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substance requires registration as a biocide under BPR or may be used as a technical additive under ECHA. As I’ll discuss in this article, the concept of intention can be dodgy. Although any MWF formulation that includes a biocide might be considered a treated product, UEIL was able to persuade ECHA’s Biocidal Products Committee to waive the requirement for MWF registration as treated products.
In this article, I will use the term microbicides when referring to biocides that target microorganisms.
Why use microbicides? When used in MWF, microbicides provide the follow four performance benefits:
• Fluid preservation – microbicides prevent microbes from eating MWF molecules (hydrocarbons, phosphate esters, amines, organosulphur molecules, etc.). This contributes to the United Nations Department of Economic and Social Affairs Goals for Sustainable Development, Goal 9 for sustainable industrialisation.
• Surface protection – microbiologically influenced corrosion (MIC) causes an estimated $50 billion U.S. to the petroleum industry. By inhibiting biofilm formation of MWF system surfaces, microbicides protect the system. This also contributes to sustainability Goal 9.
• Bioaerosol mitigation – recirculating MWF are major bioaerosol reservoirs. Controlling microbial loads in the MWF reduces bioaerosol generation. Reduced bioaerosol exposure reduces the risk of allergenic diseases among workers, contributing to Sustainable Development Goal 3, good health and well-being.
• Odor control – microbes produce thousands of different microbial volatile organic compounds (MVOC). During normal operations, MVOC are oxidised before they escape the recirculating fluid. When systems are shut down on weekends, MVOC – especially organosulphur compounds accumulate. When pumps are restarted on Monday mornings, malodorous gases are released to produce noxious Monday morning odors. Effecting system management and microbicide use can mitigate this phenomenon. Odor control contributes to Sustainable Development Goal 3, good health and well-being.
Why not formulate MWF with biostable additives?
Globally, MWF compounders are selecting biostable functional additives for use in their formulations. There is a growing list of such additives. However, among bona fide functional additives, there are also unregistered biocides. As Illustrated in Figure 1a, a biostable additive has negligible effect on microbes living in a MWF and is not used as food. These substances are recalcitrant. In contrast, a biodegradable additive (Figure 1b) can be used as food by microbes in the MWF. Its concentration is likely to decrease relative to that of other chemistries in the formulation. Biocidal additives kill microbes (Figure 1c) but offer no other functional benefits. One commonly used amine – Dicyclohexylamine (DCHA – CAS 101-83-7) is my poster child for unregistered biocidal additives. Figure 2 compares DCHA’s oral, inhalation, and dermal acute toxicities with those of Hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine (HHT – CAS 4719-04-4). For decades, HHT has ranked among the most used MWF microbicides globally.
Figure 1: Relationship between additive function and metalworking fluid bioburden – a) recalcitrant (bioresistant) functional additive: when added to a MWF, the additive has negligible impact on bioburden and its concentration remains constant; b) biodegradable functional additive: the additive is a good food source supporting biomass increase as the additive’s concentration is depleted; c) biocidal additive: bioburden drops after MWF is dosed with the additive. Additive concentration decreases at a rate related to chemical and biological demand.
Some MWF compounders have replaced HHT with DCHA so that they can claim their formulations are biocide free. To date, no data have been published to demonstrate that DCHA has any functionality other than its microbicidal efficacy. DCHA’s acute oral toxicity is 7.6x HHT’s (200 mg kg-1 1520 mg kg-1
for DCHA versus HHT’s (260 mg kg-1
for HHT). Its dermal toxicity is >7.8x versus >2000 mg kg-1
). Are MWF
formulated with DCHA safer than those formulated with HHT? It is disingenuous to use chemicals that are more toxic than the biocides they are replacing, unless the non-biocidal chemicals have other performance properties that exceed those of alternative chemistries.
10 LUBE MAGAZINE NO.181 JUNE 2024
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