Aging Effects in Suspensions of Silica Particles

In our study of the aging phenomena in silica suspensions we immersed precipitated silica particles in methyl-terminated PDMS (silicone oil), mineral oil, and glycerol/water and measured their viscoelastic properties as a function of time. We found that the samples based on silicone oil, initially of a pasty consistence, became fluid-like after a period of about three weeks. Aqueous samples aged much faster than the oily samples. Samples containing mineral oil did not exhibit any changes in viscosity and elasticity. In this paper we discuss a possible mechanism for these aging behaviors. INTRODUCTION Fumed silica forms a viscoelastic gel when suspended in silicone oil and in mineral oil, and these suspensions are widely used as filling compounds for fiber-optic cables [1]. Because these filling compounds provide mechanical support for the optic fibers, their viscoelastic properties are of great interest to researchers. The viscoelastic properties of a gel are determined largely by the interactions between the particles that form the gel, and the chemical environment of these particles plays an important role in these interactions. The surface of native silica particles has a large amount of silanol groups (Si-OH), which are hydrophilic. When immersed in a hydrophobic base fluid, silica particles form a gel network because it is energetically favorable for them to form hydrogen bonds with each other. When immersed in a polar medium, however, molecules from the base fluid form a solvation layer around the silica particles due to hydrogen bonding, preventing silica particles to bond with each other, and the suspension is a viscous fluid [2]. For obvious reasons, the stability of the viscoelastic properties of a fiber-optic cable filling compound over time is also of great importance. Reports on the aging behavior of fumed silica in silicone oil have been mixed. Some suspensions showed no significant aging behavior [1,3], while others softened with time [4,5]. In our study of silica gels, we discovered that suspensions of precipitated silica in silicone oil showed drastic aging behavior, turning from a gel to a fluid in a few weeks. In this paper, we report our preliminary observations of this aging behavior and suggest a possible mechanism for it. For comparison, we also used mineral oil and a glycerol/water mixture as base fluids. EXPERIMENTAL DETAILS The precipitated silica particles Zeodent 165, provided by Huber Engineered Materials, are highly polydisperse and amorphous, with a particle diameter ranging from 10-16 μm . The P7.11.1 Mat. Res. Soc. Symp. Proc. Vol. 790 © 2004 Materials Research Society