Polyethylene photo-oxidative degradation

H. Qin, C. Zhao, S. Zhang, G. Chen, M. Yanga, Photo-oxidative degradation of polyethylene/montmOTillonite nanocomposite. Polym. Degrad. Stab. 81, 497-500 (2003)... 

Roy PK, Surekha P, Rajagopal C, Chatterjee SN, Choudhary V. Studies on the photo-oxidative degradation of LDPE films in the presence of oxidised polyethylene. Polym Degrad Stab 2007 92 (6) 1151-1160. 

Plastic garbage made of polyethylene is biologically degraded in the environment only if the photo-oxidative degradation initiated by UV radiation and oxygen has progressed to a large extent already. Unexposed to... 

H. L. Qin, Z. G. Zhang, M. Feng, F. L. Gong, S. M. Zhang, and M. S. Yang, The influence of interlayer cations on the photo-oxidative degradation of polyethylene/ montmorillonite composites. Journal of Polymer Science Part B Polymer Physics, 42 (2004), 3006-12. 

Polybutylene terephthalate and polyethylene terephthalate, even without the addition of stabilizers, exhibit good weathering resistance, which can be improved by carbon black. Figure 5.89. During photo-oxidative degradation, mostly chain cleavage, but no crosslinking occur. The main photo-oxidation products are CO, COj, and carbonic acids. 

During photo-oxidative degradation of polymers, e.g. polyethylene, formation of unsaturated groups (—CH=CH—) has been observed. The presence of these groups can be detected by UV absorption spectroscopy, e.g. tetracene (310 nm), trienyl (325 nm), pentacene (342 nm), tetraenyl (360 nm) and penta-enyl (394nm) [711]. 

Titanium ions (Ti" ) are effective photoinitiators of photo-oxidative degradation polypropylene films and fibres [63,390]. However, in the case of polyethylene, the unsaturation level in commercial samples was found to be more important than metal residues, during thermo- and photo-oxidative degradation [447]. 

Photo-oxidative degradation of polyethylene results in chain scission and crosslinking. On photoirradiation in air, gel formation is discernible after 2h of irradiation however, after 15h of photoirradiation nearly 50% gel fraction can be found in irradiated polyethylene samples. The effects of cross-linking on the formation of oxidation products and the mechanical properties of polyethylene have been studied [2117, 2128]. 

Even very low amounfs of the ketone chromophoric groups in polypropylene can be responsible for the initiation of polymer photo-oxidative degradation which may further occur by mechanisms similar to those described for polyethylene (cf section 3.1.2). 

The photo-oxidative degradation of poly(ethylene-co-propylene-co-l,4-hexadiene) (3.133) occurs by a combination of reactions characteristic of the degradation of polyethylene and polypropylene (section 3.1) and polydienes (section 3.12). Quantum yields of different photochemical processes which occur in this terpolymer are shown in Figure 3.68. 

Qin, H., Zhang, Z., Feng, M., Gong, F., Zhang, S., Yang, M. The influence of interlayer cations on the photo-oxidative degradation of polyethylene/montmorillonite composites. J. Polym. Sci., Part B Polym. Phys. 42, 3006-3012 (2004)... 

Several of the more common commodity polymers like the polyolefins are susceptible to photo-oxidation. For a polymer like polyethylene, photo-oxidation leads to increasing amounts of carbonyl compounds. In-chain ketone groups act as sensitisers by UV light absorption. Through the well-known Norrish type I and II degradations radicals, end-vinyl and ketone groups are formed. Other products often observed in photo-oxidised low-density polyethylene (LDPE) are esters [5]. Scheme 1 shows one mechanism for abiotic ester formation. By Norrish type I cleavage the radical formed can react with an alkoxyl radical on the polyethylene (PE) chain. 

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. 

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... 

Large number of dicarboxylic acids, keto-acids and/or lactones indicated severe degradation of the polyethylene matrix [37, 38, 95]. Dicarboxylic acids were the most abundant products formed during photo-oxidation and their amount increased especially after long irradiation times, i.e. in severely degraded samples [96]. The relationship between the degree of oxidation/ degradation in the polymer matrix and the amount of dicarboxylic acids... 

Polyethylene oxide) (PEO) is a semicrystalline water-soluble polymer [64, 65], with a crystallinity that is very sensitive to the thermal history of the sample, making this property interesting as an indicator of degradation. Because it is biodegradable and biocompatible, PEO is a good candidate for environmental and medical applications [66-68]. The mechanisms of thermo- and photo-oxidation of PEO have already been investigated [69, 70] on the basis of IR identification of the oxidation products and are summarized in Scheme 10.1. 

During photo-oxidation, dicarboxylic acids were the class of products that clearly increased in the most severely degraded samples. As during thermooxidation, the most abundant of the dicarboxylic acids was butanedioic acid. Comparison between the number average molar mass and the relative amount of butanedioic acid, Fig. 6, showed a connection between the formation of butanedioic acid and the degree of degradation in the polyethylene matrix. However, the relative sum of all the carboxylic acids correlated even better with the number of chain scissions than the amoimt of only butanedioic acid. Fig. 7. 

---