Oxidation Effect on Tribological Properties of Rapeseed Oil and Lard Mixtures Containing Monoglycerides and Fatty Acids

Vegetable oils and animal fats are increasingly popular base material for producing environmentally friendly lubricants. This material is renewable and easily biodegradable in the natural environment. Main disadvantage of vegetable oils and animal fats as raw material and its lubricants is poor oxidation stability. There is already a wide range of environmentally friendly lubricants in the market, whereas the variety of grease offer is not so high. This research aims to explore the properties of prepared compositions of lubricating grease produced from rapeseed oil and lard, modifying it with monoglycerides, stearic and oleic acids. Plastic properties (penetration) and oxidation influence on tribological and corrosive properties of these compositions have been studied. It is found that modifying rapeseed oil and lard with monoglycerides, oleic and stearic acids a few lubricating compositions of NLGI grades can be obtained, namely, soft or very soft rapeseed oil based grease and medium or nearly hard consistency lard based compositions. The studies of oxidation have shown that it decreases tribological properties of both base and monoglycerides modified lubricants. Oxidation has a greater negative impact on lard and lard based compositions. Oleic and stearic acids reduce or completely eliminate negative influence of oxidation. Corrosion studies have shown that both fresh and oxidized lubricant compositions have no significant effect on copper strip corrosion.


Introduction
Vegetable oils and animal fats were one of the first materials used for lubrication.Since the 19th century however, they have been gradually replaced by lubricants produced from petroleum hydrocarbons.The latter have not always met technical requirements.It has stimulated us to analyze synthetic lubricants and introduce them to industry as they have become the equivalent to mineral lubricants.
Nowadays great attention is being paid to natural triglycerides, to fatty acids produced from them and to their applications.This raw material is renewable and does no harm to the environment.
The use of environmentally friendly products is valued due to their low soil pollution, minimal threat to the human health and safety and also to their biodegradability.Lubricants make an impact on the environment in all stages of their production, use and utilization.Accidental lubricant spills or their leakage when used in the environmentally sensitive places, such as agriculture, forestry, mining districts, construction sites, water ways and reservoirs, may cause ecological disasters.Taking it into account most European countries have set the requirements stimulating the use of environmentally friendly lubricants.This situation has created a unique possibility to develop environmentally friendly lubricants from natural esters whose quality and price may compete with synthetic lubricants produced from petroleum (Gryglewicz et al. 2003;Kodali 2002).
The base material for biological grease is either of vegetable (rapeseed, sunflower, soy, flax or other oil) or animal (lard, beef tallow, fish and other oils) origin.It is environmentally friendly and 80 % biodegradable, while only 15-20 % of mineral oils degrade during the same period (Bartz 2006).
Grease is made of base stock lubricant (oil or fat), thickeners and multi-purpose additives improving their exploitation properties (Fig. 1).The test oil sample of 22 ml was poured into the sample compartment, fully submerging the stationary balls.Under the applied load of 150 N, at the rotation speed of 1420 rpm, the machine was run for 1 hour.Prior to each experiment, all the appropriate parts of the machine, i.e. bottom and upper ball holders, oil vessel and test balls were washed in an ultrasonic bath with hydrocarbon solvents and then dried.
The wear scar diameter on three stationary balls was measured with an optical microscope.The results were recorded and reported as the medium of the wear scar diameter (WSD) of the three balls in millimeters.During the test the torque between the balls and the temperature change of sample oil were recorded.
The penetration was determined according to the ASTM D 217-97.The test temperature was 25 o C. The 102.5 g weight penetration cone was used.
Accelerated oxidation of lubricating mixtures was tested according to the standard AOCS CG-5-97 when storing them in an oven.The samples, 10 ml each, were stored in closed 250 ml volume glass vessels.During testing the constant temperature of 70ºC was maintained.The samples were kept in the oven for 50 days.The test of the copper corrosion was performed according to the standard LST EN ISO 2160: 1998.The copper plates were kept in the tested lubricating mixture at the temperature of 70ºC The test duration was 24 hours.The corrosion effect was evaluated by a comparative method using standard copper plates.

Results and Discussion
Results of penetration measurements are given in Table 1.They indicate that in all cases lubricating mixtures were of sufficient thickness, the thickest was that of lard (80 %) and monoglycerides (20 %).The mixtures composed on the basis of RO were less thick.It is of interest that application of SA or OA to lubricating mixtures based on LF and RO had a different effect on their thickness (penetration).In the mixtures based on RO the acids increased their thickness, while in the mixtures based on LF they reduced it.Tribological results of modification of rapeseed oil and lard by stearic and oleic acids and by monoglycerides are given in Figs 2-4.The diagrams show that using fresh rapeseed oil with monoglycerides the wear of balls is the same as that when using rapeseed oil with monoglycerides and oleic acid, while when using rapeseed oil with monoglycerides and stearic acid it is 1.1 times lower.After 50 days of storage oxidation the lowest wear was noticed when lubricating with rapeseed oil containing monoglycerides and stearic acid.Comparing the lubricating properties of fresh and 50 days aged, pure and modified with monoglycerides rapeseed oil, it is determined that when lubricating with the fresh mixture the wear was 1.2 times lower.Only one version was an exception, when lubricating with monoglycerides and oleic acid modified, fresh and 50 days aged rapeseed oil, the ball wear was practically the same.Whereas lubricating with fresh rapeseed oil containing monoglycerides and stearic acid the ball wear was 1.07 times lower than lubricated with a 50 days aged sample.The analysis of the diagrams shows that when lubricating with pure and modified lard it is evident that pure lard loses its lubricating properties due to oxidation.Lubricating with oxidized lard the ball wear is 2.6 times greater than that lubricated with fresh lard.
An oxidation effect on wear reduction decreases considerably when adding monoglycerides into lard: when lubricating with the oxidized sample the wear scar is 1.3 times larger than that observed when lubricating with a fresh one.Addition of oleic or stearic acids into the lard had no essential effect on the oxidation properties: lubricating with the oxidized sample the wear scar was from 1.2 to 1.3 times larger than that observed when lubricating with a fresh one.
In summarizing the oxidation effect of rapeseed oil and lard modified with monoglycerides, oleic and stearic acids on wear reduction properties, main results are as follows: a) the use of monoglycerides, oleic and stearic acids reduces the wear reduction properties of fresh lard (1,2 times), nevertheless they reduce the oxidation effect on wear reduction; b) the mixtures of fresh lard, monoglycerides, oleic or stearic acids provide better protection against wear than the same mixtures of rapeseed oil; on the other hand, the rapeseed oil mixtures are less affected by oxidation, i.e. their wear reduction properties are more stable.The reason is likely to be the natural antioxidants in rapeseed oil which stabilize the oxidation process.In animal fats the appearance of natural antioxidants is very limited.The greatest wear reduction efficiency has been reached by modifying rapeseed oil and lard with monoglycerides and stearic acid.Both monoglycerides and stearic acid have no double bond in hydrocarbon chains, thus they increase oxidation stability.Corrosion tests have been also carried out.A corrosive effect of the mixtures was evaluated after having stored them in the oven at 70 o C for 1 and 4 days.The modified rapeseed oil and modified lard are estimated to correspond to the 1a corrosiveness category which indicates that the oxidation products of the tested samples have no significant impact on corrosiveness of the tested lubricating mixtures.
Diagrams of the torque variation of the tested lubricating mixtures during testing are given in Figs 3  and 4. On the basis of the torque analysis the mean torque when lubricating with fresh rapeseed oil containing monoglicerides is found to be 1.15 times lower than that observed when lubricating with the 50 days stored in oven rapeseed oil.In the case of rapeseed oil modification with monoglicerides and oleic or stearic acids this difference diminishes up to 5 %.
The medium torque of pure lard after oxidation compared to the fresh lard has increased 1.3 times; however situation changes to the opposite when lard was modified by monoglycerides -the mean torque of the oxidized mixture is 1.3 times lower than that of the fresh one.With addition of oleic or stearic acids in the lard containing monoglycerides the torque decreases.When comparing the torque variation observed during the testing period it is clear that oxidation has a lower effect on rapeseed oil mixtures (Fig. 3) than on lard ones (Fig. 4).
Typically, after oxidation the rapeseed oil mixtures containing oleic or stearic acids distinguish themselves for a higher torque, while the analogous lard mixtures after oxidation have a lower torque than the fresh ones.It should be noted that during testing in all cases when lubricating with rapeseed oil and lard mixtures containing oleic or stearic acids a gradual decline in the torque has been determined.
In the analysis of worn surfaces different traces have been noticed (Figs 5 and 6).In the picture of a wear scar observed when lubricating with fresh rapeseed oil containing monoglycerides a lot of small scars are seen, while after storage oxidation the wear scar changes -a number of small groves decreases considerably and on the scar surface a blurred relief is seen (Fig. 5 b).With addition of oleic acid the number of groves decreases greatly and the scar relief becomes more distinct, especially after oxidation (Fig. 5 d).The analogous picture is obtained when lubricating with rapeseed oil modified with stearic acid (Fig. 5 e and f).The possible reason may be that when deformed surface is subjected to fatty acids the strength and microhardness of the surface layer decrease, especially lubricating with oxidized samples.Due to these reasons the Rebinder's effect (reduction in hardness of an adsorption surface layer) may take place.In all cases, after 50 days of storage oxidation the wear scars are larger.
Fig. 2. The ball wear observed when testing the samples modified with additives: a) -rapeseed oil b) -lard

Fig. 4 .
Fig. 3.The torque variation during the four ball test lubricated with a) RO+MG; b) RO + MG + OR; c) RO + MG + SA

Table 1 .
Penetration of lubricating mixtures