Initiation polymer oxidation

Cage Effect in Solid Polymers Migration of Free Valence in Solid Polymers Initiated Polymer Oxidation Diffusion of Dioxygen in Polymer Diffusion Regime of Polymer Oxidation Isomerization of Alkyl and Peroxyl Radicals of Polypropylene... 

The main function of metal deactivators (MD) is to retard efficiently metal-catalyzed oxidation of polymers. Polymer contact with metals occur widely, for example, when certain fillers, reinforcements, and pigments are added to polymers, and, more importantly when polymers, such as polyolefins and PVC, are used as insulation materials for copper wires and power cables (copper is a pro-oxidant since it accelerates the decomposition of hydroperoxides to free radicals, which initiate polymer oxidation). The deactivators are normally poly functional chelating compounds with ligands containing atoms like N, O, S, and P (e.g., see Table 1, AOs 33 and 34) that can chelate with metals and decrease their catalytic activity. Depending on their chemical structures, many metal deactivators also function by other antioxidant mechanisms, e.g., AO 33 contains the hindered phenol moiety and would also function as CB-D antioxidants. 

Some traces of metal and metal ions may initiate the decomposition of hydroperoxides even at room temperature. Traces of metal ions are present in almost all polymers and they may affect considerably the polymer oxidation and its subsequent degradation. The sequence of efficiency of metal ions to enhance degradation depends on its valence state and the type of its ligand, but may be postulated as follows ... 

One important point should be stressed here the efficiency of any stabilizing system depends very much on the removal or depletion of the defect structures in the polymer. An example is shown in Figure 1 where the case 5 mechanism vide supra) of a synergistic mixture is theoretically depicted [7]. The line 2 shows that the effect of the synergistic mixture of two antioxidants on thermo-oxidation stability is negligible, if there occurs a relatively significant initiation of oxidation reaction in a way independent from the route taking place via hydroperoxides. 

CL accompanies many reactions of the liquid-phase oxidation of hydrocarbons, ketones, and other compounds. It was discovered in 1959 for liquid-phase ethylbenzene oxidation [219,220]. This phenomenon was intensively studied in the 1960s and 1970s, providing foundation for several methods of study of oxidation, decay of initiators, and kinetics of antioxidant action [12,17,221], Later this technique was effectively used to study the mechanism of solid polymer oxidation (see Chapter 13). 

The encounter of two free valences as a result only of diffusion of segments of a macromolecule. Since the radius of segmental diffusion is limited in the real time, this mechanism can be efficient at the high initiation rate and intense mobility of the polymer segments. Under the conditions of polymer oxidation, this mechanism is possible at the chain length close to unity. Some examples are given in Table 13.2. 

The important characteristics of polymers oxidation were obtained as a result of the study of their initiated oxidation. In the presence of initiator (I) which generates the chains with the rate v, = /c,[I], the oxidation of polymer PH occurs with the constant rate v. When the macroradical P of the oxidized polymer reacts with dioxygen very rapidly (at [02]... 

The data described above proved that isomerization of alkyl and peroxyl radicals plays a very important role in polymer oxidation. They influence the composition of products of polymer oxidation including the structure of hydroperoxy groups. The competition between reactions of alkyl radical isomerization and addition of dioxygen appeared to be very important for the self-initiation and, hence, autoxidation of PP (see later). 

Like the oxidation of hydrocarbons, the autocatalytic oxidation of polymers is induced by radicals produced by the decomposition of the hydroperoxyl groups. The rate constants of POOH decomposition can be determined from the induction period of polymer-inhibited oxidation, as well as from the kinetics of polymer autoxidation and oxygen uptake. The initial period of polymer oxidation obeys the parabolic equation [12]... 

Compelling evidence suggesting that the breakdown of hydroperoxyl groups is not related to polymer destruction, at least in the initial period of oxidation at temperatures below 400 K, comes from experiments on the initiated oxidation of polymers. It was found that the destruction of polymers develops in parallel with their oxidation from the very onset of the process, but not after a delay related to the accumulation of a sufficient amount of hydroperoxyl groups [129]. These experiments also demonstrated that it is free macroradicals that undergo destruction. Oxidation of polymers gives rise to alkyl, alkoxyl, and peroxyl macroradicals. Which radicals undergo destruction can be decided based on the kinetics of initiated destructive oxidation. 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

The health impairing and toxic elfects of oxidation of lipids are due to loss of vitamins, polyenoic fatty acids, and other nutritionally essential components formation of radicals, hydroperoxides, aldehydes, epoxides, dimers, and polymers and participation of the secondary products in initiation of oxidation of proteins and in the Maillard reaction. Dilferent oxysterols have been shown in vitro and in vivo to have atherogenic, mutagenic, carcinogenic, angiotoxic, and cytotoxic properties, as well as the ability to inhibit cholesterol synthesis (Tai et ah, 1999 Wpsowicz, 2002). 

In the patent literature, there are several reports of the cationic polymerization of tetrahydrofuran (THF) with Nafion-H. In most cases, small amounts of acetic anhydride were added so the initial polymer had a terminal acetate group that could be hydrolyzed to the free hydroxyl. THF has also been homopolymerized936 938 and copolymerized with ethylene oxide and propylene oxide in the presence of Nafion-... 

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. 

It is thus apparent that hydrogen donors AH and hydroperoxide decomposers, such as PR"3, can act synergistically to inhibit radical initiated polymer chain oxidations. 

In initiation of oxidation, the important role may also be played by reactive bonds in a polymer. With polyethylene which is the most simple structurally, such reactive sites may be small concentrations of double bonds or more numerous sites of branching of a main chain to side alkyls. Investigation of the oxidation reaction of different types of polyethylene has. however, revealed that the degree of polyethylene branching from 0 to 20 branches for each 1000 carbon atoms of the main chain does not affect the induction period of oxidation [13]. 

On the other hand, a considerably lower level of C = C bonds brings about a significant shortening of the induction period of oxidation at a concentration of C = C bonds below 0.1/1000 C the induction period is about 100 min while at 0.6C = C/1000C twenty min only at 160 CC. This leads to the suggestion that allyl hydrogen or double bond C = C should be taken seriously as initiators of the first stages of polymer oxidation. 

The formation of oxygen anion radicals and molecules of ozone also should be counted with at the ionization initiation of oxidation [26]. Initiation reaction caused by oxygen anion radicals may play an important role within the polymer bulk while the effect of ozone forming in the surrounding air atmosphere will include only the formation of radicals on the polymer surface. The latent effect of ionization initiation on polymer oxidation which is very distinct may be documented on a relatively fast increase of concentration of carbonyl groups, observed over 1 year after irradiation crosslinking of polyethylene [27]. 

In spite of low concentration of these reactive particles, they may start the initiation of thermo-oxidation. The dependence of the course of the polymer oxidation on the residual amount of these initiating particles may be exemplified... 

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