Polyethylene photo-oxidation

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. 

Cruz-Pinto JJC, Carvalho MBS, Ferreira JEA (1994) The kinetics and mechanism of polyethylene photo-oxidation. Angew Makromol Chem 216 113—133 Darwin PRK, Abdefilah A, Elise D, Josefina LC, Sebastian MG (2003) Synthesis, characterization, and properties of poly(ethylene terephthalate)/poly(l, 4-butylene succinate) block copolymers. Polymer 44 1321—1330... 

The main products of polyethylene photo-oxidations are hydroxyl (OH), carbonyl (C=0) and vinyl groups, whereas trans-vinylene groups are formed but only in small amounts. 

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... 

Addition of tristearyl phosphite to polyethylene containing ferric acetylacetonate has been shown to decrease the stability of the polymer towards photo-oxidation.8... 

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. 

The rate of oxidation can be determined by measuring the oxygen uptake at a certain temperature. Such measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It can be concluded from this result that the thermal oxidation, like photo-oxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydro peroxide is now activated by heat. 

The most Importeuit Initiation process involved in the early stages of the photo-oxidation of polyethylene is shown to be hydroperoxide photolysis eissociated with the decay of virylidene groups. This is followed by carbonyl photolysis occurring primarily by the Norrish type II process. 

The inflexion of the carbonyl formation curve for LDPE oxidised during processing illustrated in Figure 2 always occurs at about the same time of photo-oxidation (see Figure 6). In heavily thermally oxidised polyethylene the carbonyl index actually decreases initially before increasing again. Ketone carbonyl is the main product formed in the... 

One additive that improves both long-term photo and thermal stability of polyethylene Is carbon black. The ability of carbon black to retard destructive thermal oxidation In polyethy-lenes at elevated temperatures Is well known(17) and, as was seen In the earlier section of this paper on photo oxidation. It Is effective at lower temperatures also. Our studies of the thermal oxidation of low-density polyethylene show black samples to be outstanding, even In the presence of copper. For example black low-density polyethylene wire Insulation Is still Intact after 7 years at 80°C while all other colors. Including unplg-mented, failed mechanically due to oxidation after only about 3 months. 

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. 

Fig. 2 GC-MS chromatograms showing the products extracted from photo-oxidized polyethylene by traditional HS extraction (above) and by HS-SPME with polar car-bowax/divinylbenzene fiber (below). A few short-chain carboxylic acids were detected in the chromatogram after HS extraction, while carboxylic acids, ketones and furanones were detected after HS-SPME...

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