Degradation Photo-oxidation

Ranby, B. and Rabek, J., Photo Degradation, Photo Oxidation and Photo Stabilization of Polymers, Wiley, New York, 1975, 75. 

Polymer Thermal degradation Photo-oxidation Ozone... 

How does commercial polypropylene degrade photo-oxidatively ... 

Degradation processes during weathering involve photo-radiation, thermal degradation, photo-oxidation and hydrolysis. These irreversible processes can cause... 

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. 

Protection of polymers against thermal and photo-oxidative degradation is achieved with appropriate stabilizers that ensure the desirable polymer properties throughout the entire service life of the polymer. Compatible and polymeric stabilizers usually give the best protection. In order to avoid migration and evaporation, polymeric stabilizers are used. 

What is mechanical degradation Why is it less commonly encountered relative to thermal and photo-oxidative degradation ... 

The photo-oxidative degradation of polypropylene (PPH) can be largely summarized by the reactions 1 to h ( l,2j. During the... 

Water, methanol, and n-hexane do not influence the photooxidation of PVC (43), but the photodegradation is accelerated by ferric chloride (70,71) and certain other compounds containing iron (70,71,72). Purification of the polymer might be expected to enhance its photostability by removing deleterious impurities such as iron compounds that are derived from metal equipment. This type of result was obtained in one recent study (58) but not in others (30,59). In contrast, the photo-oxidative degradation of PVC should be enhanced by admixture of the polymer with materials that are unusually susceptible to photooxidation themselves. Such behavior has been observed for impact-modified PVC containing polybutadiene-based polyblends (69,73). 

As discussed earlier under Section 2.3, Carbonyl index, in one relatively recent comparison of the photo-oxidative and thermal (oven-aged) degradation behaviour of different polyethylenes, additive free grades of a metallocene (mPE), an HDPE and a linear low-density PE (LLDPE) were analysed by a combination of mid-IR spectroscopy, TGA and CL [13]. The mid-IR... 

Some carotenoids have structures containing fewer than 40 carbon atoms and derived formally by loss of part of the C40 skeleton. These compounds are referred to as apocarotenoids when carbon atoms have been lost from the ends of the molecule or as norcarotenoids when carbon atoms have been lost formally from within the chain. These modifications are caused by oxidative degradation at the level of the terminal rings either by nonspecific mechanisms (lipoxygenase, photo-oxidation) or by... 

Agemian and Chau [55] have described an automated method for the determination of total dissolved mercury in fresh and saline waters by ultraviolet digestion and cold vapour atomic absorption spectroscopy. A flow-through ultraviolet digester is used to carry out photo-oxidation in the automated cold vapour atomic absorption spectrometric system. This removes the chloride interference. Work was carried out to check the ability of the technique to degrade seven particular organomercury compounds. The precision of the method at levels of 0.07 pg/1, 0.28 pg/1, and 0.55 pg/1 Hg was 6.0%, 3.8%, and 1.00%, respectively. The detection limit of the system is 0.02 pg/1. 

Oxidative degradation can be the most serious problem in the use of plastics at higher temperatures. At ambient temperature oxidation proceeds relatively slowly on its own, but can be stimulated by light (photo-oxidation), ionising radiation (radio-oxidation), certain gaseous and liquid environments and by the presence of transition metals. The rate at which oxidation occurs will therefore depend on the intensity of these agents, on temperature, and on the availability of oxygen, which in turn depends upon its solubility, its rate of diffusion (see Section 4.12.2) and the rate at which it is consumed. 

This type of degradation can also be referred to as photodegradation or ultraviolet (UV) degradation. It includes photo-oxidation. It produces some of the more familiar signs of degradation of plastics embrittlement, discoloration and loss of transparency. 

Investigation turned then to chemical and spectroscopic means to obtain the needed mechanistic understanding. Stephenson et al. [17] looked at gas evolution versus exposure, while Pacifici and Straley [18] used UV fluorescence spectroscopy to identify a photo-oxidation product which was later isolated by Valk et al. [19]. In addition, Valk and co-workers [19-21] isolated a number of additional photolysis products by a combination of hydrolysis and chromatography, Marcotte et al. [22] used ESR to look at radicals generated during degradation, and Day and Wiles [23-26] carried out extensive IR and fluorescence spectroscopic investigations on this subject. 

The end result of these studies showed very clearly that two major processes were important, i.e. photolysis and photo-oxidation. Photolysis reactions were posited to be the result of the well-known Norrish Type 1 and Norrish Type 2 cleavage reactions. As we shall see, the Type 1 cleavage followed by several subsequent reactions can account for many of the observed degradation products. 

Scheme 18.4 Photo-oxidation reactions of PECT [11], Reprinted from Polymer, 41, Grossetete,T., Rivaton, A., Gardette, J.-L., Hoyle, C. E.,Ziemer, M., Fagerburg, D. R. and Clauberg, H., Photochemical degradation of poly(ethylene terephtha-late)-modified copolymer, 3541-3554, Copyright (2000), with permission from Elsevier Science...

A particularly relevant thermo-oxidative study on PET degradation and PBT reported the degradation products observed for ethylene dibenzoate [39], The products observed paralleled those of the photolysis and photo-oxidation reports discussed above with benzoic acid, vinyl benzoate, 2-hydroxyethylene dibenzoate, 2-carboxymethoxy benzoate and the coupling product, 1,4-butylene dibenzoate, being reported. The 2-hydroxyethylene dibenzoate and 2-carboxymethoxy... 

Knoevenagel, K. and Himmelreich, R. Degradation of compounds containing carbon atoms by photo-oxidation in the presence... 

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