PROCESSING | QUALITY
material batches with highly variable quality and varying residual stabiliser content. The approach of simply adding ‘enough’ stabiliser is unsuitable or too costly for a consistent circular economy.” It said: “There is also an upper limit for the
compatibility of stabilisers with the polymer. This is because the plastic should not only have a second, but also a third, fourth etc life. During each life cycle, the stabilisers are used up as intended, with their by-products remaining in the plastic. This makes it even more important to add only the necessary amount of stabiliser during recyclate production. Online rheological tests accelerate the process of achieving exact stabilisation and optimising costs.” The institute said that meaningful information on
the processing behaviour of plastics can be obtained using an in-line rheometer. This technol- ogy measures the flow curves of both shear and elongational viscosity. Fraunhofer LBF carried out initial tests in which a
marginally stabilised virgin PP served as a model polymer. A clear correlation was established between the viscosity values and the stabiliser concentration or the viscosity values and the molecular weight. Then one PIR and one PCR PP material were analysed. The PIR PP came from start-up material and
sprues from injection moulding. As this material still had a high stabiliser content, no significant improvements in preserving the molecular weight could be achieved with stabiliser additions of more than 0.1%. “The flow curves for the stabiliser additions investigated are therefore almost on top of each other,” said the institute. When testing PCR PP, the antioxidants were
Right: VTT is using an adaptive process control for rheological upgrading of recycled
plastics in its advanced mechanical recycling pilot plant
Flow curves of the shear viscosity with different stabiliser content for a post-consumer recycled PP Source: Fraunhofer LBF
consumed to a high degree, which was noticeable in a low viscosity/flow curve. For optimised stabili- sation and minimised damage to the recyclate, Fraunhofer LBF said a loading of 0.5% additive is required (see chart above). “The online rheological measurements can be used to draw meaningful conclusions about the number of antioxidants that should be added to the respective batch of used plastic,” said Fraun- hofer LBF, which has made this in-line rheology technology available to Fraunhofer LBF project partners. “Plastics processors thus receive immedi- ate information on the effect of a process stabiliser and can produce their formulation more cost-effec- tively and make it more profitable.”
Viscosity Additionally, the flow curves of the elongational viscosity reflect the stability of the melt, it said. “In contrast to shear viscosity, extensional viscosity is very sensitively influenced by the fibre content and fibre distribution. As melt stability is an important criterion for the use of a given compound for blow moulding processes, online rheology can also support the compound developer in this respect.” Finland-based VTT Technical Research Centre
is offering in-line rheology technology as an answer to the problem of viscosity variations when com- pounding recyclate. The institute provides industry partners a range of technical expertise in plastics sustainability R&D, covering mechanical and chemi- cal recycling as well as bio-based polymers and other compounding areas. Its mechanical recycling work includes: improved recyclability of plastic products by design and reuse; optimised use of recyclates in plastic products; and pre-treatment of waste through optimised sorting and washing for various recycling routes.
24 PLASTICS RECYCLING WORLD | November/December 2024
www.plasticsrecyclingworld.com
IMAGE: VTT
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