Metalworking fluids (MWFs) perform very important roles including lubricity, cooling, chip transport and corrosion protection¹ to ensure that machining operations conducted by end-users meet specifications that are becoming more precise. Many of the MWFs fulfilling these functions are formulated with water and/or diluted with water before use. 

These water-based MWFs face a series of challenges that can readily lead to premature failure. Included among the concerns is contamination by microbes that include bacteria and fungus. The reason for the vulnerability of MWFs is they are formulated with many types of components that are basically organic in nature. This “cocktail of carbon-based components” represents a very desirable food for bacterial and fungal growth, which can interfere with the operation of the MWF. 

An important additive type formulated into MWFs to control microbial growth is known as microbicides. From a terminology standpoint, biocides are considered by the main regulatory agencies (U.S. and EU) to cover not just microbes but all pests, which include nematodes, insects, rodents, etc. Active substances that specifically are targeted to control microbial growth are formally known as antimicrobial pesticides and informally known as microbicides. In this article, the term microbicides will be used exclusively. 

The purpose of this article is to provide basic information on microbicides, challenges faced in their use and what the future holds for their continuing use. 

Insight on microbicides was obtained from the following industry experts. 
1. Dr. Fred Passman, Biodeterioration Control Associates, Inc. 
2. Jeff Long, Buckman Laboratories International Inc. 
3. Dr. Griselle Montanez, LANXESS Corp. 
4. Dr. Uwe Falk, The Lubrizol Corp. 
5. Richard Butler, New Age Chemical, LLC 
6. Jennifer Lunn, Independent. 

What is the function of a microbicide? 
STLE Fellow Dr. Fred Passman, president of Biodeterioration Control Associates, Inc. in Princeton, N.J., says, “The primary function of a biocide is to kill living organisms. Microbicides target pests, which are grouped categorically, as implied in the name of the U.S. legislation that regulates biocidal use: The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). In the U.S., pesticide regulations are detailed in Code of Federal Regulations detailed in 40 CFR Chapter I, Subchapter E Pesticide Programs, Part 152, Pesticide Registration and Classification Procedures through Part 180, Tolerances and Exemptions for Pesticide Chemical Residues in Food.”² 

Passman indicates that the U.S. EPA’s Office of Pesticide Programs (OPP) regulates biocide use and establishes product registrations defined by the targeted pests (e.g., all microbes, bacteria or fungi) and applications (e.g., example, lubricants, MWFs, water treatment, etc.). 

The EU regulates microbicide use under the Biocidal Products Regulation [Regulation (EU) No. 528/2012—BPR], which categorizes microbicide based on their intended applications. Microbicides intended for use in MWFs are all listed under Annex V, Part 13, Working or cutting fluid preservatives.³ 

Under Article 3.1 of the BPR,⁴ a biocidal product has the following definition: “Any substance or mixture, in the form in which it is supplied to the user, consisting of, containing or generating one or more active substances, with the intention of destroying, deterring, rendering harmless, preventing the action of, or otherwise exerting a controlling effect on, any harmful organism by any means other than mere physical or mechanical action.” 

Dr. Griselle Montanez, applications technical manager for LANXESS Corp. in Wilmington, Del., states that microbicides are chemical additives that impart biological activity on microorganisms, such as bacteria and fungi, and are used to prevent the biological deterioration of a MWF. She says, “There are two types of microbicides: preservatives and rapid kill antimicrobials. Preservatives prevent the proliferation of microbes and can render activity for months or years, while rapid kill antimicrobials are microbicides that have a quick kill action to treat fouled MWFs that can last minutes to hours.” 

Jeff Long, technical director – emergent markets at Buckman Laboratories International Inc. in Richmond, Texas, says, “The function of a microbicide is to control or eliminate the growth of microorganisms, such as bacteria, fungi, algae and viruses. Microbicides are used to prevent microbial contamination and subsequent growth of microorganisms in various systems.” 

How is microbial growth controlled? 
Long indicates there are five potential paths to control the growth of microbes. 
• Blocking of metabolic pathways. Respective microbicides interfere/ block specific metabolic pathways, which are necessary for microbial survival. By inhibiting these pathways, microbicides starve microbial metabolism and remove energy production, preventing further growth/proliferation. 
• The cell membranes are disrupted. A microbicide damages or disrupts the cell membranes of microbes where it penetrates the cell wall or membrane, promoting leakage of cellular contents, loss of vital nutrients and ultimately leading to cell death. 
• Inhibition of enzyme activity. A microbicide type that targets specific enzymes essential to microbial metabolism or cellular processes, disrupting crucial biochemical reactions within the microorganisms. 
• Damage to DNA. The applicable microbicide causes damage to the genetic material (DNA) of microorganisms. This can occur through processes such as DNA crosslinking, strand breaks or interference with DNA replication and transcription. Impaired DNA function disrupts microbial growth and reproduction. 
• Oxidative stress. Some microbicides generate reactive oxygen species (ROS) within microbial cells. ROS can damage cellular components, including proteins, lipids and DNA, leading to oxidative stress and cell death. 

Dr. Uwe Falk, global commercial manager biocides for The Lubrizol Corp. in Hamburg, Germany, says, “Microbicides are specifically designed to interfere with microbe growth and survival, making proliferation of the organisms unlikely to occur. Different chemistries are approved and specified to control different types of microbes, including both bacteria and fungi. A balanced chemical treatment plan in both the MWF concentrate, and a tankside maintenance program as needed, can yield a successful avoidance of lost productivity for end-users.” 

Montanez says, “Microbicides can either kill a microbe or prevent it from proliferating by hindering its metabolic activity, and thereby reducing its speed of reproduction. A commercially available microbicide contains one or more active ingredients that, depending upon the chemical composition, have a specific mechanism of action on the microbial cell. When a microbicide interacts with a microbe, it may affect the physical integrity (membrane active microbicide), or it may interfere with one or more molecular processes (electrophilic microbicide).” 

Montanez provides details on the mode of action for specific microbicide types. She says, “Phenolics, quaternary ammonium salts and alcohols disrupt the physical integrity of a microbial cell by protein denaturation. This destabilizes the microbe cell’s membrane structure, causing cellular rupture. Electrophilic compounds (oxidants such as chlorine, bromine and peroxides) cause enzyme inhibition when reacting with specific functionalities contained in amino acids (amino, thiol and amide groups). Electrophiles based on aldehydes and those releasing formaldehyde bind to amino groups, inhibiting and inactivating proteins and nucleic acids. Isothiazolinone-based electrophiles are known to inhibit specific enzymes such as lactate dehydrogenase, alcohol dehydrogenase and ATPase, among others.”

Microbicide types approved for use in MWFs 
Passman indicates that ASTM E2169,⁵ “Standard Practice for Selecting Antimicrobial Pesticides for Use in Water-Miscible Metalworking Fluids,” lists the active microbicides currently approved for use. He says, “Tables 2 and 3 in ASTM E2169⁵ list the active substances that are approved for use in water-based MWFs by U.S. EPA’s OPP, and in BPR’s active substances with Product Type 13 dossiers, respectively. Table 2 in ASTM E2169⁵ lists 52 active substances and Table 3 lists 27, with these lists being current as of the date of this article being published.” 

Passman next discusses the main types of microbicides approved for use in water-based MWFs. 
• Formaldehyde condensates. This remains the largest category of microbicides even as manufacturers withdraw products from the market. 
• Isothiazolinones. Since the product’s introduction in the late 1970s, the number of isothiazolinone active substances has grown to half a dozen. 
• Hindered phenols. The first microbicide used as a MWF preservative was the hindered phenol, ortho-phenylphenol (OPP). When phenol became a regulated substance under the Clean Water Act, many compounders abandoned phenolic microbicides. However, now that hindered phenols can be differentiated from toxic, chlorinated phenols, they have been regentrified. Their renewed acceptance was helped by a theory that phenolic microbicides were more effective than other chemistries against Mycobacterium immunogenum—one of a dozen microbial contaminants of MWFs known to cause hypersensitivity pneumonitis. 

Passman says, “In tests performed by professor Ed Bennett’s students in accordance with inactive ASTM E686, “Standard Method for Evaluation of Antimicrobial Agents in Aqueous Metal Working Fluids,” over the course of more than a decade (mid- 1970s to mid-1980s), several formaldehyde condensates were effective in greater than 80% of the MWF formulations in which they are tested. However, most other active substances were effective in fewer than 50% of the formulations in which they were tested. These tests rated microbicides by the number of weeks they prevented culturable microbial populations (i.e., populations detectable by culture tests) from exceeding the upper control limit under Test Method E686 conditions. The Test Method E686 evaluations did not assess microbicides’ speeds of kill, and the decrease in number of culturable microbes as a function of time.” 

ASTM E2275,⁶ “Standard Practice for Evaluating Water-Miscible Metalworking Fluid Bioresistance and Antimicrobial Pesticide Performance,” has proven to be an effective method for evaluating how microbicides can affect the bioresistance of MWFs. Figure 1 illustrates a study conducted with 200 MWFs evaluated with one of 14 additives (A through P). 


Figure 1. ASTM E2275 was used to evaluate the ability of 14 additives (A through P) in improving the bioresistance of 200 MWFs. Figure courtesy of STLE.

Passman says, “Additives (A through F) enhance bioresistance in 100 or more of the MWF formulations to which they were added. Additive A was particularly effective with over 90% of the treated MWFs. In contrast, Additives O and P were effective in less than 5% of the MWF formulations evaluated in this study.” 

Passman cautions that it is imprudent to offer generalized statements about whether a given microbicide will work in all MWFs. Figure 2 presents results from a study using the same Additive F that was part of the Figure 1 study. He says, “In this case, Additive F does not show effectiveness in 30 MWF formulations as bioburdens are higher than what is seen in untreated controls.” 


Figure 2. Additive F from the study shown in Figure 1 did not improve the bioresistance of some MWFs. The chart shows that bioburdens for 30 MWF formulations are higher than what is seen in untreated controls. Figure courtesy of STLE.

Montanez advises that microbicides must be used according to EPA guidelines, where the end-use application must be approved, plus lower and maximum dosing levels of the active ingredient must be followed. She adds, “It is important to note that not only the active but the commercially available product must be registered for a specific use application. The use of microbicides in the marketplace is heavily dominated by products containing one or more actives classified as formaldehyde releasers [i.e., hexahydro-1,3,5-tris (2-hydroxyethyl)- s-triazine—HHT], isothiazolinones, morpholines, phenolics, carbamates and oxazolidines.” 

Falk says, “Approvals and registration are key considerations. To maintain compliance with products placed in the marketplace, geographic regulations must be considered. The range of approved and licensed products varies greatly around the world. Ignoring compliance with local market regulations can present liability problems.” 

Long says, “Basically, industry possesses four common types: isothiazolinones (family), triazines, 2,2-dibromo-3-nitrilopropionamide (DBNPA) and quaternary ammonium compounds.” 

Microbicide use in MWFs 
Passman states that microbicides are used in MWFs in the following three ways: 
• Disinfect MWF formulator’s equipment 
• Preserve MWF concentrates in their packaging prior to use 
• Control microbial populations in end-use diluted MWFs. 

Passman says, “The most important aspect for microbicide performance in MWF concentrates prior to use (in packaging such as drums) is compatibility with other MWF components. Speed of kill and persistence of effect are relevant characteristics for microbicides in end-use MWF applications. But note that formulating into concentrates with the objective of providing antimicrobial performance is a delicate balance act. To be effective, the microbicide concentration in the diluted MWF must exceed the minimum concentration at which it is effective (known as the critical concentration). However, making sure this occurs is difficult because the end-use concentration of many MWFs can vary by application and typically ranges from 3% to 10%.” 

Adding to the challenge is that EPA limits the maximum concentration for microbicides permitted in MWFs when used by the end-user. He says, “The chances are good that a microbicide may be present in a MWF at a concentration that is less than the critical concentration. For example, consider a MWF formulated with a microbicide that is optimized to perform when the MWF is end-use diluted to 10% by volume (e.g., end-use diluted MWF contains 2,000 ppm microbicide). If the MWF is used at 3%, then the microbicide concentration is most likely to be below the critical concentration (e.g., critical concentration = 1,000 ppm, but 0.3 x 2,000 = 600 ppm).”

The problem for end-users is that microbial populations exposed to microbicides present at less than the critical concentration are likely to become resistant not just to the specific additive but tolerate an entire chemical class. This means that microbes tolerant to formaldehyde are typically tolerant to formaldehyde-releasing microbicides. 

Falk states that it is usually advisable to begin by using microbicides in an appropriate fluid concentrate treatment. He says, “This is a proactive approach to creating a program to control microbial proliferation and avoid future problems. Always use microbicidal and fungicidal products at treat rates in concentrates that will yield appropriate active treat rates in the recommended fluid dilutions.” 

Falk also indicates microbicide addition tankside may be advisable for many operators, and a proactive approach also is recommended. He says, “Always treat at the recommended active chemistry rate, but note that reliance on tankside additions to correct problems that have escalated to problematic levels is never ideal. With fungal contamination in particular, it may virtually be impossible to remove biomass formation with chemical methods alone. Physically removing the biomass may be the only viable option.” 

Long indicates that microbicides are used for preservation, added at a specific maintenance dosage, for system cleaning and disinfection and for fluid treatment. Each of these subjects is further explained below. 
• Preservation. Microbicides are added to MWFs during the manufacturing process to provide initial microbial protection and prevent contamination before the fluid is used. MWFs remain free of microorganisms during storage and transportation with successful applications. 
• Maintenance dosage. MWFs are susceptible to microbial growth over time due to the presence of water, organic materials and favorable conditions for microbial proliferation. Microbicides are added at regular intervals or as needed to maintain a proper concentration of active ingredients in the fluid, effectively controlling microbial populations. 
• System cleaning and disinfection. Periodic cleaning and disinfection of MWF systems, including tanks, reservoirs and associated equipment to eliminate existing microbial contamination and prevent reoccurrence is required. 
• Fluid treatment. Microbicides can be dosed directly into the MWF system to treat the fluid and inhibit microbial growth. This may involve continuous dosing or intermittent applications, depending on the specific requirements of the system. ‘

When selecting and utilizing microbicides with MWFs, it is crucial to consider their compatibility with the fluid formulation and other additives along with overall stability. 

Montanez indicates that microbicides are used in MWF concentrates and/or added tankside to fluids running at their use dilution. She says, “Microbicides are added to MWF concentrates to preserve a fluid during packaging, transport and in use after dilution (where applicable). Tankside addition is conducted to either prevent biodeterioration during the lifetime of the MWF or to reduce the microbial population of a fluid that has spoiled. The dosing concentration of a microbicide tankside is typically lower than in concentrates, but more frequent dosing may potentially be required.” 

Microbicides used in specific MWF types 
Table 1 lists some of the microbicides used in MWFs and indicates whether they can be used in the MWF concentrate or tankside. In some cases, microbicides can be added to MWF in both fashions. 


Table 1. The main types of microbicides used in MWFs can be formulated into a concentrate, added tankside or used in both ways. Table courtesy of LANXESS Corp.

Most of the microbicides listed in Table 1 target bacteria and are known as bactericides. 2-N-octyl-4-isothiazolin-3-one (OIT), iodopropynyl butylcarbamate (IPBC), propiconazole and sodium pyrithione are four microbicides designed specifically against fungus and are known as fungicides. 

Montanez says, “When choosing a microbicide for MWFs, there is not a one-size-fits-all approach. The criteria for choosing a microbicide includes aspects of chemistry, microbiology and practicality. First, the fluid type (emulsifiable oil, semisynthetic or synthetic) needs to be considered, and a determination made about whether a specific microbicide active is stable and compatible with the MWF formulation. The next step is to determine what type of microbial contamination is occurring in a specific MWF system. A final consideration is whether the microbicide will be used in a MWF concentrate or added tankside. 

Questions that a MWF formulator or end-user need to ask before using a microbicide are listed below. 
• Is the objective in using the microbicide to preserve a MWF concentrate? 
• Will the MWF to be treated be diluted for end-use? 
• Will tankside addition be needed to maintain the fluid? At what dosage and frequency? 

Montanez provides examples of how to select specific microbicides based on chemical compatibility, type of microbial contamination, dealing with bioburden and to work synergistically. 

Chemical compatibility. Members of the class of microbicides known as isothiazolinones exhibit different properties based on pH and based on the presence of alkanolamines in the MWF concentrate. A microbicide blend containing 5-chloro-2- methyl-4-isothiazolin-3-one + 2-methyl-4- isothiazolin-3-one (CMIT/MIT) degrades at pH values above 8.0 and also is sensitive to primary and secondary alkanolamines. 2-methyl-4-isothiazolin-3-one (MIT) has a broader pH stability but is sensitive to primary and secondary amines. 

Due to these chemical compatibility concerns, CMIT and MIT are seldom used in MWF concentrates because most formulations contain alkanolamines; hence, they are primarily used tankside. 

In contrast, 1,2-benzisothiazolin-3-one (BIT) is stable in the alkaline pH range where MWFs are used and is compatible with alkanolamines. BIT is versatile enough to be used in both MWF concentrates and tankside. 

Type of microbial contamination. Montanez indicates that the types of microbial species in the MWF system and the level of bioburden need to be identified. She says, “Are multiple microbicide actives required to provide synergy such as when visible fungal mold is observed in a system? This may require the use of several fungicides. The end-user must keep in mind that microbicides do not work against all types of bacteria and fungus. For example, HHT has an efficiency gap against sulfate reducing bacteria, and mycobacteria while BIT is not effective against some Pseudomonas bacteria species. In contrast, bronopol is effective against Pseudomonas, and isothiazolinones are highly efficacious for slime control (biofilm).” 

Bioburden. To deal with bioburden, a microbicide package that provides longer term preservation (BIT or MIT) or rapid kill microbicide (the biocide 2,2-dibromo-3-nitrilopropionamide— DBNPA) can be used. DBNPA is a fast acting, short-term preservative that can quickly kill the microbes but is not stable over the long term. 

Synergism. Montanez indicates that some microbicide active ingredients can be used in combination to achieve good perseveration without sacrificing chemical stability. Such an approach can bring advantages to the forefront in terms of safety, cost and/ or closing efficacy gaps. Figure 3 demonstrates how the combination of BIT and nitromorpholine can achieve excellent bactericidal activity in a synthetic fluid when the concentration of both actives is reduced to 250 ppm. This combination has the added benefit of reducing the effects of lachrymation that can occur when using nitromorpholine. 


Figure 3. Adding the microbicides, BIT and nitromorpholine, to a synthetic MWF produces a synergism where lower treat rates of both actives improve resistance to bacteria compared to using each of the microbicides individually. Figure courtesy of LANXESS Corp.

Mycobacteria control is illustrated in data shown in Figure 4. CMIT/MIT and MIT are much more effective than oxazolidine, HHT and BIT. 


Figure 4. Mycobacteria control can be achieved with the use of CMIT/MIT and MIT compared to other microbicides. Figure courtesy of LANXESS Corp.

Long lists three types of microbicides that are used in specific MWF types. 
• Polymeric, water-soluble quaternary ammonium polymer—compatible in synthetic MWFs only. 
• Aqueous solution of BIT—effective in most water-based MWF formulations. 
• Synergistic blend of mixed isothiazolinones and a cationic quaternary ammonium polymer— compatible in synthetic MWFs only. 

Falk provides guidelines on using microbicides in MWF types. He says, “The range of available microbicides are generally used in all water-based fluid types. Selection for concentrate treatments requires evaluation of compatibility and stability. Microbicides with better oil solubility profiles may be easier to incorporate into emulsifiable oils and semisynthetic fluids. Certain microbicidal products are unstable in fluid concentrates and only designed for tankside application.” 

Changes in the number of microbicides 
Long indicates that the number of approved microbicide options has been curtailed due to the following: 
• Regulatory concerns. Regulatory agencies, such as the EPA, continue to scrutinize the microbicidal offerings in the industry, and with the process, update their guidelines and requirements regarding microbicides. The outcome has either restricted or banned the use of certain biocides due to environmental concerns or safety considerations. 
• Microbicide development. Industry continues to research and develop new microbicidal compounds or formulations that are more effective, have improved environmental profiles and address specific challenges in metalworking applications. This ongoing development can lead to the introduction of new microbicides into the market. 
• Environmental considerations. There is an increasing focus on the environmental impact of microbicides with new policies and green initiatives. Additionally, there are associated liabilities considered in the selling of various microbicides along with market demands and registration costs; therefore, situations arise for discontinuing the option. Today there is greater emphasis on development of microbicides with improved environmental profiles, such as those that are biodegradable or have lower toxicity. 
• Industry changes. The microbicide industry has experienced major consolidation, with larger companies acquiring smaller ones or merging with competitors, especially post- COVID-19 pandemic. This consolidation has affected the availability of specific microbicides in product portfolios due to restructuring or strategic decisions, and allowances on supplementals. 

Passman reveals that the number of microbicides available for use in MWFs has shrunk dramatically over the past 20 years. He says, “Some microbicide manufacturers have opted to no longer support product registrations unless sales volume in non-MWF applications can support the $2 million to $10 million U.S. cost of developing additional toxicological data required by U.S. and EU regulators. Fortunately the MWF industry has been able to introduce new microbicides that were approved initially for other applications. In the future, no new active substances will be registered just for use in MWFs.” 

Additionally, far fewer microbicidal products blended with two or more active substances are now available to the MWF industry. Passman says, “There used to be countless blends available, but now with the added product registration costs, mostly single component microbicides are approved for use in MWFs.” 

Falk says, “Approved microbicidal products have declined over time. A large investment in local regulatory compliance has reduced the number of chemistries and products that microbicide suppliers can offer to the MWF industry.” 

Montanez says, “While shifts have occurred in the availability of microbicides due to cycles of shortage of raw material supply (such as for BIT), changes in regulations in regions such as Europe and assiduousness in reviewing and reducing use and dosing levels of regulatory agencies, in general, the same marketplace players have remained successful.” 

Status of formaldehyde-releasing microbicides 
Formaldehyde has been under regulatory scrutiny for some time and was designated by EPA as a human carcinogen in a draft risk assessment published in April 2022.⁷ Montanez says, “The EU classified formaldehyde as suspected of causing cancer (category 2 under the Classification, Labelling and Packaging [CLP] Regulation) until June 2014 when EPA reclassified formaldehyde as ‘may cause cancer’ (category 1B under the CLP Regulation).” 

These regulations have put formaldehyde- releasing microbicides under scrutiny. Passman says, “The dominant active substance used in MWFs is the formaldehyde- releasing microbicide HHT. The EPA has been slowly making it more difficult to register new formaldehyde-releasing microbicides over the past 20 years. In the most recent (2008) Reregistration Eligibility Decision cycle, EPA reduced the maximum permitted concentration for HHT in end-use MWFs from 1,500 ppm to 500 ppm for five suppliers. Only one company retains the 1,500 ppm maximum permissible concentration for HHT.” 

Passman indicates that EPA’s decision is based on faulty data that assumes HHT hydrolyzes completely to its starting materials (formaldehyde and monoethanolamine at alkaline pH values of end-use diluted MWFs) and the misreading of a research report that stated HHT was ineffective at concentrations less than 500 ppm. He adds, “¹³C nuclear magnetic resonance (NMR) spectroscopy data demonstrated that formaldehyde concentrations were below detection limits in MWFs with pH values above 8 and treated with formaldehyde-releasing active substances.” 

Falk says, “There is elevated pressure against formaldehyde-releasing products by governmental agencies in many parts of the world. Developing nations with less restrictive regulatory systems still widely use formaldehyde-based products. Acceptance of formaldehyde varies greatly from region to region.” 

Long says, “Formaldehyde-releasing microbicides have been widely used in various industries due to their antimicrobial properties over the decades. However, formaldehyde is classified as a hazardous substance due to its potential health and environmental impacts. Consequently, there have been regulatory restrictions and efforts to reduce the issue of formaldehyde and formaldehyde-releasing compounds. In some regions, such as the EU, formaldehyde and formaldehyde-releasing microbicides have faced increased scrutiny and regulatory measures. For example, in the EU, formaldehyde is classified as a substance of very high concern (SVHC) under the REACH regulation, which requires authorization for certain uses and imposes strict limitations. As a result, the use of formaldehyde-releasing microbicides has been gradually restricted or replaced with alternative solutions.” 

Long continues, “HHT is one specific formaldehyde-releasing microbicide that has commonly been used in various applications, including MWFs, paints and coatings. However, due to the concerns associated with formaldehyde, there have been efforts to reduce or eliminate the use of HHT and other formaldehyde-releasing compounds. In recent years, there has been a shift toward alternative microbicides that provide effective microbial control without the use of formaldehyde or formaldehyde-releasing compounds. These alternatives include isothiazolinones, triazines, quaternary ammonium compounds and other chemistries that offer similar or improved antimicrobial properties while addressing environmental and health concerns.” 

MWF formulator perspective 
Two representatives from MWF formulators were asked for their perspective on using microbicides. STLE member Richard Butler, director of technology for New Age Chemical in Delafield, Wisc., is concerned that two of the three major broad spectrum biocides used in product concentrates, HHT and isothiazolinones, may be unavailable in the future. He says, “These microbicide classes are primarily used as bactericides. Fungicide options are much more varied and under less pressure. Microbicides used tankside also are more readily available and under less pressure than concentrate compatible microbicides.” 

STLE member Jennifer Lunn, independent, is pleased with the currently available microbicide options. She says, “The cost, ease of handling and overall field performance of HHT still makes it the most attractive microbicide active. If regulations prohibit the use of HHT, then other options such as methylene-bis-morpholine will be considered.” 

In using microbicides, both Butler and Lunn work with multiple ones in formulating. Lunn says, “A widely used broad spectrum bactericide and a fungicide is preferred for use in MWFs. This is combined with a couple of pH buffers to prepare a robust formulation at the proper pH that can help stave off bacteria because it provides an inhospitable environment for them to thrive.” 

Butler says, “A combination of a broad spectrum bactericide and fungicide is used in MWF concentrates. Tankside additions involve the use of just a broad spectrum microbicide.” 

Lunn considers the biggest issue with microbicides is the lack of education in how to properly use them. She says, “When a microbicide is used properly, the beneficial properties done to stifle microbial growth outweigh any potential negatives, which are alleviated because of the testing done to maximize performance and minimize any negative effects from handling. A specific sweet spot, which is defined as the concentration range where microbicide performance is optimal, is known for each microbicide. The biggest deficiency is not using microbicides at their proper concentration. Too little microbicide treatment will enable a small segment of microbes to survive in the Darwin situation ‘where the fittest survive and proliferate’ leading to resistance. Too much microbicide treatment can lead to health and safety concerns. Fortunately most microbicides are used in MWF concentrates at a 1%-3% treat rate minimizing contact with those end-users responsible for handling them.” 

Butler is concerned that most microbicides only have a short shelf life in MWF concentrates with an efficacy of only six months. He says, “One other need is for better tankside additive containers that are graduated for measuring amounts with ‘tip and pour,’ anti-splash back spouts.”

Butler is somewhat concerned that no new microbicides may be approved for use in MWFs in the near future. He says, “This is unlikely to happen in the next 10-15 years. If for some reason microbicides are not permitted in MWFs, end-users will probably go back to adding bleach or swimming pool additives out of necessity. A second consequence is that the popularity of straight oils will increase at the expense of water-based MWFs.” 

Lunn believes that carrying out a diligent fluid management program with pH buffers will be a decent option, if microbicides are no longer available. She says, “For this to be a viable strategy, fluid management must be a No. 1 priority for an end-user to implement and maintain.” 

Lunn finds the ASTM E2275 jar test to be very useful in evaluating microbicides. She says, “This test can provide formulators with a good idea about microbicide performance. A second procedure to consider is the recirculating fish tank test that simulates a MWF sump. Both tests should be run with more than two MWFs and compared in a side-by-side manner.” 

Lunn indicates that both ASTM E2275 and the fish tank test can correlate with field results. She adds, “As with any accelerated test, failure is seen more quickly in the lab than in the field. But good correlation was found in ranking fluid performance in the field versus results using these two lab methods.” 

Butler does not feel microbicide lab tests correlate with field results. He says, “Each sump has a unique combination of microbes that have resistance to the long-term, in-use biocides. Bench tests using generic MWFs cannot adequately predict specific sump responses.” 

Future for antimicrobial pesticides 
Most of the respondents believe microbicides will continue to play a critical role but predict that no new actives will be introduced in the near future. Montanez says, “Bringing a microbicide to commercialization requires broad effectiveness, compatibility with MWFs and safety for the environment and human health. The financial cost and time investment needed for development and compliance registration outweighs the benefit. Since current microbicides are effective and accessible, no new actives will probably be brought to the market in the near future.” 

Still, Montanez maintains that microbicide use in MWFs will continue, and this additive will play a critical role as end-users move to more sustainable platforms, where fluids have longer longevities and recycling practices are incorporated. Montanez says, “To compensate for the decrease of available microbicides or the reduction of use levels by regulatory agencies, we see more strategies implemented that incorporate multiple active ingredients in a single fluid formulation or system.” 

Falk says, “The investment and regulatory challenges will drive continued decline in available microbicide options. Current new product development in this area is unlikely for the foreseeable future.” Most believe microbicides will continue to play a critical role but predict that no new actives will be introduced in the near future. 

The relatively small size of the MWF microbicide market (estimated to be less than $500 million globally) will make it difficult for companies to invest in developing and registering a new active just for use in MWFs. Passman says, “New actives will be introduced to the MWF market only if they are developed for a major market. But the number of existing microbicides eligible for use in MWFs appears to have stabilized because manufacturers have submitted BPR dossiers that meet not just EU but also U.S. EPA requirements for the foreseeable future.” 

Industry consolidation has left the few remaining microbicide manufacturers with a broad portfolio of active substances. Passman expresses concern about whether these companies will continue to support the use of microbicides in the MWF market because their contribution to corporate balance sheets continues to decline. 

The regulatory landscape plays a crucial role in the availability of microbicides, according to Long. He says, “There is an increasing focus on environmental sustainability and human health, which may lead to more stringent regulations and restrictions on certain microbicides. This could result in a decrease in the number of available options, particularly for microbicides with higher toxicity or environmental concerns.” 

Long continues, “Advancements in microbicide research and development will lead to the introduction of new and innovative options. With ongoing scientific research, there is potential for new chemistries and formulations being developed that offer improved efficacy, reduced environmental impact and increased specificity for target microorganisms. In addition to the utilization of artificial intelligence (AI), these advancements are being expedited at an astounding rate.” 

Long indicates that market demand and consumer preferences are influencing the development and availability of microbicides. There is growing interest in environmentally friendly and sustainable solutions across most industries, which forces companies to develop options with better environmental profiles, such as those that are biodegradable, renewable or derived from natural sources. 

Collaboration between biocide manufacturers, formulators and research institutions will continue to foster innovation and the development of new biocide options. Post-COVID-19 pandemic, there is greater collaboration and focus on AI generated discovery that will accelerate the introduction of novel microbicidal solutions to meet evolving needs and challenges. 

Long believes that the number of today’s microbicidal options will initially decline; however, this situation is believed to reverse as the challenges of today will be the evolution to tomorrow’s environmentally responsible options. He adds, “There are innovative projects ongoing, which entail quat combo and plant-based microbicide options that will be further explored.” 

Microbicides remain as a very important additive needed to deal with microbial contamination in MWF systems. The continuing challenge for MWF formulators and end-users is to find ways to use microbicides effectively while also continuing to meet global regulations that appear to be restricting their use.

REFERENCES
1. Byers, J. (2018), Metalworking Fluids, Third Edition, CRC Press, Boca Raton, Fla.
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