Investigation of the Effect of Particle Size on Dispersion Stability and Viscosity in Kaolin Particles-Doped Bio-Based Palm Nanolubricants

The addition of various nano- and micro-sized particles into lubricants leads to improvements in the tribological and thermal properties of the produced nanolubricants. However, dispersion stability is essential in the preparation of nanolubricants. Ensuring dispersion stability contributes to the stability of various properties of the lubricant, such as viscosity, wear, and thermal stability. In this study, kaolin nanoparticles were ground using an axial mill to obtain three different (950 nm, 650 nm, and 350 nm) particle sizes. It was subjected to oleic acid modification to improve kaolin nanoparticle dispersion in the lubricant. Oleic acid-modified nanoparticles were doped to palm oil at rates of 0.125%, 0.25%, 0.5%, 0.75%, 1.0%, 1.25% and 1.5% by weight. The dispersion stability of the nanolubricants was determined using sedimentation and ultraviolet–visible spectroscopy tests, and their viscosities were determined using a rotational rheometer. It has been determined that kaolin-palm oil-based nanolubricants maintain dispersion stability for 14 days. It has been observed that lubricants have good dispersion stability up to 0.75 wt% nanoparticle concentration and decrease above this rate. It was concluded that nanoparticles with small particle sizes exhibited better dispersion stability than large particles. It was concluded that the increase in nanoparticle size reduces the viscosity of lubricants. Moreover, the viscosity value of the kaolin nanoparticle-doped lubricant with a size of 950 nm at the 1.5% doping rate was determined as 69.7 mPa s, while for the 650 and 350 nm particle sizes, these values were determined as 70.93 and 72.04 mPa s.