Degradation of Unstabilized Fiber

The overall oxidation scheme of an unstabilized polypropylene fiber has been the subject of much investigation [112]. Polypropylene, as produced, contains trace impurities which, in the presence of heat or light, permits a polypropylene radical (PP) to be formed. This radical will react with oxygen as shown in the reaction represented by Equation 3.4. 

The PPO2 radical reacts inter- or intramolecularly with polypropylene to form PP (Equation 3.5), which will react with oxygen as shown in the reaction represented by Equation 3.4, so that the oxidation reaction will repeat itself. The hydroperoxide group formed in the reaction shown by Equation 3.5 reacts with UV light or heat to yield a tertiary alkoxy radical and a hydroxy radical (Equation 3.6). The tertiary alkoxide may react as shown in the reactions represented by Equation 3.7a or Equation 3.7b and the hydroxy radical can also react with polypropylene as shown in the reaction represented by Equation 3.8, in order that, in each case, the product is another polypropylene radical that can react with oxygen as presented in Equation 3.4. Equation 3.7b leads to chain scission and reduction in the molecular weight of the polymer 

The increase in oxygen concentration accompanies the deterioration of fiber physical properties, but the deterioration does not necessarily relate to the increase directly. Factors such as fiber orientation and the conditions, primarily temperature at which the fiber is drawn, can affect the rate of deterioration [113], The reduction of elongation that accompanies exposure to UV light is less for a highly oriented fiber because the photooxidation reaction is initially limited to the fiber surface. The photooxidation reaction is accompanied by the formation of cracks on the thin surface [114]. Cold-drawn fiber reacts throughout, degrades faster, but does not form surface cracks. Fiber wettability increases with increased UV exposure [112], which is additional evidence of the formation of surface oxygen bonds. 

To avoid or at least reduce the rate of deterioration of polypropylene fiber, a variety of stabilizers are added. These are sometimes categorized [115] as  

Radical terminators that react with active polymer radicals to form harmless products 

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