Aluminium flake alignment

A number of application test methods are discussed covering black jetness measurement, aluminium flake alignment and the usefulness of zeta potential is demonstrated in assessing dispersion stability. 

The use of microgels has been particularly successful in solvent-borne and water-borne metallic basecoats used in motor car production. Here the metallic visual effect requires viscosity control during the high film shrinkage that occurs on solvent evaporation, so that the metallic aluminium flakes align parallel to the substrate in the dry film. 

Since most aluminium formulations contain waterbased thickeners to avoid settlement of the large particles, the dispersant/thickeners/resin solids interaction is important in producing the optimum rheology ciuve whilst achieving good flake alignment at the highest acceptable resin solids. 

Circulation studies of water-borne metallic basecoats demonstrate a few reasons why specular reflectance is lost during circulation. The flow induced stress of circulation reduces flake size and produces cycles in liquid surface tension. Surface tension in turn controls amount of picture framing and film thickness. If rust is present in the circulation system, it can react with the paint resins, creating gel lumps which under circulation trap the metal flake hindering flake alignment. Smaller flakes, thicker film thickness and poor aluminium alignment all reduce specular reflectance within the sprayed basecoat paint film [109-115]. 

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