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A Simple Method to Quantify and Study the Effects of Lubricity Additives on the Misting Characteristics of Metalworking Fluids

March 01, 2014
Selim Erhan
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During metal working operations a certain amount of mist is generated from the metalworking fluid which can cause occupational health problems. More than 1.2 million workers are exposed to machining fluids. Therefore, it is desirable to keep misting as low as possible.  The three formation mechanisms of misting are impaction, centrifugal force, and evaporation/condensation.  These conditions may be difficult to control. Therefore, if the  fluid properties are such that it can minimize misting, it will have a significant impact. 

Misting of a fluid is affected by external factors such as velocity, aeration, turbulence, air flow and internal factors such as viscosity, surface tension, size, shape and the polarity of the fluid molecules. Several papers have been published on various aspects of misting and how to measure it and look at how the fluid composition affects the mist composition.  One that was particularly interesting was the 2006 STLE presentation from Doug Hunsicker who had reported this study from Caterpillar machine shops using an impeller type humidifier to determine the misting characteristics of various synthetic, semisynthetic and soluble oil type metalworking fluids (MWF). This was a simple, easy to use, low cost instrument.  They had been using several different types of fluids and noticed that there was a significant difference between the misting characteristics of these fluids. They were then able to duplicate this difference in the laboratory.  In this method a weighed amount of fluid is placed in the humidifier chamber and the humidifier is operated for a precisely measured time, in this case 60 minutes, taking care the bottom cone  is always in the fluid, and then weighed again to gravimetrically determine the fluid loss.  If the experiment is done carefully they showed that they could accurately duplicate the order of fluid loss they observed in the field. In their conclusions they reported that the type of MWF had little to do with the tendency to mist and the individual fluid properties dominated. Minor changes in the MWF composition had dramatic effects on the fluid’s tendency to mist. They also observed that the MWF’s tendency to mist can and does change during use, both with tankside additions and component degradation.

These findings showed that one could use this low cost but effective screening method and by changing a certain additive in a MWF can study the effect of an individual additive on the fluids misting behavior. It was also a good tool to verify reports showing high molecular weight vegetable oil based polar molecules were effective in lowering misting in machine shops. It can be theorized that as the polarity increases the attraction of molecules increase which results in lower volatility. A suitable group of molecules for this study was lubricity additives.  In formulations they exist at a level where their contribution would be noticeable.  They come in a variety of structures that allow studying the differences in molecular shape, size and polarity. The formulas that were chosen for this study were stable with all the different additives at the same amounts. This way changes in various additive amounts that would  introduce variability was avoided. 

At first an oil free synthetic formula with 10% lubricant was studied. Eight lubricity additives were selected. A polymeric vegetable oil based acid (Poly Acid), two polymeric vegetable oil based esters (Poly Est 1 and Poly Est 2), two straight chain esters (Syn Est 1 and Syn Est 2), two synthetic branched esters (Syn Brch Est and Succ Brch Est) and a PAG. Results seen in Figure 1 show that the larger more cross-linked polymers did have lower misting and higher surface tensions. As the number of hydroxyl groups increased the misting started to increase and the surface tension decreased. We also looked at a relation with foaming but could not see a clear relation.

Figure 1. Surface Tension and Misting Relation in a Synthetic Metalworking Fluid.

Next a high oil semisynthetic formula which had 45% mineral oil and 7% lubricity additive was chosen. The 7% lubricity additives were changed using a chlorinated paraffin, polymeric vegetable oil based acid (Poly Acid), two polymeric vegetable oil based esters (Poly Est 1 and Poly Est 2) and two synthetic branched esters (Syn Brch Est and Succ Brch Est). The results are seen in Figure 2.  It was seen that some higher molecular weight additives had lower misting and higher surface tensions. Two of the high molecular additives had higher misting but their formulas had  lower surface tensions, and confirmed what  had been seen before in the relation between surface tension and misting.  The study also showed that the interaction of the lubricity additives with the other components is clearly very important because a significant change was seen in the order of performance.  The foaming study was also completed and again we could not see a clear relation between misting and foaming.

Figure 2. Surface Tension and Misting Relation in a Semisynthetic Metalworking Fluid.

In summary, we have seen that an impeller type humidifier can be used to differentiate the contribution of the various additives to the misting of a metalworking fluid.

Selim M. Erhan works as a Senior Research Scientist at Afton Chemical Companies’ Metalworking Division. He is the Vice-President of the STLE Chicago Section. You can find his contact information in the membership database.

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