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| Five commercially available amines: m-phenylenediamine (A1), m-xylylenediamine (A2), isophorone diamine (A3), trimethylhexamethylenediamine (A4), and 4,4'-diaminodiphenyl sulfone (A5), were examined as a nucleophilic hardener for bis-benzoxazine monomers based on aniline paired with bisphenol-A (BA-a) or bisphenol-F (BF-a). The reactivity and reaction mechanism of their reactive mixture with BA-a were investigated using FTIR and NMR spectroscopy, DSC, and HPLC techniques. It is found that BA-a cured with amines is much faster than the ring-opening polymerization of bulk BA-a upon heating at 120℃ or 150℃. The possible reaction mechanism was supported by the experimental results and includes three successive steps: (i) nucleophilic substitution at the carbon atom (O-C-N) in the oxazine ring by the amine, (ii) thermal decomposition of the resulting aminomethanaminium structure, and (iii) electrophilic addition of the newly formed iminium ion with the aromatic ring to form stable aminomethylphenol structures. These findings are helpful to improve thermosetting resins in terms of their chemical structure, material properties, and processability. |
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| Keywords:materials science; benzoxazine; polycondensation; thermosetting resin |
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