Effect Of Several Catalysts On The Ring-opening Polymerization Of Benzoxazine And The Thermal Properties Of Polybenzoxazine

Benzoxazine is a kind of novel phenolic resin synthesized through the Mannich reaction of phenolic,primary amines and formaldehyde.Compared with traditional phenolic resins,polybenzoxazine not only has low water absorption,surface energy,dielectric constant and flexible molecular design but also possesses excellent thermal stability,mechanical properties and flame retardancy.However,it also has some shortcomings,such as high curing temperature,high brittleness and the thermal stability needing to be further improved.Therefore,how to reduce the ring-opening polymerization(ROP)temperature of benzoxazine has always been an important topic in this field.At present,there are two main methods of lowering the ROP temperature of benzoxazine.One is synthesizing benzoxazine with special functional groups by molecular-designing,and the other is adding catalyst into benzoxazine.Compared with the former,the addition of the catalysts is more convenient and practical.Moreover,Looking for a type of convenient,economic and green catalyst is always one of the main directions of decreasing the ROP temperature of benzoxazine.Therefore,4-dimethylaminopyridine(DMAP),ethanolamine,phosphotungstic acid(PWA)and phosphomolybdate(PMA)were selected as catalysts to reduce ROP temperature of benzoxazine,and their effects on the ROP process of benzoxazine and the thermal properties of polybenzoxazine were investigated in this paper.The main contents are as follows:Firstly,DMAP and ethanolamine were selected as catalysts,and DMAP,ethanolamine or their mixtures were added to phenol-aniline-based benzoxazine(Pa),respectively.Then,the effect of DMAP and ethanolamine on the ROP process of Pa was studied by differential scanning calorimetry(DSC),Fourier transform infrared spectroscopy(FTIR)and in situ FTIR.The results show that DMAP,ethanolamine and their mixtures could significantly reduce the initial and peak temperatures of the ROP of Pa,and the initial temperature could even be reduced to below 80~oC.The studies of kinetics show that the activation energy of Pa significantly decreased with the addition of DMAP and ethanolamine,but they hardly affected the chemical structures evolution during the ROP process of Pa and the structures of the final cured benzoxazine.Thermogravimetric analysis(TGA)results show that DMAP and ethanolamine hardly affected the thermal stability of the cured Pa.Therefore,DMAP and ethanolamine only change the ROP kinetics of benzoxazine and hardly affect the structure and properties of polybenzoxazine.In addition,the effects of DMAP and ethanolamine on the ROP of bisphenol A-aniline-based benzoxazine(Ba)and phenol-4,4'-diaminodiphenylmethane-based benzoxazine(Pmda)were also investigated.The results show that DMAP and ethanolamine could also decrease the ROP temperature of Ba and Pmda,but the thermal stability of polybenzoxazines decreased with the addition of DMAP and ethanolamine.Secondly,phosphotungstic acid(PWA)was chose as catalyst,and the mixures of PWA and Pa were prepared at room temperature.FTIR,~1HNMR,DSC and in situ FTIR were used to study the effect of PWA on the ROP of Pa.The results show that the initial and peak temperatures of ROP of Pa were significantly decreased with the introduction of PWA due to the interaction between hydrogen ions,tungsten atoms in PWA and O,N atoms in Pa,and the ROP of Pa could take place at room temperature.Furthermore,the curing kinetics of benzoxazine was studied by Kissinger and Ozawa methods.The results show that the activation energy of ROP of Pa decreased with the introduction of PWA,further proving that PWA could catalyst the ROP of benzoxazine.At the same time,PWA also changed the structure of the cured Pa,resulting in that there were more 1,4-disubstituted arylamine structure formation and the unstable aniline structures were introduced into the crosslinked network of the polymer.Besides,the GPC test of p-cresol-aniline-based benzoxazine shows that PWA could also increased the molecular weight of the polybenzoxazine,bringing about that the thermal stability of polybenzoxazines increased with the incorporation of PWA.Moreover,PWA also had a good catalytic effect on the ROP of Ba and Pmda,resulting in their ROP temperature effectively decreasing.In addition,the thermal stability of the cured Ba was improved with the incorporation of PWA,but PWA hardly had effect on the thermal stability of the cured Pmda.Finally,phosphomolybdate(PMA)was selected as the catalyst of ROP of benzoxazine,and the DSC and FTIR tests were performed to study the influence of PMA on the ROP of Pa,Ba and Pmda.The results show that the initial and peak temperatures of ROP of Pa,Ba and Pmda decreased significantly with the addition of PMA,and the mechanism of PMA catalyzing the ROP of benzoxazine was similar to that of PWA,which was initiated by H ions and heteroatoms.According to the curing kinetics study,the activation energy of ROP of Pa was reduced with the introduction of PMA,which further demonstrated the catalytic effect of PMA on the ROP of benzoxazine.TGA test results show that the thermal stability of the cured Pa,Ba and Pmda increased with the addition of PMA.