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Metal effect pigments showing reduced flow-line vulnerability

Problem

Experimental comparison of the flow-line vulnerability of a) conventional platelet pigment, and b) pigment with novel geometry
Lupe

Bei einer großen Zahl von Kunststoffprodukten werden dem Kunststoff während der Verarbeitung Metalleffektpigmente zugesetzt, um ein hochwertiges optisches Erscheinungsbild zu erzielen. Sichtbare Fehler stehen diesem Ziel entgegen.

Conventionally, metallic appearance is generated due to the orientation of planar metal effect pigment particles in the flow direction of the polymer melt. Their orientation is usually parallel to the part surface in order to reflect light to the viewer. Nevertheless, where polymer fronts merge, the particle orientation is disturbed, resulting in locally enhanced light scattering and thus creating dark streaks in the solidified polymer part, called flow lines (Fig. a).

This diminishes the design freedom for injection moulded polymer parts if flow lines have to be shifted to non-visible areas under aid of previous simulation. Extrusion of hollow profiles often leaves no choice other than accepting flow lines. Those are generated by the holders of the extrusion head mandrel, because polymer passes around these holders and the flow fronts converge afterwards.

Industrial users are looking for a possibility to reduce flow lines in injection moulding and extrusion.

Goal

The aim of the development is to produce metallic effect pigments with a reduced tendency to form flow lines (Fig. b).

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Nils Demski, M.Sc.
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