Polymer Durability - American Chemical Society

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch011

Physical Spreading and Heterogeneity in Oxidation of Polypropylene M. Celina , G. A. George and N. C. Billingham 1,4

2,4

3

Department of Chemistry, The University of Queensland, 4072 Brisbane, Australia School of Chemistry, Queensland University of Technology, Brisbane, Australia School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, United Kingdom 1

2

3

The (CL)

oxidation of solid polypropylene has been interpreted

measured

as involving

by

chemiluminescence

heterogeneous

initiation

leads to high oxidation rates in localized zones and is followed physical

spreading

of oxidation.

Evidence

of the high activity of oxi-

dizing centers to promote further oxidation of even physically PP powder particles has been obtained.

that

by the

separated

In such a model of highly re-

active centers existing from the earliest onset of oxidation, an induction period was related to the physical characteristics than the chemical interpretation fore, failure

in liquid-state

of the material

rather

kinetic models.

There-

of the material may be related to the spreading

centers of initial oxidation of the

of a few

polymer.

T T H E THERMAL AND PHOTOCHEMICAL OXIDATION of solid polyolefins has traditionally been studied within a kinetic framework developed for the autooxidation of liquid hydrocarbons. The chemical analysis of polymer films during oxidation by spectroscopic methods, particularly transmission and at tenuation total reflectance-IR spectrophotometry, has produced concentration profiles of oxidation products such as ketones, aldehydes, acids and alcohols as a function of time. These profiles show an induction period before a rapid increase in concentration to a steady increase with time, which has been in terpreted as the limiting oxidation rate of the polymer (I). Current Address: G.A. George and M. Celina, School of Chemistry, QUT GPO Box 2434, Brisbane 4001, Australia.

4

0065-2393/96/0249-0159$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


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POLYMER DURABILITY

Oxygen uptake experiments confirmed this general oxidation profile for polyethylene and polypropylene (PP) powders and film. The similarity of these curves to those from liquid hydrocarbons during autooxidation supported the application of homogeneous free-radical kinetics in which the degenerate branching agent is the polymer hydroperoxide and the hmiting oxidation rate is controlled by the rate of the propagation reaction (2). The steady-state approximation may then be applied to determine the kinetic parameters and thus predict the extent of oxidation at any time. Such an approach, if applied to the solid oxidizing polymer, would theoretically enable the ultimate service fife of the material to be determined from the kinetic curve provided a failure criterion is established and the appropriate rate coefficients can be determined accurately enough. The attractiveness of such an approach had led to the adoption of very sensitive analytical methods such as XPS (X-ray photoelectron spectroscopy) (3), microoxygen uptake using sensitive pressure transducers (4), and chemiluminescence (CL) (5) to determine the rate at the earliest stages of oxidation. Measurement of the ultra-weak C L that accompanies oxidation has been of interest because of the high sensitivity with which weak emitted fight can be measured when compared with the difference between two intense trans mitted beams as in absorption spectrophotometry. However, the process is intrinsically inefficient and has quantum yields for the overall production of fight from the polymer oxidation around 10~ . Controversy has surrounded the nature of the light-emitting reaction in the free-radical oxidation sequence, but it has generally been regarded as the exoenergetic termination reaction of peroxy radicals by the Russell mechanism (6). This mechanism requires one of the terminating radicals to be either primary or secondary so that a sixmembered transition state can be formed by the two peroxy radicals, which would lead to an alcohol, singlet oxygen, and a triplet excited carbonyl chromophore. The emission from this excited state is the measured C L . Even though it is easy to envisage such a mechanism for polymers such as polyethylene and polyamides, in the case of P P the chain-carrying radical is tertiary and the usual termination reaction of the peroxy radicals involves an intermediate tetroxide that cannot lead to an emissive carbonyl. This re striction has led to the proposal of a variety of light-emitting reactions (7). Similarly, P P hydroperoxide prepared by controlled oxidation will produce C L when heated under nitrogen. Several reaction mechanisms have been sug gested to account for this phenomenon, and they do not involve peroxy radical intermediates. By heating a sample to around 180 °C, all of the hydroperoxides may be decomposed. The integral of the glow curve was shown (8) to be a measure of the concentration of peroxides, although the absolute correlation has been questioned at all concentrations (9). Although measurements in inert atmosphere provide one method of measuring oxidation product concentration in the early stages of thermal (10) or photooxidation (8), the most common C L experiment involves the contin9

In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


11.

CELINA ET A L .

Spreading and Heterogeneity in PP Oxidation

161

uous measurement of the C L intensity during isothermal oxidation. A family of such C L curves for the thermal oxidation of P P powder is shown in Figure 1. They show features of instantaneous oxidation rate curves generated by a method such as oxygen uptake. A low but nonzero rate is maintained for a time, and the rate decreases with temperature before it increases exponentially to a limiting value. The integral of these curves is analogous to the concentration-time profile that is obtained by IR analysis of carbonyl group concentration. The integral also shows the features expected of a homogeneous, branching chain reaction, such as an apparent induction period. This induction period is frequently used as a measure of the stability of the polymer, because it increases when sta bilizers such as free-radical scavenging antioxidants are included in the sohd polymer. In the hquid state the induction time is taken as that time for the total consumption of antioxidants, after which the oxidation proceeds at the uninhibited rate. The linear part of the concentration-time curve is considered to be a measure of the steady oxidation rate. The application of this approach to the integral C L curve for PP powder during oxidation at 150 °C immediately reveals a difficulty in the definition of both the induction period and the steady rate of oxidation. Figure 2 shows that as the sensitivity of the C L analysis is increased, the apparent induction

0

1

2

3

4

Oxidation t i m e

5

6

[h]

Figure 1. Typical CL signals from the oxidation of unstabilized PP powder samples at different temperatures under oxygen. (Reproduced with permissi from reference 13. Copyright 1993 Elsevier Science Ltd.)



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POLYMER DURABILITY

0

1

2

3

4

Oxidation time [ 1 E 3 s ]

0

1

2

3

4

Oxidation time [ 1 E 3 s ]

Figure 2. Integrated CL signals from the oxidation of an unstahilized FF powder sample at 150 °C presented in relation to different instrumental sensitivities. time and the limiting rate decrease due to the continuing expansion of what is actually an exponential growth curve. The true nature of the induction period is revealed in the highest sensitivity curve as a short region of linear increase in integrated C L intensity with time before the exponential growth. This increase corresponds to the region of low but steady emission intensity seen in the C L curves of Figure 1. Consequently, the magnitude of the induction period in any oxidation experiment depends on the sensitivity of the method used to measure it. The sensitivity of C L measurements has enabled the early stages of both thermal and photooxidation to be studied in detail. In this chapter we wish to present some of our recent results on the oxidation of P P powder and film that bring into question the often-used interpretation of the oxidation curves in Figure 2 in terms of homogeneous free-radical chain reactions. These results support a view that the oxidation of even single particles of powder is highly hetero geneous and requires a new interpretation of the kinetic data.


11.


CELINA ET A L .

163


Evidence for Heterogeneous Oxidation from CL Studies of Photooxidation C L has been applied as a technique to measure the hydroperoxide formation during the photooxidation of PP-film samples (8). The integrated C L emission from a ramped temperature experiment is related to the concentration of hydroperoxides present in the sample. PP-film samples, when subjected to U V irradiation at 340 nm, immediately formed hydroperoxides at a concentration that could not be detected by ATR-IR, XPS, or a related general increase in the carbonyl index. A detailed analysis of hydroperoxide concentration during these very early stages of photooxidation indicated a possible kinetic scheme of a consecutive reaction involving a rapid formation of hydroperoxides fol lowed by their photolysis to secondary oxidation products. Figure 3 shows this rapid initial hydroperoxide buildup in relation to the overall oxidation of the sample as measured by the carbonyl index. Interestingly, further studies of stabilized samples revealed that this peak was not affected or inhibited by the presence of many different stabilizers. A

120

100

80

60 0.0

0.5

1.0

1.5

2.0

Exposure time

2.5

3.0

[h]

40

20

10

15

20

25

30

Exposure time

35

40

45

50

[h]

Figure 3. Changes of the integrated CL signal from ramped temperature experiments (Φ) and carbonyl index (ψ) of unstabilized PPfilmsamples upon UVA photooxidation. Inset shows an expansion of the early stage of photooxidation. (Reproduced with permission from reference 8. Copyright 1991 Elsevier Science Ltd.)


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phenolic antioxidant or hindered amine light stabilizer (HALS) was unable to suppress the initial reactivity of the sample during irradiation (II). Therefore, we concluded that the hydroperoxides were formed i n localized centers of high reactivity. Limited concentrations and mobility of the additives were seen as the reasons for uninhibited reactions taking place in very small zones of the material. Such heterogeneous behavior during photooxidation had been discussed before (12).

Thermal Oxidation The C L emission from the isothermal oxidation of P P is fundamentally related to the formation of an oxidation product (excited carbonyl) and therefore sim ilar to other techniques of oxidation measurements such as carbonyl index or oxygen uptake. The main advantage of using C L has to be seen as the larger dynamic sensitivity range enabling a continuous monitoring of the oxidation from the very weak early stages to the main oxidation of the material (see Figures 1 and 2). Isothermal C L curves were obtained from the oxidation of P P powder and film samples in the sohd state at various temperatures between 90 °C and 150 °C. Some of the curves are presented in Figure 1. The analysis of such curves over a wide temperature range resulted in the following conclusions (13): 1. The maximum intensity (i ) appeared at a nearly constant total emission or extent of oxidation. 2. In the very early stages of the oxidation and at the highest instrumental sensitivity, it was possible to measure an induction period during which the signal was on a constant level and significantly above the baseline. This finding indicated oxidation and the immediate formation of second ary oxidation products commencing from the earliest time possible. 3. Arrhenius plots of parameters such as induction period (t ), maximum signal time (t ), initial intensity during induction period (I ), and max imum signal intensity (I ) were possible. Activation energies for I and I were similar (113 kj/mol for P P powder), whereas I was 149 kj/ mol for P P powder. max

ind

ini

mm

max

ini

max

ind

Similar results were obtained for unstabilized P P powder and film sam ples, and we concluded that the induction period is a separate process from the remaining oxidation. In particular, the different activation energies of in itial C L emission intensity (I ) and induction period (i ) make it impossible to predict the oxidative stability of the material from a simple measurement of I without knowledge of the temperature behavior of the actual induction period. The product of I and f at different temperatures is not a constant, ini

ind

ini

ini

ind


11.

CELINA ET A L .


165

which means that the end of the induction period corresponds to different extents of oxidation of the polymer. Once the induction period is regarded as a separate process and removed from the C L curves, it is possible to plot the remaining oxidation curves in reduced coordinates. A presentation of ί as a fraction of J versus t as a fraction of t (after subtraction of t^) results in a master curve for all oxi dation curves over a wide temperature range (Figure 4). From the appearance of a universal sigmoidal curve shape, we suggest that all oxidation curves fol lowing the induction period are governed by a common fundamental process. Mathematical analysis of the master curve showed that statistical functions such as parts of Gaussian or Weibull distributions could be easily fitted to the sigmoidal increase of oxidation intensity. Therefore, we concluded that the max


max

Figure 4. CL signals from the oxidation of unstabilized PP powder samples at different temperatures presented in relative units after subtraction of the induction period and resulting in a master curve. (Reproduced with permission from reference 13. Copynght 1993 Elsevier Science Ltd.)



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main oxidation may in fact be closely related to a statistical progress or spread ing of the oxidation. Such a process would lead to a continuously increasing fraction of oxidizing material until a maximum was reached that originated from a depletion of readily oxidizable material. This process is then followed by a final decaying signal to reach complete oxidation of the sample. The observed master curve may also be closely fitted with mathematical models of fractal growth or percolation through the solid polymer. In such a model of heterogeneous oxidation based on statistical progress and increase of oxi dizing sites, the induction period has been proposed as the time required before the oxidation starts to spread from initially localized centers (13). The occurrence of the Russell mechanism of bimolecular termination of alkyl peroxy radicals requires the participation of at least one primary or sec ondary alkyl peroxy radical in the fight-producing reaction (14). The obser vation of C L emission in itself during the induction period requires, therefore, a degenerately branched chain reaction leading to either primary or secondary alkylperoxy radicals. The usual picture of P P oxidation is that tertiary alkylperoxy radicals are the chain carrier in the early stages of oxidation. If this were the case, C L would not be observed (5). Thus, higher extents of oxidation must be occurring that involve these other radical species. Heavy oxidation during the induction period may quickly lead to secondary oxidation products, such as water or C 0 . Such oxidation products were observed (15) from the earliest onset of oxidation. However, this high extent of oxidation must be confined to only a small fraction of the polymer powder or film for the du ration of the apparent induction period. After this induction period, the oxi dation spreads and leads to the characteristic sigmoidal growth curve (Figure 4) that represents an increasing fraction of the polymer that is oxidizing. 2

Heavily oxidized centers that suddenly start spreading will also lead to rapid crack formation soon after the end of the induction period. Mechanical failure due to microcrack formation is closely related to the actual induction period (13). In a heterogeneous model, heavy oxidation confined only to cer tain locations can easily explain the observation of secondary oxidation prod ucts such as water and C 0 and crack formation commencing from the very early stages of the main oxidation of the material that would be inexplicable by a classical homogeneous kinetic model. In the classical homogeneous model, such as that applied to liquid-state oxidation kinetics, the induction period is only seen as the time necessary to produce a critical concentration of a certain species before chain branching and autoacceleration commences. Homogeneous interpretation of the integrated C L curve as arising from an autoaccelerating chain reaction may also result in misleading conclusions about an average oxidation rate as the slope of parts of the curve. Tiny centers may in fact oxidize much more rapidly and cause immediate failure. Different materials may only be compared by measuring true induction periods and 2


11.

CELESTA ET A L .


167

initial oxidation rates rather than comparing average rates, for example, de termined by carbonyl index or oxygen uptake.


Physical Spreading Photo- and thermal oxidation studies as discussed in the preceding sections led to conclusions about a highly heterogeneous oxidation in which the phys ical spreading from an initial center may play an important role. This finding is in agreement with many other studies indicating heterogeneous behavior (16). Unlike other methods, C L is sensitive enough to allow study of very small (

Polymer Durability - American Chemical Society

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