Polyolefin thermal destruction

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... 

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

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. 

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... 

Analogous cycles are observed in the polyacetal-steel and polyolefin-steel friction pairs. Physical-chemical processes can exert a favorable effect on the metal-polymer friction pair as well. For instance, metal-containing compounds of copper stearate and oleate types are formed in the contact zone as a result of tribochemical reactions when CFD copolymer rubs against copper. These compounds retard thermally destruction processes in CFD, serving as wear inhibitors (WI) for this system. 

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. 

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. 

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