Photo-oxidative thermal degradation

Free radicals generated by metal ion catalysis and photo or thermal degradation of buffer or other excipients can accelerate the oxidative degradation reactions. 

The standard ASTM D6954-04 is used for the exposure of plastics that can degrade by a combination of oxidation and biodegradation (87). This standard should permit the comparison and ranking of the overall rate of the environmental degradation of plastics in the course of thermal or photo oxidation. Each degradation stage is independently evaluated to allow a combined evaluation of a polymer s environmental performance. 

Degradation processes during weathering involve photo-radiation, thermal degradation, photo-oxidation and hydrolysis. These irreversible processes can cause... 

Costa L, Luda MP, TrossarelU L. Ultra high molecular weight polyethylene-II. Thermal-and photo-oxidation. Polym Degrad Stab 1997 58 41-54. 

Protection of polymers against thermal and photo-oxidative degradation is achieved with appropriate stabilizers that ensure the desirable polymer properties throughout the entire service life of the polymer. Compatible and polymeric stabilizers usually give the best protection. In order to avoid migration and evaporation, polymeric stabilizers are used. 

Increase the oxidation rate of polymers, e.g. metal ions which increase the hydroperoxide decomposition rate. Photodegradation and thermal degradation are enhanced by transition metal ion containing pro-oxidants, such as iron dithiocarbamate (as opposed to nickel dithiocarba-mate, which acts as a photo-antioxidant). 

The y-initiated oxidation of polyolefins produces a product mix which is less complex than that resulting from photo-or thermally initiated degradation. This results from the mild conditions in the Y-cell, where the major initial oxidation product, the -OOH group, is stable. Although the derivatization methods are applicable to all types of oxidation, for simplicity only the y-irradiated systems will be considered here. 

What is mechanical degradation Why is it less commonly encountered relative to thermal and photo-oxidative degradation ... 

As discussed earlier under Section 2.3, Carbonyl index, in one relatively recent comparison of the photo-oxidative and thermal (oven-aged) degradation behaviour of different polyethylenes, additive free grades of a metallocene (mPE), an HDPE and a linear low-density PE (LLDPE) were analysed by a combination of mid-IR spectroscopy, TGA and CL [13]. The mid-IR... 

Polyethylene oxide) (PEO) is a semicrystalline water-soluble polymer [64, 65], with a crystallinity that is very sensitive to the thermal history of the sample, making this property interesting as an indicator of degradation. Because it is biodegradable and biocompatible, PEO is a good candidate for environmental and medical applications [66-68]. The mechanisms of thermo- and photo-oxidation of PEO have already been investigated [69, 70] on the basis of IR identification of the oxidation products and are summarized in Scheme 10.1. 

A polymer may be degraded by chemical changes due to reaction with components in the environment. The most important of these degrading reagents is oxygen. Oxidation may be induced and accelerated by radiation (photo-oxidation) or by thermal energy (thermal oxidation). 

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