Water can be used as an ecological lubricant base if it is possible to select additives which can beneficially modify its tribological and corrosion properties. Additionally, those additives should not be harmful to human health and the natural environment. These conditions limit or even eliminate the possibility for the application of the additives used in traditional oil bases as they are insoluble in water and often toxic. Alkyl polyglucosides (APGs) have been suggested as additives improving lubricating properties of water. They are biodegradable and do not have to be recycled. They exhibit surface activity. They produce micelles at low concentration and lyotropic liquid crystals at high concentration. Two types of alkyl polyglucosides differing in alkyl chain lengths and degrees of polymerization were used in this investigation. Tribological tests were carried out using a ball-on-disc T-11 tester. The balls were made of steel, whereas the discs were made of steel, aluminium oxide, zirconium oxide, polyamide and poly(methyl methacrylate). The description of the device and the methods has been given in the literature (Szczerek and Tuszyński in TriboTest 8:273–284, 2002). The addition of APGs improves the lubricating properties of water. The relative decrease in motion resistance and wear depends both on the type of friction couple and on the kind of alkyl polyglucoside used. The tribological test results obtained were correlated with the activity of APGs measured as wettability of friction couples by their solutions.
The objective of this study was to formulate new ecological lubricating substances, primarily water-based, and to verify their tribological and physicochemical properties. Initially, simple binary solutions were investigated. Then, various additives were added depending on application targets. Two alkyl sulfates were selected as additives modifying lubricating properties of water: sodium lauryl sulfate (SLS) and ethoxylated sodium lauryl sulfate (ESLS). They have an identical hydrophobic part in the form of an alkyl chain consisting of 12 carbon atoms. The SO 4 2-anion forms the hydrophilic part in SLS molecules, whereas an ESLS molecule also contains two mers of ethylene oxide which cause an increase in its hydrophilicity relative to SLS. Both SLS and ESLS exhibit high surface activity measured by their surface tension. Micelles form in aqueous solutions of alkyl sulfates at low concentrations of the order of 1%, whereas the presence of liquid crystalline phases can be found at the concentrations of 40 and 70%. High surface activity and formation of structures in the solutions (micelles, mesophases) formed the basis for application of the compounds as additives modifying lubricating properties. Tribological properties of aqueous solutions of alkyl sulfates were verified with a four-ball machine (T02 tester) at a constant load of 2 kN. The values of friction coefficient (l) were a measure of motion resistance, while the wear scar diameter (d) was a measure of wear. Alkyl sulfates significantly improve tribological properties of water. The coefficient of friction decreased sixfold and the wear scar diameter decreased by as much as twofold relative to the base. Non-monotonic changes in the tribological properties measured were observed as a function of concentration of additives. An attempt was made to relate those atypical changes with the presence of micelles and mesophases in both the surface phase and the bulk phase. In the model proposed the whole concentration range was divided into four areas in which tribological properties correspond well with physicochemical properties, particularly with the structures formed in solutions and at the interface.
Amphiphilic compounds composed of a hydrophilic part and a hydrophobic alkyl chain were investigated. The compounds-sodium lauryl sulfate (SLS) and ethoxylated sodium lauryl sulfate (ESLS)-exhibit strong affinity for solid surfaces and form liquid crystalline structures in water. It is expected that they may become effective additives that could significantly modify antiseizure properties of water. They may extend the possibilities for applying water as an ecological base for lubricating substances. Rheological and structural X-ray studies as well as measurements of conductance and wetting angle were carried out. Their aim was to verify and validate the structures being formed in aqueous solutions of the compounds studied. Tribological studies were conducted to measure friction torque as a function of linearly increasing load by means of a four-ball apparatus. The tests were followed by measurement of the wear scar diameter on the balls. Based on the results obtained, antiwear properties were characterized by seizure and scuffing loads as well as by limiting pressure of seizure. The test results are highly surprising. The systems in which simple one-component aqueous solutions were the lubricating substance did not undergo seizure up to the maximum load of 8 kN designed for the tribological tester used. A number of typical antiwear additives in suitable oil bases do not exhibit such good characteristics. Also, the quantities characterizing seizure show relatively high values. Local maxima for the two compounds tested are formed in the area of low concentrations in the dependences of antiseizure properties as a function of concentration. It can be assumed that this is due to ordered structures being formed in the surface phase. The hexagonal phase of high viscosity of the order of several thousand PaAEs which forms in the bulk phase for ESLS and SLS solutions has no visible effect on antiseizure properties, whereas the existence of the lamellar phase for a 70% solution of ESLS affects an increase in the values being measured.
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