I. L. Hosier

Structure-property relationships in polyethylene blends: the effect of morphology on electrical breakdown strength

Blends of linear and branched polyethylene were prepared covering the composition range 1–20% linear polyethylene, and three thermal treatments were subsequently chosen to produce a range of different morphologies. Isothermal crystallization at 124 °C gives rise to compact linear inclusions within a matrix of branched polyethylene, isothermal crystallization at 115 °C produces an open, banded spherulitic morphology and, finally, quenching leads to a continuous spherulitic texture. Ramp testing was then employed to investigate the effect of morphology on electrical strength. It was found that the electrical strength of the blend depends primarily on the morphology and that, by optimizing thermal treatment and linear polyethylene content, substantial improvements in properties can be obtained.

On the structure and chemistry of electrical trees in polyethylene

The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp2 G and D Raman bands, the constituent graphitic domains are estimated to be ~4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm−1. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

On the effects of morphology and molecular composition on the electrical strength of polyethylene blends

The effects of morphology and molecular composition on the electrical strength of blends of linear and branched polyethylenes were investigated. A range of blend systems were considered, in which both the molecular mass of the linear polymer and the comonomer in the branched component were varied. All the blends contained 10% linear polyethylene and 90% branched polymer and, in each system, three crystallization procedures were employed to modify the morphology. Isothermal crystallization at 124 °C generally resulted in compact linear inclusions within a branched matrix; isothermal crystallization at 115 °C produced a space-filling network of open, spherulitic structures; and quenching gave a banded spherulitic morphology. In these systems, the electrical strength, as measured by ramp testing, was dependent on the morphology of the material but was not influenced per se by significant changes in the molecular composition of the blend. The effect of crosslinking was also examined; the inclusion of a network did not, in itself, affect the breakdown strength or the morphology.

On the dielectric response of silica-based polyethylene nanocomposites

The dielectric response of silica-based polyethylene nanocomposites is studied by dielectric spectroscopy. The results indicate that nanocomposites absorb significantly more water than unfilled polyethylene, with the consequence that both permittivity and loss tangent increase with increasing duration of water immersion. However, appropriate surface treatment of nanosilica is found to reduce the water absorption effect and to modify the dielectric response of the nanocomposites compared with those containing untreated nanosilica. While water absorption may not be a technologically desirable characteristic, our results indicate that water molecules can act as effective dielectric probes of interfacial factors.

On the morphology of polyethylene crystallized from a sheared melt

The morphology of linear polyethylene, crystallized from a sheared melt in the form of shish-kebabs, has been investigated by transmission electron microscopy following permanganic etching. The microstructure of lamellae crystallized transversely on a central linear thread changes systematically with crystallization temperature and shear rate. Higher strain rate produces more and longer filaments and aligns lamellar normals along their rows. At lower strain rate, lamellar habits resemble, but differ from, those characteristic of quiescent growth. Fold surfaces that are inclined to the chain axis c at lower shear rates become normal to c with higher shear, thereby adopting a state of higher energy. It is inferred from this and the high crystallization temperatures that their crystallization was not strain-free. The separation of adjacent lamellae and its decrease at higher strain rate are consistent with pressure from molecular cilia emerging from fold surfaces, in a similar way to the basic cause of lamellar divergence in spherulites.

On morphology of consolidated UHMWPE resin in hip cups

The microstructure of replacement hip cups made from ultra-high molecular weight polyethylene (UHMWPE) has been revealed by permanganic etching to bring out the fine details. Specimens are examined by optical microscopy according to the Nomarski technique, and by scanning and transmission electron microscopy. In all cups the original reactor particles are visible. Optical microscopy is most productive in revealing variations not only in the texture of different individual particles, but also in the degree of consolidation of a particle with its neighbours. Both these factors differ according to the material and process. Electron microscopy reveals the existence of pockets of lower molecular weight material which has been expelled from the particles by the original sintering process.

Aging of biodegradable oils and assessment of their suitability for high voltage applications

In many items of high voltage plant, a mineral or synthetic oil is used in conjunction with paper as the dielectric medium. However, increasing awareness of the environmental impact of human activity and increasing disposal costs have encouraged researchers to direct their attention to renewable and biodegradable alternatives. Originally used in capacitors, vegetable oils are now finding widespread use in some transformer installations, particularly in the United States. Therefore, it seems prudent to begin systematic investigations of the aging behavior of a number of vegetable based oils and assess their potential for application in high voltage systems. A total of five food grade vegetable oils, an oil specifically formulated for high voltage applications (Envirotemp FR3) and dodecylbenzene (DDB) were aged at various temperatures in air. Their aging behavior was assessed by ultra-violet/visible, infrared and dielectric spectroscopies supplemented with measurements of viscosity. It was found that olive oil offered excellent resistance to aging (comparable to Envirotemp oil), rapeseed oil offered intermediate properties whereas corn and sunflower oil oxidized appreciably after aging. Despite being more prone to oxidation, all the vegetable oils offered dielectric properties which were better than dodecylbenzene and similar to mineral oil.

The effects of measuring technique and sample preparation on the breakdown strength of polyethylene

The effects of specimen thickness, testing conditions and preparation technique on the measured ramp-breakdown strength of 2 polyethylene systems have been investigated. The obtained data have been found to be highly reproducible, enabling results under different test conditions to be correlated with one another. In all cases, an increase in ramp rate results in an increase in the measured breakdown strength, as does a reduction in sample thickness. Irrespective of the test parameters, the short-term performance of a polyethylene blend containing a designed microstructure is always improved compared to that of the base low density polyethylene alone. Impulse testing leads to an identical conclusion. The effect of the blending method has been shown to have a negligible effect on the breakdown strength, despite consequent changes in nucleation density, implying that it is the local lamellar texture that is responsible for the enhanced properties of the blend.

On the space charge and DC breakdown behavior of polyethylene/silica nanocomposites

Space charge occurs in a dielectric material when the rate of charge accumulation is different from the rate of removal, which arises due to moving or trapped charges. Inevitably, the local electric field is increased at some point within the material, which then leads to faster degradation and premature failure. The determination of space charge behavior has seen wide implementation in characterizing novel dielectric materials, especially in connection with the newly emerging field of nanocomposites. In this paper, we report on an investigation into space charge dynamics in silica-based polyethylene nanocomposites. The various systems differed with respect to the amount of filler and its surface chemistry; the pulsed electro-acoustic (PEA) technique was used to evaluate the space charge distribution in each. Experimental results indicate that the incorporation of nanosilica into polyethylene results in a significant amount of homocharge development near both electrodes. With appropriate surface treatment of the nanofiller, homocharge formation was successfully suppressed, indicating less severe space charge development in the nanocomposite materials. The mechanisms leading to the observed space charge development and direct current (DC) breakdown properties of the nanocomposites are discussed.

Chemical, physical and electrical properties of aged dodecylbenzene 2: thermal ageing of single isomers in air

The linear isomer of dodecylbenzene (DDB), 1-phenyldodecane, was aged at temperatures of 105 and 135 degC in air and the resultant products were analyzed using a range of analytical techniques. On ageing, the 1-phenyldodecane darkened, the acid number, dielectric loss and water content increased and significant oxidation peaks were detected in the infrared spectrum. When aged in the presence of copper, a characteristic peak at 680 nm was also detected by UV/visible spectroscopy but, compared with previous studies of a cable-grade DDB, the strength of this peak was much increased and no appreciable precipitate formation occurred. At the same time, very high values of dielectric loss were recorded. On ageing in the absence of copper, an unusually strong infrared carbonyl band was seen, which correlates well with the detection of dodecanophenone by gas chromatography/mass spectrometry and nuclear magnetic resonance spectroscopy. It was therefore concluded that the ageing process proceeds via the initial production of aromatic ketones, which may then be further oxidized to carboxylic acids. In the presence of copper, these oxidation products are present in lower quantities, most of these oxidation products being combined with the copper present in the oil to give copper carboxylates. The behavior is described in terms of a complex autoxidation mechanism, in which copper acts as both an oxidizing and a reducing agent, depending on its oxidation state and, in particular, promotes elimination via the oxidation of intermediate alkyl radical species to carbocations.