Synthesis and curing of amino-containing benzoxazine as a novel hardener for diglycidyl ether of bisphenol-A

Benzoxazines containing various additional functional groups have been extensively reported to improve the properties of polybenzoxazines. In this work, a novel amino-containing benzoxazine (PDETDA-NH2) was conveniently synthesized from diethyltoluenediamine (DETDA), 2-hydroxybenzaldehyde, and paraformaldehyde and was used as a hardener for diglycidyl ether of bisphenol-A (DGEBA). The curing behaviors of PDETDA-NH2 and PDETDA-NH2/DGEBA systems were studied by DSC, FT-IR, and H-1 NMR. When curing, PDETDA-NH2 was firstly polymerized to N,O-acetal-type polymer and then rearranged to Mannich-type polymer at elevated temperature, while the addition reaction between amino and benzoxazine was discouraged because of the steric hindrance of alkyl substituents. During PDETDA-NH2/DGEBA curing, it was found that the reactions happened in the order of addition polymerization of amino and epoxide, ring-opening polymerization of benzoxazine, etherification between phenolic hydroxyl of the polymerized benzoxazine, and epoxide. Compared with DETDA cured DGEBA, PDETDA-NH2 cured DGEBA showed higher modulus, higher char yield, and much lower water uptake.