A kind of ablation-resistant thermosetting phenolic resin and preparation method thereof

Abstract

The invention discloses a kind of ablation-resistant thermosetting phenolic resin and its preparation method, and belongs to the technical field of phenolic resin preparation. The preparation method comprises the following steps: after mixing 40-60 parts of phenol, 10-20 parts of polyformaldehyde, 2-3 parts of 2-methylaniline and 4-6 parts of toluene, 2-3 parts of ammonia water are added thereto for heat preservation reaction, and then the temperature is lowered for dehydration; 4-6 parts of meta-cresol and 0.2-1 parts of alkaline pH regulator, 4-6 parts of naphthol and 0.2-1 parts of alkaline pH regulator, 0.6-3 parts of alkaline pH regulator and 20-40 parts of formaldehyde solution are added in sequence, after the water solubility of the reacted resin reaches the requirement, the temperature is lowered and the pressure is reduced for dehydration, and then 5-15 parts of ethylene glycol or propylene glycol are added, and the dehydration state is continued until the moisture detection is qualified, and the temperature is lowered and discharged, that is, the preparation method provided by the present invention has easy-to-obtain raw materials, low cost, simple preparation process, and the prepared phenolic resin has good high temperature resistance and ablation resistance, and is suitable for industrial production.

Description

Technical Field

[0001] The present invention relates to the technical field of phenolic resin preparation, in particular to an ablation-resistant thermosetting phenolic resin and a preparation method thereof. Background Art

[0002] The information disclosed in the background of the invention is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] Resin-based composites reinforced with fibers and fabrics are currently one of the primary forms of ablative materials. The resin matrix significantly influences the performance of ablative composites, and therefore, selecting a superior resin matrix plays a crucial role in improving the ablative properties of composites. Phenolic resins have long been used as the matrix material for high-temperature, ablation-resistant composites in the aerospace industry due to their excellent ablation resistance, particularly their outstanding transient high-temperature ablation resistance and good erosion resistance. However, the development and advancement of rocket and aerospace technology in recent decades has placed even higher demands on the heat and ablation resistance of the matrices of ablation-resistant composites.

[0004] Due to the limitations of its molecular structure, traditional phenolic resins have a relatively low charring rate, which limits their ablation resistance. Furthermore, they degrade severely during high-temperature ablation, easily creating numerous holes and cracks in the material, causing rapid material loss and making it difficult to meet higher requirements. Currently, most composite precursor matrix resins use borides to modify phenolic resins. However, boron phenolic resins have a high viscosity during curing, require high curing temperatures and pressures, and have poor prepreg adhesion. This results in greater brittleness in composite materials, low interlaminar shear strength, and poor water resistance. Furthermore, the high process production costs have prevented their large-scale application.

[0005] Patent CN 113621215 A (publication date: November 9, 2021) discloses a method for preparing an organic-inorganic hybrid biomass phenolic resin ablation-resistant material. The method first prepares a silicone-modified biomass phenolic resin, then prepares a borosilicate resin oligomer, and finally dissolves the two in a co-solvent to obtain an organic-inorganic hybrid biomass phenolic resin. The organic-inorganic hybrid biomass phenolic resin prepared by this method has a residual rate of 58.23% by weight at 800°C in an air atmosphere and a residual carbon rate of 73.68% by weight at 800°C in an argon atmosphere. However, the preparation process of this patent is complex, requiring two independent reaction units, and the addition of boride and silicone, which is costly. In addition, the co-dissolution of the two resins inevitably leads to compatibility issues compared to a single resin.

[0006] Therefore, how to obtain a thermosetting phenolic resin with low cost, simple preparation process and good ablation resistance is an urgent problem to be solved. Summary of the Invention

[0007] In view of this, the present invention provides an ablation-resistant thermosetting phenolic resin and a preparation method thereof. The preparation method of the present invention has a simple process and low raw material cost, and the prepared phenolic resin has excellent high temperature resistance and ablation resistance.

[0008] In a first aspect, the present invention provides a method for preparing an ablation-resistant thermosetting phenolic resin, comprising the following steps:

[0009] (1) 40-60 parts of phenol, 10-20 parts of paraformaldehyde, 2-3 parts of 2-methylaniline and 4-6 parts of toluene are mixed and then heated to reflux, 2-3 parts of ammonia water are added thereto and the mixture is kept warm for reaction;

[0010] (2) Add 4-6 parts of m-cresol and 0.2-1 parts of alkaline pH regulator, heat up and then conduct constant temperature reaction, and cool down after the reaction is completed;

[0011] (3) adding 4-6 parts of naphthol and 0.2-1 parts of alkaline pH regulator, heating and then conducting a constant temperature reaction, and cooling after the reaction is completed;

[0012] (4) adding 0.6-3 parts of alkaline pH regulator and 20-40 parts of formaldehyde solution in sequence, raising the temperature and then conducting a constant temperature reaction;

[0013] (5) After the water solubility of the resin reaches the requirement, cool down and reduce the pressure for dehydration. After the dehydration amount reaches the requirement, add 5-15 parts of ethylene glycol or propylene glycol, continue to maintain the dehydration state until the moisture test is qualified, cool down and discharge the material.

[0014] In a second aspect, the present invention provides an ablation-resistant thermosetting phenolic resin prepared by the above preparation method, wherein the residual carbon rate of the resin at 800° C. in a nitrogen atmosphere is above 75%.

[0015] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0016] The preparation method provided by the present invention has readily available raw materials, low cost, and a simple preparation process. The reaction can be completed in one reactor. In addition, the prepared thermosetting phenolic resin has good high-temperature resistance and ablation resistance, which makes up for the defect of low carbon residue rate of the precursor matrix of traditional phenolic resin as ablation-resistant material. After high-temperature treatment, the graphitization performance is enhanced, the degradation during high-temperature ablation is reduced, and the generation of a large number of holes and cracks in the material is avoided. The carbon residue rate at 800°C in a nitrogen atmosphere is above 75%, which has good economic benefits and is suitable for industrial production. DETAILED DESCRIPTION

[0017] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0018] As pointed out in the background art, the method for preparing ablation-resistant thermosetting phenolic resin in the prior art is cumbersome and costly. Therefore, the present invention provides a method for preparing ablation-resistant thermosetting phenolic resin, comprising the following steps:

[0019] (1) 40-60 parts of phenol, 10-20 parts of paraformaldehyde, 2-3 parts of 2-methylaniline and 4-6 parts of toluene are mixed and then heated to reflux, 2-3 parts of ammonia water are added thereto and the mixture is kept warm for reaction;

[0020] (2) Add 4-6 parts of m-cresol and 0.2-1 parts of alkaline pH regulator, heat up and then conduct constant temperature reaction, and cool down after the reaction is completed;

[0021] (3) adding 4-6 parts of naphthol and 0.2-1 parts of alkaline pH regulator, heating and then conducting a constant temperature reaction, and cooling after the reaction is completed;

[0022] (4) adding 0.6-3 parts of alkaline pH regulator and 20-40 parts of formaldehyde solution in sequence, raising the temperature and then conducting a constant temperature reaction;

[0023] (5) After the water solubility of the resin reaches the requirement, cool down and reduce the pressure for dehydration. After the dehydration amount reaches the requirement, add 5-15 parts of ethylene glycol or propylene glycol, continue to maintain the dehydration state until the moisture test is qualified, cool down and discharge the material.

[0024] In the synthesis process of the ablation-resistant thermosetting phenolic resin of the present invention, phenol, paraformaldehyde and 2-methylaniline are first used as raw materials and ammonia water is used as a catalyst to synthesize an intermediate containing a heterocyclic structure, as shown in the following formula. The intermediate can achieve ring-opening polymerization during curing without generating small molecules.

[0025]

[0026] To avoid affecting the formation of the heterocyclic intermediate and to enable the prepared thermosetting resin to undergo ring-opening polymerization during application, the present invention introduces m-cresol with a functionality of 3 after the formation of the heterocyclic intermediate. This provides a network structure with sufficient crosslinking density for the resin system and increases the molecular weight. Naphthol is then introduced to increase the carbon content and reduce the oxygen content, thereby reducing carbon consumption and further improving the residual carbon content of the resin. The present invention adds m-cresol and naphthol in stages to gradually increase the molecular weight.

[0027] In the present invention, high-boiling-point solvent ethylene glycol or glycerol is added in the later stage of the reaction to improve the fluidity of the resin, so as to facilitate the use of different processing techniques when processing the composite material.

[0028] The present invention first modifies the intermediate to produce a special structure, improving the oxidative vulnerability of conventional phenolic resins. The molecular weight is then gradually increased, allowing the phenolic compounds to better bond with the aldehyde groups, forming a well-defined network structure. The resulting thermosetting phenolic resin retains a high residual carbon content while also maintaining the composite material's excellent structure and performance.

[0029] The present invention does not impose any particular limitation on the concentrations of ammonia water and formaldehyde. The ammonia water concentration and formaldehyde concentration commonly used in the art can be used. The concentration of ammonia water in the present invention is preferably 25-28%, more preferably 25%; the concentration of formaldehyde solution is preferably 35-40%, more preferably 37%.

[0030] The present invention imposes no particular limitation on the type of alkaline pH adjuster; any alkaline pH adjuster commonly used in the art can be used. Preferably, an aqueous solution of any one of sodium hydroxide, potassium hydroxide, and barium hydroxide is used to catalyze the phenol-formaldehyde condensation reaction. More preferably, an aqueous solution of barium hydroxide is used. Resins prepared using barium hydroxide catalysis have low viscosity, fast curing speed, low molding and curing temperature, and can be molded under relatively low pressure after pretreatment. The mass concentration of the base is preferably 30-50%.

[0031] In step (1) of the present invention, the temperature of the heat preservation reaction is preferably 125-130°C, and the heat preservation time is preferably 20-40 minutes, so that the three substances first form a six-membered heterocyclic compound containing nitrogen and oxygen. After the heat preservation reaction, the temperature is lowered to 50-70°C and dehydrated under reduced pressure.

[0032] In step (2) of the present invention, the temperature of the isothermal reaction is 70-72°C, the time of the isothermal reaction is 1.8-2.2h, and the addition reaction with the remaining aldehyde group in the first step reaction is continued. After the reaction is completed, the temperature is lowered to 42-48°C.

[0033] In step (3) of the present invention, the temperature of the isothermal reaction is 75-80°C, the time of the isothermal reaction is 2.2-2.8h, the addition reaction with the aldehyde group continues, and the temperature is lowered to 42-48°C after the reaction is completed.

[0034] In step (4) of the present invention, to avoid a large amount of heat released by the reaction, 0.6-3 parts of an alkaline pH regulator are added dropwise for 30-40 minutes. The formaldehyde solution is also added dropwise for 50-70 minutes. After the formaldehyde solution is added, the temperature is raised to 68-72°C and the reaction is carried out at a constant temperature for 2.5-3.5 hours. The addition of formaldehyde is to promote the production of more hydroxymethylphenol, thereby better forming a cross-linked network structure.

[0035] In step (5) of the present invention, after the water solubility of the resin reaches 320-420%, the temperature is lowered to 45-55°C and dehydrated under reduced pressure. After the dehydration amount reaches 45-55% of the water content of the formaldehyde solution, 5-15 parts of ethylene glycol or propylene glycol are added.

[0036] The water solubility value of the present invention is detected by measuring 20 mL of resin into a conical flask, titrating with distilled water, and accurately reading the milliliters of distilled water consumed when the resin liquid becomes turbid. The reading is divided by 10 to obtain the water solubility value.

[0037] The moisture content in step (5) of the present invention is determined by the Karl Fischer method (JJG1154-2018), and the water content is less than 2%.

[0038] The present invention also provides an ablation-resistant thermosetting phenolic resin prepared by the above preparation method, wherein the residual carbon rate of the thermosetting phenolic resin at 800° C. in a nitrogen atmosphere is above 75%.

[0039] The technical solution of the present invention is further described below with reference to specific embodiments. Example 1

[0040] This embodiment provides a method for preparing an ablation-resistant thermosetting phenolic resin, comprising the following raw materials:

[0041] 50 parts of phenol, 2 parts of ammonia water, 2.5 parts of 2-methylaniline, 4 parts of toluene, 4 parts of m-cresol, 5 parts of naphthol, 30 parts of formaldehyde, 10 parts of paraformaldehyde, 4 parts of 40% barium hydroxide aqueous solution, and 10 parts of ethylene glycol.

[0042] The preparation method is as follows:

[0043] (1) Weigh each component according to the ratio, divide the 40% barium hydroxide aqueous solution into three parts, and use them in steps (3), (4) and (5) respectively. The usage ratio of the 40% barium hydroxide aqueous solution in steps (3), (4) and (5) is 1:1:3;

[0044] (2) Phenol, paraformaldehyde and toluene are placed in a reactor, first heated to maintain reflux, then ammonia is added, heated to 125-130°C, kept warm for 30 minutes, then cooled to 60°C and adjusted to a reduced pressure state for dehydration;

[0045] (3) Add m-cresol to the reactor in step (2) and stir, then add 40% barium hydroxide aqueous solution, heat to 70-72°C and react at this temperature for 2 hours, then cool to 45°C;

[0046] (4) Add naphthol to the reactor of step (3), continue to maintain the reflux state, add 40% barium hydroxide aqueous solution, heat to 76-78 ° C and react at this temperature for 2.5 hours, then cool to 45 ° C;

[0047] (5) Add 40% barium hydroxide aqueous solution to the reactor in step (4) dropwise for 35 minutes;

[0048] (6) Add formaldehyde solution dropwise to the reactor in step (5) for 60 min, maintaining the temperature at 65-67°C;

[0049] (7) The reactor in step (6) was heated to 70°C and the reaction was continued at a constant temperature for 3 hours. The water solubility of the resin was tested. When the water solubility reached 320-420%, the circulating water was immediately turned on and the temperature was lowered to 50°C.

[0050] (8) The reactor of step (7) is set to a dehydration state, and reduced pressure distillation is started to remove part of the toluene and part of the water, and the toluene is recycled for standby use;

[0051] (9) When the dehydration amount of the reactor in step (8) reaches 50% of the water content of the formaldehyde solution, ethylene glycol is added;

[0052] (10) The reactor in step (9) is kept in a vacuum distillation dehydration state until the moisture content is qualified (water content is less than 2%), and then the temperature is lowered and the material is discharged. Example 2

[0053] Compared with Example 1, the difference is that the raw materials are different. This example includes the following raw materials:

[0054] 60 parts of phenol, 2 parts of ammonia water, 3 parts of 2-methylaniline, 5 parts of toluene, 6 parts of m-cresol, 4 parts of naphthol, 25 parts of formaldehyde, 15 parts of paraformaldehyde, 5 parts of 40% barium hydroxide aqueous solution, and 15 parts of glycerol. Example 3

[0055] Compared with Example 1, the difference is that the raw materials are different. This example includes the following raw materials:

[0056] 55 parts of phenol, 3 parts of ammonia water, 2.8 parts of 2-methylaniline, 6 parts of toluene, 4 parts of m-cresol, 4 parts of naphthol, 20 parts of formaldehyde, 20 parts of paraformaldehyde, 5 parts of 40% barium hydroxide aqueous solution, and 15 parts of ethylene glycol.

[0057] Comparative Example 1

[0058] This comparative example provides a commercially available ablation-resistant thermosetting phenolic resin.

[0059] Comparative Example 2

[0060] Compared with Example 3, the difference is that the preparation method is different.

[0061] The required raw materials are prepared as follows: 55 parts of phenol, 3 parts of ammonia water, 2.8 parts of 2-methylaniline, 6 parts of toluene, 4 parts of m-cresol, 4 parts of naphthol, 20 parts of formaldehyde, 20 parts of paraformaldehyde, 5 parts of 40% barium hydroxide aqueous solution, and 15 parts of ethylene glycol.

[0062] The preparation method is as follows:

[0063] (1) Phenol, naphthol, 2-methylaniline, m-cresol and toluene are put into a reactor and stirred and mixed;

[0064] (2) adding ammonia water and barium hydroxide solution into the reactor in step (1) in sequence;

[0065] (3) adding solid formaldehyde (paraformaldehyde) and liquid formaldehyde into the reactor of step (2);

[0066] (4) Slowly raise the temperature to 70°C, start heat preservation, and keep the temperature constant for 3 hours. Start testing the water solubility of the resin. When the water solubility reaches 320-420%, immediately start circulating water to cool down to 50°C;

[0067] (5) The reactor in step (4) is set to a dehydration state and vacuum distillation is started to remove part of the toluene and part of the water, and the toluene is recycled for standby use;

[0068] (6) When the dehydration amount of the reactor in step (5) reaches 50% of the water content of the formaldehyde solution, ethylene glycol is added;

[0069] (7) The reactor in step (6) is kept in a vacuum distillation dehydration state until the moisture content is qualified (water content is less than 2%), and then the temperature is lowered and the material is discharged.

[0070] Comparative Example 3

[0071] Compared with Example 3, the difference is that the raw materials do not contain meta-cresol, naphthol, ammonia water, 2-methylaniline and toluene.

[0072] The required raw materials are as follows:

[0073] 55 parts of phenol, 20 parts of formaldehyde, 20 parts of paraformaldehyde, 5 parts of 40% barium hydroxide aqueous solution, 15 parts of ethylene glycol.

[0074] The preparation method is as follows:

[0075] (1) Phenol is put into the reactor;

[0076] (2) adding the barium hydroxide solution into the reaction kettle in step (1) in sequence;

[0077] (3) Add solid formaldehyde into the reactor of step (2), heat it to 60°C, and start adding liquid formaldehyde dropwise. The addition is completed within 60 minutes.

[0078] (4) Slowly raise the temperature to 70°C, start heat preservation, and keep the temperature constant for 3 hours. Start testing the water solubility of the resin. When the water solubility reaches 320-420%, immediately start circulating water to cool down to 50°C;

[0079] (5) The reactor in step (4) is set to a dehydration state and vacuum distillation is started;

[0080] (6) When the dehydration amount of the reactor in step (5) reaches 50% of the water content of the formaldehyde solution, ethylene glycol is added;

[0081] (7) The reactor in step (6) is kept in a vacuum distillation dehydration state until the moisture content is qualified (water content is less than 2%), and then the temperature is lowered and the material is discharged.

[0082] Test example

[0083] Pyrolysis analysis was performed on the ablation-resistant thermosetting phenolic resins prepared in Examples 1-3 and Comparative Examples 1-3. Six samples were pre-cured and then heated to 800°C at a heating rate of 10°C per minute. The cured samples were subjected to thermogravimetric analysis under nitrogen protection. The results are shown in Table 1.

[0084] Table 1 Performance parameters of phenolic resins in Examples and Comparative Examples

[0085]

[0086] Note: a. The pyrolysis point temperature refers to the initial temperature at which decomposition occurs due to heat. A high pyrolysis point temperature indicates high thermal stability of the resin. b. The peak pyrolysis temperature refers to the temperature at which carbonization begins. The degradation rate of the resin slows down after 600°C. After 600°C, the resin undergoes cracking, dehydrogenation, coking, and carbonization.

[0087] As can be seen from the table above, the technical solution of the present invention can reduce the degradation of phenolic resin during high-temperature ablation, avoiding the formation of numerous pores and cracks in the material. It also exhibits high pyrolysis temperatures and high carbon residue rates. The carbon residue rates of the phenolic resins in Examples 1-3 can reach over 75%. Their pyrolysis temperatures are above 440°C, indicating high thermal stability, and their peak pyrolysis temperatures are below 570°C, indicating a low carbon formation temperature. In contrast, the commercially available ablation-resistant phenolic resin (Comparative Example 1) and the phenolic resin prepared by single-step addition of the reaction raw materials (Comparative Example 2) exhibit lower pyrolysis temperatures, higher peak pyrolysis temperatures, and lower carbon residue rates. The phenolic resin prepared without the addition of 2-methylaniline, m-cresol, and naphthol (Comparative Example 3) exhibits significantly reduced ablation resistance.

[0088] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing an ablation-resistant thermosetting phenolic resin, characterized in that: The steps include: (1) 40-60 parts of phenol, 10-20 parts of paraformaldehyde, 2-3 parts of 2-methylaniline and 4-6 parts of toluene are mixed and heated to reflux, 2-3 parts of ammonia water are added thereto and then the mixture is kept warm for reaction; (2) adding 4-6 parts of m-cresol and 0.2-1 parts of alkaline pH regulator, heating up and then conducting a constant temperature reaction, and cooling down after the reaction is completed; (3) adding 4-6 parts of naphthol and 0.2-1 parts of alkaline pH regulator, heating up and then conducting a constant temperature reaction, and cooling down after the reaction is completed; (4) adding 0.6-3 parts of alkaline pH adjusting agent and 20-40 parts of formaldehyde solution in sequence, raising the temperature and conducting a constant temperature reaction; (5) When the water solubility of the resin reaches the required level, cool down and reduce the pressure to dehydrate. When the dehydration amount reaches the required level, add 5-15 parts of ethylene glycol or propylene glycol, continue to dehydrate until the moisture test is qualified, cool down and discharge the material.

2. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: The concentration of the ammonia solution is 25-28%, and the concentration of the formaldehyde solution is 35-40%.

3. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: The alkaline pH regulator is selected from an aqueous solution of any one of sodium hydroxide, potassium hydroxide and barium hydroxide, and the mass concentration of the alkali is 30-50%.

4. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: In step (1), the temperature of the insulation reaction is 125-130° C., and the insulation time is 20-40 min.

5. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: In step (1), after the heat preservation reaction, the temperature is lowered to 50-70°C and dehydrated under reduced pressure.

6. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: In step (2), the temperature of the isothermal reaction is 70-72°C, the time of the isothermal reaction is 1.8-2.2h, and the temperature is lowered to 42-48°C after the reaction is completed.

7. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: In step (3), the temperature of the isothermal reaction is 75-80°C, the time of the isothermal reaction is 2.2-2.8h, and the temperature is lowered to 42-48°C after the reaction is completed.

8. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: In step (4), the method of adding 0.6-3 parts of alkaline pH adjusting agent is dropwise addition, and the dropwise addition time is 30-40 minutes; the method of adding formaldehyde solution is dropwise addition, and the dropwise addition time is 50-70 minutes; after the formaldehyde solution is added, the temperature is raised to 68-72°C for constant temperature reaction for 2.5-3.5 hours.

9. The method for preparing an ablation-resistant thermosetting phenolic resin according to claim 1, characterized in that: In step (5), after the water solubility of the resin reaches 320-420%, the temperature is lowered to 45-55°C and dehydrated under reduced pressure. After the dehydrated amount reaches 45-55% of the water content of the formaldehyde solution, 5-15 parts of ethylene glycol or propylene glycol are added.

10. The ablation-resistant thermosetting phenolic resin prepared by the method for preparing an ablation-resistant thermosetting phenolic resin according to any one of claims 1 to 9, characterized in that: The ablation-resistant thermosetting phenolic resin has a residual carbon rate of more than 75% at 800° C. in a nitrogen atmosphere.

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