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pinking of Ti0,-pigmented WC can be observed for profiles exposed to the mild environmental conditions of Northern European countries. In this paper, the pinking phenomenon is accounted for at the molecular level by the formation of polyene sequences which are not completely photooxidatively bleached under conditions of oxygen starvation. Photoreduction of TiO, to give Ti(II1) species accounts for greying which cannot be converted into pinking under mild thermooxidative conditions (65°C). The polyenic sequences which account for an initial greying after brief exposure in the SEPAP 12.24 apparatus can be converted into shorter, modified polyene sequences which explain the pinking. From the basic understanding gained, a predictive test has been designed and validated by comparison with results obtained under natural exposure conditions. n the mild climatic conditions typical of Northern

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A polymeric system based on LDPE would be qualified as a "photo( bio)degradable synthetic polymer" for use as films or thin systems in plasticulture and later in packaging where severe specific criteria should be respected. The evolution of such a system in environmental conditions should present three phases. In Phase I, corresponding to storage and use, in the presence of physicochemical and biological aggression, chemical evolution should be very limited and resistance to any microorganism should be observed. In Phase I1 a rapid abiotic degradation should occur until the complete destruction of physical (mechanical) properties and spontaneous fragmentation of the thin systems into more and more di-73 1 Copyright 0 1995 by Marcel Dekker, Inc.Downloaded by [Michigan State University] at 15:20 09 April 2015 LEMAIRE ET AL.vided parts. Phase 111 corresponds to bioassimilation of heavily transformed (oxidized) solid particles. Phase I should be predicted and controlled on the basis of artificial photoaging or thermoaging experiments. Depending on the desired lifetime of the system, nonaccelerated, accelerated, or ultra-accelerated photoaging techniques could be used. The earliest fragmentation, which should be observed in Phase 11, should be predicted within the same experiment. The prediction of the long-term fate of the polymeric materials should be based not only on the variations of physical properties but on a full analysis of the chemical evolution, i.e., determination of the major final transformed groups of the macromolecular chains (and especially the acidic end groups) and the molar mass distribution. In a recent BRITE-EURAM European contract, we developed an experimental protocol for the control of Phase 111 based on the use of pure cultures of strains from collections or selected adapted wild strains (from industrial polyethylene site dumpings) which had been examined. Abiotically oxidized LDPE was the only carbon source in a starving mineral medium. Bioerodibility caused by the carboxylic acid formed throughout abiotic degradation has been observed.