Oxidative degradation Photooxidation

Finishing. AH acetal resins contain various stabilizers introduced by the suppHer in a finishing extmsion (compounding) step. The particular stabilizers used and the exact method of their incorporation are generally not revealed. Thermal oxidative and photooxidative stabilizers have already been mentioned. These must be carefully chosen and tested so that they do not aggravate more degradation (eg, by acidolysis) than they mitigate. 

Water, methanol, and n-hexane do not influence the photooxidation of PVC (43), but the photodegradation is accelerated by ferric chloride (70,71) and certain other compounds containing iron (70,71,72). Purification of the polymer might be expected to enhance its photostability by removing deleterious impurities such as iron compounds that are derived from metal equipment. This type of result was obtained in one recent study (58) but not in others (30,59). In contrast, the photo-oxidative degradation of PVC should be enhanced by admixture of the polymer with materials that are unusually susceptible to photooxidation themselves. Such behavior has been observed for impact-modified PVC containing polybutadiene-based polyblends (69,73). 

Oxidation is, of course, the dominant reaction. For example, vaporized trifluralin ( a, a, < -trifluoro-2,6-dinitro-ll,ll-dipropyl-p-toluidine) was demethylated (Figure 7) (26), and its atmospheric half-life was found to be 8 minutes (27). However, the reaction occurred to a small extent even at night, and oxidation by ozone was implicated. In fact, there is evidence (28) that parathion photooxidation actually required the presence of ozone or other highly reactive oxidants. Degradation not requiring external reagents also may proceed rapidly trifluralin was cyclized to a substituted benzimidazole (11, 26), and dieldrin again formed photodieldrin (29). 

During weathering, phenolic antioxidants are photooxidized into hydroperoxycy-clohexadienones, such as 59 (Pospisil, 1993 Pospisil, 1980). The presence of peroxidic moieties in 57 and 59 renders them thermolabile at temperatures exceeding 100 °C and photolysable under solar UV radiation. Both processes account for homolysis of the peroxidic moieties. As a result, the oxidative degradation of the polymeric matrix is accelerated by formed free-radical fragments (tests were performed with atactic polypropylene and acrylonitrile-butadiene-styrene terpolymer (ABS) (PospiSil, 1981 PospiSil, 1980). Low-molecular-weight products of homolysis, such as 60 to 63 are formed in low amounts. 

The two major catalytic applications of OMS and OL materials involve oxidations and photooxidations. Some amorphous manganese oxide (AMO) systems have been prepared that are outstanding photooxidation catalysts for degradation of CHsBr and conversion of isopropanol to acetone.76 Catalytic data for several OMS and OL systems are summarized in Table VI. 

During oxidative degradation, a concentration gradient always develops at a film surface. Inasmuch as the depth profile depends on permeabilities and reaction rates, the effect is more noticeable in photooxidations than in thermal oxidations. An unusually marked skin effect observed in photooxidized polypropylene has been ascribed (14) to the action of chronophores located at or near the surface. 

Volume 14 deals with all aspects of polymer degradation, classified on the basis of the method of initiation for the process. Thus, Chapter 1 covers thermal degradation, Chapter 2 radiolysis initiated by high-energy radiation such as X- and 7-radiation and electrons, Chapter 3 photodegradation arising from exposure of polymers to visible or ultraviolet radiation and Chapter 4 discusses oxidative degradation, oxidation and photooxidation. 

THERMOOXIDATION, PHOTOOXIDATION, OXIDATIVE DEGRADATION, AND PRODUCT CRUMBLING AND FAILURE... 

Photooxidation of plastics and wood-plastic composites (WPCs) was described in principal detail in the preceding Chapter 15. It was emphasized that photooxidation acts in a synergism with thermooxidation of the materials, speeding up an oxidative degradation of WPC products, particularly being exposed to direct sunlight. 

PROBABLE FATE photolysis, direct photolysis is improbable, indirect photolysis is too slow to be important, aqueous photolytic half-life 100 yrs oxidation could occur, but could probably not compete with degradation, photooxidation half-life in air 2.5 days hydrolysis too slow to be important, half-life is greater than 100 days volatilization not expected to be a likely transport process, will volatilize under windy conditions or from shallow waters sorption sorption onto particulates and complexation with organic substances are dominant transport processes, expected to adsorb if released to soil biological processes bioaccumulated and metabo-... 

PROBABLE FATE photolysis insufficient data, but photolysis may be very important, atmospheric and aqueous photolytic half-lives 21 hrs-2.6 days, in the unadsorbed state, it will degrade by photolysis with a half-life of a few days to a week oxidation chlorine and/or ozone in sufficient quantities may oxidize fluoranthene, photooxidation half-life in air 2.02-20.2 hrs hydrolysis not an important process volatilization not an important transport process sorption adsorption onto suspended solids and sediments is probably the dominant transport process, when released to water, it will quickly adsorb to sediment and particulate matter in the water... 

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