Restrictions Mount

The fate of formaldehyde-releasing micro-biocides was the topic of a number of presentations and a panel discussion at the Society of Tribologists and Lubrication Engineers Annual Meeting in May. The concern over these chemicals is that formaldehyde is classified as a human carcinogen. However, as Alan Eckard of Monroe Fluid Technology pointed out, There have been very few cases where human carcinoma can be related to formaldehyde exposure, especially in the metalworking environment where worst-case exposure levels are quite low.

As noted by Fred Passman of Biodeterioration Control Associates Inc., These products represent 70 to 80 percent of the total volume of antimicrobials used in metalworking fluids. Nonetheless, many countries are placing regulatory pressure on suppliers and users to replace these microbiocides.

Peter Wachtler of Lanxess summed up the regulatory landscape in Europe, saying, European nations, especially France, are determined to move formaldehyde to a more severe labeling category. Overall, the metalworking fluids industry has a lot of challenges to deal with.

Passman, who is based in Princeton, New Jersey, U.S., added, The fate of formaldehyde-releasers in the U.S. remains uncertain. The U.S. Environmental Protection Agency has determined that the maximum permissible dose for formaldehyde-condensing biocides should be 500 parts per million, compared to the current usage of up to 2,500 ppm. Existing products are completely ineffective at the lower dosage, he said, adding that EPA has tabled plans to reduce the limit while awaiting a report on the effects of formaldehyde.

Passman added that Canadas Pest Management Regulatory Agency has also agreed to postpone [its] planned action to restrict the maximum dose of triazine to 500 ppm. The agency will review the data presented to it in the months following its decision to require a label change. The postponement buys the industry more time in Canada. The reprieve could be a few months or greater than a year, he said.

The label regulation is on hold, in part due to strenuous objections from the metalworking industry, biocide manufacturers and many health and safety professionals. They realized that removal of the most widely used and cost-effective pesticide could easily increase microbial growth in many large MWF systems. This would cause a subsequent increase in exposure to microbes and endotoxins that many feel is a much higher risk than formaldehyde, said Eckard.

In Europe, the Biocidal Products Directive will significantly influence the future use of biocides, said Wachtler, who is based in Leverkusen, Germany. The requirements of the BPD were explained by Christine McInnis of Dow Microbial Control. The BPD requires biocides for metalworking fluids to be registered under BPD PT 13. The first step was registration of the active substance. Dossiers were submitted in 2007, and all are still under evaluation by European Union competent authorities. The second step is registration of individual biocidal products containing approved active ingredients.

As long as dossiers are under evaluation, said McInnis, all biocidal products containing these actives can be marketed in Europe. However, national requirements in EU member states, such as those in France, must be observed.

Wachtler emphasized that long-term maintenance of water-based metalworking fluids is an indispensable requirement to control process costs and optimize used oil management. It is widely recognized that the proper use of preservatives is a critical factor in achieving these goals.

According to Eckard, The most widely used antimicrobial compounds function by releasing formaldehyde once inside the microbe cell. And some, especially triazines, release detectable levels of formaldehyde into the air space above MWFs, especially when pH has dropped. This is often due to excess growth of micro-organisms that can generate organic acid as a byproduct of growth.

Formaldehyde-releasers have been used for decades as biocides in metalworking fluids because they are economical and effective against a wide variety of microbes. They are still an essential tool for reliable protection of circulating oil emulsions and solutions, said Wachtler. A ban would have severe consequences for industry, but we must be prepared for this worst-case scenario and search for alternatives.

A further complicating issue is that some microbes may develop tolerance or resistance to certain pesticides or may be inherently less sensitive. This has been observed for the triazines typically used for bacterial control in MWFs, Eckard said. Unfortunately, yeasts, molds, fungus and mycobacteria are difficult to control with conventional triazine chemistry. He explained that yeasts, in particular, generate acid rapidly and can decompose in the presence of triazine to release formaldehyde.

Alternative Chemistries

In Wachtlers opinion, in highly regulated areas such as Europe and the U.S., the quest for new biocides must focus on existing and approved biocides. In Europe, this is a rather small set of compounds, he said. From the almost 30 listed actives currently supported under BPD for use in metalworking fluids, approximately 10 are formaldehyde-releasers and five are isothiazolinone derivatives.

Dows McInnis reviewed the critical properties required of an effective biocide. She concluded that benzisothiazolinone (BIT), morpholines, methyl isothiazolinone (MIT) and phenolics were viable alternatives although each had pros and cons that users must evaluate prior to use. McInnis also indicated that the combination of BIT and nitro-dimorpholine is easy to formulate into concentrates and is effective against both bacteria and fungi.

Wachtler expanded on the antimicrobial efficacy of phenolic biocides, which he said is based on interactions with the cell membrane of the target microorganisms. Compared to so-called reactive biocides, this interaction does not destroy the phenolic structure. As a result, he said, they provide effective long-term microbe management and require fewer post additions. Of the phenolics, Wachtler explained that testing at Lanxess showed that parachlorometacresol (PCMC) provided the broadest antimicrobial activity spectrum, and ortho-phenylphenol (OPP) was strong against fungi and yeast.

To demonstrate the effectiveness of PCMC, Lanxess researchers conducted trials in mineral oil based emulsions. In the test, 5 percent emulsions were inoculated weekly with a micro-organism mixture known to contaminate oil based fluids. The inoculations were repeated up to a maximum of 10 times.

The results showed that a 1,500-ppm active dosage of PCMC protected against all the organisms (bacteria, mold and yeast) over the 10-week test period. Triazine at the same dosage was a good bactericide but was not able to combat mold and yeast.

Lanxess also conducted trials with OPP alone and in an amine-enhanced formulation. As expected, a 1,500-ppm dosage of OPP protected against mold and yeast, but not bacteria. Wachtler explained that while OPP provides excellent antifungal efficacy, it typically is not strong enough alone to provide complete protection against the whole range of microorganisms, including bacteria.

There are two ways to improve the effectiveness of OPP and convert it into a broad spectrum biocide. It can be either combined with other biocides, or it can be enhanced by an amine. And Lanxess testing showed that active OPP at concentrations of 1,000 ppm, when combined with an amine package, provided much better protection against bacteria, as well as mold and yeast.

A similar Lanxess experiment examined the effect of amines on the performance of PCMC. The results showed that while PCMC alone required a dosage of 1,500-ppm active for effective protection, an amine package with a dosage of only 1,000-ppm PCMC active provided good protection against bacteria, mold and yeast.

The chemical stability of both phenolic additives makes this group of biocides particularly well suited to be combined with amines, said Wachtler. On the other hand, he added, combining most other reactive biocides with amines would be too aggressive chemically, particularly if added to concentrated fluids.

Both phenolic compounds are covered by broad documentation and approvals for many applications. For example, PCMC and OPP are both approved for use in cosmetics and pharmaceutical products, Wachtler noted. OPP is widely used to protect citrus fruits to prevent mold growth during storage. In addition, PCMC is on the list of Food Contact Substances and can be used as an NSF-H1 approved antimicrobial in lubricants with incidental food contact.

Wachtler concluded, Whether in the framework of the U.S. EPA or the European Biocidal Products Directive, both phenolic actives enjoy the same high level of regulatory support, resulting in a very broad coverage of possible target markets.

In the search for alternative chemistries, Lubrizols Uwe Falk, Hamburg, Germany, said, There may be no significant changes in biocide chemistry due to high registration costs, including extensive data generation concerning health, safety and environmental impacts. These requirements could effectively limit new molecule development.

He went on to describe a biocide based on methylenebismorpholine (MBM) chemistry that is currently available in 36 countries. The available formaldehyde from this EPA-registered chemistry is estimated to be 1.1 parts per billion with maximum estimated concentration of 3.3 ppb. These levels are below the agencys occupational level of concern of 100 ppb of formaldehyde, said Falk, and are expected to pose little to no environmental exposure in MWFs.

The MBM biocide is for use as a concentrate in closed and semi-enclosed machines, Falk said. It forms stable emulsions at a treat rate of 1,000 to 1,500 ppm and is stable for more than six months at elevated temperatures.

Nonferrous Metals: A Special Challenge

Also at the STLE conference, Monroes Eckard, based in Hilton, New York, U.S., examined the unique challenge faced by biocide users in nonferrous metal removal operations. There is increasing interest in using renewable or vegetable-based fluids that can be effective lubricants especially with metals such as aluminum. However, these fluids can act as nutrients that facilitate microbial growth, he said. Therefore, reducing the number of possible biocide candidates will be a challenge to users of these fluids.

MWFs for nonferrous applications can be more difficult to protect from microbial growth because these metals respond especially well to the vegetable-oil-based esters, fats and triglycerides often used as lubricity additives. While vegetable oils can be very effective especially for aluminum forming and removal, their esters are excellent nutrients for microbes, Eckard said.

Microbe growth is usually much less likely in applications involving copper alloys because they usually form soluble complexes or soaps that are inherently biostatic. Aluminum, on the other hand, does not produce salts or reaction products that retard microbial growth.

With possible restrictions on triazine chemistry, other pesticides must be considered for nonferrous applications, as well as biostatic agents. While vegetable oils are not inherently biostatic, they can be made so by incorporating biostatic compounds. One of the most common is boron in the form of boric acid salts, complexes or esters, Eckard said. However, concerns about the toxic effects of boron may lead to restrictions, especially in Europe.

Eckard explained that other bio-static agents typically have alkyl or alkanolamine structures that enable them to interfere with the metabolic processes of most microbes. In many cases these agents show low to moderate toxicity to humans and, at typical use levels, appear to be of low risk, he said. Some of these agents also appear to increase the potency of certain pesticides that are registered for use in MWFs. Using these combinations is a promising approach to microbial control.