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MARCH 12-14,1991 /DEARBORN INN HOTEL DEARBORN (DETROIT) , MCI HGI AN

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Society of Manufacturing Engineers

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CLOSER LOOK AT SEMI-SYNTHETIC COOIANTS

Bob Rauth President Pillsbury Chemical and Oil Company Detroit. MI

i PRESENTED AT:

"Metalworking Fluids'' March 12-14, 1991 Dearborn. MI

SPONSORED BY:

The Society of Manufacturing Engineers Courses and Clinics Department One SME Drive, P.O. Box 930 Dearborn, MI 48121-0930 Phone: (313) 271-1500 FAX: (313) 271-2861

CLOSER LOOK AT SEMI-SYNTHETIC COOLANTS

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by Robert K. Rauth, President Pillsbury Chemical & Oil, Inc., Detroit, MI Introductioq

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The metalworking coolant field provides a wide array of fluids which the user may select. The general categories of coolants available include straight oil, soluble oil, semisynthetic, and synthetic fluids, and there are numerous products that fall within each of these classifications. Narrowing the selection can be difficult, but a careful, studied approach can lead to the best fluid for the job. This paper takes a closer look at semi-synthetic coolants - a less widely used category -- detailing their application capabilities, composition, and benefits. The physical and chemical characteristics of semi-synthetics, their advantages as well as disadvantages, technique for evaluating and selecting a particular fluid, and coolant maintenance procedures are a11 discussed.

majority of particle size distributions ranging from 2 0 to 6 0 micrometers, are weakly dispersed in the fluid. Their relatively large and irregularly shaped particle suspensions in water create the opacity characteristics of wacro-emulsion coolants. Translucent to transparent fluids are the result of microemulsion formation of uniform coolant particles that are tightly dispersed and suspended in water. Statistically, the particle size populations for the micro-emulsion will observe a bell-shape distribution, with the majority of the population falling between 1 to 10 micrometers in size.

As might be expected, macro-emulsion semi-synthetics closely resemble the familiar, “milky” soluble oil coolants, while micro-emulsion fluids are very similar to true solution synthetic coolants. Both types of products fill a niche in metalworking applications. ! Semi-synthetic fluids may be formulated with or without water in their neat form. Although this item by itself has little bearing on the selection of semi-synthetic coolants, it does have an effect on the type of additive formulated into the product and the final cost of the neat material. Chemical Characteristics Semi-synthetic coolants are basically hybrids of soluble oil and synthetic fluid chemistries. The’amount of intermixing of the two chemistries is dependent upon the final characteristics desired for the semi-synthetic. To achieve soluble oil traits, soluble oil chemistries would predominate, with a minor input of synthetic fluid chemistry. The result would be a semi-synthetic coolant that behaves similarly to soluble oil without the significant petroleum oil content. At the other end of the spectrum, semi-synthetics that are required to behave like synthetic fluids predominantly utilize solution-type chemistry. The result is a coolant exhibiting primarily synthetic fluid tendencies, with a minimal presence of mineral oil (Figure 2 ) . Like all metalworking coolants, semi-synthetics are required to lubricate, cool, and protect metal parts and machinery. Various generic chemistries are used to accomplish these objectives.

To provide lubrication, the semi-synthetic coolant first and foremost employs petroleum oil, the common denominator for all semi-synthetic fluids. Not only does mineral oil provide a diluent for the coolant chemicals, it also provides a certain degree of boundary lubrication. Typically, the lubrication properties are enhanced by other chemical lubricating materials common to soluble oil or synthetic fluids. For example, synthetic hydrocarbons, polyols, and esters are generally used not only to augment the lubrication properties of the semisynthetic, but also to provide a diluent f o r additives.

Semi-Svnthetics:

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Ideal Comuromise

The first step in making the choice of a suitable metalworking coolant is to determine the general category of fluid required or desired. The hpplicable factors in determining whether to use a straight oil, soluble oil, semi-synthetic, or synthetic coolant are derived from a number of considerations. These typically include the specific metalworking application, the age and requirements of the equipment, metallurgy of the workpiece, waste treatment and environmental concerns, cost, and the personal preference of the user. What is it that makes semi-synthetic coolants an advantageous choice over other fluid categories? In the author's opinion, the primary reason is the presence of a small percentage of mineral oil formulated in with the synthetic chemicals. Many users feel much more comfortable and confident of the SUCC@ss of their application using a metalworking coolant which contains at least some mineral oil because it applies a protective and lubricating film of oil on the machinery. The selection O f a semi-synthetic coolant offers an ideal compromise to those users who would like to diminish the use Qf straight oil and soluble oil coolants in their plants, yet are unwilling or unable to take advantage of true synthetic fluids (Figure 1). Other important reasons for selecting semi-synthetic coolants will be discussed later. It should be emphasized here, however, that semi-synthetics offer a viable, practical alternative to those users who are not ready to switch to synthetic chemical solutions, but do desire to minimize or replace their straight oil and soluble oil coolants. Before addressing the advantages and disadvantages of semisynthetics and how to go about the evaluation and selection of a fluid, it is useful to gain an understanding of the basic physical and chemical characteristics of these coolants. acteristics Definitions vary as to which are the basic properties that constitute a semi-synthetic coolant. In general, however, one can define a semi-synthetic coolant as a fluid which contains less than 20 percent petroleum oil. The fluid may be aqueous or non-aqueous, water soluble or insoluble. The key characteristic, though, is that semi-synthetics include some amount of mineral oil. The majority of semi-synthetic coolants available are water soluble. When mixed with water, the semi-synthetic material actually forms an emulsion rather than a solution. The appearance of the emulsion varies from opaque to transparent. Opaque fluias are the result of macroscopic emulsification of coolant components. These macroscopic particles, with the

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protection. This can prevent moving parts from seizing as well as minimize tacky build-up. Semi-synthetics are generally more waste treatable than conventional synthetic coolants. Waste treatability is increasingly becoming a major factor in selecting a coolant. A s with many compromises, there are some disadvantages to consider in the selection of a semi-synthetic fluid. The presence of mineral oil exposes the coolant to oxidation, with the possibility of sludge formation on surfaces. The emulsifiers required in the formulation of semi-synthetics can tie up significant quantities of tramp oil that may leak into the coolant system, further promoting sludge formation. This can also lead to decreased cleanliness, detergency, and oil misting. Significant emdlsifier presence in the semi-synthetic fluid leaves it susceptible to biodegradation and eventual rancidity. If not properly maintained, the fluid can deteriorate substantially, causing potential machining and environmental problems.

Finally, due to the characteristics of macro-emulsions/ micro-emulsions, semi-synthetic fluids cannot be fine filtered by such methods as diatomaceous earth. Emulsified lubricants and additives are likely to be stripped out by filtration finer than the particle sizes of the coolant. Once the decision has been made to use a semi-synthetic coolant, possible products must be selected that meet the needs of the individual machining operation.and part metallurgy. Like oil type coolants and synthetic fluids, semi-synthetic coolants are formulated for a variety of applications and metals. These range from narrow applicability to broad-purpose usage. Depending on their formulation and properties, semisynthetic fluids are capable of accommodating such metalworking processes as grinding, drilling, tapping, gun reaming, turning, milling, and broaching, to name a few, involving a range of metals including cast iron, steel, stainless steel, aluminum, and brass. The user should consult the coolant suppliers under consideration for their recommendations. tina and Selectina a F1uid Once a group of several possible fluids has been identified, the coolant buyer should bench test and evaluate the candidates according to the user's established test procedures and criteria. Each plant has its own methods of testing and particular parameters that must be met. There are, however, some basic evaluations, appropriate to any water-soluble metalworking coolant, which should be applied to semi-synthetic fluids. 1.

Waste Treatability This factor is of considerable importance; more and more metalworking plants are establishing waste treatability as

I n order to solubilize or emulsify the lubricants into the water, emulsifying chemicals are necessary. Most of the commonly used materials are borrowed from soluble oil chemistries. Typical emulsifiers used are non-ionic surfactants such as polyethers, amine soaps, petroleum sulfonates, and amine condensates. Many of these chemicals provide both emulsification of the lubricant, and also the anti-corrosion properties required for any water-dilutable coolant. The total chemical package for a semi-synthetic fluid also includes miscellaneous additives which are formulated into the coolant to provide specific properties. These can vary from phosphorous, sulfur, or chlorine additives for EP (extreme pressure) lubrication to wetting agents, defoamers, and biocides as required.

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Thus, the formulation of semi-synthetic metalworking coolants is plainly a smorgasbord of chemistries selected from both soluble oil and synthetic fluid technology. The types and influences of the two technologies when combined form a hybrid coolant exhibiting various degrees of characteristics from these two classifications of fluids. Semi-Svnthetic Advantaaes /Disadvantaaeq As previously mentioned, metalworking coolant users must first determine which category of fluid is required or desired for the metalworking operation. Typically, the fact that semisynthetic coolants contain petroleum oil is considered advantageous for those machining operations that may not be suited to synthetic coolant application. Many users believe that by selecting a semi-synthetic coolant, the positive characteristics of synthetic fluid chemistry can be realized without some of the drawbacks. Also, a semi-synthetic is often viewed as an improvement to the undesirable properties of straight oil and soluble oil coolants. It can be argued that selection of a semi-synthetic coolant offers a worthy compromise to the metalworking fluid user. What other advantages of semi-synthetic fluids are there? Because of the several borrowed properties from synthetic coolant chemistry, semi-synthetics can potentially run cooler and cleaner than oil type coolants. Synthetic lubricating agents formulated into the semi-synthetic can enhance machinability and tool life. The lower oil content and increased surfactancy as compared to straight and soluble oil can provide improvement in the cleanliness of the machining operation and the surrounding plant environment. Better surface tension properties can reduce coolant drag-out by parts and chips, decreasing coolant usage. At the same time, the properties borrowed from soluble oil chemistry provide advantages over conventional true solution synthetics. A semi-synthetic fluid deposits a light, oily film on machinery surfaces, providing lubrication and surface

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temperature exposure tests for non-ferrous metals. It is recommended that if a metalworking operation contains nonferrous fixtures, the semi-synthetic coolant should he evaluated for degree of corrosion resistance on those metals. 4.

Foam Resistance

The foaming resistance of semi-synthetic fluids should always be evaluated for operations such as grinding or those utilizing high-pressure coolant delivery systems. A variety of tests evaluating this property have been developed, such as high-speed blender tests and forced aeration (Figure 4). Generally, the foam height and rate of foam decay should be compared to the user's requirements and against the performance of other fluid candidates. 5.

Health and Safety Evaluating the possible health hazards of a metalworking coolant is mandatory in today's regulated environment. Virtually every manufacturing facility has some form and method to evaluate a coolant's material safety data sheet and to determine its potential health hazards. Semi-synthetic coolants should be scrutinized regarding the type of mineral oil formulated into the product. Certain classes of mineral oils are considered potential carcinogens, and if present in the product, may prompt the user to reject that particular metalworking fluid. The user should also be aware that some semi-synthetics have been formulated with materials that may be listed, reportable chemicals under the EPA's SARA, Title 3 , Section 313 reporting requirements. What this means is that, if the user has a product that contains a chemical on the Section 313 list and the amount used meets or exceeds the threshold level for that chemical, then the user'must report annually to the EPA the presence of that material in their facility, how much they receive, and how much was discharged into the environment. The use of a reportable level of a listed chemical in any prospective coolant should therefore be looked at by the user to see if it would pose a problem. Many coolant manufacturers are developing newer formulations of semi-synthetics that will eliminate all use of listed chemicals. Obviously, the potential health hazards of a,metalworking fluid should be carefully explored and evaluated according to the plant's established safety standards.

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Product Stability This property is one that is not always considered by users, and should be. An indication of neat material stability helps in determining the shelf life of the product under the storage conditions at the manufacturing facility. Of even

the primary consideration in selecting a coolant. Most manufacturing facilities must pre-treat their waste water prior to discharge. Because metalworking coolants can represent a significant percentage of a plant's waste water, it has become critical that the prospective coolant be evaluated to determine its impact on the facility's waste treatment process and its compatibility with that particular method of treating waste.

I n recent years, the EPA has been putting increasing pressure on local publicly owned treatment works (POTWs), via the various state environmental agencies, to enforce the Clean Water Act. Therefore, local POTWs have been increasingly vigorous in monitoring industrial sites for their discharges. Because of this, many metalworking managers are placing the main emphasis on whether their facilities can treat the spent coolant before they even consider any other criteria as to whether it may be a good coolant far their application. Semi-synthetics, as noted earlier, are typically more treatable than conventional synthetic fluids. They can be as treatable as soluble oils. Parameters to consider are COD (chemical oxygen demand) and FOG (fats, oils, and grease) content. The user may need to examine the cost of waste treatment chemicals for the coolant. One may also need to consider the amount of sludge generated in the waste treatment process. 2.

Lubricity Determining the ability of a metalworking coolant to lubricate and machine the parts is, of course, essential. Typically, coolant users evaluate the relative lubricity of fluids compared to one another as well as to some minimum level of performance determined by the plant. Bench tests sometimes utilized are tap and torque, Falex, 4-ball, and Timken tests (Figure 3 ) . On occasion, actual field tests will be conducted on the plant floor, where small-scale operations can be run to evaluate prospective coolants.

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Corrosion Resistance The amount of corrosion resistance provided by a watersoluble coolant is critical and should always be evaluated. There are a number of test variations that have been developed by the metalworking industry. Typically, an evaluation should be conducted in which various concentrations of semi-synthetic dilutions are rated for their degree of corrosion inhibition. The results are then evaluated by assessing the coolant's corrosion inhibiting performance compared to other fluid candidates, or by determining its acceptability in light of the user's requirements, or according to both of these measures. Examples of typical corrosion resistance tests are 24-hour cast iron chip exposure, Herbert rust tests, and elevated

monitoring and maintenance procedures. This point cannot be emphasized enough. There are several basic reasons why coolant systems need regularly scheduled monitoring and maintenance. These include prevention of coolant deterioration, prevention of production shutdowns, detection of mechanical support equipment malfunction, maintenance of part quality, avoidance of OSHA violation, avoidance of pollution problems, worker morale, and optimization of coolant costs. Coolant control procedures should, of course, be discussed with the semi-synthetic supplier who can outline specific methods for measuring and maintaining the fluid. Therefore, only the basic methods generally applicable to semi-synthetics will be presented here. Tests for monitoring semi-synthetic coolan+.s fall into two categories: tests that should be performed daily and those that should be conducted weekly. Daily tests monitor coolant concentration, pH level, suspended solids content, tramp oil content, and the concentration of the coolant premix system (if used). Coolant concentration can be measured in a number of ways, ranging from simple tests of alkalinity to more complex tests such as cationic-anionic titrations. In any case, the test should not be affected by the normal contaminants that exist in typical metalworking operations. Any deviations that may occur from the normal concentration level indicate a problem. Lower concentrations can lead to reduced tool life, workpiece corrosion, and/or biological problems. Higher concentrations can cause residues, foaming, health problems, and, of course, increased coolant costs. Regarding pH level, semi-synthetics typically have a pH between 8.5 and 9 . 4 . The specific level depends on the particular coolant as well as plant operating conditions. Any change in the pH level may indicate an increase in contaminants or a new source of contamination. It could also indicate significant activity by detrimental aerobic bacteria. Suspended solids content in the metalworking coolant consists primarily of metallic particles. coolant systems generally should operate with as minimal a suspended solids content as practical. Excessive amounts of such solids use up important components of the coolant which leads to decreases in concentration and fluid performance. This condition of excessive suspended solids indicates that enhanced filtration should be considered. Excessive tramp oil content in the semi-synthetic fluid can cause bacterial growth and related problems, including oil misting. In the most extreme cases, it can also result in dirty machine tools and operator complaints. In general, coolant systems can handle 1 to 3 percent tramp o i l . Any increases in

greater importance is dilution stability. This property is related not only to the quality of the product, but also to the quality of the water to be used in the metalworking coolant system. Hard water, in particular, can cause performance deterioration with semi-synthetics if precipitation is not inhibited. Stability tests typically used are 24-hour static stability or a longer term circulating stability test. In both cases, observations are made for the degree of precipitation or separation that might occur at various dilutions. 7.

Miscellaneous Factors The general evaluations recommended above are by no means comprehensive. Other properties that might be assessed are residue characteristics, pH, bio-stability, and tramp oil rejection. Each individual user of metalworking coolants should establish a complete set of evaluation procedures for prospective semi-synthetic fluids. With this type of approach, a more sound decision can be made in choosing a coolant.

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cost One more consideration in evaluating coolants, naturally, is the unit cost of the product. It's mentioned last here because, in the author's opinion, unit cost should not be the overriding concern in the initial screening of prospective semi-synthetic coolants. During the evaluation process, the data generated might indicate that a more expensive product is actually more cost effective than a less expensive material. This may be borne out by a comparison of the minimum coolant concentrations required for two or more coolants to perform equally, thereby indicating which fluid exhibits the lower use cost. Or lubricity data could indicate potential productivity gains and total cost reductions in using one semi-synthetic over another. Coolant evaluation should include some type of total cost analysis comparing the contending metalworking fluids

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After careful studv of all the data qenerated in the evaluation process, the-best possible semi-synthetic coolant should emerge. If two or more fluids still favorably compare, assessing external criteria could prove helpful. This data would include the vendor's field service capability, shipping/delivery, or even the personal preference of the user for a particular coolant manufacturer. Upon completion of this thorough appraisal, users will be in a position to select the proper semisynthetic fluids for charging their metalworking operations. Monitorino and Maintenance Procedures

In order to gain the maximum benefits of using semisynthetic metalworking coolants, it is necessary to set up proper

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probability of success f o r any metalworking operation. By subjecting potential coolant candidates to a battery of evaluating tests, the user can make a sound selection of a semisynthetic fluid that meets or exceeds the requirements of the metalworking application. Regular monitoring and maintenance of the selected product will assure long-term, reliable performance of the coolant.

tramp 011 content indicate excessive hydraulic oil leakage into the coolant system which can decrease the life of the semisynthetic. The coolant variables that should be monitored on at least a weekly basis are the biological contaminants -- bacteria, fungus, and yeast -- as well as rust and corrosion control. Coolant suppliers can usually provide testing services for biological contamination. If the bacteria level should begin to rise, the addition of a biocide may be necessary. Fungus can be difficult to monitor with measuring procedures since fungus occurs in independent masses n o t evenly dispersed in the coolant. Fungus is most easily detected visually and by smell. Fungicides can be used to retard fungal activity. However, if a system does become seriously infested, it may have to be flushed and the fungus removed physically. Rust and corrosion control properties should be tested, again looking for any changes in system condition. Keeping a record of these daily and weekly monitoring tests over a period of time will provide the user with a valuable guide in determining the level of contamination that can be tolerated by a coolant system before destabilization occurs.

In monitoring coolants, especially for large central systems or for critical machining applications, the user may find it valuable to perform statistical analyses on the test measurements. The use of statistical control methods such as XMR (X moving range) charts could aid in detecting a possible out-ofcontrol condition before it becomes a major problem, particularly in regard to concentration and pH level measurements. If finishes are critical, the user may also want to do a statistical analysis on suspended solids. It is also important for the user to establish a plan of action for system treatment specifying corrections to be made for each type of deviation that may occur. If coolant makeup additions are performed on a daily basis, less problems are likely to arise. Keeping the system fluid circulating continuously is also recommended; this helps prevent stagnation which promotes biological growth. Finally, maintaining the concentration of the semi-synthetic coolant on a daily basis is advisable. Whenever lost coolant is to be replenished, it is recommended that the coolant concentrate be added simultaneously with water. This can be done by adding prediluted coolant from storage containers or by using automated proportionating equipment. The reason for maintaining coolant concentration in this manner is to minimize large fluctuations in the integrity of the semi-synthetic coolant. Conclusion

In conclusion, a systematic approach to the evaluation and selection of a semi-synthetic coolant will increase the

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A Closer Look at Semi-synthetic Coolants by Rauth

Figure 1. Semi-synthetic coolants are an ideal compromise for users who want to minimize the use of oil in their metalworking operations but are not able or do not wish to use synthetics.

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A Closer Look at Semi-synthetic Coolants by Rauth

Figure 2. Semi-synthetics are a combination of soluble oil and synthetic chemistries. The amount of intermixing depends on the final coolant properties desired. A predominance of solutiontype synthetic chemistry, for example, would yield a coolant that behaves like a synthetic.

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A Closer Look at Semi-synthetic Coolants

by Rauth

Figure 3. Bench tests such as the Falex test can be used to determine the relative lubricity of semi-synthetic fluids.

A Closer Look at Semi-synthetic Coolants by Rauth

Figure 4. Foam resistance should be evaluated for operations such as grinding or those using high-pressure coolant delivery. After high-speed blender testing, the fluid on the left shows virtually no foaming while the one on the right foams excessively.