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In August, 2012 a member of the LinkedIn Metalworking Fluids Group asked for a recommendation for the best biocide/fungicide package to be used to protect a semisynthetic metalworking fluid from biodeterioration. His posting has generated nearly 50 responses. Some of the suggestions were clearly based on limited experience; experience with few MWF, a limited number of MWF systems or both. I posted a number of comments to the string and have compiled them in this article. I've edited my original posts to create an order by topic, minimize redundant comments, spell out acronyms on first use and provide appropriate trademark attribution when I've used product brand names. Although my comments are drawn primarily from ASTM consensus documents and STLE course material, the opinions that I've expressed herein are the ones I've expressed as an individual participant in the LinkedIn MWF Group.

Microbicide Selection - General Principles

Depending on where you are located, be mindful of local pesticide registration requirements (EU's biocidal products directive, USA's Office of pesticides programs, etc.). ASTM E2169 discusses the various considerations that contribute to optimized MWF microbicide selection and use.

Microbicidal versus Biostable Additive Use

From ASTM E2523 Standard Terminology for Metalworking Fluids and Operations:

"biocide, n-any chemical intended for use to kill organisms (sources E2169, E2275)

"Discussion-Biocide is a term usually used synonymously with the preferred antimicrobial pesticide or microbicide."

If there's a choice between using a potentially toxic "non-biocidal" chemistry for which there is a fraction of the toxicological data than that required for pesticide registrations, or using registered microbicides, I'll argue that it's substantially safer to work with the registered products. There are a number of molecules being formulated into MWF to confer biostability. The only documented performance properties are their ability to knock-down microbial contamination, yet they are being used as if their antimicrobial properties are incidental to other putative, but undemonstrated, performance properties. I'm waiting for the first litigation in which someone does something foolish with a MWF, discovers that it contains one of these unregistered biocides and sues whoever was involved with formulating, delivering, managing or allowing the use of the MWF in the plant where they were exposed. I expect that the litigation will have a significant chilling effect on the use of unregistered biocides.

MWF, by their very nature, are biodegradable. 5% concentrate, in well agitated and aerated water, at temperatures that approximate body temperature, provide an optimal environment in which microbial communities can flourish. Formulating with recalcitrant molecules (chemistry that resists biodegradation, but which isn't toxic to microbes or other living things), system design, good housekeeping and industrial hygiene practices all help to control microbial problems, but antimicrobials - when used effectively - are part of the prevention effort. In fact, using microbicides to knock-down populations that are already out of control is a waste of time, energy and good microbicidal product. I never recommend microbicide use as a band-aide solution to microbial contamination.

I'm all for bioresistant/biostable performance additives. But the key here is to demonstrate non-microbicidal performance properties and confirm that the additive doesn't rapidly knock-down the population (say from 6Log to <2Log CFU/mL in <8h).

Corrguard™ (Trademark of Dow Chemical, Midland MI) and Synergex? (registered trademark of Taminco, Inc. Allentown, PA) {NOTE: these is not a product endorsements; just examples of a legitimate approach to offering biostable amines into the MWF market} are good examples of products that are manufactured and marketed by responsible companies. Both have data sets demonstrating: biostability, corrosion inhibition and their inability to knock-down heavy bioburdens without augmentation by a microbicide. I contrast these products to DCHA for which no real data other than its antimicrobial activity has been published. IMHO, DCHA is an unregistered microbicide being used in violation of 40CFR 152-189 regulations.

A point about which I have offered my opinion in several peer reviewed papers and countless education courses. The toxicological data required for pesticide registration under BPD in Europe and FIFRA in the US are substantially more comprehensive (and expensive) than those required under REACH or TSCA, respectively. I suggest - as general guidance :

a) if a sample with >1E6 microbes per mL is treated with a chemical, and that chemical causes the population to die off in 24h or less, then the product is microbicidal.

b) if a sample with >1E6 microbes per mL is treated with a chemical, and that chemical does not causes the population to die off in 24h or less, then the product is not microbicidal.

c) If the concentration of product tested in (b) is monitored for a period of at least a week, but preferentially for 30 days in a MWF formulation that is either challenged with bacteria (or fungi or both) and the product's concentration doesn't diminish by >5% greater than it diminishes over the same time period in a microbe-free control, then the product is bioresistant.

d) If there are strong data demonstrating important performance properties (for example corrosion inhibition) other than microbicidal activity, AND neither the product's manufacturers nor users claim microbicidal performance, AND the users & manufacturers use/market the product based on its non-microbicidal properties, THEN a reasonable case for not having a pesticide registration can be made.

I strongly recommend AGANST using additives for which the only verifiable data are for antimicrobial activity, but for which "reason to believe" claims are made (for example, there are some microbicidal amine chemistries that are allegedly used as corrosion inhibitors or pH buffer, but without any demonstration of more than mediocre performance; the only real data are for microbial inhibition performance).

Conversely, there are a number of new amine chemistries for which the manufacturers have developed and reported data that demonstrate that when the amine is used by itself at designated concentrations, it is not microbicidal, but when used with registered microbicides, lower concentrations of the microbicidal products are needed. These products meet the criteria I listed as 9b) and (c).

A final note for this post: not all microbicidal chemistries need a pesticide registration. For example, 2N sulfuric acid will kill microbes quite effectively. As a strong inorganic acid, its industrial use is not as a microbicide. Therefore it doesn't need to have a BPD or FIFRA or other similar pesticide registration.

Of course the eternal search is for a MWF formulation that is:

a) Universally applicable;

b) Fully biostable in application;

c) Rapidly biodegraded during waste treatment and

d) Is environmentally and toxicologically benign.

I anticipate that it will be a couple of more years before the industry develops that product...

Although industrial microbicides are similar to antibiotics, in that they are both developed to kill infecting microbes, they differ in terms of specificity. Unlike antibiotics are designed to target specific microbes with minimal collateral harm to the patient or other microbes, microbicides are less specific. Most bactericides will kill genetically diverse bacteria. Fungicides will kill any species of fungus contaminating the MWF and broad spectrum products will kill diverse bacteria and fungi. Knowing what species are present is scientifically interesting, but beyond knowing whether your contaminant microbes are bacteria or fungi, doesn't do much to guide biocide selection. The only caveat might be mycobacteria which respond better to biocides with good lipid solubility.

Microbicide Selection

Many years ago, most of the MWF microbicide manufactures offered one or two products and did their best to convince MWF compounders and endusers to use one of those options. However, the primary microbicide manufactures serving the MWF industry today all have substantial product portfolios. I recommend that in order to determine what microbicide or combination of microbicides will work best for a given MWF formulation, there are three reliable options: a) develop an in-house capability to run microbicide performance tests (see my earlier post about ASTM E2275); b) work with a MWF microbiology expert to have candidate microbicides evaluated; or c) work with the manufacturer to get a recommendation based on what works best for the formulation and application. I'm a strong fan of option b, but that's my bread & butter business. I do not recommend following anecdotal recommendations from individuals based on their personal experience, since (again as I've posted previously), microbicide performance is affected by a variety of factors. Products that work perfectly under one set of conditions can be ineffective, or create problems under others

Bactericide Selection

I always recommend the lowest cost to treat product that works in the target application; recognizing that a product that meets that goal for one application may not for another. No universal answers here. Although I believe that the oxazolidines are fine antimicrobial chemistries, I'll repeat my mantra: NO ANTIMICROBICAL CHEMISTRY IS UNIVERSALLY APPLICABLE. Not to be harsh, but anyone who believes that this is an inaccurate statement simply lacks sufficient experience, knowledge of the subject or both.

When a microbicide isn't properly evaluated for use in both a specific MWF formulation and application, and subsequently fails to perform to expectations, customers make comments about the value of the product. It's like trashing a screwdriver because you can't use it to loosen a 2cm hex nut. There are >75 products approved by the US EPA as MWF microbicides; a similar number under the EU's Biocidal Products Directive. Each has a niche for which it is the most cost effective option. When compared against other chemistries, Triazine (HHT) and several of the oxazolidines have a much broader range of MWF & applications than most. However, no chemistry is universal. Don't take my word for it. Look up papers written in the 1960's by E. O. Bennett and H. W. Rossmoore in which they (actually their respective students) screened microbicides in 100's of MWF formulations. Having been evaluating MWF microbicides since 1981, my personal experience has been exactly what the Bennett papers and Rossmoore papers reported. This is why we have two ASTM standards on the issue. E 2196 - mentioned above - addresses strategic and tactical considerations for microbicide selection. E2275 provides guidance on how to evaluate MWF microbicide performance. Another indication of the veracity of my contention is that all of the major biocide manufacturers serving the MWF sector have considerable portfolios of active ingredients.

I recommend a slightly modified version of ASTM E2694 (ATP in MWF) to detect biofilm populations. The modified method detects biomass on surfaces and in deposits. You can also try using a sterile swab to transfer surface samples to dip-slide surfaces. One of the major causes for apparent microbicide performance problems is that only microbes in the bulk fluid are monitored. If you don't control the biofilm community, it's MUCH more expensive to control the free-floating (planktonic) community.

As noted by another member of the MWF Group, CIT/MIT has a short half-life in the high MEA fluids, but I'm not aware of a similar impact on NOIT. I'm glad to hear that a member of the Group is getting good performance with BBIT. Regarding price are you referring to cost/L of BBIT or cost-to-use per 1,000 L of MWF concentrate. Also, is a less expensive product really less expensive if it doesn't provide comparable performance to the more expensive product? I'd be interested in looking at impact on annualized MWF costs in order to keep from being misled by false economies.

Fungicide Selection

The most commonly used products are Na Pyrithione (NaP), 3-Iodo-2-propynyl butylcarbamate (IPBC), and N-octylisothiazolin-3-one (NOIT).

I wasn't aware of MEA accelerating IPBC's half-life, but IPBC's heat lability has been recognized since it was first introduced as a MWF microbicide. If drums will be stored at temperature much above 40? C, then IPBC won't be your fungicide of choice. NaP or NOIT are your best candidate chemistries.

I've been using NaP since 1981, and have never had a water issue. High concentrations of dissolved iron can cause NaP to become FeP - which is actually more fungicidally active, but insoluble in water. The addition of some chelating agent will sequester the Fe so that it doesn't react with the NaP. NaP is sold as blends with triazine and with oxazolidines. Given that it is stable in these multi-active ingredient products, I'm not sure why a Group member think it would be unstable in MWF with triazine or MEA (triazine is simply a condensate of MEA & formaldehyde).

ZnP is a very effective fungicide. That's one reason why it's the dominant active ingredient in the dandruff shampoo market. Perhaps, counter-intuitively, it is not registered for use as a MWF fungicide in the U.S. (nor in the BPD, but don't quote me on the latter). The problem in MWF (the reason why ZnP's manufacturers have never sought the MWF end-use cite) is that ZnP, CuP, FeP are not water soluble. Only the sodium salt is. The metallic salts (CuP is used as an antifouling coating active-ingredient in marine coatings; we've already discussed ZnP) are used effectively in colloids and other suspensions in which the carrier is sufficiently viscous to keep the insoluble pyrithione particles in suspension. As I wrote, several posts ago, FeP forms spontaneously in MWF with high concentrations of Fe-fines (primarily ferrous grinding). It's a more effective fungicide than NaP, but tends to settle out. Once it has precipitated out of the recirculating MWF, it's no longer available. That and its tendency to discolor the MWF are minor limitations, but explain why it's NaP that's marketed into the MWF sector. Be certain to check the pesticide regulations in your locality. A Group member wrote that DMC, Brazil report that NOIT's half-life decreases at pH >9. I'll reach out to my friends at DMC and ask for the data showing the relationship between NOIT half-life and pH.

Condition Monitoring and Tankside Treatment

A Group member posted: "You can often get massive fungal mats and still get zero counts on dip-slides". Spot on! The parts of fungi that carry spores can generate 100's of colonies (1/spore) but only amount to <100 ng of actual biomass. Conversely the filamentous mass we call vegetative hyphae can contain billions of cells, weigh tonnes and potentially yield a single colony. Moreover, if the fungal mat is on surfaces and you sample the recirculating fluid, you are likely to miss the fungi. CIT/MIT is not a fungicide, even though it can kill fungi at very high doses in a lab flask.

I am aware of systems that have run for extended periods without the use of tankside additions of microbicides. Per my earlier comments, I am also aware of MWF that are formulated without US EPA registered microbicides that also achieve extended life. IF fluid managers follow all of the well documented best practices (dare I put in a plug for STLE's Certified Metalworking Fluid Manager program here?) then tankside microbicide use potentially can be minimized. The problem is that best practices aren't always followed. Foreign fluids and solids find their way into MWF systems and overburden it with more microbial contamination than the formulation can handle without augmentation (READ: microbicide additions). In the real world, best practices are too often dismissed as either being too expensive, too time consuming or both. At least that's my experience. Of course, my experience is biased by the fact that folks with problem free operations don't call on my services.

Microscopy can be a useful tool, but as a Group member noted, it requires a skilled technician to get reliable direct microscopic observation results. Fungi are relatively large and can be observed using a low-power (10x to 200x) stereoscope. Bacteria are trickier. You need high power magnification (1,200x) to see them. At that magnification you need >1E6 cells/mL to see one per microscope field. Direct observation by phase-contrast is difficult because bacteria, micelles and swarf all look very similar at that magnification. More advanced microscopic methods involve separating the microbes form the MWF and staining them (numerous options here) for microscopic observation. Although I'm a professional microbiologist, it's been nearly 3-decades since I last used a microscope routinely. The ley is to carefully define the questions you are trying to answer, and then select the tool or tools (methods) that will best answer those questions. Microscopy is a very powerful tool for certain applications. Just not for routine MWF condition monitoring.

I've never advocate intentionally infecting or maintaining so-called good (non- biodeteriogenic) bacteria as a strategy for suppressing biodeteriogenic ("bad") bacteria. Have discussed the fact that there is typically (let's say invariably) interaction among different microbes in an ecological niche (MWF is a collection of niche from the perspective of microbiologists). These interactions include synergies (the activities of each member of the community provide benefits to others), commensalism (no apparent harm or benefit - might just reflect the limits of our understanding), or competition (some cause harm to others). Most commonly I discuss the impact of misuse of biocides. A product that only kills the fastest growing microbes creates opportunities for slower growing ones to dominate. In the past, this has been misunderstood as the result of mutation rather than the impact of unintentional, selective elimination of part of the population.

Regarding one company's use of intentionally infected MWF, I'm ambivalent. I am very impressed with the level and amount of R &D they have done. However, even if they effectively prevent biodeterioration, they may not be reducing the health risks associated with bioaerosol exposure. There's simply not enough information about this. Although mycotoxins (certain fungal metabolites) can inhibit bacteria , I suspect that differences in MWF formulation and dilution water chemistry, and contamination source, are the primary factors in determining whether a given formulation will be more susceptible to fungal, rather than bacteria contamination.

Most microbicide manufacturers have protocols for quantifying the concentration(s) of their product(s) in MWF. Depending on the microbicide there may be simple colorimetric tests (there are several commercially available formaldehyde test kits), but as John noted, most have HPLC protocols. I suggest contacting microbicide suppliers for product-specific methods.

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Dr. Frederick Passman, PhD is a Certified Metalworking Fluids Specialist with over 35 years experience in Environmental & Industrial Microbiology. His company, Biodeterioration Control Associates, Inc. (BCA) provides clients with unparalleled expertise in Microbial Contamination Control.

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