Influence of multiple extrusions of blends made of polyethylene terephthalate and an oxygen scavenger on processing and packaging-related properties
We investigated the influence of multiple extrusions of poly-(ethylene terephthalate) (PET) blended with a polymer-based oxygen scavenger. PET and PET blended with an oxygen scavenger additive were extruded up to four times to polymer strands. They were chilled in a water bath and cut to granules. Between the extrusions, the granules were dried at (only) 60°C to avoid the oxygen scavenger losing its reactivity. The water content was up to 0.5 wt.-% and therefore up to factor 100 higher than the recommended water content for PET extrusion. For further analysis, films were extruded and bottles were stretch-blow-molded. Due to polymer degradation and the recycled PET blend viscosity being too low, this material had to be blended with virgin PET in order to be processable. We used 33 wt.-% recycled PET and 66 wt.-% virgin PET ratio. The impact of the thermomechanical stress by multiple extrusions was investigated by several test methods: analysis of the melt pressure in the extruder barrel during extrusion, intrinsic viscosity and melt flow rate, microscopy, differential scanning calorimetry, color value measurements (L*a*b* values), tensile testing, compression testing, and oxygen absorption measurements. The intrinsic viscosity reduced after four re-extrusions in a compounder from 0.82 dl/g to 0.35 dl/g for PET with scavenger and to 0.41 dl/g for pure PET. This, even though low, difference can be explained by the slightly higher PET degradation due to the oxygen scavenger. We found slightly better mechanical properties (yield stress, Young's modulus) for pure PET films compared to PET blended with an oxygen scavenger. At up to three extrusions, there was little influence on these properties. The extruded material with 3 wt.-% oxygen scavenger additive has an oxygen absorption capacity of 9.5 mg oxygen per 1 g of granules, i.e., the scavenger additive absorbed about 300 mg oxygen per 1 g of additive. Our results are relevant for in-house recycling processes of PET with oxygen scavenger.