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384 Lubricant Additives: Chemistry and Applications
resistance to microbial degradation simply because they do not contain water of dilution. To ensure
acceptable long-term performance of any uid, protection against microbial degradation should be
an integral part of the metalworking system.
15.2 MICROBIOLOGICAL GROWTH
The two types of microorganisms that contaminate metalworking uids, bacteria and fungi, often
proliferate independently, but they can also coexist in a uid. The types of organisms present, and
their ability to coexist in the same uid, depend on uid composition and environmental conditions.
15.2.1 BACTERIA
Bacteria, single-cell organisms that lack the internal cell organelles found in higher forms of life,
are classi ed as either gram-positive or gram-negative depending on their cell wall structure, and
as aerobic or anaerobic according to their requirement for oxygen. Anaerobic bacteria cannot grow
in the presence of oxygen; however, strict anaerobes can tolerate very short exposure to oxygen
before dying. Another group of bacteria, facultative anaerobes, is able to use metabolic pathways
similar to those of aerobes when oxygen is present and then switch to anaerobic metabolism when
oxygen becomes limited. The compounds produced from anaerobic pathways create unpleasant
odors associated with the degradation of uid quality and performance. The compounds produced
through anaerobic pathways may include organic acids, as well as poisonous and explosive gases
such as H
2
S and H
2
.
Not all bacteria are able to survive in the environment of metalworking uids, and their occur-
rence is often speci c to certain uid formulations. However, Pseudomonas aeruginosa, Entero-
bacter spp., Escherichia coli, Klebsiella pneumoniae, and Desulfovibrio spp. are commonly found
in all types of uid. Because these bacteria are the species most predominantly found in rivers,
streams, lakes, and soil, it follows that makeup water is viewed as a signi cant source of bacterial
contamination. Other contamination sources include air, raw materials, and animals, including
humans. When bacteria are present, they are generally distributed throughout the metalworking
system and multiply rapidly when conditions are favorable.
15.2.2 FUNGI
Fungi may occur as either yeasts or molds. Yeasts, like bacteria, are unicellular and usually spheri-
cal in shape. Conversely, molds are composed of more than one cell and form complex mazes of
lamentous hyphae with spore-bearing structures, which give them their powdery, matlike appear-
ance. They are widespread in the environment and are commonly seen on decaying foods such as
bread, fruit, and cheese. Fungi are often found in synthetic metalworking uids but are usually
found in lower concentrations and distributed less evenly than bacteria. They may be found in the
uid itself (planktonic), but they also tend to grow on solid surfaces (sessile) such as splash areas and
in sluices, troughs, lters, and pipes, providing an important constituent of bio lm. Although many
different species of fungi have been isolated from metalworking uids, Fusarium sp., Candida sp.,
and Cephalosporium sp. occur most frequently.
15.2.3 BIOFILMS
Although microorganisms can exist in the free- owing metalworking uids in central systems, a
large proportion of the microbial population exists within bio lms. Bio lms, often composed of
diverse populations of microorganisms and complex in structure, adhere to system surfaces. Both
chemical and biological materials can become trapped in the secretions of a bio lm’s living cells.
Bio lms, or slime, can vary in thickness but usually range from a few millimeters to several cen-
timeters. The environment within the bio lm is highly in uenced by the microorganisms present
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Antimicrobial Additives for Metalworking Lubricants 385
and may be very different from the conditions in the uid itself. Because the microorganisms
growing within a bio lm are protected from the conditions that may be affecting the bulk uid,
the ability of antimicrobial agents to effectively control their growth is severely limited. This
explains why chemical-control treatments shown to be effective in laboratory testing sometimes
fail in eld situations [1].
15.3 A MICROBE-FRIENDLY ENVIRONMENT
The primary factors affecting microbial growth are the availability of water and nutrients, system
pH and temperature, and the presence of inhibitory chemicals [2]. With the introduction of water
as an important component of metalworking uids, microbial growth has become a major con-
cern. Like all living organisms, bacteria and fungi require water for survival, which is present in
all use-diluted emulsi able, semisynthetic and synthetic metalworking uids, as well as in some
concentrates. In regard to nutrients, metalworking uids are capable of providing all the essential
nutrients needed to support microbial growth [3]. These include carbon, nitrogen, phosphorus, sul-
fur, and other trace elements. Small quantities of inorganic salts are also crucial for microbiological
growth, which means variances in water hardness and quality can in uence microbial survival.
And, although at least some level of inorganic salts must be present, very high salt concentrations
may actually inhibit growth.
In regard to pH, the ideal level for optimum bacterial growth is a neutral or slightly acidic pH
of 6.5–7.5. Fungi prefer a slightly lower pH of ∼4.5 to 5.0. However, both bacteria and fungi can
survive in uids with pH levels outside these ranges. A large contingent of microorganisms thrives
in typical metalworking uid pH ranges; however, species diversity decreases in uids with pH
levels of 8.5 and higher.
The optimum temperature range for mesophiles, the largest contingent of microorganisms, is
30–35°C, the bulk-temperature range of most metalworking uid systems. However, many organ-
isms can exist at extreme temperatures; for example, bacteria have been found growing in both
Arctic climates and at the mouths of geysers and hot springs.
By manipulating these various growth factors, microbial survival and proliferation can be
controlled and even prevented. Water quality and pH are the two factors most easily managed.
However, the availability of nutrients can be regulated through the strategic selection of raw mate-
rials that are inherently more resistant to microbial degradation. Components such as borates,
certain amines, and biocides can all inhibit microbial proliferation. Although little can be done
economically to control bulk uid temperature, factors such as oxygen availability can, if not slow
microbial growth, at least alter microbial populations. For instance, a shift from aerobic bacteria
to facultative or obligate anaerobe populations is likely to occur if metalworking uids are not
aerated or circulated.
15.4 DESIGN, MAINTAIN, AND MONITOR
In the ongoing quest for microbial control in metalworking uids, system design, proper housekeep-
ing, uid maintenance, and microbiological monitoring programs all play an essential role. Micro-
organisms are less able to establish healthy populations in areas of high, uniform ow, therefore a
well-designed system, one that eliminates or minimizes dead legs and stagnant zones, is ideal. For
example, anaerobic bacteria are particularly likely to establish populations where ow is minimal,
as this often limits the diffusion of O
2
into the uid.
Good, consistent housekeeping practices at metalworking uid end-use sites can signi cantly
reduce the occurrence and proliferation of microbes. By preventing contaminants from entering the
system, good housekeeping practices reduce the amount of chemical and human effort needed to
maintain a system. An all-too-common scenario occurs when metalworking uid splashes around
machines. These small puddles of uid can harbor concentrations of l0
8
–l0
9
microbes/mL. When
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386 Lubricant Additives: Chemistry and Applications
debris or built-up material is washed off or falls into central sumps, an extremely high microbiolo-
gical burden is placed on the metalworking uid’s preservation package. Therefore, machine surfaces
and work areas should be kept clean and free of debris or residue buildup. Operators should not rinse
or sweep debris into return sluices, as this can inoculate central systems with contaminated material.
Tramp oil is another signi cant cause of contamination that should be removed, as it introduces its
own microbial population into the uid and helps prevent oxygen contact with the uid surface. High
levels of tramp oil are associated with microbiological preservation problems in metalworking uids
and are known to reduce the ef cacy of chemical and biological control agents [4].
Good housekeeping practices also require that workers wear clothing that is cleaned and kept
on-site. This reduces the risk of workers introducing contaminants such as dirt into the plant and
transporting contaminants and other possible chemical and biological irritants into their homes.
Food and human waste should also be prohibited from entering uid systems, as they both are likely
sources of contamination.
15.5 COMPLICATIONS AND CHALLENGES
Uncontrolled microbiological growth can signi cantly impact both the performance of metalwork-
ing uids and the operation of the central system. The poor uid quality and performance that
microbial degradation causes can result in corrosion of machines, tools, and workpieces; loss of
lubricity and uid stability; and the formation of slime, which can plug lters and delivery lines and
cause unacceptable odors. Microbial growth may also contribute to skin irritation and respiratory
problems.
Fluid deterioration can be de ned as any change that adversely affects a uid’s utility. Direct
deterioration of metalworking uids can be caused by various microorganisms with the capability
to degrade a number of organic compounds that make up the intricate formulation of a metalwork-
ing uid [5,6]. Although components such as petroleum sulfonates, fatty acids and fatty-acid esters,
organophosphorus products, and nitrogen compounds may or may not remain unaffected by any
one singular group of organisms, they are all susceptible to attack from combinations of microbes.
The degradation of these uid components is responsible for loss of uid performance. Resourceful
groups of microorganisms may even develop the ability to attack and degrade biocides, especially
if they are used at suboptimum levels.
Microbial contamination is also an indirect source of problems in uid applications, including
the generation of odors. The odors thus produced are associated with volatile organic acids (primarily
acetate, propionate, and butyrate) and hydrogen sul de—by-products of microbial metabolism.
These compounds are formed when oxygen levels drop, allowing anaerobic bacteria to proliferate.
A complication of weekend shutdowns occurs when uids are not circulated, causing uid oxygen
levels to drop; odors generated by this means are often referred to as Monday morning odor, as uid
agitation at start-up volatilizes them. The odor can become so offensive that employees refuse to
work until it is resolved.
Another complication of microbial contamination is a decrease in uid pH. The organic acids
generated from microbial metabolism have the potential to decrease the pH of the uid, and a uid
with a lower pH can become corrosive to ferrous materials. A decrease in pH can also result in
emulsion splitting and decreased ef cacy of other uid components, including antimicrobial agents
or biocides. To maintain proper performance, the alkalinity of the uid must be buffered at the
desired pH.
Microorganisms can also have a direct effect on corrosion through a process known as microbi-
ally in uenced corrosion (MIC). Here, certain bacterial populations either establish an electrolytic
cell or stimulate anodic or cathodic reactions on metal surfaces. This can cause pitting and corro-
sion on both machining tools and workpieces, although it is more often associated with the pipeline,
chemical process, and water-treatment industries [7]. Microorganisms can also degrade corrosion
inhibitors, leaving metals more vulnerable to corrosion.
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Antimicrobial Additives for Metalworking Lubricants 387
The formation of bio lm is a source of signi cant problems within the central system. Bio lm
growth on machine surfaces can affect the heat-exchange characteristics of the system. Also, if it
sloughs off, it can interfere with mechanical operations such as uid ow and ltration. Fungal
contamination, in particular, plays a major role in the clogging of lines and lters. Considerable
time and effort are required to remove established bio lm from the system and resume ef cient
operations.
Microbial contamination is sometimes blamed for worker’s skin irritation and dermatitis, but
such issues may turn out to be chemically caused, instead. In recent years, respiratory problems
among machine-shop workers have been thought to be linked to metalworking uid microbial con-
tamination. Hypersensitivity pneumonitis (HP), the most severe condition, has been attributed to the
presence of Mycobacterium immunogenum in contaminated uid mists [8]. Other possible causes
of respiratory impairment include endotoxins, which are cell wall fragments of gram-negative bac-
teria that may be found in metalworking uids, and mycotoxins, produced by fungi in contaminated
uids. These situations continue to be explored; Passman [9] has very recently provided a detailed
discussion of worker health issues. The American Society for Testing and Materials (ASTM) Sub-
committee E34.50 on Health and Safety Standards for Metal Working Fluids has produced standard
test methods for enumeration of mycobacteria (E2564-07) [10] and determination of endotoxin
concentration (E2250-02) [11] in metalworking uids.
15.6 TESTING FOR CONTAMINATION
Test methods are vital in the ght to control and prevent microbiological contamination in uid
concentrates and coolant systems through the timely and accurate diagnosis and treatment of the
problem. Many test methods exist, and each has its own set of advantages and limitations. The best
method to use will vary by situation, but ideally it should balance ease of use with accurate, practi-
cal, and timely results. It is important to remember that different testing methods measure different
aspects of microbial contamination.
Changes in uid or system properties such as odor or slime formation, corrosion, emulsion
splitting, and color changes are all good indicators of microbiological problems. However, by the
time these symptoms are noted, microorganism populations may be very high and uid properties
signi cantly impaired. Other methods estimate the amount of microbial growth present through
either direct or indirect measure of the biomass growing in the uid.
The most common direct procedure includes taking microscopic counts. Requiring observation
by a trained professional, this method is tedious and time-consuming but yields results within a
matter of minutes. Another direct method that requires a skilled professional is staining. This iden-
ti es both living and dead cells among speci c groups of organisms. Direct methods can also help
account for organisms that will not grow on the various nutrient media speci cally designed for
enumerating microorganisms. However, because of their level of sophistication, direct test methods
are not typically implemented in plants using metalworking uids.
Indirect measures include obtaining estimates of the number of organisms present through the
replication of these organisms and obtaining measurements of microbiological activity that trans-
late into the amount of microbial biomass present in the system. Standard plate counts and dip-slide
counts are the most common indirect methods for enumerating numbers of bacteria or fungi. Here,
counts are estimated based on the development of bacterial or fungal colonies on the surface of
solid nutrient media. In a standard plate count, a serial dilution is made of the metalworking uid
onto sterile petri dishes. The dishes are then lled with nutrient agar. Agar is a liquid above 45°C
but solidi es at room temperature. After several days of incubation, bacteria and fungi form visible
colonies on the agar and can be visually counted. Dilutions are taken into account in determining
the number of organisms in the original sample.
Because plate-count methodology can be expensive and time-consuming, a similar method
has been developed that uses paddles coated with the agar media to form a dip-slide. The dip-slide
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388 Lubricant Additives: Chemistry and Applications
is immersed in a uid sample for a few seconds, returned to its sterile holder, and incubated for a
few days at ambient temperature. Dip-slides generally contain a color indicator that changes color
as bacteria or fungi begin to grow, providing a simple visual way to determine colony counts. An
additional bene t of many dip-slide paddles is their ability to test for bacteria and fungi at the same
time by offering a different growth medium on each side of the paddle.
In both standard plate-count and dip-slide methodology, the assumption is made that each col-
ony comes from a single cell; however, cells often aggregate together, invalidating this conclusion.
In addition, a given nutrient medium will only support the growth of certain microbes and may
actually inhibit the growth of others, thereby limiting the ability to gain an accurate count of the
total organisms present. It has been estimated that direct count methods may recover as few as 10%
of the organisms present. Another major drawback to plate-count methods is the time required for
the organisms to grow. Typically, results are available in 24–72 h, but they may take longer. This
provides ample time for microorganism levels to reach detrimental levels in plant environments.
The long lag times associated with both plate-count and dip-slide methods have prompted a search
for more rapid techniques. Rapid test methods focus on the determination of the concentration of
cell constituents such as proteins, enzymes, lipids, or adenosine triphosphate (ATP, the energy unit
of living cells). Although these methods provide reliable estimates of microbial growth, they often
require costly and complex instruments. However, as they become more commonplace, the cost of
these technologies is likely to decline.
15.7 TREATMENT OPTIONS
Because metalworking uids provide an environment conducive to microorganism growth, preven-
tion techniques are vital to minimize the threat of contamination and facilitate trouble-free, con-
sistent operations. Both chemical and physical methods are available, and often a combination of
various methods is warranted when trying to establish and maintain microbiological control.
15.7.1 CHEMICAL METHODS
The use of biocides (also known as antimicrobial pesticides) is the most common method for con-
trolling microbiological growth in metalworking uids. The use of biocid reduces or maintains
bacteria and fungi at acceptable levels in water-based concentrates and nal use-diluted uids, and
ultimately maintains the integrity of the nal product. Although they are capable of eradicating
microbial growth at high concentrations, biocides are more often used at levels that control growth
and manage contamination due to economic and toxicological considerations.
Governmental agencies in most countries regulate the use of biocides. In the United States,
biocides are regulated by the Environmental Protection Agency (EPA), under the Federal Insecticide,
Fungicide and Rodenticide Act (FIFRA) and must meet certain toxicological and environmental
impact standards. Biocides are approved based on the relative risk resulting from their use. In the
European Union, the Biocidal Products Directive (BPD, Directive 98/8/EC) performs a similar func-
tion. The EPA requires that all biocide labels include speci c end-use applications, and it is up to the
user to determine whether a registered biocide is appropriate for a speci c application. Metalworking
uids are a speci c EPA end-use application, and of the ∼60 chemicals registered as biocides for this
end use, fewer than two dozen are routinely used by uid formulators and end users.
Two main biocide categories exist: bactericides and fungicides. Although some biocide
chemistries possess both capabilities, it is common to use both antibacterial and antifungal
functionality to preserve a uid.
A brief summary of the most common chemistries follows. For a more complete listing of bio-
cides and their properties, several excellent references on the subject exist, including Rossmoore
[12], Ash and Ash [13], and Paulus [14]. Biocide suppliers are another valuable source of information
and can supply product information and material safety data sheets (MSDS) that provide detailed
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Antimicrobial Additives for Metalworking Lubricants 389
information on biocide chemistry, ef cacy, toxicological properties, and handling. Additionally,
appropriate biocide product labels must be consulted in determining dosages for a particular met-
alworking uid application, as Federal law requires that biocide use-levels be within label limits.
Dosage rates will also vary, depending on uid formulation, housekeeping practices, time of year,
operating temperature, pH, and type of contamination likely to be encountered. Testing should also
be performed to determine the optimal biocide dosage.
15.7.2 POPULAR CHEMISTRIES
Formaldehyde condensates are by far the most popular and proven biocide chemistries for metal-
working uid applications. They control microbiological growth through their ability to generate
formaldehyde in situ. Like other aldehydes, the antimicrobial activity of formaldehyde is derived
from an electrophilic active group that reacts with nucleophilic cell sites. These targets include
amino acids or proteins, which are often important components of enzymes or other functional pro-
teins critical to cell function. Because of this more-generalized mode of action, formaldehyde-con-
densate biocides exhibit a broad spectrum of antimicrobial activity but are viewed as more effective
against bacteria than against fungi. A general disadvantage of formaldehyde is the tendency of
microorganisms to develop resistance to it [15,16].
Hydroxyethyltriazine (hexahydro-1,3,5-tris[2-hydroxyethyl]-s-triazine, CAS# 4719-04-4) is the
most commonly used and most cost-effective formaldehyde-condensate biocide for metalworking
uids. Water-soluble and stable at moderately alkaline pH levels, it is the cyclic trimer made from
formaldehyde and monoethanolamine (MEA). It is usually supplied as a 78% active aqueous solution,
and its use can add signi cant alkalinity to a system. Hydroxyethyltriazine can be added tankside
to an in-use uid or incorporated into an aqueous concentrate. Viewed primarily as an antibacterial
agent, its customary dose rate is ∼1500 ppm (0.15%) in a use-diluted uid.
Another frequently used formaldehyde-condensate biocide class is the oxazolidines, oxygen–
nitrogen heterocyclic products formed by the reaction of formaldehyde with various alkanolamines.
Representative materials include 4,4-dimethyl-1,3-oxazolidine (CAS# 51200-87-4) and 7-ethyl-
bicyclooxazolidine (CAS# 7747-35-5). Being oil-soluble as well as water-soluble, these products
can easily be incorporated into metalworking uid concentrates or added tankside. Their alkaline
nature will also aid in ferrous metal corrosion inhibition. Known primarily as antibacterial agents,
customary dose rates for oxazolidine biocides are ∼750 to 1500 ppm in working uid systems.
Tris(hydroxymethyl)nitromethane (CAS# 126-11-4) is a water-soluble condensation product of
formaldehyde and nitromethane. Chemically stable at acidic pH, it will decompose and release
formaldehyde under alkaline conditions. Supplied as a 50% active aqueous solution, or sometimes
in one-ounce solid tablets, its decomposition rate increases with increasing alkalinity levels, and it
shows a rapid rate of kill against bacteria. Owing to its instability under alkaline conditions, result-
ing in lack of persistence of effect, this biocide is not incorporated into concentrates, but only added
tankside to in-use uids. It is often used in nonferrous applications such as aluminum rolling, due to
its acidic, halogen-free nature and rapid kill rate. Tris(hydroxymethyl)nitromethane has been shown
to work very well together with isothiazolinone biocides, especially in aluminum-rolling applica-
tions [17]. Recommended use rates are between 1000 and 2000 ppm active material.
Another reaction product of nitromethane and 2 mol of formaldehyde, and 1 mol of bromine,
is known as bronopol (2-bromo-2-nitro-1,3-propanediol, CAS# 52-51-7). Bronopol, an antibacterial
product, does not release free formaldehyde in its mechanism of antimicrobial action, and it is con-
sidered especially effective against resistant strains of P. aeruginosa. This water-soluble product is
not chemically stable, long term, at pH levels above ~8.0. Its usual dosage rate is ∼50 to 400 ppm
active material.
A biocide consisting of 4-(2-nitrobutyl)morpholine (CAS# 2224-44-4) and 4,4´-(2-ethyl-2-
nitrotrimethylene)dimorpholine (CAS# 1854-23-5) has proven effective as an antibacterial and
antifungal metalworking uid biocide. Although it is produced from formaldehyde, morpholine,
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390 Lubricant Additives: Chemistry and Applications
and nitropropane, this blend does not exert its antimicrobial effect through the release of formalde-
hyde. With limited water solubility and high oil solubility, the active ingredients formulate easily
into many metalworking uid concentrates, especially soluble oils. In fact, it is one of the few oil-
soluble fungicides available. To optimize both its fungal and bacterial ef cacy, it should be dosed at
500–1000 ppm active ingredient.
Phenolic compounds, including the commonly used phenolic active ingredient 2-phenylphenol
(OPP, CAS# 90-43-7) as well as the sodium salt of OPP (CAS# 6152-33-6), have long been used
as biocides in metalworking uids. Although environmental concerns have at times lessened their
popularity, these compounds have seen resurgence due to their proven favorable human toxicity
pro le and restrictions prohibiting the release of metalworking uids into the environment. As a
biocide, OPP can be used either tankside or in metalworking uid concentrates. The sodium salt
of OPP has greater solubility in aqueous systems. OPP is more easily formulated into concentrates
with low water contents. The compound exhibits a very broad-spectrum, having ef cacy against
bacteria and fungi including both yeast and molds. Both OPP and sodium OPP can be used over a
broad pH range; however, their performance and solubility are best at a pH > 9. Dosage rates range
from ~500 to 1500 ppm active ingredient for OPP and from 500 to 1000 ppm active ingredient
for the sodium salt of OPP. These active ingredients are available for various formulations in both
solid and liquid forms. Another phenolic compound is p-chloro-m-cresol or 4-chloro-3-methylphe-
nol (PCMC, CAS# 59-50-7). A very broad-spectrum biocide, PCMC is effective against bacteria,
yeasts, and molds at dosages of 500–2000 ppm active ingredient. The active ingredient 4-chloro-3-
methyl phenol is effective in a pH range of 4–8 but is less stable at an alkaline pH than either OPP
or sodium OPP. It can be added to metalworking uid concentrates or used as a tankside additive.
In tankside addition, it is generally preferable to make a solution rather than adding the dry powder
directly. A closely related compound, p-chloro-m-xylenol (PCMX, CAS# 88-04-0), will behave in
the same fashion as PCMC. Both PCMC and PCMX have received renewed interest recently as a
result of their ef cacy against mycobacteria.
The biocide 2,2-dibromo-3-nitrilopropionamide (DBNPA, CAS# 10222-01-2) has recently seen
increased use in metalworking applications. This biocide chemistry is viewed as a bactericide with
quick-killing properties that are achieved at very low dosage rates and is meant for use only as a
tankside treatment. Because of the short half-life of DBNPA at a pH of >8, this chemistry is not
persistent enough to provide prolonged preservation when added to metalworking uid concen-
trates. An effective dosage rate for DBNPA in tankside applications is 1–2 ppm active ingredient.
More recently, a timed-release tablet [18] has been developed that provides for a slow, continuous
dose of DBNPA of 0.5–1 ppm active ingredient to central systems. The tablet eliminates the need
for pumping liquid material tankside and is especially useful in smaller sumps.
A dialdehyde, known as glutaraldehyde (1,5-pentanedial, CAS# 111-30-8), also offers quick-kill
properties and can be used in the cleanup of contaminated systems or as a tankside additive in uids
that do not contain amines, because its activity is impeded by the presence of amines. Its customary
dose level in use-diluted uids is 60–300 ppm active material.
In uids that do not contain anionic emulsi ers such as sodium petroleum sulfonate, a polymeric
quaternary-amine compound, poly[oxyethylene(dimethylimino)ethylene(dimethylimino)ethylene
dichloride] (CAS# 31512-74-0) has shown ef cacy against microbial contamination, at typical
active levels of 180–300 ppm. The cationic nature of quaternary compounds is not considered
compatible with anionic materials.
Several isothiazolinone chemistries have also seen extensive use in metalworking uid appli-
cations. A blend of the active ingredients 5-chloro-2-methyl-4-isothiazoline-3-one (CMIT, CAS#
26172-55-4) and 2-methyl-4-isothiazolin-3-one (MIT, CAS# 2682-20-4) in a ratio of about 2.7:1
(CMIT/MIT) is the most commonly used isothiazolinone chemistry. Its typical dosage range for
the total actives is 10–21 ppm. Offered in the form of a stabilized aqueous solution, this biocide is
effective against bacteria and fungi, including both yeasts and molds, and can be used in all uid
types. Like formaldehyde condensates, isothiazolinones are strong electrophilic agents that react
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Antimicrobial Additives for Metalworking Lubricants 391
with nucleophilic cell entities to exert their antimicrobial effect. Although resistance of microbial
populations to isothiazolinones has been reported [19], its development was strongly affected by the
dosing pattern of the biocide. When a regimen of fewer high-dose biocide treatments is compared
with more frequent low-dose treatments, with the total biocide addition being equal, the low-dose
treatment regimen is more likely to result in the selection of resistant microbial populations. Stable
over a pH range of ∼3 to 9, this chemistry shows greater stability at the lower end of the pH range.
Although isothiazolinone chemistry is compatible with a wide variety of metalworking uid addi-
tives, it is incompatible with reducing agents, various amines, mercaptobenzothiazole corrosion
inhibitors, and the antifungal agent sodium 2-pyridinethiol-1-oxide.
Owing to the incompatibility of isothiazolinones with certain amines (especially at higher con-
centrations) and the issues of sensitization that could result from using high levels of isothiazoli-
nones in concentrates, the CMIT/MIT blend is used only in tankside applications and is not added
to metalworking uid concentrates. Combining CMIT/MIT with formaldehyde condensates or cop-
per [20] has been shown to stabilize the isothiazolinone molecules, indicating that these combina-
tions may be better suited for uid-concentrate incorporation. Recently, MIT alone has been offered
as a uid-concentrate biocide. While requiring a higher effective dose level than the CMIT/MIT
blend (20–150 ppm active), it demonstrates signi cantly greater stability in aqueous solution and is
less aggressive in human contact.
An aromatic isothiazolinone derivative, 1,2-benzisothiazolin-3-one (BIT, CAS# 2634-33-5), is
well established as a metalworking uid biocide. It can be incorporated into uid concentrates or
added tankside to use-diluted uids. Its customary dosage level is ∼40 to 360 ppm active ingredient
in the nal use-diluted uid.
Another isothiazolinone chemistry, 2-n-octyl-4-isothiazolin-3-one (OIT, CAS# 26530-20-1) is
also appropriate for use in metalworking uids. Unlike CMIT/MIT, which demonstrates broad-
spectrum activity, OIT functions only as a fungicide. Stable in the pH range of 2–10 and compatible
with a wide variety of metalworking uid additives, OIT products are often used in combination
with other metalworking biocides including CMIT/MIT, triazine, oxazolidines, and sodium pyri-
thione. Its use with sul des, mercaptans, bisul tes and metabisul tes, and strong oxidizing agents,
however, should be avoided. It may be added to either concentrates or use-diluted uids as a tankside
addition. Dosage rates range from 25 to 75 ppm active OIT based on the nal use-diluted uid.
Another biocide chemistry used as a fungicide in metalworking uid applications is 3-iodo-2-
propynylbutylcarbamate (IBPC, CAS# 55406-53-6). Available in both powder and liquid forms, it
has very limited water solubility but is miscible in both alcohols and aromatics. Effective dosage
rates for IBPC range from 100 to 300 ppm active, and it can be formulated into metalworking uid
concentrates or used as a tankside additive.
Sodium 2-pyridinethiol-1-oxide (sodium pyrithione, CAS# 15922-78-8), a broad-spectrum
fungicide, can be added either to water-based metalworking uid concentrates or tankside into
use-diluted uids in metalworking uid sumps. It has an effective pH range of 4.5–9.5 and is
compatible with most metalworking uid formulations; however, it is not recommended for use
in combination with CMIT/MIT-based biocides. Strong oxidizing or reducing agents also impair
the ef cacy of sodium 2-pyridinethiol-1-oxide. The appearance of a blue color or black specks is
common with this chemistry, as it reacts with ferric ions to form insoluble ferric pyrithione, which,
incidentally, also has antifungal properties. Dosage rates range from 46 to 64 ppm active sodium
2-pyridinethiol-1-oxide.
A nonexhaustive listing of major producers of metalworking uid biocides (ones that offer
several different chemistries) includes Acti-Chem Specialties, Inc. (Trumbull, Connecticut); Arch
Chemicals, Inc. (Norwalk, Connecticut); BASF Corporation (Florham Park, New Jersey); Buckman
Laboratories International, Inc. (Memphis, Tennessee); Clariant Corporation (Mt. Holly, North
Carolina); The Dow Chemical Company (Midland, Michigan); Lanxess Corporation (Pittsburgh,
Pennsylvania); Rohm and Haas Company (Philadelphia, Pennsylvania); and Troy Corporation
(Florham Park, New Jersey).
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392 Lubricant Additives: Chemistry and Applications
15.7.3 BIOCIDE COMBINATIONS
The use of more than one biocidal active ingredient is a well-known practice and works to broaden
or complement the antimicrobial spectrum of a single product. Here, an antifungal product may
be combined with an antibacterial product to provide a dual mode of action, or in cases where
an antibacterial biocide may have limited ef cacy against certain bacteria, another antibacterial
agent may be added to expand the system’s overall antibacterial spectrum. For example, sodium
2-pyridinethiol-1-oxide, a proven fungicide, is often blended with triazine or oxazolidines, both
proven bactericides, to form a product effective against both types of organisms. An added bene t
of commercial blends is that they reduce the number of products that an end user must hold in
inventory.
Biocides may also be used together to improve the overall performance of either biocide. This
concept, known as synergy, can boost ef cacy in several different ways. One biocide molecule may
stabilize another, while broadening the spectrum of antimicrobial coverage or providing additional
modes of action for killing or inhibiting microorganisms. The use of isothiazolinones with certain
formaldehyde-condensate biocides is an example of this type of synergistic blend [21]. Some isothi-
azolinone molecules, when used alone, have limited persistence in metalworking uids. However,
these formaldehyde condensates stabilize the isothiazolinones and increase the persistence of the
blend.
It is important to remember that, in the United States, regulations regarding blending of reg-
istered biocides are stricter than current regulations in some other parts of the world. If a blend of
registered active ingredients is sold in the United States, the blend itself must be separately regis-
tered by the EPA as a pesticide. The recently enacted European regulatory initiative known as the
BPD will also result in more-restrictive biocide regulation and possibly fewer choices of biocide
candidates.
15.7.4 METHODS OF BIOCIDE APPLICATION
Biocides can be either formulated into the metalworking uid concentrates or added to a system
tankside. Each strategy has advantages and limitations, and the biocide chemistry may dictate
which application method is best. A biocide added into a concentrate will not only protect the con-
centrate but will also provide antimicrobial protection in the use-diluted uid. This method of bio-
cide addition reduces handling of the concentrated biocide at plant locations and end-use sites and
ensures that the biocide is added proportionally to the systems in the correct ratio to the uid. The
limitation to this type of addition is that different metalworking uid systems may exert different
demands on the biocide. For instance, there may be uid ingredients in a central system which
are not compatible with a certain biocide chemistry. Thus, the biocide in the newly diluted uid
may be deactivated and consequently fail. In systems that are already heavily contaminated, the
organisms may create a heavy demand on the biocide and rapidly deplete it, leaving the remain-
ing formulation components unprotected. Furthermore, adding a biocide to the concentrate for
preservation of the use-diluted uid assumes that the uid will be diluted properly at the end-use
site. If for any reason the uids are over-diluted, biocide levels may not be suf cient to control
microbiological growth.
Alternatively, tankside addition of a biocide at the plant site allows a more direct and speci c
way to address individual plant situations. With this method, biocide selection can be based on
unique plant-operating parameters such as water quality (microbiological and chemical), house-
keeping, uid residence time, degree of expertise in personnel, and waste treatment processes used
in the plant. Tankside treatment does require close, if not constant, monitoring, but the added bene t
of this is the ability to respond immediately to problems as they arise. Routine tankside addition
of a biocide, with speci c targeted levels of biocide in the uid, is more likely to provide effective
control if these levels are maintained at all times. It is better to integrate biocide use into a planned
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Antimicrobial Additives for Metalworking Lubricants 393
program than to just use it on an as-needed basis. Waiting until a microbiological problem is noted
before providing treatment may result in large amounts of dead microbial biomass, which can plug
pipes and lters and release odors associated with decaying microbial cells. Limitations associated
with tankside treatment include user concerns about maintaining and handling undiluted (concen-
trated) biocide products due to the health and safety issues associated with these products, and the
labor-intensive aspect of having to constantly monitor and maintain biocide levels. Tankside addi-
tion also requires more in-depth training in the proper handling of biocides, as well as a deeper
understanding of how to properly calculate and add a given dose of biocide. When using tankside
treatment methods, it is very important that all personnel in the plant be informed of the necessary
precautions to take when neat biocides are present.
Many metalworking uid operations use both methods of biocide application to provide a
comprehensive treatment program. Although biocides added to uid concentrates help preserve the
concentrate and reduce the volume of tankside preservative that must be kept on-site, they may also
become depleted in the working uid over time. The routine addition of a tankside biocide, once the
biocide provided in the concentrate has been depleted, prevents uncontrolled microbial growth and
ultimately helps extend uid life. However, compatibility between the concentrate biocide and the
tankside biocide must be established.
15.7.5 BIOCIDE SELECTION
Although a seemingly straightforward task, biocide selection can prove challenging because of the
number of options available and the fact that no single biocide will provide optimum performance in
all situations. Of the various factors to consider when choosing a biocide, the rst should be whether
the product has an EPA end-use registration for metalworking uid applications. In addition, the
compatibility of the biocide with the uid must be considered. The biocide must not affect the
functional properties of the uid, including lubricity, corrosion inhibition, and emulsion stability.
Other factors such as cost, known chemical incompatibilities, and mode of application also need to
be examined before laboratory or eld evaluation of the biocide can begin. The type of metal on
which the uid is used should also be considered, as ferrous and nonferrous metals may have differ-
ent compatibilities with various biocides. The ASTM document E2169-01 (2007), entitled Standard
Practice for Selecting Antimicrobial Pesticides for Use in Water-Miscible Metalworking Fluids
[22], provides a detailed discussion of the issues to be considered in matching the preservative with
coolant chemistry and performance requirements. This document also lists the 57 chemicals regis-
tered by the EPA for use in the preservation of metalworking uids.
Biocides must be cost-effective and also provide protection against the spectrum of microor-
ganisms affecting the uid. Although various laboratory test methods are available for measur-
ing biocide performance, simulating real-use conditions in metalworking systems is dif cult. The
most rigorously documented laboratory test procedure is that published by ASTM. It is designated
as ASTM E2275-03e1, Standard Practice for Evaluating Water-Miscible Metalworking Fluid Bio-
resistance and Antimicrobial Pesticide Performance [23]. It replaces two previous ASTM methods,
D3946 and E686, and can be used to evaluate initial uid bioresistance, biocide speed of kill, and
uid resistance to repeated microbial challenges. In this test procedure, use-diluted uid samples
of <1 L in volume are inoculated with microbes and then aerated to simulate recirculation condi-
tions. The samples may also contain metal chips. The test duration may range between 24 h and
3 months; for longer test periods, a portion of the test uid is periodically replaced with fresh uid
to simulate uid turnover in use. Fluid performance is determined by measuring properties such
as increase in viable-cell recovery or biomass and changes in physical and chemical uid proper-
ties. This test procedure is meant to compare relative performance of uids and biocides, not as an
absolute predictor of performance in the eld.
Despite the best efforts and assumptions, laboratory
testing is not a perfect indicator of real-use conditions. Consequently, when a biocide has been
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