Boric acid and is compounds have been used as corrosion inhibitors in metalworking fluids (MWFs) for a number of years. However, boric acid has come under attack in recent years as a potential hazard to human reproduction and as dangerous to plants and animals. While these findings have been disputed in some quarters, the European Union classified boric acid as a Substance of Very High Concern (SVHC) that requires special labeling. So, despite boric acids benefits, MWF suppliers and formulators are searching high and low for replacement chemistries.

Boric Acid – The Good and the Bad

Robert Stubbs, Sea-Land Chemical Europe Ltd., gave a presentation titled Boric Acid – What happens next? at the 4th Symposium on Metal Removal Fluids, in Barcelona, Spain, in September 2011. Stubbs noted that the use of boron technology in metalworking fluids dates to the 1970s when boron compounds were developed as replacements for nitrosamines for use in synthetic fluids. The technology grew through the 1980s especially in semi-synthetic fluids.

Combined with amines, boric acid is an excellent ferrous metal corrosion inhibitor, said Stubbs. It also provides good pH buffering and resistance to bacterial degradation. On the other hand, the compounds can form persistent residues on metal surfaces, and they have poor resistance to fungal degradation.

Concerns about boron first originated in Southern Europe over borates in powdered detergents entering groundwater and irrigation canals, causing harm to citrus fruits. Boron is unusual in that it is an essential plant nutrient; however, at concentrations above 1 ppm, it causes leaf necrosis, and above 2 ppm, plant failure.

Also, in the early 1970s, studies showed a potential link between boric acid and reproductive issues in humans. In the early 1990s, a number of studies revealed potential reproductive toxicity effects in rats and mice. In addition, in 2008, the U.S. Environmental Protection Agency issued a Drinking Water Health Advisory for Boron (Document Number: 822-R-08-13). Because of these concerns, a number of European countries adopted classification requirements for products containing boric acid and its compounds.

At the same time, several reports refuted the findings of reproductive concerns. For example, a study of mineworkers in the U.S., Turkey, and China showed no real evidence of reproductive effects. The reports also pointed out that boron is an essential human nutrient, is used as a preservative in some food, and is used in eyewash.

The United Kingdom Lubricants Association (UKLA) Metalworking Fluid Product Stewardship Group voiced its concerns in a position paper on boric acid in 2008 that states in part: The position of the major European borates suppliers is that classification is unwarranted. They assert that borates do not pose any risk to human health. The EC decision was based on potential hazards, determined by exposures that are not relevant to people, even those who work with borates every day. Nevertheless, the borates industry is fully committed to compliance with the new requirements and is working with downstream user groups and customers, including manufacturers of metalworking fluids and additives.

Labeling Requirements

As of December 1, 2010, the European Classification, Labeling and Packaging of Chemicals (CLP) regulation classified boric acid and preparations that contain more than 5.5 percent boric acid as toxic to reproduction, Category 2. Also, according to the EU Dangerous Preparations Directive, these substances must be labeled with skull and crossbones until June 1, 2015. After that date, mixtures containing 5.5 percent or more of boric acid must be classified and labeled according to the CLP, and the pictogram for target organ toxicity – the so-called exploding chest – must appear on the container label.

Since June 2010, boric acid is also on the Substances of Very High Concern (SVHC) list of candidates according to Articles 57 and 59 of the REACH regulations. This list contains materials that will be subject to special examination to assess the risk to human health and the environment.

According to at least one metalworking fluid supplier, substances that are subject to approval must be registered under all of their uses. Under certain circumstances, approval may not be granted for one, for several or for all applications so that, from a certain point in time, sales may be restricted or even completely forbidden. The possibility that boric acid may become a substance subject to approval in the future cannot be ruled out. At present, it is difficult to assess whether its use in metalworking fluids would be approved, but it is not unlikely.

John Neale, Managing Director, John Neale Ltd., a Midlands England specialty lubricants formulator and supplier, pointed out, The key thing to consider in deciding whether MWF containing boric acid is categorized as toxic to reproduction is the amount of free boric acid. If the level is below 5.5 percent but above 0.1 percent, boric acid is to be displayed as an SVHC substance, according to Chapter 15. Also, according to Chapter 3, boric acid must be displayed as a substance toxic to reproduction marked with T R60/61. Regardless of these labels, the product need not be labeled as a hazardous preparation.

Currently, said Stubbs, labeling is required if a product contains the concentrations of boric acid shown in the table above.

Despite these requirements, UKLAs position paper states: The nature of the active ingredients in specific corrosion inhibitors is complex, and they can vary enormously in their chemistry. In most metalworking fluids, the borate compounds are present at a level below the threshold for classification of boric acid. In additives and a small proportion of fluids, the concentration of the active ingredient is higher, and the potential for the presence of unreacted boric acid has to be considered.

Looking for Replacements

Neale said that regardless of the industrys position on the hazards posed by boric acid, it is likely that more and more boron-free products will come on the market in the future, if for no other reason than to avoid the labeling requirements. As a result, product manufacturers will likely reduce and, in many cases, cease to develop with boron based technology in Europe. And product manufacturers will avoid handling boric acid if possible. Thus, it will be incumbent upon additive manufacturers to develop alternative chemistries or find ways to avoid having their customer handle boron-containing products.

Neale noted that citrus fruit producing countries in Europe have already moved away from using boric acid in preparations, but it is still common throughout the rest of Europe. However, because of the concern, it is becoming a marketing advantage to say your products contain no boric acid, said Neale.

Unfortunately, said Stubbs, there are no obvious direct replacements for boric acid because of the properties it imparts to the finished fluid. Alternative chemistries include lactic acid, phenoxyethanol/propanol, dicyclohexylamine (DCHA), and various cyclic/branched amines. However, none of these has proven to provide the beneficial effects of boric acid at comparable treat rates.

Several companies are offering boron-free technology. However, said Neale, traditional boron-free products are not very biostable. They allow the growth of so-called good bacteria, but the U.K.s Health and Safety Executive (HSE) has done extensive studies that show these good bacteria produce toxins just as dangerous as the bad bacteria. As a result, the HSE requires bacteria levels be maintained below 1 million organisms per milliliter to ensure worker safety. Unfortunately, Neale noted, experience shows that bacteria levels in boron-free products can easily exceed 10 million to 100 million organisms per ml. Therefore, the idea of just allowing a population of bacteria to exist in the MWF is no longer viable.

According to Neale, much effort is focusing on developing boron-free products that are biostable with low bacterial populations. For example, phenoxy ethanol and phenoxy propanol have exhibited biostable performance. However, while these compounds keep MWFs stable, they are not capable of controlling bacteria below 1 million organisms per ml.

Other chemicals being examined as boric acid replacements include lactic acid, DCHA, and other biostable amines. To date, none has proved a satisfactory replacement.

Lactic acid, while biostable, does not provide the corrosion protection of boric acid. It also is not an effective buffering agent, said Neale.

DCHA has good biostability and buffering properties; however it must be used at 7 percent (vs. 5 percent for boric acid) to produce these characteristics. In addition, concerns have been raised that DCHA may be aggressive to aluminum. Also, Neale said, there is a question of whether these compounds may form nitrosamines, which are also hazardous. Finally, other biostable amines have shown promise; however, they must be used at relative high treat rates (above 7 percent).

Boric acid is particularly difficult to replace, said Neale. It has about three times the corrosion resistance of amines. And in areas with chloride in the water, amines can cause corrosion problems. In addition, there is a significant cost penalty. We can produce a biostable boron-based product with excellent corrosion resistance and buffering for a raw material cost of 1.20 (1.3) per liter vs. 2 per liter for an amine-based product, said Neale.

Another alternative, said Neale, is to use non-biostable materials but add significant levels of biocides to control bacteria. This route is usually not favored due to the increasing concern over the use of biocides.

According Neale, An important thing to remember about the new regulations is that the use of boric acid in metalworking fluid concentrates and the use of water-mixed metalworking fluid containing boric acid is allowed, and will remain so for the time being. A few changes will, however, be required of both the makers and users of metalworking fluid, in terms of categorization, labeling and risk assessment, and of communication along the supply chain.