Diisocyanates

Chemistry of Diisocyanates

Q: What are diisocyanates?

A: Diisocyanates (DII) are a family of chemical building blocks used to make polyurethane products, such as rigid and flexible foams, coatings, adhesives, sealants and elastomers. Many of the products that we rely upon every day are safer, tougher and more comfortable through the use of polyurethanes made possible through diisocyanates.

Polyurethane chemistry is complex, but the basics are relatively easy to understand. Polyurethanes are produced from the reaction of specially formulated diisocyanates with polyols and other chemical additives tailored to specific manufacturing techniques. Because a variety of diisocyanates and a wide range of polyols can be used to produce polyurethane (and the reaction speed can be adjusted), a broad spectrum of materials can be produced to meet specific application needs.

While diisocyanates are used in many applications, they are particularly important to the transportation and construction industries.

Q: What is toluene diisocyanate (TDI) and why is it important?

A: TDI is a chemical used in the production of polyurethanes, primarily for flexible foam applications, including furniture, bedding and carpet underlay, as well as packaging applications. TDI is also used in the manufacture of coatings, sealants, adhesives and elastomers. In transportation applications, TDI helps produce lighter automobile parts, saving weight, which leads to improvements in fuel efficiency and thus energy conservation. TDI, along with MDI, are considered the primary aromatic diisocyanates.

Q: What is methylenediphenyl diisocyanate (MDI) and why is it important?

A: MDI is a chemical used in the production of polyurethanes for many applications. MDI is used primarily in the production of rigid polyurethane foams used for insulation for your home or refrigerator, and many other uses. Insulation made with MDI can help consumers save on their heating and cooling costs and conserve energy. Some additional uses of MDI in polyurethanes include coatings, adhesives, sealants, and elastomers found in items such as paints, glues, and weather resistant materials. It is also used to make many types of footwear, sports and leisure, truck bedlining products and to a much lesser extent, some specialty flexible foams. MDI can also be used as a binder for wood and to produce mold cores for the foundry industry. MDI, along with TDI, are considered the primary aromatic diisocyanates.

Q: What is hexamethylene diisocyanate (HDI) and why is it important?

A: In addition to aromatic diisocyanates, there is a family of diisocyanates known as aliphatics. One of the aliphatic diisocyanates is HDI. It is primarily used as a chemical building block in the manufacture of higher molecular weight polyisocyanates. HDI polyisocyanates are used in paint and surface coatings such as automotive and truck refinishing, industrial, maintenance and performance coatings. Chemical resistant coatings made with HDI help military vehicles maintain the durability and resistance needed to withstand harsh combat environments.

Other aliphatic diisocyanates include methylene dicyclohexyl diisocyanate or hydrogenated MDI (HMDI) andisophorone diisocyanate (IPDI). Visit the ACC's Aliphatic Diisocyanate website to learn more.

Extensive Regulatory Review

Q: How are diisocyanates regulated?

A: The United States chemical industry is committed to complying with all federal, state and local regulations, and evaluates products before they reach the marketplace for health, safety and environmental compliance. Diisocyanates have been used since the late 1940s and their safety and environmental impact have been well studied. Diisocyanates are highly regulated chemicals under the authority of the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA) and other government agencies. Federal authorities have set exposure safety limits for diisocyanate emissions to protect both workers in production facilities and surrounding communities. These limits are based on science and reviewed by government officials with the goal of protecting workers and communities. Companies can face civil and criminal penalties for noncompliance.

Q: Are there health effects associated with exposure to diisocyanates in the workplace?

A: At the recommended workplace concentrations, generally no short or long-term health effects would be expected¹. However, at levels above recommended workplace exposure limits, diisocyanates can be strongly irritating to eyes, nose, throat, skin and lungs. If diisocyanate overexposure occurs, respiratory sensitization and asthma are the greatest health concerns. Less frequently, prolonged contact with skin can lead to skin allergy. Industry follows important workplace exposure limits that are established and enforced by OSHA.

Extensive safety precautions are undertaken by the diisocyanates and polyurethane industries to protect worker and consumer health and to comply with all government regulations.

¹Buyantseva L., Tarlo S. et al. Reduction in Diisocyanate and Non-Diisocyanate Sensitizer-Induced Occupational Asthma in Ontario 2011. JOEM. Paris C. et al. Work-related asthma in France: recent trends for the period 2001-2009. 2012. Occ Env Med.

Q: How will the polyurethanes and diisocyanate industries respond to the EPA’s Chemical Action Plans for MDI and TDI?

A: ACC strongly supports programs to provide guidance on safe use and handling of diisocyanates products. ACC's Center for the Polyurethanes Industry has a full library of product stewardship documents, videos and training modules to address questions about polyurethane raw materials related to environmental, health and safety, distribution, use, emissions, and waste issues. Most of these materials are available free on this website or at www.polyurethane.org, www.spraypolyurethane.org, and www.spraytruckbedliner.com. ACC also supports research programs to better understand key issues such as potential emissions from products as they are hardening or “curing,” which can help inform safety guidance. In cooperation with government agencies and other interested parties, the chemical industry will continue to work on product stewardship efforts for all diisocyanate products.

Q: Have the polyurethanes and diisocyanate industries been working on any of the issues the EPA mentions in the TDI and MDI Chemical Action Plans?

A: Yes. In fact, the MDI action plan references an industry-hosted website providing guidance on the use of spray polyurethane foam (accessible at http://www.spraypolyurethane.org). It also acknowledges that the “federal agencies are working with the polyurethanes industry to ensure accessible hazard communication, applicator training, and best workplace practices to prevent exposure to [di]isocyanates and other SPF chemicals.” Several important industry-sponsored research projects are also well under way to promote the further development of safe handling guidance. These are just some of the industry initiatives underway in response to the EPA TDI and MDI Chemical Action Plans.

Worker Safety

Q: How are workers protected from diisocyanate-induced occupational asthma?

A: The vast majority of diisocyanates-based products are produced and used in an industrial setting. Workers are protected through workplace controls, personal protective equipment, training, and provision of safety information, as well as other workplace practices that keep exposures well below levels of concern. Diisocyanates manufacturers and users often have medical surveillance and industrial hygiene programs to monitor for respiratory symptoms. Studies, including a study by the University of Toronto² that reviewed occupational asthma claims in Ontario during a five year period, have shown that there has been a reduction over the years in occupational asthma associated with diisocyanates. Also, according to an International Isocyanates Institute review of various national data collection programs on worker exposure and disease incidence, a fairly consistent picture is presented showing a reduction of diisocyanates related asthma cases over the last decade. Click here for an infographic on the "Decrease in Diisocyanate- Related Occupational Asthma Aided by Enhanced Industry Stewardship."

Consumer Safety

Q: How are consumers protected from potential exposures to diisocyanates?

A: The vast majority of diisocyanates manufactured are for industrial use. Overall, consumer exposures to unreacted diisocyanates are expected to be of very low magnitude and frequency. EPA notes that “polyurethane products, such as mattresses, pillows, and bowling balls, are considered completely cured products before they are sold.” EPA also states that “[c]ompletely cured products are fully reacted and therefore are considered to be inert and non-toxic.” Diisocyanates are known to cause respiratory sensitization at concentrations above allowable workplace limits; however exposure to such air concentrations are highly unlikely to occur in consumer products. Consumer products containing uncured diisocyanates (e.g. certain adhesives and glues) generally are accompanied by product safety information like warning labels, including the characteristics of the chemicals, their approximate cure time and how to properly protect yourself while handling the product.

Q: How does “curing” take place during formation of a polyurethane product?

A: Curing refers to the reaction that occurs between the two primary chemicals used to form a polyurethane product. These primary chemicals are commonly referred to as a diisocyanate (A-side material) and a polyol (B-side material). The A-side material, or diisocyanate, is highly reactive and curing begins immediately upon mixing with the B-side material. The cure time varies depending on the type of polyurethane product being produced, the ingredient formulations and other factors in the manufacturing process.

Many polyurethane products are completely cured and therefore considered “inert” before they are sold, such as mattresses, pillows, furniture cushions, car seating, refrigerator insulation, footwear, ski bindings or inline skates. This means that the original reactive ingredients, the diisocyanates and polyols, in the fully cured polyurethane product are no longer present in their original form. As a result of the reaction, they were transformed during production into the finished polyurethane product.

Q: Are diisocyanates contributing to the increased rate of asthma in the general population?

A: There is no scientific consensus on the causation for the increase of asthma. A large majority of people with asthma have allergies to airborne substances such as trees, grass, weed pollens, mold, animal dander, dust mites, and cockroach particles. Such allergens can act as triggers for asthmatic attacks. The EPA advises that the most common indoor asthma triggers include secondhand tobacco smoke, dust mites, mold, cockroaches and other pests, household pets, and combustion byproducts.

A 2011 study conducted by the Agency for Toxic Substances and Disease Registry (ATSDR) concluded, “We did not find a scientific connection between respiratory problems and exposure to TDI…Overall, we did not find that people living near the plants that emit TDI have recent or current exposure to TDI at levels of health concern.” EPA under their School Air Monitoring Program recently monitored the outdoor air around several schools for diisocyanates and found that the levels were non-detectable and well below levels of concern. There is no scientific evidence to support that diisocyanates are contributing to the increased rate of asthma in the general population, including children.

Q: Are children more susceptible to asthma due to their physiology?

A: The case is often made that children are more susceptible to asthma, and the exacerbation of pre-existing asthma, than adults. However, the physiological differences between children and adults (e.g., breathing rates, lung size) can result in the lungs of children receiving a higher dose of asthmogen at any given air concentration. Thus, the incidence of asthma in children may be more reflective of higher asthmogenic doses rather than an inherently higher susceptibility to asthma.

With regard to diisocyanates, it is becoming increasingly clear that the macromolecular and cellular pathways that are associated with childhood asthma and predominate in early childhood are different from those associated with the full manifestation of diisocyanate asthma in adults. This dichotomy in pathophysiology indicates that children are likely to be less susceptible to diisocyanate-induced asthma than adults.

The special needs and safety of children is an integral consideration in the establishment of community exposure limits. Children live safer, healthier lives thanks in part to the development of many products and technologies made with diisocyanates chemistry that improve public health and safety.

Q: Does the Jan et al. 2008 study investigating an MDI-based athletic track application provide accurate information linking MDI exposure and asthma-like symptoms in children?

A: The Jan study has been used as a basis for concluding that schoolchildren experienced asthma-like symptoms when exposed to an athletic track application containing MDI. According to an International Isocyanate Institute (III) critique, the reported Reactive Airways Dysfunction Syndrome (RADS)-like symptoms (e.g., cough, wheeze, headache) are likely due to xylene, a known central nervous system depressant and upper respiratory tract irritant that was used as a solvent for the applied MDI.

III noted this conclusion based on several facts. First, a closer inspection of the claim by Jan and coworkers that MDI was detected in the air near the polyurethane track proves this statement to be untrue. There is no evidence in this publication or an earlier one that MDI was detected. Second, no biomarker was detected in the urine of school children purportedly exposed to MDI. Third, the symptoms described by Jan and coworkers are likely due to the inhalation of xylene, the major component in the applied product (0.1% MDI in xylene) that is 1 million-fold more volatile than MDI. This incident reinforced among athletic track installers the importance of following all safety precautions during every stage of a track installation.