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The resulting semi-synthetic micro emulsion droplet size distribution (DSD) showed some interesting differences versus the milky Soluble oil emulsion, Figure 5. All four oils, in hard or soft water, display droplet sizes below 10 µm. For the NYNAS™ T 22 in soft water (both om Day 0 and Day 7), most the droplet sizes are below 1 µm (the absorption peak below 1 µm). In contrast, in hard water, NYNAS™ T 22 displays two peaks, at 0.3 µm and one about 1.3 µm. The SN 100 in hard water displays a broad peak around 0.5 µm (both Day 0 and Day 7). In soft water, the peak is shifted up towards 0.8 µm and is narrower in shape. The Group II oil (HP 4) display narrow peaks centred around 1 µm in both hard and soft water. The NYBASE®
100 at Day 7
Figure 3. DSD development over 7 days versus TSI (10 minutes) for a NYNAS™ T 22 based milky emulsion
For the more paraffinic oils the optimum HLB was close to 10, but the value of the minimum droplet size was in no case below 10 µm, more than 20 times larger than for the naphthenic NYNAS™ T 22 oil. One example is shown in Figure 4.
displays broader peaks centred around 1.2 µm, in hard water, a broad peak (0.5 µm to 5 µm) above 1.1 µm, possibly obscuring a bi-phasic behaviour, which is not baseline separated. In contrast, in soft water the peak at 1.2 µm is (somewhat) more narrow.
An attempt to summarize the above results would be that NYNAS™ T 22 in soft water yields the most stable emulsion with the smallest mean DSD. For hard water, the result spread is larger, and both SN 100 and NYNAS™ T 22 display notable peak maximum shifts, but in opposite directions of change with water hardness.
Figure 4. Emulsion based on NYBASE® minimum droplet size at HLB 10.5.
100 in soft water (°dH 0) displaying a
Emulsion phase thickness and stability determination The emulsion phase thickness was determined by light scattering determination at different time intervals utilizing a Turbiscan LAB, measurements at actual concentration “as-is”. The Turbiscan Stability Index (TSI) was utilized to characterise emulsion stability. In Figure 3, the TSI value versus HLB is also shown (the thin purple line).
The TSI development during the first ten minutes after sonication is shown for nine samples with HLB from 9.5 to 13. The most stable properties for the NYNAS™ T 22 based emulsion were found around HLB 12, similar to what the droplet side distribution (DSD) experiment indicated. A good correlation was found between the DSD established, and the TSI calculated from the emulsion phase thickness measurements utilising the Turbiscan instrument, for those oil and emulsifier combinations that gave good (small) droplet sizes. In Figure 3, the “U-shape” of the DSD is mirrored by the TSI values.
Semi-synthetic translucent micro emulsions As a second phase of the study, we made semi-synthetic translucent micro emulsions of the same four base oils. The emulsion concentrate contained 36% water, 30 % base oil, and a range of additives (34% in all): Tall Oil Fatty Acid (TOFA) as the main emulsifier, a non-ionic Fatty alcohol alkoxylate as co-emulsifier, aminic bases, steel and yellow metal corrosions inhibitors, coupling agents and biocide. The concentrate was added to the water at ca. 5 v/v-%, and sonicated at low power for three minutes.
Figure 5. The Semi-Synthetic formulation, droplet size after 7 days under hard (°dH 20) and soft (°dH 0) water conditions
Results and discussion In this study, we set out to investigate different parameters affection the primary emulsion stability of model metalworking fluids. We could determine the optimal HLB value for the different base oils, and could also observe large differences in emulsion stability. The primary contribution to stability, as demonstrated by a low (1 µm or less) mean droplet size, was found to follow solvency; the lower the aniline point, the more stable the emulsion formed in the Soluble oil coarse (milky) emulsion system based on non-ionic emulsifiers. The Naphthenic base oil emulsions display the highest stability, followed by the Group I and Group I replacement base oils, then Group II. The solvency, as indicated by the aniline point, mirrors this order, and thus apparently plays an important role for emulsion stability in the systems investigated.
The second part of the study was on semi-synthetic translucent formulations, based on anionic and non-ionic surfactants. For these samples, the DSD in general was much smaller, indicating even better emulsion stability. This system did display sensitivity towards water hardness, as expected from the anionic surfactant chemistry. The extent and character of the effect was different for the different base oils. The clear bi-phasic nature of the NYNAS™ T 22 based micro emulsion in hard water, warrants
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LUBE MAGAZINE NO.139 JUNE 2017
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