LDPE carbonyl formation

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... 

Figure 11. Effect of metal chelates (0.1%) carbonyl formation during irradiation of LDPE. Hours to embrittlement in parenthesis.

Polyethylene and polypropylene blended with iron carboxylate complexes, for example, acetylacetonate (FeAcAc) and stearates (FeSt), and irradiated by UV light under accelerated aging conditions were shown to act as effective phtoactivators giving rise to rapid photoxidation as shown from the rapid rate of carbonyl formation without any induction period (see Fig. 16.4a for FeAcAc in HDPE) and with a reduction in molar mass (see Fig. 16.2a for FeSt in LDPE). However, these complexes have been shown to cause considerable oxidation to both PE and PP during processing reflected in a sharp increase in the polymer s melt flow index (reflecting chain scission and drop in molar mass) (Fig 16.4b) and act, therefore, as thermal prooxidants and cannot be used without the use of additional antioxidants in the system [2,3,17-19,48,49]. 

Figure 10. Effect of processing time on carbonyl formation in LDPE. (1) open chamber (2) closed chamber (3) closed chamber purged with argon.

Table I. Induction Periods to Onset of Carbonyl Formation in the Thermal Oxidation of LDPE Films (110°C)°...

Thermo-oxidation increases the degradation rates considerably. During this type of degradation both molecular reduction and molecular enlargement reactions occur [9]. In starch-filled LDPE containing pro-oxidant in addition to starch it was shown that oxidation at 100°C initiates carbonyl formation after 5 days compared to pure LDPE which is unaffected under the same conditions at [10]. 

Figures 3.14 and 3.16 show that the maximum rate of initial carbonyl formation in polyethylene (LDPE) observed in Figure 3.15 is associated with a higher initial hydroperoxide concentration (Fig. 3.14) and a higher rate of hydroperoxide formation during subsequent thermal oxidation (Fig. 3.16). Figure 3.14 and 3.16 also show that the hydroperoxide concentration rises to a maximum and then decays with heating time both in the melt and the solid phase, and that the maximum concentration achieved increases with decreasing temperature. In the absence of oxygen, hydroperoxide concentration decayed to zero in less than 20 h at 110°C [414]. The half-life of polyethylene hydroperoxide is 6.4 h at 100 °C [989].

Layered double hydroxides, intercalation in, 12, 828 LCBs, see Long chain branches LC compounds, see Liquid crystalline silicones LCST, see Lower critical solution temperatures LDHs, see Layered double hydroxides LDPE, see Linear low-density polyethylene Lead complexes in C-C bond formations, 3, 887 chromium carbonyl complexes, 5, 208... 

Starch and cellulosic materials are frequently used as fillers in degradable materials. The addition of starch to LDPE in combination with a pro-oxidant increases the photooxidation rate and the formation of hydroperoxides and carbonyl groups. Starch alone does not increase the photooxidation rate. The addition of starch to LDPE increases its stability in 80°C water. Slower degradation in water is due to leaching out of the pro-oxidant. The addition of starch causes biodegradation process under soil burial conditions. Further increase in the degradation rate can be achieved by preheating polyethylene filled with starch. ... 

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. 

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