Peter Greenwood

Light Scattering in Pigmented Coatings:: Experiments and Theory

Abstract Small silica particles uniformly coated by a shell of titania, were produced by wet chemical methods. The pigments were dispersed in polymeric binders and were applied as paints to glass substrates. Total reflectance and transmittance were determined by integrating sphere measurements. Luminous and solar reflectance of paints containing the novel pigments were comparable with paints containing commercial titania. The spectral transmittance and reflectance of the paints were modelled by a four-flux radiative transfer theory. Theory and experiment could be brought into good agreement, if it was assumed that the particles were slightly porous. The studied pigments may find applications in sunscreens or in foils for daylighting and radiative cooling.

Aqueous silane modified silica sols: Theory and preparation

The purpose of this paper is to study methods of reacting the surface of the particles of silica sols with silanes, primarily gamma-glycidoxypropyltrimethoxysilane (GPTMS) and study some basic properties of the modified sols and the nature and structure of the silane groups attached to the particle surface. Design/methodology/approach - The surface of the silica particles was modified by reacting the silica sols with aqueous solutions of silanes, chiefly GPTMS. The presence and structure of silane groups on the particle surface were established by Si-NMR and C-NMR, respectively. Findings - Several silanes were studied but silica sols could be readily modified only with GPTMS and glycidoxypropylmethoxydiethoxysilane (GPMDES), most readily if the silanes were pre-hydrolysed in water. Higher degrees of silylation were preferably done by continuous addition of silane. Lower degrees of modification can be achieved at room temperature by the stepwise addition of the silane solution. The silylation of the silica surface with GPTMS significantly reduces the number of charged surface groups and silanol groups. GPTMS binds covalently to the silica surface and the epoxy ring opens and transforms into a diol. Silica sols modified with GPTMS and GPMDES are stable toward aggregation. Research limitations/implications - Only organo-reactive silanes were studied. Originality/value - This is the first work to study the modification by silanes of silica aquasols with high concentrations of silica. The silane modification can extend the use of silica to areas of applications previously inaccessible to silica sols.

Modified silica sols: titania dispersants and co‐binders for silicate paints

Purpose – The purpose of this paper is to investigate epoxysilane‐modified silica sols as surfactant‐free inorganic pigments dispersants and as co‐binders/reinforcing agents for silicate paints.Design/methodology/approach – The performance of epoxysilane‐modified silica sols as dispersants for titania was studied using a polyacrylate‐based dispersant as reference. Furthermore, the effect of the addition of silica sols, with or without silane modification, to potassium silicate on binder properties was investigated.Findings – Significant improvements were obtained in stability towards settling in water‐based titania pigments pastes and in light‐scattering efficiency (as much as 50 per cent) for the optimal size of the silica particle of 5 nm. The number of silane molecules per nm2 silica particle surface must exceed a critical value of at least 1 molecule of epoxysilane per nm2 particle surface. Additionally, improved stability towards gelling, water resistance and film‐forming properties of sol‐silicate b...

Novel nano-composite particles: titania-coated silica cores

Purpose - The purpose of this paper is to develop methods to produce white composite pigments consisting of a silica core with a titania shell. Design/methodology/approach - Silica cores were coated with titanium dioxide (TiO2) via forced hydrolysis of a solution prepared from titanium tetrachloride (TiCl4). The morphology, surface charge and particle size of obtained composite particles were studied. Findings - Dispersions of well-dispersed composite particles, having silica cores of uniform size in the range from 300 to 500 nm with a homogeneous titania coating are obtained. The coating thickness corresponded to 150-400 per cent by weight of titania based on the core. Modification of the silica core by incorporation of 1.5 aluminosilicate sites per square nanometre of core surface proves to be favourable in achieving a homogeneous coating on the silica core. Deposition of such titania coating is also favoured by agitating the dispersion well, keeping electrolyte content low, maintaining pH at 2.0 and the temperature at 75 degrees C during the coating process. Research limitations/implications - Only TiCl4 is used as titania source. In addition, only silica cores obtained by Stober synthesis are used while commercially available silica solutions made from sodium silicate are not used. Practical implications - The process offers a method of producing a white composite pigment with a narrow particle size distribution in order to maximise light scattering as well as using a core with lower density than the shell. This kind of particle would be of interest for coating applications and white inorganic inks. Originality/value - The developed method provides a straightforward process to produce well-defined composite particles.