Sustainable additives and inorganic fillers in Ethylene Propylene Rubber for high voltage cables accessories

Polymeric insulating materials are the basis of the electric power system. Eco-friendly polymeric insulation takes a relevant role in this direction, and it has a promising role in advanced R&D, also to improve the sustainability of dielectric materials and electrical insulating accessories. The primary goal of the present thesis is to understand the role and mechanism played by additives (micro- and nano-oxides, peroxides, and antioxidants) in polyolefin polymer composites (used in electrical MV and HV industry) to increase sustainability and performance through substitution, reduction, heterogenization and recycling. In contrast, the polymeric insulating materials have a relatively slow rate of eco-friendly transformation. This finding is mainly related to the fact that proper selection of the ingredients of a polymeric composite in each category requires that consideration be given to the desired physical, electrical and environmental properties, as well as cost, ease of mixing, chemical stability and ease of processing of materials, so that compromises are normally adopted which increase the variability of the mixture composition and its technological use and recycling. Therefore, the full life cycle of polymeric insulating materials, devoted to constructing ecofriendly electric power systems by maintaining sustainability and high electrical performance, is a hard task to reach but needs urgent applied studies and quite specific technological advancements. This work is concerned with the preparation and characterization of EPR composites useful to model setup of medium and high voltage electrical cables and related joints. Some specific aspects investigated were the influence of micro-and nano-fillers like SiO2, TiO2, ZnO, and Pb3O4 on water stability, thermal, mechanical, and electrical performances in the absence and presence of dicumyl peroxide (DCP). The impact of thermal decomposition of DCP on EPR composites crosslinking was also briefly investigated. Functionalization of SiO2 and TiO2 by decyl-triethoxysilane was studied to produce intermediates functionalized fillers, useful for linking (through different ionic approaches). Some pure natural antioxidants were further investigated in EPR composites, with and without DCP, to prove their potentialities as thermal and electrical stabilizers. Oxidation onset temperature (OOT) and oxidation induction time (OIT) measurements on a composite containing pure natural antioxidant showed activity in autoxidation process similar to that of Vulkanox (a standard antioxidant for these matrices). Moreover, electrical measurements including tan delta and relative permittivity on the same samples support their potential use in HV accessories. Some natural antioxidants (caffeic acid (CA), rosmarinic acid (RA) and eugenol) and synthetic antioxidants (2-mercaptobenzimidazole (MBI)) were also tested through derivatized silicon dioxide (silica) in EPR matrix. Their antioxidant efficiency (OOT and OIT) was also tested. An investigation was performed to determine the role of minium (micro-Pb3O4); (a) in industrial EPR composites in relation to the phenomena of discoloration under different conditions and (b) in model systems (cyclododecane) under peroxide crosslinking conditions. In the first direction, it was proved that the change of color occurs via three different mechanisms i.e. including the thermal degradation of EPR rubber by the oxidant properties of Pb3O4, water absorption which induces the Pb3O4 filler disproportionation with dissolution of Pb(II) in water and masking of the Pb3O4 color by crystals of ZnO filler when oriented preferentially. On the latter direction, kinetic measurements allow us to prove that Pb3O4 catalyzes the decomposition of DCP inducing at lower temperature and lower yield of C-C crosslinking products, with minor oxidation of intermediate radicals and their significant trapping by the lead oxide. The newly formed C-O-Pb increases the hydrophobicity of the surface and creates a new organic-inorganic network, which induces better moisture resistance and reduces the electrical loss. Moreover, the analysis of reaction products allows to identify and quantify the homo and cross-coupling derivatives and the trapping of methyl radicals by the aromatic products acetophenone and cumyl alcohol. The last reaction has a potential for preparative access to methylacetophenone isomers when DCP decomposition is carried out in acetophenone as solvent. After a general introduction on the background of electrical cable insulation and accessories, the intrinsic properties of polymeric insulation, their constituents and their analysis, the base of sustainability was discussed. Then the following aspects are considered to be the significant original contributions of this research: (a) It was determined that metal oxide nanofillers (SiO2, TiO2, ZnO and lead oxide) in low concentration (0,1-1%) improve the electrical, mechanical, thermal and water-resistant properties of EPR composites but at different degrees under same peroxide crosslinking condition. These results showed that the impurities distribution is influenced strongly by the nanoparticles due to the coulombic or molecular interactions. (b) Natural antioxidants, caffeic acid and rosmarinic acid work as effective antioxidants and thermal stabilizers in EPR composites as proved by OIT and OOT measurements. Chemical derivatization of mesoporous SiO2 by the same phenols induces lower antioxidant properties at similar phenol concentration (except for the eugenol). (c) By a careful analytical survey on industrial samples of EPR/Pb3O4 and EPDM/Pb3O4 composites, it was proved that the color change occurs by three different mechanisms: a) thermal degradation of EPR rubber by the oxidant properties of Pb3O4; b) water absorption which induces the Pb3O4 filler disproportionation at surface (1-2 mm) by dissolving Pb(II) ions and increasing the PbO2 and other oxides/carbonates content; c) anisotropic crystals of ZnO filler when oriented preferentially in these composites (i.e., when produced by injection molding) induce the masking the Pb3O4 color, so inducing the appearance of discolored areas and possibly dielectric inhomogeneity in so manufactured cables. (d) Pb3O4 works as catalyst in the decomposition of dicumylperoxide and improves the crosslinking of EPR by an alternative C-O-Pb linked network superimposed to the common C-C network. This helps to better explain the use of this oxide as stabilizer for moisture resistance and as a reducer of dielectric loss in EPR composite assemblies in the cable industry. This finding will also open new opportunities for the reduction (and possibly the substitution) of this toxic oxide, maintaining the above-mentioned peculiarity.