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.

Compound Structure Reactivity
Primary aliphatic amine R-NH2 100,000
Secondary aliphatic amine R2-NH 20,000 - 50,000
Primary aromatic amine Ar-NH2 200 - 300
Primary hydroxyl R-CH2-OH 100
Water H-OH 100
Carboxylic acid RC(O)-OH 40
Secondary hydroxyl R2CH-OH 30
Ureas RNH-C(O)-NHR 15
Tertiary hydroxyl R3C-OH 0.5
Urethane RNH-C(O)-OR 0.3
Primary amide R-C(O)-NH2 0.1
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
1° Amine MACM <10 15 18
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.

Dytek® HMD

Dytek® A

Dytek® EP

Dytek® DCH-99