*4.1. Environmental Implications of CO2 Coolants*

Given that CO2 is a greenhouse gas, it is reasonable to assume that should it displace conventional emulsion coolant strategies, the net release of CO2 into the atmosphere would invariably increase in concert. This is a particularly concerning proposition given that the UN's Intergovernmental Panel on Climate Change (IPCC) has stated that a 45% reduction in global net CO2 emission is necessary (by 2030) to meet global warming targets [9]. Clearly, on the surface, these concerns make intuitive sense. Unrestricted CO2 usage would seem to be a significant backwards step in meeting the UN's lofty climate change goals; however, it is the argument of the author that these concerns are a consequence of a fundamental misunderstanding of the CO2 processing route. If CO2 was indeed purposefully generated in order to meet coolant needs, this would be a pertinent point; however, this is not the case. In reality, CO2 is generated in a range of essential industrial processes (i.e., in the production of ammonia and alcohol), and is ordinarily released into the atmosphere at the point of production. By instead reclaiming this CO2 for use in MWF's (amongst other things), the

rate of CO2 production is unchanged relative to its baseline industrial production, and as such, the otherwise wasted CO2 now serves a purpose prior to being released into the atmosphere (as it ordinarily would be). Ultimately, the only additional energy expenditure associated with CO2 as an MWF comes from that which is consumed in the recycling process; otherwise, its use corresponds to a net zero change in emissions relative to the industrial baseline. The use of CO2 coolants could also lead to an overall lower carbon footprint of the machining process, since the targeted application of such coolants could lead to increased throughput of the machining process. Furthermore, carbon capture techniques could be used to recycle and reuse the CO2 used in the machining process.

In contrast with the sentiments of the previous paragraph, the use of emulsion and oil-based coolants can be extremely problematic from an environmental sustainability standpoint. Whilst mineral oils were once regarded as an inconvenient by-product of petroleum production, this is no longer the case. In fact, it can be reasonably suggested that the vast demand for mineral oils serves as an ancillary driving force for the crude oil industry. Of course, many of the concerns associated with the crude oil industry can be subverted by formulating MWF's that are comprised of oils which are non-derivative of mineral oil, i.e., vegetable oils and synthetic formulations. Indeed, whilst synthetic and vegetable oil-based MWF's are generally more expensive, this statement is largely correct, and in fact, many high performance MWF's are formulated without the use of mineral oil. Despite the improved solution offered by synthetic oil formulations, there nonetheless persists a range of problems that apply to both synthetic, mineral and vegetable oil-based MWF's, not least of which the issue of their ultimate disposal.

Whilst cryogenic MWF's re-enter the atmosphere after a given machining cycle, conventional MWF's are repeatedly recirculated in the machining centre. Although this is a comparatively environmentally frugal system of coolant delivery relative to single use, oil-based coolant strategies, it fails to address the issue of the ultimate disposal of the MWF. This disposal becomes necessary as the coolant is recycled throughout the machine owing to a degradation in quality by means of microbial, and processing by-product contamination [10]. In fact, in a 1988 article, this degradation is explained as an intuitive consequence of the underlying MWF chemistry, whereby "mineral oil base stocks, glycols, fatty acid soaps, amines and other metalworking fluid concentrates" are said to "provide all of the essential nutrients required for growth" of bacterial and fungal organisms [11]. Whilst this ultimate microbial spoilage is necessary to facilitate the ultimate biodegradation of the fluid, it equally accompanies a range of adverse implications for machine operation, some of which include:


In addition to these consequences, microbial spoilage corresponds to an increased incidence of dermatitis and skin irritation; however, this is explored in greater detail in Section 4.2.

Evidently there exists a range of problems associated with the prolonged use of spent coolant. In order to subvert these concerns, machine coolant reservoirs must frequently be drained, cleaned and replenished with fresh MWF, whilst the spent coolant should be disposed of, or recycled in an environmentally cognisant manner. The process of cleaning an MWF system is, however, not without its concerns. A common cleaning protocol involves the use of compressed airlines and/or the administration of synthetic cleaning products, which are often harmful to life [12]. Whilst this use of compressed air is effective in displacing MWF biofilm and clogged particulates, it equally leads to the mobilising of a rancid MWF mist, corresponding to the generally poor ergonomics of the cleaning process. This phenomenon is equally concerning given the often small, or confined working areas

associated with machine shop floors, whereby fresh, uncontaminated air is often unable to easily recirculate. Moreover, this cleaning protocol is generally iterative and may have to be repeated several times until the extent of bacterial contamination is satisfactorily reduced (as tested by a dip slide). In addition to the challenging cleaning protocol, the recycling process by which MWFs are treated is typically extremely convoluted and, in general, requires the adoption of a robust separation procedure. Whilst there are currently a range of developments being explored in the treatment of spent MWF (i.e., nanofiltration, ion exchange resins, etc.), the general processing route by which spent MWF's are treated still largely fails to generate a reusable MWF, but rather, a less hazardous waste substance which can be disposed of in a more financially, or environmentally sustainable manner [13]. For these reasons, the use of cryogenic MWFs, which return to the atmosphere post use, is of clear benefit in terms of the relative reduction in environmental, financial and time cost to the manufacturing sector.
