Preparation Of Carbon Fiber Composite Material With Integrated Functional Structure In Low-temperature Forming By "Combustion Method"

Carbon fiber reinforced resin matrix composites have the highest specific strength and specific modulus,and have broad application and development prospects.It is of great significance to prepare new and higher-performance composite materials and to study the relationship between the structure and performance of carbon fiber composite materials.In this paper,benzoxazine with excellent comprehensive properties is selected as the matrix resin,and the modification method and preparation process of benzoxazine carbon fiber composite materials are explored,and the material structure is optimized through modification to achieve the purpose of improving material performance.(1)Benzoxazine is a new type of thermosetting resin with excellent thermodynamic properties,stable curing dimensions and good weather resistance.However,its high curing temperature and high brittleness of the cured product severely restrict its development and application.In this paper,a curcumin-based polyurethane was synthesized and blended with benzoxazine resin as a toughening modifier.By adjusting the molecular weight and content of polyurethane,the effect on the thermodynamic properties of the resin matrix was studied.The study found that the flexural strength of the blended system was increased by 22.1%,and the impact strength was increased by 64.6%.Thermal stability decreased slightly(10%thermal decomposition temperature decreased by 3?7?),glass transition temperature increased slightly(increased by 11?),two glass transitions appeared at 3%polyurethane addition Temperature,and micro-phase separation structure was observed under the scanning electron microscope.The improvement of thermodynamic properties is attributed to the reaction between the polyurethane terminal isocyanate group and the benzoxazine,the hydrogen bond formed by the curcumin dione structure and the formation of the microphase separation structure.In addition,in order to further reduce the curing temperature of the blending system,indium nitrate was added as a catalyst to the blending system,although the addition of a small amount(0.6%)significantly reduced the curing temperature(by 14.5%compared to pure BA-a),but the degradation of thermodynamic performance is also obvious.Reducing the curing temperature and improving the mechanical properties cannot be balanced in this system.This is because the bound water in the indium nitrate reacts with the polyurethane to extend the chain,which not only destroys the original terminal isocyanate groups,but also increases the molecular weight.It causes the polyurethane to be free from the crosslinking network and hinders the benzoxazine crosslinking.(2)In order to further study the high-efficiency catalytic effect of indium nitrate on the blending system.The nitrate-based combustion synthesis system is used in the curing process of the resin.On the one hand,the self-exothermic heat of combustion synthesis is used to promote the curing of benzoxazine,and on the other hand,combustion products are generated in-situ to strengthen the resin matrix.By blending indium nitrate(oxidant)and acetylacetone("fuel")with benzoxazine,the curing temperature of the modified resin obtained is significantly reduced,the minimum curing initiation temperature is as low as 100°C,and the curing exothermic enthalpy is also significantly reduced.Using DSC and FTIR to study the curing mechanism,it is found that the curing process is divided into two steps:before the combustion reaction,In3+catalyzes the benzoxazine to generate more aromatic amine Mannich bridge structures;after reaching the combustion reaction temperature(about 150?),The reaction releases a large amount of heat to promote the thermal curing of benzoxazine to generate more phenolic Mannich bridge structures,and at the same time generate combustion product In₂O₃in situ.As more arylamine Mannich structures and In₂O₃ particles are formed during the curing process,the thermal stability and mechanical properties of the modified resin are improved.The coatings prepared with modified resins were used to evaluate the impact resistance,hardness and adhesion of up to 6.86 J,6H and 5B,respectively.The carbon residue rate at 800?increased by 25.0%at the maximum.(3)Preliminary research on the processing performance of the above two systems.Among them,the viscosity of polyurethane-modified benzoxazine is below 1000 Pa·s,and the processing window temperature is similar to that of benzoxazine(about 80??220?).In addition,the gel time of BA-a at 180?is about 35 min,while the gel time of B-PU2-3 and B-PU2-5 is about 40 min and 50 min.When the polyurethane content is low(<10%),the blending system maintains good processing performance.The B-PU2-3 system with the best modification effect is used to prepare carbon fiber composite materials.Compared with the composite materials prepared by pure BA-a,the flexural strength is increased by 12.3%,and the flexural modulus is slightly decreased.On the other hand,the"combustion method"modified benzoxazine resin system has a higher viscosity.Among them,the softening point temperature of B-In-A-5 and B-In-A-10 is between 80°C and 150°C,and the viscosity rises rapidly with the increase of indium nitrate and acetylacetone.However,using the"combustion method"modified benzoxazine resin system as a carbon fiber sizing agent not only avoids the disadvantage of its high viscosity and the solvent is not easy to remove,but also exerts its advantages as a coating with high strength and high adhesion.The use of sizing agent improves the flexural performance of the pure BA-a composite by 9.7%,and the flexural modulus is also improved.The flexural performance of the modified polyurethane resin after the sizing agent treatment was further improved by 4.5%,and the flexural modulus was increased by 10.1%.The coating state of the carbon fiber sizing agent was observed by SEM,and the reinforcing effect of the carbon fiber sizing agent was discussed by comparing the surface morphology of the cross-section fibers of the composite material.