Carbodiimide is a functional group represented by -N=C=N- and compounds having carbodiimide groups are used in various applications thanks to their excellent reactivity. For polyester and other general-purpose resins and biodegradable resins such as polylactic acid, hydrolysis is a big issue. Carbodiimide compounds are known to have an effect of suppression by supplementing acid in the polymer which is a catalyst for decomposition reaction.
On the other hand, when adding conventional carbodiimide compounds to polymers, melt-kneading them, or processing them by re-melting the polymers, isocyanate gas known as a substance with high irritation and sensitization is generated. As a result, it is necessary to take measures against the work environment such as exhaust equipment of this gas, so carbodiimide was sometimes refrained from use.

Capture of carboxylic acid end group by carbodiimide

Teijin challenged to develop carbodiimide compounds that do not fundamentally generate isocyanate gas, in response to technical challenges of improving the hydrolysis resistance of polymers and handling workability. With the repeated synthesis and evaluation of various carbodiimide compounds, the idea of introducing carbodiimide unit into a cyclic molecule has come up. This means that the isocyanate formed by the reaction with the polymer remains in the polymer through the structure that constitutes the ring, so that gasification of the isocyanate is suppressed.

Capture of carboxylic acid end group by cyclic carbodiimide

As a result of investigations, we have developed a cyclic carbodiimide that can be stably added to various resins by introducing carbodiimide group into cyclic structure with high thermal stability. As a result of actually adding this cyclic carbodiimide into polylactic acid, it was found that hydrolysis resistance can be greatly improved and its effect is higher than that of conventional carbodiimide compound. (see left graph)
Furthermore, the generation of isocyanate gas in the melt state of the polymer to which the carbodiimide compound was added was investigated, and it was found that isocyanate gas was not detected in the cyclic carbodiimide system developed. (see right graph)

Hydorolysis of PLLA

GC-MS analysis of the carbodiimide-added PLLA

The cyclic carbodiimide developed by Teijin has been actively developing markets since 2018. First, cyclic carbodiimide was introduced as a hydrolysis agent for polyester and polylactic acid. Since then, it became clear that specific effects such as improvement of heat resistance and compatibility are improved in other various polymers. Teijin's cyclic carbodiimide is expected to become a useful material for improving polymer properties in a wide range of applications in the future.

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