Analysis of the Deterioration of Nylon-66 Immersed in GTL Diesel Fuel Part 2. Analysis of Model Fuel and Nylon Before and After Immersion

In a previous paper (Part 1 of this series), nylon-66 specimens were immersed in two GTL diesel fuels (GTL-A and GTL-B) and then subjected to tensile testing. The tensile test results revealed that the elongation of the specimen immersed in GTL-A was dramatically reduced. The GTL diesel fuels and nylon specimens before and after immersion were analyzed to determine the cause of the decline in elongation. It was found that the poor elongation was caused by penetration and oxidation of low molecular-weight paraffins and that the ease of penetration and oxidation of paraffin depended on the structure of paraffin.

In this paper, the low molecular-weight paraffins detected in GTL-A were mixed to produce model fuels. Then, pieces of nylon cut from the tensile test specimen, were immersed in the model fuels. In addition, partial oxidation products of the paraffin (alcohol, aldehyde or ketone and acid) were used in immersion tests of the nylon pieces. It was found that: (1) The quaternary carbon atom in i-paraffin hardly oxidizes, while the tertiary carbon atom in i-paraffin readily oxidizes to give alcohol. (2) 2-Methyl-paraffin produced 100 times as many oxidation products as n-paraffin. (3) The lower the carbon number of a paraffin is, the easier it penetrates the nylon samples. (4) The degree of oxidation of nylon corresponded with the tendency of paraffin to oxidize. (5) The composition of the oxidation products of paraffin, which were detected in the nylon after the immersion test, was nearly the same as that of the oxidation products of paraffin itself. (6) Oxidation products of the paraffin outside of the nylon had no influence on the oxidation of the inner layer of nylon. Rather, the free radicals that oxidized the inner layer of nylon were generated from the paraffin that entered the nylon.