Abstract
This study investigates crystallization, melting and glass transition of Li- and Ca-12-hydroxystearate greases in relation to the pour point of the corresponding oils. The base oils for the greases are mineral oil, polyalphaolefin, alkylated naphthalene, propylene glycol, and trimellitate. For the mineral oil-based greases the crystallization temperature Tc increases and the melting temperature Tm decreases upon addition of thickener. The pour point of the mineral oil then is 3 K below Tc and does not properly define the lowest application temperature for mineral oil (MO) based greases. Both thickeners induce a small increase of the glass transition temperature (1–3 K) of the synthetic oils polyalphaolefin, alkylated naphthalene, propylene glycol. The pour point of the base oils correlates well with the onset of the glass transition in the corresponding grease indicated by a sharp increase in grease viscosity. Pure trimellitate with unbranched alkyl chains does not crystallize upon cooling but shows noticeable supercooling and cold crystallization. As the percentage of thickener in corresponding greases increases, more oil crystallizes upon cooling 20 K above the crystallization temperature of the trimellitate without thickener (−44 °C). Here, the thickener changes the crystallization behavior from homogeneous to heterogeneous and thus acts as a crystallization nucleus. The pour point of the base oil does not provide information on the temperature below which the greases stiffen significantly due to crystallization.
Highlights
Concerning the lubrication conditions in operation, the higher viscosity of the base oils and higher consistency of the greases at low temperatures, the friction factor increases slightly [1]
Crystallization and Melting of Lubricating Greases Based on Mineral Oil (Group I)
This study investigates the influence of thickener type and concentration on the crystallization, melting and glass transition of greases made from three different base oil types (Groups I–III) using rheological and thermoanalytical measurements
Summary
Concerning the lubrication conditions in operation, the higher viscosity of the base oils and higher consistency of the greases at low temperatures, the friction factor increases slightly [1]. Ester oils crystallize to the extent of blocking the rheometer [3] It would block a tribological contact during operation. Practical test methods such as the low-temperature torque test for wheel bearings (ASTM D4693 [4]) or ball bearings (ASTM D1478 [5]) determine the suitability of greases for low temperatures. These standards are close to practical experience, they do not provide information on whether the base oil in lubricating greases precipitate crystals and change their flow properties upon cooling. Some oils can crystallize at temperatures above the pour point if given enough time [3]
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