In the manufacturing of greases, the selection of the base oil has an impact both on the production process and on the properties of the formulated greases. In the case of naphthenic oils, they have traditionally been associated with higher solvency and better low temperature properties than paraffinic oils with the same viscosity.
In the present work, the behavior of greases manufactured with naphthenic oils having different degrees of refining, as well as with Group I paraffinic oils and paraffinic/naphthenic blends, is investigated. The goal is to compare the effect of the type of mineral oil and the degree of refining on selected grease production and final product properties. In particular, tests on solubility of common thickener precursors in oils, soap consumption, elastomer compatibility, and oil separation from grease were carried out. It was observed that the superior low temperature properties and solvating power of naphthenic oils could significantly affect several important properties of the lubricating grease.
2. Base Oils Tested
The properties of the base oils used in the tests are summarized in Table 1 and 2. The naphthenic oils used include an oil having a viscosity of 110cSt at 40°C (N500), and three oils having a viscosity of 150cSt at 40°C and different degrees of refining (N750CA13%, N750CA7% and N750CA3%). A blend of paraffinic and naphthenic oil (NP750) was also used, which was prepared by mixing 80wt% of the naphthenic oil N500 and 20wt% of the paraffinic oil SN 2500.
When selecting a base oil for grease production, several factors must be taken into consideration. A base oil property that is particularly relevant is its solvating power or solvency. The most widely used methods to test the oil’s solvency are the Aniline Point (AP) and the Viscosity Gravity Constant (VGC). The aniline point depends solely on the chemical nature of the base oil. The lower the aniline point, the higher the solvating power of the oil. The VGC provides a weighted value between the viscosity effect on the solvating power and the chemical nature of the oil. The higher the VGC value, the higher the solvating power of the oil.
It can be observed in Table 1 and 2 that all naphthenic oils have a higher solvating power than the paraffinic oils with the same viscosity.
Table 1. Main properties of naphthenics base oils used
Table 2. Main properties of paraffinic and naphthenic paraffinic base oils mixtures used
3. Grease Production Method
NLGI grade 2 greases were prepared using Lithium 12 hydroxystearate as a thickener and the base oils listed in the previous section. The formulation was made from oil, 12-hydroxystearic acid and lithium hydroxide, using a 5 kg steel cooker. Three times as much oil as acid was initially used. The acid was dissolved in the oil (~96°C) by gentle stirring. A slurry of lithium hydroxide in water was slowly added to the oil/acid mixture. After about 20 minutes the temperature was raised to 103°C and the mixture was cooked for approximately one hour. The temperature was then gradually raised to 220°C to allow the complete evaporation of the water. At this stage, the soap had already melted and a clear oil/soap mixture was formed. The temperature was then progressively lowered to 50°C by adding oil. The grease was finally homogenized between rollers under vacuum. The desired consistency was reached by step-wise dilution with oil. The consistency of the grease was measured after each dilution step.
4. Test Methods
4.1 Soap Consumption
Two naphthenic oils with different degrees of refining (N750 CA13% and N750 CA3%), one paraffinic (P750), and a blend of paraffinic and naphthenic oils (NP750), all having approximately a viscosity of 150 cSt at 40°C were used to produce greases with the same consistency (NLGI grade 2). The amount of soap required to reach the desired grease consistency was measured.
4.2 Fatty Acid Dissolution Temperature
The temperature required to dissolve 30% of a fatty acid (12-hidroxystearic acid) in two naphthenic oils with different degree of refining (N750CA13% and N750CA3%), two paraffinic oils (P750, and P2500), and a naphthenic-paraffinic oil blend (NP750) was measured. The dissolution of the fatty acid was followed by optical inspection, and the temperature at which the last flake of solid had dissolved was recorded.
4.3 Elastomer Compatibility
The interactions between two different types of rubber, chloroprene rubber (CR) and nitrile butadiene rubber (NBR) with a nitrile content of 28%, with base oils and greases were studied. To this respect the rubber samples were completely submerged in the base oils and greases. Then, the samples were aged at 100°C for 168 hours. Three naphthenic oils (N750CA13%, N750CA7%, and N750CA3%) and a paraffinic oil (P750), as well as greases based on them were used for the tests. The changes in hardness and weight for all the samples at different temperatures were measured. The hardness was determined by IRHD (International Rubber Hardness Degrees) method, where rubber’s resistance to indentation is measured by pressing a rounded steel peak connected to a calibrated spring towards the material.
4.4 Low Temperature Behavior
In order to determine the low temperature pumpability of the greases, flow pressure tests were performed according to DIN 51 805. In these tests a standard conical nozzle is filled with deaerized grease. The nozzle is subsequently placed in the measuring instrument and equilibrated for 2-3 hours at the desired temperature. At each temperature an increasing pressure is applied to the nozzle and the threshold pressure at which the grease starts to flow through the nozzle is recorded.
5. Results and Discussion
5.1 Influence of the Base Oil on Grease Production Parameters
The solvating power of the base oil plays an important role in grease production, as it affects several parameters, such as soap consumption, solvency towards additives, as well as the dissolution temperature of the fatty acids.
5.1.1 Soap Consumption
In grease production, the higher the base oil’s solvating power the better the soap yield (i.e. the amount of soap necessary to reach certain grease consistency). It was observed that greases prepared with the naphthenic oils N750CA 13% and N750CA3% needed 20% less soap than the one prepared with the paraffinic oil P750. Also the paraffinic-naphthenic blend NP750 provided lower soap consumption than the paraffinic oil.
5.1.2 Fatty Acid Dissolution Temperature
The oil’s solvating power was seen to affect the dissolution temperature of 30% 12-hydroxystearic acid. The dissolution temperature of the fatty acid was measured on the oils having a viscosity of 150 cSt at 40°C. It was observed that the dissolution temperature followed the oil’s solvency and was lowest for the oil with lowest aniline point (highest solvating power), the naphthenic N750CA13%, and highest for the oil with highest aniline point (lowest solvating power), the paraffinic base oil P750.
Figure 1. Dissolution temperature of 30% 12-hydroxystearic acid in different naphthenic and paraffinic base oils.
When comparing the dissolution temperature in the naphthenic-paraffinic blend NP750 and in the base oils used to blend it, it can be seen the addition of naphthenic oil to a paraffinic oil positively impacts the paraffinic oil’s solvency, bringing the solvating power to levels typical of a mid-refined naphthenic oil.
5.2 Influence of the Base Oils on Grease Properties
The base oil choice does not only affect some production parameters, but it also influences several grease properties. For instance, it is well known that oils with a high solvating power are used in order to obtain a smooth grease structure and in some cases even a nearly transparent appearance. However, the base oil affects also other grease properties, such as their compatibility with elastomers and their low temperature behavior.
5.2.1 Elastomer Compatibility
Base oils have a relevant impact on the grease compatibility with the boot material and the risk of deterioration of the boots. In fact, when a lubricant and an elastomer are in prolonged contact at high temperatures, a migration of plasticizer out of the rubber and a migration of mineral oil from the lubricant into the rubber occur. The extent of each of these phenomena depends on the chemical composition of the elastomer. For some elastomers, the migration of plasticizer from the rubber is prevailing over the migration of mineral oil into the rubber. In such cases, when the rubber is in prolonged contact with lubricant or oil, it tends to shrink and harden. In other cases, the migration of oil into the rubber is prevailing, causing excessive swelling and softening of the rubber itself. Both shrinkage and excessive swelling are undesired as they compromise the stability of the sealing boot, ultimately resulting in failure. Generally, mineral oils with a higher solvency display a higher affinity towards the elastomer and therefore a higher tendency to migrate into the rubber. It is therefore preferable to use high solvency oils in contact with elastomers that display a tendency to shrinkage, while low solvency oils should be preferred in contact with elastomers that display a tendency to swelling.
Ageing tests were performed on nitrile-butadiene rubber, NBR (Figure 3 and 4) and on chloroprene rubber, CR (Figures 5 and 6). It is interesting to observe that even though there are differences in the absolute values of shrinkage and hardening when the rubber is aged in the presence of oil or the respective grease, they still follow the same trend. This means that the behavior of the elastomer in contact with grease can be predicted by studying the behavior of the elastomer in contact with the base oil from which the grease is produced, which brings practical advantages when conducting tests.
It is evident by observing Figures 3 and 4, that all the NBR samples become harder and shrink during the ageing process. For such elastomers, greases should be based on high solvency oils, as this minimizes the shrinkage and hardening effects. In fact, in our tests the lowest shrinking and hardening effects can be observed when the NBR samples are in contact with the naphthenic N750CA13%, which is oil with the highest solvating power (lowest aniline point).
Figure 3. Weight changes in NBR aged in the presence of different greases and base oils.
Figure 4. Hardness changes in NBR aged in the presence of different greases and base oils.
The opposite behavior was observed for the CR samples, as seen in Figures 5 and 6. This elastomer in fact tends to swell and soften in the presence of the lubricants. In this case therefore the oils with the low solvating power (high aniline point), should be preferred.
Figure 5. Weight changes in CR aged in the presence of different greases and base oils.
Figure 6. Hardness change in CR aged in different greases and base oils.
5.2.2 Low Temperature Behavior
The low temperature properties of the oils affect the grease behavior at low temperatures. Upon cooling the long n-alkanes chains (often referred to as waxes) that may be present in the mineral oil crystallize thereby impeding the free flow of the oil. When the cloud point occurs (i.e. the crystallization point is reached), the oil is no longer a Newtonian fluid but a two-phase system. Unlike Group I paraffinic oils, which present a relatively high wax content, naphthenic oils are virtually free from n-alkanes, leading to superior low temperature properties. The low temperature behavior of a mineral oil is commonly indicated by its pour point, which is the temperature at which the oil does not flow anymore. As it can be seen in Table 1 and 2, the pour point of the naphthenic oils is much lower than that of the paraffinic oils.
The impact of oil selection on the low temperature behavior of the greases can be observed in Figure 7, where the results of the flow pressure tests (DIN 51 805) are displayed. At 0°C, there are not significant differences between the paraffinic- and the naphthenic-based greases, but already at -10°C differences start to show up. The differences become larger as the temperature is further decreased, and at – 20°C the pressure needed for the paraffinic grease to start to flow is already almost twice the pressure required by the naphthenic based greases, while at -30°C the flow pressure value for the paraffinic based grease is 5 times higher.
Figure 7. Low temperature flow pressure test DIN 51805 for naphthenic and paraffinic greases.
This study shows how the choice of base fluid has an important effect both in the production parameters and final properties of lubricating greases.
The oil solvency proves to be a key parameter, impacting for instance soap consumption, grease structure and elastomer compatibility. Another important role played by the base oil is its effect on the grease low temperature behavior, where the absence of waxes in the mineral oil leads to a better low temperature performance of the lubricating grease.
Dr. Valentina Serra-Holm is a Manager at Nynas AB, in Sweden. Her contact information can be found in our member directory.
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