Superficial degradation

The rate of wood degradation by bacteria under anaerobic conditions is slow. Benner et al. (22) reported only 1.5 and 4.1% degradation after 246 days under strictly anaerobic conditions. Holt and Jones (23) demonstrated that test blocks of beech wood buried in anaerobic sediments are superficially degraded within months by a variety of rod-shaped bacteria. Others, however, report no measurable degradation of lignin in the absence of molecular oxygen. Little information has been published about the chemical composition of wood that has been degraded by known types of bacteria under anaerobic conditions (24). 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

The attack must be superficial and the parts must be thoroughly rinsed to avoid subsequent degradation. 

Despite the superficially structural complexity of the CPC molecule, it is prepared with comparative ease (15) (see PHTHALOCYANINE COMPOUNDS). The preferred manufacturing process starts with phthalic anhydride and is carried out in a variety of solvents, trichlorobenzene having been the preferred solvent, but it has been replaced with high boiling hydrocarbons or glycols in order to avoid the formation of the hazardous and poody degradable polychlorinated biphenyls. 

There is a limiting thickness, Lc 2TDL, such that for L < Lc, aging is homogeneous, whereas for L > Lc, it is heterogeneous (the degradation rate is lower in the core than in the superficial layers). From... 

Interest in the susceptibility of polypropylene (PP) to degradation, especially by heat, light, oxygen and combinations of these agencies, has kept pace with the increasing commercial application of this material. The purely photo (1-5) and thermal reactions (0 which occur, have been studied in isolation from one another, and it is clear that similar radicals are involved as intermediates in the overall processes. Previous work on acrylate and methacrylate polymers and copolymers (7-8) has demonstrated that the superficial differences in overall degradation mechanism may be attributed to the different ways in which the same primarily produced radicals react in the solid and liquid... 

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