Detection of Super-Adsorbent Polymer in Jet Propulsion Fuel Using a Sodium Ion Selective Electrode

Abstract

In 1996, the United States Air Force began using water-adsorbing filters in response to changes to the fuel additives being used. These filters were to provide a last means of water filtration before the fuel was placed on the aircraft. The principal component of the water-adsorbing filter is a membrane which contains either a sodium or potassium salt of a cross-linked polyacrylic polymer, commonly referred to as super-adsorbing polymer (SAP). When the SAP filters were used, a gelatinous substance was found downstream of the filters in both the fuel and water layers. This substance has been identified as polyacrylic polymer. It has been determined from analysis of postuse filters that the polymer salt has reverted to poly(acrylic acid) and thus potentially released a metallic cation into the fuel. A filter test apparatus, using a filter containing sodium-based polyacrylate, was used in varying situations to detect the sodium loss from the polymer. The most dramatic result occurred while using a jet propulsion fuel that had been injected with a 2% water slug containing military grade fuel additives. Within 30 min of flow, the fuel sodium level escalated from ∼5 ppm sodium to 480 ppm sodium. Additionally, poly(acrylic acid) was detected outside the filter and in the water layer. The following two-stage experiment was designed to determine the release rate of the sodium cations into jet propulsion fuel and the chemical mechanism for the polymer decay.