Degradation carbonyl index

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... 

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...

Whether biotic or abiotic, degradation is essentially lydiolytic or (photo/thermo)-oxidative (other modes exist such as acid attacks by Thiobacillus). For example, for the hydrolysis of poly(lactic add) (PLA), the reaction is veiy slow at low temperatures (20-25°C), but very fast at temperatures around 60°C (Figure 14.2). We can also mention the abiotic degradation of poly(ethylene) by photo- and thermo-oxidation (Figure 14.3) which results in a drastic decrease in molar masses, a loss of mechanical properties of the material, an increase in the carbonyl index up to 1713 cm and the progressive appearance of degradation molecules, such as ketones, alcohols, acids and so on. 

The conditions were chosen to mimic a general crop season in the mediterranean region. The samples showed a relatively low extent of degradation as monitored by the carbonyl index, molecular weight variation, extractability by solvent, changes in the onset of the decomposition temperature and the crystallinity. 

Andrady AL, Pegram JE, Tropsha Y. Changes in carbonyl index and average molecular weight on embrittlement of enhanced photo-degradable polyethylene. J Environ Degrad Polym 1993b l 171-179. 

Fig. 1 Simultaneous modifications in accumulated CL photons and carbonyl index for the degradation of polypropylene. The data were taken from [93G1]. ( ) carbonyl indexes ( ) CL records.

In the case of PE irradiation, it has been established that the superficial oxidized layer undergoes essentially chain scission, whereas the behavior of nonoxidized core layers is dominated by crosslinking therefore, the samples become more and more heterogeneous as the dose increases and a skin core structure appears. If in an inert medium, y-rays penetrate in all directions of the material in an oxidative medium, the phenomenon is oxygen diffusion-controlled. A depth distribution will exist in the variation of carbonyl index, crystallinity, melting point, degradation products, and so on. 

In products with a large wall thickness, the influence of weathering is often limited to a surface layer, either due to limited oxygen diffusion or to limited UV penetration. The depth of this layer may be small ( 0.5 mm) compared with the whole wall thickness ( 4 mm) but it can causes brittle fracture. Mechanical behavior depends on the oxidation (degradation) profile, a critical degradation profile accounted for the failure of samples, that is time of failure. Tensile tests were performed on films microtomed from the samples [39]. The density of weathered samples can increase because of chemicrystallization, increase in polar groups, oxygen uptake or the loss in volatile products. The decrease in the nominal strain corresponded to an increase in the vinyl index and in the density. The carbonyl index showed too much scatter. 

The photo-oxidation of polypropylene is characterized by an induction period in which oxygen uptake (whether measured directly or by carbonyl index) occurs at a very low rate, followed by autoacceleration to a steady rate of oxidation. Measurements of mechanical properties and polymer molecular weight [2187] have shown that extensive degradation occurs in the induction period and the useful life extends little beyond this point [1443]. The surface of photo-oxidized polypropylene is highly degraded to a depth of 0.5 ym during the induction period, leading to microcracks and loss of mechanical properties [390]. 

Figure 10.43. Carbonyl index of LLDPE film containing Chimassorb 944 and two types of silica (A B) vs. time of exposure to mercury lamp. [Data from Liauw, C. M. Childs, A. Allen, N. S. Edge, M. Franklin, K. R. Collopy, D. G, Polym. Degradation Stability, 65,207-15,...

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