Prooxidants polymers

Where there is a danger of contamination of a hydrocarbon polymer with such ions it is common practice to use a chelating agent which forms a complex with the metal. It is, however, important to stress that a chelating agent which effectively slows down oxidation initiated by one metal ion may have a prooxidant effect with another metal ion. Table 7.5 summarises some work by... 

Another explanation was the following. The organomontmorillonite used was a natural montmorillonite that contained iron. Chemical analysis of the clay confirmed the presence of a low amount of iron. It was recalled that iron and, in more general terms, metals are likely to induce the photochemical degradation of polymers. Iron at low concentration had a prooxidant effect that was due to the metal ion of iron that can initiate the oxidation of the polymer by the well-known redox reactions with hydroperoxides [93]. It was concluded that the transition metal ions, such as Fe, displayed a strong catalytic effect by redox catalysis of hydroperoxide decomposition, which was probably the most usual mechanism of filler accelerating effect on polymer oxidation. A characteristic of such catalytic effect was that it did not influence the steady-state oxidation rate, but it shortened the induction time. 

Polyethylene and polypropylene blended with iron carboxylate complexes, for example, acetylacetonate (FeAcAc) and stearates (FeSt), and irradiated by UV light under accelerated aging conditions were shown to act as effective phtoactivators giving rise to rapid photoxidation as shown from the rapid rate of carbonyl formation without any induction period (see Fig. 16.4a for FeAcAc in HDPE) and with a reduction in molar mass (see Fig. 16.2a for FeSt in LDPE). However, these complexes have been shown to cause considerable oxidation to both PE and PP during processing reflected in a sharp increase in the polymer s melt flow index (reflecting chain scission and drop in molar mass) (Fig 16.4b) and act, therefore, as thermal prooxidants and cannot be used without the use of additional antioxidants in the system [2,3,17-19,48,49]. 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Prooxidant effects were observed at higher antioxidant concentrations and at higher temperatures. Reversal of the direction of the isotope effects observed under these conditions showed that initiation by direct reaction of the antioxidant with oxygen is an important initiation reaction. Peroxide decomposition is quite slow at 90 °C and begins to contribute significantly to initiation only at the start of a second stage of more rapid, but still retarded, autoxidation. We have suggested (4) that some oxidation product of polymer or antioxidant may induce hydroperoxide decomposition. 

The formation of polymers leads to an increase in viscosity. The various lipids that can leach into the frying oil change the properties and the performance of the frying oil. Colored lipids solubilized in the oil contribute to the darkening. Phospholipids are emulsifiers. Traces of liposoluble metal compounds may act as prooxidants. Liposoluble vitamins and phenolic compounds are antioxidants. Volatile compounds (e.g., from fish or onions) contribute to off-flavors. 

Four main types of polymer are currently accepted as being environmentally degradable. They are the photolytic polymers, peroxidisable polymers, photo-biodegradable polymers and hydro-biodegradable polymers. Commercial products may be composite materials in which hydrolysable and peroxidisable polymers are combined (e.g. starch-polyethylene composites containing prooxidants). The application, advantages and limitations of each group will be briefly discussed. 

Transition metal prooxidants cause problems during both the manufacture and use of plastics products. Firstly they catalyse rheological changes in the polymer during processing and reduce shelf-life before use. Secondly it is difficult to control the induction time to photooxidation. It will be seen below that control of peroxidation is essential to the application of degradable plastics in agriculture. 

Polyolefins such as PE and polypropylene (PP) are usually not accessible to direct microbial attack. For such polymers, biological degradability is achieved hy the addition of starch, prooxidant additives or photosensitive components. Starch, a natural polymer, can he degraded by microorganisms which enhances the defragmentation of the polyolefins (if the starch is accessible to the microbes). The additives increase the initial reduction of polymer chain length by chemical... 

Figure 3.2 shows two examples of CL analyses on aged polymers. Figure 3.2a presents CL recorded from thermally aged samples of starch-prooxidant filled LDPE compared with pure LDPE and LDPE with only starch [10]. The large content of unsaturation (oxidizable sites in the SBS-phase), which is part of the prooxidant formulation, is the explaination for the rapid initial oxidation in the filled LDPE, yielding hydroperoxides, compared with pure LDPE. 

Stage B, a rapid catalysed photooxidation to embrittlement (fragmentation) of the polymer which continues to oxidize in Stage C, due to the presence of prooxidant species which catalyse thermal oxidation and lead to rapid incorporation of the plastic into the soil structure with ultimate conversion to carbon dioxide and water by the combined effects of abiotic and biotic oxidation. 

They are converted to polymer-bound metal carboxylates in photooxidizing polyolefins which act as highly effective photo- and thermal prooxidants [9, 13, 24], while they themselves all behave initially as thermal and photoantioxidants. The zinc and nickel dithiocarbamates (II. M = Zn, Ni, n = 2) by... 

The transition metal dithiocarbamates contain both a peroxide decomposer function and a latent prooxidant function. The latter is liberated by photolysis at the end of the induction period and in the case of the photoactive transition metal ions (notably iron), very rapid thermal and photooxidation of the polymer ensues. 

According to the actual trends in additive use in polymers, a system composed of starch and a prooxidant such as manganese stearate with a styrene-butadiene copolymer as compatibilizing agent was developed [70, 73]. 

Oxodegradable polymers Polyethylene, prooxidants Piaster, TDPA, (EPI)... 

Typical prooxidant transition metal compounds (e.g. iron, cobalt or manganese stearates) are used commercially to induce peroxidation in degradable plastics. However, such prooxidants alone have no practical utility in commercial products unless the prooxidants are deactivated during polymer fabrication, since oxidative degradation begins during... 

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