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Environmental fate & pathways

Hydrolysis

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Description of key information

 Hydrolysis of 1,5-naphthylene diisocyanate results in formation of the main hydrolysis products 1,5-naphthylene diamine and carbon dioxide. Half-life of the parent compound 1,5-naphthylene diisocyanate is less than 1 hour at pH values between 4 and 9 under ambient conditions (Bayer Industry Services, 2006b).

Key value for chemical safety assessment

Half-life for hydrolysis:
1 h
at the temperature of:
25 °C

Additional information

The study on hydrolysis of 1,5-naphthylene diisocyanate which was performed in accordance with OECD 111 reported of rapid hydrolysis (T1/2 < 1 hour) with mainly the formation of 1,5-naphthylene diamine and carbon dioxide. The test item had been stirred in the study which allowed dispersion of particles in the water phase and by this increased the surface which may be subject to hydrolysis compared to conditions without stirring. The formation of amines is commonly known for isocyanates under such laboratory test conditions and in particular when the test item is treated by ultra turrax which results in extremely fine dispersed test item with huge reaction surface and small particle volume. Under such test conditions the formation of amines is assumed to be dominant.

However, it is also commonly known that amines being formed from hydrolysis reactions of isocyanates are not the final reaction product. In fact amines use to react with isocyanates spontaneously in a secondary exothermal reaction (already at room temperature) resulting in the formation of poly-urea. This process was e.g. described by Meier-Westhues (Polyurethane – Lacke, Kleb- und Dichtstoffe, Vincentz Network, 2007, ISBN 3-86630-896-5).

Taking into account the laboratory test conditions with artificial stirring and particle dispersion which does not reflect environmentally relevant conditions the above mentioned allows to interpret the study results of 1,5-naphthylene diisocyanate in a specific light. This test item was treated by stirring which preferably leads to the formation of small particles with huge surfaces. Hydrolysis takes only place at the particle surface and 1,5-naphthylene diamine as well as carbon dioxide are formed. The smaller the particles due to the stirring process the less likely that the amine may spontaneously react with 1,5-naphthylene diisocyanate as less of the latter one is available inside the particle.

Under environmentally more relevant conditions, however, clearly lower or even no “stirring” effect is to be expected and isocyanate particles remain in original size. The same hydrolytical reaction as described above would be expected at the particle surface resulting in the formation of 1,5-naphthylene diamine. However, in a second step the amine will spontaneously and exothermically react with 1,5-naphthylene diisocyanate isocyanate groups forming poly-urea. Poly-urea is a chemically inert substance which is not classified (no H or P phrases according to Regulation (EC) No 1272/2008).

Heimbach, Jäger and Sporenberg (Ecotoxicol Environ Saf. 1996 Mar; 33(2):143-53) investigated on fate and biological effects of polymeric 4,4’-diphenylmethanediamine (MDI) - another isocyanate - in small artificial ponds (mesocosm). They reported of the formation of carbon dioxide (released as bubbles) which fits to the theoretical chemical reaction expected and described above. Furthermore, they reported of the formation of poly-urea from the isocyanate. Thus, this study supports the more theoretical assumption on commonly known chemical reactions of isocyanates.

Another supporting aspect is that the reaction of amines with isocyanates under formation of poly-urea is rapid enough to cause problems for the use of isocyanates in varnish applications as e.g. reported by Müller and Poth (Lackformulierung und Lackrezeptur, Vincentz Network, 2005, ISBN 3-87870-170-5).

Concluding, based on theory of isocyanate chemical reaction, mesocosm studies as well as practical experience with vanisher applications the formation of amines (i.e. 1,5-naphthylene diamine) is likely but should be understood as an intermediate reaction. The final reaction product, in particular under environmentally relevant conditions, is the chemically inert poly-urea. Any evaluation of isocyanates, i.e. of 1,5-naphthylene diisocyanate, as well as classification and regulatory decisions in general should be based on those findings.