Ageing of thermoplastic umbilical hose materials used in a marine environment II: Nylon

Measurements of nylon 11 pipes subjected to ageing at 400C, 700C and 1000C in water, methanol and xylene using both a constant pressure of 200 bar and a cyclic pressure regime are reported. Gravimetric measurements indicate the rate at which the solvent is absorbed by the polymer and differential scanning calorimetry follows the changes in the crystallinity as the materials are aged. Dramatic changes in the tensile properties are observed when the pipes are subjected to high pressure and reflect a relaxation of the stress in the matrix introduced by the quenching process when the pipes were extruded. The magnitude of the change varies with the fluid and reflects their relative abilities to be absorbed by the polymer. Measurements of the relative viscosity for the polymer indicated that in the case of water and methanol, hydrolytic degradation of the polymer is taking place with time. The impact of the morphological changes on the dynamic mechanical properties revealed movement of the elasticity transition to higher temperatures and reduction in the chain mobility with time. Changes in the mechanical properties are a function of the initial stress relaxation and chain scission as a consequence of degradation and thermodynamically driven morphological changes increasing the crystallinity and embrittling the polymer. As the pipes aged so the burst pressures progressively decreased. Examination of the failure surfaces indicated brittle failure and clear evidence of environmental stress cracking. Whilst the data indicate the pipes might be used effectively for hydrophobic fluids, hydrophilic fluids and in particular methanol can significantly shorten their effective life.