Gel-like Dispersions Based on Cellulosic Derivatives and Castor Oil Applicable as Biodegradable Lubricating Greases

This study deals with the design of gel-like dispersions formulated using castor oil and ethyl cellulose/α-cellulose or ethyl cellulose/methyl cellulose blends, potentially applicable as bio-lubricating greases. The influences of ethyl cellulose molecular weight and cellulosic derivatives concentration ratio (W) on the thermal behaviour, linear viscoelasticity and mechanical stability were studied. Experimental results show that small-amplitude oscillatory shear (SAOS) functions are not significantly influenced by ethyl cellulose molecular weight below a critical threshold value (Mw<70000 g/mol). On the contrary, a significant increase in SAOS functions is noticed using ethyl cellulose with Mw values higher than the critical one. Temperature does not have a significant influence on SAOS functions, which is opposite to the behaviour found with standard lubricating greases. On the other hand, all oleogel formulations studied display much higher decomposition temperatures than standard lubricating greases, independently of both ethyl cellulose molecular weight and concentration ratio. Formulations containing ethyl cellulose/methyl cellulose blends show excellent mechanical stability enhanced by increasing ethyl cellulose molecular weight. A decrease in the values of the linear viscoelasticity functions down to a minimum value was found by increasing ethyl cellulose/α-cellulose or ethyl cellulose/methyl cellulose weight ratios (W) up to a critical value. Above this, the linear viscoelastic functions increase with W, at temperatures in the range 0-75oC, and continuously decrease at higher temperatures, i.e. 125oC.