A low-viscosity solvent-free high-carbon phenolic resin and preparation method thereof

Abstract

The present invention relates to a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof, belonging to the technical field of phenolic resins. The invention solves the problems in the prior art of low residual carbon rate of traditional phenolic resins and high viscosity in solvent-free systems, which makes process operation difficult. A preparation method of a low-viscosity solvent-free high-carbon phenolic resin comprises the following steps: (1) heating at least two phenolic substances to melt, adding a first portion of an aldehyde substance and stirring and mixing, and then adding a composite catalyst, and performing a first reaction under heating and continuous stirring; after the first reaction is completed, adding a second portion of an aldehyde substance, and performing a second reaction under heating and continuous stirring; (2) cooling to room temperature after the second reaction is completed, adding a polymerization inhibitor to terminate the reaction, and obtaining a phenolic resin solution; (3) removing the solvent by vacuum heating, and naturally cooling to room temperature to obtain a low-viscosity solvent-free high-carbon phenolic resin. The synergistic optimization of the processing performance and heat resistance of the phenolic resin is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of phenolic resins, and in particular to a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof. Background Art

[0002] Phenolic resins, due to their excellent heat resistance and ablation resistance, have become a widely used ablation-resistant material matrix in the aerospace industry. Phenolic resins are composed of long chains of phenol molecules connected by methylene bridges. However, conventional preparation methods produce phenolic resins with excessively high polymerization rates, resulting in high viscosity and requiring solvent dilution to improve molding properties. During the curing and drying process, solvents evaporate from the resin system, creating pores and defects within the composite material, reducing its mechanical properties and adversely affecting its antioxidant and ablation resistance.

[0003] The residual carbon content of phenolic resins is a key indicator of their thermal stability. Existing phenolic resins have low residual carbon content, making it difficult for composites to form a dense protective carbon layer during ablation, effectively preventing oxidative corrosion and thus reducing their ablation resistance in extreme environments. Therefore, developing a phenolic resin with low viscosity, solvent-free properties, and a high residual carbon content is crucial for improving the ablation resistance of phenolic resin-based composites and will also provide important material support for technological advancements in the aerospace field. Summary of the Invention

[0004] In view of the above analysis, the embodiments of the present invention aim to provide a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof, so as to solve at least one of the problems of low residual carbon rate of traditional phenolic resin and high viscosity in solvent-free system, which makes process operation difficult.

[0005] On the one hand, an embodiment of the present invention provides a method for preparing a low-viscosity solvent-free high-carbon phenolic resin, the specific steps of which are as follows:

[0006] (1) heating at least two phenolic substances separately until they are melted, and mixing them to obtain a mixed phenolic substance, adding a first portion of an aldehyde substance and stirring to mix them, and then adding a composite catalyst, and performing a first reaction under heating and continuous stirring; after the first reaction is completed, adding a second portion of an aldehyde substance, and performing a second reaction under heating and continuous stirring;

[0007] (2) After the second reaction is completed, the mixture is cooled to room temperature and an inhibitor is added to terminate the reaction to obtain a phenolic resin solution;

[0008] (3) The phenolic resin solution prepared in step (2) is heated in a vacuum to remove the solvent, and then cooled naturally to room temperature to obtain a low-viscosity solvent-free high-carbon phenolic resin.

[0009] Furthermore, in step (1), the phenolic substance includes phenol and aromatic substituted phenol.

[0010] Preferably, the aryl-substituted phenol is one or both of 2-phenylphenol and 2,4-diphenylphenol.

[0011] Furthermore, in step (1), the first part of aldehyde substances and the second part of aldehyde substances are both formaldehyde aqueous solution or furfural, or a mixture of the two.

[0012] Specifically, the molar ratio of the sum of the first part of aldehydes and the second part of aldehydes to the mixed phenols is greater than 1.4.

[0013] Illustratively, in step (1), the composite catalyst is a mixed solution of sodium hydroxide and barium hydroxide.

[0014] Specifically, the mass ratio of the sodium hydroxide, barium hydroxide and mixed phenolic substances is (0.02-0.1):(0.02-0.1):1.

[0015] Preferably, the mass ratio of sodium hydroxide to barium hydroxide in the composite catalyst system is ≥1.0.

[0016] Furthermore, the process conditions for removing the solvent by vacuum heating in step (3) are: temperature 80-90° C., vacuum degree 200 mbar-300 mbar.

[0017] On the other hand, an embodiment of the present invention further provides a low-viscosity solvent-free high-carbon phenolic resin, wherein the low-viscosity solvent-free high-carbon phenolic resin has a degree of polymerization of 3-5, a weight-average molecular weight of <400 g / mol, and a viscosity of <300 mPa·s at 50°C.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0019] 1. The preparation process of the phenolic resin of the present invention adopts a staged addition method of aldehydes, which effectively avoids the addition reaction of the phenolic hydroxyl group to the para position caused by excessive aldehydes in the early stage of the reaction and improves the ortho-selectivity of the phenolic resin. In addition, the staged addition method is beneficial to improving the reaction conversion rate and reducing the free phenol content in the resin system.

[0020] 2. The preparation of the phenolic resin of the present invention constructs a high-carbon, high-ortho structure of the phenolic resin through the efficient introduction of aromatic groups and the synergistic effect of the composite catalyst system, thereby increasing its cross-linking density and the stability of the cross-linked network, and significantly improving the thermal stability of the phenolic resin;

[0021] Using phenol and aromatic substituted phenol as reactants can introduce aromatic groups into the cross-linked network structure of phenolic resin, enhance the stability of the overall structure, and promote the condensation process during high-temperature carbonization, significantly improving its thermal stability.

[0022] By rationally controlling the amount and ratio of the two catalysts, sodium hydroxide and barium hydroxide, the high ortho selectivity of the phenolic resin is effectively promoted, which is conducive to the formation of a rigid three-dimensional network structure with a high cross-linking density during the thermal polymerization process, thereby improving the thermal stability and ablation resistance of the phenolic resin.

[0023] 3. The liquid phenolic resin of the present invention has a degree of polymerization of 3-5, a weight-average molecular weight of <400 g / mol, and a viscosity of <300 mPa•s (50°C). It can achieve a viscosity of <200 mPa•s (50°C), fully meeting the resin fluidity requirements of liquid molding processes for composite materials such as RTM. It solves the technical problem of the incompatibility between the process operability (low viscosity) and thermal stability (high carbon) of phenolic resin, and achieves the coordinated optimization of processing performance and heat resistance.

[0024] In the present invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of the present invention will be described in the following description, and some advantages will become apparent from the description or be learned through practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are only used for the purpose of illustrating specific embodiments and are not to be considered as limiting the present invention. Like reference symbols denote like components throughout the accompanying drawings.

[0026] Figure 1 This is a photo of the low-viscosity, solvent-free, high-carbon-residue phenolic resin after curing according to Example 2 of the present invention;

[0027] Figure 2 Thermogravimetric curves of low-viscosity solvent-free high-carbon residue phenolic resins of Examples 1 to 9 and Comparative Examples 1 to 6 of the present invention;

[0028] Figure 3 This is the molecular weight distribution curve of the low-viscosity solvent-free high-carbon residue phenolic resin of Example 2 of the present invention;

[0029] Figure 4 This is the viscosity-temperature curve of the low-viscosity solvent-free high-carbon residual phenolic resin according to Example 2 of the present invention. DETAILED DESCRIPTION

[0030] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, and are not used to limit the scope of the present invention.

[0031] On the one hand, a specific embodiment of the present invention discloses a method for preparing a low-viscosity solvent-free high-carbon phenolic resin, the specific steps of which are as follows:

[0032] (1) Phenolic substances are heated to melt respectively, and mixed to obtain a mixed phenolic substance, and the first part of the aldehyde substance is added and stirred thoroughly, and then the composite catalyst is added, and a first reaction is carried out under heating and continuous stirring; after the first reaction is completed, the second part of the aldehyde substance is immediately added, and a second reaction is carried out under heating and continuous stirring;

[0033] (2) After the second reaction is completed, the mixture is cooled to room temperature and an inhibitor is added to terminate the reaction to obtain a phenolic resin solution;

[0034] (3) The phenolic resin solution prepared in step (2) is heated in a vacuum to remove the solvent, and then cooled naturally to room temperature to obtain a low-viscosity solvent-free high-carbon phenolic resin.

[0035] Specifically, in step (1), the phenolic substances include phenol and aromatic substituted phenol.

[0036] Furthermore, the aryl-substituted phenol is one or both of 2-phenylphenol and 2,4-diphenylphenol.

[0037] For example, if the phenolic substance includes 2,4-diphenylphenol, 2-phenylphenol and phenol, the components are mixed in any proportion; preferably, the molar ratio of 2,4-diphenylphenol, 2-phenylphenol and phenol is (0-0.5): (0-0.5):1.

[0038] Using phenol and aryl-substituted phenols as reactants can introduce aromatic groups into the cross-linked network structure of the phenolic resin, enhancing the overall structural stability. This also promotes the condensation process during high-temperature carbonization, significantly improving its thermal stability. When the proportion of aryl-substituted phenols is too high, the phenolic resin chains lack sufficient active cross-linking sites, hindering the formation of a high-crosslink density three-dimensional network structure. The resulting network structure becomes less complete, thus affecting the mechanical properties and heat resistance of the resin.

[0039] For example, in step (1), the first part of aldehyde substances and the second part of aldehyde substances are both formaldehyde aqueous solution or furfural, or a mixture of the two, and the mass fraction of formaldehyde in the formaldehyde aqueous solution is 37%.

[0040] When the aldehyde substance is a mixed solution of formaldehyde aqueous solution and furfural, the molar ratio of furfural to formaldehyde in the mixed solution is (0.1-0.5):1. When the furfural ratio is low, the degree of introduction of aromatic groups on the cross-linked network of the phenolic resin is relatively low, and the thermal stability of the product phenolic resin is reduced.

[0041] Furthermore, in step (1), the molar ratio of the mixed phenolic substance, the aldehyde compound in the first part of the aldehyde substance, and the aldehyde compound in the second part of the aldehyde substance is 1:(0.8-1.5):(0.1-0.5).

[0042] The staged addition effectively avoids the addition reaction of the phenolic hydroxyl group at the para position caused by the excess of aldehydes in the early stage of the reaction, thereby improving the ortho-position selectivity of the phenolic resin; in addition, the staged addition is beneficial to improving the reaction conversion rate and reducing the free phenol content in the resin system.

[0043] Furthermore, the molar ratio of the sum of the first part of the aldehydes and the second part of the aldehydes to the mixed phenols is the feed ratio of the aldehydes to the phenols. Increasing the aldehyde ratio can accelerate the addition reaction rate, effectively reduce the free phenol content, and improve the cross-linking density during solidification. However, when the aldehyde addition amount is too much (aldehyde / phenol>1.6), a large amount of para-addition products will be generated. These products will hinder the formation of a cross-linked network during the polycondensation process due to large steric hindrance and high activation energy. Therefore, the molar ratio of the aldehydes to the phenols should be>1.4, such as 1.5, 1.6, 1.7, 1.9, preferably 1.6.

[0044] Preferably, in step (1), the composite catalyst is a mixed solution of sodium hydroxide and barium hydroxide, wherein the mass fractions of sodium hydroxide and barium hydroxide are both 5-20%.

[0045] Furthermore, the mass ratio of sodium hydroxide to barium hydroxide in the composite catalyst system is ≥1.0, such as 1, 1.2, 1.4, 1.5, 1.6, 1.7, 1.8, and the preferred ratio is 1.8.

[0046] Sodium hydroxide provides an alkaline environment. When the alkalinity is too low, the phenolic addition and condensation reactions are less active; when the alkalinity is too high, formaldehyde is prone to self-disproportionation. Barium hydroxide catalyzes the ortho-addition reaction of formaldehyde to phenol and simultaneously promotes the ortho-polycondensation reaction between the polymers. By properly controlling the dosage of both catalysts, the efficient ortho-addition and polycondensation reactions can be ensured, resulting in a high-ortho-phenolic resin. This facilitates the formation of a rigid three-dimensional network with a high cross-link density during thermal polymerization, thereby improving the thermal stability and ablation resistance of the phenolic resin.

[0047] Preferably, the mass ratio of sodium hydroxide, barium hydroxide and mixed phenolic substances in the catalyst is (0.02~0.1):(0.02~0.1):1, ensuring high ortho selectivity during the reaction process.

[0048] Furthermore, the polymerization inhibitor in step (2) is a non-oxidizing strong acid, preferably hydrochloric acid, and the mass fraction of the hydrochloric acid is 20-35%.

[0049] Specifically, the reaction process of step (1) is as follows:

[0050]

[0051] It should be noted that in step (1), the termination points of the first reaction and the second reaction are determined by controlling the reaction time; the process conditions for the first reaction are: stirring at 400-600 r / min at 50-90°C for 0.5-1.5 hours; the process conditions for the second reaction are: stirring at 400-600 r / min at 50-90°C for 0.5-1 hour.

[0052] The degree of polymerization and molecular weight of the phenolic resin increase with increasing reaction temperature and prolonged reaction time. By strictly controlling the reaction time and terminating the reaction in a timely manner, the degree of polymerization and molecular weight of the phenolic resin can be effectively reduced, which is conducive to maintaining a low viscosity after solvent-free treatment. For example, the phenolic resin prepared by the present invention has a degree of polymerization of 3 to 5 and a weight-average molecular weight of <400 g / mol. The degree of polymerization and weight-average molecular weight of the phenolic resin are linearly correlated, and the degree of polymerization can be derived from the weight-average molecular weight and the molecular weight of the repeating unit.

[0053] Furthermore, the process conditions for removing the solvent by vacuum heating in step (3) are: temperature 80-90°C, vacuum degree 200-300 mbar. By controlling the vacuum heating temperature, the condensation reaction can be prevented from continuing at high temperature, thereby avoiding excessive viscosity of the phenolic resin.

[0054] On the other hand, a specific embodiment of the present invention further discloses a low-viscosity solvent-free high-carbon phenolic resin, wherein the low-viscosity solvent-free high-carbon phenolic resin has a degree of polymerization of 3 to 5, a weight-average molecular weight of <400 g / mol, a viscosity of <300 mPa•s (50°C), and a carbonization yield of ≥64.1% at 900°C.

[0055] It should be noted that the carbonization yield of the phenolic resin of the present invention at 900°C is ≥64.1%, the residual carbon rate is high, the thermal stability at high temperature is good, and it is not easy to decompose; the thermal gravimetric test results show that the mass retention rate at 800°C is ≥68.8%, and the mass retention rate at 1200°C is ≥66%, the mass ablation rate is low, and the anti-ablation ability in a high temperature environment is strong.

[0056] In summary, the preparation process of the phenolic resin of the present invention adopts a staged feeding method of aldehyde substances, which effectively avoids the addition reaction of the phenolic hydroxyl group at the para position caused by excessive aldehyde substances in the early stage of the reaction, thereby improving the ortho-selectivity of the phenolic resin; through the efficient introduction of aromatic groups and the synergistic effect of the composite catalyst system, a high-carbon-content, high-ortho-position structure of the phenolic resin is constructed, its cross-linking density and the stability of the cross-linked network are increased, and the thermal stability of the phenolic resin is significantly improved; by reasonably controlling the amount and ratio of the two catalysts of sodium hydroxide and barium hydroxide, the high ortho-selectivity of the phenolic resin is effectively promoted, which is conducive to the formation of a rigid three-dimensional network structure with a high cross-linking density during the thermal polymerization process, thereby improving the thermal stability and ablation resistance of the phenolic resin. The liquid phenolic resin of the present invention has a degree of polymerization of 3-5, a weight-average molecular weight of less than 400 g / mol, a viscosity of less than 300 mPa•s (50°C), and a carbonization yield of 64.1% or more at 900°C, fully meeting the resin fluidity requirements of liquid molding processes for composite materials such as RTM. It also solves the technical problem of the incompatibility between the process operability (low viscosity) and thermal stability (high carbon) of phenolic resin, and achieves the coordinated optimization of processing performance and heat resistance.

[0057] The low-viscosity solvent-free high-carbon phenolic resin and its preparation method of the present invention are described below with reference to specific embodiments.

[0058] Example 1

[0059] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0060] Raw materials: Phenolic substances include 77.8g of phenol, 70.3g of 2-phenylphenol, and 101.8g of 2,4-diphenylphenol; the first part of the aldehyde substance includes 71.5g of formaldehyde aqueous solution and 42.4g of furfural; the second part of the aldehyde substance includes 71.5g of formaldehyde aqueous solution and 42.4g of furfural. The mass fraction of formaldehyde in the formaldehyde aqueous solution is 37%.

[0061] Catalyst: sodium hydroxide and barium hydroxide. Weigh 12.5 g of sodium hydroxide solid and 12.5 g of Ba(OH)2•H2O solid and fully dissolve them in 100 g of water as catalysts.

[0062] The preparation process is as follows:

[0063] (1) Phenolic substances are heated to melt respectively and mixed to obtain a mixed phenolic substance, the first part of the aldehyde substance is added and the mixture is stirred thoroughly, and then the catalyst is added. The mixture is heated and stirred at 50°C for a first reaction for 1.5 hours. When the first reaction is completed and the system is still at a relatively high temperature, the second part of the aldehyde substance is immediately added. The mixture is heated and stirred at 80°C for a second reaction for 40 minutes.

[0064] (2) After the second reaction, the mixture was cooled to room temperature and 7.44 g of polymerization inhibitor was added to terminate the reaction to obtain a phenolic resin solution;

[0065] (3) removing the solvent from the phenolic resin solution prepared in step (2) by vacuum heating and naturally cooling to room temperature to obtain a low-viscosity solvent-free high-carbon phenolic resin;

[0066] The dehydration process is as follows: temperature 80°C, vacuum degree 200 mbar, and time 90 minutes.

[0067] Performance test: The low-viscosity solvent-free high-carbon phenolic resin obtained in step (3) was cured at 110°C for 8 hours, and the curing yield was calculated; a thermogravimetric test was performed in the range of 30-1200°C and a heating rate of 10°C / min to obtain the mass retention at 800°C and 1200°C; a carbonization experiment was performed in a tubular furnace in a N2 atmosphere in the range of 30-900°C and a heating rate of 5°C / min to obtain the carbonization yield at 900°C; the specific results are shown in Table 4, and the thermogravimetric curve is shown in Figure 2 .

[0068] Example 2

[0069] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0070] The raw materials and catalysts are shown in Tables 2 and 3 respectively. The difference from Example 1 is that the addition ratio of sodium hydroxide and barium hydroxide in the catalyst is different, as shown in Table 3.

[0071] The preparation process is the same as that in Example 1, as shown in Table 1.

[0072] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0073] Product photos such as Figure 1 As shown, the molecular weight distribution curve is Figure 3 As shown in the figure, the viscosity-temperature curve is Figure 4 shown.

[0074] Examples 3 to 5

[0075] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0076] The raw materials and catalysts are shown in Table 2 and Table 3 respectively. The difference from Example 2 is that the addition ratio of aldehydes and phenols is different, as shown in Table 3.

[0077] The preparation process is the same as that in Example 1, as shown in Table 1.

[0078] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0079] Example 6

[0080] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0081] The raw materials and catalysts are shown in Table 2 and Table 3 respectively. The difference from Example 2 is that the phenolic raw material is a mixed phenol of phenol and 2-phenylphenol, as shown in Table 3.

[0082] The preparation process is the same as that in Example 1, as shown in Table 1.

[0083] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0084] Example 7

[0085] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0086] The raw materials and catalysts are shown in Table 2 and Table 3 respectively. The difference from Example 2 is that the phenolic material raw material is a mixed phenol of phenol and 2,4-diphenylphenol, as shown in Table 3.

[0087] The preparation process is the same as that in Example 1, as shown in Table 1.

[0088] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0089] Example 8

[0090] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0091] The raw materials and catalysts are shown in Table 2 and Table 3 respectively. The difference from Example 2 is that the first part of aldehyde substances and the second part of aldehyde substances are both formaldehyde aqueous solutions, as shown in Table 3.

[0092] The preparation process is the same as that in Example 1, as shown in Table 1.

[0093] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0094] Example 9

[0095] This embodiment provides a low-viscosity solvent-free high-carbon phenolic resin and a preparation method thereof.

[0096] The raw materials and catalysts are shown in Table 2 and Table 3 respectively. The difference from Example 2 is that the first part of aldehyde substances and the second part of aldehyde substances are both furfural, as shown in Table 3.

[0097] The preparation process is the same as that in Example 1, as shown in Table 1.

[0098] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0099] Comparative Example 1

[0100] This comparative example provides a phenolic resin and a preparation method thereof.

[0101] The raw materials and catalyst are shown in Table 2 and Table 3 respectively. The main difference from Example 1 is that the catalyst is only sodium hydroxide, as shown in Table 3.

[0102] The preparation process is the same as that in Example 1, and the specific process conditions and parameters are shown in Table 1.

[0103] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0104] Comparative Example 2

[0105] This comparative example provides a phenolic resin and a preparation method thereof.

[0106] The raw materials and catalyst are shown in Table 2 and Table 3 respectively. The main difference from Example 1 is that the catalyst is only barium hydroxide, as shown in Table 3.

[0107] The preparation process is the same as that in Example 1, and the specific process conditions and parameters are shown in Table 1.

[0108] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0109] Comparative Example 3

[0110] This comparative example provides a phenolic resin and a preparation method thereof.

[0111] The raw materials and catalysts are shown in Table 2 and Table 3, respectively, which are the same as those in Example 2.

[0112] The preparation process is basically the same as that of Example 2, except that the first reaction time in step (1) is too long, which is 2.5 hours. The specific process conditions and parameters are shown in Table 1.

[0113] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0114] Comparative Example 4

[0115] This comparative example provides a phenolic resin and a preparation method thereof.

[0116] The raw materials and catalysts are shown in Tables 2 and 3, respectively. The main difference from Example 1 is that the mass ratio of the catalyst sodium hydroxide to barium hydroxide is less than 1, as shown in Table 3.

[0117] The preparation process is the same as that in Example 1, and the specific process conditions and parameters are shown in Table 1.

[0118] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0119] Comparative Example 5

[0120] This comparative example provides a phenolic resin and a preparation method thereof.

[0121] The raw materials and catalysts are shown in Table 2 and Table 3, respectively, which are the same as those in Example 2.

[0122] The preparation process is basically the same as that of Example 2, except that the aldehyde substance is added at one time in step (1). The specific process conditions and parameters are shown in Table 1.

[0123] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0124] Comparative Example 6

[0125] This comparative example provides a phenolic resin and a preparation method thereof.

[0126] The raw materials and catalysts are shown in Table 2 and Table 3, respectively, which are the same as those in Example 2.

[0127] The preparation process is basically the same as that of Example 2, except that the second reaction time in step (1) is too long, which is 2.5 hours. The specific process conditions and parameters are shown in Table 1.

[0128] The product performance test method is the same as that in Example 1. The specific results are shown in Table 4 and the thermogravimetric curve is shown in Table 4. Figure 2 .

[0129]

[0130]

[0131]

[0132]

[0133] Compared with Example 2, Example 1 and Comparative Examples 1-2 mainly changed the ratio of sodium hydroxide to barium hydroxide in the composite catalyst system. Among them, Comparative Example 1 only used sodium hydroxide as a catalyst. 2+ The catalytic driving effect of the ortho-addition and polycondensation reactions results in a large amount of accumulation of the para-addition product in the system. During the thermal polymerization process, the cross-linking degree is limited by steric hindrance, which in turn affects the thermal stability of the phenolic resin. On the contrary, Comparative Example 2 uses only barium hydroxide as a catalyst. Due to its insufficient alkalinity, the addition reaction is not complete. During the thermal polymerization process, there is a lack of sufficient phenolic resin molecular chains to participate in the reaction, resulting in a significant reduction in thermal stability. Compared with Example 2, the mass ratio of sodium hydroxide to barium hydroxide in Example 1 is on the low side, and the alkalinity of the system is slightly insufficient relative to the requirements of the efficient catalytic addition reaction, resulting in a phenolic resin thermal stability lower than that of Example 2. The mass ratio of sodium hydroxide to barium hydroxide in Comparative Example 4 is less than 1, which does not meet the requirements of the present invention. The alkalinity of the system is completely unable to meet the requirements of the efficient catalytic addition reaction, resulting in poor thermal stability of the phenolic resin. By constructing a composite catalytic system of barium hydroxide and sodium hydroxide and rationally regulating the ratio of the two, the efficient generation of high-ortho-phenolic resin can be achieved, thereby significantly improving its cross-linking degree and thermal stability. The mass ratio of sodium hydroxide to barium hydroxide in the composite catalyst system should be ≥1.0, and the optimal ratio is 1.8.

[0134] Compared with Example 2, the reaction time of the first reaction in Comparative Example 3 was too long, and the second reaction time in Comparative Example 6 was too long. The extension of the reaction time resulted in chain growth of the polycondensation product, which made it easier to break bonds and release small molecules at high temperatures, resulting in a decrease in the residual carbon rate.

[0135] Compared with Example 2, in Comparative Example 5, the aldehyde substance was not added in stages, resulting in poor ortho-selectivity of the reaction, low crosslinking density and stability of the crosslinked network, and lower thermal stability of the phenolic resin than that of Example 2.

[0136] Examples 2-5 primarily vary the feed ratio of aldehydes to phenols. Increasing the aldehyde ratio accelerates the addition reaction rate, effectively reduces the free phenol content, and improves the crosslink density during curing. When the aldehyde content is high (aldehyde / phenol ratio > 1.6), more para-addition products are generated. These products, due to their high steric hindrance and activation energy during the polycondensation process, have a certain impact on the formation of the crosslinked network. The aldehyde to phenol ratio should be greater than 1.4, with an optimal ratio of 1.6.

[0137] Examples 2, 6, and 7 primarily varied the ratio of the three phenols in the phenolic compound. As shown in Table 4, aryl-substituted phenols can introduce aromatic groups into the cross-linked network of the phenolic resin. However, when the ratio of aryl-substituted phenols is too high, the number of active cross-linking sites on the phenolic resin chains is relatively insufficient compared to the optimal example, limiting the formation of a high-crosslink density three-dimensional network structure and reducing the integrity of the network structure. In this case, the mechanical properties and heat resistance of the resins are slightly different from those of the optimal example. The optimal molar ratio of 2,4-diphenylphenol to 2-phenylphenol to phenol is (0-0.5):(0-0.5):1.

[0138] Examples 2, 8, and 9 mainly changed the ratio of the two aldehydes in the aldehyde substances. As shown in Table 4, when the furfural ratio is low, the degree of introduction of aromatic groups on the cross-linked network of the phenolic resin is relatively small. Compared with the best example 2, its thermal stability performance is slightly different, but it still shows good performance overall.

[0139] In summary, the preparation process of the phenolic resin of the present invention adopts a staged feeding method of aldehyde substances, which effectively avoids the addition reaction of the phenolic hydroxyl group at the para position caused by excessive aldehyde substances in the early stage of the reaction, thereby improving the ortho-selectivity of the phenolic resin; through the efficient introduction of aromatic groups and the synergistic effect of the composite catalyst system, a high-carbon-content, high-ortho-position structure of the phenolic resin is constructed, its cross-linking density and the stability of the cross-linked network are increased, and the thermal stability of the phenolic resin is significantly improved; by reasonably controlling the amount and ratio of the two catalysts of sodium hydroxide and barium hydroxide, the high ortho-selectivity of the phenolic resin is effectively promoted, which is conducive to the formation of a rigid three-dimensional network structure with a high cross-linking density during the thermal polymerization process, thereby improving the thermal stability and ablation resistance of the phenolic resin. The liquid phenolic resin of the present invention has a degree of polymerization of 3-5, a weight-average molecular weight of less than 400 g / mol, a viscosity of less than 300 mPa•s (50°C), and a carbonization yield of 64.1% or more at 900°C, fully meeting the resin fluidity requirements of liquid molding processes for composite materials such as RTM. It also solves the technical problem of the incompatibility between the process operability (low viscosity) and thermal stability (high carbon) of phenolic resin, and achieves the coordinated optimization of processing performance and heat resistance.

[0140] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a low-viscosity solvent-free high-carbon phenolic resin, characterized in that: The specific steps are as follows: (1) Phenol and at least one aromatic substituted phenol are heated to melt, mixed to obtain a mixed phenolic substance, a first portion of an aldehyde substance is added, stirred and mixed, and then a composite catalyst is added. A first reaction is carried out under heating and continuous stirring for 0.5 to 1.5 hours. After the first reaction is completed, a second portion of an aldehyde substance is added, and a second reaction is carried out under heating and continuous stirring for 0.5 to 1 hour. The first part of aldehyde substances and the second part of aldehyde substances are both furfural or a mixture of formaldehyde aqueous solution and furfural; The composite catalyst is a mixed solution of sodium hydroxide and barium hydroxide, and the mass ratio of sodium hydroxide to barium hydroxide in the composite catalyst system is ≥1.0; The molar ratio of the mixed phenolic substance, the aldehyde compound in the first part of the aldehyde substance, and the aldehyde compound in the second part of the aldehyde substance is 1:(0.8-1.5):(0.1-0.5); The molar ratio of the sum of the first part of aldehydes and the second part of aldehydes to the mixed phenols is ≥1.6; (2) After the second reaction is completed, the mixture is cooled to room temperature and an inhibitor is added to terminate the reaction to obtain a phenolic resin solution; (3) The phenolic resin solution prepared in step (2) is heated in a vacuum to remove the solvent, and then cooled naturally to room temperature to obtain a low-viscosity solvent-free high-carbon phenolic resin.

2. The preparation method according to claim 1, characterized in that The aryl-substituted phenol is one or both of 2-phenylphenol and 2,4-diphenylphenol.

3. The preparation method according to claim 2, characterized in that The molar ratio of 2,4-diphenylphenol, 2-phenylphenol and phenol is (0~0.5):(0~0.5):

1.

4. The preparation method according to claim 1, characterized in that The molar ratio of the sum of the first part of aldehyde substances and the second part of aldehyde substances to the mixed phenolic substances is greater than 1.

7.

5. The preparation method according to claim 1, characterized in that When the aldehyde substance is a mixed solution of formaldehyde aqueous solution and furfural, the molar ratio of furfural to formaldehyde in the mixed solution is (0.1~0.5):

1.

6. The preparation method according to claim 1, characterized in that The mass ratio of the sodium hydroxide, barium hydroxide and mixed phenolic substances is (0.02-0.1):(0.02-0.1):

1.

7. The preparation method according to claim 6, characterized in that The mass ratio of sodium hydroxide to barium hydroxide in the composite catalyst system is greater than or equal to 1.

2.

8. The preparation method according to claim 1, characterized in that The process conditions for removing the solvent by vacuum heating in step (3) are: temperature 80~90°C, vacuum degree 200mbar~300mbar.

9. A low-viscosity solvent-free high-carbon phenolic resin, characterized in that: The low-viscosity solvent-free high-carbon phenolic resin is prepared by the method according to any one of claims 1 to 8, wherein the polymerization degree of the low-viscosity solvent-free high-carbon phenolic resin is 3 to 5, the weight-average molecular weight is less than 400 g / mol, and the viscosity at 50°C is less than 300 mPa•s.

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