Isocyanates

Introduction

Isocyanates are used in a wide range of industrial products, including paints, glues and resins. They are potent respiratory and skin sensitisers and a common cause of asthma and allergic contact dermatitis, see Occupational allergens article. A range of other adverse health effects are also associated with isocyanate exposure including cancer. Where isocyanates are used or unintentionally generated, for example when polyurethanes are heated, it is important that workers’ exposures are properly controlled. There are various ways of achieving this, and the way that the isocyanate is used or generated often dictates which control strategy is needed. All exposure controls require maintenance if they are to remain effective, and the article provides information on how to achieve this for isocyanates.

Background

Isocyanates are a family of organic chemicals which have one or more N=C=O functional groups bonded to the molecule^[1]. The most common isocyanates found in industrial settings are based on molecules with 2 such functional groups, and are generally referred to as diisocyanates, these include:

Toluene diisocyanate (TDI)

Methylene bis(phenylisocyanate) (MDI) or methylene diphenyl diisocyanate

Napthalene diisocyanate (NDI)

Hexamethylene diisocyanate (HDI)

Isophorone diisocyanate (IPDI)

TDI, MDI and IPDI exist as a mix of isomers In their most simple form, these substances exist as monomers. However, many industrial isocyanate preparations have molecular structures based on 2 or more monomer molecules chemically bonded together. These are generally referred to as pre-polymers or oligomers. These substances still contain the N=C=O functional group and hence they still carry the health risks associated with isocyanates. Pre-polymers are less volatile than their associated monomer so they are less likely to become airborne as a vapour. However, very high inhalation exposures can still occur when these materials are sprayed and the health risks associated with skin exposure are still present.

More complex forms of isocyanates are also marketed, containing other functional groups which may reduce the isocyanate exposure potential. These are often referred to as blocked or stoved isocyanates. For the N=C=O functional group to take part in the polymerisation reaction, and the paint, glue etc. to cure, the isocyanate must be free to react and so at some point in the process there is still isocyanate exposure potential associated with these materials.

Commercially available isocyanate preparations are either solids or viscous liquids.

Health hazards

A range of serious, adverse health effects are associated with isocyanate exposure. ^[2] These include effects on the respiratory system^[3][4], and the skin^[5]

The health hazards of MDI and TDI are summarised in Table 1. Other isocyanates will have similar health effects. This information can be found on the safety data sheet supplied with the chemical.

Exposure routes

Isocyanate exposure generally occurs through inhalation and/or dermal routes. Depending on the isocyanate type and the application method, there may be significant exposure potential from either, or both, of these routes and this should be considered in the risk management approach.

Inhalation exposure can occur when isocyanates are present in the workplace air, either as a vapour or an aerosol. In some instances, airborne isocyanates can be present in both of these forms simultaneously.

Vapours can be generated from passive processes by evaporation, and the volatility (aka vapour pressure) of the isocyanate will influence the degree of airborne vapour which it generates. Evaporation will increase as the process temperature increases, and so the heating of isocyanates will increase airborne vapour levels. Liquid isocyanates are often very viscous at ambient temperature and are usually heated to help them flow better, and hence make them easier to handle. It should be borne in mind that this will increase the rate of isocyanate vapour generation. It should also be borne in mind that the isocyanate-polyol reaction which takes place to form a polyurethane is highly exothermic, generating a lot of heat. Again, this will increase vapour generation, even if no external heat is added to the process.

Aerosols can be generated by deliberate means, such as spraying, or inadvertently when isocyanates are mechanically agitated or vigorously disturbed. For example, fine aerosol particles will be generated when liquids are brush applied or poured from one vessel to another. However, the amount of aerosol generated in this way will usually be much lower than from spraying processes. Where solid isocyanates are handled, there is potential for airborne dust to be generated.

Dermal (skin) exposure can occur wherever there is potential for workers’ skin to come into contact with isocyanates. The main mechanisms by which dermal exposure to isocyanates occur are:

• Direct contact with workers’ skin
• Deposition of aerosol from the air onto workers’ skin
• Splashing, during pouring or mixing activities for example.
• Handling contaminated items such as tools or used personal protective equipment (PPE)
• Contact with contaminated surfaces, such as control panels or process plant, during maintenance for example

Common applications

Some common industrial uses of isocyanates are listed below:

• Paint hardener. Many industrial paints use isocyanates as a hardener. These are frequently ‘2-pack’ products, where 2 components are mixed together immediately prior to use. In these cases the isocyanate is present in the hardener component of the paint. Some ‘1-pack’ paints do contain isocyanates, and these do not require mixing, thus removing one task with exposure potential. The safety data sheet supplied with the paint will provide information as to whether isocyanates are present. These paints are commonly used in motor vehicle repair (MVR), and in the painting of large commercial vehicles and structural steelwork^[7]. They can be spray, brush or roller applied. The highest exposure potential is associated with the spray application. Inhalation exposures associated with brush or roller application would be much lower, although the potential for dermal exposure would still exist^[8]. There is a high prevalence of occupational asthma in workers in the MVR sector using these paints. The paints are generally based on pre-polymeric forms of HDI, with the isocyanate being present in the hardener component of the mixture. The sanding and polishing of fully cured isocyanate based paints does not liberate airborne isocyanate^[9]. However, when exposed to higher temperatures, such as from grinding and welding, cured paints have been shown to liberate airborne isocyanate^[10].

• Production of polyurethane rubbers and thermoplastic elastomers. These are generally based on an aromatic isocyanate, most commonly MDI or TDI, reacted with a polyfunctional alcohol (polyol) or other organic material. The isocyanates are often manually mixed and poured. There are generally no processes involving spray application of isocyanates in this industry sector. Provision of exposure controls is variable in this industry^[11].

• Production of soft polyurethane foam. This is manufactured from TDI and a polyol, with other additives used to modify the properties of the finished product. The isocyanates are usually mixed with an automated system, with initial curing inside an extracted enclosure. Airborne isocyanate concentrations within the enclosure can be high and respiratory protective equipment RPE needs to be worn if the enclosure has to be entered for maintenance purposes. There is further exposure potential when the partially cured foam is removed from the enclosure, and cut into smaller blocks, where the uncured interior can liberate airborne isocyanate.

• Thermal insulation of buildings, domestic appliances and refrigerated transport. This involves spray application of a polyurethane foam, with the isocyanate component usually being based on MDI^[12]. This work is often site based, and can be conducted in environments with restricted ventilation. There is a high potential for exposure, and often exposure control strategies rely almost entirely on PPE.

• Industrial flooring. MDI is a component in the production of high quality, low porosity industrial resin flooring. This is commonly used in food factories and other environments where easily cleaned, hygienic flooring is required. The resin is usually mixed in an open system and the flooring is manually laid using hand tools. Large areas, up to several hundred square metres, can be laid in a single session. There is no potential for aerosol generation, and extremely low vapour pressure of the pre-polymeric MDI results in very little airborne isocyanate and hence little risk of inhalation exposure. However, there is significant potential for dermal exposure.

• Foundry binders. Urethane binder systems, containing MDI, are commonly used to form moulds and cores from sand in foundries. There is potential for exposure when the moulds and cores are being made, and also to thermal degradation products when the hot metal is poured into the moulds^[13][14].

This is not an exhaustive list, and there will be other industrial applications. The presence of an isocyanate in a raw material should be indicated on the material safety data sheet. Processes which involve heating of polyurethanes have the potential to generate isocyanate. As with any industrial process, a thorough risk assessment and implementation of an appropriate exposure control strategy should be conducted before work begins with dangerous substances.

Risk management

Given the toxicity of isocyanates, it is important to control worker exposures to these chemicals wherever they are used or generated. A thorough risk assessment is part of the process of achieving adequate control. This will allow an appropriate exposure control strategy to be defined and implemented. Risk assessment for dangerous substances is a legal requirement^[15]. The hierarchy of control should be observed when designing exposure control strategies, see also Substitution of hazardous chemicals article.

Occupational exposure limits (OELs) for isocyanates exist in various EU member states but these do not necessarily represent safe levels of exposure. In the case of isocyanates, exposures should be controlled to be reduced to minimum. Some individuals are more susceptible to sensitisation effects than others, and even exposures substantially below OELs can lead to serious health effects.

In terms of respiratory effects, processes which generate high airborne levels of isocyanates, such as spray application, carry the greatest risk. It is important to remember that all airborne isocyanates, whether they are monomeric or polymeric, in either aerosol or vapour phase, are harmful. Even where airborne levels are likely to be very low, such as brush or roller application of low volatility polymeric isocyanates, the potential for skin effects still exists and must be taken into consideration when developing an exposure control strategy.

Exposure controls

Elimination/substitution

According to the principles of good occupational hygiene practice, and the hierarchy of control, elimination of a hazard, or substitution with a less hazardous material or a less hazardous application technique is a preferable control option to solutions based on engineering controls and PPE. Control solutions based on substitution include:

• The replacement isocyanate based paints with other, less hazardous products which still achieve acceptable quality and durability of finish.

• The use of pre-polymeric isocyanates rather than monomers. In this instance, although the isocyanate is still present, it is in a less volatile form and so the potential for vapour generation is reduced.

• The adoption of different application techniques, which lower process emissions. The use of brush or roller application for paints, instead of spraying, significantly reduces the potential for inhalation exposure.

Engineering control

Where substitution is not possible, engineering control solutions based on separating the worker from the exposure source are seen as the next best option. Engineering controls can take various forms, with the following being most relevant for controlling isocyanate exposures:

• Containment. This would include the use of sealed handling systems for transferring bulk material from storage tanks to the point of use, or the use of lids on containers when not in use, to prevent vapour emission into the workroom.

• Process modification. High volume low pressure (HVLP) spray guns are available for spraying isocyanate paints. These reduce the amount of paint used, and minimise aerosol generation.

• Local Exhaust Ventilation (LEV). This would include the use of fume cupboards and ventilated cabinets for the storage and handling of small to medium quantities of isocyanates and the use of ventilated spray booths for the application of 2 pack paints in MVR.

• Segregation. In some situations it may not be possible to apply LEV effectively to control exposure. In such instances, segregating the workplace to contain the isocyanate in designated, clearly signed areas will reduce the spread of contamination and protect workers who are not directly involved in the process.

• Safe working distance. The use of tools to increase the distance between the worker and the exposure source can significantly reduce dermal and inhalation exposure. Examples would include the use of long handled rollers for the smoothing of isocyanate flooring and the use of a spatula rather than a gloved hand to remove viscous isocyanates from tins.

Personal protective equipment

PPE is generally seen as a less reliable exposure control than those discussed above and should be used as the last resort only. However, PPE still has a part to play and there may be processes with a high potential for exposure, even after the implementation of engineering controls, where PPE is the only means of achieving adequate control. The following issues are of specific relevance to isocyanates.

• Chemical protective gloves should be used as splash protection only, processes should not be designed such that gloves are used as a primary barrier against direct contact with isocyanates or isocyanate contaminated work equipment. Gloves should be selected which offer the appropriate level of chemical protection whilst also taking into consideration other factors such as the need for thermal protection or manual dexterity.

• Work overalls and oversuits should provide whole body coverage and not leave susceptible body parts, such as forearms, open to exposure. Disposable coveralls may offer a better solution than re-usable garments which can become heavily contaminated over time, and potentially act as an additional exposure source.

• Respiratory protective equipment (RPE) must be selected taking into account the ‘control challenge’ (i.e. airborne concentrations of isocyanate outside the RPE) and usage factors such as the length of time for which it will be worn and the need for other PPE, such as eye protection. Airborne isocyanates can be present in the atmosphere at harmful levels and not be detectable by smell, hence it would not be immediately obvious to the wearer if a filtering respirator were to fail. For this reason, the use of air supplied RPE is generally the preferred option for processes with high potential for inhalation exposure. This would apply to all manual spraying processes, such as paint spraying or the application of polyurethane foam insulation. Filtering respirators may be acceptable for processes with lower airborne emissions. Exposure monitoring can play a key role in RPE selection. If RPE is selected which requires a good seal to the workers face for effective operation, it is important that the RPE fits the worker correctly. Face fit testing is required to ensure this.

In all cases PPE must be selected, used, stored and maintained correctly in order to obtain maximum protection.

The practicalities of achieving adequate control

It is almost always the case that a practical, effective exposure control strategy will use a combination of exposure controls. In designing a control strategy, all exposure routes should be considered and the hierarchy of control applied for each exposure route. Processes should be designed to limit the potential for workers to come into contact with isocyanates. PPE for controlling dermal exposure should be provided for splash protection and not as a primary barrier against direct contact with isocyanates and heavily contaminated work equipment.

LEV will often be a necessary part of achieving control, and preventing the spread of airborne contamination into areas occupied by other workers not directly involved with the isocyanate process. However this control approach can fail due to poor design, incorrect use or inadequate maintenance. The design and implementation of an effective LEV system requires the specialist expertise of ventilation engineers and occupational hygienists. It is vital to establish that the system provides adequate control when it is commissioned.

For some processes involving spray application of isocyanates, LEV systems alone cannot provide adequate control of inhalation exposure, even where they are well designed and properly used. RPE will also be required in these circumstances^[16]. In MVR, the role of the ventilated booth is to reduce the airborne isocyanate levels as far as possible during spraying, to remove airborne isocyanate from the spray space as quickly as possible after spraying, and to contain the airborne contamination within the spray space to prevent other workers being exposed. It is essential to consider that all spray booths take time to clear airborne isocyanates after spraying is complete. Even when the visible spray has cleared, which usually happens quite quickly, dangerously high levels of airborne isoscyanate can remain for several minutes. It is common practice amongst spray painters to lift the visor of full face RPE immediately after spraying to inspect the paint finish. This results in peaks of very high inhalation exposure and adds significantly to the risk of developing asthma. The manual cleaning of spray guns can also give rise to high isocyanate exposures, in addition to the cleaning solvents. Spray guns should not be cleaned in the open workshop or paint mixing room.

Wherever possible exposure controls should be designed and built into the process. It is always more difficult to achieve adequate control when measures are retro fitted to existing plant and machinery.

All exposure controls require maintenance if they are to offer sustained exposure control. LEV systems should be tested frequently, and filters changed at recommended intervals. PPE requires appropriate checking and maintenance, where air fed RPE is used it is important to ensure that the breathing air is clean and supplied at an adequate flow rate and pressure. This also applies to ‘software’ controls, where regular refresher training of workers is appropriate.

Exposure monitoring

Exposure monitoring can play a key part in the risk management approach to handling isocyanates^[1]. This can be broadly segregated into two areas, air sampling and biological monitoring.

Air sampling

From an occupational hygiene viewpoint, the most common and useful form of air sampling is personal monitoring. This allows the best estimation of worker exposure, and can be an essential element in ascertaining the adequacy of control and informing RPE selection. The measurement of airborne isocyanates is complex and requires specialist expertise^[18][19]. Some measurement methods only quantify certain isocyanate species, most commonly monomers. Industrial isocyanate preparations are frequently a mixture of pre-polymers, all of which are harmful to health. Other techniques are only applicable to vapour phase or particulate phase airborne isocyanate. To be of value to the risk assessment process, the measurement method must identify and quantify all isocyanates in monomeric and polymeric forms, whether in the vapour phase or present as airborne particulate. In particular, methods which only quantify monomeric isocyanates can grossly underestimate exposure, and give an impression that risk is low when harmful levels of airborne isocyanate are present. Where possible, measurement methodology which is accredited by a reputable organisation should be employed. A number of isocyanate measurement methods have ISO accreditation^{[20][21][22][23]}.

Where large volumes of isocyanates are handled under containment, continuous fixed point gas monitors and alarms are appropriate. These are generally only applicable to vapour phase monomeric isocyanate. The consequences of a large scale leakage of isocyanate to atmosphere are potentially very serious. One of the most catastrophic industrial accidents in history occurred in Bhopal, India. In 1984, the loss of containment on plant containing methyl isocyanate resulted in the deaths of several thousand people living in the local area.

Biological Monitoring

Biological monitoring offers a useful approach to exposure assessment^[24] and can provide a reliable indication of recent occupational exposure. Biological monitoring can be cheaper and easier to administer than air sampling, and can provide information on total exposure by all routes, and on the efficacy of PPE in controlling exposure. Certain amines which are used with isocyanates in some industrial processes can interfere with the biological monitoring method.

Health surveillance

Health surveillance plays a key part in the risk management approach for isocyanates^[25]. Regular, targeted surveillance by a competent individual can identify the early stages of skin and respiratory disease, and hence allow interventions on an individual and company wide basis to be made.

Summary

Isocyanates are important and useful industrial chemicals, with wide ranging applications. However, they have the potential to cause a range of serious health effects, and a rigorous and robust exposure control strategy must be employed wherever isocyanates are used. The specialist skills of a professional occupational hygienist may be needed to ensure that all risks are adequately controlled.