Polyethylene chemiluminescence

Figure 3 shows how the elementary structure of the polymer affects the observed patterns of chemiluminescence response in oxygen at 120°C. As expected, the oxidizability decreases in the order polyisoprene < polybutadiene < polypropylene < polyethylene. However, it is difficult to understand why the maximum light emission is almost 2x lower for polybutadiene than for PP. The oxidation of polybutadiene occurs via secondary peroxyl radicals, while in... 

Figure 4 Chemiluminescence oxidation runs at 140°C in oxygen for films of polypropylene (PP), polyamide 66 (PA 66) and polyethylene (PE) (logarithmic time scale).

Figure 6 (A) Non-isothermal chemiluminescence runs for oxidation of polystyrene (PS), polyethylene terephthalate) (PETP) and polyfmethyl methacrylate) (PMMA), in oxygen, heating rate 2.5°C/min. (B) Non-isothermal chemiluminescence runs for oxidation of polypropylene (PP), polyamide 6 (PA 6), poly(vinyl pyrrolidone) (PVP), polyethylene (PE) and polyamide 66 (PA 66), in oxygen, heating rate 2.5°C/min.

Figure 7 Arrhenius plots for non-isothermal chemiluminescence runs of oxidized polymers, (1) polypropylene, (2) polyethylene, in oxygen, heating rate 2.5°C/min. 

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. 

Keywords Chemiluminescence Imaging chemiluminescence Polyethylene Polypropylene Oxidation PP PE... 

Generally, polyethylene exhibits a double sigmoidal shape on the isothermal chemiluminescence curve. Setnescu et al. interpreted the behavior as oxidation of very active sites (double bonds, branches, etc.) of PE and oxidative crystalline-phase conversion [34]. Zlatkevich suggested that overlaid sigmoids reflect two oxidation processes related to two kinds of peroxides with different stability assumed to exist in the oxidized polyolefins [35]. 

FIGURE 3.2 Plot of the reciprocal of the square root of chemiluminescence intensity versus time obtained at 170°C under nitrogen for free and stabilized low-density polyethylene films with the different phenolic antioxidants (0.1% w/w). 

Phenolic antioxidants are well known for being melt processing stabilizers as well as long-term thermal stabilizers. In the chemiluminescence measurements on the polyethylene films under oxygen (Fig. 3.3), the antioxidant effect of the phenols is clear when these induction times are compared with those of the free additive polyethylene film (0.73 h) (Table 3.2). The results showed that the structure of the phenolic moiety will be a cmcial factor influencing the stabilization performance. 

The thermal activity of a series of nano- and micron-particle grade anatase and rutile titanium dioxide pigments, with various densities of surface treatments, particle size, and surface area, have been determined by chemiluminescence in monomodal metallocene polyethylene [57]. 

TABLE 3.3 Chemiluminescence Under Nitrogen at 170°C of Metallocene Polyethylene Films Containing the Nano and Micron Grade Titania Pigments at 0.5% w/w Concentrations with 0.05% w/w Irganox 1010 Antioxidant Together with Control Unpigmented Samples... 

FIGURE 3.4 First and second chemiluminescence peak curves versus time of metallocene polyethylene films free, stabilized with Irganox 1010 (0.05% w/w), and pigmented with micron- and nanoparticle titanium dioxide (0.5%), determined at 170°C under oxygen. 

The origin of chemiluminescence in polyolefins has been profoundly analyzed and it has contributed to the better understanding of their complex mechanism of thermooxidation The thermal oxidation of polyethylenes with different manufacturing histories has been compared, which allowed to establish a relationship between CL and some structural characteristics of the polymers. Modification of their stability in the presence of antioxidants, or other additives such as the activity of nano- and micron particles of pigments has been evaluated. 

Finally, a few articles have appeared on chemiluminescence of polymers. This technique has been used to detect hydroxy radicals in wood oxidation,y-irradiation effects on polyethylene, oxidation of nitrile-butadiene rubber, rubber under stress,antioxidant efficiencies in polyethylene, reactions of peroxy radicals, stereoregularity in poly(propylene), colour development in epoxy resins and structural changes in thermally aged poly(phenylene sulfide). ... 

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