Thermal oxidation polyolefins

Mechanisms involved in the photolysis of thermally oxidized polyolefins during processing... 

At present, high-temperature stabilization of polyolefins is still misunderstood besides, this problem presents serious difficulties. Stabilization of thermal oxidation and photoinduced destruction with the use of stabilizers in this case is inefficient, since at high temperatures these stabilizers are easily evaporated out of the polyolefin melt and decomposed with the formation of radicals capable of initiating additional kinetic chains of destruction. 

Hindered phenols and dialkylhydroxylamines protect plastics against thermal oxidation. Alkylated benzo[b]furane-2(3H)-ones was introduced very recently as a processing stabilizer for polyolefins. 

Extensive studies of polyolefin thermal oxidation at elevated temperatures (above the melt) have been reported, but extrapolation of these data to lower temperatures (below the melt) have led to over optimistic estimates of the life-time of these polyolefins.In order to obtain more realistic values for polyolefin lifetimes, samples were tested at temperatures from llO C to 40°C at 10 C Intervals. Small, Individual static air ovens were used to Isolate Individual types of samples and prevent cross contamination. 

The content of aldehyde and ketone groups reaches 42% during the thermal oxidation of polypropylene, and 35—44% in polyethylene [5, 7]. This content depends on the conditions of the oxidation, particularly the temperature. The quantitative distribution of various functional groups formed during the oxidation of polyolefines is shown in Table 2. 

The origin of chemiluminescence in polyolefins has been profoundly analyzed and it has contributed to the better understanding of their complex mechanism of thermooxidation The thermal oxidation of polyethylenes with different manufacturing histories has been compared, which allowed to establish a relationship between CL and some structural characteristics of the polymers. Modification of their stability in the presence of antioxidants, or other additives such as the activity of nano- and micron particles of pigments has been evaluated. 

A pioneering and comprehensive study of the effect of copper on the thermal oxidative degradation of polyolefins has been made by a Bell Laboratories research group (1-6). They found that surface reactions at the interface between metal and polymer are important factors in many applications including metal-polymer composites, polyolefin-insulated... 

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. 

Monohydric phenols, the most applied group of phenolic antioxidants, are changed via phenoxyls into more types of products. The C-C coupling reactions leading to the antioxidation-efficient multinuclear phenols play a favourable role. The formation of quinone methionid compounds which stain polyolefins cannot be avoided in the transformation process. However, these compounds exhibit a retardation effect in thermal oxidation and are able to quench singlet oxygen. The least favourable properties have alkylperoxycyclohexadienones and dioxycyclohexa-dienones, which initiate both the thermal and photochemical oxidation. Products of their subsequent transformations are either inactive or have a weak retardation effect. 

In the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which through a complex series of reactions result in polymer degradation. Oxidative degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet radiation (photooxidation). Unsaturated polyolefins are particularly susceptible to attack by oxygen or ozone (Equation 9.6). 

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 most widely used antioxidants are sterically hindered phenols and bisphe-nols other additives are combined with phenols mostly in synergistic mixtures. The most recommended structures for stabilization of polyolefins against thermal oxidation and degradation are listed in Table 12.1. 

Shibryaeva L.S.,Popov A. A., Zaikov G.E. (2006). Thermal Oxidation of Polymer Blends. Ch.2"Structure effects in thermal oxidation of polyolefines".Leiden, Boston VSP P.35-55. ISBN-13 978-90-6764-451-8 ISBN-10 90-6764-451-X. 

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. 

The mechanism developed originally by Holland and Gee [1] to explain the thermal oxidation of rubbers and polyolefins has been successfully applied to the same situation in various polymers. A generalised form of the reaction sequence may be set out as follows ... 

Polyolefins In addition to stabilization of polyolefins against thermal oxidation to reduce the sensitivity to light, stabilization against exposure to light is required for articles to be used outdoors as well as those intended for indoor use [93]. Light stabilizers include UV absorbers of the benzotriazole and the benzophenone types (except for thin sections), HALS, and nickel-containing stabilizers. The latter are used for thin sections such as tapes and films and for surface protection. The type of stabilizer is dictated by the type of polyolefin, its thickness, application, and desired lifetime of the article [20]. 

Polymers may be attacked by molecular oxygen, ozone, or by indigenous free radicals in the polymer. Thermal-oxidative degradation of polyolefins in air is autocatalytic, i.e., the rate is slow at first but gradually accelerates to a constant value. According to the three-step mechanism outlined below, the RO2 peroxy radicals formed (Step 1) are sufficiently reactive to attack some primary CH bonds of the chain R H (Step 2). The peroxy radical RO2 is thus reformed (Step 3) and can attack another CH bond. This chain reaction continues until termination occurs (Step 4) [1-11]. 

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