How does prepolymerization affect the curing and properties of the thermosetting resins-benzoxazine resin as an example

Most polymers used in coatings are thermosetting resins because of their good adhension, protective capability, thermal and mechanical properties. Prepolymerization is a common procedure in the application of thermosetting resins, in order to adapt the specific processing condition, processing equipment or to achieve desired performance. However, the effects of prepolymerization on the curing behavior, chemical structure and properties of the resultant resins haven't been systematically studied. Among various thermosetting resins used as coatings, benzoxazine is a relatively new member, but it is being intensively studied due to its good processability, adhesive strength, low water absorption, as well as good anticorrosive property. Hence, in this work, 1,3benzoxazine, a kind of high -performance thermosetting resin, was used as an example to probe the influence of prepolymerization. The chemical structure variation was characterized, the curing activity and the rheological behavior were investigated, and the thermal, mechanical and dielectric properties of the resultant polymers were also studied. It was found that the content of benzoxazine groups decreased while the molecular weight and the melt viscosity increased with prelonged prepolymerization time, which may release more active phenolic hydroxyls that could promote the curing of benzoxazine, but also caused worse molecular moveability. The glass transition temperature, initial decomposition temperature and char yield of most polymers decreased consequently, with few exceptions. However, the mechanical property was improved, as suggested by higher tensile strength and elongation at break value after prepolymerization.