Improving ablation-resistant properties of novolac epoxy coatings via boric acid or POSS-modified expandable graphite with a PDA bridge

Inspired by the fact that nanostructured expandable graphite (denoted as EG), when functionalized with silicon (Si) and boron (B) elements, can significantly enhance thermal insulation performance and improve the strength of char residue layers, the study controllably synthesizes EG supported with boron acid (H 3 BO 3 ) or polyhedral oligomeric silsesquioxane (POSS), using polydopamine (PDA) as a bridging agent. The resulting composites possess specific particle sizes of 80–90 nm and 100–120 nm for H 3 BO 3 -PDA-EG and POSS-PDA-EG, respectively. By formulating ablative-resistant coatings incorporating these composites, significant improvements are achieved in the mechanical properties of the resulting ceramic layers formed during ablation compared to control formulation. The compressive strength of the char residue layer is 0.13 MPa for EG alone, 8.85 MPa for H 3 BO 3 -PDA-EG, and 12.86 MPa for POSS-PDA-EG. Additionally, it is observed that the back surface of the coating with POSS-PDA-EG is approximately 180 ℃, while the back surface of the coating with H 3 BO 3 -PDA-EG is approximately 300 ℃ in a thermal test by a FD-04D/E Wind Tunnel, and compared to the coating with H 3 BO 3 -PDA-EG, the coating with POSS-PDA-EG yields more uniformly expanded and dense residue, which indicated that the coating with POSS-PDA-EG is characterized by better heat insulation and heat-resistant parameters. This is attributed to the improved silicon-boron coordination effect facilitated by the presence of POSS-PDA-EG and boron oxide, promoting ceramization. The resulting graphitized carbon layer, formed through fusion with porous expanded graphite at high temperatures, effectively resists high heat flow erosion over an extended duration. This work introduces an effective approach for enhancing ablation resistance by synergistically catalyzing boric acid or POSS-modified EG with a PDA bridge. The initial decomposition temperature was increased from 189 to 223 ℃, the maximum decomposition temperature was increased from 282 to 287℃, and the residual weight was increased from 51.6 to 63.6%. This can be concluded that the mixture of epoxy resin with either H 3 BO 3 -PDA-EG or POSS-PDA-EG demonstrates outstanding thermal resistance. The utilization of these novel composites and the insights gained from their performance contribute to advancements in the field of ablation-resistant coatings.