In the last few years there has been a trend in the market to supply bactericide-free water-mix metal working fluids to the more discerning customer. This is due to the potential risks associated with formaldehyde release biocides (US EPA consider formaldehyde a possible human carcinogen), and isothiazolinone types which generally lead to the metal working fluid concentrate being labelled as a potential skin sensitizer (R43).
More recently boric acid has been designated as a substance of very high concern (SVHC) in Europe, and this has encouraged the development and marketing of boron-free coolants to markets where boron was not before considered an issue.
In the UK, bio-concept fluids – a common style of boron and biocide free coolants – are now considered less suitable, tests carried out by the Health and Safety Executive (HSE) have shown that the bacteria found in these bio-concept fluids are no different to those found in traditional boron bio-stables when it comes to allergic alveolitis.
There does now seem to be a fairly narrow approach to producing boron-free, bactericide-free metal working fluids which have adequate control of bacteria and give products with good general properties.
DCHA – dicyclohexylamine is a commonly used amine in these types of fluids, giving very good bacteria control when used at 3500ppm in the diluted emulsion (7% DCHA in a product designed for use at 5%). There is however a potential issue with this amine as it is a secondary amine and as such may form nitroso compounds which could be of concern. Synergex T is another good candidate for these fluids, again utilised at 3500ppm in the emulsion. Corrguard EXT is another good amine which gives excellent bacteria control at dilutions as low as 1250ppm.
To optimise the bio-control of these amines, other additives can be used to give broader performance and stability. Common additives for this are alcohols such as benzyl alcohol, and modified alcohols like phenoxyethanol. Addition of these types of additive on top of the bio-stable amines gives products with significantly better bio-stability.
These approaches to boron- and bactericide-free cutting fluids are more expensive than traditional boron-containing fluids.
Cutting fluids using these amines and combined with alcohols required modification in the formulation approach, with different emulsifiers sometimes required to give suitable concentrate and emulsion stability.
It seems likely over the next few years there will be more companies designing products based on these few amines and we will see all small- to medium-sized manufacturers of coolants developing a range of boron and bactericide-free cutting fluids to mirror their old boron bio-stables.
John Neale is Owner of John Neale, Ltd. You can reach him at firstname.lastname@example.org.
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