Beyond degradation of fluid appearance and function, there can also be adverse health effects caused by microbial contamination, and a number of industrial health syndromes have been linked to systemic infections by and adverse allergic reactions to microbes present in contaminated industrial fluids. With respect to metalworking fluids, recent concerns about allergic reactions to certain species of aerosolized mycobacteria are a perfect example.

The best way to prevent the microbial colonization of a fluid is through the addition of a biocide to the fluid. Many classes of biocides are known, and large amounts of data have been collected concerning their minimum inhibitory concentrations – the minimum concentration of the biocide necessary for control of a particular species of bacteria or fungi.

However, toxicity, regulatory, economic and chemical stability issues oftentimes complicate the seemingly simple strategy of adding a biocide to a fluid. When everything is considered, the minimization of biocide concentrations usually emerges as the best policy.

So how does the formulator deal with the contradictory goals of absolute biological control and minimization of biocide concentrations? One possible approach, in many cases just a start, is the use of biocide synergy. To properly describe and illustrate the utility of biocide synergy, it is necessary to review formulation synergy in general.

The Ancient Art of Formulation

The art of formulating functional fluids has developed steadily from prehistoric times to the present. Prehistoric man mixed natural pigments with a variety of natural binders to produce paint. The pigments were chosen mostly from easily accessed inorganic ore deposits (e.g., iron oxides, pyrites, galena, cinnabar) and the binders came from a variety of natural sources. For example, Dr. Carolyn Boyd of the SHUMLA School (Studying the Human Use of Materials, Land, and Art) in Comstock, Texas, reports that Native Americans in the southwestern United States experimented with binder ingredients as diverse as blood, egg whites, plant sap, animal fat and even urine. The first paint formulations were not optimal, and early formulators developed successive iterations of formulas in an attempt to get the best possible product.

As is true today, these early formulators used all the knowledge and technology at their disposal to accelerate the development process and converge on the best formula as fast as possible. Modern formulators do pretty much the same thing, albeit with a far greater array of tools and ideas to draw from in their efforts.

Formulating a Metalworking Fluid

We are not aware of prehistoric efforts to formulate metalworking fluids, but lubricant materials were used in ancient times. The ancient approach to developing a lubricant would have been an empirical one. The process starts with accidental and random observations of useful materials followed by random mixing with other known materials. The process continues until a successful product is obtained. If certain methods seem to be successful, then subsequent work is usually based on the same approaches.

The modern formulator still uses empirical methods, but empiricism is supplemented with conceptual knowledge. For example, the modern formulator understands the function of all the components in a formulation. We could call this the function concept. The function concept allows for the rapid selection of candidate materials to test in a given formula. Even with the function concept, the formulation process can be time consuming. Without it, the process is impossible.

By way of example, consider a simple semi-synthetic metalworking fluid containing the following eight components:

1) Water Solvent: solvent carrier for components of formulation, heat transfer fluid;

2) Oil Lubricant: lubricating phase to aid in machining of parts;

3) Emulsifier: creates emulsion of oil in water;

4) Nonionic Surfactant: modifies surface tension and possibly defoams system;

5) Coupler: allows for complete solubility of all additives;

6) Neutralizing Alkanolamine: raises pH to optimal value (8.9 to 9.3);

7) Corrosion Inhibitor: prevents corrosion of metal parts immersed in fluid;

8) Biocide: prevents microbial growth in the solution.

The function concept allows us to think in terms of candidate materials to fill each of the necessary functions within the fluid. Even in a simple fluid like this the formulator has a great many choices. The solvent can be pure water, or a mixture of water and an alcohol (e.g., isopropanol). The lubricating oil can be a Group II refined base oil, Group III refined base oil, polyalphaolefin, diester, polyolester or more exotic type of synthetic lubricant. The emulsifier can be a natural material produced by the sulfonation of refined base oil, or a synthetic material produced by the sulfonation of a purified alkylated aromatic compound. The nonionic surfactant can be chosen from thousands of possible ethoxylated alcohols, polypropylene glycols, alkylated phenols and/or other hydrophobes. The coupler, neutralizing alkanolamine, corrosion inhibitor and the biocide also present the formulator with long lists of choices.

What additional concepts can the formulator use to accelerate discovery? One important idea could be called the range of physical properties concept.

The formulator can define a necessary range for a given physical property based on a knowledge of the intended application of the fluid. The defined range of a given property is then superimposed on the candidate materials to further win-now the candidate list. Luckily, copious amounts of physical data are available in the literature. The choices for nonionic surfactant can be limited by consideration of cloud point, hydrophile-lipophile balance, molecular weight, solubility, surface tension and other properties. The choice of the appropriate base oil can be limited by consideration of viscosity, solubility, hydrolytic stability and chemical structure.

After an assessment of function and the range of physical properties needed, the formulator considers a variety of other issues like availability, cost, biodegradability, toxicity and general customer acceptance. After all this, a much smaller group of possible candidate components will emerge, and these materials can then be obtained in sample quantities for laboratory testing. After the final formula components are chosen and the appropriate level of each component is defined, a functional fluid will hopefully result.

But what other concepts, beyond those mentioned above, might have been useful in accelerating formulation development? One very useful idea is that of off-diagonal effects. In simple terms, an off-diagonal effect is the influence of a component on a property of the fluid for which it was not added – such as the influence of a corrosion inhibitor on a fluids emulsion stability.

Formulation and the Modern Mind

If we think about the formulation process mathematically, the techniques of linear algebra immediately suggest themselves. For an eight-component system, we generate an eight-by-eight matrix. Every position in the matrix represents the effect of an additive on a functional property of the fluid. For simplicity, lets imagine that every contribution of a component of the fluid to the actual properties of the fluid can be represented on a scale of zero to 10, with zero being the worst possible impact and 10 being the best. A value of five would represent no effect. We assume that the concentrations of all the components of the formula have been appropriately optimized. The rows of the matrix represent the eight components listed above while the columns of the matrix represent the physical properties of the fluid. The formulation matrix is shown in figure 1.

Within this matrix, position a6,3 represents the impact of the alkanolamine neutralizing agent on the emulsion stability of the fluid; a1,8 represents the impact of the solvent on the biostability of the fluid; a6,6 represents the impact of the alkanolamine on the pH, and so on.

Every component of the fluid is chosen to have the best possible impact on the property it is supposed to influence. Thus, assuming a good formulation has been developed, the fluid formulation matrix should have 10s all the way down the diagonal – implying that each additive has the best impact possible on the property it is meant to influence. If the formulation matrix does not have a diagonal of high values, then the formulation is not optimized.

However, the off-diagonal effects of the additives are harder to predict. The formulator may decide to use pure water as the solvent for its good solvency and heat transfer properties – but water also will have a less-than-optimal impact on the fluids bioresistance.

To illustrate a formulation matrix, consider the following typical formula for a kilogram of semi-synthetic cutting fluid concentrate:

*

The formulation matrix for the above fluid might look as shown in figure 2.

The true art of formulation lies in the optimization of the off-diagonal effects. Much like backgammon, formulation takes only a few months to learn but a lifetime to master. The few months are necessary to learn how to handle the diagonal effects. The lifetime is necessary to learn how to handle the off-diagonal effects.

To help formulators understand off-diagonal effects, Arkema, in King of Prussa, Pa., has carried out a significant amount of research into one cell in the matrix – a6,8, the impact of the neutralizing alkanolamine on the efficacy of the biocide in the generic formulation matrix above. The term used to describe a6,8 is biocide synergy. A compound used purposely for biocide synergy is known as a biocide adjuvant, biocide synergist or most simply as a biosynergist.

Historic Understanding

The first use of biocide synergy is prehistoric. Early Man made soap by boiling ground animal bones with animal fat and/or vegetable oil. The soaps produced in this way display biocide synergist properties when used in combination with natural astringents like witch hazel. General descriptions of the biocide synergist effect are also found in the writings of Pliny the Younger, late First Century historian and scientist.

This intuitive use of biocide synergy in metalworking fluids was put on a firmer footing in the late 1970s, when E.O. Bennett published a series of papers describing the effects of many non-biocides on the overall biostability of metalworking fluids (International Biodeterioration Bulletin, 1978, v.14, p.21-9; Lubrication Engineering, 1979, 35, 137-44). Since then, many additional publications have discussed various aspects of biocide synergy in metalworking fluids.

Arkemas contribution to the general work in this field has been the development, in collaboration with Microbe Inotech Laboratories in St. Louis, Mo., of standardized methods for the comparison of various alkanolamines as biocide synergists. We used high throughput optical absorbance measurements of bacterial growth in transparent broth media to compare various alkanolamines as synergists.

In brief, the Arkema/MIL method involves dosing an appropriate pH buffered transparent broth with various levels of biocide (e.g., triazine, BIT, CMIT/MIT) and alkanolamine. The prepared broths are dosed into the wells of a microtiter plate followed by standardized inoculation of all wells with a bacterial suspension of approximately 107 CFU/ml (about 106 CFU/ml after dilution).

The prepared microtiter plate is placed in a temperature controlled instrument chamber (typical inoculation temperature: 25 degrees C) and precise absorbance measurements are made on all wells once every 15 minutes. The incubation is allowed to continue for two or more days. After two days, the absorbance data is analyzed to determine how rapidly the bacteria grew in each of the wells.

Typically, at least three replicates of each solution are used to ensure that statistical abnormalities did not influence the data. The absorbance data is grouped into simultaneous sets of 15 points, starting from the first point and proceeding to the end. Each 15-point data set for a given well is subjected to a least squares analysis that yields a slope. The maximum slope for a given well is taken as the Maximum Growth Slope. The r2 value is checked to ensure that the slope value came from a reasonably linear portion of the data. The data can be fit to a partial log model, but such analysis is not necessary for quantification of relative growth rates.

Finally, the raw data is examined to ensure that the maximum growth slope was taken at a reasonable point and that the data processing was generally correct. Once an average maximum growth slope is determined, it can be plotted versus the concentration of various additives present in the different growth media. It is through an examination of maximum growth slope values for different alkanolamines in otherwise identical solutions that an assessment of relative biocide synergy can be made. For example, Figure 3 shows a typical graph of maximum growth slope versus the concentration of the biocide benzoisothiazolinone (BIT).

The development of standardized synergist data gives the formulator an enhanced ability to cull candidates from the initial list. When sufficient off-diagonal data is available, the formulator can define ranges for both base physical properties (diagonal effects) and off-diagonal efficacies (off-diagonal effects).

While the biocide synergist data that we have collected addresses only one off-diagonal term in the formulation matrix, it is still hoped that it will be useful in accelerating formulation efforts throughout the metalworking industry. Given sufficient time, all of the off-diagonal effects will eventually be completely described.