Oxidation of polymers

In the depolymeri2ed scrap mbber (DSR) experimental process, ground scrap mbber tines produce a carbon black dispersion in ok (35). Initially, aromatic oks are blended with the tine cmmb, and the mixture is heated at 250—275°C in an autoclave for 12—24 h. The ok acts as a heat-transfer medium and swelling agent, and the heat and ok cause the mbber to depolymeri2e. As more DSR is produced and mbber is added, less aromatic ok is needed, and eventually virtually 100% of the ok is replaced by DSR. The DSR reduces thermal oxidation of polymers and increases the tack of uncured mbber (36,37). Depolymeri2ed scrap mbber has a heat value of 40 MJ/kg (17,200 Btu/lb) and is blended with No. 2 fuel ok as fuel extender (38). 

Metal deactivators (MD) act, primarily, by retarding metal-catalyzed oxidation of polymers they are, therefore, important under conditions where polymers are in contact with metals, e.g., wires and power cables. Metal deactivators are normally polyfunctional metal chelating compounds (e.g.. Table la, AO 19-22) that can chelate with metals and decrease their catalytic activity [21]. 

The oxidation of polymers is most commonly depicted in terms of the kinetic scheme developed by BoUand [14]. The scheme is summarized in Figure 15.1. The key to the process is the initial formation of a free-radical species. At high temperatures and at large shear forces, it is likely that free-radical formation takes place by cleavage of C-C and C-H bonds. 

Retard efficiently oxidation of polymers catalysed by metal impurities. Function by chelation. Effective metal deactivators are complexing agents which have the ability to co-ordinate the vacant orbitals of transition metal ions to their maximum co-ordination number and thus inhibit co-ordination of hydroperoxides to metal ions. Main use of stabilisation against metal-catalysed oxidation is in wire and cable applications where hydrocarbon materials are in contact with metallic compounds, e.g. copper. 

Atomic oxygen oxidation of polymers has been reported by a few authors (46,50). Experiments were limited to the measurements of weight-loss data and changing of the wetteability (46-48), and only two papers were devoted to the study mechanism of atomic oxygen oxidation of polydienes (49,50). 

Scheme 2 Bolland-Gee scheme of free radical oxidation of polymer pH. P denotes macromolecular chain, InH is chain-breaking inhibitor, D peroxide decomposer and parameters above arrows are the corresponding rate constants.

A direct oxidation of polymer additive, which may occasionally give a much stronger signal than the oxidation of polymer itself. This is very important as it may lead to an erroneous relation between the rate of polymer oxidation and chemiluminescence intensity. 

The effect of molar mass and stereoregularity on the thermo-oxidation of polymers and on the chemiluminescence-time patterns has been investigated only rarely [44-46]. 

The chemiluminescence intensity accompanying the oxidation of polymers is usually dependent on the concentration of oxygen in the surrounding atmosphere (Figures 21 and 22). This is confirmed by restricting the diffusion... 

If we presume that oxidation of polymer PH takes place according to the Scheme 2 of Bolland-Gee mechanism, in which reactions 2, 3, 5, 6, 7, 8, 9 and 10 predominate, involving initiation, propagation and termination of the free radical process. The production of primary radicals by the reaction 1 is governed rapidly by decomposition of hydroperoxides. From the shape of kinetic runs it may be assumed that decomposition of hydroperoxides has the character of a bimole-cular reaction (reaction 5) even in a very short time after the start of experiment. 

G. Geuskens, D. Baeyens Volant, G. Delaunois, Q. Lu Vinh, W. Piret, and C. David, Photo oxidation of Polymers II. The Sensitized Decomposition of Hydroperoxides as the Main Path for Initiation of the Photo oxidation of Polystyrene Irradiated at 253.7 nm, Eur. Polym. J., 14, 299 303 (1978). 

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 reactions of intramolecular isomerization occur and are important in the oxidation of natural and synthetic rubbers. The peroxyl radical addition to the double bond occurs very rapidly. For example, the peroxyl radical adds to the double bond of 2-methylpropene by 25 times more rapidly than abstraction of hydrogen atom from this hydrocarbon (see Chapter 4). Therefore, the oxidation of polymers having double bonds proceeds as a chain process with parallel reactions of P02 with double and C—H bonds including the intramolecular isomerization of the type [12] ... 

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. 

At the same time, quinones do not practically retard oxidation of hydrocarbons, since alkyl radicals react very rapidly with dioxygen (see Chapter 4) to give alkylperoxyl radicals, which scarcely react with quinones. Quinones exhibit their inhibiting properties as alkyl radical acceptors only in the oxidation of polymers (see Chapter 19). However, quinones were found to decelerate the oxidation of alcohols very efficiently and for long periods by ensuring cyclic chain termination via the following reactions [31-34] ... 

Oxidation of polymer in the presence of dioxygen acceptors is limited by the diffusion of dioxygen into the polymer bulk. The lifetime of polymer does not depend on the acceptor concentration at [acceptor] > [acceptor]min. The lifetime of a polymer sample t depends on p02, A, D, and the thickness of the sample / according to the parabolic equation [14-18]... 

Chemiluminescence is the emission of light in the 300-500 nm region due to exothermic chemical reactions. With a suitable detection system this can be used to monitor the chemical reactions that occur when materials age. This technique is sensitive enough to produce results more rapidly and at lower temperatures. Chemiluminescence has been used to investigate the effect of different additives on the oxidation of polymers as well as to determine oxidation induction times which are more meaningful than those obtained from the DSC method. 

Let us consider the easiest example of oxygen diffusion and reaction in polymer film having the thickness 2/. When oxygen pressure is low (see Chapter 2), the rate of chain oxidation of polymer... 

---