Preparation And Properties Of Bio-benzoxazines Based On Renewable Furan-containing Diamine

Polybenzoxazine,a new type of thermosetting resin,has been widely concerned because of its characteristics of product dimension stability,low water absorption,high glass transition temperature and zero volume shrinkage during polymerization.With the depletion of fossil resources and the increasingly serious environmental problems,benzoxazine resin also faces the challenges of feedstocks selection and sustainability of produce.Therefore,the preparation of biobenzoxazine resin from renewable materials through green process has become the research frontiers in this field.In this paper,fully biomass-based benzoxazine monomers and main-chain benzoxazine precursor were prepared from renewable furfuryl diamine,eugenol,cardanol and salicylaldehyde under solventless condition.The curing kinetics and the properties of the corresponding polymers were studied.Propane-2,2’-diyldi（furan-5,2-diyl）dimethanamine（PDFA）,a furan-containing bio-diamine was first prepared from renewable furfurylamine and acetone catalyzed by concentrated hydrochloric acid.Two fully bio-based benzoxazine monomers,BBOZ-Ca（derived from PDFA/Cardanol）and BBOZ-Eu（derived from PDFA/Eugenol）,were synthesized under a solvent-free condition.The chemical structures of two bio-based benzoxazine monomers were characterized by Fourier Transform infrared spectroscopy（FT-IR）,nuclear magnetic resonance spectroscopy（NMR）and high-resolution mass spectroscopy（MS）,the results indicated that these two monomers were successfully synthesized.The curing kinetics of BBOZ-Ca and BBOZ-Eu were studied by differential scanning calorimeter（DSC）and the results showed that the two monomers were cured through two stage curing recations.The activation energies of BBOZ-Ca and BBOZ-Eu were calculated by Kissinger method,Ozawa method and Flynn-Wall-Ozawa method,respectively,and the curing mechanism were autocatalytic reaction deduced by Friedman method.The curing behavior of BBOZ-Ca and BBOZ-Eu was also studied by variable temperature FT-IR.With the increase of temperature,the characteristic peaks of oxazine ring and furan were weakened gradually.It was further proved that furan groups also participated in the polymerization of benzoxazine,which was consistent with the conclusion of DSC.Meanwhile,the double bonds on the long alkyl chain of BBOZ-Ca and the allyl groups of BBOZ-Eu were also underwent polymerization under curing conditions.The thermal stability of the two crosslinked polymers was characterized by thermogravimetric analyzer（TGA）.The results showed that the cured BBOZ-Ca had excellent thermal stability,the T_maxof 240℃-cured sample was 434.5℃,and the char yield at 800℃was 28.8%.The T_maxof cured BBOZ-Eu sample was 390.1℃,and the char yield at 800℃was 65.4%.Copolymerization with BBOZ-Eu can significantly improve the thermal stability of cured BBOZ-Ca.The apparent morphology and hydrophilicity of P（BBOZ-Ca）,P（BBOZ-Eu）and copolymer were characterized by SEM and contact angle tester.By taking three-steps Schiff base avenue,a fully biomass-based main-chain benzoxazine precursor（PBOZ-FS）was prepared from PDFA and biomass-based dialdehyde of 3,3’-methylenebis（6-hydroxybenzaldehyde）（MBS）,which was obtained from the renewable salicylaldehyde.The chemical structure and the number-average molecular weight of PSB-FS、PRSB-FS and PBOZ-FS were characterized by FT-IR、NMR and gel permeation chromatography（GPC）.DSC results showed that both PSB-FS、PRSB-FS and PBOZ-FS were have exothermic peaks after 250℃.The curing kinetics of PBOZ-FS was studied by non-isothermal DSC.It was found that PBOZ-FS had a shoulder peak after 270℃.The activation energy of PBOZ-FS was calculated by Kissinger method,Ozawa method and Flynn-Wall-Ozawa method,and its curing mechanism was verified as autocatalytic reaction by Friedman method.The curing behavior of PBOZ-FS was studied by variable temperature infrared spectroscopy.The results showed that the oxazine ring began to polymerize at 180℃,and with the temperature increasing,the related characteristic peaks of furan also decreased,indicating that furan participated in the polymerization of PBOZ-FS.Based on the results of DSC and variable temperature infrared,a curing mode different from furfurylamine-based benzoxazine was proposed.TGA results showed that the cured PBOZ-FS had good thermal stability,its T_5%was 370.7℃,and the carbon yield at 800℃up to 69.7%after curing 240℃.The surface properties of the polymer films were studied by the contact angle test and SEM,indicating that the polybenzoxazine films with smooth surface could be obtained by controlling the curing process.