Polypropylene, photodegradation

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]. 

The photooxidation of polypropylene involves the initiation of free radical chain reaction by the photolysis of hydroperoxides, producing peroxyl radicals as well as alkoxy radicals. In the last decades many studies dealing with the types of stabilizers used to delay the polypropylene photodegradation were reported as well as several studied on the experimental methods for assessing their photostability effectiveness [73-78]. 

Vychopiiova, J., Cernak, R., Obadal, M., Raab, M., Vemey, V., Commtaeuc, S. The role of specific nucleation in polypropylene photodegradation. Polym. Degtad. Stab. 92, 1763-1768... 

PL can be used as a sensitive probe of oxidative photodegradation in polymers. After exposure to UV irradiation, materials such as polystyrene, polyethylene, polypropylene, and PTFE exhibit PL emission characteristic of oxidation products in these hosts. The effectiveness of stabilizer additives can be monitored by their effect on PL efficiency. 

Photodegradation of polyethylene waste can be markedly accelerated via its grafting with acrylamide [98], In contrast, photostabilization of polyethylene and polypropylene can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2-hydroxy-propoxy) benzophenone using gamma radiation [ 147], In this case, the grafted compound acting as a U V stabilizer is chemically bound to the backbone chain of the polymer and its evaporation from the surface can be avoided. 

Quinones are effective compounds for the photodegradation of polystyrene (105-10 )pPolyisoprene (108.109).polypropylene (110),polyamides (111),polyurethanes (97) and oellulose (112). 

The skin is cleaned and anesthetized as for other similar procedures, and lOmL of fluid is aspirated into a syringe connected to the spinal needle that is typically used. Sterile containers such as polypropylene test tubes or urine cups are used to transport the fluid to the laboratory. If a specimen is for the determination of the lecithin-sphingomyelin (L/S) ratio, the container is immediately placed in ice if it is for spectrophotometric analysis, the specimen should be transferred to a brown tube or bottle to prevent photodegradation of bilirubin. Alternatively the specimen container may be wrapped in aluminum foil. 

Fig. 10 Strain at break Vs. UV exposure time for polypropylene copolymer bars made from (a) virgin polymer (V) (b) virgin polymer + undegraded recyclate (U) (c) virgin polymer + photodegraded recyclate (P). Results for blends with recyclate contents of 10% and 25% are shown. (More details in Craig, I.H. White, J.R. Mechanical properties of photo-degraded recycled photo-degraded polyolefins. J. Mater. Sci. 2006, in press.)...

Rabello, M.S. White, J.R. The role of physical structure and morphology on the photodegradation behaviour of polypropylene. Polym. Degrad. Stab. 1997, 56, 55-73. 

The polymers obtained by polymerization in the presehce of metal catalysts contain metal residues which cannot be removed so readily. It is also well known that transition metal ions act as sensitizers for the photooxidation of polyolefins (29). Kujirai et al. (30) found that photodegradation of polypropylene depends on the oxygen concentration and on the residues of the polymerization catalyst, and they concluded that oxidative photodegradation is sensitized by the initiator metal residues (ash). Very recently Scott (31) used transition metal ions as sensitizers to develop photodegradable polymers. 

Polypropylene usually exhibits an ultraviolet absorption extending beyond 290 nm, although the pure polymer is not expected to show an absorption beyond 200 nm. Photodegradation of polypropylene is thus likely to be initiated by impurities such as hydroperoxides or ketonic oxidation products formed during the processing of the sample. Such behaviour is similar to that already discussed for polyethylene. 

Hydroperoxide photolysis by sunlight in the presence of air is considered to be the major source of free radicals and backbone scission during the photodegradation of polypropylene [37]. 

The photodegradation of other polyolefins has not been studied extensively but the conclusions drawn from work with polyethylene and polypropylene are likely to be valid in general. ESR investigations of the free radicals produced in the photolysis of poly-l-butene [31], poly-4-methylpentene-1 [31, 40] and poly-1-dodecene [31] have been published. 

Many workers have reported the importance of intramolecular propagation in polypropylene oxidation [144, 148, 390, 550, 633] and photodegradation [125]. Dulog et al. [193, 194] asserted that only a crystalline polymer reacts in this way. Tobolsky et al. [615] concluded, from a study of the oxidation of ethylene—propene rubber, that intramolecular propagation is involved. The importance of intramolecular... 

FUNCTIONAL GROUPS IN PHOTODEGRADED POLYPROPYLENE AND POLYETHYLENE AFTER UV EXPOSURE[6]... 

Fig. 6. Carbonyl and hydroxyl content of photodegraded polypropylene, from IR spectra of 20 mil sheet taken after volatiles removed at 70°C/100 mm [6].

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