MATERIALS | POLYOLEFIN ADDITIVES
Researchers at Fraunhofer LBF are using online rheometry to speed up
development of optimally stabilised compounds
Image: Fraunhofer LBF
method for accelerating development of antioxi- dant formulations. Plastics degrade by auto-oxida- tion when in contact with air, a process initiated by elevated temperature or light that propagates as a radical chain reaction, causing cleavage of the polymer chains. The Fraunhofer researchers say the polymer chains are primarily attacked by the OH-radical resulting in the formation of hydroper- oxide moieties, which trigger follow up reactions leading to OH-radical regeneration. For optimum protection of the polymer, the
researchers say two different types of antioxidant must be added. A primary antioxidant, often containing a phenolic structure, quenches the OH-radical. A secondary antioxidant, consisting of sterically hindered alkyl-derivatives of functional groups such as phosphites or thioethers, react with the hydroperoxide without OH formation. Both types of antioxidant act in a synergistic way and a typical commercially available stabiliser package containing both antioxidants in equal amounts was used in the institute’s experiments. Commercially available virgin plastic grades are typically formulated with appropriate stabiliser packages for the expected use for reasons of resource efficiency and economy. As has been mentioned previously, when recycling plastics some of the stabilisers will have been depleted during the previous life cycle. Fraunhofer LBF says the traditional way to optimise stabiliser content is to compound a series containing varying amounts of antioxidants, then characterise them offline using tests such as melt volume rate (MVR, DIN 1133-1) or oxidative induction time (OIT, ASTM D3895-19). As a result, the first reliable results are obtained only after the compounding step has been completed. The LBF researchers aim to gain an indication of
the efficacy of the stabiliser content during the compounding step. To achieve this, the viscosity of
24 FILM & SHEET EXTRUSION | December 2023
the melt is used as a response and recorded while varying the recipe using an online rheometer fitted behind the screw tips of the twin-screw extruder. It measures the flow curves of the shear as well as the elongational viscosity. First experiments were carried out on a minimally stabilised virgin polypropylene (PP) with the amount of stabiliser added varied at selected screw speeds. The reduced process-related degradation was immediately reflected in an increase in viscosity in the flow curves. Above a certain additive level there was no further increase in viscosity, meaning that for the actual processing conditions the stabiliser concentration had reached the limit above which no further improvement could be achieved. The researchers say the experiments show that online rheology provides the formulation devel- oper with high value information regarding the efficacy of a processing stabiliser during com- pounding. They add that, because the flow curves of different polymers are not identical, their information content is much higher than a single value from an MVR measurement. It is also possible to include the flow curves of the elongational viscosity in the evaluation. The Fraunhofer team concludes that, when
supported by an appropriate AI-based system, online rheology could provide a promising tool to implement stabilisation during the production of recyclates, offering the ability to make real-time adjustment according to the ageing stage of recycled feedstock.
CLICK ON THE LINKS FOR MORE INFORMATION: �
www.doverchem.com �
www.clariant.com �
www.siigroup.com �
www.plasticadditives.basf.com �
www.baerlocher.com �
www.lbf.fraunhofer.de
www.filmandsheet.com
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