Synthesis And Properties Of Low-temperature Curable,Reprocessable,and High Heat-resistant Benzoxazine Resins

Benzoxazine resins is a new type of commercialized thermosetting phenolic resin.They have been widely used in printed circuit boards,aerospace,brake pads,and other fields due to their high heat resistance,good flame retardancy,low water absorption,and flexible molecular design.However,high curing temperature is a major problem for them.Many methods have been developed to reduce the curing temperatures of benzoxazine resins including the addition of catalysts or the synthesis of benzoxazine monomers containing catalytic groups.Although these methods decrease the curing temperatures of benzoxazine resins to some extent,low-temperature curing remains a challenge for them.In this dissertation,different from the traditional "one component" synthetic route,we proposed a "two component" synthetic route based on benzoxazine oligomers to achieve low-temperature curing of benzoxazine resins.Firstly,benzoxazine oligomers bearing reactive functional groups were synthesized from monofunctional benzoxazine monomers.Subsequently,the second component were added and the low-temperature curing of benzoxazine resins were achieved by using some low-temperature and high-efficiency reaction,such as the click reaction between thiol groups and double bonds,the reaction between hydroxyl groups and isocyanate groups.Furthermore,this method can also facilitate the introduction of other efficient reactions.It generates some dynamic covalent bonds or other heat-resistance structures in situ,imparting benzoxazine resins with reprocessability or further improve their heat resistance.The main contents and results of this dissertation are shown as follows.1.Benzoxazine resins via UV curingRoom-temperature fast-curing benzoxazine resin has been synthesized via the UV irradiated thiol-ene click reaction.Allyl-containing monofunctional benzoxazine monomer(ABZ)was synthesized using para-cresol,allylamine,and paraformaldehyde as raw materials.Allyl-containing benzoxazine oligomer(O-ABZ)was synthesized after the ring-opening polymerization of ABZ.Thiol-containing siloxane oligomer(O-TS)was synthesized by the co-hydrolysis and condensation of 3-mercaptopropylmethyldimethoxysilane with dimethyldimethoxysilane.The structures and molecular weights of ABZ,O-ABZ,and O-TS were characterized by 1H NMR,FT-IR,SEC,and MALDI-TOF,respectively.Subsequently,rapid cross-linking reaction occurred between O-ABZ and O-TS under UV irradiation.The curing kinetics of this reaction was studied by in situ FT-IR.The results showed that the curing reaction was nearly complete within 3 min of UV irradiation at room temperature.The thermal and mechanical properties of the cured resin can be further regulated by adjusting the content of thiol groups in O-TS.As the thiol content increased,their Tg,G" max and tensile strength increased from 47℃ and 27.5 MPa to 96 ℃ and 47.0 MPa,respectively.Meanwhile,their elongation at break decreased from 7.5%to 3.7%.The room temperature rapid curing technology developed in this work is expected to be applied in the field of low-temperature molding of benzoxazine resin.2.Low-temperature curable and reprocessable benzoxazine resinsThe low-temperature curable benzoxazine resin has been synthesized via the low-temperature and high-efficiency reaction between hydroxyl groups and isocyanate groups.Meanwhile,the urethane bonds formed in situ impart benzoxazine resin with reprocessability.Hydroxyl-containing monofunctional benzoxazine monomer(HBZ)was synthesized using cardanol,diethylene glycol amine,and paraformaldehyde as raw materials.Hydroxyl-containing benzoxazine oligomer(O-HBZ)was synthesized after the ring-opening polymerization of HBZ.In addition,polycaprolactone diol(PCL)was modified by isoflurone diisocyanate(IPDI),yielding isocyanate end-capped polycaprolactone(PCL-NCO).The structures and molecular weights of HBZ,O-HBZ,andPCL-NCO were characterized by 1H NMR,FT-IR,SEC,and MALDI-TOF,respectively.Subsequently,O-HBZ was crosslinked by PCL-NCO,giving rise to urethane-linked benzoxazine resin(C-PCL-HBZ)at low temperature.The dynamic characteristics of C-PCL-HBZ were verified by stress relaxation experiments,and the activation energy(Ea)of dynamic bond exchange was 98.27 kJ mol-1.On this basis,the self-healing,thermal reshaping,and reprocessing of C-PCL-HBZ at 150℃ were studied.After three reprocess,C-PCL-HBZ still maintained good mechanical performance,and the corresponding recovery ratios of tensile strength were 95.3,85.9 and 61.0%,respectively.The heat resistance decreased slightly and the thermal stability remained almost unchanged.Finally,the reprocessing mechanism of C-PCL-HBZ was confirmed by choosing polytetrahydrofuran diol(PTMO)as a control experiment.It was found that the reprocessing mechanism of C-PCL-HBZ was mainly due to the exchange of urethane bonds.This work provides a new route for the preparation of low-temperature curable and recyclable benzoxazine resin.3.High heat-resistant benzoxazine resins using amide bonds as crosslinkagesBenzoxazine resin crosslinked by amide bonds has been synthesized by the classical reaction between carboxyl groups and amino groups.Carboxyl-containing monofunctional benzoxazine monomer(CBZ)was synthesized using p-hydroxyl benzoic acid,aniline,and paraformaldehyde as raw materials.Carboxyl-containing benzoxazine oligomer(O-CBZ)was synthesized after the ring-opening polymerization of CBZ.The structures and molecular weights of CBZ and O-CBZ were characterized by 1H NMR,FT-IR,SEC,and MALDI-TOF,respectively.Cured benzoxazine resin C-HDA-CBZ was synthesized using aliphatic 1,6-hexanediamine as curing agent.The heat resistance and thermal stability of C-HDA-CBZ were better than commercial bisphenol A-aniline benzoxazine resin(C-BABZ).Finally,the effects of different aromatic and aliphatic diamines on the thermal properties of cured resin were studied.Among them,the C-pPDA-CBZ using para-phenylene diamine as curing agent has the best heat resistance and thermal stability.The corresponding Tg,tan δ,Td5,Td10 and char yield at 800℃ were 274℃,382 ℃,408 ℃and 59.9%,respectively,which were 97℃,68℃,75℃and 31.7%higher than those of C-BABZ.This work provides a simple and efficient method for preparing benzoxazine resin with high heat resistance and high char yield.4.High heat-resistant benzoxazine resins using imine bonds as crosslinkagesBenzoxazine resin crosslinked by imine bonds has been synthesized by the Schiff base condensation reactions between aldehyde groups and amino groups.Aldehyde-containing monofunctional benzoxazine monomer(FBZ)was synthesized using p-hydroxybenzaldehyde,aniline and paraformaldehyde as raw materials.Aldehyde-containing benzoxazine oligomer(O-FBZ)was synthesized after the ring-opening polymerization of FBZ.The structures and molecular weights of FBZ and O-FBZ were characterized by 1H NMR,FT-IR,SEC,and MALDI-TOF,respectively.Imine-linked benzoxazine resin(C-pPDA-FBZ)was synthesized using p-phenylenediamine as crosslinking agent.Its Tg,taδ,Td5,Td10 and char yield at 800℃were 340 ℃,377 ℃,418℃ and 63.6%,respectively,which were 163 ℃,63℃,85℃ and 35.4%higher than those of commercial bisphenol A-aniline benzoxazine resin(C-BABZ).On this basis,the effects of different diamine on the thermal properties of cured resin were studied.When 1,12-diaminododecane was used as curing agent,the Tg,tan δ of C-DDA-CBZ was up to 355℃.This work provides a simple and efficient method for preparing benzoxazine resin with high heat-resistance and high char yield.This work provides a simple and efficient method for preparing benzoxazine resin with excellent comprehensive thermal properties,and the resulting resin with high char yield is expected to be applied in the field of flame-retardant materials.5.Weldable,reprocessable,and water-resistant polybenzoxazine vitrimer crosslinked by imine bondsPolybenzoxazine vitrimer(C-FBZ)crosslinked by imine bonds has been synthesized using aldehyde-containing benzoxazine oligomer(O-FBZ)and 1,12-diaminododecane as raw materials.The dynamic characteristics of imine bonds impart C-FBZ with weldability and reprocessability,and meanwhile polybenzoxazine backbone significantly improved the water-resistance of imine bonds.After welding,the tensile strength of the welded C-FBZ was 76.7 MPa,and the tensile strength recovery ratio was 74.1%.After hot-pressing three times,the tensile strengths of the reprocessed C-FBZ were 81.3,70.8,and 58.1 MPa,and the corresponding tensile strength recovery ratios were 78.6%,68.4%,and 56.1%,respectively.In addition,the weight increase of C-FBZ was less than 1%after immersing in deionized water for 30 days at room temperature,and the tensile strength was 59.5 MPa with a tensile strength retention ratio of 57.5%.The heat-resistance of C-FBZ decreased slightly with the increase of hot-pressing times,yet it still retained a glass transition temperature(Tg,tan δ)of 150℃.This work provides a new member to the family of polymer vitrimer,and shows better comprehensive properties.