Photopolymerization behaviors of hyperbranched polyphosphonate acrylate and properties of the UV cured film

A novel hyperbranched polyphosphonate acrylate (HBPPA), used as a reactive-type flame retardant in UV curable systems, was successfully synthesized by the reaction of di(acryloyloxyethyl) benzenephosphonate (DABP) with N-(2-aminoethyl)-piperazine, and characterized by FTIR, 1H NMR and GPC measurements. HBPPA was blended with DABP as a monomer in different ratios to obtain a series of flame retardant resins. Their maximum photopolymerization rates (Rpmax) and final unsaturation conversion (Pf) in the cured films in the presence of a photofragmenting initiator were investigated. The results showed that the Pf increased along with HBPPA content and the pure HBPPA has the maximum value of 81.0% in the photo-DSC analysis. Their flame retardancy was monitored by the limiting oxygen index (LOI), and showed that the UV cured films greatly expanded when burning, and the degree of expansion increased along with HBPPA content. However, the LOI values varied from 36.0 to 39.0, which can be ascribed to the condensed phase mechanism. Their thermal degradation behaviors were investigated by thermogravimetric analysis and in situ FTIR spectroscopy, and showed that the phosphonate group of HBPPA first degraded to form poly(phosphoric acid)s at around 300 °C, which had a major contribution to form the compact char to protect the sample from further degradation. The dynamic mechanical thermal properties were studied by dynamic mechanical thermal analysis and showed a good miscibility between HBPPA and DABP. The crosslinking density and Tg of the cured films decreased along with the content of HBPPA in the blend.