Boron carbide and phthalonitrile synergistically modified phenolic resin as well as preparation method and application thereof

Abstract

The invention discloses boron carbide and phthalonitrile synergistically modified phenolic resin as well as a preparation method and application thereof. The preparation method comprises the following steps: dissolving a biphenyl bisphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, heating, stirring and dissolving; adding phenolic resin and boron carbide into the cooled biphenyl bisphthalonitrile prepolymer solution according to a mass ratio, and mixing under a vacuum condition to obtain a blend; and injecting the blend into a mold, carrying out stepped curing, and cooling to room temperature to obtain the modified phenolic resin. The modified phenolic resin has higher cracking temperature and carbon residue rate and also has excellent mechanical properties, and the phenolic resin can catalyze the curing of phthalonitrile, lower the curing temperature, improve the process properties and further optimize the heat resistance and processability of the material, and can meet the requirements of thermal protection of the solid rocket engine jet pipe.

Description

Technical Field

[0001] The invention relates to the field of thermal protection of solid rocket engine nozzles, and in particular to boron carbide and phthalonitrile synergistically modified phenolic resin and a preparation method and application thereof. Background Art

[0002] At present, the main power unit in the missile and aerospace fields is the solid rocket engine. The solid rocket engine has a simple structure and is characterized by mobility, reliability, and strong survivability. It is widely used in both military and civilian aerospace fields. With the rapid development of new aerospace vehicles, the reentry speed is getting higher and higher, and the thermal environment faced by the nozzle of the solid rocket engine is becoming more and more severe: the aerodynamic heating amount has increased significantly, large areas are facing high temperatures, and the instantaneous temperature of key parts can reach more than 3000℃. In order to improve the performance of solid rocket engines, high-performance, high-efficiency, and powerful advanced materials must be selected to withstand the effects of various loads under high temperature, high pressure, high speed, and special chemical atmospheres. This not only brings difficulties to structural design, but also puts forward stringent requirements on material engineering. Thermosetting phenolic resin has excellent mechanical properties, thermal stability, ablation resistance and high carbon residue rate. It is currently the most widely used thermal protection material. However, as a pure ablative material, resin-based composite materials have a large ablation rate and a low carbon residue rate at high temperatures. The high temperature resistance grade and ablation resistance of traditional ablative thermal protection materials no longer meet the requirements. It is necessary to improve and perfect their thermal stability and ablation resistance based on the existing preparation process.

[0003] At present, the modification methods are mainly divided into two categories: inorganic nano-doping modification and organic structure modification. The process of inorganic nano-material modification of phenolic acid is simple, the raw materials are cheap and easy to obtain, and the modification effect is obvious. It has become one of the research hotspots. However, excessive addition of inorganic ions will deteriorate the mechanical properties of the resin matrix.

[0004] Since traditional phenolic resin contains only three elements, C, H, and O, its organic structure is relatively simple and the intermolecular bond energy is small. Researchers are considering introducing chemical bonds with larger bond energy into the molecular structure of phenolic resin. By increasing the initial cracking temperature of the material and the residual carbon rate at high temperature, the high temperature resistance and ablation resistance of the material can be improved, while effectively improving its mechanical properties.

[0005] Therefore, how to obtain a low-cost, simple preparation process, good mechanical properties, and higher residual carbon rate thermosetting phenolic resin to meet the wider application of phenolic resin in aerospace thermal protection materials is an urgent problem to be solved. Summary of the invention

[0006] In order to overcome the technical problems in the prior art that the phenolic resin has insufficient initial high temperature resistance, low residual carbon rate at high temperature, high brittleness, and is difficult to meet the growing requirements for aerospace heat protection materials, the purpose of the present invention is to provide a preparation method and application of boron carbide and phthalonitrile synergistically modified phenolic resin. This method not only improves the cracking temperature and residual carbon rate of the phenolic resin, but also significantly improves the mechanical properties of the resin, so that it has a higher high temperature resistance level and better ablation resistance, and the material has good bending performance, which can meet the thermal environment faced by the nozzle of a solid rocket engine.

[0007] The present invention is achieved through the following technical solutions.

[0008] One aspect of the present invention provides a method for preparing a phenolic resin modified by boron carbide and phthalonitrile, comprising the following steps:

[0009] a. Dissolve the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, and heat and stir to dissolve;

[0010] b. Adding phenolic resin and boron carbide to the cooled biphenyl type diphthalonitrile prepolymer solution in a mass ratio of (30-90):100:(1-5), and mixing under vacuum conditions to obtain a blend;

[0011] c. Inject the blend into a mold, perform step-wise curing, and cool to room temperature to obtain a modified phenolic resin.

[0012] Preferably, the biphenyl type diphthalonitrile prepolymer comprises the following raw materials in mass ratio:

[0013] 50-60 parts of biphenol, 100-120 parts of 4-nitrophthalonitrile, 100-120 parts of anhydrous potassium carbonate, 1000-1400 parts of dimethyl sulfoxide.

[0014] Preferably, the biphenyl type diphthalonitrile prepolymer is prepared according to the following method:

[0015] In terms of mass ratio, 50-60 parts of biphenol, 100-120 parts of 4-nitrophthalonitrile and 100-120 parts of anhydrous potassium carbonate and 1000-1400 parts of solvent dimethyl sulfoxide are heated and stirred at 85-95°C for 5-6 hours; deionized water and anhydrous ethanol are filtered to obtain an intermediate powder; dried to obtain a high-purity biphenyl type diphthalonitrile monomer; the biphenyl type diphthalonitrile monomer is dissolved in a solvent, prepolymerized at 180-220°C for 5-6 hours, washed with deionized water, filtered, and a biphenyl type diphthalonitrile prepolymer is obtained.

[0016] Preferably, the phenolic resin is prepared according to the following method:

[0017] In terms of mass ratio, 70-80 parts of phenol, 100-120 parts of formaldehyde and 3-5 parts of sodium hydroxide are mixed, the temperature is raised for reaction, and the pH value of the solution is adjusted to be neutral to obtain a phenolic resin mixed solution; the mixed solution is dehydrated under reduced pressure, impurities are removed repeatedly, and vacuum distillation is performed to obtain high-purity phenolic resin.

[0018] Preferably, phenol is melted at 60-80° C. for use; sodium hydroxide particles need to be dried before use; and the temperature is raised to 80-90° C. for reaction for 2-3 hours.

[0019] Preferably, in step a, the biphenyl type diphthalonitrile prepolymer is dissolved in 1-methyl-2-pyrrolidone and heated at 120-150° C. for 1-2 hours.

[0020] Preferably, in step b, the particle size of the boron carbide is 1 to 2 μm.

[0021] When the biphenyl type diphthalonitrile prepolymer solution is cooled to 60-70°C, the solution is kept warm and stirred at a high speed for 1-1.5 hours in a vacuum below 0.1 MPa.

[0022] Preferably, in step c, the stepwise curing is 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, and 240°C / 2h.

[0023] Another aspect of the present invention provides a boron carbide and phthalonitrile synergistically modified phenolic resin prepared by the method.

[0024] The present invention adopts the above technical solution, which has the following beneficial effects:

[0025] The micron-sized boron carbide in the present invention has the characteristics of high melting point, hardness, large specific surface area, etc., which can effectively improve the residual carbon rate of phenolic resin at high temperature, thereby improving its anti-ablation performance; the chemical reaction between the cyano group in phthalonitrile and the hydroxyl group of the phenolic resin combines the phenolic resin and biphenyl-type diphthalonitrile, thereby increasing the decomposition temperature and residual carbon rate of the phenolic resin, thereby improving the high temperature resistance level and ablation resistance of the material, and is expected to be used as a thermal protection material for the nozzle of a solid rocket engine.

[0026] The advantages of this method are mainly reflected in:

[0027] 1) Boron carbide modification of phenolic resin is easy to operate, the raw materials are cheap and easy to obtain, and boron carbide reacts chemically with oxygen-containing volatiles in the phenolic pyrolysis products to generate amorphous carbon and boron oxide. The low-melting point boron oxide penetrates into the pores of amorphous carbon in liquid form at high temperature, fills the gaps generated during pyrolysis, and reduces the channels for oxygen to diffuse into the material. Therefore, the material's antioxidant properties can be improved, the phenolic resin pyrolysis reaction rate can be reduced, and the high-temperature residual carbon rate of the phenolic resin can be increased, thereby improving the material's anti-ablation properties.

[0028] 2) Through molecular-level structural design and process optimization, a phthalonitrile-modified phenolic resin copolymer was synthesized. Although phthalonitrile resin has excellent heat resistance and mechanical properties, its curing temperature is too high and it is difficult to spontaneously cure without a curing agent, which makes it difficult to use directly. The introduction of phenolic resin allows the cyano group to react with the hydroxyl group to generate triazine rings, phthalocyanine rings, etc. at 240°C, which reduces the curing temperature and curing reaction conditions of phthalonitrile.

[0029] 3) The biphenyl type diphthalonitrile used contains a polyether segment, which can effectively improve the bending performance of the resin by introducing it into the phenolic resin. At the same time, the phthalonitrile is connected to the main chain of the phenolic resin to complete chemical copolymerization, which can effectively improve the initial cracking temperature and high-temperature residual carbon rate of the phenolic resin, thereby improving the temperature resistance and ablation resistance of the material. The initial cracking temperature of the modified phenolic resin reaches 389.9°C, and the residual carbon rate at 800°C in a nitrogen atmosphere reaches more than 74%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute an improper limitation of the present invention. In the drawings:

[0031] Figure 1 : are TGA curves of the cured products in Example 4 of the present invention and Comparative Example 1 under nitrogen atmosphere;

[0032] Figure 2 The microscopic morphology of the bending fracture of the solidified material in Comparative Example 1 of the present invention;

[0033] Figure 3 The bending fracture microstructure of the solidified material in Example 4 of the present invention. DETAILED DESCRIPTION

[0034] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0035] The present invention provides a method for preparing a boron carbide and phthalonitrile synergistically modified phenolic resin, comprising the following steps:

[0036] Step 1, dissolving the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, heating and stirring at 120-150° C. for 1-2 hours.

[0037] Wherein, the biphenyl type diphthalonitrile prepolymer is prepared according to the following method:

[0038] (1) In terms of mass ratio, 50-60 parts of biphenol, 100-120 parts of 4-nitrophthalonitrile, 100-120 parts of anhydrous potassium carbonate, and 1000-1400 parts of dimethyl sulfoxide as solvent are heated and stirred at 85-95° C. for 5-6 hours to obtain a brown-red solution;

[0039] (2) adding appropriate amounts of deionized water and anhydrous ethanol to the solution, filtering 5 to 8 times, and finally obtaining a light yellow powder, which is dried in a forced air oven at 60 to 80° C. for 24 to 48 hours to obtain a high-purity biphenyl-type diphthalonitrile monomer;

[0040] (3) Using 1-methyl-2-pyrrolidone as solvent, adding biphenyl type diphthalonitrile monomer, prepolymerizing at 180-220° C. for 5-6 hours, washing with deionized water after prepolymerization, and repeatedly filtering 3-5 times to obtain a dark green biphenyl type diphthalonitrile prepolymer.

[0041] Step 2, when the prepolymer solution is cooled to 60-70° C., the prepolymer solution is kept warm for 1-2 hours, and phenolic resin and boron carbide are added to the cooled biphenyl type diphthalonitrile prepolymer solution according to a mass ratio of 30-90:100:1-5, and the mixture is stirred at a high speed of 2000-3000 r / min for 1-1.5 hours at a vacuum degree of less than 0.1 MPa to obtain a blend.

[0042] Wherein, phenolic resin is prepared according to the following method:

[0043] (1) Melt phenol at 60-80°C for later use; dry sodium hydroxide particles in an oven at 80-90°C for 2 hours for later use.

[0044] (2) By mass ratio, add 70-80 parts of phenol, 100-120 parts of formaldehyde, and 3-5 parts of sodium hydroxide into a three-necked flask preheated at 60-80° C., mix thoroughly, heat to 80-90° C., react for 2-3 hours, stop heating, and discharge;

[0045] (3) adding a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring to adjust the pH of the solution to 6-7 to obtain a phenolic resin mixed solution;

[0046] (4) After the mixed solution is dehydrated under reduced pressure, an appropriate amount of deionized water is added to remove the precipitate in a centrifuge, the impurity removal step is repeated multiple times, and finally the water is removed by vacuum distillation to obtain a high-purity phenolic resin.

[0047] Step 3, inject the blend into a mold, and perform stepwise curing at 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, 240°C / 2h, and then naturally cool to room temperature. When the temperature is lowered to room temperature, a boron carbide and phthalonitrile modified phenolic resin cured product is obtained.

[0048] The present invention is further described below through different embodiments.

[0049] Example 1

[0050] Step 1, dissolving the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, heating and stirring at 130° C. for 1 hour to completely dissolve it.

[0051] Biphenyl type diphthalonitrile prepolymer is prepared according to the following method:

[0052] In terms of mass ratio, 50 parts of biphenyl diphenol, 120 parts of 4-nitrophthalonitrile and 110 parts of anhydrous potassium carbonate were added to a three-necked flask, and then 1200 parts of dimethyl sulfoxide were added as a solvent, and heated and stirred at 90°C for 5 hours to obtain a brown-red solution. Appropriate amounts of deionized water and anhydrous ethanol were added to the solution and filtered 5 times to finally obtain a light yellow powder; it was dried in a blast oven at 80°C for 24 hours to obtain a high-purity biphenyl-type diphthalonitrile monomer; 1-methyl-2-pyrrolidone was used as a solvent, biphenyl-type diphthalonitrile monomer was added, and prepolymerized at 180°C for 6 hours. After prepolymerization, it was washed with deionized water and filtered 5 times to obtain a dark green biphenyl-type diphthalonitrile prepolymer.

[0053] Step 2, when the prepolymer solution is cooled to 60°C, it is kept warm for 2 hours, and phenolic resin and boron carbide with a particle size of 1 μm are added to the cooled biphenyl type diphthalonitrile prepolymer solution according to a mass ratio of 30:100:1, and the mixture is stirred at a high speed of 2500 r / min for 1 hour under a vacuum degree of less than 0.1 MPa to obtain a uniform bubble-free blend.

[0054] Phenolic resin is prepared as follows:

[0055] Place phenol in an oven at 70°C for melting and set aside; add 75 parts of phenol, 120 parts of formaldehyde, and 3 parts of sodium hydroxide to a three-necked flask preheated at 60°C by mass ratio, stir until completely mixed, heat to 90°C for reaction for 2h, stop heating, and obtain a dark red mixed solution. Add a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring, adjust the solution pH to 7, and obtain a phenolic resin mixed solution; dehydrate the mixed solution under reduced pressure, add an appropriate amount of deionized water, remove the precipitate in a centrifuge, repeat the impurity removal steps several times, and finally remove water by vacuum distillation to obtain high-purity phenolic resin.

[0056] Step 3, injecting the resin mixed solution into a polytetrafluoroethylene mold, placing it in an oven, and performing step-by-step curing. The curing reaction procedure is 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, and 240°C / 2h, and then naturally cooling to room temperature to obtain a boron carbide and phthalonitrile modified phenolic resin cured product.

[0057] Example 2

[0058] Step 1, dissolving the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, heating and stirring at 120° C. for 2 hours to completely dissolve it.

[0059] Biphenyl type diphthalonitrile prepolymer is prepared according to the following method:

[0060] In terms of mass ratio, 55 parts of biphenyl diphenol, 110 parts of 4-nitrophthalonitrile and 100 parts of anhydrous potassium carbonate were added to a three-necked flask, and then 1000 parts of dimethyl sulfoxide were added as a solvent, and heated and stirred at 85°C for 6 hours to obtain a brown-red solution. Appropriate amounts of deionized water and anhydrous ethanol were added to the solution and filtered 6 times to finally obtain a light yellow powder; it was dried in a 60°C blast oven for 48 hours to obtain a high-purity biphenyl type diphthalonitrile monomer; 1-methyl-2-pyrrolidone was used as a solvent, and biphenyl type diphthalonitrile monomer was added, prepolymerized at 200°C for 5 hours, and washed with deionized water after prepolymerization and filtered 4 times to obtain a dark green biphenyl type diphthalonitrile prepolymer.

[0061] Step 2, when the prepolymer solution is cooled to 70°C, it is kept warm for 1 hour, and phenolic resin and boron carbide with a particle size of 1 μm are added to the cooled biphenyl type diphthalonitrile prepolymer solution according to a mass ratio of 50:100:3, and the mixture is stirred at a high speed of 2000 r / min for 1 hour under a vacuum degree of less than 0.1 MPa to obtain a uniform bubble-free blend.

[0062] Phenolic resin is prepared as follows:

[0063] Place phenol in an oven at 60°C to melt for later use; add 80 parts of phenol, 110 parts of formaldehyde, and 3 parts of sodium hydroxide to a three-necked flask preheated at 80°C by mass ratio, stir until completely mixed, heat to 90°C for reaction for 2.5 hours, stop heating, and obtain a dark red mixed solution. Add a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring, adjust the solution pH to 7, and obtain a phenolic resin mixed solution; dehydrate the mixed solution under reduced pressure, add an appropriate amount of deionized water, remove the precipitate in a centrifuge, repeat the impurity removal steps several times, and finally remove water by vacuum distillation to obtain high-purity phenolic resin.

[0064] Step 3, injecting the resin mixed solution into a polytetrafluoroethylene mold, placing it in an oven, and performing step-by-step curing. The curing reaction procedure is 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, and 240°C / 2h, and then naturally cooling to room temperature to obtain a boron carbide and phthalonitrile modified phenolic resin cured product.

[0065] Example 3

[0066] Step 1, dissolving the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, heating and stirring at 140° C. for 1.5 hours to completely dissolve it.

[0067] Biphenyl type diphthalonitrile prepolymer is prepared according to the following method:

[0068] In terms of mass ratio, 60 parts of biphenyl diphenol, 100 parts of 4-nitrophthalonitrile and 120 parts of anhydrous potassium carbonate were added to a three-necked flask, and then 1300 parts of dimethyl sulfoxide were added as a solvent, and heated and stirred at 95°C for 5 hours to obtain a brown-red solution. Appropriate amounts of deionized water and anhydrous ethanol were added to the solution and filtered 7 times to finally obtain a light yellow powder; it was dried in a 75°C blast oven for 36 hours to obtain a high-purity biphenyl type diphthalonitrile monomer; 1-methyl-2-pyrrolidone was used as a solvent, biphenyl type diphthalonitrile monomer was added, and prepolymerized at 220°C for 5 hours. After prepolymerization, it was washed with deionized water and filtered repeatedly 5 times to obtain a dark green biphenyl type diphthalonitrile prepolymer.

[0069] Step 2, when the prepolymer solution is cooled to 65°C, it is kept warm for 2 hours, and phenolic resin and boron carbide with a particle size of 1 μm are added to the cooled biphenyl type diphthalonitrile prepolymer solution according to a mass ratio of 90:100:3, and the mixture is stirred at a high speed of 3000 r / min for 1 hour under a vacuum degree of less than 0.1 MPa to obtain a uniform bubble-free blend.

[0070] Phenolic resin was prepared as follows:

[0071] Place phenol in an oven at 75°C to melt for later use; add 75 parts of phenol, 100 parts of formaldehyde, and 5 parts of sodium hydroxide to a three-necked flask preheated at 65°C by mass ratio, stir until completely mixed, heat to 85°C for reaction for 3 hours, stop heating, and obtain a dark red mixed solution. Add a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring, adjust the solution pH to 6, and obtain a phenolic resin mixed solution; dehydrate the mixed solution under reduced pressure, add an appropriate amount of deionized water, remove the precipitate in a centrifuge, repeat the impurity removal steps several times, and finally remove water by vacuum distillation to obtain high-purity phenolic resin.

[0072] Step 3, injecting the resin mixed solution into a polytetrafluoroethylene mold, placing it in an oven, and performing step-by-step curing. The curing reaction procedure is 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, and 240°C / 2h, and then naturally cooling to room temperature to obtain a boron carbide and phthalonitrile modified phenolic resin cured product.

[0073] Example 4

[0074] Step 1, dissolving the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, heating and stirring at 150° C. for 1 hour to completely dissolve it.

[0075] Biphenyl type diphthalonitrile prepolymer is prepared according to the following method:

[0076] In terms of mass ratio, 60 parts of biphenyl diphenol, 120 parts of 4-nitrophthalonitrile and 110 parts of anhydrous potassium carbonate were added to a three-necked flask, and then 1400 parts of dimethyl sulfoxide were added as a solvent, and heated and stirred at 85°C for 5 hours to obtain a brown-red solution. Appropriate amounts of deionized water and anhydrous ethanol were added to the solution and filtered 8 times to finally obtain a light yellow powder; it was dried in a 70°C blast oven for 30 hours to obtain a high-purity biphenyl type diphthalonitrile monomer; 1-methyl-2-pyrrolidone was used as a solvent, and biphenyl type diphthalonitrile monomer was added, prepolymerized at 210°C for 5.5 hours, and washed with deionized water after prepolymerization and filtered repeatedly 3 times to obtain a dark green biphenyl type diphthalonitrile prepolymer.

[0077] Step 2, when the prepolymer solution is cooled to 70°C, it is kept warm for 2 hours, and phenolic resin and boron carbide with a particle size of 2 μm are added to the cooled biphenyl type diphthalonitrile prepolymer solution according to a mass ratio of 70:100:5, and the mixture is stirred at a high speed of 2000 r / min for 1.5 hours under a vacuum degree of less than 0.1 MPa to obtain a uniform bubble-free blend.

[0078] Phenolic resin is prepared as follows:

[0079] Place phenol in an oven at 65°C to melt for later use; add 70 parts of phenol, 110 parts of formaldehyde, and 3 parts of sodium hydroxide to a three-necked flask preheated at 70°C by mass ratio, stir until completely mixed, heat to 80°C for 2h, stop heating, and obtain a dark red mixed solution. Add a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring, adjust the solution pH to 6.5, and obtain a phenolic resin mixed solution; dehydrate the mixed solution under reduced pressure, add an appropriate amount of deionized water, remove the precipitate in a centrifuge, repeat the impurity removal steps several times, and finally remove water by vacuum distillation to obtain high-purity phenolic resin.

[0080] Step 3, injecting the resin mixed solution into a polytetrafluoroethylene mold, placing it in an oven, and performing step-by-step curing. The curing reaction procedure is 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, and 240°C / 2h, and then naturally cooling to room temperature to obtain a boron carbide and phthalonitrile modified phenolic resin cured product.

[0081] Comparative Example 1

[0082] Step 1, melt phenol in an oven at 70°C for standby use, then add 75 parts of phenol, 120 parts of formaldehyde and 3 parts of sodium hydroxide into a three-necked flask preheated at 60-80°C, stir until completely mixed, heat to 90°C for reaction for 2h, stop heating, and obtain a dark red mixed solution. Discharge, dropwise add a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring, adjust the solution pH to 7, and obtain a phenolic resin mixed solution; after decompressing and dehydrating the mixed solution, remove the precipitate in a centrifuge, repeat the impurity removal step multiple times, and finally remove water by vacuum distillation to obtain high-purity phenolic resin.

[0083] Step 2, stirring the phenolic resin under vacuum to obtain a uniform, bubble-free blend.

[0084] The phenolic resin needs to be kept at 60°C and stirred at a high speed below 0.1Mpa for 2 hours.

[0085] Step 3, injecting the resin into a polytetrafluoroethylene mold, placing it in an oven, and curing it in a stepwise manner, and then cooling it to room temperature to obtain a phenolic resin cured product.

[0086] Comparative Example 2

[0087] Step 1, melt phenol in an oven at 70°C for standby use, then add 70 parts of phenol, 110 parts of formaldehyde and 3 parts of sodium hydroxide into a three-necked flask preheated at 80°C, stir until completely mixed, heat to 90°C for reaction for 2h, stop heating, and obtain a dark red mixed solution. Discharge, dropwise add a corresponding proportion of dilute hydrochloric acid to neutralize sodium hydroxide under mechanical stirring, adjust the solution pH to 7, and obtain a phenolic resin mixed solution; dehydrate the mixed solution under reduced pressure, add an appropriate amount of deionized water and introduce it into a centrifuge tube, remove the precipitate in a centrifuge, repeat the impurity removal steps for multiple times, and finally remove water by vacuum distillation to obtain high-purity phenolic resin.

[0088] Step 2, stirring the phenolic resin and boron carbide under vacuum to obtain a uniform, bubble-free blend.

[0089] The phenolic resin needs to be kept at 70°C, and the ratio of phenolic resin to boron carbide is controlled to be 100:5, and the mixture is stirred at a high speed below 0.1 MPa for 1 hour.

[0090] Step 3, injecting the resin mixed solution into a polytetrafluoroethylene mold, placing it in an oven, performing step-by-step curing, and then cooling to room temperature to obtain a modified phenolic resin cured product.

[0091] Test Case

[0092] 1. The boron carbide and phthalonitrile modified phenolic resin prepared in Comparative Examples 1 and 2 and Examples 1 to 4 were subjected to bending performance test. The bending strength of the resin casting was tested using an electronic universal testing machine, using a three-point bending method, and the test standard was referenced to GB / T1040-2006. The maximum load during the fracture of the sample was recorded, and the results are shown in Table 1.

[0093] 2. The boron carbide and phthalonitrile modified phenolic resin prepared in Comparative Examples 1 and 2 and Examples 1 to 4 were subjected to pyrolysis measurement. Six samples were first dried and then heated to 800°C at a heating rate of 10°C / min. The samples were subjected to thermogravimetric analysis (TGA) under nitrogen protection. The results are shown in Table 1.

[0094] Table 1 Comparative Examples and Examples Performance Parameters of Boron Carbide and Phthalonitrile Synergistically Modified Phenolic Resin

[0095]

[0096] From Comparative Example 1 and Comparative Example 2 in Table 1 above, it can be seen that boron carbide has no obvious effect on the cracking process of the material; however, the residual carbon rate of Comparative Example 2 at 800°C is significantly improved. The higher the residual carbon rate, the higher the retention rate of the resin under high temperature conditions, so the better the ablation resistance. It can be seen that the introduction of boron carbide can change the overall ablation resistance of the material;

[0097] Comparative Examples 1, 2 and Examples 1 to 4 show that the initial pyrolysis temperature and pyrolysis peak temperature of the biphenyl-type diphthalonitrile-modified phenolic resin are significantly improved; the initial pyrolysis temperature refers to the temperature at which decomposition occurs when heated, and a high initial pyrolysis temperature indicates that the resin has high thermal stability, so the temperature resistance grade is higher; the pyrolysis peak temperature refers to the temperature at which the cracking reaction is most intense, and the higher the pyrolysis peak temperature, the more difficult the pyrolysis reaction is to occur, the higher the resin retention rate, the higher the residual carbon rate, and therefore the better the ablation resistance;

[0098] Figure 1 The TGA curves of the cured products in Comparative Example 1 and Example 4 of the present invention under a nitrogen atmosphere show that, compared with the unmodified phenolic cured product, the initial cracking temperature of the modified cured product is significantly increased, and the residue rate at 800°C is also significantly increased, which is beneficial to improving the heat resistance grade and ablation resistance of the material.

[0099] Figure 2 The bending fracture microstructure of the solidified material in Comparative Example 1 of the present invention is shown in FIG. Figure 2It can be seen that there are still many holes inside the phenolic resin after curing, the bending section is flat, and it is more prone to brittle fracture. Therefore, the material has low bending strength, low bending modulus, and poor mechanical properties.

[0100] Figure 3 The bending fracture microstructure of the solidified material in Example 4 of the present invention is shown in FIG. Figure 3 It can be seen that there are no obvious holes in the cured product of biphenyl type diphthalonitrile: phenolic formaldehyde: boron carbide = 70:100:5, and the cross section presents a mountain-like pattern. This pattern is more conducive to the transmission of force and is more inclined to ductile fracture. Therefore, the modified resin has high bending strength and bending modulus, and good mechanical properties.

[0101] Example 4 has excellent thermal properties and the best mechanical properties among Comparative Examples 1, 2 and Examples 1 to 4, indicating that the boron carbide and phthalonitrile synergistically modified phenolic resin prepared in this example has excellent heat resistance and ablation resistance while the mechanical properties are also significantly improved.

[0102] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solution disclosed in the present invention, technicians in this field can make some substitutions and deformations to some technical features therein according to the disclosed technical content without creative labor, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A method for preparing a phenolic resin modified by boron carbide and phthalonitrile, characterized in that: The following steps are involved: a. Dissolve the biphenyl type diphthalonitrile prepolymer in 1-methyl-2-pyrrolidone, and heat and stir to dissolve; b. Adding phenolic resin and boron carbide to the cooled biphenyl type diphthalonitrile prepolymer solution in a mass ratio of (30-90):100:(1-5), and mixing under vacuum conditions to obtain a blend; c. Inject the blend into a mold, perform step-wise curing, and cool to room temperature to obtain a modified phenolic resin.

2. The method for preparing the boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 1, characterized in that: The biphenyl type diphthalonitrile prepolymer comprises the following raw materials in mass ratio: 50-60 parts of biphenol, 100-120 parts of 4-nitrophthalonitrile, 100-120 parts of anhydrous potassium carbonate, 1000-1400 parts of dimethyl sulfoxide.

3. The method for preparing the boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 1, characterized in that: The biphenyl type diphthalonitrile prepolymer is prepared according to the following method: In terms of mass ratio, 50-60 parts of biphenyl diphenol, 100-120 parts of 4-nitrophthalonitrile and 100-120 parts of anhydrous potassium carbonate and 1000-1400 parts of solvent dimethyl sulfoxide are heated and stirred at 85-95° C. for 5-6 hours; deionized water and anhydrous ethanol are filtered to obtain an intermediate powder; dried to obtain a high-purity biphenyl type diphthalonitrile monomer; the solvent dissolves the monomer; The biphenyl type diphthalonitrile monomer is prepolymerized at 180-220° C. for 5-6 hours, washed with deionized water, and filtered to obtain a biphenyl type diphthalonitrile prepolymer.

4. The method for preparing the boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 1, characterized in that: The phenolic resin is prepared as follows: In terms of mass ratio, 70-80 parts of phenol, 100-120 parts of formaldehyde and 3-5 parts of sodium hydroxide are mixed, the temperature is raised for reaction, and the pH value of the solution is adjusted to be neutral to obtain a phenolic resin mixed solution; the mixed solution is dehydrated under reduced pressure, impurities are removed repeatedly, and vacuum distillation is performed to obtain high-purity phenolic resin.

5. The method for preparing the boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 4, characterized in that: Phenol is melted at 60-80°C for use; sodium hydroxide particles need to be dried before use; the temperature is raised to 80-90°C for reaction for 2-3 hours.

6. The method for preparing the boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 1, characterized in that: In step a, the biphenyl type diphthalonitrile prepolymer is dissolved in 1-methyl-2-pyrrolidone and heated at 120-150° C. for 1-2 hours.

7. The method for preparing the boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 1, characterized in that: In step b, the particle size of the boron carbide is 1 to 2 μm; When the biphenyl type diphthalonitrile prepolymer solution is cooled to 60-70°C, the solution is kept warm and stirred at a high speed for 1-1.5 hours under a vacuum of less than 0.1 MPa.

8. The method for preparing boron carbide and phthalonitrile synergistically modified phenolic resin according to claim 1, characterized in that: In step c, the step curing is 70°C / 2h, 90°C / 16h, 100°C / 10h, 120°C / 10h, 140°C / 10h, 220°C / 2h, and 240°C / 2h.

9. Boron carbide and phthalonitrile synergistically modified phenolic resin prepared by the method according to any one of claims 1 to 8.

10. Use of the boron carbide and phthalonitrile synergistically modified phenolic resin as claimed in claim 9 in thermal protection materials for nozzles of solid rocket engines.

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