Polyolefins photo-oxidation

Polyolefins.— The mechanism(s) of polyolefin photo-oxidation continue to be shrouded in complexities. Recently Geuskens has attempted to answer questions concerning the sensitized photolysis of hydroperoxides in these polymers in the presence of carbonyl groups. In contrast with the theory proposed above,it is suggested that a complex is formed and photolysed to produce a carboxylic acid and an ether directly (Scheme 19) rather than free hydroxyl radicals. 

The content of aldehyde groups in the final product of polypropylene photodegradation is 7%. In the case of polyethylene degradation these groups are completely absent [6]. The relatively lower amount of aldehyde groups in the products of polyolefine photo-oxidation is due to the fact that they absorb radiation and subsequently react as above [278]. 

Polyolefins.—Polyolefin oxidation continues to be a subject of considerable controversy. Wiles and co-workers have written a comprehensive review of polyolefin photo-oxidation mechanisms with particular emphasis on the role of... 

Wiles and co-workers have examined the role of peroxy-radicals in polyolefin photo-oxidation. They suggest that radical recombination processes have a high probability even if they escape the primary polymer cage. The occurrence of secondary cage-recombination processes was considered. Vasilenko et have also studied the role of free radicals in polyethylene photo-oxidation. 

Scoponi M, PradeUa F, Carassiti V. Photodegradable polyolefins. Photo-oxidation mechanisms of innovative polyolefin copolymers containing double bonds. Coord Chem Rev 1993 125 219-230. 

Regarding the outdoor application of commercial polymers such as polyolefins, photo-oxidation reactions are of prominent importance. Photooxidation processes will be discussed in the next subsection. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

NO Reactions. The most informative derivitization reaction of oxidized polyolefins that we have found for product identification is that with NO. The details of NO reactions with alcohols and hydroperoxides to give nitrites and nitrates respectively have been reported previously, and only the salient features are discussed here (23). The IR absorption bands of primary, secondary and tertiary nitrites and nitrates are shown in Table I. After NO treatment, y-oxidized LLDPE shows a sharp sym.-nitrate stretch at 1276 cm-1 and an antisym. stretch at 1631 cm-1 (Fig. 1), consistent with the IR spectra of model secondary nitrates. Only a small secondary or primary nitrite peak was formed at 778 cm-1. NO treatment of y-oxidized LLDPE which had been treated by iodometry (all -OOH converted to -OH) showed strong secondary nitrite absorptions, but only traces of primary nitrite, from primary alcohol groups (distinctive 1657 cm-1 absorption). However, primary products were more prominent in LLDPE after photo-oxidation. 

Norrish type II mechanism for photo-oxidation of polyolefins. 

The time to embrittlement of polyolefins during photo-oxidation is not directly related to initiation carbonyl concentration (9-11). This is exemplified for HOPE in Figure 3 and a similar relationship exists for LDPE and... 

Vink, P. Loss of UV stabilizers from polyolefins during photo-oxidation. In Developments in Polymer Stabilisation Scott, G., Ed. Applied Science London, 1980 Vol. 3, 117-138. 

Damage by light and oxidation to polyolefin fibres, including photo-oxidation... 

Since UV absorbers cannot completely prevent the initiation reactions above, we felt that better antioxidants were necessary to obtain improved long-term photo-oxidative stability of polyolefins. We reasoned that stable radicals which scavenge radicals formed in the oxidative process might compensate for the imperfections of the absorbers. On the basis of the assumption that the diaryl nitroxyl radicals, rather than their parent diarylamines, are the active antifatigue species in rubber protection, we first attempted to obtain new stable nitroxyl radicals. 

A number of volatile compounds are obtained apart from the crosslinked products in the final oxidation process. With polyolefines, water, formaldehyde, acetaldehyde, acetone, methanol, hydrogen peroxide, carbon monoxide, and carbon dioxide were identified [Refs. 75, 76, 122, 292, 424, 425, 449, 487, 560, 608]. Table 5 shows the percentage content of particular volatile products formed during the photo-oxidative degradation of polypropylene [122]. 

The involvement of singlet oxygen in the photo-oxidation of polyolefins was first considered by Trozzolo and Winslow [618]. The mechanism is as follows. 

Vink has produced evidence in total conflict with previous experience which is suggested to show that the photo-oxidation of polypropylene is a bulk reaction rather than a surface phenomenon. ESCA studies, however, have shown that the photo-oxidation of polypropylene is clearly a surface phenomenon.In a recent study by Kollmann and Wood the photo-oxidation of polypropylene was found to be dependent upon the intensity of the light source. Thus, for unstabilized polymer the rate was proportional to I , whereas for stabilized polymer the rate was proportional to 1O 8-0.9 Thej-e appears to be some conflict in the literature as to whether chemical changes during the photo-oxidation of polyolefins correlate with the changes in mechanical properties.This has always been a difficult... 

A. Carton, D.). Carlsson, D.M. Wiles, Photo-oxidation Mechanisms in Commercial Polyolefins, in N.S. AUen (ed.). Developments in Polymer Photochemistry - I, Applied Science, London (1980), Chapter 4, p. 93. 

Different polyolefin structural forms react differently to UV. Highly branched LDPE tends to degrade more readily than LLDPE or HOPE, and overall, degradation takes place more easily within the amorphous phase of a polymer than in the crystalline phase. Moreover, PE and PP photo-oxidation behaviors are different enough that the same additive approach for protecting PE may not work the same in PP, even in the same applications. For example, a common... 

W.K. Busfield, T. Paulina, Photo-oxidative degradation of mechanically stressed polyolefins. Polym. Degrad. Stab. 51, 185-196 (1996)... 

What causes photo-oxidative degradation and how does it take place in polyolefins ... 

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