Destruction of polyolefins

Organic phosphites POR(OR )OR where R=Ci—C30 represents aliphatic, cycloaliphatic, or aromatic radical, are also able of inhibiting the thermal destruction of polyolefin [19]. Of light stabilizers, benzo-phenone derivatives have the ability for inhibiting thermal destruction of polyolefins, too. 

Phthalic anhydride also shows the ability to inhibit thermal destruction of polyolefins [21]. Among the organometallic compounds may be quoted organotin compounds R2Sr(OR )2, where R2 means alkyl, aryl, or cycloalkyl OR means alkoxyl, acyl, or R2Sn(CH2COORi)2, where Rj—Ci—Cm means alkyl, allyl, or benzyl Ro represents chloro-, mono-, or triorga-notin mercaptans [22,23]. 

It should be noted that the aforementioned few compounds behave as stabilizers of thermal destruction of polyolefins only at temperatures from 200-250°C. 

Hydroxides of alkali metals are effective as inhibitors of thermal destruction of polyolefins even without the carrier, yet at lower temperatures (Table 4). 

According to the ionization potential and electron-transfer work, alkali metals form the following series Li > Na > K, and their hydroxides are arranged in the sequence KOH > NaOH > LiOH as to their inhibitive efficiency relative to thermal destruction of polyolefins. And the efficiency of alkali metals can be represented by the sequence Na > K > Li. This seems to be due... 

During heating to 150-220° C polyolefins isolate thermooxidative products of macromolecules with pronounced toxic properties, namely organic acids, ethers, unsaturated hydrocarbons, peroxide and carbonyl compounds (formaldehyde and acetaldehyde), carbon oxide and dioxide, etc. A mixture of such products of the thermooxidative destruction of polyolefins may result in acute chronic poisoning on inhalation. 

Thus, the kinetics of the oxidation of the overwhelming majority of pol5miers is described by S-shaped curves, as is the case, for example, in the thermooxidative destruction of polyolefins, polyethylene oxide, polyamides, polycarbonates, polyarylates, epoxide resins, rubbers, etc. 

At the present time there is a comparatively large number of works in the literature studying the mechanism of the photooxidative destruction of polyolefins. Unfortunately, most of them are devoted to high-pressure polyethylene, in view of which it does not seem possible to make any comparison between individual types of this class of polymers. However, the general picture of the photooxidation possesses much in common with the thermal process of oxidation and differs from the latter mainly in the stage of initiation. 

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. 

It has been only found that some antioxidants and light stabilizers show the ability for partial inhibition of thermal destruction of the polyolefins. 

Thermal stabilization of polyolefins has been first demonstrated for low-molecular models-normal structure alkanes [29]. It has been shown that metallic sodium and potassium hydroxide with absorbent birch carbon (ABC) as a carrier are efficient retardants of thermal destruction of n-heptane during a contact time of 12-15 s up to the temperature of 800°C [130]. Olefins and nitrous protoxide, previously reported as inhibitors of the hydrocarbon thermal destruction, are ineffective in this conditions. 

Table 3 Inhibition of Polyolefin Thermal Destruction by KOH with ABC Carrier in Nitrogen Atmosphere...

Hydroxides of alkali metals and alkali metals provide for inhibition of polyolefin thermal destruction following the radical-chain pattern. These substances fail... 

Photochemical destruction of polymers runs together with formation of oxide compounds, low-molecular-weight and structurized products. These results in the generation of cracks on the surface of polyolefin articles, and their mechanical and dielectric properties are noticeably impaired. 

In most polymer applications oxygen is present. Consequently, the stability is less than in a vacuum (Fig. 10.1) or that in an inert gas. Many oxidation studies use films, a few jxm thick, in which there is a uniform oxygen concentration. The oxidation of polyolefins is auto-catalytic, since the main product (hydroperoxides) initiates the reaction (Section 10.2.1). An induction period is followed by a constant rate of oxygen consumption (Fig. 10.3). In the latter, the rates of hydroperoxide destruction and formation are equal. Antioxidants increase the induction period, but they are eventually consumed. The process is exactly the same as in the melt, except that the rate is lower The activation energy for the maximum oxidation rate in polyethylene is 146 kj mol . It appears that all hydrogen atoms on the chain are equally vulnerable to oxidation. 

The adherence to the polymer and elimination of diffusion to the environment are both essential however, they should be mobile enough to move to the surface layer. Novel stabilizers, based on steric hindered amines (HALS) are considered to be very efficient, albeit there is lack of agreement regarding their stabilization mechanism. It is assumed that there exists a mutual reaction of antioxidation and hydroperoxide decomposition combined with the destruction of free radicals. These stabilizers are essential for protecting polyolefins and other polymers. 

The data available in the literature, mostly published in the last decade, pertain chiefly to the aging of high-pressure polyethylene, and to a lesser degree to polyolefins produced on complex catalysts. There are practically no data on the destruction of copolymers. 

This chapter cites data characterizing the processes of oxidation and photodestruction of polyolefins of the series polyethylene—copolymers of ethylene and propylene—polypropylene, and methods of their stabilization against the destructive action of oxygen and light. 

As can be seen from Table 4, the properties of polyolefins change sharply as a result of oxidative destruction. The content of carbonyl, carboxyl groups, and unsaturated compounds increases. The content of insoluble (cross-linked) Structures increases sharply. Structuring is manifested especially strongly in high-pressure polyethylene, least of all in polypropylene. This fact is weighty evidence that oxidation leads to structuring in the case of polyethylene, while it leads mainly to destruction to low-molecular products in the case of polypropylene. 

Fig. 59. Variation of tan 5 of polyolefins during the process of photo -destruction. 1) High-pressure polyethylene 2) low-pressure polyethylene 3) polypropylene 4) copolymer of ethylene and propylene.

Oxidation is the main cause of the rapid destruction of PP during its service and processing. The oxidation of polyolefins proceeds by the radical chain mechanism with degenerate branching on hydroperoxide [15], It is known that, at the initial stage of uninhibited oxidation of polyolefins, the oxygen uptake kinetics is described by the parabolic law ... 

Hydrocarbons oxidize to give a complex mixture of products which include hydroperoxides, alcohols, ketones, acids, esters, etc. (1). Polyolefins similarly can be oxidized by heat, radiation or mechano-initiated processes. The precise identification and quantification of these oxidation products are essential for the complete understanding and control of these destructive reactions. Conventional methods for the identification of oxidation products include iodome-... 

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