Reaction of Isocyanates with Some Important Compounds

In the production of polyurethane materials, Toluene diisocyanate (TDI) is widely used. During the use of this chemical, its toxic effects on human health and the environment must be carefully examined and addressed.

During the use of TDI, it is essential to neutralize the reactive isocyanate groups in uncontrolled spills and to convert the residual TDI chemical into harmless compounds.

Below, you can see the reactions of the TDI molecule with compounds such as functional groups, amines, alcohols, and water, as well as methods for neutralizing waste Toluene Diisocyanate.

Usage Areas and Reaction Mechanism of Toluene Diisocyanate

TDI is used in polyurethane applications. It is obtained through a condensation and addition reaction of an isocyanate and a substance with hydroxyl group functionality, such as a polyol. Polyurethane materials have found wide applications in areas such as elastomers and flexible foam materials.

The commercial grade of Toluene Diisocyanate is a mixture of the 2,4-toluene diisocyanate and 2,6-toluene diisocyanate isomers. This chemical mixture is called Toluene Diisocyanate 80/20.

Due to its structure, TDI, together with a polyol, is used in rocket propellant formulations by providing structural integrity to the propellant particles.

Some Important Chemical Reactions of Isocyanates

Isocyanates tend to react strongly with water, ammonia, alcohols, and amines. This is due to the presence of active hydrogen on the molecule, which makes it reactive towards the isocyanate.

Reaction with Alcohols;

Isocyanates react with alcohols. The reaction with primary alcohols occurs vigorously, forming a urethane. The reaction of secondary alcohols is relatively slower, but this reaction also forms a urethane. The reaction with tertiary alcohols is the slowest. The reactions are exothermic.

Reaction with Amines;

Primary and secondary amines react strongly with isocyanates. The reaction of primary aliphatic amines with isocyanates is thought to be due to their strong nucleophilic and autocatalytic properties. In some reactions, the second active hydrogen of a primary amine can react with another isocyanate.

The amine groups reacted with isocyanates are as follows. These include highly reactive aliphatic amines. Catalysis is not necessary for this reaction. Highly reactive aliphatic amines act as very fast chain extenders. The reactions occur very quickly due to the lack of significant steric hindrance.

Reaction with Water;

The reaction of isocyanates with water rapidly forms urea. This reaction is complex and involves the formation of an unstable intermediate.

Reaction with Urea;

The primary reason for the reaction between urea and isocyanate is the presence of active hydrogen on urea, making it more reactive. The reaction can proceed to form chains and branches.

Reaction with Urethane;

Urethane can be formed from the reaction of an alcohol and an isocyanate. The urethane contains an active hydrogen. This reaction leads to the formation of an allophanate.

How are Isocyanates Neutralized?

Isocyanates should be neutralized and disposed of by reacting them with certain substances to form less harmful compounds.

The compounds used for the neutralization of TDI are called a neutralization solution.

The reaction of TDI with water yields urea, a harmless compound. However, this reaction is exothermic. Below 50 °C, the reaction proceeds slowly. It is recommended not to increase the temperature further, as this could cause volatilization of TDI and lead to additional problems.

To ensure effective neutralization, a basic medium is created, leading to the formation of intermediates.

What are the Advantages of Using Toluene Diisocyanate in the Production of Polyurethane Materials?

It provides products with high strength, hardness, and insulation value. A wide range of products can be created by reacting it with different types of polyols. It is relatively low-cost.