The cables that are used to transport electricity (and which are often seen as black 'power cables' near roads) are surrounded by a insulating sheath of polymer (usually polyethylene). One of the reasons for this is to prevent the electricity from leaking out into the environment. It is therefore important that this insulating sheath does not break down. One of the most common mechanisms by which this sheath breaks down is via the formation of 'water trees'. These are channels of water that form from the outer surface of the sheath and tunnel there way towards the conducting metal cable.
Understanding of the formation and propagation of water trees will help us to prevent this process and hence increase the lifetime of the insulating cable. To gain a fundamental understanding of polymers and polymer degradation we used Monte Carlo methods to study water under eqiulibrium and water-polymer systems at equilibrium.
As a first step in the project, we studied the formation of water vapour phase clusters under vapour-liquid equilibrium conditions. The simulations show that more clusters, as well as larger clusters, are formed at higher temperatures, mainly due to the increase in pressure under the equilibrium conditions. Also, the most stable cluster topologies change from cyclic to open structures with increasing temperature (due to entropic effects). This is illustrated in Fig. 1. These changes allowed us to calculate the change in enthalpy and entropy when going from the closed to open topologies.
Figure 1. Percentage of the four most common tetramer (a) and pentamer (b) topologies between 400 and 600 K.
We extended studies to include adsorption of water into a polyethylene matrix, both in the presence and absence of an external electric field (found under high voltage conditions) and when charged impurities are found in the polymer sheath (which are always present due to production and processing of the polymer). We observed that the presence of an external electric field decreases the amount of water that is adsorbed in the polyethylene, whereas the presence of charge impurities increases the water adsorption. This is seen as a rod-like water structure that extends between the charge ions (see the figure below) and indicates that these impurities should be removed (or decreased) from the polymer sheath to prevent water treeing.
The rod-like water cluster (water in red and white) between two ions (dark green and yellow) in a hydrocarbon matrix (light green)
More
information can be found in our publications.
This
work was funded by The
National Graduate School in Materials Science.