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Polyethylene (PE) insulations have a wide applicability in the insulation of both underground pipelines and underground power cables. In this context, by coupled techniques of thermal analysis (TG/DTG+DTA) and microbiological determinations, have been studied thermooxidability and resistance to moulds action of some polyethylene sorts. Following the processing of the experimental data obtained by thermal analysis it was found that during the applied heat treatment (100 grd C), in the first approx. 380 h, there is a growth of LDPE (low density polyethylene) polymerization degree by elongation of the aliphatic chains, after which the predominant process consists in the structure crosslinking. For MDPE (mean density polyethylene) samples, during the thermal treatment applied, it was found that the crosslinking degree of polyethylene (PE) increased without significant molecular weight change (with all the related consequences of increasing the weight of the tertiary and quaternary carbon atoms in the molecule). Microbiological determinations have highlighted that the resistance to filamentous fungal action of LPDE is higher than that of the investigated MDPE. It was found that after heat treatment applied (1000 h and 100 oC), both at LDPE and at MDPE, decreases the resistance to moulds action is decreased. It has also been found that moulds action resistance is substantially decreased when inoculated culture media and PE samples are exposed to an alternative electric field of 50 Hz - 6 Vrms/cm.

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