Unstabilized polypropylene film

Whilst all the polymers with intrinsic chromophores are inherently sensitive to degradation, at least some are regarded as relatively stable materials, able to resist solar exposure for long periods. In contrast, polymers like the polyolefins and the hydrocarbon rubbers have remarkably poor lifetimes when exposed to solar light. Unstabilized polypropylene film, for example, has... 

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

In less than 50 hours, the unstabilized film cracked when flexed 90°. The film containing 0.25% PPP was protected for 152 hours, while the combination of 0.4% DLTDP 4 0.2% PPP failed after 274 hours. The commercial hydroxybenzophenone at 0.5% prevented embrittlement for 580 hours, while at 0.25% this compound protected the film for approximately 300 hours. By adding 0.25% PPP and reducing the amount of hydroxybenzophenone to 0.1%, the film lasted for 525 hours before embrittlement. The two commercial benzotriazole ultraviolet absorbers at 0.25% protected the polypropylene films for 290 hours and 425 hours, respectively. 

Termination. Just as peroxy radicals are key to the propagation sequence, so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (25). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. As a standard, we have assumed a value close to that determined from oxygen absorption (26) and by ESR spectra (27) for oxidized polypropylene films. 

Trubiroha [43] reported an action spectrum for development of carbonyl groups (as measured by the change in absorbance at 5.8 microns) of unstabilized polyethylene and polypropylene films. A measurable effect after 1 MJ/sqm of radiant exposure was obtained only for X = 380 nm between 280 and 380 nm the absorption changed logarithmically with wavelength of irradiation. 

Polypropylene Formulations Unstabilized, Crystalline Polypropylene + the Following Oven Life, Hr for 5-Mil Film Samples ... 

Lacey, D.J., Dudler, V. Chemiluminescence from polypropylene. Part 1 Imaging thermal oxidation of unstabilized films. Polymer Degradation and Stability, 51 (1996), p. 101- 108... 

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