Polyolefin oxidation inhibition

Hitachi Cable Ltd. (35) has claimed that dehydrogenation catalysts, exemplified by chromium oxide—zinc oxide, iron oxide, zinc oxide, and aluminum oxide—manganese oxide inhibit drip and reduce flammability of a polyolefin mainly flame retarded with ATH or magnesium hydroxide. Proprietary grades of ATH and Mg(OH)2 are on the market which contain small amounts of other metal oxides to increase char, possibly by this mechanism. 

The solution oxidation technique allows the study of polyolefin autoxi-dation under conditions where the temperature, concentration of reactants, and rates of radical initiation can be controlled. The results should be considered as a useful prelude to any fundamental understanding of the autoxidation processes which occur in neat polymers where the effects of very high viscosity, partial crystallinity, and oxygen diffusion rates are included. The objective of our work was to determine the kinetics and stoichiometry of the inhibited autoxidation of polypropylene in solution. A relatively detailed study of the oxidation of polypropylene inhibited by 2,6-di-terf-butyl-4-methylphenol [butylated hydroxytoluene (BHT)] has been made for comparison with data obtained in polypropylene oxidations inhibited by a variety of other stabilizers which include commercial polyfunctional antioxidants. Singly hindered phenols appeared to be superior in the inhibited-solution oxidation of polypropylene, and the application of this finding to stabilization technology was investigated briefly. 

These results demonstrate some interesting chemical principles of the use of acrylic adhesives. They stick to a broad range of substrates, with some notable exceptions. One of these is galvanized steel, a chemically active substrate which can interact with the adhesive and inhibit cure. Another is Noryl , a blend of polystyrene and polyphenylene oxide. It contains phenol groups that are known polymerization inhibitors. Highly non-polar substrates such as polyolefins and silicones are difficult to bond with any technology, but as we shall see, the initiator can play a big role in acrylic adhesion to polyolefins. 

VA Roginskii. Oxidation of polyolefines inhibited by sterically hindered phenols, Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1983. 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

L.Yu. Smoliak, N.R. Prokopchuck, Y.P. Losev, V.P. Prokopovich, and I.A. Klimotsova, The role of sulphur containing groups in inhibition of oxidative degradation of polyolefins by HALS, Polym. Blends Polym. Compos. 2002, 1, 151-159. 

Transformation mechanisms of the most important classes of phenolic antioxidants are given for the conditions of inhibited oxidation. Properties and importance of the main types of formed products are reviewed with respect to the long-term properties of polyolefins. 

This review analyzes the data on most important antioxidants for polyolefins, i. e. the data on phenols. To learn consistently the whole mechanism of polyolefin stabilization, it is not possible to consider only the facts about the kinetics of the process and relationships between the chemical structure of stabilizers and their observed efficiency. It is necessary to understand the mechanism of stabilizer action on the basis of knowledge of transformations which occur in the inhibited oxidation and of properties of resulting products. The product analyses were obtained above all from the study of models and from independent syntheses. The confirmation of results under real conditions cannot be always carried out consistently because of low concentrations, difficult isolation, and reactivity of transformation products. It is also difficult to analyze such reaction mixtures directly in a polymer6 ... 

Earlier data on transformations of phenolic antioxidante have been compiled in reviews3,4> 7 This article deals with the transformations of typical and technically important phenolic antioxidants and includes the literature data available until end of April, 1979. The emphasis is laid on transformations occurring under the conditions which simulate an inhibited thermal oxidation and photochemical processes during ageing of polyolefins. The chemical and photochemical properties of main products and available data concerning the subsequent transformations and the effects of transformation products on the oxidation of hydrocarbon systems are included. 

The presence of the above-mentioned metal ions increases the decomposition rate of hydroperoxides and the overall oxidation rate in the autoxidation of a hydrocarbon to such an extent that even in the presence of antioxidants, the induction period of oxygen uptake is drastically shortened. In such a case, sterically hindered phenols or aromatic amines even at rather high concentrations, do not retard the oxidation rate satisfactorily. A much more efficient inhibition is then achieved hy using metal deactivators, together with antioxidants. Metal deactivators are also known as copper inhihitors, because, in practice, the copper-catalyzed oxidation of polyolefins is by far of greatest importance. This is due to the fact that polyolefins are the preferred insulation material for communication wire and power cables, which generally contain copper conductors. 

Though there are metals other than copper (such as iron, manganese and cobalt) that can accelerate thermal oxidation of polyolefins and related polymers such as EPDM, in practice, however, the inhibition of copper-catalyzed degradation of polyolefins is of paramount importance because of the steadily increasing use of polyolefin insulation over copper conductors. Among polyolefins, polyethylene is still the most common primary insulation material for wire and cable. In the United States, high-density polyethylene and ethylenepropylene copolymers are used in substantial amounts for communications wire insulation. 

The sterically inhibited amines (HALS) were the most important new development in the field of photoprotectants of the past 10 years. The low molecular types have now been supplemented by highly effective polymeric types. They are particularly effective with very oxidation-sensitive PPs and other polyolefins - even in products with minimum wall thicknesses (foils, fibers). The polymeric HALS types have proven highly resistant to extraction and migration. 

As it is seen from Figure 10(b), tlie formation of stabihzed radicals occurs with pronounced induction period which is related to antioxidant properties of MWNCT. Such a type of kinetic dependence is coincided with an oxygen uptake kinetics observed during inhibited polyolefines thermal oxidation. Moreover, no EPR signals were observed in the samples of the PP/MCWNT samples heated at 350°C in inert Ar. 

In the condensation of alkyl derivatives of hydroquinone with di-haloalkanes, compounds recommended for the inhibition of oxidation processes in polyolefins are formed [201] ... 

Of the compounds containing a sulfur atom in the molecule, mer-captans possess comparatively high inhibiting effectiveness. Hence, the use of mercaptobenzimidazole [50, 51], mercaptobenzothiazole, (S-naphthylthiol, and certain aliphatic mercaptans, for example, dodecyl mercaptan [41], is recommended for polyolefins. However, to guarantee the desired stability to oxidation under the conditions of reprocessing of the polymer at temperatures of 200-250°C, a comparatively high concentration of such stabilizers (of the order of 1% by weight) is required,... 

There are indications [41 ] that a number of organic disulfides can be used as stabilizers of polyolefins. Many of them (for example, 1-dodecylsulfide [41], or 2,2 -dithio-bis-benzimidazole [56]) are more effective than the corresponding mercaptans. As a result of this, we might assume that the mechanism of the inhibiting action of disulfides reduces to their dissociation into two thiyl radicals, followed by termination of the oxidation chain according to reaction (11). 

The macroradical formed to reaction (16) interacts with oxygen, leading to a normal chain reaction and the formation of hydroperoxides. Accordii to the data of [85], the quantum yield of carbonyl groups in the photolysis of polyethylene is no greater than 0.1. This means that the kinetic chains in photooxidation are not very long. In view of this, termination of the chains by means of antioxidants is hindered in practice, since in the case of short chains and a high rate of initiation, effective inhibition is impossible. The formation of unsaturated compoimds during photolysis [reactions (15) and (16)] facilitates further oxidation of the polyolefin. 

The principles obtained confirm the complex mechanism of the process of oxidation of polyformaldehyde. Although the introduction of inhibitors that terminate chain oxidation processes is sufficient to decelerate (inhibit) decomposition processes for a number of polymers (polyolefins, polyamides, etc.), for polyformaldehyde, additives-are needed which, on the one hand, might inhibit chain oxidation processes, and, on the other, would prevent acceleration of the decomposition of the polymer, by tying up the monomeric formaldehyde. 

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