Facing Down Microbes


Phenolic compounds have been historically used in disinfectant applications, due to their ability to eradicate bacteria, fungi and viruses. But how effective are they against acid-fast microbes such as bacteria and mold in metalworking fluids?

Microorganisms can readily proliferate in metalworking fluids because of the various constituents that are present – oils, water, fats and additives. These fluids can contain a wide array of microbial contaminants such as gram positive ( Corynebacteria, Staphylococcus) and gram negative ( Pseudomonas, Citrobacter, Legionella pneumophila). Common fungal strains present in metalworking fluids include ones like Acremonium, Geotrichum and Fusarium. Other strains such as Alternaria sp. and Penicillium sp. can also be found.

Microbes selectively deplete specific metalworking fluid components including petroleum base stocks, glycols, amines, emulsifiers, sulfonates, fatty acids, soap and oils. This biodeterioration contributes to the loss of lubricity efficiency, corrosion problems, biomass buildup, drop in pH, emulsion instability, foaming, decrease in machine and tool life, poor filter performance and fluid flow, and reduced thermal conductivity.

In addition, unpleasant odors can develop due to waste products such as hydrogen sulfide, ammonia, mercaptans and volatile dicarboxylic acids from the presence of Desulfovibrio sp., which can adversely affect plant employees.

We can prevent microbial contamination by following good housekeeping and industrial hygiene practices. Routine monitoring of metalworking fluid pH, concentration, dirt, fines, conductivity, and tramp oil is necessary to ensure parameters are controlled at their proper performance specifications. Proper biocide selection and application is a critical element of microbial contamination and control. Biocide selection is based on a variety of considerations including target microorganisms, fluid chemistry, and application treatment strategies as well as the conditions pertinent to the application.

Even though there are at least 250 EPA registered biocides, the four primary actives in the forefront of the metalworking fluid industry have been orthophenylphenol, isothiazolones, thiocyanobenzothiazoles and nitromorpholine. No particular biocide can meet all needs, but there has been a renaissance in the use of antimicrobial additives like parachlorometacresol (PCMC), due to health and safety issues. PCMC, which has non-polar characteristics, lends itself to be more oil soluble, whereas other biocides may function primarily in the aqueous portion of a metalworking fluid.

Although PCMC has been used as an industrial microbicide for 50 years, and is registered with the U.S. EPA for use in metalworking fluids, there are no previously published reports of its antimicrobial performance in such fluids. This article presents the results of laboratory microcosm tests used to evaluate the efficacy of PCMC against a mixed challenge population comprised of a blend of 14 bacteria and fungi. The purpose is to demonstrate the broad-spectrum effectiveness of PCMC as an antimicrobial pesticide in metalworking fluids.

10 Tough Weeks

Using both soluble oil and semi-synthetic metalworking fluids , tests were conducted at various PCMC concentrations. The soluble oil was formulated using approximately 75 percent mineral oil; sodium petroleum sulfonate (10%), amines (less than 5%), plus fatty acid components (5%). The semi-synthetic contained mineral oil (20%), emulsifiers (20%), amines (6%) and fatty components (5%). These test fluids were challenged with field strains ( Psuedomonas fluorescens, Psuedomonas stutzeri, Alcaligenes faecalis, Corynebacterium sp., Acremonium sp., Fusarium sp., and Rhodotorula rubra) and International Biodeterioration Research Group strains of bacteria and fungi ( Psuedomonas aeruginosa, Psuedomonas oleovorans, Pseudonomonas rubescens, Citrobacter freundii, Acremonium strictum, Fusarium solani, and Geotrichum candidum).

PCMCs effectiveness in the fluids was evaluated in accordance with ASTM E2275-03 (Standard Practice for Evaluating Water-Miscible Metalworking Fluid, Bioresistance, and Antimicrobial Pesticide Performance) in testing that continued for 10 weeks rather than the usual six. Using sterilized water, the soluble oil metalworking fluid concentrates were diluted to 1.5 percent, and the semi-synthetics to 5.0 percent. For each ready-to-use fluid, 100-gram samples of the diluted metalworking fluid were poured into sterile 250 ml wide-necked bottles with a cap and magnetic stirrer. PCMC was added one time only, at the designated levels of 1000, 1500, and 2000 mg/L.

After two hours the samples were separately inoculated with bacteria or mold and yeast so that one gram of the inocula contained 106 to 107 cfu of bacteria/ml or 104 to 105 cfu of mold/ml and 105 to 106 cfu of yeast/ml. (There was also a control sample used for each metalworking fluid, with no PCMC.) During the 10-week challenge test, samples were recontaminated with microorganisms every seven days. Microbial counts were determined before each new inoculation, using the standard plate count, and seven days after the final count to establish whether the microorganisms growth had been eliminated or inhibited.

In this test, adequate efficacy is achieved when during the course of 10 inoculations microorganisms disappear or do not reach the following levels in the diluted metalworking samples: 105 to 107 cfu Bacteria per ml, and 105 to 106 cfu Molds and Yeast/ml.

Samples fail if these levels are exceeded and no further testing is done.

How Effective?

Test results for the evaluation of PCMCs effectiveness against the bacterial challenge population in the soluble oil are depicted in Figure 1. It shows that a level of 2000 mg/L of PCMC provided bacterial control in the soluble oil formulation over the entire 10-week testing period, while 1500 mg/L of PCMC in the same formulation provided seven weeks of bacterial control. (We also found a minimum of 1000 mg/L of PCMC was needed for yeast control in all fluids tested, but these results are not shown here due to space limitations).

In Figure 2, 2000 mg/L of PCMC provided control of the mold for the first eight weeks. Thereafter, some growth was observed.

Figures 3 and 4 for bacteria and mold, respectively, present analogous data for PCMC performance against field strains in the semi-synthetic formulation, and show that these can be controlled with 1500 mg/L for the 10-week testing period. In contrast, Figures 5 and 6 (page 18) show that 1000 mg/L was able to control the International Biodeterioration Research Group strains of bacteria and mold in the semi-synthetic metalworking fluid throughout the 10 weeks.

These efficacy studies demonstrate PCMCs ability as a broad-spectrum biocide in semi-synthetic and soluble oil metalworking fluids. Tests were not performed in replicate; however, lab results have been substantiated by field studies. This limited test data provide positive results; however, independent testing should be performed with each metalworking fluid formulation to validate the efficacy of PCMC.

Stable, broad-spectrum biocide is needed to prevent rancidity in metalworking fluids and to prevent an array of problems that inhibit fluid performance and damage fluid handling systems, machinery and forming tools.

PCMC has been evaluated widely in other industries (see below). Testing demonstrates that PCMC also is effective against yeast, mold and bacteria in soluble oil and semi-synthetic MWF. (It is not used in synthetic metalworking fluids due to its limited water solubility.)

The renaissance of PCMC in the metalworking fluids industry is also due to its ability to control Mycobacterium immunogenum. The latter has been found in all metalworking fluid systems with diagnosed worker Hypersensitivity Pneumonitis. PCMC has successfully controlled M. immunogenum in both in a lab and extensive field trial.

Further research with PCMC is being conducted to show its ability to control Mycobacterial biofilms and new MIC data on M. immunogenum. In the future, more data will be published showing the ability of PCMC to control biofilms associated with M. immunogenum.