Stabiliser diffusion in long-term pressure tested polypropylene pipes analysed by IR microscopy

Abstract

Pipes of random polypropylene was (PP-R) were hydrostatic pressure tested and the distribution of the primary stabiliser, Irganox 1010, was measured in the radial direction over the pipe wall by Infrared (IR) microscopy. Parabolic concentration profiles of the stabiliser develop during testing indicating a loss of stabiliser, at both the inner and outer pipe walls. Raising the temperature of the water bath leads to a uniformly accelerated loss of stabiliser. An increase of the hoop stress accelerates the stabiliser migration at the inner pipe wall. The concentrations of Irganox 1010 averaged over the pipe wall as determined by IR microscopy were in excellent agreement with those obtained from High Performance Liquid Chromatography and Oxidation Induction Time (OIT) analysis. Diffusion constants were calculated from the stabiliser concentration profiles based on the Fickian equations using appropriate initial and boundary conditions. A literature value is compared to these experimental results. The developed IR technique allows monitoring the stabiliser migration faster and more reproducibly than the conventional approach by manual abrasion of layers and measurement of the OIT. Additionally, this brings a tremendous improvement with regard to spatial resolution. As a result the impact of hoop stress and extrusion rate on the migration of stabiliser in PP-R pipes can be shown for the first time.