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Metalworking fluid emulsion stability and base oil properties - some critical correlations

Product News Tuesday, June 30, 2020: Nynas South Africa

The make-up of a typical metalworking fluid emulsion is a dilution of 5 to 10 volume-% mineral oil concentrate in water. This water could be tap water, with whatever water hardness the local source offers, of demineralised (Demin) or reverse osmosis (RO) water which is very soft. The mineral oils content is high, typically 60-80% of the concentrate, and the remainder being oil soluble additives: emulsifiers, lubricity additives, corrosion inhibitors, biocides, antifoams and mist suppressants.

 

Naphthenic base oils provide several advantages to MWF formulations. High solvency allows for the dissolution of high amounts of additives and contributes to increased emulsion stability. Emulsion stability is key to metalworking fluid (MWF) usefulness - if the emulsion breaks, it has ceased to function. Investigations of the relationship between formulation and emulsion stability thus is a first step towards a better understanding of the complex chemistry of a fully formulated MWF. We sought to understand how the properties of the base oils, especially solvency (as indicated by the Aniline Point) would influence emulsion stability over test period up to one week.

Experimental work

 

Metalworking fluid emulsions

A MWF soluble oil emulsion stability study was set up, comparing the properties of a naphthenic base oil, versus three paraffinic type base oils of similar viscosity, ca. 20 cSt (100 SUS) at 40 °C. NYNAS® T 22 is a good example of the quintessential “100/100” metalworking fluid oil, having a viscosity of 100 SUS at 100 °F. As Group I oils we picked a traditional SN 100 base oil, and the NYBASE® 100, which is a Group I replacement base oil. A Group II base oil if similar viscosity was also included.

 The solvency, as indicated by the Aniline Point (AP), varies across the base oils studied:

1. Naphthenic NYNAS® T 22 (~100 SUS), AP = 76 °C
2. SN 100, AP = 100 °C
3. NYBASE® 100, AP = 101 °C
4. HP4, a Group II base oil, 20 cSt @ 40 (4 cSt @ 100 °C), AP = 108 °C

 

 

Two non-ionic emulsifiers, Span 80 (Sorbitan monooleate), with a Hydrophile-Lipophile Balance (HLB) number of HLB 4.3 and Tween 80 (Polyethylene glycol sorbitan monooleate), HLB 15, were utilized to make nine different blends with HLB:s ranging from 9 to 13, in half-steps. Butyldiglycol was employed as solubiliser (co-emulsifier, coupling agent).

All emulsion concentrates were of the same oil content, with surfactants to make up the required HLB value. The concentrate was added to the water at ca. 5 v/v-% and sonicated at low power for three minutes.

 

Droplet size distribution experiments

 

The soluble oil emulsion droplet size distribution (DSD) was determined at three different times. The droplet size was measured at high dilution by a Malvern Mastersizer 3000 E. In Figure 1, the droplet size distribution statistical Dx50 mean value is plotted versus HLB. These graphs typically will show an “U”-shaped minimum where the emulsion droplet size, is the smallest, and hence at which HLB the most stable emulsions are formed. Similar plots were obtained for the Group I, NYBASE® new range Group I replacement, and Group II formulations. 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.

An increase in mean droplet size over time is observed: Day 0 (blue bar), Day 1 (red bar) and Day 7 (green bar) in general show increasing droplet size with time. Since an increase of droplet size is the earliest warning sign and the first steps towards coalescence and emulsion break-up, this is very interesting information.

 

 

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NYNAS® and NYBASE® are trademarks of Nynas.

 

Figure 1. DSD development over 7 days versus TSI (purple line) for a NYNAS® T 22 based milky emulsion, soft water (0 °dH).