Embrittlement photo-oxidation

The nitroxide radical (from processes 5 and 6 and attack by other radicals on the parent piperidine) is found in photo-oxidizing PPH samples in concentrations of M. x 10 M (initial piperidine level 5 x 10-3M) up to the embrittlement point of the PPH film (7.). Nitroxides are well known to scavenge carbon centered radicals (but not peroxyl radicals) in both polymers and liquid alkanes (reaction 7) (10, 8). In the liquid phase k7 is... 

This type of degradation can also be referred to as photodegradation or ultraviolet (UV) degradation. It includes photo-oxidation. It produces some of the more familiar signs of degradation of plastics embrittlement, discoloration and loss of transparency. 

Table 3 Comparison of embrittlement times during photo-oxidation with catalyst residues for polypropylene samples ...

The time to embrittlement of polyolefins during photo-oxidation is not directly related to initiation carbonyl concentration (9-11). This is exemplified for HOPE in Figure 3 and a similar relationship exists for LDPE and... 

PP. Embrittlement times always change most rapidly in mildly oxidised polymers containing little or no carbonyl. This finding is consistant with the observation that added long chain dialkyl ketones are not sensitisers for polypropylene photo-oxidation (30) although aromatic ketones are (22, 30). 

Photo-oxidation of the additive-free PP film gave similar oxidation product yields and embrittlement as exposure to the 1.35 Mrad.h-1 y-cell (Table 1). 

When the peroxyl population was quickly destroyed by 2-methyl -2-nitrosopropane treatment, post-y oxidation still occurred (Figure 7). In addition, film which had been pre-oxidized by xenon-arc photo-initiated oxidation showed an identical, slow, thermal oxidation during storage, as well as a rapid embrittlement in the accelerated aging at 60°C (Figure 6). The photo-oxidation of PP is well... 

Embrittlement of the surface as a result of photo-oxidation is another major cause of fracture problems in polymers. The photons that make up the ultraviolet component of sunlight have enough energy to split chemied... 

Figure 5.178 shows property curves and temperature-time curves for various properties of non-reinforced ABS. It illustrates that ABS embrittles very rapidly due to oxidation, but its dielectric strength remains hardly affected. This rapid embrittlement is caused by destruction of the butadiene. Figure 5.179. Thermal-oxidative degradation of ABS progresses similarly to photo-oxidative degradation (see Section 5.2.2.2.2.2).

Failure of LDPE by photo-oxidation consists of discoloration, embrittlement, extensive circumferential microcracking, and brittle fracture. 

These are called Narish type I and II (NI and Nil) degradations. Develop the reaction mechanism. Show how an ester group could be formed. You can see that the photo-oxidation embrittles the polymer and makes the polymer hydrophilic also. 

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