By far the most researched class of materials with self-healing ability are polymers and polymer composites. Due to the high consumption of fossil fuels, numerous studies are focused on the synthesis of polymers from renewable resources in order to replace fossil fuel based polymers. Benzoxazines can be synthesized by reaction between phenol, amine and paraformaldehyde. Polymers with self-healing ability are cross-linked systems. The network of such material must be flexible enough to carry out self-healing reactions and must have suitable functional groups.
In experimental work, two benzoxazine monomers were synthesized. The first one based on diphenolic acid, stearylamine and paraformaldehyde (DK/SA-BOX) and the second one based on diphenolic acid, Jeffamine D-2000 and paraformaldehyde (DK/JF2000-BOX). Copolymers were prepaired by ring-opening polymerization at high temperatures in five different weight ratios, to study the self-healing ability of copolymers from bio-based resources. Their chemical structure and synthesis success were confirmed with nuclear magnetic resonance (NMR). On both pure benzoxazine monomers spectrum were visible peaks which confirmed the formation of the oxazine ring. Thermal properties of pure benzoxazines and copolymers were obtained by differential dynamic calorimetry (DSC) and thermo-mechanical properties by dynamic mechanical analysis (DMA). For each copolymer, the glass transition temperature, Tg, was determined using the STAR program. Self-healing of the copolymers took place at a temperature of Tg + 10 °C. The effectiveness of self-healing has been successfully demonstrated by tensile fracture toughness tests. Each copolymer and its parallel was broken four times and self-healing took place three times. The highest self-healing efficiency was expected to be achieved by a copolymer containing 50 wt. % monomer DK/SA-BOX in 50 wt. % monomer DK/JF2000-BOX (sample 50:50).
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