Metalworking fluids

Introduction

The term 'Metalworking Fluids' (MWFs) refers to a variety of oils and liquids with various additives to enhance performance that are used to lubricate and cool metals during machining and shaping processes^[1][2].and they are used in a wide range of industries: automotive, aerospace, aluminium and steel manufacturing etc, MWFs are available as non-water-miscible oils, water-miscible/emulsifiable concentrates or MWFs contain various chemical substances, some of which are well-known irritants or allergens. 

The most frequently reported health problems associated to exposure to MWFs are skin diseases and respiratory disorders such as work-related asthma, bronchitis, irritation of the respiratory tract. 

Types of metal working fluids and composition

Metalworking fluids (MWFs) have a whole range of functions that require very different ingredients. Some of the functions are the same for all types of MWF: cooling, lubricating, removing swarf and corrosion protection. The type of MWF chosen is determined by which of these functions is most important^[3][4].

The MWF is usually chosen initially 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 workpiece 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^[5] [6].

Non-water-miscible oils

Non-water-miscible MWFs, also known as neat cutting oils or straight oils contain no water and are use undiluted. They 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^{[4] [7]}.

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 aerosolised and becomes 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. 

Water-miscible oils

The concentrate itself may contain varying proportions of base oils—such as mineral oil, ester oil, chemically modified rapeseed oil, or synthetic oil—typically ranging from 20% to 80%, depending on the formulation.

Water-miscible MWFs are concentrates that are diluted with water before use, typically at concentrations of 2 to 25%, depending on the product and the type of machining. This type of MWF may contain varying proportions of base oils similar to the oils used for non-water-miscible  ranging from 20 to 80%, depending on the formulation. To combine this oil with water to yield an oil-in-water emulsion, an emulsifier is necessary^{[4] [7]}.

The emulsion contains a number of components that encourage bacterial and fungal growth, e.g. phosphorus and sulphur additives. Micro-organisms can also be introduced through the water, floor, air, humans and the work piece itself. To prevent microbial contamination and degradation of the fluid, biocides are added.

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 amines can degrade into ammonia, and various sulphur compounds can degrade into hydrogen sulphfide. 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 also water-miscible with no or a limited amount of oils and therefore do not need emulsifiers. Semi-synthetic fluids contain 5–30% oils, while fully synthetic fluids contain no oils^[2]. They can be based on water-miscible glycol compounds, for example. Mixing in water yields a transparent water-mixed MWF. The characteristics of these MWFs are similar to those of water-miscible oils^{[4] [7]}.

Minimum quantity lubrication (MQL)

The use of minimum quantity lubrication (MQL) has evolved as a more sustainable alternative to wet machining. The products recommended to be used for MQL are generally synthetic ester oils and fatty alcohols with low evaporation properties, toxicological suitability and a high flash point^[9]. 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 ‘wet machining’ where hundreds of cubic meters of fluid flood the workpiece, and they are delivered precisely to the machining zone by compressed air. Reduced consumption of cutting fluids results in fewer occupational health hazards, without compromising the quality of the work^[8]. MQL significantly reduces the amount of MWFs in workers' breathing air, on their skin and in the work environment^[9].

Exposure routes and Health risks

MWF are applied at the interface of the tool or machine and the workpiece using pressure either by spraying or through a jet although manual application when working with small metal pieces is also frequent using brushes or dipping the metal piece. 
The main exposure routes are dermal by contact with the MWF but also the oxidised components and contaminants, such as tramp oil[i], metals, biocides and microorganisms^[10], and inhalation of aerosol droplets and mist generated during the operations. 
Exposure can also occur through cuts and abrasions and by ingestion through contact with contaminated hands, objects or food. 

The main health risks associated with these exposures are skin and respiratory diseases The type and severity of the health problem depend on^{[2] [11]}:

• the type of MWF used (whether it is water-based or not and what its components are)
• the duration and frequency of exposure
• the techniques used to apply the MWF (e.g. spray, liquid)
• the level of exposure
• the exposure route (skin contact, inhalation or ingestion).

Exposure to MWFs has also been linked to an increased risk of various cancers, including those of the skin, larynx, pancreas and bladder. Due to the long latency period, studies generally focus on exposure in the 1970s and 1980s. The composition of MWFs has changed significantly since then^[2]. For example, older formulations often contained unrefined or mildly refined mineral oils with polycyclic aromatic hydrocarbons (PAHs), which are known carcinogens. Nowadays, base oils are either severely refined mineral oils with a very low PAH content, or synthetic and ester-based oils (including bio-oils), which are essentially PAH-free. However, caution is still necessary, as it is difficult to assess the long-term effects of exposure to a variety of substances.

Skin diseases

Skin exposure may occur during application and when handling parts or equipment covered in MWF, or through liquid spills or splashes if the machine is not properly guarded^[2]. Contact with MWFs can result in several different skin diseases. In general, reports link non-water-miscible MWFs to oil acne and folliculitis (inflammation of hair roots) and water-miscible MWFs to irritant contact dermatitis and, less frequently, to allergic contact dermatitis^[12]. The most commonly reported skin disease associated with MWFs is contact dermatitis (either irritant or allergic)^[10] [12]. Due to their composition, water-mixed MWFs are more hazardous to the skin than non-water-miscible MWFs. 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.

The early symptoms of contact dermatitis are itching and red rashes on the skin. Other possible symptoms include cracks, swelling, pain and, in some cases, blisters^[13]. With irritant contact dermatitis, the rash is confined to the area that came into contact with the irritating substance. In allergic contact dermatitis, however, the rash can spread beyond the area that was in direct contact with the substance. Irritant dermatitis heals slowly, and allergic dermatitis recurs if the allergen cannot be permanently avoided.

Respiratory diseases 

Exposure to mist, aerosol, and vapour from MWFs can lead to the development of respiratory diseases such as work-related asthma, bronchitis, irritation of the respiratory tract and breathing difficulties. Occupational hypersensitivity pneumonitis and occupational asthma are the most commonly reported types of lung diseases in operators^[13]. Occupational hypersensitivity pneumonitis is an allergic lung disease caused by repeated inhalation of fungi, bacteria, animal proteins, and some chemicals^[10] [13]. In the UK, exposure to water-miscible MWF is a common cause of reported cases of occupational hypersensitivity pneumonitis^[14]. Occupational asthma is caused by exposure to allergens and irritants from breathing in the mist or vapour released during metalworking processes. MWF components, such as emulsifiers, biocides and additives, can act as sensitisers, leading to allergic airway responses. Water-based fluids can support the growth of bacteria and fungi, which produce allergens and toxins that contribute to occupational asthma.

Prevention

The various hazards call for a range of prevention and protective measures. For example, enclosure can prevent the splashing of MWFs but, in the absence of a suitable ventilation system with an extractor and downstream separator, MWF vapour and aerosol are released 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.

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 principle is based on the legal obligation to follow the hierarchy of control as set out in the Chemical agents Directive 98/24/EC[^15].

Exposure limits

In the European Union there is not an occupational exposure limit (OEL) for MWF. However, the individual components of the MWF may have an OEL. Therefore, it is essential to check the information provided in the Safety Data Sheets and follow the safety recommendations. 

In Germany, a consortium of various stakeholders has compiled a list of substances used in MWFs^[16] that provides further information on each substance, such as air limit values, concentration limits and the legal basis for these limits. The list is updated annually^[16] . 

It should be noted that, regardless if OELs are available for specific substances, worker’s exposure has to be monitored, assessed and reduced to a minimum, even below the OEL level when an OEL is established.

Elimination and substitution

The OSH Framework Directive 89/391/EEC^[17] and the Chemical agents Directive 98/24/EC^[15] prioritise the elimination of hazardous substances and the substitution of dangerous substances with less dangerous ones, or with less hazardous working processes. If substances are equally technically performant, the principle is that the less hazardous substance should be preferred to the toxic one and the non-combustible substance to the combustible one. The decision must be taken on a case-to-case basis taking into account the work environment and production process, 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 and avoiding toxic bactericides.

The working methods should be designed to prevent contact with the MWF, splashes and formation of mist. For example, the use of compressed air to remove excess MWF should be avoided. The rate at which the MWF is added to the machine should be controlled to prevent the formation of mist. 

Keeping the quality of the MWF is also important to prevent health risks specially to prevent microbial growth. Therefore, checks should be carried out and they should be prepared, maintain and stored following the instructions of the manufacturer. 

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 re-circulated into the workshop an additional system is required for a sufficient supply of fresh clean air. Air ventilation systems supply fresh air and blow it upwards from floor level, thereby creating sufficient zones of fresh air^[18][19].

Using fully automatic systems, including transfer lines, eliminates the need to regularly open metalworking machines, and reduces emissions into the workshop. Skin contact and exposure are thus minimised. Modern computer numerical control (CNC) machines are equipped with safety systems designed to minimise the risk of accidents and prevent metalworking fluid (MWF) mist from being released into the workplace. However, MWF mist is generated inside the machine enclosure during machining and may escape through small gaps or if the doors are opened too soon after operation^[20]. To minimise mist formation, the delivery parameters of the MWF, such as the flow rate, volume, operating temperature and cutting speed, must be set correctly^[13] . Installing local exhaust ventilation (LEV) on the enclosure provides additional protection by capturing any residual mist^[20].

Very heavy and/or hot workpieces have to be handled with lifting gear and appropriate tools. Chips should also be removed from the machining zone with a suitable tool, e.g. a chip hook or hand brush minimising skin contact with sharp-edged chips that are covered with MWFs.

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, 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 personal hygiene measures.

Safety Data Sheets of hazardous substances as well as training and information on risks of MWFs, prevention measures and emergency procedures should 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.

Measures should also be in place to ensure a swift and efficient response in the event of a spill. Clear operating procedures should define responsibilities, and the steps to contain and clean up spills. Readily accessible spill kits equipped with appropriate absorbent materials and protective equipment, should be provided and workers should be trained in their correct use^[13] . Finally, used MWFs must be managed and disposed of safely by following the supplier’s guidance and safety data sheets. Proper handling, storage, and disposal of used MWFs help prevent risks to both human health and the environment^[13] .

Personal protective equipment

If technical and organisational measures are not sufficient to control the risks of exposure to MWFs, appropriate personal protective equipment (PPE) has to be provided.

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 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 for working with the MWF in question. Also, a skin protection plan must be drawn up and suitable skin protection agents, hand cleaners and skin care products must be made available. Details of their effectiveness can be obtained from suppliers.

Information and training 

It is important workers are trained on how to work safely with MWF, how to use the exposure controls and how to put on, remove and check the PPE. 

Workers should also be informed on the health risk of exposure to MWF and trained to perform checks on their skin and to be able to identify the symptoms that may indicate a skin or lung problem so they can report them to the doctor or authority in charge of the health surveillance programme. 

Health surveillance Early detection of health problems is crucial to prevent serious health problems and also for regulatory compliance. Employers should put in place a health surveillance programme that looks at dermal and respiratory symptoms. Workers and workers representatives should be consulted about the health surveillance programme,