MECHANICAL ASPECTS OF WATER TREEING

An attempt is made to correlate the initiation and growth of water trees in a clear polyester resin with the mechanical properties of the material, The mechanical properties of the resin were varied systematically by the addition of controlled amounts of a plasticizing polyester. Tensile strength, modulus of elasticity, stress intensity factor and fracture energy were measured in the pure original resin and in mixtures up to 75% by volume of plasticizer. Using the water needle method, water tree initiation times and growth rates were determined for each degree of plasticization. The polymer was in contact with 0.1 M NaCl solution and a 50 Hz ac voltage was applied to each specimen. It was found that the initiation time for water trees decreased (and the growth rate increased) with the plasticizer content. A correlation has been made between the initiation time and the tensile strength, both of which decreased due to plasticization, based on the assumption that initiation occurs when the osmotic pressure in a cavity at the water needle tip exceeds the mechanical strength of the material. The increase in propagation rate correlates well with the decrease in fracture toughness as plasticizer is added.