Bio-based Benzoxazine-Containing Polycyclosiloxanes as Flame-Retardant Adhesive Materials

The combination of polybenzoxazines and siloxane-based polymers has been exploited to merge the advantages of the two types of polymers via two main methods: (1) synthesizing polybenzoxazine-polysiloxane copolymeric structures and (2) introducing polysiloxane into a polybenzoxazine matrix. The polymer composites demonstrated enhanced mechanical, thermal, and hydrophobic properties. Herein, monofunctional furfurylamine-based benzoxazine (FBz) side groups were incorporated into polycyclosiloxane (pCS) to prepare prepolymers (pCS-FBzs) via a facile one-pot, two-step hydrosilylation reaction. In the polymer structure, the content of FBz side groups can be linearly controlled with different substitution ratios (from 21 to 65 mol %) by controlling the feeding ratios in the second step of the hydrosilylation reaction. Widely tunable T g values were obtained from −67.8 to −1.7 °C with increasing FBz content in the pCS-FBz prepolymer. Reaction efficiency was determined by using 1 H NMR and FT-IR measurements. The curing behaviors of the prepolymers were investigated using the DSC results. Both the Si–H group and the benzoxazine ring in the prepolymers can undergo thermal cross-linking at 210 and 243 °C, respectively. Interestingly, the two exothermic peaks shifted to lower temperatures in the pCS-FBz prepolymers and merged into a single peak as low as 156 °C for pCS-FBz 21 (21 mol % FBz content). The other exothermic peaks of each prepolymer were also demonstrated, and possible effects on reducing curing temperatures were discussed. The cured pCS-FBz polymers were found to have outstanding thermal stability ( T d5 > 420 °C, limiting oxygen index (LOI) > 45, and the char yield >70%) without the addition of any flame-retardant additives as well as strong lap shear adhesion. Results indicate that the pCS-FBz polymers are promising thermal noncombustible materials with high thermal and physical stability.