LDPE carbonyl index

Fig. 4 Changes in carbonyl index for pre-oxidised LDPE-PO degraded at 50 °C in air, in sterile mineral medium and in biotic mineral medium with compost microorganisms...

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 6. Variation of carbonyl index with irradiation time for LDPE processed for varying times. Processing times in parenthesis.

Figure 1.2 Carbonyl index as a function of depth below the exposed surface of sandwich specimens of LDPE exposed in the tropics for 177 days (A) to fluorescent lamps In air (x) to fluorescent lamps in oxygen (O). (Reproduced from A. Davis and D Sims, Weathering of Polymers, App. Sci. Pub., 1983, Chap. 6 with permission).

Figure 1.4 Decay of dithlocarbamate absorbances and corresponding increases in carbonyl index in LDPE during photooxidation. 1, 2, 3 = carbonyl index growth curves for FeDMC, ZnDEC and NiDEC, respectively i = disappearance of visible absorbance of FeDMC 2 decay of ZnDEC, 280 nm absorbance 3 = decay of NiDEC 315 nm absorbance. (Reproduced from S. Al-Malaika, A M. Marogi and G. Scott, J. App. Polym. Sci., 31, 685 (1986), with permission).

The early stages of the photo-oxidation of low density polyethylene (LDPE) were studied by the polymerization of vinyl monomers [180]. It has been shown that evaluation of the changes in the polymer is more easily made using this method than by use of the carbonyl index. 

Fig. 2 Variation of gel content (a) and carbonyl index (b) in the systems consisting of LDPE and EOP in different loadings, (white) 0 % (light grey) 5.8 % (dark grey) 8.8 % (black) 10.3 %...

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]. 

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