A Raman microprobe study of electrical treeing in polyethylene

Abstract

The chemical nature of electrical tree growth in a blend of high and low density polyethylene has been studied by confocal Raman microprobe spectroscopy. The observed spectra, which are easily perturbed by the Raman probe beam, can be described in terms of three components, the relative intensities of which vary from place-to-place on the sample. Throughout the body of the tree, the usual Raman bands of polyethylene are seen, but superimposed upon a pronounced fluorescent background. This suggests a degree of material degradation throughout the structure, even where there is no visible evidence of electrical discharge damage or ageing. The individual channels that make up the fractal structure of the tree can then be divided into two distinct categories. Within the core of the tree, their Raman spectra are made up of two elements; fluorescence, plus the G and D bands of sp 2 hybridized carbon. Here, the tree channels are best thought of as hollow tubules surrounded by a carbonaceous shell. At the tips, the channels are characterized simply by the Raman bands of polyethylene superimposed upon a reduced fluorescent background, irrespective of their size. That is, in this region, the tree channels are simply hollow tubules within the dielectric. The transition between these two structures occurs relatively abruptly. These observations are related to the treeing process. C 2004 Kluwer Academic Publishers

A Raman spectroscopic study of bulk and surface ageing phenomena in polyethylene

“…However, from our results, local fields of this magnitude are likely to exceed the breakdown strength of the polymer and, then, the bulk degradation process will be very different from that considered here. Indeed, our studies of treeing processes in polyethylene [10] have demonstrated that internal discharges can generate precisely the same Raman fingerprints as those previously associated with ageing through electromechanical chain scission [3]. We believe this to be the origin of the discrepancy.…”

On the use of photobleaching to reduce fluorescence background in Raman spectroscopy to improve the reliability of pigment identification on painted textiles

“…A similar exponential decrease of background fluorescence has been reported in a study of extended photobleaching on fluorescent samples of polyethylene. 41 Further analysis of the signal to noise ratio of the spectrum showed an approximately 10-fold improvement in signal to noise after 2 h photobleaching at 100%, with the result that many more of the smaller peaks became visible.…”

“…40 Finally, health and safety considerations dictate that conservators handling fragments from friable samples during conservation treatments must take appropriate precautions should artefacts contain toxic materials such as lead, arsenic and mercury, as was common in early pigments. 39 The large background fluorescence often seen on Raman spectra from painted textiles is typically considered to be due to surface contamination (soiling) or due to the organic binders used in painting the textiles, though it may be an inherent aspect of aged samples 41 and may be indicative of the age of the sample as has been found in certain glass artefacts. 42 Many other factors, including storage conditions, may also be significant in determining the magnitude of the fluorescence.…”

Tree growth in a propylene / ethylene copolymer: relationships between electrical activity during growth and the structure, chemistry and properties of the trees that result

“…Given that the larger path length of the discharges in the nonconducting tree tubules will produce larger amplitude partial discharge pulses in the external circuit, much of the discharge activity that was detected and hence the calculated energy dissipation must relate to activity in those non-conducting tree segments. Raman Characterization In our previous Raman studies of electrical tree structures in polyethylene [7], we only considered a single surface cut through each tree. While this has the advantage of being non-destructive and, therefore, allows each tree to be examined repeatedly using a number of complementary techniques, it does mean that it is not possible to obtain a complete 3-dimensional "map" of a tree's chemistry.…”

“…In a recent publication [7], we reported on preliminary studies in which we explored the potential of confocal Raman microprobe spectroscopy as a technique for characterizing electrical trees. This work has clearly demonstrated that this technique is capable of extracting chemical information from surfaces with sufficient spatial resolution to permit the chemical characterization of electrical tree structures.…”