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It is the aim of the Biocidal Products Regulation (BPR) which came into force within the European Union on September, 1st, 2013, that all biocides are risk assessed for their toxicity to humans and the environment before they are permitted to be placed on the market, and that they are sufficiently active against the harmful organisms they are designed to target. This is done via processes of approval for biocidal active substances (the active ingredients) and via authorisation of biocidal products (substances, formulations or articles that contain the active substances and are intended to be used as biocides). The approval and authorisation processes are very cost-intensive and many manufacturers of biocides for metalworking fluids spared the expense. This resulted in a drastic reduction in the number of biocides that can be used for MWF to currently only 25 substances.

Formaldehyde: Moreover 13 of them are formaldehyde releasing biocides (FRBs) that continuously release traces of formaldehyde into the metalworking fluid. Formaldehyde is labelled as potentially causing cancer according to the globally harmonised system of classification and labelling of chemicals. Additionally the three FRBs; MBO, HPT and MBM are expected to be classified soon as category 1B carcinogenic and category 2 mutagenic based on read-across to the specific toxic effects of formaldehyde. Other FRBs may follow. Despite the concentration of free formaldehyde in water mixed MWF and also in the concentrates in most cases is far below the limit of 0.1%, many users of metalworking fluids refuse to use FRB containing types.

Further limitation of suitable biocides is caused by a low occupational exposure limit for the fungicide sodium pyrithione and the classification of 3-iodo-2-propynyl-butylcarbamate (IPBC) as toxic if inhaled. Many isothiazolinones which are effective at very low concentrations are classified according to CLP as sensitising substances which can cause allergies.

Boric acid: Boric acid, which has been commonly used as corrosion inhibitor in the past and which also inhibits the growth of micro-organisms, is classified as reproductive toxic category 1B (may damage fertility, may damage the unborn child). This classification limits the presence of boric acid in watermiscible metalworking fluid concentrates to 5.5%. However boric acid is also listed on the candidate list of substances of very high concern (SVHC) which is published by the European Chemicals Agency (ECHA) because of its supposed toxicity for reproduction. Therefore many users of metalworking fluids do not further accept any content of boric acid.

The number of substances that can be added to a watermiscible MWF to avoid excessive growth of bacteria or fungi in the machine is quite low meanwhile and may be more reduced in future. This has a strong influence on the overall composition and further development of the products. To achieve a long stability of the water mixed metalworking fluid in use, synergistic effects between the remaining biocides have to be exploited and all other components have to be selected for their resistance to microbiological degradation. The use of higher amounts of specific amines raises the pH of the MWF, which can significantly reduce microbial growth. The number of amines which can be used for the formulation however is already limited and will undergo a further limitation in future due to new or tightened legal restrictions.

Figure 2. Water mixed metalworking fluids need biocides to prevent bacteria growth

But also the users of MWFs are able to increase the lifetime of the fluid by taking care of its cleanliness, by avoiding contamination of tramp oil or biological materials and by controlling and maintaining important parameters like concentration or pH-value.

Accumulation of heavy metals The occupational exposure limits of several heavy metals are expected to be reduced by a number of national authorities within the next few years. This will also affect MWF aerosols containing dissolved heavy metals. Heavy metals are defined as metallic elements that have a density of more than 5 g/cm3. Some of them have been reported to affect cellular components or enzymes of humans, animals or plants. They can also interact with cell components such as DNA and nuclear proteins, causing DNA damage followed by cancer or apoptosis. On the other hand, heavy metals like lead, copper, cobalt, zinc, chromium or nickel are used in industrial applications.

Cobalt: Cemented carbides are sintered metals usually consisting of tungsten carbide particles and cobalt as a binder. They are commonly used for metalworking tools but also for components for the chemical or petrochemical industry. By improper metalworking fluids (watermiscible as well as neat oils) the cobalt binder can be leached out of the surface of the cemented carbide and dissolved in the MWF. When cobalt containing MWF mist is swallowed or breathed it can cause nausea, visual disorders and as a long-term effect, heart trouble and several types of cancer. Additionally, cobalt is a major cause of contact dermatitis.

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