(111e) The Effect of Hydrophobic Surface Modification on Bulk Cohesive Strength

Current industry trend is to add small amounts of very fine or nano-particles to already existing products in hopes that the combination of particles will exhibit some enhanced behavior. A science is developing around the addition of these small particles and the appearance of certain chemical behaviors that they cause in the bulk. Techniques are also being developed that allow surface modification of many products. It is becoming routine to coat particles with hydrophobic or hydrophilic coatings to achieve certain behaviors in solutions or gels. However, little attention is paid to the effect that these surface modifications and small particle interactions may have on the cohesive nature of the bulk particulate material. One intriguing question is: can methodologies or theories be postulated to predict the bulk cohesion of a given particle assembly after a prescribed surface modification or the addition of custom nano-particles? This paper examines the strength behavior of hydrophobic particles in moist environments. At times, the effect of liquid addition to hydrophobic surfaces dramatically reduces the bulk unconfined yield strength. This is likely due to the reduction in inter-particle friction from hydrodynamic effects. However, some particle sizes and degrees of hydrophobicity show an increase of cohesive behavior as moisture is added to the system. Finer particles tend to first exhibit a reduction in strength followed by an increase asmoisture content is increased. In any event, there is a non-linear relationship between particle size, liquid binder content, and the hydrophobic nature of the surface which is examined in this paper.

KEYWORDS: yield strength, hydrophobic, force chains, attenuation, CT scan