A comprehensive approach to microbial control in metalworking systems must account for all facets of the biocide program – from design, to implementation and trouble shooting. Best practices for selecting a biocide treatment program must consider cost, efficacy, compatibility, stability, waste discharge, etc.

Biocides may be added to MWF concentrates, which provides a convenient method for treatment of the in-use recirculating system. As makeup fluid is added, more biocide is also added via the concentrate. However, the biocide level in the concentrate is fixed, so the amount added to the fluid cannot be significantly modified during use.

Thus, the only way to shock a metalworking system for microbial control is by tankside addition. Biocides may be dosed tankside for individual treatment of a system within the manufacturers recommended use range to control the microbial problems. The flexibility of adjusting the dose as needed is one advantage of tankside treatment.

Concentrate Options

Stability and compatibility of biocides in concentrates is critical to their effective use. Based on past studies, isothiazolone biocides generally have insufficient stability if added indiscriminately to concentrates. However, novel stabilization technology, along with definition of a preferred set of amines, has been developed by Rohm and Haas to enable these effective biocides to be used in various concentrate formulations. Benefits to formulators and end-users include enhanced stability and efficacy during extended storage and elevated temperature conditions.

Figure 1 compares the efficacy of seven types of biocides when dosed in MWFconcentrates and heat-aged. Samples of the treated concentrates were diluted over time and inoculated with microorganisms. As shown, a stabilized isothiazolone (Rohm and Haas trademarked Kordek LX 5000 biocide) provided excellent long-term efficacy versus bacteria, even when aged for six months at two temperatures. Others, including oxazolidine, triazine, dimorpholine and benzisothiazolone (BIT), lost efficacy over time, especially with heat aging, and one product (polyquaternium) was ineffective.

In addition to being free of VOCs and formaldehyde, Kordek LX 5000 has broadspectrum efficacy. It is based on the 2- methylisothiazolone (MIT) chemistry and is efficacious against bacteria, mold and yeast. MIT functions by rapidly inhibiting critical enzymes of microorganisms. This causes a massive disruption of key metabolic processes, including growth, respiration, and energy generation.

Another non-formaldehyde biocide choice for MWF concentrates is Rocima BT 2S, a trademarked Rohm and Haas product. This is based on the BIT active and is especially good for bacterial control. BIT has excellent stability at high pH with amines and is thus very stable in MWF concentrates.

Efficacy versus key problem-causing microorganisms in MWF is also a critical feature for selecting a biocide. Studies using field samples, conducted at Biosan Laboratories Inc., ranked Kordek LX 5000 as most effective against mycobacteria (followed by Kathon 886 MW, another trademarked Rohm and Haas product) in the highest percentage of fluids tested (see Figure 2). Both provided at least a 90 to 99 percent kill in the majority of fluids tested. Triazine and oxazolidine biocides were significantly less effective in the range of fluids tested, and BIT showed no kill against mycobacteria in any fluid.

MWF concentrates also require addition of high-performing fungicides, to provide rapid kill of yeasts and mold and to prevent fungal slimes on surfaces, which otherwise may block filters and cause musty odors. One example is Kathon 893 MW, a broadspectrum fungicide based on octylisothiazolone, in a propylene glycol base. When stabilized in concentrates, it provides extremely long-lasting fungal control at very low dosage in the use-diluted fluids.

Like the other isothiazolones, Kathon 893 MW also inhibits key enzymes involved in microbial metabolism, resulting in rapid inhibition of growth followed by cell death. Additionally, due to its stability in diluted fluids, it provides weeks of fungal control from a single addition. It is most effective in synthetic and semi-synthetic fluids. Efficacy studies in six MWF dilutions showed this formulation to have equal or better control of fungi – at lower product use rates – versus commercial fungicides in most of the fluid types (Table 1).

Tankside Tactics

Keeping the metalworking fluid in the best possible condition will ensure that the fluid does its job well and will help keep the workplace operating smoothly. A fluid in good condition can also reduce potential worker health risks.

One preventive measure is to establish a fluid management program to continuously maintain high fluid quality. The program should continuously remove metal chips and tramp oil, use good quality water, and – very importantly – keep microbial growth under control by making timely concentrate or biocide additions before problems develop. Periodic tankside treatment with biocides is typically required to provide an additional level of control over microbial growth in the end-use fluid.

Among the questions to ask when selecting a tankside additive is whether the biocide is fast acting against a broad spectrum of microorganisms. Does it contain or release formaldehyde? Is it capable of controlling mycobacteria? Is it effective for control of biofilm (slime) formation on metal surfaces within industrial processes? And if a system develops serious microbiological problems, what can be done to bring it back under control?

All of the biocide products mentioned previously for use in concentrates may also be added tankside, as a part of a total systems management for bacterial and fungal control. Some other biocides are used exclusively as tankside additives, due to their poor compatibility in concentrates.

The most widely used tankside biocide is Kathon 886 MW, due to its high efficacy – remember Figure 2 – and lowcost performance. This water-based formulation of chloro-methylisothiazolone (CMIT) and methylisothiazolone (MIT) is used across all fluid types for broadspectrum microbial control. It is compatible with most MWF additives, with the exception of sodium pyrithione and thiocyano-methyl-thiobenzothiazole biocides, mercaptobenzothiazole, and zinc dialkyldithiophosphate (ZDDP). It is most effective in end-use fluids with pH below 9.5.

Other tankside biocide products, such as Kathon MWC and Kathon CC, are used mostly for troubleshooting purposes when serious microbiological problems arise in the system. These are based on CMIT/MIT biocide, but also contain copper salts to provide added stability to the active ingredients and odor control in more aggressive fluids and adverse conditions.

Disposal Questions

Metalworking fluids are designed to last for a long period of time if proper maintenance procedures are carried out. The industry has been well educated on how to properly take care of metalworking fluid systems, so they can conceivably last for years. However, at some point in time, depending upon the fluid, the system and the application, no further measures can be carried out to keep the metalworking fluid in operation. The fluid manager is then faced with determining the best approach for ultimate disposal.

There are a number of options for treatment of the MWF wastewater. Regulatory issues come into play as components in the spent fluid may be considered hazardous. Fluid managers may seek to discharge the wastewater (effluent) into a waterway, such as a river or stream, or through a sanitary sewer to a publicly owned treatment works. But this can only be done if the treated MWF wastewater meets certain guidelines covering organic and inorganic pollutants. Biocide-treated metalworking fluids may also require additional measures prior to discharge.

The selection of MWF biocides must take into account the ultimate environmental fate and ecotoxicity of the active ingredient. Some biocides (such as phenolics) are recognized as difficult to waste treat. The isothiazolone biocides mentioned in this article have been rigorously evaluated for their environmental impact.

Key environmental toxicity values for isothiazolone biocides have been determined (Table 2). These biocides are stable to hydrolysis and photolysis, but are rapidly biodegraded to carbon dioxide in natural waters containing microorganisms (that is, they have short half-life values). There is no effect of these biocides on sewage treatment (activated sludge respiration) when diluted below 1 ppm on an active ingredient basis.

Since biocides are toxic materials, their safe use and handling is critical to protect workers from exposure. In concentrated form, isothiazolone biocides are corrosive to the skin and eyes and may cause sensitization upon repeated contact. However, they are safe and non-irritating at use levels in the recirculating MWF. Other biocides used in MWF systems are also corrosive, sensitizers, and/or may release formaldehyde vapors. It is always important to consult with the biocide manufacturer and materials safety data sheets to obtain the correct equipment and procedures for their safe use.

Selection of the most effective biocide for a particular operation is a key decision factor for a successful microbiological control strategy for MWF management. Formulators and end users need to have not only highly effective, formaldehydefree biocide solutions, but also comprehensive knowledge about the specific problem- causing microorganisms, to help find synergistic combinations and best practices for their biocide treatment needs in different MWF systems.