(317r) The Effect of Metal-Catalyzed Oxidation on Diffusion in Polybutadiene

Although the use of polymers in packaging applications is growing rapidly, polymers are not replacing glass and metal in certain applications because they do not have sufficiently low permeation properties to oxygen. The presence of oxygen at even ppm levels can cause certain foods and beverages to rapidly experience deleterious oxidation or other aerobic degradation processes. Incorporation of oxygen scavengers into otherwise passive barrier polymers can markedly reduce oxygen permeability and extend the shelf-life of oxygen sensitive products to levels rivaling that of glass and metal packaging over time spans of several years.

Effective oxygen scavengers include polybutadienes (PBs) doped with low levels (e.g., 100 ppm) of a transition metal salt. These materials have been shown to sequester significant amounts of oxygen (as much as 10 weight percent of the polymer) while retaining mechanical strength and transparency. In this study, the oxygen scavenging properties of PB materials doped with cobalt neodecanoate were studied. A non-invasive oxygen sensor was used to measure oxygen sorption in PB films as a function of time, and the effect of catalyst concentration and film thickness on initial rates of oxidation and equilibrium mass uptake were reported. Oxidation of these materials was also documented with FTIR.

The sorption experiments suggest significant modification of the polymer network occurs during oxidation. Extensive oxidation and cross-linking on the film surface layers reduce the ability of oxygen to diffuse into the bulk polymer. Specifically, CO₂ permeability was found to decrease by two orders of magnitude over the course of the oxidation experiment (e.g., 30 days), and tapping mode atomic force microscopy phase profiles show the formation of a highly oxidized boundary layer. XPS was also used to characterize the heterogeneous oxidation of PB films.