What are isocyanates?

Isocyanates are chemical compounds that contain a specific functional group referred to as the isocyanate group. Isocyanates are highly reactive compounds and are used in various industrial applications. A common representative of this is methylene diphenyl isocyanate (MDI) in reactive PUR hot-melt adhesives.

The chemical structure

The chemical structure of an isocyanate consists of a nitrogen atom bonded to a carbon atom and an oxygen atom. The carbon atom is connected to a further oxygen atom via a double bond. The general structure formula can be represented as follows:  


Here, „R“ stands for an organic residue or an organic group that is bonded to the nitrogen atom. For this reason, isocyanates are often referred to as NCO.

Production of PUR hotmelts

Polyurethane hot-melt adhesives are produced by polyaddition, in which the polyurethane resins are formulated from polyols and polyester resins or their mixtures with an excess of diisocyanates. The polymer chains therefore consistently have terminal, reactive isocyanate groups that are available for subsequent cross-linking/curing using atmospheric moisture. The majority of the isocyanates are bound to the polyols / polyesters; free diisocyanate components remain(which are not polymer-bound. According to the hazardous substances regulations, their content leads to the corresponding classifications:

The concentration of these free diisocyanate components is crucial for the classification and labelling of the products in accordance with the hazardous substances regulations. There are three main categories:

  1. Classic PUR adhesives: These contain more than 1.0 % and less than 5.0 % free diisocyanate and are subject to labelling. They can cause allergic skin reactions, breathing difficulties and are suspected of being carcinogenic. (P and H phrases)
  2. LE (Low Emission): These contain between 0.1 % and 1.0 % free diisocyanate and also require labelling. They can also cause breathing difficulties and allergic reactions.  The H351 label (can cause cancer) is no longer applicable here.
  3. ME (Micro Emission): These contain less than 0.1 % free diisocyanate and therefore do not require labelling.

It is important that employees working with reactive PUR hot-melt adhesives are properly trained and understand the required safety precautions. Training is mandatory, especially for classic and LE products. However, since 24th August 2023, the training requirement for ME products has been lifted in accordance with REACH.

It is crucial to emphasise that reactive hot-melt adhesives containing isocyanates can be used safely as long as appropriate safety precautions are followed and the products are handled in accordance with regulations. Proper training and understanding of potential risks are the key to handling these materials safely.  

The training courses are offered by FEICA, among others. We will be happy to provide you with an information flyer for download.

Tim Charzinski

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