Aliphatic Diisocyanates

Chemistry of Aliphatic Diisocyanates
Extensive Regulatory Review
Worker Safety
Consumer Safety

Chemistry of Aliphatic Diisocyanates

Q: What are aliphatic diisocyanates?

A: Aliphatic diisocyanates (ADI) are specialty intermediate chemicals often reacted to form polyisocyanates, which act as building blocks to form color-stable and durable polyurethane coatings, adhesives, sealants and elastomers. These coatings and elastomers can significantly enhance a product’s appearance, lengthen its lifespan and offer high abrasion and UV resistance. Volumes of ADI products used in North America are significantly smaller than the aromatic diisocyanates.¹

ADIs are typically not sold directly for consumer use. Rather, they are sold primarily to industrial customers who use them as binders or hardeners during manufacturing processes. The most common types of aliphatic diisocyanates include:

• hexamethylene diisocyanate (HDI),
• methylene dicyclohexyl diisocyanate or hydrogenated MDI (HMDI)
• and isophorone diisocyanate (IPDI)

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

A: The most widely used ADI is hexamethylene-1,6-diisocyanate (HDI).¹ It is derived from hexamethylene diamine, which is used in the production of nylon. HDI is an intermediate chemical used to produce HDI-based products (i.e., polyisocyanates). These HDI-based products are primarily used in the manufacture of industrial coatings where high performance capability, such as UV and weather resistance, is required. HDI-based coatings are used to manufacture a variety of products, including automobiles, aircraft, flooring, furniture, safety equipment, machinery, medical devices and infrastructure projects.

Q: What is methylene dicyclohexyl diisocyanate or hydrogenated MDI (HMDI) and why is it important?

A: Dicyclohexylmethane-4,4’diisocyanate (HMDI) serves as a building block for the preparation of chemical products, reactive intermediates and polymers such as polyurethane dispersions (PUDs), elastomers, and thermoplastic polyurethanes (TPUs). Products based on HMDI may be useful in coatings for flooring, roofing, textiles, elastomers, optical products, adhesives, and sealants.

Q: What is isophorone diisocyanate (IPDI) and why is it important?

A: Isophorone diisocyanate (IPDI) is an intermediate chemical used to produce IPDI-based products (i.e., polyisocyanates, polyurethane dispersions) that are primarily used in certain polyurethane coatings. These IPDI-based products are used by industrial customers to manufacture various coatings for automobiles, flooring, roofing, machinery, and textile applications. They are also used in cast elastomers, adhesives, sealants, and as crosslinkers for powder coatings.

Q: What are aromatic diisocyanates and how do they differ from aliphatic diisocyanates?

A: There are two primary aromatic diisocyanates: toluene diisocyanate (TDI) and methylenediphenyl diisocyanate (MDI). Together, they comprise over 90% of the overall isocyanate consumption in the North American polyurethane industry, with aliphatic diisocyanates accounting for the balance.² TDI is used primarily in the production of flexible foams. MDI, the second type of DII, comes in two forms: Pure MDI and polymeric MDI (PMDI). Pure MDI is used in the production of a variety of polyurethane products like elastomers, sealants, adhesives and coatings. PMDI is a highly versatile product used to produce a wide variety of rigid, flexible, semi-rigid and polyisocyanurate and thermoset foams.

While aromatic diisocyanates are primarily used to make polyurethane foam products, aliphatic diisocyanates (ADIs) are primarily used to make various color-stable and durable coatings. To a lesser degree, they are also used to make adhesives and sealants. ADIs are sold primarily to industrial customers who use them as binders or hardeners during manufacturing processes. Both aromatic and aliphatic diisocyanates are manufactured in closed-loop systems that are carefully monitored for compliance with environment, health and safety regulations.

Visit the ACC diisocyanates website for more information about aromatic diisocyanates.

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Extensive Regulatory Review

Q: How are aliphatic 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. Aliphatic diisocyanates have been used since the late 1940s and their safety and environmental impact have been well studied. ADIs are regulated under the authority of the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA) and other government agencies as needed. Federal authorities have set exposure safety limits for aliphatic diisocyanates emissions to protect both workers in production facilities and surrounding communities. The environment, health and safety benchmarks are based on science and reviewed by government officials with the goal of protecting workers and communities. Companies can face significant civil and criminal penalties for noncompliance.

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

A: There are potential health effects. At the recommended workplace concentrations, generally no short or long-term health effects would be expected.³ However, at levels above recommended workplace exposure limits, aliphatic diisocyanates can be strongly irritating to eyes, nose, throat, skin and lungs. If ADI 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 have been established by organizations such as the American Conference of Governmental Industrial Hygienists (ACGIH). Extensive safety precautions are undertaken by the ADI and polyurethane industries to protect worker and consumer health and to comply with all government regulations.

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.

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Worker Safety

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

A: The vast majority of aliphatic 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. Aliphatic 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 Toronto⁴ that reviewed occupational asthma claims in Ontario during a 5 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."

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Consumer Safety

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

A: The vast majority of aliphatic diisocyanates (ADIs) manufactured are for industrial and commercial use. Products made using ADI chemistry, such as durable coatings that protect automobiles, commercial/industrial equipment, aircraft, and bridges, contribute to the safety of modern conveniences. These finished products contain fully cured ADIs. The U.S. Environmental Protection Agency (EPA) states that “[c]ompletely cured products are fully reacted and therefore are considered to be inert and non-toxic.” ²

Aliphatic diisocyanates are known to cause respiratory sensitization at airborne concentrations above the allowable workplace limits; however, exposure to such airborne concentrations is highly unlikely during the use of consumer products. Consumer products containing uncured aliphatic diisocyanates are very limited (i.e., certain coatings) and are accompanied by product safety information such as warning labels, including the characteristics of the chemicals, their approximate cure time and how to properly protect yourself while handling the product. Additionally, for professionally applied products (i.e., floor coatings) where there may be public interaction, return to service times are often set to ensure the public does not return to the space until the coating is cured. Thus, overall consumer exposures to unreacted ADIs are expected to be of very low magnitude and frequency.

The chemical industry makes the safety and responsible use of its products a priority. A robust system of laws and industry initiatives oversees the development and use of chemical products, enhances scientific understanding and makes safety information available to the public.

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

A: This can be explained by looking at how aliphatic diisocyanates are among the building blocks used to make many polyurethane (PU)-based products, such as coatings, sealants and adhesives. Curing refers to the reaction that occurs between the two primary chemicals used to form a PU product. These primary chemicals are commonly referred to as an aliphatic diisocyanate and a polyol. The aliphatic diisocyanate material is highly reactive and curing begins immediately upon mixing with the polyol 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 PU products are completely cured and therefore considered “inert” before they are sold. This means that the original reactive ingredients, the aliphatic diisocyanates and the 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 product.

Q: Are aliphatic 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 pollen, 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.

EPA under their School Air Monitoring Program monitored the outdoor air around several schools for diisocyanates, including HDI, and found that the levels were non-detectable and well below levels of concern. There is no scientific evidence to support that aliphatic 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 (i.e., 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 aliphatic diisocyanates chemistry that improve public health and safety.

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