Degradation and recovery of polydimethylsiloxane (PDMS) based composites used as high voltage insulators

Abraham Berhane, Teclesenbet (2004-12)

Thesis (MSc)--Stellenbosch University, 2004.

Thesis

ENGLISH ABSTRACT: Polydimethylsiloxane (PDMS) compounds are utilized in outdoor high voltage insulation due to their low weight, vandalism resistance, better anti-contamination performance and their superior hydrophobic nature. Under severe environmental conditions and over prolonged service time, however, the hydrophobic surface can gradually become hydrophilic and then recover with adequate resting period. In this study, room temperature vulcanized (RTV) PDMS samples were prepared with different formulations and then exposed to corona discharge to evaluate its effect. The influence of different additives, such as different types and amount of fillers and additionally added low molar mass silicone oils, on the hydrophobicity recovery of the material was investigated. The effects of two types of corona treatment were also evaluated. Hydrophobicity recovery of corona and UV-C aged PDMS samples was evaluated by means of static contact angle measurements. Positron annihilation spectroscopy (PAS) gave important information on the micro structural change after corona treatment of RTV PDMS as well as naturally aged high temperature vulcanized (HTV) PDMS samples. The different formulations of the RTV PDMS samples and the effect of the additives were studied with this technique. The formation of a thin, highly crosslinked inorganic silica-like (SiOx) layer was confirmed even at the early stage of degradation. It was also possible to estimate the thickness of the silica-like layer formed during corona exposure that is responsible for the loss and recovery of hydrophobicity. The surface hardness and hydrophilicity change of PDMS samples due to corona treatment were studied simultaneously with force distance measurements by atomic force microscopy (AFM). The adhesive force calculated from the pull-off force-distance curves showed that the adhesive force between the probe and the sample decreased with increasing corona treatment time, indicating hydrophobicity recovery. In addition to this, the increase in hardness after corona exposure provides indirect evidence of the formation of a silica-like layer. In all cases the hydrophilicity and the surface hardness of the PDMS samples increased directly after corona treatment and recovered with time. Two types of FTIR spectroscopy were used to analyse the surface of the polymer.

AFRIKAANSE OPSOMMINGS: Polidimetielsiloksaan (PDMS) word in buitelug hoogspanninginsulasie gebruik as gevolg van sy lae massa, weerstand teen vandalisme, verbeterde anti-kontaminasie werkverrigting en superieure hidrofobiese karakter. Die hidrofobiese oppervlakte kan egter gelydelik hidrofillies word onder uiterste omgewingsomstandighede en oor langdurige dienstyd. PDMS materiaal herstel egter nadat dit genoeg rustyd toegelaat is. Kamertemperatuur-gevulkaniseerde (KTV) PDMS met verskillende formulasies is in hierdie studie voorberei, aan korona ontlading blootgestel, geëvalueer en vergelyk. Die invloed van bymiddels soos verskillende tipes en hoeveelhede vuiler, asook addisionele lae molekulêre massa silikoonolie, op die herstel van hidrofobisiteit van die materiaal is ondersoek. Twee verskillende metodes van korona behandeling is ook geëvalueer. Die herstel van hidrofobisiteit van korona en UV-C verouderde PDMS monsters is met statiese kontakhoekmeting geëvalueer. Positronvernietigingspektroskopie (PVS) is 'n kragtige tegniek wat belangrike inligting oor die mikrostrukturele verandering van korona behandelde van KTV PDMS sowel as natuurlik-verouderde hoë temperatuur gevulkaniseerde (HTV) PDMS monsters gee. Die verskillende formulasies van die KTV PDMS monsters, sowel as die effek van die vullers, is met behulp van hierdie tegniek ondersoek. Die vorming van 'n dun, hoogskruisgebinde, anorganiese silika-agtige (SiOx) laag op die PDMS oppervlak, selfs tydens die vroeë stadium van degradasie, is bevestig. Dit was ook moontlik om die dikte van die silika-agtige laag wat gedurende die korona blootstelling gevorm het, en wat verantwoordelik is vir die verlies aan hidrofobisiteit, te bepaal. Die oppervlakhardheid en hidrofilisiteit verandering van PDMS monsters as gevolg van korona behandeling, was gelyktydig met krag-afstand metings deur middel van atoomkragmikroskopie (AKM) bestudeer. Die kleefkrag, soos bereken van aftrek kragafstandkurwes, dui daarop dat kleefkragte tussen die taster en die monster afneem met toenemende korona behandelingstyd, wat beduidend is op die herstel van hidrofobisiteit. Daarbenewens is die toename van oppervlakhardheid na korona blootstelling "n indirekte bewys van die formasie van 'n silika-agtige laag. In alle gevalle het die hidrofilisiteit en die oppervlakhardheid van die PDMS monsters toegeneem direk na afloop van korona behandeling en gevolglik herstel met tyd. Twee tipes IR spektroskopie metodes is gebruik vir die chemiese-oppervlak analises

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