Kinetics photo-oxidative degradation

The photo-oxidative degradation of polypropylene and stabilization by hindered amines has been reviewed. A study has appeared of the effect of P-carotene on the photoreactivity of anthracene in hexane solution and a kinetic scheme has been proposed to account for the photochemical and photophysical processes that occur on irradiation at 365 nm. Quenching rate constants have been determined between /S-carotene and singlet oxygen. Some characteristics have been communicated of the sensitized photo-oxidation of abietic acid contained in a vinyl butyl ether-butyl methacrylate-methacrylic acid copolymer. At 400 nm... 

The kinetic processes involved in photo-oxidative degradation are complex and their mathematical expression in still far from accurate and consequently cannot yet be applied for predicting long-term properties. 

Degradative processes have long been known to be promoted by the produets of solvent degradation. Tetrahydrofuran is oxidized to form a peroxide whieh then dissoeiates to form two radicals initiating a chain of photo-oxidation reactions. Figure 12.1.35 shows the kinetics of hydroperoxide formation. Similar observations, but in polymer system, were made in xylene by direct determination of the radieals formed using ESR. An inereased concentration in trace quantities of xylene eonlributed to the formation of n-oetane radieals... 

It was shown that the kinetics of photo-oxidation of polyethylene is characterized by the superposition of two phenomena. The first corresponds to an exponential increase in the concentration of the carbonyl groups with time and is observed when the kinetics are controlled by the diffusion of oxygen. The second one is not controlled by diffusion, but corresponds to a linear increase of the carbonyl concentration with time and takes place in degraded samples. This is explained in terms of chain rupture in the amorphous regions of the polymer, allowing free access of oxygen [567]. 

Clearly, kinetic studies alone are silent as to photo-oxidations are surface processes or solution processes. Some studies have sought chemical evidence and inferences to ascertain whether or not the oxidation is a surface process. A considerable fraction of pollutant degradation may occur by -OH radical desorption from the photocatalyst surface [40-43]. By contrast, recent pulse radiolysis studies [20] and experiments on photocatalytic degradation of tenaciously adsorbed compounds on inert materials [44] strongly support that major oxidants are adsorbed -OH radicals. Free hydroxy radicals probably play a minor role, if any. 

The equations are especially valid for thermo-oxidative degradation, as in photo-oxidation the role of hydroperoxides is less important, their curve of formation showing a maximum an improvement of the theory for other situations is also required. For the kinetics of hydroperoxide decomposition in a general case no stationary state for radical concentration is postulated. The induction period tends to a finite value when the initial ROOH concentration tends towards zero. The rate of hydroperoxide decomposition in the PE/PP blends is an increased function of the PP content. 

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