Isocyanates are compounds containing one or more -N=C=O (Nitrogen, Carbon, Oxygen) groups which combine with other compounds containing alcohol groups to produce polyurethane polymers. Isocyanates are used in manufacturing materials such as polyurethane foams, rubbers, plastics, varnishes, adhesives and paints. The isocyanate pre-polymers are included in polyurethane paint formulations which, after curing, form durable films, these durable films are well known for their longevity in harsh environments. Due to their durability, polyurethane paints are used as corrosion protection coatings for machinery and facilities that are exposed to extreme conditions.
Polyurethane paints normally have a two-component product consisting of a curing agent and a base which is mixed just before application. The base is based on polyols and are not considered very hazardous, while the curing agent consists of various molecules containing isocyanate groups. The type of isocyanate compounds that are used is dependent upon whether the paint is used for a topcoat or for an intermediate coat. Paint used for topcoat applications are based on hexamethylendiisocyanate (HDI), and intermediate coat applications are based on the aromatic isocyanate compound diphenylmethane-4 (MDI).
It is important to be aware that when there is hot work being conducted on the polyurethane material that there is potential for thermal degradation to occur. Thermal degradation of polymers is a molecular deterioration as a result of overheating. Paint systems such as epoxy and polyurethane consist of chemical compounds containing nitrogen (N), carbon (C) and oxygen (O), from this we can assume that smoke from these materials will contain amines, aldehydes, alcohol, aromatic compounds with various functional groups and anhydrides. When inhaled these compounds can be very harmful to an individual’s health. During thermal degradation of the polyurethane paint, the urethane bond will depolymerise and form chemical compounds with reactive isocyanate groups (-NCO). For paints, glue and varnishes, this depolymerising starts at 150-200 degrees Celsius, while depolymerisation has been registered between 100-150 degrees Celsius for soft polyurethane foam. A general degradation mechanism for polyurethane can be described as:
Polyurethane -> isocyanate -> amine -> nitrile -> nitrous gasses.
150 0C ——————————————————————————-à 500-600 0C
Isocyanates can enter the body through inhalation and absorption. This can occur from vapours, mists and aerosol particles that become airborne or come in to contact with the skin. This has led to many industry’s restricting spray painting of isocyanate containing paints as this creates high risk of exposure to the workers. It also requires high level controls to mitigate the exposure to the worker in the area. Isocyanates are known as powerful irritants when in high concentrations, having the ability to cause health effects to the skin and mucous membranes in the body including the eyes, gastrointestinal and respiratory tracts. Isocyanates are also a sensitiser to workers, making them extremely receptible to severe asthma attacks. Occupational asthma can potentially be caused by just one exposure to isocyanates and will cause the exposed individual to suffer asthma attacks for their lifetime, even if they have never had it before. If this happens the individual will no longer be able to work with isocyanates again, as this will trigger ongoing asthma attacks.
Ways to mitigate Isocyanate exposure:
- Eliminate or Substitute
- Source an alternative isocyanate-free product (i.e. polysiloxane paint), and eliminate the use of isocyanates in your workplace
- Eliminate the use of spray painting for the application process (Only use brush or roller)
- Can a negative pressure unit be set up around the working area?
- Can ventilation be improved?
- Restrict access to non-essential personnel
- Restrict hot works on the product
- Administrative Controls
- Develop a Ventilation Plan
- Develop an Emergency Clean-up Plan
- Personal Protective Equipment
- Ensure high level PPE (airline breathing apparatus, Tychem coveralls etc.) are discussed in a risk assessment
- Conduct air sampling during the task to determine exposure to worker
- Conduct biological sampling pre and post task to determine a baseline and then the potential exposure (to be conducted immediately post shift given the short half-life)
- All PPE and contaminated materials to be removed prior to leaving the area and to be double bagged and disposed of hazardous waste
If you have isocyanate containing products in the workplace be sure to get in contact with a qualified Occupational Hygienist to discuss the controls necessary to ensure exposure to isocyanates is kept to ALARP.
Relevant documentation available to assist with managing Isocyanates include:
- SafeWork Australia: Guide to handling isocyanates
- NOHSC: Isocyanates
- SafeWork Australia: Isocyanates – Baseline health monitoring before starting work in an isocyanate process
- DMIRS: Isocyanates Health Surveillance Form
- DMIRS: Guidance Note: Controlling isocyanate hazards at work