Irradiated polypropylene stability

Polypropylene, stabilized irradiated. Radiation-stabilized grades,... 

Stabilizing Activity of Pyrocatechols in Thermal Oxidation and in y-Irradiation of Polypropylene. It is interesting to compare the relationships found on stabilizing isotactic polypropylene oxidized over the melting temperature with the results of our previous study (22, 23) of the stabilizing properties of some derivatives of pyrocatechol in y-irradiated polypropylene. 

In this study we measured chemiluminescence of polypropylene stabilized with different combinations of antioxidants and irradiated to different extents, and made correlations with conventional impact strength measurements of the same materials. 

The thioether ester (DLTDP) shows a very strong synergistic effect with the phenolic stabilizer at room temperature, and is active alone at 60°C for protection from loss of impact strength of the irradiated polypropylene. 

Meligi, G., Yoshii, F., Sasaki. T., Makuuchi, K., Rabie, A. M. and Nishimoto, S-L, 1995. Comparison of the degradability of irradiated polypropylene and poly(propylene-co-ethylene) in the natural environment. Polymer Degradation and Stability, Vol. 49, No. 2, (No information 1995), pp 323-327, ISSN 0141-3910. 

In this paper we compare the stabilization effectiveness of some less usual phenolic compounds in y-irradiated polypropylene with that of a conventional hindered phenolic antioxidant and hindered amine stabilizers (HALS). EXPERIMENTAL... 

PL can be used as a sensitive probe of oxidative photodegradation in polymers. After exposure to UV irradiation, materials such as polystyrene, polyethylene, polypropylene, and PTFE exhibit PL emission characteristic of oxidation products in these hosts. The effectiveness of stabilizer additives can be monitored by their effect on PL efficiency. 

Better mechanical stability can be expected from irradiated polyethylene or mi-croporous polypropylene ( Celgard ) membranes, but these account for increased electrical resistance values. 

However, several articles in the area of microwave-assisted parallel synthesis have described irradiation of 96-well filter-bottom polypropylene plates in conventional household microwave ovens for high-throughput synthesis [16-19]. While some authors have not reported any difficulties associated with the use of such equipment [19], others have experienced problems in connection with the thermal instability of the polypropylene material itself [17] and with respect to the creation of temperature gradients between individual wells upon microwave heating [17, 18]. While Teflon (or similar materials such as PFA) can eliminate the problem of thermal stability, the issue of bottom-filtration reaction vessels has not yet been adequately addressed. 

The activities of all compounds from the hydroquinone series studied as antioxidants in isotactic polypropylene were lower than that of pyro-catechol. A similar conclusion was reached even when comparing both groups of substances as stabilizers in y-irradiated isotactic polypropylene... 

Influence of UV Irradiation on the Stability of Polypropylene and Blends of Polypropylene with Polymethyl methacrylate... 

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... 

The loss of Impact strength of polypropylene was followed from sheets stored In air at 25°C and 60°C after irradiation with electron beams. A marked difference in efficacy of phenolic and thioether-based stabilizers at the two temperatures was found, with the thioether active alone at 60°C but only synergistically at 25°C. This difference was also reflected qualitatively in differences in chemiluminescence emission from the samples. 

Irradiation of polymers with y- or electron beams is an attractive alternative to chemical sterilization because of its speed, ease of control, and the absence of residue. Radiation treatment of polypropylene, however, also initiates chemical changes which lead ultimately to embrittlement. These changes in physical properties may not become apparent until some time after the treatment. The ability of antioxidants to prevent radiation damage does not always follow the trends observed in thermal oxidation, which has stimulated efforts to develop new stabilizers or optimized combinations of existing ones. 

N.S. Allen, A. Chirinos-Padron, and J.H. Appleyard, Photo-stabilizing action of metal chelates in polypropylene. Part I. Excited state quenching versus UV antioxidant action under polychromatic irradiation, Polym. Degrad. Stab. 1982, 4, 223-237. 

Pure polypropylene does not discolor at the 2.5 Mrad required for sterilization. However, the stabilizers added to improve the polymers thermal and irradiation stability often discolor upon irradiation. This yellowish discoloration increases with irradiation dose. This problem can be resolved by using stabilizers that do not discolor when they are irradiated. 

The photo-oxidative degradation of polypropylene and stabilization by hindered amines has been reviewed. A study has appeared of the effect of P-carotene on the photoreactivity of anthracene in hexane solution and a kinetic scheme has been proposed to account for the photochemical and photophysical processes that occur on irradiation at 365 nm. Quenching rate constants have been determined between /S-carotene and singlet oxygen. Some characteristics have been communicated of the sensitized photo-oxidation of abietic acid contained in a vinyl butyl ether-butyl methacrylate-methacrylic acid copolymer. At 400 nm... 

In spite of the high carotenoid protective effect In natural biopolymers as well as In man-made polymer matrices [demonstrated, e.g., by Inhibition of self-sensltlzed oxygenation of tetracene In polystyrene (127) or by measured rate constant kq = 4.3 X 10 M 1 sec l In polystyrene (135)], B-carotene proved to be Ineffective as a UV stabilizer In polypropylene films under conditions of xenonarc Irradiation (136). The possible explanation Is that the polyene chain of B-carotene Is quickly... 

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