Polyolefins photo-oxidative degradation

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]. 

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]. 

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 ... 

Light radiation increases the brittleness of polyolefins and seriously lowers their physical properties, mainly mechanical properties. Such degradation is more pronounced in the presence of oxygen. All modifications produced by photo-oxidative degradation are similar to those produced by thermal degradation and, are accelerated by an increase in light intensity [19] and temperature [20]. 

On the other hand, dithiocarbamate nickel and zinc complexes are very effective light stabilizers for polyolefins. Using a combination of iron(pro-oxidant) and nickel or zinc (stabilizers) dithiocarbamates, it is possible to control the rate of photo-oxidative degradation very accurately [1931]. For long induction periods a combination of iron and nickel dithiocarbamates is recommended, whereas for shorter times it is more usual to combine iron and zinc dithiocarbamates [1932]. The combination of different metal ions decreases the induction period and increases the rate of photo-oxidation. At higher concentrations of stabilizing (Ni, Zn) dithiocarbamates, much longer lifetimes can be achieved. 

B. Dolezel and L. Adamirova, Method of hygienically safe stabilization of polyolefines against thermoxidative and photo oxidative degradation. Czechoslovakian Socialist Republic Patent. 403,1982. 

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. 

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... 

The degradation products of synthetic polymers due to photo-oxidation of polyolefins, hydrolysis of polyesters, and so on. 

Geuskens G, Kabamba MS (1982) Photo-oxidation of polymers—part V a new chain scission mechanism in polyolefins. Polym Degrad Stab 4 69-76... 

The oxygen uptake curve for polyolefin degradation has an S shape (similar to some curves for the evolution of products (see Figs. 3 and 4) with a pronounced induction period whose duration increases with the stability of polymer and varies with the reaction conditions [92]. The initiation by heat leads to a longer induction period than in photo-oxidation (Fig. 5) and this induction period decreases with increasing temperature [4, 92, 117]. 

Hydroxyl region - The hydroxyl absorption in the IR spectrum of PP has a broad band centred at 2.90 pm (associated alcohols) with a definite shoulder at 2.77 pm (non-associated alcohols). At a similar extent of degradation, thermally oxidised polyolefins show hydroxyl bands of approximately half the absorbance values of the photo-oxidised polyolefins. Thus, thermal oxidation produces about half as many hydroxyl groups as photo-oxidation in polyolefins. 

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