Polypropylene photooxidative degradation

Bauer I, Habicher W D, Korner S and Al-Malaika S (1997) Antioxidant interaction between organic phosphites and hindered amine light stabilizers effect during photooxidation of polypropylene, Folym Degrad Stab 55>217-224. 

Girois, S. Delprat, P. Audouin, L. Verdu, J. Oxidation thickness profiles during the photooxidation of non-photostabilized polypropylene. Polym. Degrad. Stab. 1997, 56, 169-177. 

J.-L. Gardette, C. Sinturel, and J. Lemaire, Photooxidation of fire retarded polypropylene, Polym. Degrad. Stab. 1999, 64, 411—417. 

Polyethylene (PE) is inherently less sensitive to oxidative attack than PP, but stabilization of PE is also mandatory for outdoor use. The stability varies with the type of polyethylene and manufacturing process. Linear low-density polyethylene (LLDPE) (1-octene comonomer) is significantly less sensitive to photooxidation than low-density polyethylene (LDPE) with comparable density and molecular weight [20, 21]. Generally, LDPE is less susceptible to photooxidation than high-density polyethylene (HDPE). The most fundamental difference between polyethylene homopolymers and polypropylene is the behavior of hydroperoxides toward photolysis. On photooxidation, hydroperoxides accumulate in PP, but decrease rapidly on UV exposure of PE. In copolymers of polyethylene with vinyl acetate, the stabihty depends on the content of vinyl acetate. The higher the content, the more the copolymers act like polyvinyl acetate, which is more susceptible to photooxidative degradation than polyethylene. 

Balaban L, Majer J, Vesely K. Photooxidative degradation of polypropylene. J Polym Sci Part C Polym Symp 1969 22 1059-1071. 

Carlson, D.J., Wiles, D.M. The photooxidative degradation of polypropylene. Part 1. Photooxidation and photoinitiation processes. J. Macromol. Sci. C-14, 65-106 (1979) Salamone, J.C. (ed.) Concise pol5nneric materials encyclopedia. CRC Press, Boca Raton (1999)... 

Phifippart, J.-L., Sinturel, C., Amaud, R., Gardette, J.-L. Influence of the exposure parameters on the mechanism of photooxidation of polypropylene. Polym. Degrad. Stab. 64, 213-225 (1999)... 

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. 

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. 

Low molecular weight dicarboxylic acids, keto acids and hydroxy acids have been shown to form as photooxidation products of polyethylene and polypropylene. These are almost certainly formed by intramolecular reactions of alkylperoxyl and peracyl radicals shown typically in Scheme 3.7. Back-biting along the aliphatic chain gives rise to unstable hydroperoxides and the elimination of small molecular fragments. It will be seen in Chapter 5 that these low molar mass oxidation products, which are already present in the environment from natural sources, are the first point of microbial attack in the surface of environmentally degraded polymers, leading to oxidation initiated bioerosion (Chapter 5). 

Yakimets I, Lai D, Guigon M. Effect of photooxidation cracks on behavior of thick polypropylene samples. Polym Degrad Stab 2004 86 59-67. 

The use of polypropylene (PP) outdoors is limited by its weatherability. The main cause of degradation is photooxidation, promoted by ultraviolet (UV) irradiation, and in hot sunny dimates components made from PP may fail within a few days outdoor service unless they are suitably protected. Failure is usually by brittle fracture resulting from surface embrittlement. Chemical stabilizers added to the polymer can increase the lifetime very considerably and should be used even if a component is to be exposed to sunlight only occasionally. Much of the research into polymer weathering and stabilization has been conducted on PP because of its importance and its sensitivity to outdoor conditions. 

In the absence of light, most polymers are stable for very long periods at ambient temperatures. However, above room temperature many polymers start to degrade in an air atmosphere even without the influence of light. For example, a number of polymers show a deterioration of mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g., polyethylene, polypropylene, poly(oxy methylene), and poly(ethylene sulfide)). Measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It was concluded that thermal oxidation, like photooxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen (Van Krevelen and Nijenhuis 2009) ... 

It has also been suggested that peroxy acid groups and perester groups [2315, 2316] formed during the photo-oxidation of isotactic polypropylene may participate in the photo-oxidative degradation of the polymer. Polypropylene films of low atactic content were found to undergo faster photooxidation than films of high atactic content, irradiated under identical conditions [1163]. 

It is well known (11) that transition metal ions act as senatizers promoting the photooxidation of polyolefins. Japanese authors (38) have recently found that the degradation of polypropylene induced by U V light (3650, 2537, 1800 A) depends on the oxygen concentration and on the residues of the polymerization catalyst. They concluded that degradation in an oxygen atmosphere is a photooxidative process sensitized by metal residues. 

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