Metal effect pigments for reducing flow line visibility

摘要

Metallic appearance of injection moulded polymer parts is favoured in many industries, e.g. packaging or automotive. Filling polymers with metal effect pigments avoids additional coating steps, however, visible inhomogeneities in the metallic appearance suggest low part quality. Metallic appearance is generated due to the orientation of planar pigment particles in the flow direction, usually parallel to the part surface. Nevertheless, where polymer fronts merge, this orientation is disturbed, resulting in light scattering and creating dark streaks called flow lines. This paper aims at reporting novel tetrahedron shaped metal effect pigment particles for reducing flow lines, as well as novel particle manufacturing methods. Two manufacturing methods for tetrahedral particles were examined. In the first method, a file tool is used featuring a cutting surface consisting of pyramidal cutting edges with tetrahedral interstices in between. The file is linearly moved across the surface of bulk AI99.5 under normal force. The aluminium fills the tetrahedral interstices, forming tetrahedral particles, which are then removed. In the second manufacturing method, molten tin is poured onto an aluminium coated silicone rubber patch featuring tetrahedral recesses, and covered by a polyimide lid. After cooling, the particles are removed using a brush. In flow tests, a compound featuring tetrahedral particles (edge length 75-90 μm) in a silicone fluid matrix (viscosity 97 N·s·m~(-2)) was compared to a second compound featuring conventional planar particles (diameter 100 μm) in the same matrix. Both compounds were injected into transparent moulds, letting polymer flow fronts merge and granting in-process observation of the flow line visibility during flow. Particle orientation in the flow line region was examined in a 3D x-ray tomoscopy (time-resolved tomography). The examined tetrahedral particles did not create the characteristic flow lines of conventional planar pigment particles. By the described manufacturing methods, the manufacturability of tetrahedral metal particles was proved in the range of grams.