Study On The Molecular Design, Synthesis And Properties Of High Performance RTM Resins Based On Benzoxazine

Resin Transfer Molding (RTM) has the potential to manufacture high quality, geometrically complex composite parts. RTM offers the advantages of relatively low tooling and processing equipment cost, short cycle times, and the ability to make high quality, more precision complex composite parts with high fiber content and low voids. The polybenzoxazines are a newly developed class of thermosetting resins. The oxazine ring of benzoxazine precursor can be opened thermally or catalytically to form a cross-linked network like a phenolic resin. The polybenzoxazines have various unique properties such as high mechanical strength, low water absorption, ease of processing, no volatile evolution, and near-zero volumetric shrinkage upon polymerization. Based on the specific requirements of RTM and the characteristics, several novel high performance benzoxazine resins were prepared and their process performance, polymerization reactions and polymer properties were investigated. The first chapter of this dissertation is the background of study on high performance RTM benzoxazine resins and literature review. The goal, significance and works should be done were proposed. Chapter 2 refers to the synthesis of benzoxazine monomers. According to the flexibility of molecular design of benzoxazine, four benzoxazine monomers were designed and synthesized. The four monomers were 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (S-BOZ) which synthesized from formaldehyde, phenol and amine, bifunctional benzoxazine (B-BOZ) which synthesized from aromatic diamine, phenol and formaldehyde, 3-(3-ethynylphenyl)-3,4-dihydro-2H-1,3-ben-zoxazine (EP-BOZ) which synthesized from formalde-hyde, phenol and 3-ethynylaniline, and 8-allyl-3-phenyl-3,4-dihydro-2H -1,3-benzoxazine (AP-BOZ) which synthesized from amine, formaldehyde and 2-allylphenol. The structures of these monomers were characterized by means of FTIR and 1H NMR. Chapter 3 concerned the design and choice of formulae of high performance RTM resins based benzoxazine. Take the B-BOZ as the basic component of resin, the process characteristics of the BB precursor were investigated by means of Brookfield viscometer and gelation time. It was soften and can be flowed when heated to temperature above 70℃, its initial viscosity at 100℃was 0.22 Pa·s and was only 0.33 Pa·s after 240 min. When temperature was in the range of 100 and 180℃, its viscosity was below 0.3 Pa·s. These results showed that the BB can be used as resin matrix for RTM technology. When BB mixed with reactive diluent S-BOZ by 6/4 (wt), the BS64 resin was obtained. The viscosity of BS64 was very low. It is concluded that the viscosity of resin can be modulated by adding reactive diluents. When 0.5% of catalyst was added to BB and BS64, we can get BBC05 and BS64C05 resins. The catalyst had little influence on the process properties of resin, but it can accelerate the ring-open reaction rate of benzoxazine. When BMI was reacted with allyl of AP-BOZ and obtained the low viscosity oligomer, then was mixed with BB, the BMI modified benzoxazine resins, BAPB121 and BAPB111 can be obtained, and the process property of resin can be adjusted by changing the ratio of AP-BOZ and BMI. When the BB was toughened with liquie rubber, the rubber content exerted a tremendous influence on resin viscosities. For the rubber had different reactivity with BB, the ATBN had more great effect on viscosity than CTBN. By adding liquid cycloaliphatic epoxy resin to the rubber modified BB resin, the low viscosity RTM resins can be obtained. Chapter 4 dealt with the curing reactions of several novel RTM benzoxazine resins. The curing behaviors of resins were studied by means of DSC, isothermal DSC and FTIR. The test results showed that the BB can be cured at 180℃, which conversion at 180℃for 5 hrs was 90%. If we want to get higher conversion, the resin should be post cured at elevated temperature, such as at 200℃for 60 min, the conversion was 95%. The curing reaction of BB was a autocatalyzed reaction, which activation energy was 110 kJ/mol, logA was 11.8 min-1, and reaction orders m and n were 1.84 and 1.33, respectively. When S-BOZ was added to BB, the ring-open reaction temperature become lower, and this phenomenon similar to the catalyzed BB resin, which initial reaction temperature was more lower. But the reaction stop temperatures were almost same.These results showed that catalyst and S-BOZ can accelerate the ring-open rate, but has little effect on polymerization reaction. The added components had a few of effect on the enthalpy of curing reaction. The allyl of AP-BOZ reacted with BMI when AP-BOZ and BMI were mixed and heated under 130℃for 30 min, and the self-polymerization peak at 349℃disappeared on DSC trace. The temperature of "ene"reaction of allyl and double bond of BMI was overlap with the temperature of polymerization reaction of benzoxazine. By changing the ratio of AP-BOZ and BMI, we can control the process properties of resins. When the pre-reacted mixtures were added to BB, the RTM resins with good process behavior based on BMI modified benzoxazine were obtained. When liquid rubbers were added to BB, their content had effect on the polymerization reaction, the more the rubber content, the lower of ring-open reaction temperature. The cyclo-aliphatic epoxy resin reacted at higher temperature; it should select suitable catalyst to catalyzed epoxy and made the epoxy react temperature range closed to the temperature of benzoxazine polymerization. Chapter 5 studied the properties of cured resins by means of TGA, DMA and INSTRON tester. The heat resistant temperature index was 233.57℃. The heat decomposition reaction was a 1st order reaction which the activation energy was 147.5 kJ/mol. BB cast had the good DMA and mechanical properties, which Eo′n set was 194.0℃, Tg ( Em′′ax) was 214.4℃, the tensile strength, modules and elongation at break were 93.6 MPa, 4.6 GPa and 2.2%, respectively, and flexural strength and modules were 160.1 MPa and 4.9 GPa, respectively. These results, except the elongation at break, are better than those of some commercial RTM epoxy resins (such as RTM6, which Tg is 183℃, the tensile strength, modules and elongation at break are 75 MPa, 2.89 GPa and 3.4%, respectively, and flexural strength and modules were 132 MPa and 3.30 GPa, respectively), and except the Tg, are better than those of some commercial RTM BMI resins (such as QY8911-4, which Tg is 230℃, the tensile strength, modules and elongation at break are 81 MPa, 4.5 GPa and 2.2%, respectively, and flexural strength and modules were 119 MPa and 4.2 GPa, respectively). When 4 parts of S-BOZ was added to 6 parts of BB, the Tg of BS64 was approximate 50℃lower than that of BB. The catalyst can reduce the curing time of BB, accelerated the curing rate, but the Tg of catalyzed resin BBC05 was approximate 15℃lower than BB. However, the diluent and catalyst components had little effect on the mechanical properties of resins. The BMI was reacted with allyl and was introduced into benzoxazine, andthe obtained BMI modified benzoxazine resins had good properties and can be used as the high performance RTM resin matrices. Lastly, the liquid rubbers had great influence on the resin properties. When the rubber content increased from 2% to 20% (wt) of BB, the modules of cured resins decreased from 3.79 GPa to 1.94 GPa ( Ei′n itial of DMA) for ATBN modified BB, for CTBN modified BB, the modules of cured resins decreased from 3.43 GPa to 2.42 GPa. Compared with the BB, the mechanical properties of rubber modified BB also decreased, but the tensile strength reached the maximum value when the 10% of ATBN and 10% of CTBN were added to BB resins, and then decreased dramatically as the rubber content increased. But the added liquid rubbers had little effect on the Tg of rubber-modified resins. The study results showed that the obtained benzoxazine resins had well integration properties, and can be used as high performance resin matrices for RTM process, and some of them are in practice for using. Chapter 6 investigated the morphology of fracture surfaces of rubber modified benzoxazine resins by SME. The results showed that ATBN can react with benzoxazine, for the rubber particle did not become large as the rubber content increased. But in CTBN modified resin, the rubber particle size enlarged obviously with the rubber content increased, this phenomenon indicated that CTBN had weak reaction with benzoxazine. When ATBN and CTBN content was 10% of BB, the morphologies were not similar to the fracture surfaces of other rubber content resin, the results showed that the resin reached an equilibrium point of rubber content, and the resins ruptured in a roughness mode, and rubber particles played a important role in the fracture. The cycloaliphatic epoxy resin could improve the miscible of rubber and resins. For the epoxy could react with rubber molecules, the rubber particles can disperse into resin matrix, and enlarged more slowly than resins without cycloaliphatic epoxy resin, the rubber particle sizes were littler than those resins without epoxy.