Study On Preparation And Characterization Of Heat-resistant Benzoxazine Resin For Resin Transfer Molding

Based on the specific requirements of resin transfer molding(RTM) process for matrix resin and the characteristics of polybenzoxazine,novel high performance matrix resins with low viscosity and improved thermal properties were prepared.The brief contents are shown as follows:1.Two Low viscosity benzoxazine monomers 3-allyl-3,4-dihydro-2H-1,3-benzoxazine(HBA) and 4,4'-bis(3,4-dihydro-2H-1,3-benzoxazin-3-yl)diphenol methane(BOZ) were synthesized from allylamine and aromatic diamine with phenol and formaldehyde.The chemical structures were identified by Fourier-transform infrared(FTIR) spectroscopy,¹H nuclear magnetic resonance(¹H NMR) spectroscopy and gel permeation chromatography(GPC).In order to meet requirement of RTM process and heat resistant resin,the two monomers were mixed at different ratio,and resin blends BA41,BA12,BA11,BA12,and BA14 were obtained.The curing reaction of benzoxazine monomer and resin blends was monitored by Differential scanning calorimetry(DSC),FTIR and Gel time.The relation between composition of resin blends and curing reaction was studied.And the result showed that the curing rate of the mixed resin increased with the rising concentration of HBA.The gel time of all resin is less than 20 min at 160℃,and less than 10 min at160℃.The process characteristics of benzoxazine monomers and resin blends were investigated by means of viscometer.The result shows that the viscosity of all resin blends did not increased a lot at the temperature below 80℃.The initial viscosity of BA14 is 121mPa·s and is only 160mPa·s after 300 min.The initial viscosity of BA41 is 345mPa·s and still less than 500mPa·s after 300 min.It means that all the resin blends can be used as matrix resin for RTM process.The thermal properties of obtained polybenzoxazines were evaluated by DSC,dynamic mechanical analyses(DMA),and thermogravimetrie analyses(TGA).The influence of the composition of the resin on the thermal properties of obtained polybenzoxazine was studied.It shows that the T_g of polybenzoxazine increased with the rising concentration of HBA.The T_g of polybenzoxazine BA14 is as high as 290℃.While the thermal stability of obtained polybenzoxazines showed different result.The polybenzoxazine BA21 showed highest 5%and 10%weight-loss temperatures,413℃and 514℃,respectively.The ultimate char yield was up to 79%at 800℃under N₂.2.Novel benzoxazine monomers containing two different functional groups were synthesized.3-allyl-3,4-dihydro-6-cayno-2H-1,3-benzoxazine(NBA) was synthesized from p-cyanophenol,allylamine and formaldehyde.3-allyl-3,4-dihydro-6-formyl-2H -1,3-benzoxazine(FBA) was synthesized from p-formylphenol,allylamine and formaldehyde.Their chemical structures were identified by FTIR,¹H NMR,and elemental analysis.The influence of different functional groups on curing reaction of these monomers was studied by DSC,FTIR and gel time.The results showed that incorporation of nitrile or formyl groups into benzoxazine structures effectively decreased the polymerization temperature of the monomer.While introduction of allyl groups into benzoxazine structures further decreased the curing temperature of the monomer The initial polymerization temperature of NBA and FBA were as low as 140℃and 149℃,respectively.The processing characteristics of benzoxazine monomers were investigated by means of viscometer.The result shows that the viscosity of novel benzoxazine monomers did not increasing a lot at the processing temperature.The initial viscosity of NBA at 90℃is 128 mPa·s and is only 185 mPa·s after 300 min,while the initial viscosity of FBA is 142 mPa·s and is only 174 mPa·s after 300 min,It means both of the two novel resin can be used as matrix resin for RTM process.The thermal properties of obtained polybenzoxazines were evaluated by DSC,dynamic mechanical analyses(DMA),and thermogravimetric analyses (TGA).The polybenzoxazine prepared from NBA exhibited the T_g at 297℃,the 5 and 10%weight-loss temperatures at 340 and 370℃,respectively.The ultimate char yields under N₂ were up to 57%.As for FBA,the figure was 266℃,345℃,393℃and 61%.