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by Dom Ruggeri

March 2001:


I have always held the opinion the central systems are a life form unto themselves. They are as different as people and have as many personalities as the human race; further no two are exactly alike. Even systems built on the same day, using the same specs, and the same equipment will perform differently.

Back in the mid eighties my career was progressing well at the large formulating house. One day I was called out to a major automotive manufacturing facility, the coolant was performing well in one central system however, the second system identical to the first, there were no end of problems. Normally I would not have been very concerned but the coolant in question was a PAO based soluble of my design, as was the rule your product your problem.

When I arrived at the facility the chief engineer and I did a complete system analysis of both systems. The systems were performing within tolerances. There was no evidence of unusual wear on the pumps or any other area of the system. To further complicate matters both systems were charged from the same tanker so the coolant was identical, no lot-to-lot variance.

We struggled with this problem for 3 days.  Finally, engineering, sales, and I agreed to dump this new high-tech synthetic fluid and recharge this system with standard soluble oil. The problems disappeared. Thus, confirming my theory that these systems are alive.

When I first began in this business, a synthetic was defined as a Metalworking Fluid in which all the components are water soluble.  This was a very neat definition that was accurate in the early eighties but not today. In today’s metalworking marketplace a synthetic is defined as any metalworking fluid that does not contain mineral oil. 

This complicates matters for the formulator as well as the end user for a few reasons:
        Different base carriers yield different performance.
        Additives perform differently depending on the base carrier.
        Emulsification packages must change, these changes can lead to other production problems e.g. tramp oil rejection.
        Certain natural oils like Soya do not have the lubrication properties of mineral oils.
        New and different microbiological problems.

Note well, the term base carrier refers to the base lubricant in a metalworking fluid formulation.  In the case of a soluble oil, the base carrier would be mineral oil.

Why is the base stock so important?  The base stock determines the properties of the metalworking fluid.  For example a Polyalpha Olefin (PAO) is more expensive then mineral oil.  While a synthetic ester is about the same price as a PAO, which would you choose?  Well it depends on the properties you’re looking for.  If your looking for extreme pressure lubrication additives, one would look first at chlorinated paraffins.  A good choice, however, certain esters will perform as good and in some instances better then the old standby.  As with all newer technologies there is a cost and many times the cost cannot be justified using value added techniques.


Regarding the PAO, my research has shown that most PAO’s enhance the lubricating effects of fatty acid amides. However, the trade off is that most amides pull tramp oil into the metal working fluid emulsion. This may increase the likelihood of microbiological problems. I remember a formulating house developing a formulation using amides as the base carrier that formulation was a tramp oil nightmare.

Vegetable Oils:

These have been around for ages, only within the past 10 years has their use in metalworking fluids been studied. I have worked with rapeseed, canola, and soybean oils of these each has their own special uses. Rapeseed oil is perhaps the best natural product boundary lubricant in today’s market. Canola oil has excellent overall lubrication. Soybean oil right now is very inexpensive. Each of these requires a special emulsification package that must be carefully controlled. However since these are natural oils made from renewable resources, and as we move toward more green coolants, their use will expand in the metalworking marketplace.

As always there are downsides to these green coolants.  These are agro products and a crop could fail sending prices through the roof. Microbiological programs must be strictly followed, as these materials are a gourmet feast for bugs. Concentration control must be maintained, as coolants formulated with these oils need the additive packages at optimum levels.


As I mentioned earlier esters can be an excellent choice for a base carrier. As with all other synthetic materials these too have a drawback. The biggest drawback is the more water-soluble the ester, the better the chance for it to under go base hydrolysis.  Now I fully realize from my very old days in Physical Chemistry that thermodynamically, this type of hydrolysis is not favored. However, I was able to measure the rise in acid number of a water-soluble ester held at a pH of 8.7, this increase in acidity is indicative if the ester breaking down to it’s components, an alcohol, and an organic acid.

We have now covered PAOs Vegetable Oils and Esters, the formulations of these should be left to the experts.  In closing I will rank these bases.  This ranking is only my opinion, and is made on the basis of ’lubrication with no additives,’ ranked from most to least:

        1.  Esters
        2.  Vegetable Oils
        3.  Mineral Oils
        4.  PAO’s

As always, this is once again a broad-brush approach to synthetics. Your metalworking fluid supplier will give you a more in depth overview of their particular synthetic chemistry and how it can be of service to you. As always should you have any questions or comments feel free to e-mail me at the magazine.

All Best