Polypropylene photooxidation

Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene . Photooxidation processes in blends of polypropylene with poly(butylene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester . This effect is shown in Scheme 1. 

Both thermooxidation and photooxidation of polyolefins can be prevented by using the same antioxidants as those employed for the stabilization of polypropylene, ie, alkylated phenols, polyphenols, thioesters, and organic phosphites in the amount of 0.2—0.5% (22,25). 

Photooxidation Diffusion Chemical changes due to photochemical reactions Introduction of contaminants from man-made materials, such as solvents from polyvinyl chloride (PVC) materials and PVC cement, plasticizers, and phthalates from polyethylene and polypropylene materials Protection from exposure to light, use of amber glass bottles Use of inert materials (PTFE, fiberglass-reinforced epoxy materials) steam-cleaning of groundwater well components prior to installation... 

To improve the fire retardancy of polypropylene, beyond the UL 94 V-2 level, it is necessary to use blends of aromatic bromine fire retardants with antimony trioxide as a synergist. The usual loading is between 35% and 40% fire retardant however, the additional cost may prohibit commercialization. Moreover, the presence of aromatic bromine increases the photooxidation of polypropylene67 69 inactivating hindered amines. To reduce the cost without losing in efficacy the combination of brominated flame-retardant/antimony trioxide system with magnesium hydroxide... 

Sinturel, C. Philippart, J. L. Lemaire, J. Gardette, J. Photooxidation of fire retarded polypropylene. I. Photoageing in accelerated conditions, European Polymer Journal, 1999, 35(10), 1773-1781. 

Hydroperoxides are much more efficient than ketones for initiating photooxidation of ethylene-propylene copolymers [19]. This fact was confirmed by the results from photolysis of low-molecular model compounds and isotactic polypropylene [20]. 

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. 

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. 

The photooxidation rates of nylon polymers have been monitored using Fourier Transform Infra-red spectroscopy. Norrish type-II reactions of carbonyl groups were considered to be the most important process. Another group of workers have studied the photooxidations rates of a nylon 6- polypropylene glycol copolymerIn this case polyether sequences are the major source of free radical attack resulting in high levels of hydroperoxides. Irradiation of nylon and polyester fibres with an excimer laser resulted in... 

Hexahydropyrene sensitized chain scission of polypropylene and polyisobutylene during light irradiation Q20).Polycyclio hydrocarbons have a important role in sensitized photooxidation of polyisoprene (122).polys tyrene (123) poly(methyl methacrylate) (123-126). It is quite probable that these reactions can also occur with participation of singlet oxygen. 

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. 

Detailed ESR studies on the photooxidation of isotactic polypropylene at different temperatures showed that decomposition of tertiary hydroperoxides is the key step in initiation and that the Norrish type I photocleavage of... 

A.Z. Margolin, M.A. Menendes Tomassevitch, V.la Shliapintokh Peculiarities of solid-phase photooxidation of isotactic polypropylene and efficiency of photoinitiators///7zg/2ffJo/ecM/ r compounds (1987), v. 29 (A), 1067-1073 (in Russian). 

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. 

The mechanisms of the photooxidation of polyethylene and polypropylene have been discussed in depth with particular emphasis on the importance of hydroperoxides as the precursor to free radical formation . Both the kinetics and nature of the photooxidation products of the polymers are markedly controlled by these species especially polypropylene. On the other hand the density of polyethylene has been found to play an important role on the photooxidation rate of the polymer . Here the photostability of the polymer decreased with decreasing film density indicating that oxygen diffusion is impaired by the crystallites and therefore improves stability. In fact, other workers have found that the crystalline regions of polyethylene are unaffected by irradiation in air . These workers also found new crystalline regions are formed on irradiation due to the smaller polymer fragments... 

At the present time, a discussion of the results of our model investigations in terms of possible consequences for polypropylene must be completely speculative. Apart from the differences expected between liquid phase and solid polymer photooxidation kinetics, differences in the chemical structure between our model substance, isooctane, and the structural unit of polypropylene have to be also considered. With respect to the number of CH,-groups per structural unit, isooctane and polypropylene differ by a ratio of 5 1. 

The polymers obtained by polymerization in the presehce of metal catalysts contain metal residues which cannot be removed so readily. It is also well known that transition metal ions act as sensitizers for the photooxidation of polyolefins (29). Kujirai et al. (30) found that photodegradation of polypropylene depends on the oxygen concentration and on the residues of the polymerization catalyst, and they concluded that oxidative photodegradation is sensitized by the initiator metal residues (ash). Very recently Scott (31) used transition metal ions as sensitizers to develop photodegradable polymers. 

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