Development of new polybenzoxazine-coated cellulose strips for extreme acid–base indicator kit applications

A new bisphenol-BA/aliphatic amine-based hydrophobic polybenzoxazine-coated cellulose paper was developed for proximate pH indicator kit applications. Bisphenol-BA (BBA) was prepared using phenol and benzaldehyde in the presence of an acid catalyst. New BBA-based benzoxazine resins were prepared using bisphenol-BA, aliphatic amines (n-butyl amine (ba), n-hexyl amine (ha), 2-ethylhexyl amine (2-eha), dodecyl amine (dda), and octadecylamine (oda)) and paraformaldehyde. The structures of prepared bisphenol-BA and the corresponding benzoxazines were characterised using FTIR, ¹H NMR and mass spectroscopic techniques. The polymerisation temperatures of benzoxazine resins were characterised using DSC analysis. Ring-opening polymerisation of BBA-ba, BBA-ha, BBA-2eha, BBA-dda and BBA-oda resin was observed at 234, 236, 236, 240 and 243 °C, respectively. Polybenzoxazine-coated cellulose paper samples were prepared, and the hydrophobic behaviour was studied using a goniometer. The photophysical behaviour of BBA and the polybenzoxazines was studied using UV-visible spectroscopic techniques at different pH levels (acid–base). Furthermore, the prepared polybenzoxazine-coated cellulose paper was used in a pH indicator kit capable of detecting highly acidic solutions with pH values below zero, as well as strongly basic solutions in the pH range of 11 to 14. The polybenzoxazine-coated cellulose paper kit shows an orange colour at pH 0.0, dark orange at pH −1.0, light red at pH −1.5, and dark red at pH −1.8. The polybenzoxazine-coated cellulose paper kit shows a light violet colour at pH 11, dark violet at pH 12, light blue at pH 13 and dark blue at pH 14.