Decoding isocyanates: A deep dive into isocyanates

21st October 2023
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ISOCYANATE, including diisocyanates, serve as essential raw materials in the production of numerous polyurethane products.  These compounds are fundamental building blocks of polyurethane chemistry that have been applied in various industries, bringing lots of benefits to our daily life.

 What is Isocyanate?

Isocyanates are a group of highly reactive chemical compounds containing isocyanate functional group with the formula R−N=C=O.  Isocyanates are valued for their ability to create polyurethane products with numerous properties, such as flexibility, hardness, and thermal stability. They are primarily used in the production of polyurethane foams, coatings, adhesives, sealants, and elastomers, which find a wide range of applications in industries such as construction, automotive, aerospace, and furniture manufacturing.

Isocyanate chemical structure

Chemical structure of isocyanate

What is Isocyanate Made From?

The most common method to produce isocyanate is through the reaction of amines and phosgene. In this reaction, the phosgene reacts with the amine functional group (NH2) of the aromatic amine, resulting in the formation of the isocyanate group (-N=C=O) and hydrochloric acid as a byproduct. The resulted isocyanate compound will then processed through distillation and filtration to remove impurities. The pure form of isocyanate is now suitable for use to produce polyurethane products.

Amine+Phosgene→Isocyanate+HCl (Hydrochloric Acid)

 

Isocyanates Reactions

Isocyanates exhibit a strong nucleophilic nature, making them highly reactive with compounds containing active hydrogen atoms, such as amines, water, alcohols, acids, and bases.

Reaction with Amines:

Isocyanates react with amines, yielding urea derivatives. This reaction is widely applied in the synthesis of polyurethanes, a vital class of polymers used in various applications including plastics, elastomers, coatings, and adhesives.

RNCO+R’NH2​→RNHCO(NHR′)

Reaction with Water:

Isocyanates react with water to produce carbon dioxide and an amine. In this reaction, the isocyanate group (-NCO) reacts with water, resulting in the formation of an amine group (-NH2) and the release of carbon dioxide (CO2).

The isocyanate reaction with water is rapid while also releasing heat during the process. Thus, the exothermic property is essential in the production of polyurethane foams, as it contributes to their expansion and curing.

R-N=C=O+H₂O→R-NH₂+CO₂

Reaction with Alcohols:

The reaction between an isocyanate group (-N=C=O) and an alcohol group (-OH) results in the formation of a urethane linkage.

R2-N=C=O+R1-OHR2-NH-C(=O)-O-R1

Reaction with Acids:

Isocyanates react with acids to form compounds like carbamoyl chlorides.

R-N=C=O+R’-COOH→R-NH-C(=O)-O-R’

Reaction with Bases:

Isocyanates react with bases, leading to the formation of urea compounds.

R-N=C=O+R’-OH→R-NHC(=O)O-R’

 

5 Types of Isocyanates

Isocyanates feature different chemical structures that give their unique characteristics and applications. The most commonly used compounds are diisocyanates, which refer to isocyanates with two isocyanate functional groups. When diisocyanates are combined with polyols, they undergo a chemical reaction to formulate polyurethanes. There are commonly 5 types of diisocyanates, namely TDI, MDI, HDI, IPDI and HMDI.

TDI

TDI, short for Toluene Diisocyanate, is widely used in polyurethane chemistry due to its durability, insulation and superior adhesion. However, TDI feature relatively poor resistance to UV light and thus it is normally used in indoor applications.  Its widespread application lies in the production of flexible polyurethane foams and coatings found in furniture, bedding, automotive seats, mattresses and construction.

applications-of-TDI

MDI

MDI, short for Methylene Diphenyl Diisocyanate, is another key ingredient in polyurethane production with excellent thermal insulating properties, chemical and moisture resistance. Therefore, MDI is widely used for rigid polyurethane foams, insulation materials, and adhesives for automotive, furniture, household appliances and more.

remote unit and glasses applied with PU coatings formulated with MDI

HDI

Hexamethylene Diisocyanate, commonly known as HDI, consists of a hexamethylene chain with two isocyanate functional groups at the ends in its chemical structure. Well known for its excellent durability, weather and chemical resistance, high abrasion, HDI is widely used to formulate lightfast polyurethane coatings and adhesives including automotive finishes, industrial anti-corrosion coatings, and architectural coatings that even need to withstand the harsh environmental conditions.

outdoor wood furniture applied with PU coatings based on HDI

IPDI

IPDI, namely Isophorone Diisocyanate, belong to aliphatic diisocyanate. IPDI is used in high-performance coatings, sealants, and elastomers due to its outstanding weather resistance, flexibility and adhesion.

HMDI

HMDI, Hydrogenated MDI, is also a type of aliphatic diisocyanate. It is used in many specialty applications including enamel coatings with resistance to abrasion and UV light.

Isocyanate Exposure

Isocyanates are indeed a useful materials to produce products that are widely applied in our daily use. However, it’s necessary to be aware of the risks and danger of isocyanate exposure. Inhalation or skin contact with isocyanates can lead to respiratory issues such as asthma, bronchitis, skin sensitization, and even severe allergic reactions. Jobs that involve working with or around materials containing isocyanates are at risk of exposure, including automotive technicians, workers producing foam, coatings, adhesives, furniture as well as those spraying polyurethane foam or coatings. It’s crucial for these workers to use protective equipment to minimize the harmful effect of isocyanate exposure in the workplace.

Effective safety measures include wearing disposable suits and goggles to protect skin and eyes, , respirators to prevent breathing in harmful substances.

man-with-suit-disinfecting-shelf-under-isocyanate-exposure

In conclusion, isocyanates are versatile chemicals with a wide range of applications, for polyurethane production of coatings, adhesives and sealants. Understanding their chemistry, types, and reactions, including the intriguing isocyanate water reaction, is crucial for safely and effectively utilizing these compounds in actual practice.

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