Metal-catalyzed oxidation of polybutadiene in oxygen scavenging packaging applications
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To better characterize the fundamentals of oxygen scavenging as a means to prepare high oxygen barrier polymer films, the oxidation of 1,4-polybutadiene, in the presence of a transition metal salt catalyst, cobalt neodecanoate, was studied at 30oC. 1,4-Polybutadiene was subjected to several purification steps to remove oxidation antioxidants that are typically added during the industrial scale preparation of this polymer. The importance of the purification method and residual antioxidant on oxidation was determined. Oxygen uptake of 1,4-polybutadiene films was measured as a function of cobalt neodecanoate concentration. In these samples, oxygen mass uptake values as high as 15 weight percent were observed, and the oxidation process occurred over approximately one week. Oxygen mass uptake was measured in 1,4-polybutadiene films of different thicknesses undergoing cobalt-catalyzed oxidation in air at 30ºC. FTIR and XPS analysis suggest that the oxidation was heterogeneous, with the film surface being highly oxidized and the film center being less oxidized. Interestingly, the oxygen uptake exhibited a maximum with catalyst loading, which is believed to be related to the heterogeneous nature of the oxidation process. Films thicker than approximately 50 µm showed a decrease in oxygen uptake per unit polymer mass as film thickness increased, while oxygen uptake per unit film area remained independent of thickness, suggesting that oxidation was heterogeneous and proceeded essentially as an oxidized front penetrating into the film from the surfaces exposed to oxygen. In contrast, oxidation in thin films appears to proceed homogeneously, with oxygen uptake per unit mass being essentially independent of thickness. The dividing line between thick and thin films, the so-called critical thickness for oxidation, appears to be about 28 µm. In oxidized samples, oxygen and nitrogen permeability decreased by more than two orders of magnitude relative to permeability values in unoxidized samples. A two-phase model was used to describe the permeability data. Experiments were also conducted at different temperatures and oxygen partial pressures. Thick films oxidized at 45ºC showed heterogeneous oxidation similar to that reported above, while films oxidized at 5ºC showed a much longer oxidation time scale and higher oxygen mass uptake value. SEM images demonstrated that the structures of cross sections in films oxidized at different temperatures were also different; the oxidized layer structure was not observed in samples oxidized at 5ºC. Oxygen partial pressure experiments were conducted under the conditions that environmental oxygen content was less than 21%. It is observed that increasing oxygen partial pressure leads to faster oxidation kinetics and higher oxygen mass uptake in polybutadiene films.