Water treeing using the water needle method: the influence of the magnitude of the electric field at the needle tip

The initial growth kinetics of water trees in polyethylene is measured for different values of the radius of curvature of the needle tip and for different applied voltages. The initial propagation rate is shown to be determined by the magnitude of the electric field at the needle tip, except at its highest values. The local electric field at the solution/polymer interface or in its vicinity plays an essential role in the growth mechanisms of water trees; it determines the initial propagation rate up to values of about 100 to 300 V/μm. Above these field values the water tree growth is found to be controlled by slower processes, e.g. the migration of the liquid through the polymer. These results are used in discussing some experimental conditions which are required to get reproducible results in water needle tests