Aromatic vs Aliphatic Diisocyanates: The Ultimate Comparison

6th November 2023

ISOCYANATES are a group of highly reactive chemical compounds containing isocyanate functional group with the formula R−N=C=O. Based on the number of ester groups in the molecule, they can be classified into monoisocyanates and diisocyanates, among others.

A chemical containing two isocyanate functional groups refer to DIISOCYANATES, which are essential building blocks for polyurethane products through the polymerization with polyether polyols or polyester polyols. The most commonly used chemical compounds are aliphatic diisocyanates and aromatic diisocyanates. With different chemical structures, these two compounds render unique properties that result in their distinct applications in the world of polymers and coatings.

Isocyanate chemical structure

Isocyanate Functional Group

Gain an in-depth understanding through our comprehensive guide of isocyanates.


What are aliphatic diisocyanates?

Aliphatic diisocyanates, also known as ADIs, are diisocyanates where the NCO group attached to an aliphatic carbon structure. Compared to aromatic diisocyanates, aliphatic diisocyanates lack the aromatic ring and have a more linear or branched structure. The absence of aromatic rings in their structure gives aliphatic diisocyanates excellent weather resistance and UV stability, which makes it widely used in industries requiring lightfast and durable material, especially outdoor applications.


 3 types of aliphatic diisocyanates

There are 3 common types of aliphatic diisocyanates including hexamethylene diisocyanate (HDI),  Hexamethylene diisocyanate(HMDI) and isophorone diisocyanate (IPDI). These diisocyanates play a vital role in polyurethane systems, providing versatility and performance in different formulations.

Hexamethylene diisocyanate (HDI)

HDI is the most widely used aliphatic diisocyanates by formulators. Hexamethylene diisocyanate, commonly known as HDI, consists of a hexamethylene chain with two diisocyanate functional groups at the ends in its chemical structure. HDI is commonly used for the manufacturing of durable industrial coatings that need to withstand the harsh environmental conditions.

HDI Chemical Structure

Chemical Structure of HDI

Isophorone diisocyanate (IPDI)

IPDI, namely Isophorone diisocyanate, are mostly used for high-performance PU coatings for automotive, architectural, industrial equipment and flooring. Besides, IPDI is also used for the production of elastomers, sealants.

IPDI Chemical Structure

Chemical Structure of IPDI

Hydrogenated MDI (HMDI)

HDMI, Hydrogenated MDI is also an aliphatic chemical compound with excellent durability, mechanical properties and chemical resistance. It is produced by hydrogenation of methylene diphenyl diisocyanate (MDI) and is commonly manufactured in small quantities. Hydrogenated MDI (HMDI) is widely consumed by construction, automotive and furniture industries.

HMDI Chemical Structure

Chemical Structure of HMDI

What are aromatic diisocyanates?

Aromatic diisocyanates, widely dominant in the market due to their cost-effective use of inexpensive raw materials like toluene, have rapidly advanced in various applications. Aromatic diisocyanates are diisocyanates where the NCO group are attached to a benzene ring or aromatic ring, which makes them susceptible to oxidation that can lead to issues such as yellowing and poor weather resistance in products like polyurethane coatings.

However, this benzene ring structure of aromatic diisocyanates also bring exceptional mechanical strength, reactivity and flexibility. These properties render them indispensable in enhancing the structural integrity and performance of thermoplastic elastomers (TPE). Aromatic diisocyanates also find wide applications in PU coatings and more.


2 types of aromatic diisocyanates

Toluene diisocyanate (TDI)

TDI is one of the most vital diisocyanates in polyurethane synthesis. Its widespread use spans across various applications, including flexible polyurethane foam plastics, coatings, elastomers, adhesives, sealants, and other polyurethane products.

TDI Chemical Structure

Chemical Structure of TDI

Methylene Diphenyl diisocyanate (MDI)

MDI can be divided into two types according to their chemical structures. Pure MDI or MDI-100 refers to MDI containing over 99% of 4,4'-diphenylmethane diisocyanate. It is widely utilized in the polyurethane industry, primarily in the field of fibers (such as nylon), and can also be used in the synthesis of prepolymers (elastomers). Its main applications include pulp slurries and TPU manufacturing.

Polymeric MDI, on the other hand, is a mixture of pure MDI and polymeric polymeric polyols. It is extensively used in the production of rigid polyurethane foams and finds wide applications in refrigerators, freezers, refrigerated containers, thermal insulation materials.

MDI Chemical Structure

Chemical Structure of MDI