Optical properties of metalized cenospheres

This report describes the optical properties of metal-coated cenospheres, used as a pigment for low emissive camouflage paint. A cenosphere is a lightweight, inert and hollow sphere. Due to their unique combination of spherical shape, relatively high strength, good thermal and acoustical insulating properties, many applications can benefit from this material. For military applications it is of interest to paint, e.g. a vehicle, with a low emissive paint in order to adapt the apparent temperature of the body to that of the background. A decrease in emissivity can be achieved by adding some heat reflecting pigment, like metal powder, to the paint. One advantage of using cenospheres as a pigment in such a camouflage paint is due to their light weight, since they are hollow, which may be of interest for special applications when the weight should be minimized. Two different types of techniques were used in order to cover the cenospheres with metals. The copper coating was achieved by using reactive magnetron sputtering. For the silver coating a wet chemical method was used. The pigment was then mixed with a commercial polymer binder, Acronal A706. Reflectance measurements were made, first on the metal-coated cenospheres and then on the mixed paint, both in the visual and in the IR region.