Metalworking Fluids (MWF) have never been easy to formulate. There have always been many factors to consider including the application type (cutting, grinding, stamping, etc.), the metal being machined, biological growth, and worker exposure. As regulatory, sustainability, and performance pressures increase from both governmental organizations and end users, MWF formulating is harder than ever.
One major challenge for formulators is the need to replace certain biocides in their fluids. In particular, formaldehyde releasing biocides have come under increasing pressure, leading some suppliers to stop production of key products, including some Oxazolidine chemistries. This leaves formulators with the question of which biocides to use for the preservation of fluids. This article will offer tips on non-formaldehyde releasing biocide options that are effective for metalworking fluids.
While formaldehyde releasing biocides are the most commonly used products, there are many more products that do not release formaldehyde. Many of these products offer additional protection from fungi, a group of organisms not usually well controlled by formaldehyde releasing biocides. Sodium ortho-phenylphenate and ortho-phenylphenol (NaOPP and OPP) have been used as preservatives since the 1940s and are highly effective, broad spectrum biocides. Although NaOPP and OPP can be processed by municipal waste water treatment plants, they do give a positive reading on a phenol test, which can cause some extra trouble in removing fluids preserved with this chemistry.
Two additional formaldehyde free chemistries are methyl isothiazolone (MIT) and glutaraldehyde. MIT is a good bactericide and effective at controlling mycobacteria. However, it can be difficult to stabilize in a MWF concentrate. Glutaraldehyde can cross link amines and is best suited for aluminum hot rolling formulations, which do not typically utilize amines.
Other formaldehyde free biocides include dibromonitrilopropionamide (DBNPA) and bromo nitropropanediol (Bronopol). Both of these chemistries degrade quickly in a typical MWF and are better suited for a tankside addition than a fluid concentrate.
One of the most popular formaldehyde free biocides is benzisothiazolone (BIT). BIT is very stable in a variety of MWF concentrates and is able to preserve a fluid for a long time. However, BIT has efficacy gaps, including a gap against Pseudomonas and fungi. Since pseudomonades and fungi are commonly found in contaminated MWF, it is recommended that BIT be combined with another biocide. One good option for a combination with BIT is the nitromorpholine chemistry. Nitromorpholine has a very broad efficacy spectrum and can make up for the efficacy gaps with BIT. Nitromorpholine does not release formaldehyde under the conditions found in most MWF formulations. In some high misting cases, the nitromorpholine chemistry may cause lachrymation or eye irritation/tearing. However, by using it in combination with the BIT chemistry, a lower level of nitromorpholine may be used, mitigating the potential for lachrymation.
One of the first questions formulators ask when adopting BIT and nitromorpholine is what levels of each product they should use. Because fluid compositions vary so much, it is hard to offer one size fits all recommendations. However, an experiment carried out by the authors offer a starting point for formulators. This experiment showed that low levels of BIT combined with modest levels of nitromorpholine provided excellent control of microbial growth.
The experiment examined the microbial control achieved in a model synthetic metalworking fluid dosed with varying leves of BIT and nitromorpholine. The level of microbial growth was determined through an optical density reading at 630 nm. This method measures the turbidity in the fluid and correlates more turbid (cloudy) fluids to those having more growth. This experiment was completed twice. The first time only bacteria were used to test the combination of BIT and nitromorpholine, using five strains commonly found in MWF in equal ratios. In a second test, a combination of five bacterial species and five fungal species were used to test the combination of BIT and nitromorpholine chemistry. This second case represents a realistic field situation, while the first case is similar to the level of control obtained from formaldehyde releasing biocides.
Using the biocide efficacy testing information, the charts in Figure 1 and Figure 2 were made. Green cells stand for biocide combinations that allowed very little growth. Red cells are biocide combinations that allowed approximately the same level of growth as the undosed control. Yellow cells are those combinations that might offer an intermediate level of microbial control. As with all biocides and combinations, the biocide combination should be verified in a fluid before commercialization of the fluid.
Figure 1. Biocide Efficacy Testing with Bacteria Only.
Figure 2. Biocide Efficacy Testing with Mixed Bacteria and Fungus.
Many formulators are experiencing frustration at the need to formulate out formaldehyde releasing biocides due to regulatory drivers and end user requirements. However, there are several options for biocides that do not release formaldehyde under MWF conditions. Sometimes a combination of biocides, such as BIT and nitromorpholine, can be the best option. This combination can provide the desired efficacy of microbial control, while maintaining the high performance fluid that customers expect.
Dr. Christine E. McInnis and Jenni Allen work at Dow Microbial Control. Their contact information can be found in the membership database.
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