Titanium dioxide is the main white pigment used in the paint industry. It is the most efficient broad-band light scattering material due to the high refractive index and its ability to scatter light across the entire visible range of the electromagnetic spectrum. However, the high production costs and the high carbon footprint associated with this compound constitute a motivation for investigating alternative materials. Polymeric particles have the potential to become a competitive alternative considering both their physical properties and the comparatively low production costs. The objective of this work was the identification of a simple and cost effective system for the production of bio-mimetic multi-voided polymeric particles, to be introduced in paint formulation as a partial replacement for the industrial white material titanium dioxide.
Nature does not rely on titanium dioxide to produce striking examples of white and is particularly ingenious in designing structures that are capable of maximising the amount of scattering in very little material, producing a remarkable white in very thin tissues. Some of the most relevant structures displaying exceptional white are the scales of the Cyphochilus and Lepidiota stigma beetles, consisting of isotropic networks of rod-like filaments of chitin and air-filled voids, and the foam-like structures found in the feathers of the Garrulus glandarius (Eurasian jay) bird, consisting in spherical air-filled voids within a keratin matrix. Those were identified as the target structures for the synthetic work. An effective way of producing porous structures in polymer materials is based on the possibility of inducing phase separation in an initially homogeneous polymer solution. The phase separation process can be initiated by the introduction of a non-solvent (NIPS) into the system and can proceed by a mechanism of nucleation and growth, producing closed pores within a polymer matrix resembling the foam-like structures of the Garrulus glandarius, or by spinodal decomposition, producing interconnected polymer-pore domains, similar to the networks found in the Cyphochilus and Lepidiota stigma beetles’ scales.
Polystyrene was the polymer selected for the synthetic work, due to a good compromise between costs, refractive index and a large selection of good solvents. The first attempt at fabricating polystyrene multi-voided particles by NIPS involved a simple ternary system with tetrahydrofuran as the solvent and deionised water as the non-solvent. The process was successful in achieving porosity, but particles size and shape were difficult to control. The fabrication process was upgraded by employing an Ink-Jet apparatus, capable of reducing and standardising the size of the polystyrene solution droplets. The Ink-Jet provided a way to control particles’ shape and size, but presented a fatal flaw in the fact that the production was limited to a very small amount of material and the process was not industrially scalable. In order to overcome these limitations, a modification of the fabrication process was necessary, whereby the ability to control shape and size of the polymer solution droplets would not rely on the mechanical capabilities of the equipment employed, but rather on the intrinsic properties of the chemical system. An emulsion system was considered, where the polystyrene in toluene solution was finely dispersed into a continuous phase of deionised water. The spherical shape of the droplets was achieved by the action of the surface tension and the droplets’ size could be controlled by the level of shear applied to the emulsion mixing. The emulsion method allowed the production of particles with the desired specifications and was a scalable process. The scale up was performed in the AkzoNobel® laboratories. The polystyrene multi-voided particles, along with titanium dioxide and the extender Ropaque, were implemented in paint formulations. The paints were spread on substrates to produce paint films of a desired thickness. The reflectance of the paint films was measured using a spectrophotometer and their density calculated. The reflectance and density data allowed the calculation of the scattering coefficient of the paints. A factorial design analysis of the scattering results was performed in order to compare the scattering power of the polystyrene multi-voided particles with those of titanium dioxide and Ropaque, and to highlight any synergistic effects between these materials. The analysis concluded that polystyrene multi-voided particles have a scattering power of 57-60% that of the Ropaque extender and of 6-8% that of titanium dioxide.
