Irradiated polypropylene chemiluminescence

Comparison of Chemiluminescence with Impact Strength for Monitoring Degradation of Irradiated Polypropylene... 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

The loss of Impact strength of polypropylene was followed from sheets stored In air at 25°C and 60°C after irradiation with electron beams. A marked difference in efficacy of phenolic and thioether-based stabilizers at the two temperatures was found, with the thioether active alone at 60°C but only synergistically at 25°C. This difference was also reflected qualitatively in differences in chemiluminescence emission from the samples. 

In this study we measured chemiluminescence of polypropylene stabilized with different combinations of antioxidants and irradiated to different extents, and made correlations with conventional impact strength measurements of the same materials. 

Table II. Chemiluminescence (counts/2 min.) at ambient temperature from polypropylene irradiated at day 0 with a dose of 5MR.

Figure 7. Top and Middle Loss of impact strength of polypropylene, irradiated to 5MR, vs. storage time at 25 and 60°C. Bottom Ambient chemiluminescence from irradiated (5MR) polypropylene samples vs. storage time at 25°C.

Chemiluminescence at ambient temperature and at 150°C of several irradiated (electron-beam) polypropylene formulations can be qualitatively correlated with loss of impact strength. The correlation with chemiluminescence is preserved at high vs. low temperatures, even though the ranking changes. 

Figure 10.52 shows chemiluminescence curves obtained from UV irradiated unstabilized polypropylene film. 

Fig. 10.51, Chemiluminescence curves from polypropylene film (a) UV irradiated for 6 min and heated under nitrogen immediately (b) as for (a) but stored for 4 days in darkness before heating under nitrogen and (c) unirradiated film heated under nitrogen [723]. (Reproduced with permission from [723] published by Elsevier Science Publishers Ltd, 1991.)...

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