Practical AppLications

Wxd@g, S&f dMXg four si'gnificant factors can help contm#m8avhiol contam @ation 'm metalwrking operations

A 11 metalworking fluid formulationsshare the common problem of sus-

,at ceptibility to microbial attack. Thisis not all bad news, since we need the useddilute fluid to be biodegradable for dispos-al purposes. However, the challenge forboth formulators and metalworking facilityoperators, using water-based fluids, is tominimize the adverse economic impact ofuncontrolled microbial contamination inmetalworking operations. The following dis-cussion will focus on water-extended fluids.A short glossary is provided in Table I toassist the reader who is unfamiliar with themicrobiological terms used in this article.

Metalworking fluid spoilage can bedefined as any change in the fluid whichadversely affects its utility. Table 2 lists themost common indications of spoilage inmetalworking fluids.

When any of these listed problemsbegins to occur with a coolant that has pre-

viously performed well, uncontrolledmicrobial contamination should be sus-pected and corrective action taken.

In order to determine what action isappropriate, a manager must have a feel forthe fundamental nature of the problem. Wewill discuss some of the more salient prin-ciples of metalworking fluid microbiology.

Metalworking fluids become "rancid"because they contain rich blends of nutrientsthat encourage microbial growth. Mineral oilbase stocks, glycols, fatty acid soaps, aminesand other nearly universal constituents ofmetalworking fluid concentrates provide all ofthe essential nutrients required for growth.Not all fluids contain the same nutrients norare they used under identical conditions. Con-sequently, the types of microbial contami-nants which predominate may vary.

Types of microorganismsTwo types of microorganisms contaminate

CONTINUED ON PAGE 26TRIBOLOGY & LUBRICATION TECHNOLOGY

By Frederick J. Passman(Member, STLE)

Editor's Note: This articlewas originally published asa compilation of articlesfrom the series "Startinafro m Scratch: TribologyBasics," previously pub-lished in Lubrication Engi-neering (now TLT).

24 APRIL 2004

TABLE 1GLOSSARY OF USEFUL MICROBIOLOGICALTERMS FOR THE METALWORKIG INDUSTRY

: | Aerobe. A microorganism that can only grow whenadequate oxygen is present Oxygenserves as theter-rninal electron acceptor

Anaerobe: A microorganism that can only grow ini I the absence of oxygen. Nitrate,.sulfate or organic

compounds serve as terminal electron acceptors indifferent species.Bacteriurm: A single-cell organism. Bacteria are thesmallest living units capable of carrying out, inde-pendently,all of the basicfunctionsthat characterizeliving beings (respiration, growth, assimilation,metabolism, excretion and reproduction). Bacteriamay beassmaH as 0.1 pm (4x 10-6 in.) in diameterand may appear as spheres, rods, commas or spiralsunder a microscope. As a kingdom,the bacteria rep-resent a vast diversity of nutritional and survivalcapab-itfies.Biodide A chemical that kills living organisms.Biofilrm Complex layer comprised of microorgan-isms and their secretions as well as detritus trappedwithin the biofilm matrix. Bioflm thickness may

* 7 range from a few microns to several centimeters.Physical and chemical conditions within biofilms arecontrolled bythe microorganisms growing there andmay be very different from conditions in the bulkfluid. Many species of microorganisms form thebiofilm communiuty.

Biomass- The total amount of living organisms in agiven volume of material Difficultto measure direct-ly, biomass is generally extrapolated from the meas-

I urement of a chemical component of the living cellswithin the mass. Optimally, the average conrientra-tion (per cell) of the chemical being measured iseither known or a standard conversion factor hasbeen established.Biostat; A chemical substance that prevents thegrowth or proliferation ofliving organisms, but doesnot necessa(ily prevent metabolism.Colony: The mass formed on the surface or withinthe matrix of microbial growth media as a result ofthe reproduction of ostensibly one cell. A bacteriumwith a generation tine of I hour (population doubleseach hour), will form a colony containing over 2 bil-

* lion cells in about 30 hours.The colony, visible to thenaked eye, is easier to count than individualmicrobes.Problemsarisebecausenot all microorgan-isms grow on the same nutrients or under the sameconditions.Bacteria andfungi grow in a coolant oftenfal to form colonies on solid growth media.

I Corrosion-enhaning microbial activities: Somebacteria, likethesulfate-reducing bacteria, contain theenzyme "hydrogenase," which scavenges hydrogenions and creates a galvanic cell.Many microbes manu-facture organic acids which attack metal surfacesdirectly. The very presence of nonuniform bioffimscauses electropotential gradients to develop. All ofthese activities tend to accelerate corrosion rates.Detritus: Unwanted partiles floating on the sur-

face, suspended in the bulk mass or precipitated outto the bottom of metalworking fluid. Swarf, flocs ofbiomass, detached rust deposit fragments all makeup typical detritus in coolant systems.Dip stickkdip slide: One of a number of paddle-likedevices either coated or saturated with a growthmedium.They are dipped into the coolant to be test-ed, iricubated for 1-2 days and observed for thedevelopment of colonies. They provide a simplemeans of getting very approximate plate count data.DishnfectTo destroy or inactivate harmful bacteria.Not equivalent to sterilization.Electron-acceptor molecule: A molecule like oxy-gen that captures electrons and becomes reduced towater AII cells derive their energy from asequence ofreactions involving the transfer of electrons along a"cascade" of molecules. The last molecule in thissequence is the terminal electron acceptor. For aerobicorganisms, oxygen is the terminal electron acceptor.Sulfate serves this function for sulfate-reducing bac-teria.Enzyme: A molecule or cluster of molecules com-posed of long chains of amino acids.The enzymes arethe cell's metabolic factories.They act as catalysts forthe metabolic reactions of all biochemical pathways.Active enzymes can cause fluids to become rancideven though the cells may be judged"dead" by someother criterion, such as plate counts.Essential elements:Carbon,hydrogen,oxygen,phos-phorous and sulfur are the elements withoutwhich life,as we know it cannot exist. All of these elements arepresent in abundance in metalworking fluids.Facultative anaerobe: A bacterium that lives likean aerobe when oxygen is present, and like an anaer-obe when oxygen is absent. Facultative anaerobesplay a key role in creating and maintaining environ-ments in which anaerobes can grow.Fungicide: A chemical that preferentially or selec-tively kills fungi.Fungus:The simplest microorganisms having a truecell wall.They appear as single-cell yeasts (approxi-mately 1 0,000 times the volume of most bacteria) oras fiamentous "molds.' These filaments are longstrands (hyphae) of cells which form the fibrous net-work of growth one sees when looking at moldyfood. The colored bodies found in a filamentous matcontain spores, One of these bodies may contain sev-eral hurndred spores, each of which can give rise to afungal colony.Growth: Growth is the measurable increase of anindividual's or population's biomass. See definition of"bi'omass."Growth rate: The amount by which the biomassirireases per unit period of time (usually hors).Often growth rate is reported as the amount of timeit takes for the population to double, assuming thatbiomass per organism is constant.

Inhibition: The prevention of any particular activity.Corrosion inhibition, ricrobial growth inhibition andfoam inhibition are examples of inhibiting functionsrequired of metalworking fluids. Inhibition is rarelyabsolute, so cost-benefit ratios should be computedto determine the merits of various inhibitor products.

Metabolism- The enzymatic reactions by which cellsbreak down food sources (anabollsm} and create newcell material (catabolism), giving off heat and waste"metabolites"in the process.

Microbially-mediated process: Processes such ascorrosion and pH drop which are the direct or indirectresult of microbial activity. For example, bacteriasecrete organic acids which react with pH buffers,leading to a loss of pH control.The consequent drop inpH is microbially-mediated.Microbicide: (Often written "microbiocide") is anagent which is designed to kill microorganisms, ie.,both bacteria -and fungi.

Mold:See'Furigus."Nutrient: Any substance that an organism needs inorder to grow and proliferate. In order for a chemicalto function as a nutrient, the organism must be ableto assimilate it (bring it into contact with the appro-priate enzymatic machinery). Nutrients are "essen-tialr if an organism cannot survive without them."Non-essential" nutrients are important for healthygrowtb, but not for survival.Pasteurize: To heat-treat a fTluid (usually withsteam) in order to kill-off potentially pathogenicmicroorganisms. Pasteurization requires exposure to61-63 C (142-145 F) for 30 minutes.Note:manynon-patho.enicicroorganisms generaffysurvive pasteur-Ization.

Plate counts: A standard method for enumeratingbacteria and fungi. A small sample portion is spreadonto the surface of a suitable nutrient-containinggel. After incubation, colonies develop. (see 'Dip-sticks 4) This traditional technique is called the platecount" because the nutrient.gel is generally con-tained in a standard 100 mm mpetri dish" or"plate.'Like dipstick methods, microbes must grow on thenutrients provided to form visible colonies. Conse-quently, these methods are often referred to as"viable counts."

Proliferation: The increase in the number of indi-viduals in a population.lt may or may not be propor-tional to growth:for example, if cells are dividing, butthere is no net biomass increase, the population isproliferating butnot growing.Often,in metalworkingsystems, biomass increases are not accompanied byincreased cell numbers. If viable counts are the onlymeasuremrents of microbial contamination beingused,the data may be dangerously mnisleading.

Sterilize: The complete destruction of biologicalactivity.

Yeast See"Fungus."

TRIBOLOGY & LUBRICATION TECHNOLOGY APR IL .20 04 25

CONTINUED FROM PAGE 24

metalworking fluids: bacteria andfungi. Bacteria are single-celledmicroorganisms which lack a true cellwall. Unlike higher organisms, theirshape and nutritional requirementsoften reflect the environment fromwhich they are recovered, rather thantheir genetic makeup. Consequently, abroad spectrum of genetic, physiolog-ical and morphological tests are gen-erally needed for identifying a givenbacterium. Gross classification canoften be accomplished with as few aseight tests, but complete identifica-tion may require over 300 tests.

In contrast to bacteria, fungi havetrue cell walls, like the cells of all ofthe higher plants and animals. Incoolant systems, fungi may appear assingle-celled yeasts or as filamentousmolds. Many fungi spend part of theirlife cycle as yeasts and part as molds.The factors, which determine whethera fungus will propagate in one form orthe other are beyond the scope of thisdiscussion. Some of the bacteria andfungi most commonly found in metal-working fluid are listed in Table 3.

In reality, the specific identity of abacterial or fungal contaminant is oflimited practical value in managingcontamination problems. It is muchmore valuable to know:

o What are the contaminants doingto the system?

o What can be done to prevent thatfrom happening?

Understanding Microbial lifeBy understanding a few basic con-cepts about microbial life, the plantmanager can take effective measuresto monitor microbial contaminationand control it intelligently.

Microbiologists concerned withspoilage prevention try to definemicrobial activities. Populations canbe defined by their impact on thecoolant or system. Table 4 lists somemicrobial activities that can be moni-tored in metalworking fluids andmethods used to measure these activ-ities, Gross observations of the phe-nomena listed in Table 2 are alsoimportant for monitoring microbialactivity. Control is achieved by taking

LTABLEZ2i PRIMARY SYMPTOMS OF

MICROBIAL SPOILAGE INMETALWORKING FLUIDS

* OdordevelopmentCloggedfiltersscreensand linesDecrease in pH

* Increased work piece rejection ratesChanges in emulsion stabflity

* Decreased tool life* Increase in corrosion rates

Unpredictable changes in coolant chemistry* Increased incidence of dermatitis and skin

irritations* Surface-finish blemishes

I TABLE3BACTERIA AND FUNGIFREQUENTLY RECOVEREDFROM METALWORKING FLUIDSBACrERIA FUNGIPseudomonas aeruginosa Fusafiumsp.'Proteusmirabilis Candida sp.IEnterobacter cloacae Cephalosporiui.m sp)Escherichia coliKlebsiella pneumoniaeDesulfovibriospp.1Note:.`sp."and"spp."are abbreviations for"specileand spedes,"respecdvely.

ria require oxygen for energy metabo-lism. However, anaerobic bacteria can-not grow in the presence of oxygen.Some species use sulfate, others usenitrate, but most anaerobes use "highenergy" organic molecules as their ter-minal electron acceptors. The four-carbon organic acid, fumarate, is onesuch molecule. Facultative anaerobesgrow whether or not oxygen is present.When oxygen is present, they use itjust as other aerobes do. When oxygenbecomes depleted, they shift to ananaerobic mode of metabolism. Thus,they play a critical role in creating hos-pitable environments for odor andcorrosion-enhancing microbial activi-ties, which occur principally underoxygen-free conditions.

NutrientsOrganic compounds and mineral saltspresent in metalworking-fluid formu-lations and makeup water are listed inTable 5, and have been discussedabove. Besides serving as nutrients,inorganic salts appear to make micro-bial populations more resistant tobiocide treatment.

Typically, growth rate and metabol-measures which will minimiZe the ic activity increase as temperaturerates at which these microbially- increases, up to a point. (Livingmediated adverse processes occur. microbes have been recovered from

Four factors predominate in con- Arctic ice and from deep-sea ventstrolling microbial life: an energy where temperatures exceed 120 Csource, nutrients and acceptable ther- (248 F)). Further temperature increas-mal and pH conditions. es inactivate enzymes and cause rapid

die-off. The temperature at which die-Energy Source off occurs is species-specific. Thus,Energy comes either from light, as for pasteurization (brief exposure to 61-photosynthetic microbes and plants or 63 C (142-145 F)) inactivates mostfrom the breakdown of oxidized organic common pathogens, but prolongedmolecules. The pathways involved in super-heated steam treatment (15energy metabolism require molecules min at 121 C (250 F)) is required to killsuch as oxygen, sulfate or specialized many of the species commonly recov-oxidized organic molecules which can ered from metalworking fluids.act as Lewis acids (i.e., molecules which Despite brief exposure to extremetend to accept electrons in chemical temperatures at the tool-work-piecereaction) to drive the process along, interface, metalworking fluids do notThese molecules accept the electron normally achieve bulk temperaturesliberated at the end of a cascading sufficient to control microbial growth.series of oxidation-reduction reactions In fact, in most systems the bulk-fluidand are consequently called "terminal temperatures are optimal for growth.electron acceptors."

Cells of all higher life forms (includ- Thermal and pH environmenting the fungi) use oxygen as their ter- A number of bacterial species com-minal electron acceptor. Aerobic bacte- monly found in metalworking fluids

TRIBOLOGY & LUBRICATION TECHNOLOGY26 APRIL 2004

grow at pH 9.2-9.5. Like the faculta-tive anaerobes, which create micro-environments for the anaerobes,these pH-tolerant bacteria createmoderate pH environments for pH-sensitive microbes.

Microorganisms create "micro-environments" in which conditionsmay be very different than they are inthe bulk fluid. These micro-environ-ments are protected from changes incoolant temperature, chemistry andpH by the slime or glycocalyx matrixthat the microbial population pro-duces. In terms of pH, this means thateven though a coolant may be main-tained at pH 8.5-9.0, the pH within themicro-environment may be below 7.0.PFH control often affects symptomsrather than the underlying problem ofmicrobial growth.

ConclusionsIn summary, uncontrolled microbialgrowth can create problems rangingfrom gross odors and slimes to moresubtle effects on coolant performance.All microorganisms require an energysource, nutrients and a suitable ther-mal and pH environment. The essenceof coolant system management isoptimizing operating conditions whilekeeping the environment of the metal-working fluid inhospitable for micro-organisms. Worker health and safetyconsiderations are of paramount im-portance in this equation.

Although it is beyond the scope ofthe present discussion to delve intohealth and safety questions associatedwith metalworking, a couple of pointsshould be stressed: Metalworking fluidsare complex mixtures of chemicals,which are constantly changing due tothe chemical, physical and microbio-logical factors which typify metalwork-ing systems. Moreover, elevated con-centrations of potentially pathogenicmicroorganisms are occasionally recov-ered from aerosols in metalworkingplants. The risks posed by these condi-tions are understood only poorly. The

e existing data suggest that good person-al hygiene and health care minimize therisks due to coolant exposure in metal-working shops. People who have good

TABLE 4MONI TORING MICROBIAL PROCESSES IN METALWORKING FLUIDS

PROCESSB4ofilm development

* Orgaric acid productionOxygen consumptionOrganic compound metabolism

* Emulsifier/demulsifier productionG* rowth and proliferation

TASLE 5NUTRIENTS IN METALWORKINGFLUIDSORGANIC INORGANICMineral Oil Cations:

ChlorideMineral waxes Calcium

l Fatty oils

1 Fatty acd soapsI Synthetic esters

I Phosphate esters

SodiumMagnesiumManganeseIron

Anions:SulfateChloridePhosphate

Amines

personal-care habits do not seem to beat any greater health risk than any otherportion of the general population.

Author PostscriptAlthough sixteen years have passedsince I drafted the Starting from Scratcharticle (1), I remain comfortable withthe fundamentals as I presented themin 1988. However, there are a few issuesthat can use some updating. At thetime I wrote the article, there was littleevidence of any relationship betweenmicrobial contamination and work-place disease. Although it still seemsas though pathogenicity is a non-issue,there are recognized relationshipsbetween bioaersol exposure and respi-ratory disease. I have reviewed these inmy 2002 article co-authored with pro-fessor Harold Rossmoore (2).

In 1988 there was little talk aboutbioresistant additives. These are func-tional additives that are used to pro-vide corrosion protection, lubricity orother performance property other thanantimicrobial activity, that also renderthe fluid less readily biodegradable.During the past decade, there has been

MEASUREMENT* Surface scraping dry weightd PH,alkalinity* Oxygen-volumetrictest* Gas chromatography, liquid chromatography,

mass chromatography, radiotracers* Emulsion stability test* Viable counts, chemial assays |

a proliferation of additives for whichbioresistance claims have been made.Some of these products are valuableadditions to the additive inventory.Others are simple unregistered micro-bicides for which no performance dataexist. ASTM E2275 (3), first publishedin 2003, provided details on how toevaluate metalworking fluid bioresis-tance. ASTM E2169 (4) offers athrough discussion of the issues thatmust be considered when selecting ametalworking fluid microbicide.

One fact amazes me as I reflect onhow we monitor microbial contamina-tion in metalworking fluids. Despitetremendous advances in microbiologi-cal methods, the industry continues todepend on techniques that were devel-oped over a century ago. I'm not advo-cating that we toss out the baby withthe bath water, but it might be time thatmetalworking fluid managers take ad-vantages of some of the rapid microbi-ological methods that have found broadapplication in microbial ecology.

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TITLE: Microbial Problems in Metalworking FluidsSOURCE: Tribol Lubr Technol 60 no4 Ap 2004

WN: 0409408256007

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