Phosphorus-containing cardanol-based phenolic resin cured epoxy thermosets for flame retardant and anti-corrosion coatings

Epoxy resin is one of the most important thermosetting resins at present, and it is widely used in various industrial fields because of its excellent properties. However, most of the hardeners used in traditional epoxy resins are derived from petroleum resources, which are accompanied by problems such as greenhouse gas emissions and environmental pollution. In addition, epoxy resin also has the disadvantage of being brittle. Therefore, it is of great significance to develop new environmentally friendly bio-based curing agents to toughen epoxy resins. Phosphorus-containing cardanol-based hardeners with adjustable functional groups were designed and synthesized. First, three kinds of cardanol-phenolic resins (P1C1F, P2C1F and P3C1F) with different molar ratios were synthesized by Mannich reaction using phenol, formaldehyde solution and cardanol as raw materials. Then, three kinds of phosphorus-containing cardanol-based hydroxyl type hardeners (P1C1F-0.6DCP, P2C1F-0.6DCP and P3C1F- 0.6DCP) were synthesized by partial substitution reaction with diphenyl chlorophosphate as a raw material. These three kinds of hardeners were used to cure DGEBA and compared with DGEBA/DDM. The tensile test results showed that the elongation at break of DGEBA/P1C1F-0.6DCP, DGEBA/P2C1F-0.6DCP and DGEBA/P3C1F-0.6DCP increased from 2.9% (DGEBA/DDM) to 54.5%, 45.9% and 27.1%, respectively. The impact test results further showed that the impact strength of DGEBA/P1C1F-0.6DCP, DGEBA/P2C1F-0.6DCP and DGEBA/P3C1F-0.6DCP were 42.2%, 106.9% and 91.2% higher than that of DGEBA/DDM, respectively. The results of vertical combustion and limiting oxygen index showed that DGEBA/P1C1F-0.6DCP, DGEBA/P2C1F-0.6DCP and DGEBA/P3C1F-0.6DCP passed the V-0 level and their LOI values were around 29%. In addition, compared with DGEBA/DDM coating, DGEBA/P3C1F-0.6DCP coating showed better corrosion resistance. Phosphorus-containing cardanol-based hardeners were one kind of promising components for the fabrication of epoxy coatings with excellent toughness and anti-corrosive performance.