Positron annihilation lifetime spectroscopy and diffusion studies of large molecule penetrants into alkyd resins

Abstract

Alkyd resins can be used to create transparent films and their characteristics depend on the method used in their cure. Changing the catalysts can have profound effects on the physical properties of the films formed. Positron annihilation lifetime spectroscopy (PALS) measurements are used to explore how changes in catalyst influence the free volume for an alkyd resin system. Diffusion measurements are reported for large molecule permeants; dioctyl phthalate, 2-ethyl hexylbenzyl phthalate, nonyl phenol ethoxylate, isopropyl myristate and oleic acid into these alkyd resins. The diffusion behaviour changes as the temperature is raised through the glass transition temperature of the resin but is also sensitive to the chemical structure of the permeant. The diffusion deviates from simple Fickian behaviour in a number of cases and is indicative of plasticization influencing diffusion. Comparison of the average molecular size with the available free volume indicates that diffusion involves a complex co-operative motion in which the matrix plays an important role. Increasing the cross link density by post cure of the alkyd resin at elevated temperatures, reduces the free volume and decreases the ability for the large molecules to permeate in to the matrix. Oleic acid exhibits a two stage diffusion process; the second stage being associated with opening up of the resin structure as a result of acid catalysed degradation. The unusual behaviour of isopropyl myristate is attributed to a combination of its ability to adopt favourable conformations and a high degree of compatibility with the matrix. This study highlights the importance of the chemical structure of the permeant on its ability to enter and diffuse through the coating.