over 5000 s in CLDl.
A. Kron et al.
124
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8-
F .o ln 22 ::
!
.$
4
5
0 2.
0
0g 0
1000
500
1500
2000
2500
3000
Ageing time at 70Whr 160
(b)
158
0
0
0
156 0
$ .-
152. 154
g
150.
i? 'Z
148 -
s
0
146
0 0
144 0
142 t
Ageing time at 70’Clhr Fig. 5. (a) Chain scission (l/Mn
- l/Ma”) for PP powder during ageing at 70°C in air. (b) Changes in melting temperatures PP powder during ageing at 70°C in air.
after about 1000 h, which indicates that degradation becomes significant at this point. The decrease takes place at the same time as the peroxide concentration and the TLI start to increase. 3.2 Effects of dimethylsulfide on peroxides and CL The effect of dimethylsulfide (DMS) exposure on has been reported concentration peroxide earlier.‘9~20 We have investigated the effects of exposure of oxidised PP to DMS on the CL in nitrogen (TLI) and on the peroxide concentration. Oxidised PP samples were exposed to DMS for different periods of time with subsequent measurements of TLI and peroxide concentration. The two-stage decay behaviour found for the peroxide concentration by Zahradnickoval’
for
and by Carlssonzo was found for both the CL and the peroxide concentration. Figure 6 shows typical data, from a sample aged for 1420 h at 70°C to a peroxide concentration of 330 mmol kg-‘. Initial exposure to DMS causes a rapid decay of the peroxide concentration to about 15% of that at zero exposure time. After the initial rapid reaction, the remaining peroxides are destroyed extremely slowly and the concentration is almost constant. According to Gijsman et a1.,16 about 40% of peroxides formed under these oxidation conditions are of the fast-decomposing type. Thus it seems that both fast- and slow-decomposing peroxides are destroyed under these conditions of DMS exposure, but a significant fraction of titratable peroxide is much more resistant to DMS reaction. The interesting observation is that the TLI and
Chemiluminescence
f
0 g
0.4.: i
. . . . . . . . . . . . . . . . . ..O...................~ 0.0 0
50
1M)
150
200
250
l
300
Time of exposure to DMSl min Fig. 6. Relative concentration
of peroxides (0) and relative TLI (0) of PP powder aged for 1420 h at 70°C in air for different exposure times to DMS.
the peroxide concentration follow each other well. The speculation that CL is directly associated with the more reactive of the two peroxide fractions with different decay rates is not confirmed if we assume that the fastdecomposing peroxides are all destroyed in the early stages of DMS treatment. The fraction of the peroxides that is left unreacted after the first stage of DMS reaction is as capable of giving CL as that destroyed early in the treatment. A possible explanation for the residual slow-reacting fraction is that some peroxides are physically protected from the DMS by being so far into the bulk of the sample that the DMS can not diffuse to them. This was tested by a particle size fractionation experiment. A typical sample of PP powder had a relatively wide particle size distribution, from diameters less than 250 pm up to 1 mm. To investigate the influence of the particle size on the oxidation a series of samples aged for different periods at 70°C were fractionated by sieving. Two fractions were taken; one with diameters smaller than 250 pm and one with diameters bigger than 500 pm. The peroxide concentrations were measured iodometrically for duplicate samples, with and without exposure to DMS. The results are shown in Table 1. Before DMS treatment, the concentration of peroxides is higher in small particles than in large ones for all ageing times, with the effect most marked at the highest peroxide concentrations. This suggests that the peroxides are mostly located near the particle surface. We also found that the two groups of particles do not have the
from oxidation of PP
125
same behaviour with ageing time. The bigger particles have a later maximum in peroxide concentration than the smaller ones. The maximum is thought to appear when the peroxide concentration has reached a critical value where the decomposition rate of the peroxides is bigger than their formation rate. If the oxidation is surface limited then this critical peroxide concentration is equivalent to a certain percentage of the particle surface being oxidised. The later maximum for the larger particles may be due to the fact that there is more oxidisable surface on a large particle when the smaller particle reaches its critical value. This effect probably reflects the highly crystalline morphology of the as-synthesised particles, which restricts oxygen access. The results in Table 1 also show the effect of exposure to DMS for different particle sizes. The fractional decrease in peroxide concentration is found to be about the same for the small and the big particles for all ageing times, though the fraction of peroxides remaining after treatment increases with the ageing time. Since the effect of exposure to DMS is independent of particle size we conclude that physical hindrance of the reaction with DMS is unlikely. Another possible explanation for the DMSunreactive fraction is that oxidation products other than peroxides may contribute to the oxidation of iodide ions at these high levels of oxidation. We cannot exclude this, particularly as the fraction of ‘inert’ material increases significantly with extent of oxidation. It is significant that the same behaviour was
Table 1. Effect of partiele size on peroxide concentration in PP aged at 7O”C,with or without subsequent exposure to DMS Ageing time at Particle Peroxide 70°C (h) diameter (pm) concentration after ageing (mmol kg-‘)
500 500 1420 1420 1710 1710 2750 2750
< 250 >.500 < 250 >500 < 250 >500 < 250 >500
3 2.5 410 157 355 257 146 139
Peroxide concentration after ageing and DMS exposure (mmol kg-‘) 77 32 101 79 92 80
A. Kron
126
found for TLI versus DMS exposure time, so that even samples in which all of the fastdecomposing peroxides should have been removed by DMS treatment are luminescent. This the fastwith where contrasts samples decomposing peroxides have been removed by annealing, when the CL is reduced to near zero.15 3.3 Effect of DMS on oxidative stability Increased oxidative stability of DMS-exposed PP has been shown by Gijsman et a1.,16 using oxygen absorption measurements. We have studied oxidation of DMS-exposed PP by measuring CL in oxygen. The results are presented in Table 2 as oxidation induction times (OIT) at 100°C. Experiments were carried out on samples which had been aged for different periods at 70°C. Dual samples were measured, with and without exposure to DMS. The OIT was found to be prolonged by DMS exposure in all but one case. For the sample aged for only 500 h the effect was the opposite. This is thought to be an effect of stabiliser that remains in the sample after this relatively short ageing time. The stabiliser is thought to evaporate during the storage in vacuum which is done after every DMS exposure experiment, thus leaving the DMS exposed sample more unstabilised than the unexposed. This view was supported by examining samples washed and vacuum-treated without exposure to DMS. Without treatment the unaged powder had an OIT of 1055 min and this was reduced to 670min by washing and drying. Similarly, a sample, aged for 500 h but not exposed to DMS had an OIT of only 209min after vacuum treatment, as compared with 500min with no treatment, thus confirming the proposed explanation. The stabiliser is thought to give minor effects when the samples have reached the phase of accelerating oxidation (shown in Fig. 2 to start at about 1000 h). For ageing times of 1420 h or Table 2. Oxygen induction times (OIT) at 100°C for PP aged at 70°C with or without subsequent exposure to dimethylsulfide _._~
Ageing time at 70°C (h)
OIT for unexposed PP (min)
OIT for DMS exposed PP (min)
500 1020 1420 1710
500 180 2 1
290 205 185 130
et al. more the DMS exposure leads to a higher oxidative stability. Measurements of CL in oxygen thus confirm the results of oxygen absorption measurements.‘6 Comparison of Tables 1 and 2 shows that the effect is profound for moderately oxidised samples. For example, the OIT at 100°C for PP oxidised at 70°C for 1420 and 1710 h is extended by almost 100 times, despite the fact that between 20 and 30% of the original peroxides remain after DMS treatment. It is not clear whether the CL-active, DMS-resistant material is indeed rather stable peroxide or some other oxidation product.
4 CONCLUSIONS CL in inert atmosphere (TLI) has been found to be proportional to the concentration of iodometrically-titratable peroxide accumulated during accelerated ageing of the studied PP, for all levels of oxidation up to the maximum observed peroxide concentrations. This proportionality was observed for two different ageing temperatures, for two oven temperatures during CL measurements and for two different CL instruments. It is in contrast to earlier studies, where the proportionality was seen only for low ( < 150 mmol kg-‘) peroxide concentrations. The reason for this discrepancy is not clear, though it appears to originate in the PP samples rather than in experimental differences, since experiments on the present samples using the apparatus and techniques of the previous work show the same proportionality. DMS treatment shows a two-step decay of peroxide concentration with exposure time. Comparison with the work of Gijsman”j suggests that both fast- and slow-decomposing peroxides react with DMS under our conditions. Nevertheless, samples can be produced by DMS treatment which have significant titratable peroxides which are virtually unreactive with DMS. Particle size experiments suggest that the low reactivity of a fraction of peroxides is not due to diffusion limitations. We cannot exclude the possibility that oxidation products other than peroxides may contribute to the oxidation of iodide ions at these high levels of oxidation, particularly as the fraction of ‘inert’ material increases significantly with extent of oxidation. CL measurements in oxygen show that
Chemiluminescence from oxidation of PP
exposure to DMS has a profound stabilising effect on oxidised PP, despite the presence of significant concentrations of peroxide as measured by CL or titration. The present and previous studies agree that the CL monitors peroxide formation very well for the lower concentrations of peroxide which are relevant to all practical ageing conditions. The amount of CL measured is a function of the geometric collection efficiency of the apparatus, so that the proportionality ‘constant’ depends upon the apparatus used and the sample form. In addition, both here and previously, we find that the temperature of oxidation affects the proportionality. Thus, CL is a very simple and rapid method for monitoring peroxidation in PP, although it must be used with care, particularly in comparing samples with different forms (film, powder, plaque, etc.) or from different ageing temperatures.
2. Jellinek,
H.
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127
H.
G.,
Aspects
of
Degradation
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and
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N. S., Degradation and Stabilization of Polyolefins. Applied Science Publishers, London, 1983. 4. George, G. A., Luminescence Techniques in Solid State Polymer Research, Chap. 3, ed. L. Zlatkevich. Dekker, 3. Allen,
New York, 1989. 5. Carlsson, D. J. & Lacoste, J., Polym. Degrad. Stab., 32 (1991) 377.
6. Scheirs,
J.,
Carlsson,
D.
J.
&
Bigger,
S.
W.,
Polym.-Plast. Technol. Eng., 34 (1995) 97. 7. Ashby, G. E., J. Polym. Sci., 50 (1961) 99. 8. Schard, M. P. & Russell, C. A., J. Appl, Polym. Sci., 8 (1964) 985. 9. George, G. A., Developments in Polymer Degradation,
10. 11. 12. 13. 14. 15.
Vol. 3, Chap. 6, ed. N. G. Grassie. Applied Science Publishers, London, 1981. Reich, L. & Stivala, S. S., Die Macromol. Chem., 103 (1967) 74. Vasiliev, R. F., Die Macromol. Chem., 126 (1969) 231. Quinga, E. M. Y. & Mendenhall, G. D., J. Am. Chem. Sot., 105 (1983) 6520. Russell, G. A., J. Am. Chem. Sot., 79 (1957) 3871. Mayo, F. R., Macromolecules, 11(1978) 942. Billingham, N. C., Then, E. T. H. & Gijsman, P. J., Polym. Degrad. Stab., 34 (1991) 263.
16. Gijsman,
P., Hennekens,
J. & Vincent,
J., Polym.
Degrad. Stab., 42 (1993) 95.
ACKNOWLEDGEMENTS NUTEK is thanked for financial support to this project. The Ragnar and Astrid Signeuls Foundation and the KTH Fund for International Exchange are thanked for travelling awards to allow AK to carry out some of this work at University of Sussex, Brighton, UK.
REFERENCES 1. Scott, G., Atmospheric Oxidation and Antioxidants. Elsevier, London and New York, 1965.
17. Chien, J. C. W. & Jabloner, H., J. Polym. Sci., Al, 6 (1968) 393. 18. Zolotova, N. V. & Denisov, E. T., J. Polym. Sci., 9 (1971) 3311.
19. Zahradnickova,
A., Sedlar, J. & Dastych, D., Polym.
Degrad. Stab., 32 (1991) 155.
20. Falicki, S., Carlsson, D. J., Gosniak, D. J. & Cooke, J. M., Polym. Degrad. Stab., 41 (1992) 205. 21. Gijsman, P., Kroon, M. & van Oorschot, M., Polym. Degrad. Stab., in press (1996). 22. Patai, S., Rapoport, Z. & Stirling, C. J. M., The Chemistry of S&phones and Sulphoxides, Chap. 20. Wiley, Chichester, 1988. 23. Forsstrom, D., Kron, A., Reitberger, T., Stenberg, B. & Terselius, B., Polym. Degrad. Stab., 43 (1994) 277. 24. Matisova-Rychla, L., Fodor, Zs., Rychly, J. & Iring, M., Polym. Degrad. Stab., 3 (1980) 371. L., Polym. Degrad. Stab., 19 (1987) 51.
25. Zlatkevich,