Carbonyl index

The carbonyl index (measured at 1720-1710/cm) is commonly used for the determination of embrittlement time (mainly in polyolefins), i.e. the time (in hours) to reach 0.06 carbonyl units from the plot of absorbance of the carbonyl groups versus irradiation time [45]  

By extrapolation of the portion of the curve to the time axis at zero carbonyl, the induction period (mainly in thermal oxidative degradation) is determined. 

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The photostabilizing efficiency of polymer-bound HALS in i-PP and E-P copolymers were studied and compared with commercial HALS (i.e., Tinuvin 770 and Chimassorb 944) by measuring the carbonyl index at 1720 cm. Plots of carbonyl index versus irradiation... 

Figure 2 Plot of carbonyl index versus irradiation time in i-PP films at 0.2 wt% concentration of stabilizers.

Figure 4 Changes in (a) carbonyl index and (b) hydroxyl index versus irradiation time for polystyrene films. O-con-trol -2,4-DHBP -2H-4MBP -2H-4BB X-DHBP-F A-HMBP-F and D-HBBP-F.

Figure 6 (a) Carbonyl index (at 1740 cm and (b) hydroxyl index (at 3550 cm ) versus aging time for CTA films X-control O-DHBP D-HMEP A-HBBP -DHBP-F -HMBP-F and A-HBBP-F. 

The carbonyl index obtained from transmission and ATR spectra of PP films UV-irradiated (L2) in ozone shows that the concentration of carbonyl groups at the surface is 8 times higher than in a bulk. 

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

As part of a multi-technique investigation (see also discussion under mid-infrared spectroscopy later), Corrales et al. [13] plotted the carbonyl index for films prepared from three grades of polyethylenes a high-density PE (HDPE), a linear low-density PE (LLDPE) and a metallocene PE (mPE) (see Figure 5). In this study, the data trend shown in Figure 5 correlated well with activation energies derived from the thermal analysis, which showed that the thermal-oxidative stability followed the order LLDPE > mPE > HDPE, whereas the trend... 

Figure 5 Left thermo-oxidative carbonyl index vs oven ageing times at 90°C. Right photo-oxidative carbonyl index vs irradiation time after exposure to UV radiation (300-800 nm wavelength). In each case, the carbonyl index is based on the intensity of the mid-infrared absorption at 1,720 cm-1 normalised against sample film thickness, d, which was 290 gm. Reprinted from Corrales et al. [13]. Copyright 2002, with permission from Elsevier. 

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... 

Figure 14 Carbonyl index as a function of corona discharge treatment of PP film. Reproduced with permission from Sellin and Campos [39].

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...

Figure 7. Effect on carbonyl index during uv exposure of HDPE with 1% inhibitor processed at various temperatures (26). X, control—no additive O, I00°C, 5 min A, 200°C, 30 min , 200°C, 60 min , 200°C, 90 min t,...

Concentration Carbonyl index Time to mols / 100 g (before exposure) embrittlement (hrs)... 

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.

Fig. 6 Depth profiling in 3 mm thick polypropylene bar exposed to UV in the laboratory for 18 weeks. The exposed surface was at depth = 0. The steep gradients for both the carbonyl index and the weight average molecular weight at the back face indicate that oxygen starvation in the interior of the bar is the dominant factor rather than UV intensity. (Data from Rabello, M.S. Ph.D. thesis. University of Newcastle upon Tyne, 1996.)...

Figure 1.41. (a) Oxidation induction time, t, for various concentrations of oxidation inhibitor (antioxidant) as determined from oxygen uptake (or carbonyl index) plots, (b) Estimation of intrinsic inhibitor concentration, Cg, from a plot of t versus [Inhibitor]. The inverse of the slope of the plot reflects the rate of initiation. 

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