Recycling of blends made of polypropylene and an iron-based oxygen scavenger - Influence of multiple extrusions on the polymer stability and the oxygen absorption capacity

An often used oxygen scavenger is a blend of iron powder and additives dispersed in a polymer carrier matrix. It is applied as an additive for separate layers of multilayer film structures which are produced by coextrusion. Not much is known about the influence of multiple extrusions, for example during recycling processes, of these materials on polymer properties, film properties and process stability. Therefore the aim of this study was to assess these issues. The focus was on polypropylene because it is a common blend partner for iron-based oxygen scavenger additives. Pure polypropylene and blends of polypropylene with approximately 20 wt.-% iron-based oxygen scavenger additive (SHELFPLUS® O2 2710) were extruded up to ten times at two temperature profiles (T1: 160-230 °C; T2: 210-300 °C). Polymer degradation was evaluated by different means of measurement: MFR, DSC, colorimetry, IR-spectroscopy and oxygen absorption. The results indicate that polypropylene and polypropylene with dispersed oxygen scavenger degrade by the impact of extrusion which can be seen by increased MFR and crystallinity and color changes. Furthermore the iron particles did undergo changes regarding color and oxygen scavenging capacity. The absorption capacity at 23 °C was reduced by one third from 39 to 48 mg oxygen per one gram scavenger additive after one extrusion to 26-35 mg oxygen per one gram scavenger additive after 9 and 10 extrusion cycles, respectively. Overall the iron based oxygen scavenger has a lower impact on degradation than the temperature and the number of extrusions.