Yield Properties of Wax Crystal Structures Formed in Lubricant Mineral Oils

Abstract

On being cooled to temperatures well below the onset temperature for wax crystallization, mineral oils undergo a transition from viscous to elastic-solid (G‘ ≫ G‘ ‘) behavior, referred to as gelation. Gelation is caused by the formation of a wax crystal network and is distinct from a glass transition. We investigate the yield behavior of gelled mineral oils, linear elastic and fracture yield limits, as a function of the amount of crystallized wax and the particle size distribution of the wax by studying yield behavior as a function of temperature. By cooling the mineral oils at different constant cooling rates, we altered the surface-area-average crystal length (l s) in the wax crystal network. The observed yield behavior is compared to that reported for gels formed from flocculated or aggregated colloidal particles; gelled mineral oils exhibit a weak-link type of scaling in contrast to the strong-link scaling exhibited by flocculated colloidal gels.

Influence of the initial cooling temperature on the gelation and yield stress of waxy crude oils

“…Davenport and Somper (1971), for example, showed that the reduction of temperature enlarges significantly the material yield stress. The increase of yield stress, the storage modulus, G′, the loss modulus, G″, and the viscosity with the temperature reduction was also observed by several authors (Wardhaugh and Boger 1987;El-Gamal and Gad 1998;El-Gamal 1998;Remizov et al 2000;Chang et al 2000, Webber 2001Venkatesan et al 2003;Kané et al 2004;Visintin et al 2005;Chen et al 2006;Zhang 2007, 2010;Lopes-da-Silva and Coutinho 2007;Lee et al 2008;Li et al 2009;Oh et al 2009;Hasan et al 2010;Dimitriou et al 2011;Ghannam et al 2012;Rønningsen 2012). The rise of such properties was attributed to wax solubility reduction in oil as the temperature decreases (Venkatesan et al 2003).…”

“…Most of them concluded that the higher the cooling rate, the lower the material yield stress (Rønningsen, 1992;Venkatesan et al 2005;Chen et al 2006;Lin et al 2011) and the lower the gelation temperature (Visintin et al 2005). An opposite behavior was observed by Webber (2001) for mineral lubricant oils that showed an increase of yield stress with cooling rate. Lee et al (2008) proposed an interesting discussion to explain the physical phenomena that take place during start-up of gelled oils.…”

“…The effect of the static cooling rate was also investigated by several authors (Rønningsen 1992;Webber 2001;Kané et al 2003;Venkatesan et al 2005;Visintin et al 2005;Chen et al 2006;Lee et al 2008;Chang et al 2000, Lin et al 2011. Most of them concluded that the higher the cooling rate, the lower the material yield stress (Rønningsen, 1992;Venkatesan et al 2005;Chen et al 2006;Lin et al 2011) and the lower the gelation temperature (Visintin et al 2005).…”

Causes of Floccules Formation in Hydro-Treated Lubricating Base Oil

“…At lower temperature, the crystal growth depends largely on the molecular weight distribution, concentration of paraffinic component and the composition of the oil (Webber, 2001). The average molecular weights of the floccules, KH150BS and filtrated oils were firstly measured and the results were given in Table 2.…”

“…It indicate that its solubility decreased with an increase in average molecular weight of the floccules, so the floccules separated at higher experimental temperature were more difficult to dissolve in KH150BS. Webber (2001) pointed out that the hydrocarbon which had large molecular weight easily shaped crystals and deposited in the oil. Hence, it can be primarily concluded that the floccules were some hydrocarbon mixture which had larger average molecular weight and lower solubility compared with KH150BS.…”