Mechanisms of Photooxidation of Polyolefins - American Chemical

35

Downloaded by UNIV OF SYDNEY on April 1, 2013 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch035

Mechanisms of Photooxidation of Polyolefins: Prediction of Lifetime in Weathering Conditions Jacques Lemaire, Jean-Luc Gardette, Jacques Lacoste, Patrick Delprat, and Daniel Vaillant Laboratoire de Photochimie Moléculaire et Macromoléculaire, Unité de Recherche Associée, Centre National de la Recherche Scientifique 433 Université Blaise Pascal, 63177 Aubiere France

The fate of solid synthetic polymeric materials aging outdoors could be predicted from accelerated laboratory or a mechanistic

approach.

vance of the observed

experiments using a

In the simulation

phenomena

simulation

methodology,

is deduced from

the rele-

the

similarities

among the physical and chemical aggressions in natural and

artificial

exposures. In the mechanistic approach, the relevance of the

observed

phenomena is controlled at the molecular level through the

recognition

of the chemical changes in the solid matrix. In the mechanistic

ap-

proach,

re-

which considers the polymeric

systems as photochemical

actors, it is essential to analyze the intermediate

and final products

at

a very low reaction extent. The chemical analysis is mainly based on spectrophotometric

or

microspectrophotometric

techniques

coupled

with specific gaseous reagents. The comparison between weathering

ki-

netics and accelerated artificial aging kinetics should be used only when the nature and the spatial distribution similar.

As

examples,

photo(bio)degradable cent results are

T H E

the

of the various photoproducts

photooxidations

low-density

polyethylene

of

polypropylene

are presented

are and

and re-

emphasized.

PREDICTION O F T H E LONG-TERM LIFETIME of polymeric materials,

especially in outdoor conditions, is a very difficult problem. The progress of scientific knowledge in this field, whose development began no sooner than 1970, came up against a well-established empiricism lasting for more than 50 years. This tenet is still the background of more than 90% of the world activity in control, development, and even research. As soon as they were used, 0065-2393/96/0249-0577$12.50/0 © 1996 American Chemical Society In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

POLYMER DURABILITY


578

polymers have shown problems of durability. In the 1950s, some experimental methods of studying the phenomena of degradation urgently had to be de veloped to try to solve these difficulties. Because the phenomena involved in long-term evolution looked complex, the chosen techniques could only be empirical. At that time, polymers were considered black boxes, where physical and chemical stresses of the environment were artificially applied with the hope of observing in the laboratory the same phenomena as in natural con ditions. These phenomena were only indirecdy studied from the variations of physical properties in the use conditions of these polymers (e.g., mechanical properties, aspect of surfaces, transparency, and discoloration). The tentative approach based on a simulation of strains was both successful and unsuccessful but without any possibility to justify each case. In fact, the long-term behavior of polymers exposed to U V radiations, heat, oxygen, and water could be predicted in the laboratory with a good coefficient of security through experimental techniques based on scientific data obtained from fundamental research. In 1970, a fundamental approach to the phenomena of photoaging of polymers was not thought to be fruitful, because a very complex situation existed that might not be able to be simpli fied. The interactions of a polychromatic light, of a solid state more or less organized, and of a strong perturbation agent like oxygen were not supposed to be simple. The Laboratory of Photochemistry, University of Clermont-Fer rand, France, began to work in that particular field of fundamental research in 1972, because a bet was made on a possible stylization of these phenomena. Some 12 years later, the experience gained from studying several thou sand blends in conditions of accelerated photoaging showed that reproduction of chemical evolutions in conditions very similar to natural ones was possible in the laboratory. The exact mimesis of natural strains no longer had to be found, and it was sufficient to work in experimentally relevant conditions and to control, at a molecular scale, the similarity of the mechanisms of chemical evolution in weathering and in artificial aging conditions. A n acceleration of the detrimental phenomena resulted only from an acceleration of chemical reactions, in most cases. The relevancy of the data collected could be checked throughout these studies based on S E P A P 12.24 and S E P A P 12.24H units, which were much easier to use compared with the simulation techniques ( I 4). Research of mechanisms of photochemical, thermal, and hydrolytic aging carried out for more than 20 years uncovered the following facts : 1.

A polymer fives as a chemical reactor. Its degradation implies the appearance of generally low concentrations of chemical groups (for example oxidized groups), and this chemical evolution is responsible for the degradation of physical properties. In weathering, there are no examples of physical aging without chemical modifications.

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

35.

L E M A I R E ET A L .

2.


3.

4.

•

6. 7.

579

The chemical evolution does not depend on the mechanical stresses. These stresses modify only the physical consequences of the chemical evolution and not the kinetics of the chemical reaction; therefore, the acceleration of the chemical evolution does not depend on the external and internal mechanical stresses. By considering the chem ical evolution, laboratory data may be converted into durations of use in natural conditions on the basis of the acceleration of the chemistry (consequently, the chemical evolution is used as a base for any trans fer of data). The chemical evolution is a characteristic of the mechanism of evo lution of any specific material. A precise formulation (polymer + filler + pigments or dyes + additives) must be characterized with a specific acceleration factor. A set of various materials or various formulations classified from artificial aging experiments cannot be transferred into use conditions without taking into account the acceleration factors that are necessarily different. The acceleration of chemical evolutions is not only allowed but is a strict requirement because •

5.

Mechanisms of Photooxidation of Polyolefins

it is impossible to extrapolate the data collected in the earlier phases of the evolution of the materials (any treatment based on homogeneous kinetics is not acceptable) the material must reach a level of chemical evolution that leads to a mechanical degradation

O n the other hand, the acceleration of the chemical events should be provoked while maintaining the relevancy of the phenomena (con ditions inducing a lack of oxygen in solid polymers should be avoided, for example). Any accelerated aging corresponds to an acceleration of the chemical evolution. Eventually, the description of the chemical evolution must be asso ciated with the criteria of degradation that have been selected: •

•

description with products able to be observed by means of vibrational spectroscopy (IR, Raman) correlated with variations of mechanical properties description with products able to be observed by means of electronic spectroscopy (e.g., U V , visible, colorimetric, and emission) correlated with variations of aspect

These basic principles led to a number of consequences. 1.

When a chemical mechanism implies several processes of the same importance, there is no hope to accelerate all of them with the same



POLYMER DURABILITY

acceleration factor. The experimental tests in the laboratory misrep resent the reality. When phenomena of physical transfer are superposed on the chem ical evolution (i.e., oxygen diffusion or stabilizers migration), all these dynamic processes cannot be accelerated with the same factor of acceleration. Only the systems where evolution is controlled by only one dynamic process can be transferred from accelerated laboratory conditions to nonaccelerated conditions of use. In fact, this case has been met fairly often in weathering, where the photooxidative process is the con trolling one. The migration of stabilizer additives is a major process when eontrolling the aging phenomena. Effects of the stabilizers should be tentatively examined, and the perturbation provoked by migrations should be evaluated. O n a practical point of view, it is acceptable to consider a common acceleration factor for different formulations of a basic polymer in volving different additives with a similar mechanism of action. Standardization and specifications should take into account the na ture of the polymer under test and the nature of the formulation. The consideration of the aging conditions is also a prerequisite. Devices used to simultaneously study several physical and chemical processes (simulation devices such as the Weatherometer and Xenotest) have to be considered as compromises, as a base of language whose advantage is to be common and whose disadvantage is to be only an approximation. According to some Japanese works, some accelerated setups are de veloping based on the use of fight intensities that are 3-5 times the daylight intensity. These apparatus are close to analytical devices, which means they are only able to examine the preponderant proc esses involved in photoaging (like a S E P A P 12.24 setup). Durability studies of polymers in artificial aging (nonanalytical) and in natural aging are generally based on macroscopic criteria. These measures could be completed with analyses of the chemical evolution either of elementary layers (thickness from 5 to 40 μηι) from surface to core of the samples, or of microzones on surfaces of small sizes (e.g., 10 X 10 μηι or 10 X 100 μηι). Profiles of the degradation products and of additives can thus be determined in thick systems. The basic technique is presendy Fourier transform IR (FTIR) microspectrophotometry. However, FT-Raman microspectrophotometry will be combined with F T I R as the basic technique in the near future.


35.

LEMAIRE ET AL.


581


As emphasized in the previous sections, chemical analysis should allow the recognition of the evolution mechanisms in artificially accelerated condi tions and throughout weathering. The required information deals with the exact chemical nature of the intermediate and final groups formed on the main polymer chains or branches and with the spatial repartition of these photoproducts in the exposed systems (films or plaques). When a common mechanism has been observed, that is, when relevancy is controlled, compar ison between weathering kinetics and artificial aging kinetics supplies the re quired acceleration factor. As an example, the evolution of polymeric systems based on photostable blends of heterophasic polypropylene (PP) and on photo(bio)degradable polyethylene (PE) are described with some details.

Evolution of Heterophasic PP Systems The oxidation of isotactic and atactic PP, initiated photochemically, thermally, or radiochemically, has been studied for years by many research groups. The general features of the oxidation mechanisms are fairly well described, al though the initiation steps remain largely unknown. However, the formation of the primary radicals could have many different origins, and the analysis generally starts with the two macroradicals formed from the normal structure of PP.

-C-CH I CH 2

2

3

f •

-C-CH1 CH 3

A summary of most of the results reported in the literature (5-13) yields the mechanism of isotactic P P photooxidation represented in Scheme I. This mechanism indicates the formation and conversion of the main intermediate photoproducts (associated tertiary hydroperoxides, chain-end, and chain ke tones) and the formation of the final oxidation products that accumulate in the matrix (e.g., tertiary alcohols). Although P P appears as the polymer whose photooxidation is the best understood, several questions remain unanswered. • •

What is the exact origin of the acid groups formed? What is the relative importance of the two β-scission processes of the alkoxy radicals? (In 1973, the formation of methylated and chain-end ketones was shown to be favored on model hydroperoxides (14).


582

POLYMER DURABILITY

X

+

PP


00H 90%

-C-CH -

(Hydrogen-bonded)

2

CH

3

hi>

Δ ( θ ) 90°C

i n cage chain reaction" N

0· I

-f-CH -

-Ç-CH - + ·0Η

2

2

PP POOH OH -C-CH CH 2

CH

-scission

3

CH,

Ιβ-scission

0 II

-C-CH

+ CH 3

3

Norrish types I and I I

3

important

00·

Norrish types I and I I Scheme I,

•

•

Why are photooxidation and thermooxidation stoichiometrics so sim ilar, even though the Norrish type I and type II photoprocesses should significantly convert the ketonic intermediates? What is the exact assignment of the 1740 c m band that is observed in the IR spectrum of photooxidized P P and has been frequently understood as IR absorption of ester groups? - 1

A few more recent works on ethylene-propylene copolymers (EPR) have not provided more information on these specific questions (15, 16). The de-



35.

L E M A D Œ ET A L .


583

velopment of heterophasic PP, that is, blends of isotactic P P as major con stituents and E P R noddles, prompts new effort to understand the involved problems of P P photooxidation. Some original results are reported in this chapter. Himont Spheripol P P and Hoechst heterophasic P P have been photooxidized in the form of thin films (100 μηι) or thick plates in a S E P A P 12.24 photoaging unit. F T I R analysis was generally carried out by using a transmis sion technique with the film and photoacoustic detection to analyze the most superficial layers of the thick plates (a 10-μηι superficial layer was indeed analyzed). Films were photooxidized as homogeneous reactors, and photoproduct profiles were observed in thick unpigmented or pigmented plates (pigmented with T i 0 or carbon black). The assignment of complex absorption massives in IR spectra of oxidized samples was considerably eased by the derivatization techniques first proposed by Carlsson and co-workers (17). S F treatment of photooxidized heterophasic PP films converted the acidic groups formed in acid fluorides absorbing at 1840-1841 c m " and not at 1848 c m . A series of low molecular weight carboxylic acids of various structures was introduced into a heterophasic P P matrix and submitted to S F treatment. The 1840 c m absorption band could only be assigned to an α-methylated acid: 2

4

1

- 1

- 1

4

1

A

S

°H

Such a carboxylic structure could not be formed from the acyl groups resulting from Norrish type I processes on the intermediate ketone group. As explained in the next sections, a new route for the acid formation should be proposed. O n the basis of pulsed 300-MHz C N M R techniques, the analysis in the solid state on heterophasic P P thermooxidized at 140 °C up to a large oxidation extent was tentatively carried out. Chain-end ketones whose signal appears at 206 ppm largely prevailed over chain ketones whose signal appears at 216.2 c m " . Vinylidene groups absorbing at 112 ppm (in J-modulated echo spec trum) were also observed among more conventional oxidized groups (e.g., acid, hydroperoxide, and tertiary alcohols). In the β-scission of alkoxy radicals, which accounts for the favored for mation of chain-end ketones, a macro-alkyl radical is simultaneously produced (see Scheme II). This macro-alkyl radical, before rearrangement into vinyli dene groups or into a tertiary radical, could be oxidized into a primary hy droperoxide, then into an aldehyde, and finally into an acidic group that is α-methylated (see Scheme III). A similar radical could be formed in the Nor rish type I process observed from the chain-end ketones or from the macroketones (see Scheme IV). 1 3

1


POLYMER

584

DURABILITY

Η I

H I

-Ο Ι

2

C H 3

3

H I

-C0-CH -C-

*CH -Ç2

CH

C H 3

—

2

+

hv

3

H -CO* + *CH -ÇCH 1

>

2

CH

3

3

Scheme IV. These routes should not be considered as important because the stoichiometries of photo- and thermooxidation are very similar, as shown by the IR spectra and by the rates of production of the various oxidized products (Fig ures 1 and 2). The 1740 c m " shoulder that appears in the thermo- or photooxidation of isotactic or heterophasic P P was reassigned to an acidic group that would be hydrogen-bonded to a vicinal hydroperoxide. 1

CH I -C-CH 3

N

CH I CH

3

2

0H..300 nm and 60 °C: (a), carbonyl vibration region; (b), hydroxyl vibration region; A, 0 h; B, 1074 h; C, 1626 h; D, 1925 h; E, 2000 h; F, 2089 h; G, 2237 h; and H, 2331 h. 2500 ppm of a nonmigrating H A L S (Figure 11). The spectra of Figure 10 are identical to those observed with film samples in any conditions, whereas the spectra of Figure 11 are quite different; in the superficial layers, a compound accumulated with a band peaking at 1732 c m " . Interestingly enough, the carbonyl compound absorbing at 1732 c m " could result from a grafting of the H A L S additive (converted into nitroxy radical) to an oxidized macroradical (which could be a ketyl radical). This observation is only possible in an ultraacceleration unit in which the loss of volatile photoproducts is kept low due to the reduced time scale. When two dynamic processes coexist, that is, a photooxidation process and a diffusion process, it is important to check that the chemical evolution is controlled during the whole lifetime of the material by the same process in weathering and in accelerated artificial conditions. When the accelerated conditions favor the noncontrolling process, irrelevant results are obtained. 1

1


592

POLYMER DURABILITY


00 at 1713 cm

2000

3000

Irradiation time (hrs) Figure 8. Evolution of optical density at 1713 cm' with irradiation times of films (e =100 μηι) on irradiation at λ > 300 nm and 60 °C: X, no additive; o, 0.1% HALS 1; O, 0.25% HALS 1; * 0.5% HALS 1; 0.1% HALS 2; +, 0.25% HALS 2; and ·, 0.5% HALS 2. 1

Evolution of Photo(Bio)Degradable PE In a recent European Brite-Euram Contract (Brite-Euram Contract B R E U 170, Proposal B E 3120-89, Université Blaise Pascal, Clermont-Fd, France, and Aston University, United Kingdom), we had to evaluate the bio-assimi lation properties of highly photooxidized P E systems. The control of the abi otic degradation of a series of photodegradable systems enabled us to compare weathering, artificial nonaccelerated photoaging (in the U V cabinet of Aston University), moderately accelerated photoaging (in S E P A P 12.24), and ultraaccelerated photoaging (in S E P A P 50.24). The comparison was made on the basis of F T I R spectroscopy and on the basis of gel permeation chromatog raphy (GPC). When the photostabilization of a polymeric material has to be evaluated, the F T I R analysis of the matrix is the more informative because the oxidation occurs at a very low extent, most photoproducts are nonvolatile, and cross-linking could be important only in the very earliest phases. When the ultimate fate of a polymeric material is characterized after heavy frag mentation, G P C is the most informative. The photodegradable PEs examined were •

a photolytic system based on a copolymer ethylene-CO (1% CO) (Film A ) 2


35.

LEMAIRE ET A L .


593


0.8J

600

400

200

800

Irradiation time (hrs) Figure 9. Evolution of optical density at 1713 cm' with irradiation times of films (e =100 μπι) on ultra-accelerated irradiation at λ > 300 nm and 60 °C: X, no additive; o, 0.1% HALS 1; * 0.25% HALS 1; •, 0.5% HALS 1; +, 0.1% HALS 2; M, 0.25% HALS 2; A, 0.5% HALS 2; and ·, 0.25% HALS 1 + 0.25% HALS 2. 1

• • •

a nonpigmented P E photosensitized by iron carboxylate (Film a Ti0 -pigmented P E photosensitized by iron carboxylate (Film C ) a nonpigmented P E photosensitized by iron and nickel dithiocarbamates (Scott-Gilead system) (18) (Film B ) 2

2

5

The photooxidation was followed by the simultaneous determination of the variations of the absorbance at 1715 c m (by measuring the concentration of acid groups considered as the critical photoproduct in polyethylene ma trices) and the variations of the average weight-average molecular weight, M . The variations of M versus the extent of the photooxidation are shown on Figure 12. Large decreases of M from 3 million to 4,000 or 36,000 were observed, and demonstrated that chain scissions largely prevailed on crosslinking. The photolytic copolymer (Film Ag) initially presented the fastest de crease; however, afterward the decrease in M due to photooxidation of fragments proceeded slower than in other systems. A more extensive photooxidation was carried out in the S E P A P 12.24 with C films. After an exposure equivalent to 2 years of weathering, M decreased to around 2,000, whereas the absorbance at 1715 c m was around 2 for a 45-μπι film. The results obtained in weathering conditions and in the U V - 1

w

w

w

w

2

w

- 1



594

POLYMER

3800

3600

3400

3200

DURABILITY

3000

Figure 10. Evolution of the PAS-FTIR spectra of a copolymerized thick plaque containing 0.25% migrating HALS 1 on ultra-accelerated irradiation at λ > 300 nm and 70 °C: (a), carbonyl vibration region; (b), hydroxyl vibration region; A, 0 h; B, 154 h; C, 461 h; D, 610 h; E, 774 h; and F, 938 h. cabinet of Aston University (i.e., in nonaccelerated conditions) for the B and C systems are very consistent with the data obtained in S E P A P 12.24 (Table g

2

I).

_ The reduction of M of the C system with the oxidation extent was com pared throughout weathering, moderate accelerated photoaging (SEPAP 12.24), and ultra-accelerated photoaging (SEPAP 50.24). As shown in Figure 13, the decrease in M is very similar in the three conditions. If the decrease in M is slighdy antagonized by cross-linking, that cross-linking is not favored in ultra-accelerated conditions. w

2

w

w

Conclusions The recognition of an evolution mechanism throughout weathering and arti ficial aging allows a fairly efficient control of relevancy. When the simulation techniques were proposed, acceleration was not accepted to avoid the obser vation of irrelevant phenomena in laboratory conditions. O n the other hand,


35.


LEMAIRE ET A L .


f

UNITE PAS

1 3800

1 3600

595

Ε

! 3400

1 3200

1 3000

Figure 11. Evolution of the PAS-FTIR spectra of a copolymenzed thick plaque containing 0.25% nonmigrating HALS 1 on ultra-accelerated irradiation at λ > 300 nm and 70 °C: (a), carbonyl vibration region; (b), hydroxyl vibration region; A, O h; B, 154 h; C, 461 h; D, 610 h; E, 774 h; and F, 938 h. an aging matrix should not be considered as a homogeneous (photo)reactor at the molecular scale. By using conventional kinetics, an expression of the evolution rate of the matrix cannot be obtained, even approximately; and the data collected at the initial time, in nonaccelerated conditions, cannot be ex trapolated to the real lifetime of the polymeric material. Thus, experimental acceleration should appear as a fundamental necessity and its limitation should be controlled. Because identification of the evolution mechanism is essentially based on the nature of the intermediate and final photoproduct, like in any phenomenological approach, the following techniques should be used: • • •

F T I R and Raman spectroscopies for matrices converted into homo geneous reactors micro-FTIR and micro-Raman spectroscopies for examining the el ementary layers of matrices that present some photoproduct profiles G P C for molecular weight determinations


POLYMER DURABILITY


596

Table I. Results for B and C Film Systems 5

Film

Conditions Clermont-Ferrand

(m)

5

34

2

45

B Q

35 45

B

c

UV cabinet, Aston University

Thickness

5

2

ΔΑ at 1715 cm'

1

0.142 0.173 0.305 0.428 —

0.133 0.249 0.630 2.13 2.07

M

w

229,500 239,300 208,800 105,100 63,600 271,000 125,000 76,700 27,100 4,500 8.500

Coupling with chemical dosages or with gaseous-specific in situ reagents (like S F , N O , and N H ) is advantageous i n many circumstances. Acceleration could be used every time a single (even complex) dynamic process controls the evolution of the matrix. The acceleration level should be controlled by the invariance of this prevalent mechanism. When deviations are pointed out from experimental results, the data collected in laboratory conditions could not be directly transferred in the conditions of the material's real life. However, the data could be useful to design new accelerated conditions if deviations are accounted for. The frequent reasons for deviations are generally either b i 4

3


35.

LEMAIRE ET A L .

À

M


w χΙΟ

597

3

2801

2101


1401 70H 0 0.3

0

0.6

0.9

1.2

Absorbance at 1715 cm -1 Figure 13. M variations of C film according to M, natural IN-exposure; Φ, accelerated photoaging in SEPAP 12.24 at 60 °C; and X, ultra-accelerated photoaging in SEPAP 50.24 at 80 °C. w

2

molecular reactions between intermediate species, whose concentrations in real conditions are too low to interact, or diffusional processes of reactants (e.g., 0 , H 0 , or stabilizers) that appear to be too slow in accelerated con ditions. 2

2

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Ly, T.; Sallet, D.; Lemaire, J. Macromolecules 1981, 15, 1437. Lemaire, J.; Arnaud, R.; Gardette, J. L. Rev. Gén. Caoutch. Plast. 1981, 613, 87. Ginhac, J. M . ; Arnaud, R.; Lemaire, J. Makromol. Chem. 1981, 182, 1229. Penot, G.; Arnaud, R.; Lemaire, J. Makromol. Chem. 1981, 183, 2731. Adams, J. H . J. Polym. Sci. A 1970, 1(8), 1279. Adams, J. H.; Goodrich, J. E. J. Polym. Sci. A 1970, 1(8), 1269. Chien, J. C. W. Vandenberg, E. J. Jabloner, H . J. Polym. Sci. A 1968, 1(6), 381. Niki, E.; Decker, C.; Mayo, F. R. J. Polym. Sci. Polym. Chem. Ed. 1973, 11, 2813. Carlsson, D. J. Wiles, E. J. Macromolecules 1969, 2(6), 597. Carlsson, D. J. Wiles, E. J. Macromolecules 1969, 2(6), 587. Carlsson, D. J.; Wiles, E. J. J. Macromol. Sci. Rev. Macromol. Chem. 1976, C14(1), 65. Adams, J. H . J. Polym. Sci. A 1980, 1(8), 1077. Carlsson, D. J.; Chmela, S.; Lacoste, J. Macromolecules 1990, 23, 4934. Mill, T.; Richardson, H.; Mayo, F. R. J. Polym. Sci. Polym. Chem. Ed. 1973, 11, 2899. Lacoste, J.; Singh, R. P.; Boussand, J.; Arnaud, R. J. Polym. Sci. Polym. Chem. Ed. 1987, 25, 2799. ;

;

;

;


598

POLYMER DURABILITY

16. Singh, R. P.; Lacoste, J.; Arnaud, R.; Lemaire, J. Polym. Degrad. Stab. 1988, 20, 49. 17. Carlsson, D. J.; Dobbin, C. J. R. Jensen, J. P. T.; Wiles, D. M. In Polymer Stabilization and Degradation; Klemchuk, P. P., Ed.; ACS Symposium Series 280; American Chemical Society: Washington, DC, 1985; pp 359-371. 18. Gilead, D. Scott, G. Dev. Polym. Stab. 1982, 5, 71. ;

;

RECEIVED

for review January 26, 1993. A C C E P T E D revised manuscript December 9,


1994.


Mechanisms of Photooxidation of Polyolefins - American Chemical

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