Curatives for epoxy and isocyanate-reactive systems
Amines and other reactive N-H compounds are widely used as curatives, chain extenders, and crosslinkers for epoxy, isocyanate, and other reactive polymer systems. They present the chemist and formulator with virtually limitless opportunities for customization of reactivity profiles and final polymer properties by varying the number, type, and position of reactive N-H groups.
Reactivity Toward Isocyanate
Reactivity can be controlled over six orders of magnitude by modifying the structure of the amine, using blocking techniques, or through electronic effects.
|Primary aliphatic amine||R-NH2||100,000|
|Secondary aliphatic amine||R2-NH||20,000 - 50,000|
|Primary aromatic amine||Ar-NH2||200 - 300|
|Source: Dow “Flexible PU foams” book|
Primary aliphatic amines tend to be the most reactive toward epoxy or isocyanate groups, and unhindered primary aliphatic amine groups such as those in 1,6-hexanediamine show the highest reactivity. Introducing a methyl branch, even one carbon removed from the amine carbon as in Dytek® A amine, reduces reactivity of that amine by a factor of 2-3 compared to the less hindered amine end. A methyl branch even closer to the amine noticeably reduces reactivity even further.
Primary aromatic amines have very desirable reactivity rates but are typically limited to applications that do not need photochemical stability.
Secondary aliphatic amines offer reactivity over a very useful range in cases where primary aliphatic amines are too fast and aromatic amines are unsuitable or undesirable.
Reactivity toward TMXDI (seconds) (m-tetramethylxylene diisocyanate)
|Pot life||Gel time||Tack-free|
|1° Amine||Dytek® A||<10||15||<6|
|1° Amine||Dytek® DCH-99||<10||15||<6|
|2° Amine||Clearlink® 1000||180||320||12|
|2° Amine||Clearlink® 3000||1700||4000||>900|
|1° Aromatic Amine||DETDA||2800||6900||>900|
|1° Aromatic Amine||Unilink® 4200||3000||30000||>900|
Reductive alkylation of Dytek® Amines creates opportunities for curatives with new reactivity profiles and physical properties.