Metalworking fluids

Introduction

Metalworking fluids (MWFs) are used in workshops worldwide for the cutting and forming of metals. Their main purposes are to cool and lubricate tools, work pieces and machines, inhibit corrosion and remove swarf. MWFs are available as non-water-miscible oils, water-miscible/emulsifiable concentrates or fully synthetic, oil-free products.

MWFs contain 10 to 30 different substances, some of which are well-known as irritants or allergens. During use, chemical and biological reactions can cause the formation of carcinogens (nitrosamines, polycyclic aromatic hydrocarbons) or foul-smelling products of decomposition of the products (H₂S, NH₃). The carbon dioxide produced (by microbial respiration) lowers the original pH from about 9.5 to an acidic value, resulting in corrosion. Biocides are used to prevent these conditions arising.

The most frequently reported health problems are skin diseases, respiratory disorders and in some cases conditions associated with toxicologically relevant substances from the worked material itself (lead, chromium, nickel, beryllium).

Types of metal working fluids and composition

Metalworking fluids (MWFs) have a whole range of functions that naturally require very different ingredients. Some of the functions are the same for all types of MWF: cooling, lubricating, removing swarf, corrosion protection, and compatibility with humans and the environment. Which of these functions is foremost determines the type of MWF chosen ^[1][2].

The MWF is usually initially chosen for technical reasons, i.e. depending on the MWF recommended (or, in exceptional cases, prescribed) by the metalworking machine manufacturer, the type of machining (coarse or fine) and the type of work piece material. The technically suited MWF is then examined for its hazard potential, e.g. allergen content, emission behaviour of the base oil.

A fundamental problem with all MWFs is that the longer they are used the more they become “contaminated" with impurities, e.g. hydraulic fluid from leaky systems or corrosion protection agents from upstream processing. These are only tolerated up to a certain percentage, and beyond this machining problems can arise, e.g. lack of emulsion stability or lubricating effect ^[3][4].

Non-water-miscible oils

Non-water-miscible MWFs mainly consist of a base oil (usually over 95%). This can be a mineral oil, ester oil (e.g. unrefined or chemically modified rapeseed oil) or synthetic oil (e.g. poly-alpha-olefin). For sustainability reasons, the share of recycled oils (re-refined oils) has been growing for years. These have to satisfy the same technical specifications as primary raw materials ^[2][5].

Antioxidants, lubricity enhancers and anti-mist additives are further typical ingredients of non-water-miscible MWFs. However, the risk-determining component is usually the base oil, particularly if it is aerosolized and become airborne due to high working load (high feed pressures, high speed of work piece rotation and elevated temperatures during machining).

Since non-water-miscible MWFs are not susceptible to microbial contamination, they usually have a long service life (several years). The incursion of water or aqueous media must be prevented. The removal of fine abrasion particles (metal particles, graphite) demands greater effort with increasing service life. Contamination oils (e.g. tramp oil from hydraulics) may become emulsified in certain MWFs, in which case the latter are “emulsifying" products with surplus emulsifier. If the content of contaminants is too high, it is better to use a “demulsifying" product so that these contaminants can be separated.

Water-miscible oils

Water-miscible metalworking fluids (MWFs) are mixed with water before use, in concentrations of 2 to 25% , depending on the product and type of machining. This type of MWF is based on oil (comparable to the oil described in the preceding chapter “Non-water-miscible oils), and shares of 20 to 80% are possible. To combine this oil with water to yield an oil-in-water emulsion, an emulsifier is necessary ^[2][6].

The emulsion contains a number of components that encourage bacterial and fungal growth, e.g. phosphorus and sulphur additives. Micro-organisms can also be imported via the water, floor, air, humans and the work piece itself. Adding biocides is therefore necessary so that the MWF’s lifetime is as expected and its use is therefore economically rentable. To prevent the growth of fungi (moulds, yeasts), MWFs also usually contains fungicides. In most cases, they also contain a bactericide to prevent or minimise bacterial growth, although there are exceptions. In all cases, there is a metabolic degradation of some MWF-specific substances (e.g. sulfur- and phosphorus- containing additives) and the content of biomass increases accordingly. The number of micro-organisms that can be tolerated in the MWF depends on the application in question.

The typical pH of water-mixed MWFs ranges from 8.5 to 9.5. However, decomposition of micro-organisms can lower the pH by formation of carbon dioxide and make the water-mixed MWF become acid, which can cause the corrosion of many metals. Therefore, pH stabilisers are also very important ingredients to ensure alkaline conditions. The most frequent ingredients are caustic soda solution and various amines, buffered by using salts with relatively long-chain carboxylic acids.

Also worth mentioning are dangerous substances arising as a result of the metalworking process itself. As a consequence of microbiological activity, for instance, amines can degrade into ammonia, and various sulfur compounds into hydrogen sulfide. Both reaction products are gaseous and are released by the MWF, are foul-smelling and toxicologically relevant

Fully and semi-synthetic fluids

These types of MWFs are water-miscible that are free of oils and do not therefore need emulsifiers. They can be based on water-miscible glycol compounds, for example. Mixing in water yields a transparent water-mixed MWF. Otherwise the characteristics are very similar to those given under chapter “Water-miscible oils" ^[2][6].

Media for minimum quantity lubrication (MQL)

The application of minimum quantity lubrication (MQL) has steadily increased in the last decade. These products are based on fatty acid esters and long-chain alcohols. Since these products are water-free, the wetting of the work piece with these components is sufficient as corrosion protection. By definition, these products are only used in minimal quantities (roughly 1 litre per 8-hour shift) compared with hundreds of cubicmeters flooding the workpiece, and they are delivered precisely to the machining zone by compressed air. This type of application yields a whole range of advantages and renders large-volume filter systems (as necessary in non-water-miscible oils, water-miscible oils and fully and semi-synthetic fluids) superfluous. Instead, the metalworking machine and usually also the tool and the MQL metering system have to be designed so that the chips can be removed from the machine tool without large quantities of lubricant. Products for MQL also contain a whole range of specific additives ^[2].

If efficiently employed, this form of machining yields dry work pieces and dry chips, which in turn yields a number of benefits over wet machining as outlined under the preceding chapters in “Non-water-miscible oils", “Water-miscible oils" and “Fully and semi-synthetic fluids".

Exposure and health aspects

Not all MWF ingredients necessary for production and machining reasons are compatible with human health. Skin disorders and diseases are the most common form of health problems, followed by respiratory disorders and diseases. Usually in practice it is not possible to completely prevent skin contact with MWFs or work pieces coated with MWFs as, for example, using protective gloves is prohibited if there is a danger to be gripped by the tool ^[7].

Relatively rarely, but all the worse for the affected person, carcinogenic substances can be formed under unfavourable circumstances (e.g. nitrosamines (Carc 1B) from secondary amines and nitrite, or polycyclic aromatic hydrocarbons (e.g. Benzo(a)pyrene, Carc 1B ^[8]) when aromatic hydrocarbons are present and treated at high temperatures). These carcinogenic substances can cause serious health effects both by absorption through the skin and inhalation.

Exposure to carcinogenic metals in some alloys, e.g. beryllium, cobalt, nickel or chromium, can also cause serious health effects if inhaled ^[9].

Source: Adopted from ^[9][10]

* Cases in which it has been proved in an adjudication procedure that the person is indeed suffering from the occupational disease. NB For some diseases, the confirmation of the occupational causation must coincide with additional insurance conditions, e.g. some diseases must have forced the person to refrain from all activities which led or could lead to the development, aggravation or recurrence of the illness. If such conditions are not fulfilled, a formal OD recognition is not possible.

Skin contact

To stay healthy, the human skin requires a hydrolipidic film on its surface. This is damaged and in serious cases destroyed when the skin comes into contact with siccative and degreasing substances. Neither can be excluded when working with MWFs ^[11].

Owing to their composition, water-mixed MWFs are more hazardous to the skin than non-water-miscible MWFs (preceding chapters “Water-miscible oils" and “Fully and semi-synthetic fluids"). Water-mixed MWFs are capable of dissolving dermal lipids (especially due to the emulsifiers they contain) as well as depleting the skin’s moisture due to prolonged contact with water. The process is accelerated by the fact that the surface of the skin has a pH of about 5.5 and the MWF a pH of up to about 9.5. This can give rise to a neutralisation reaction that is capable of destroying the hydrolipidic film. This process takes place relatively slowly and it may be years before skin problems arise. Dermatologists refer here to a chronic irritant eczema or allergic eczema. More information on the differences between irritant and allergic eczema can be found in the articles on irritants and occupational allergens.

The skin-damaging process advances much more rapidly if the person comes into contact with products with a stronger effect, e.g. more powerful irritant concentrates of MWFs, biocides or machine cleaners. Owing to their higher concentration of active ingredients, these undiluted products can cause skin disorders faster. These may arise even after just a few days or, at the latest, weeks. Dermatologists refer here to primarily toxically irritant eczema. In the event of even stronger effects on the skin, burns may occur.

Allergic eczema may often occur when the skin has already suffered prior damage. Whether or not an individual worker acquires an allergy is hard to predict. However, specific circumstances that enhance this process are known, such as the frequency and intensity of exposure, and the strength or potency of the allergenic substance. Many biocide ingredients, e.g. isothiazolinones and liberated formaldehyde, or ions of cobalt, nickel or chromium leaching from the work pieces into the MWF, are MWF ingredients known to have a relatively strong allergenic potential.

Given sufficient duration and severity, skin diseases can force the affected persons to give up their hazardous work. Irritant eczema only heals slowly, and allergic eczema recurs if the allergen cannot be permanently avoided.

Inhalation

As described, MWFs consist of a multitude of components that also constantly change due to substance loss (e.g. via workpiece and chips) and due to chemical and biological reactions. Their main effect on the respiratory passages is by inhalation and subsequent deposition of MWF components from the vapour phase, e.g. amines, readily volatile components, or from the aerosol phase, e.g. solid material components, oils, emulsifiers and biomass. A large part of the inhaled vapour is exhaled again and problems are only to be expected if substances are irritating the respiratory organs, e.g. certain amines or surface-active agents. With regard to the aerosol phase, its components (fine dust, oil droplets) can become deposited on the surfaces of the respiratory passages and are physically expelled from the body by coughing.

If the rate of deposition of inhaled substances exceeds that of expulsion, the loading of the lung surfaces and/ or the lower airways increases and inevitably causes respiratory diseases.

Even cancer of the respiratory organs can occur caused by carcinogens as mentioned above in chapter 2 and table 1.

The inhalation of aerosols of MWFs contaminated with micro-organisms can, depending on the spectrum of the micro-organisms present, cause serious allergic respiratory disease known as extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP). In such cases, the affected persons cannot continue to work in the areas of risk ^[12].

Swallowing

The risk of swallowing MWFs is usually negligible. It may occur in an accident if, for instance, the MWF concentrate is mixed under pressure and supply lines are leaky or burst, or maintenance staff performs work on machine components carrying MWFs, e.g. pumps or pipelines.

Aspiration

The aspiration of MWFs constitutes a special case. Following the swallowing of MWFs, vomiting may arise, depending on the type of MWF, and the result mixture of MWF and stomach acid, possibly as a foam, may enter the lung. The greatest risk is posed by non-water-miscible MWFs of low viscosity. Above a certain aspirated quantity, this can on principle cause death, but no cases are known in practice.

Aspiration must always be distinguished from inhalation, even though the lung is the affected organ in both cases.

Prevention

The very different hazards described under the preceding “Exposure and health aspect" chapter naturally call for different protective measures. For example, enclosure can prevent the splashing of metalworking fluids (MWFs), although, in the absence of a suitable ventilation system with an extractor and downstream separator, MWF vapour and aerosol are liberated when the machine enclosure is opened. Enclosure cannot prevent risks to the skin, as work pieces covered with MWFs are usually manually removed from the machine and re-measured or stacked in the handling devices available.

Since oil mist separators are currently incapable of sufficiently intercepting the vapour phase, the vapour concentration will gradually increase, even in workshops with separators.

When planning and implementing protective measures, if the risk cannot be eliminated, reference is often made to the STOP approach, a prioritised series of measures: Substitution, Technical measures, Organisational measures, Personal protection measures. The legal base behind this principle is the legal obligation to follow the hierarchy of control measures as per directive 98/24/EC (chemical agents directive) ^[13].

For an overview of the STOP principle and MWFs see e.g. links under ^[14][15].

General exposure limits

When assessing the risk at MWF workplaces, attention should be directed to substances with occupational exposure limit values (OELs). These OELs are listed in specific, nationally established legislation and should not be exceeded. Monitoring is therefore needed.

At EU level, there are actually discussions about limiting the “oil-mist-fraction", reasonably because compositions of MWF are that different. Especially the Scientific Committee on Occupational Exposure Limits (SCOEL) recommendation SUM 163 “Aerosols of Severely Refined Mineral Oils" ^[16] can be used as a basic document for hazard assessment.

It should be noted that, independently from the fact that OELs may be available or not for a MWF used or its components, worker’s exposure has to monitored, assessed and reduced to a minimum, even below OEL level when an OEL is established.

In Germany, “technical", non-binding values established in so-called Technical Rules for Hazardous Substances (TRGS) also relate for example to MWF concentration, pH, nitrite concentration. These values are also important with regard to the safe use of MWF as not respecting these may result in harmful substances building up in the system. Thus, for example with decreasing pH, bacterial growth increased in water-mixed MWFs ^[17][18].

Elimination and substitution

EU occupational safety and health directive 89/391/EEC ^[19] and directive 98/24/EC ^[13] accord top priority to elimination of hazardous substances and | substitution of a dangerous substance]] with a less dangerous one, or with a less hazardous working process. In the event of substances being equally applicable technically, the principle holds that a less hazardous substance should take preference over a toxic one and a non-combustible substance over a combustible one. The decision must be taken on a case-to-case basis and in relation to the accompanying conditions, e.g. open or closed systems, and the result of the hazard assessment. “Socioeconomic" factors also have to be considered.

Examples of substitution in the case of MWFs are substituting secondary amines by primary or tertiary amines; mineral oil by ester oil; avoiding toxic bactericides.

Technical solutions

Modern metalworking machines are usually enclosed and can be retrofitted with a suitable waste air system (an extractor and a suitable separator). If the air leaving the separator is returned to the workshop and the machining department has a large number of machine tools, an additional system is usually required for a sufficient supply of fresh air. Modern displacement ventilators supply fresh air and blow it upwards from floor level, thereby creating sufficient zones of fresh air ^[20][21].

Using fully automatic systems, e.g. transfer lines, saves having to regularly open metalworking machines, which also means reduced emissions into the workshop. Skin contact and exposure are thus minimised.

Very heavy and/or hot work pieces have to be handled with lifting gear and “safe tools". Chips are also removed from the machining zone with a suitable tool, e.g. a chip hook or hand brush minimizes skin contact with chips with sharp edges and which are covered with MWF.

Modern computed numeric control (CNC) machines have safety controls to minimise the risk of accidents and also minimizes spraying of MWF into the workplace. In some cases machines cannot be closed, e.g. because workpieces are too large (partly more than 20 meters). In these cases additional ventilation is necessary.

Organisational solutions

After elimination, substitution and collective technical measures at source, organisational measures are the next measures to be taken according to the hierarchy of control measures, such as reducing to a minimum the number of workers exposed or likely to be exposed, reducing to a minimum the duration and intensity of exposure and appropriate hygiene measures.

Safety Data Sheets, training and information on appropriate precautions and actions to be taken in order to safeguard themselves and other workers at the workplace when working with the MWFs used, and information on emergency arrangements should be made available to workers. Information on the results of the risk assessment and the selected protective measures also has to be provided to workers. Consultation and participation of workers and/or their representatives should also be ensured.

Furthermore, first aid and a health surveillance programme adapted to work with the MWFs used in the workplace should be in place.

Personal protective equipment

In the case that the measures described before (under “Elimination and substitution", “Technical solutions"; and “Organisational solutions" chapters) are not sufficient to prevent exposure, adequate personal protective equipment (PPE) has to be made available by the employers and has to be used by the workers

At all machines without sufficient technical control measures emitting MWFs and/or chips, suitable protective goggles and work clothing must be worn. If work clothing is likely to become soaked (e.g. set-up mechanics working continuously bent-over at machines), waterproof aprons are necessary.

The wearing of gloves is only permitted if there is no risk of entanglement or being caught in the machine. MWF manufacturers must specify in their safety data sheets what kind of gloves are suitable gloves to work with the MWF in question. In all cases, a skin protection plan is to be drawn up and skin protection agent, suitable hand cleaner and skin care agents must be made available. Details of their effectiveness can be obtained from suppliers.

Depending on the rules applicable in the workshop, shoes and hearing protectors must usually be worn. The wearing of respiratory protection must not be a long-term measure and is only permitted temporarily, e.g. when a hazardous zone has to be entered for maintenance or servicing.