QUALITY CONTROL | TECHNOLOGY
important contribution to future recycling process- es by making them more transparent, more control- lable, and more stable in quality.”
Polyamide study Bernd Steinhoff and Hans Kothe at the Fraunhofer Institute for Structural Durability and System Reliability LBF in the Department Engineering Materials and Components have studied inline rheological measurements during PA66 recyclate processing. The aim of the research was to adjust the molar mass by means of coupling additives or additives for controlled chain cleavage, using online rheology for process control. The researchers reported that varying propor-
tions of bisoxazoline were mixed into a polyamide recyclate (rPA66) during extrusion. “The chosen coupler features two binding sites (imide groups), so that its reaction with the –NHâ‚‚ or –COOH end groups results in a linear build-up of the chains,” said Steinhoff.
As shown in the first chart, the viscosity initially rises with increasing coupler content, up to a proportion of 2%, but at 4% coupler, the viscosity decreased instead of increasing further. “Presum- ably, above 2%, all end groups are occupied by the coupler, so that any further build-up of molar mass would require the imide groups of two coupler molecules to react with one another. This hardly occurs, since the imide does not react with itself in a neutral, dry environment. The resulting excess additive can act as a plasticiser and thus lead to the observed reduction in viscosity. This explanation is supported by the molar mass values determined by GPC,” the researchers explained.
Chart 1: Measured data points of the flow curves for the processing of the post-industrial rPA66 with different proportions of the linear coupler. The Cross-Fit viscosity model has been fitted (solid curves) Source: Fraunhofer LBF
The second chart shows this in another way, with
a plot of zero-shear viscosity (a characteristic quantity derived from the viscosity model for the limiting case of a shear rate of zero) and the weight-average molar mass (Mw) as a function of the bisoxazoline content. “A monotonic increase in both zero-shear viscosity and molar mass with the bisoxazoline content can be seen for values below approximately 2%. For higher bisoxazoline con- tents, the zero-shear viscosity decreases again, while the molar mass remains almost constant.” Online rheology allowed the effects of adding
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