In the frame of this project, click polymerization method was used to synthesis a novel unsaturated polyester (UP) as a polymer matrix for the new tailored foaming composites. However, this attempt was failed, then a novel benzoxazine vitrimer (BO) resin was chosed to replace the UP resin. As a result, a novel tailored and recyclable foaming composites based on the BO resin was successfully prepared, which can be used as the core of sandwich composites. As a result, a lightweight BO vitrimer based foaming composites was prepared. It is bendable due to the introduction of S-S dynamic bond, which is crucial for preparing curved sandwich composites. The molding process of sandwich composites is less limited by the foam core as well. Furthermore, the BO foam can be reshaped into a curved structure under high temperature, and the curved structure can be fixed after cooling down, which based on the reversible generation-break principle of BO vitrimer dynamic S-S bonds.
The structure of foaming core can be tailored to improve the mechanical properties and fire retardant performance of foaming core and sandwich composites. The compressive strength of BO foam is lower than expected when it composed of 4 layer foaming sheets. With the increase of foaming layers, both density and compressive stress were increased. It shows that the compression stress of BO foam with 5 layers is in the same level with commercial plastic foam. This result suggests that the tailored structure of BO vitrimer foam brings good designability of the foam and the final sandwich composites. Furthermore, intumescent flame retardant was used to improve its fire performance as well.
After many attempts we found that the UP resins is not suitable for the two-step foaming process and it wasted almost 9 months of our time. Fortunately lots of experiences in foaming process were obtained and finally we found the right polymer matrix that the vitrimer with dynamic bond can be used to support the two-step foaming process. Moreover, we designed a benzoxazine vitrimer that was good match with our requirements in flame retardant performance and good recyclability. We tried our best to complete the preliminary exploratory work and prepare a prototype. Most of the major parameters of the novel recyclable and bendable foaming composites were investigated and an EU patent was submitted in July 2022. As a result, some papers are not yet published, but are planned to be submitted within 2022.
