30 S E P T E M B E R 2 0 0 6 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

The mysterious worldof MWF additives

Lubrication Fundamentals

In the June TLT we talked abouthow metalworking fluids pro-mote tool life by removing heat,

lubricating the interface between thetool and the workpiece, carryingaway chips and preventing in-process corrosion.

Those are the macro issues.Imbedded in these functions are pre-venting such phenomena as poorsurface finish, chemical attacks onthe workpiece, workpiece mechanical

damage, metallurgical change to theworkpiece, thermal damage and elec-trical changes to the workpiece.

But these are just the operationalparts. In addition, metalworking flu-ids can contribute to a number ofnegative phenomena, all of whichmust be minimized. These includemist leading to respiratory problems;dermatitis; fungal and microbialgrowth, which can lead to employeehealth and safety issues; plugging fil-

ters and other operational problems. Speaking as a chemist, this is a

formidable set of interrelated andinteracting problems. How do che-mists balance all these differentissues? Aside from choosing from aselection of base fluid materials(straight oils, soluble oils, semisyn-thetic oils and synthetic oils) andconcentration, they use a mystifyingarray of additives.

What do we mean by additives?

By Dr. Robert M. Gresham

Contributing Editor

I

When it comes to evaluating additives used in metal-

working fluids, chemists and suppliers take a close

look at compatibility and performance issues.

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Additives are a chemical component orblend used at a specific treat rate to provideone or more functions in the fluid. Ideally,components are multifunctional and gener-ally used at treat rates from < 1% to 35%.They are soluble in mineral oil, water and,in some cases, both.

Of course, there are many types of addi-tives: boundary lubricity additives, extreme-pressure additives, corrosion inhibiters, re-serve alkalinity boosters, metal deactiva-tors, emulsifiers, couplers, water condition-ers/softeners (chelating agents), antimistadditives, antimicrobial pesticides, anti-foamers and defoamers, and dyes. That’squite a list!

Thus, it is easy to understand whychemists look for multifunctional additivesand why, with so many different chemicalsin a formulation, additive compatibility,both with other additives as well as with thebase fluid, is of critical importance.

Are you mystified yet? In the future, wewill examine biocides and defoamers, butfor now let’s take a very quick look at themost common of the performance-relatedadditive types.

The world of additives Boundary lubricity additives enhance thelubricity of the fluid by adsorbing on thesurface of the metal to form a film, reducingmetal-to-metal contact. These additivestypically have a polar group that interfaceswith metal and a tail that is compatible withmineral oil or water. Examples are lard oiland canola oil.

Extreme pressure additives are a specialtype of boundary lubricity additive thatactually reacts with the metal surface,instead of adsorbing on the surface, to forma metal salt layer or a physical barrierbetween the tool and the workpiece undersevere metalworking conditions. The layeracts as a barrier to reduce friction, wear anddamage. Examples are chlorinated paraf-fins, sulfurized lard oils, phosphate estersand overbased calcium sulfonates. Theseadditives have different temperatures ofactivation. Thus, the chemist also choosesthe additive according to the applicationconditions, so that the additive will, in fact,be activated.

Corrosion inhibitors prevent the fluidfrom corroding the metal workpiece, cuttingtool and machine tool. They perform byeither forming a protective coating on themetal surface or by neutralizing corrosivecontaminants in the fluid. Examples areoverbased sulfonates (some of these alsocan act as extreme-pressure additives),alkanolamides, aminoborates and amino-carboxylates.

Reserve alkalinity additivesmaintain the fluid’s corrosionprotection by neutralizingacidic contaminants and bymaintaining the pH in a suit-able range. These additivesalso can act as form emulsifierswith other components to sta-bilize the fluid. Examples arealkanolamines like monoetha-nolamine, triethanolamine,aminomethylpropanol and 2-(2-aminoethoxy) ethanol.

Metal deactivators preventthe fluid from staining nonfer-rous alloys (such as copperand brass) and reduce corro-sion when dissimilar metalscontact each other. They actby forming a protective coat-ing on the metal surface.Examples are mer-captobenzothiazole, tolyltria-zole and benzotriazole.

Emulsifiers stabilize oil-sol-uble additives in water-dilutable metalworking fluids by reducinginterfacial tension between incompatiblecomponents by forming micelles (a submi-croscopic aggregation of molecules, as adroplet in a colloidal system). Thesedroplets then can remain suspended in thefluid. This is what you do when you washyour hands. For example, milk is an emul-sion. In metalworking fluids, examples ofemulsifiers are sodium petroleum sulfonateand alkanolamine salts of fatty acids.

Couplers assist in stabilizing water-dilutable metalworking fluids in the con-centrate to prevent separation of compo-nents. Couplers facilitate formation ofemulsions for soluble oils. Examples are

Additives are a chemical

component or blend used

at a specific treat rate to

provide one or more

functions in the fluid.

Ideally, components are

multifunctional and

generally are used at treat

rates from < 1% to 35%.

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CONTINUED FROM PAGE 31

propylene glycol, glycol ethers and nonion-ic alkoxylates.

Chelating agents (also known as watersofteners or conditioners) reduce thedestabilizing effect of hard water (calciumand magnesium ions) on metalworkingfluid emulsions. Chelating agents bind cal-cium and magnesium salts to prevent themfrom reacting with anionic emulsifiers suchas fatty acid salts of alkanolamines. Anexample might be ethylenediaminete-tracetic acid.

Antimist additives minimize the amountof lubricant that disperses into the air dur-ing machining. They are typically polymersand/or wetting agents. For oil-based sys-tems, ethylene, propylene copolymers andpolyisobutenes are used. For water-basedsystems, polyethylene oxides are typical.

Finally, dyes give the metalworking fluida specific color type desired by the cus-tomer. Their main value is in water-dilutedfluids to indicate that product is present,since these can be clear and water-like inappearance. However, dyes often carry neg-atives as well, as they can stain skin andpaint. Some water-soluble dyes are unsta-ble and can change color. And some dyescan pass thru waste treatment systemsresulting in pollution downstream.

Clearly, this is quite a list, and the skill ofthe chemist is needed to keep everythingunder control. Arbitrary use of additives atthe tank side carries substantial risk whennot done under the supervision of a chemistfamiliar with the original formulation. It isimportant to have a good working relation-ship with your fluid supplier to know howbest to manage the fluids in your valuablemachinery.

STLE members who have attained thesociety’s new Certified Metalworking FluidSpecialist certification are aware of theseissues and can keep your system in balanceand safely working at optimum levels—thisis what they do. <<

You can contact Bob Gresham, STLE’s director ofprofessional development, at rgresham@stle.org.

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