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Isocyanate
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{{Distinguish|Isocyanide}}thumb|The isocyanate functional groupIsocyanate is the functional group with the formula R–N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for the production of polyurethanes, a class of polymers.Isocyanates should not be confused with cyanate esters and isocyanides, very different families of compounds. The cyanate (cyanate ester) functional group (R–O–C≡N) is arranged differently from the isocyanate group (R–N=C=O). Isocyanides have the connectivity R-N≡C, lacking the oxygen of the cyanate groups.

Structure and bonding

In terms of bonding, isocyanates are closely related to carbon dioxide (CO2) and carbodiimides (C(NR)2). The C-N=C=O unit that defines isocyanates is a planar and the N=C=O linkage is nearly linear. In phenyl isocyanate, the C=N and C=O distances are respectively 1.195 and 1.173 Å.JOURNAL, Porphyrin Sponges: Conservative of Host Structure in over 200 Porphyrin-Based Lattice Clathrates, Marianne P. Byrn, Carol J. Curtis, Yu Hsiou, Saeed I. Khan, Philip A. Sawin, S. Kathleen Tendick, Aris Terzis, Charles E. Strouse, J. Am. Chem. Soc., 1993, 115, 21, 9480–9497, 10.1021/ja00074a013,

Production

Isocyanates are produced from amines by phosgenation, i.e. treating with phosgene:
RNH2 + COCl2 → RNCO + 2 HCl
These reactions proceed via the intermediacy of a carbamoyl chloride (RNHC(O)Cl). Owing to the hazardous nature of phosgene, the production of isocyanates requires special precautions.

Reactivity

With nucleophiles

Isocyanates are electrophiles, and as such they are reactive toward a variety of nucleophiles including alcohols, amines, and even water. Upon treatment with an alcohol, an isocyanate forms a urethane linkage:
ROH + R'NCO → ROC(O)N(H)R' (R and R' are alkyl or aryl groups)
If a di-isocyanate is treated with a compound containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are formed, which are known as polyurethanes. (File:Polyurethane synthesis.tif|thumb|center|700px|Synthesis of polyurethane from a diisocyanate and a diol.)Isocyanates react with water to form carbon dioxide:
RNCO + H2O → RNH2 + CO2
This reaction is exploited in tandem with the production of polyurethane to give polyurethane foams. The carbon dioxide functions as a blowing agent.BOOK, Fundamentals to Polymer Science, An Introductory Text, 39, Paul Painter and Michael Coleman, Second, Isocyanates also react with amines to give ureas:
R2NH + R'NCO → R2NC(O)N(H)R'
The addition of an isocyanate to a urea gives a biuret:
R2NC(O)N(H)R' + R"NCO → R2NC(O)NR'C(O)NHR"
Reaction between a di-isocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas.

Cyclization

Isocyanates also can react with themselves. Aliphatic di-isocyanates can form trimers, which are structurally related to cyanuric acid. Isocyanates participate in Diels-Alder reactions, functioning as dienophiles.

Rearrangement reactions

Isocyanates are common intermediates in the synthesis of primary amines via hydrolysis:

Common isocyanates

missing image!
- Methylene diphenyl 4,4'-diisocyanate.PNG -
frame|Methylene diphenyl 4,4'-diisocyanate (MDI)Numbering of the ring atoms shown with blue numbers
missing image!
- Isophorone-diisocyanate-2D-skeletal.png -
Isophorone diisocyanate.
The global market for diisocyanates in the year 2000 was 4.4 million tonnes, of which 61.3% was methylene diphenyl diisocyanate (MDI), 34.1% was toluene diisocyanate (TDI), 3.4% was the total for hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), and 1.2% was the total for various others.BOOK, D, Randall, 2002, The Polyurethanes Book, Wiley, 978-0-470-85041-1, A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides.

Common applications

MDI is commonly used in the manufacture of rigid foams and surface coating.BOOK, Christian Six, Frank Richter, Isocyanates, Organic, Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim, 10.1002/14356007.a14_611, 978-3527306732, Polyurethane foam boards are used in construction for insulation. TDI is commonly used in applications where flexible foams are used such as furniture and bedding. Both MDI and TDI are used in the making of adhesives and sealants due to weather resistant properties. Isocyanates, both MDI and TDI are widely used in as spraying applications of insulation due to the speed and flexibility of applications. Foams can be sprayed into structures and harden in place or retain some flexibility as required by the application.WEB,weblink Chemicals and Production of Spray Polyurethane Foam - Why It Matters, US EPA, OCSPP, 2015-08-14, US EPA, en, 2018-12-08, HDI is commonly utilized in high performance surface coating applications including automotive paints.

Health and safety

The risks of isocyanates was brought to the world's attention with the Bhopal disaster, which caused the death of nearly 4000 people. The release of methyl isocyanate (MIC) was the cause of this disaster.LD50s for isocyanates are typically several hundred milligrams per kilogram.Allport DC, Gilbert, DS and Outterside SM (eds) (2003).MDI and TDI: safety, health & the environment: a source book and practical guide. Chichester, Wiley. Despite this low acute toxicity, an extremely low short-term exposure limit (STEL) of 0.07 mg/m−3 is the legal limit for all isocyanates (except methyl isocyanate: 0.02 mg/m−3) in the United Kingdomweblink These limits are set to protect workers from chronic health effects such as occupational asthma, contact dermatitis, or irritation of the respiratory tractweblink they are used in spraying applications, the properties of their aerosols have attracted attention.WEB,weblink CDC - Isocyanates - NIOSH Workplace Safety and Health Topic, 2018-11-09, www.cdc.gov, en-us, 2018-11-21, WEB,weblink Isocyanate Exposure, Reaction and Protection - Quick Tips #233 - Grainger Industrial Supply, www.grainger.com, 2018-11-21, In the U.S., OSHA conducted a National Emphasis Program on isocyanates starting in 2013 to make employers and workers more aware of the health risks.WEB,weblink OSHA announces new National Emphasis Program for occupational exposure to isocyanates {{!, Occupational Safety and Health Administration|website=www.osha.gov|language=en|access-date=2018-11-21}} Polyurethanes have variable curing times, and the presence of free isocyanates in foams vary accordingly.JOURNAL,weblink Riedlich, C, Risk of isocyanate exposure in the construction industry, CPWR - the Center for Construction Research and Training, 1–8, 2010, Both the US National Toxicology Program (NTP) and International Agency for Research on Cancer (IARC) have evaluated TDI as a potential human carcinogen and Group 2B "possibly carcinogenic to humans"WEB,weblink Safety Data Sheet - TOLUENE DIISOCYANATE (TDI), IXOM, 2018-11-24, WEB,weblink Health Effects of Diisocyanates: Guidance for Medical Personnel, American Chemistry Council, 2018-11-24, . MDI appears to be relatively safer and is unlikely a human carcinogen. The IARC evaluates MDI as Group 3 "not classifiable as to its carcinogenicity in humans."WEB,weblink SAFETY DATA SHEET, Everchem, 2018-11-24, All major producers of MDI and TDI are members of the International Isocyanate Institute, which promotes the safe handling of MDI and TDI.

Hazards

Toxicity

Isocyanates can present respiratory hazards as particulates, vapors or aerosols. Autobody shop workers are a very commonly examined population for isocyanate exposure as they are repeatedly exposed when spray painting automobilesJOURNAL, Airborne Isocyanate Exposures in the Collision Repair Industry and a Comparison to Occupational Exposure Limits, Journal of Occupational and Environmental Hygiene, 9, 5, 329–339, 10.1080/15459624.2012.672871, 4075771, 22500941, 2012, Reeb-Whitaker, Carolyn, Whittaker, Stephen G., Ceballos, Diana M., Weiland, Elisa C., Flack, Sheila L., Fent, Kenneth W., Thomasen, Jennifer M., Trelles Gaines, Linda G., Nylander-French, Leena A., and can be exposed when installing truck bed liners.WEB,weblink Preventing Asthma and Death from MDI Exposure During Spray-on Truck Bed Liner and Related Applications, www.cdc.gov, 2018-12-07, JOURNAL, Bogaert, Pieter, Tournoy, Kurt G., Naessens, Thomas, Grooten, Johan, January 2009, Where Asthma and Hypersensitivity Pneumonitis Meet and Differ, The American Journal of Pathology, 174, 1, 3–13, 10.2353/ajpath.2009.071151, 0002-9440, 2631313, 19074616, Hypersensitivity pneumonitis is slower onset and features chronic inflammation that can be seen on imaging of the lungs. Occupational asthma is a worrisome outcome of respiratory sensitization to isocyanates as it can be acutely fatal.JOURNAL, Kimber, Ian, Dearman, Rebecca J., Basketter, David A., 2014-07-25, Diisocyanates, occupational asthma and IgE antibody: implications for hazard characterization, Journal of Applied Toxicology, en, 34, 10, 1073–1077, 10.1002/jat.3041, 25059672, 0260-437X, Diagnosis of occupational asthma is generally performed using pulmonary function testing (PFT) and performed by pulmonology or occupational medicine physicians.WEB,weblink Do You Have Work-Related Asthma? A Guide for YOU and YOUR DOCTOR, OSHA, 2018-11-21, Occupational asthma is much like asthma in that it causes episodic shortness of breath and wheezing. Both the dose and duration of exposure to isocyanates can lead to respiratory sensitization.JOURNAL, Daniels, Robert D., 2018-02-01, Occupational asthma risk from exposures to toluene diisocyanate: A review and risk assessment, American Journal of Industrial Medicine, en, 61, 4, 282–292, 10.1002/ajim.22815, 0271-3586, 6092631, 29389014, Dermal exposures to isocyanates can sensitize an exposed person to respiratory disease.Dermal exposures can occur via mixing, spraying coatings or applying and spreading coatings manually. Dermal exposures to isocyanates is known to lead to respiratory sensitization.JOURNAL, Bello, Dhimiter, Herrick, Christina A., Smith, Thomas J., Woskie, Susan R., Streicher, Robert P., Cullen, Mark R., Liu, Youcheng, Redlich, Carrie A., 2006-11-28, Skin Exposure to Isocyanates: Reasons for Concern, Environmental Health Perspectives, en, 115, 3, 328–335, 10.1289/ehp.9557, 0091-6765, 1849909, 17431479, Even when using the right personal protective equipment (PPE), exposures can occur to body areas not completely covered.JOURNAL, Ceballos, Diana M., Fent, Kenneth W., Whittaker, Stephen G., Gaines, Linda G.T., Thomasen, Jennifer M., Flack, Sheila L., Nylander-French, Leena A., Yost, Michael G., Reeb-Whitaker, Carolyn K., 2011-08-10, Survey of Dermal Protection in Washington State Collision Repair Industry, Journal of Occupational and Environmental Hygiene, en, 8, 9, 551–560, 10.1080/15459624.2011.602623, 21830873, 1545-9624, Isocyanates can also permeate improper PPE, necessitating frequent changes of both disposable gloves and suits if they become over exposed.

Flammability

Methyl isocyanate (MIC) is highly flammable.WEB,weblink Methyl isocyanate, Pubchem, pubchem.ncbi.nlm.nih.gov, en, 2018-11-21, MDI and TDI are much less flammable.WEB,weblink Dealing with fires involving MDI and TDI, ISOPA, 2018-11-21, Flammability of materials is a consideration in furniture design.WEB,weblink Is Your Couch Poisoning You?, Weiss-Hills, Samantha, 2018-05-28, Architectural Digest, en, 2018-12-08, The specific flammability hazard is noted on the safety data sheet (SDS) for specific isocyanates.

Hazard controls

Hierarchy of hazard controlsElimination and substitution seeks to eliminate a hazard directly from use in industrial processes. Elimination if possible also has the possibility of eliminating the need for other controls. If unable to make an elimination substituting a less hazardous isocyanate may also control hazards. Because of the hazards inherent isocyanates there is ongoing research for suitable replacements.NEWS,weblink Finding a substitute for methyl isocyanate, Chemistry World, 2018-11-21, en, The EPA has sponsored work on finding suitable replacements for isocyanates in polyurethane coatings.WEB,weblink Final Report {{!, Isocyanate-Free Polyurethane Coatings {{!}} Research Project Database {{!}} Grantee Research Project {{!}} ORD {{!}} US EPA|website=cfpub.epa.gov|language=en|access-date=2018-12-07}}Engineering controls seek to decrease hazards by creating barriers to hazard exposure. Using the Source-Pathway-Receptor model, an engineering control acts on the pathway to mitigate hazards emanating from the source from reaching the receptor. An automated spraying booth with a separate ventilation system would be an example of engineering controls. Appropriate ventilation is a common engineering control when using isocyanates.WEB,weblink Isocyanates - Controlling hazardous substances - Managing occupational health risks in construction, www.hse.gov.uk, en-GB, 2018-11-21, Administrative controls are policy or training based controls to decrease hazards. A quarterly training session on recognizing symptoms of occupational asthma or proper respirator use would be examples of administrative controls. Administrative controls can be effective in reducing hazards for which personal protective equipment does not exist, for example, no eating or smoking in work areas can prevent ingestion of hazardous chemicals. Training is required by OSHAWEB,weblink Safety and Health Topics {{!, Isocyanates - Additional Resources {{!}} Occupational Safety and Health Administration|website=www.osha.gov|language=en-us|access-date=2018-11-21}}Personal protective equipment (PPE) is the lowest level of hazard control. For isocyanates commonly used PPE include respirators for inhalation hazards and gloves to minimize absorption of dermal hazards. PPE like respirators are sensitive to fit and require some maintenance periodically. In some autobody paint and clear coat spraying applications exposure limits exceed the protection factor of half mask respirators and a full face mask is required. Eye protection is an important component of PPE.WEB,weblink Isocyanates - Controlling hazardous substances - Managing occupational health risks in construction, www.hse.gov.uk, en-GB, 2018-11-21, Gloves and coveralls are appropriate personal protective equipment for workers.WEB,weblink Guidance for Selection of Protective Clothing for MDI Users, American Chemistry Council, 2018-11-21, Gloves and protective clothing can be effective in reducing dermal exposures but user resistance can arise due to loss of tactile sensation or increased thermal burden. The material and thickness of gloves is an important component of protection.JOURNAL, Ceballos, Diana, Reeb-Whitaker, Carolyn, Glazer, Patricia, Murphy-Robinson, Helen, Yost, Michael, 2014-03-28, Understanding Factors That Influence Protective Glove Use Among Automotive Spray Painters, Journal of Occupational and Environmental Hygiene, en, 11, 5, 306–313, 10.1080/15459624.2013.862592, 1545-9624, 5514320, 24215135, WEB,weblink Chemical Resistant Gloves > Painters and Repairers Education Program {{!, Internal Medicine {{!}} Yale School of Medicine|website=medicine.yale.edu|language=en|access-date=2018-11-21}}

Industrial hygiene

Exposure assessment is the domain of industrial hygienists. An objective of exposure assessment is to ensure regulatory compliance with occupational exposure limits (OELs) below. OSHA guidelines provide detailed technical guidance on measuring isocyanates via sampling and analytics procedures tailored to specific chemicals. In the case of MDI, sample is via glass fiber filters at standard air flow rates and liquid chromatography.WEB,weblink Sampling and Analytical Methods {{!, Methylene Bisphenyl Isocyanate (MDI) - (Organic Method #047) {{!}} Occupational Safety and Health Administration|website=www.osha.gov|language=en-us|access-date=2018-11-22}}Occupational health surveillance is primarily the domain of medical professionals. This can include counseling, respirator fit testing, tracking of biologic exposure via biologic exposure indices (BEI) and PFT results. Biologic monitoring levels for isocyanates existJOURNAL, Hu, Jimmy, Cantrell, Phillip, Nand, Aklesh, 2017-07-29, Comprehensive Biological Monitoring to Assess Isocyanates and Solvents Exposure in the NSW Australia Motor Vehicle Repair Industry, Annals of Work Exposures and Health, en, 61, 8, 1015–1023, 10.1093/annweh/wxx064, 29028250, 2398-7308, but may not be commonly used. One example of a monitoring program by the United States Navy relies on pulmonary function testing and screening questionnaires.WEB,weblink MEDICAL SURVEILLANCE PROCEDURES MANUAL AND MEDICAL MATRIX (EDITION 11), Navy And Marine Corps Public Health Center, 2018-11-21, The combination of industrial hygiene and medical surveillance can have a significant effect on the incidence of occupational asthma.JOURNAL, Tarlo, S. M., Liss, G. M., Yeung, K. S., 2002-01-01, Changes in rates and severity of compensation claims for asthma due to diisocyanates: a possible effect of medical surveillance measures,weblink Occupational and Environmental Medicine, en, 59, 1, 58–62, 10.1136/oem.59.1.58, 1351-0711, 1740212, 11836470, Emergency management is a complex process of preparation and should be considered in a setting where a release of bulk chemicals may threaten the well-being of the public. The Bhopal disaster involving release of MIC and resulting in the deaths of thousands of people and affecting hundreds of thousands more. As a result of major industrial incidents like this, public health officials have proposed disaster preparedness programs aimed at assessing hazards, prevention via engineering and coordinated responses.JOURNAL, Rose, Dale A., Murthy, Shivani, Brooks, Jennifer, Bryant, Jeffrey, 2017-09-11, The Evolution of Public Health Emergency Management as a Field of Practice, American Journal of Public Health, en, 107, S2, S126–S133, 10.2105/ajph.2017.303947, 0090-0036, 5594387, 28892444, More recently MIC was involved in an explosion at a pesticide manufacturing plant in West Virginia.WEB,weblink CSB Issues Report on 2008 Bayer CropScience Explosion: Finds Multiple Deficiencies Led to Runaway Chemical Reaction; Recommends State Create Chemical Plant Oversight Regulation - General News - News {{!, CSB|website=www.csb.gov|language=en|access-date=2018-11-21}}

Occupational exposure limits

Exposure limits can be expressed as ceiling limits, a maximum value, short term exposure limits (STEL) a 15 minute exposure limit or an 8 hour time weighted average limit (TWA). Below is a sampling, not exhaustive, as less common isocyanates also have specific limits within the United States and in some regions there are limits on total isocyanate which recognizes some of the uncertainty regarding the safety of mixtures of chemicals as compared to pure chemical exposures. For example while there is no OEL for HDI, NIOSH has a REL of 5ppb for a 8 hour TWA and a ceiling limit of 20ppb, consistent with the recommendations for MDI.WEB,weblink CDC - NIOSH Pocket Guide to Chemical Hazards - Hexamethylene diisocyanate, www.cdc.gov, 2018-12-08, {| class="wikitable sortable"|+Methylene bisphenyl isocyanate (MDI)!Organization (Region)!Standard!Value|OSHA (USA)|Ceiling Limit
website=www.osha.govaccess-date=2018-11-24}}
|NIOSH (USA)|Recommended Exposure Limit (REL) - Ceiling Limit
LAST=CDCWEBSITE=ARCHIVE-DATE=ACCESS-DATE=2018-11-24,
|NIOSH (USA)|Recommended Exposure Limit (REL) - TWA|5ppb
|ACGIH (USA)|Threshold limit Value (TLV)
FIRST=DCFIRST2=DSFIRST3=SMYEAR=2003LOCATION=ENGLAND, 346,
|Safe Work (Australia)|All isocyanates - TWA
LAST=SAFE WORK AUSTRALIAWEBSITE=ARCHIVE-DATE=ACCESS-DATE=2018-11-21, (approximately 2.5ppb for comparison)
|Safe Work (Australia)|All isocyanates - STEL|0.07mg/m3 (approximately 10ppb for comparison)
|Heath & Safety Executive (UK)|All isocyanates - TWA
PUBLISHER=THE STATIONARY OFFICEISBN=978 0 7176 6703 1PAGES=17,
|Heath & Safety Executive (UK)|All isocyanates - STEL|0.07mg/m3
{| class="wikitable sortable"|+Toluene-2,4-diisocyanate (TDI)!Organization (Region)!Standard!Value|OSHA (USA)|Ceiling Limit
website=www.osha.govaccess-date=2018-11-24}}
|NIOSH (USA)|Recommended Exposure Limit (REL)
LAST=CDCWEBSITE=ARCHIVE-DATE=ACCESS-DATE=2018-11-24,
|ACGIH (USA)|Threshold limit Value (TLV)|5ppb
|ACGIH (USA)|Ceiling Limit|20ppb

Regulation

United States

The Occupational Safety and Health Administration (OSHA) is the regulatory body covering worker safety. OSHA puts forth permissible exposure limit (PEL) 20 ppb for MDI and detailed technical guidance on exposure assessment.The National Institutes of Health (NIOSH) is the agency responsible for providing the research and recommendations regarding workplace safety while OSHA is more of an enforcement body. NIOSH is responsible for producing the science which can result in recommended exposure limits (REL) which can be lower than the PEL. OSHA is tasked with enforcement and defending the enforceable limits (PELs). In 1992 when OSHA reduced the PEL for TDI to the NIOSH REL, the PEL reduction was challenged in court and the reduction reversed.JOURNAL, 1996-03-01, Request for assistance in preventing asthma and death from diisocyanate exposure.,weblink en-us, 10.26616/NIOSHPUB96111, The Environmental Protection Agency (EPA) is also involved in the regulation of isocyanates with regard to the environment and also non-worker persons that might be exposed.WEB,weblink Spray Polyurethane Foam (SPF) Insulation and How to Use it More Safely {{!, US EPA|last=EPA,OCSPP,OPPT,EETD|first=US|website=US EPA|language=en|access-date=2018-11-22}}The American Conference of Government Industrial Hygienists (ACGIH) is an non-government organization that publishes guidance known as Threshold Limit Values (TLV) for chemicals based research as constant work exposure level without ill-effect. The TLV is not an OSHA enforceable value unless the PEL is the same.

European Union

The European Chemicals Agency (ECHA) provides regulatory oversight of chemicals used within the European Union.WEB,weblink Restriction proposal on diisocyanates and several authorisation applications agreed by RAC and SEAC - All news - ECHA, echa.europa.eu, en-GB, 2018-11-22, ECHA has been implementing policy aimed at limiting worker exposure through elimination via lower allowable concentrations in products and mandatory worker training, an administrative control.WEB,weblink Proposed restriction of diisocyanates, KG, Vincentz Network GmbH & Co., European Coatings, en-GB, 2018-11-22, Within the European Union many nations set their own occupational exposure limits for isocyanates.

International groups

The United Nations via the World Health Organization (WHO) together with the International Labor Organization (ILO) and United Nations Environment Programme (UNEP), collaborate on the International Programme on Chemical Safety (IPCS) to publish summary documents on chemicals. The IPCS published one such document in 2000 summarizing the status of scientific knowledge on MDI.WEB,weblink Concise International Chemical Assessment Document 27: DIPHENYLMETHANE DIISOCYANATE (MDI), Sekizawa J., Greenberg M.M., 2000, 2018-11-18, The IARC evaluates the hazard data on chemicals and assigns a rating on the risk of carcinogenesis. In the case of TDI the final evaluation is possibly carcinogenic to humans (Group 2B).WEB,weblink TOLUENE DIISOCYANATES, IARC, 1987, 2018-11-18, For MDI the final evaluation is not classifiable as to its carcinogenicity to humans (Group 3).WEB,weblink 4,4′-METHYLENEDIPHENYL DIISOCYANATE AND POLYMERIC 4,4′-METHYLENEDIPHENYL DIISOCYANATE, 2018-11-18, The International Isocyanate Institute is an international industry consortium that seeks promote the safe utilization of isocyanates by promulgating best practices. WEB,weblink Welcome to the International Isocyanate Institute, 2018-11-18,

See also

References

{{Reflist}}

External links

{{Functional Groups}}

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