A high molecular weight thermoplastic phenolic resin with narrow molecular weight distribution and preparation method thereof

Abstract

The invention discloses a high-molecular-weight thermoplastic phenolic resin with a narrow molecular weight distribution and a preparation method thereof. The resin preparation method comprises two steps, namely a polymerization process and a xylene extraction process. The invention uses dihydroxydiphenylmethane as a starting raw material, thereby ensuring that the resin obtained by polymerization has a narrower molecular weight distribution than that of a traditional phenolic resin. Furthermore, xylene extraction is performed to remove part or most of the low-molecular-weight fractions in the resin, thereby increasing the average molecular weight of the resin while further narrowing the molecular weight distribution of the resin, thereby achieving the purpose of significantly improving the charring rate of the resin. The thermoplastic phenolic resin prepared by the invention has a weight-average molecular weight of more than 2600 g / mol and a molecular weight distribution index as low as 1.79. The charring rate of the resin increases with increasing molecular weight and narrowing molecular weight distribution. After the resin is cured with hexamethylenetetramine, the charring rate can reach a maximum of 63.0%, and the interlaminar shear strength of the carbon fiber reinforced composite material reaches 55 MPa.

Description

Technical Field

[0001] The invention belongs to the technical field of composite materials and relates to resins, in particular to a high molecular weight thermoplastic phenolic resin with a narrow molecular weight distribution and a preparation method thereof. Background Art

[0002] Throughout the development of thermoplastic phenolic resins, researchers have focused on the effects of thermoplastic phenolic resins and their molecular weight on their properties. This is particularly true in the field of phenolic resin-based ablation-resistant composites, where molecular weight is a significant factor influencing the charring rate of the resin. Previous studies have shown that higher molecular weight thermoplastic phenolic resins have fewer end groups, allowing for a higher degree of crosslinking and fewer end groups at lower curing agent dosages. This reduces the probability of volatile organic compounds (VOCs) (such as phenol and cresol) formed during thermal decomposition of the cured phenolic resin, thereby increasing the thermal decomposition temperature and charring rate of the resin.

[0003] With the development of science and technology and the expansion of phenolic resin application fields, people realize that the molecular weight distribution of phenolic resin is also a major factor affecting the performance of phenolic resin. It is generally believed that (2000, Zhao Tong et al., CN1124299C) relative to ordinary phenolic resin, phenolic resin with narrow molecular weight distribution has lower melt viscosity and more uniform curing behavior, and the heat resistance and toughness of the product are better. Patent (2023, CN 117164789 A point of view) points out that in the field of rubber application, the molecular weight distribution of conventional phenolic resin is wider, resulting in poor compatibility with rubber, thereby resulting in poor mechanical properties and processing properties of the rubber compound.

[0004] However, when traditionally synthesizing phenolic resins from phenol and formaldehyde as raw materials, the molecular weight of thermoplastic novolac resins gradually grows due to the characteristics of their polymerization reaction, i.e., phenol is converted into a dimer, which is converted into a trimer, ..., ultimately forming a higher molecular weight product. However, since the substituted phenol rings are more likely to react further with formaldehyde than the phenol raw material, when the raw formaldehyde is consumed, in addition to the higher molecular weight resin molecules, a large amount of phenol will remain in the system. Due to this molecular weight distribution characteristic, the molecular weight distribution of phenolic resins is wider, especially with the formation of higher molecular weight molecules. Although the average molecular weight of the resin has increased, the molecular weight distribution of the resin is also wider.

[0005] The molecular weight distribution of phenolic resin is expressed as the weight average molecular weight of the resin. and number average molecular weight The ratio of is called polydispersity index (PDI):

[0006] Due to phenol's strong polarity and low molecular weight, it can be removed industrially through washing or distillation to produce thermoplastic novolac resins with very low free phenol content (less than 1%). With the removal of phenol, the average molecular weight of the phenolic resin increases, and the PDI decreases. However, phenolic resins inevitably contain a large amount of dimers. In commercially available thermoplastic novolac resins, dimers are often the most abundant fraction relative to the polymers of all polymerization degrees above trimer, with a content as high as 10%. Due to this characteristic of the polymerization reaction between phenol and formaldehyde, a certain amount of high molecular weight fraction is necessary to obtain a resin with a higher average molecular weight. This results in a broader molecular weight distribution, i.e., a higher PDI. For example, a commercially available phenolic resin with a weight-average molecular weight of 6600 g / mol and a number-average molecular weight of 1500 g / mol has a PDI of 4.36; a phenolic resin with a weight-average molecular weight of 2350 g / mol and a number-average molecular weight of 1060 g / mol has a PDI of 2.22.

[0007] In other words, if phenol is used as a raw material to synthesize a thermoplastic phenolic resin, its strong molecular weight polydispersity is difficult to avoid. Although there are reports of thermoplastic phenolic resins with narrow molecular weight distribution, the average molecular weight of these resins is mostly not high, that is, they are lower PDIs achieved at lower average molecular weights. For example, CN 117164789 A (2023) discloses a method for preparing a narrow molecular weight distribution phenolic resin, wherein the PDI of the resin is less than 1.9, but its weight average molecular weight is between 1700 and 2300 g / mol. It can be seen that it is not easy to prepare a thermoplastic phenolic resin with both high molecular weight and narrow molecular weight distribution. Summary of the Invention

[0008] In view of the shortcomings of the prior art, the present invention aims to provide a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution and a preparation method thereof.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A method for preparing a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution comprises the following steps:

[0011] Step 1, adding dihydroxydiphenylmethane, formaldehyde aqueous solution, organic solvent and oxalic acid into a reaction kettle; wherein the molar ratio of dihydroxydiphenylmethane to formaldehyde is 1:(0.8-1.2); the mass ratio of dihydroxydiphenylmethane to organic solvent is 100:(40-100); the mass ratio of dihydroxydiphenylmethane to oxalic acid is 100:(0.5-5); the organic solvent includes an organic compound that is miscible with water and has good solubility in dihydroxydiphenylmethane;

[0012] Step 2: After purging with nitrogen, close the reactor, heat it to 80-100°C, control the pressure in the reactor to 0.105-0.110 MPa, and keep the temperature for 1-4 hours; heat it to 100-120°C, keep the temperature for 1-4 hours, and control the pressure in the reactor to no more than 0.2 MPa;

[0013] Step 3: Pass nitrogen protection, heat to 120-140 ° C, perform atmospheric distillation to remove most of the water and organic solvent; then perform reduced pressure distillation at -0.09-0.08 MPa, while heating to raise the temperature in the kettle to 160-165 ° C, and maintain for 0.2-1.0 h; finally, return to normal pressure and cool to 130-140 ° C under nitrogen protection;

[0014] Step 4: extracting the product of step 3 with xylene to obtain a high molecular weight thermoplastic phenolic resin with a narrow molecular weight distribution.

[0015] The present invention also has the following technical features:

[0016] Preferably, the mass percentage concentration of the formaldehyde aqueous solution in step 1 is 35.0-38.0%.

[0017] Preferably, the organic solvent includes any one of ethanol, methanol, acetone and tetrahydrofuran.

[0018] Preferably, the xylene described in step 4 includes any one of o-xylene, p-xylene or m-xylene, or a mixture of any of the above in any proportions.

[0019] Preferably, the method of extracting the product of step 3 with xylene in step 4 includes:

[0020] S1: Add xylene to the reactor, pass nitrogen, stir at a speed of 20-100 rpm, heat to reflux for 10-30 minutes, stop stirring, let it stand for 20-40 minutes, wait for the xylene and resin in the reactor to separate, and then suck out the upper layer of xylene; wherein the mass ratio of xylene to dihydroxydiphenylmethane is 100:(50-200);

[0021] S2 repeats the operation of S1 2 to 6 times, and at the same time determines the molecular weight, distribution and composition of the resin by gel permeation chromatography (GPC). When the molecular weight of the resin reaches the requirement, the extraction process is completed;

[0022] S3 is distilled under reduced pressure at 140°C to remove xylene, and discharged at 135-145°C under normal pressure to obtain a high molecular weight thermoplastic phenolic resin with a narrow molecular weight distribution.

[0023] The present invention also protects a high molecular weight thermoplastic phenolic resin with narrow molecular weight distribution prepared by the above method.

[0024] Compared with the prior art, the present invention has the following technical effects:

[0025] The present invention prepares a high-molecular-weight thermoplastic novolac resin with a narrow molecular weight distribution through a polymerization reaction and a refining extraction step in an autoclave. The conventional method using phenol as a raw material is abandoned. Instead, the thermoplastic novolac resin is synthesized using dihydroxydiphenylmethane (BPM) as a raw material through a polymerization reaction between BPM and formaldehyde. This reduces the polydispersity of the molecular weight of the novolac resin and makes its molecular weight distribution narrower. Furthermore, through a refining process of xylene extraction, xylene has a stronger solubility for low-molecular-weight fractions in the novolac resin, which can partially or mostly remove low-molecular-weight fractions, such as some dimers, trimers, and tetramers, thereby increasing the average molecular weight of the resin while reducing the polydispersity of the molecular weight of the resin, that is, reducing the PDI. The xylene extraction process completely overcomes the limitations of conventional methods for synthesizing high-molecular-weight novolac resins.

[0026] The thermoplastic phenolic resin prepared by the present invention has a maximum weight-average molecular weight of 2600 g / mol and a minimum molecular weight distribution index as low as 1.79. Furthermore, as the molecular weight increases and the molecular weight distribution narrows, the charring rate of the resin increases. After the resin is cured with hexamethylenetetramine, the charring rate can reach a maximum of 63.0%, and the interlaminar shear strength of the carbon fiber reinforced composite material reaches 55 MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 1 is the GPC curve of the thermoplastic phenolic resin obtained in Example 1 and Comparative Example 1;

[0028] Figure 2 1 is the GPC curve of the thermoplastic phenolic resin obtained in Comparative Example 2, Example 2, Example 3 and Example 4. DETAILED DESCRIPTION

[0029] To make the purpose, technical effects, and technical solutions of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention are clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the embodiments described here are only part of the embodiments of the present invention. Based on the embodiments disclosed in the present invention, other embodiments obtained by ordinary technicians in this field without making any creative efforts should fall within the scope of protection of the present invention.

[0030] In the following examples, the dihydroxydiphenylmethane used, abbreviated as BPM, commonly known as bisphenol F, methylene bisphenol, bis-(hydroxyphenyl) methane, was provided by Shanghai Licas Industrial Co., Ltd., with a purity of 92.5% and a melting point of 113.1°C.

[0031] The purity of the adopted organic solvent and oxalic acid is greater than 99%.

[0032] The xylene used includes any one of o-xylene, p-xylene or m-xylene or a mixture of several of them in any proportion.

[0033] The molecular weight and composition of the resin were determined using a 1260 gel permeation chromatograph (GPC) from Agilent, USA. Plgel and Plgel The columns were operated in series, with a column temperature of 40°C and a mobile phase of chromatographic-grade tetrahydrofuran (untreated) at a flow rate of 0.6 mL / min. The resin elution curve was obtained. To calculate the average molecular weight and PDI of the resin, 12 monodisperse polystyrene (PS) standards with molecular weights of 162, 445, 670, 1100, 2400, 3920, 5590, 7070, 10060, 18310, 29960, and 50800 g / mol were used. The elution curves of the PS standards were measured and fitted to obtain a calibration curve of molecular weight versus retention time. The number average molecular weight, weight average molecular weight, and PDI of the resin were calculated using the instrument's software.

[0034] Example 1

[0035] Step 1: Add BPM, a 38.0% formaldehyde aqueous solution, ethanol, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:0.95; the mass ratio of BPM to ethanol is 100:50; and the mass ratio of BPM to oxalic acid is 100:2;

[0036] Step 2: After purging with nitrogen, close the reactor, heat it to 100°C, control the pressure in the reactor to 0.110 MPa, and keep the reaction at this temperature for 4 hours; then heat it to 120°C, keep the reaction at this temperature for 4 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0037] Step 3: distill under normal pressure, pass nitrogen protection, raise the temperature to 140°C to remove most of the water and organic solvent; then distill under reduced pressure at 0.08MPa, while heating to raise the temperature in the kettle to 165°C, and maintain for 1.0h; finally, return to normal pressure and cool the resin to 140°C under nitrogen protection;

[0038] Step 4: extracting the product of step 3 with xylene. The specific operation is as follows:

[0039] Add xylene to the reactor, purge with nitrogen, stir at 60 rpm, heat to reflux (about 135°C), maintain for 20 minutes, and then stop stirring. The mass ratio of xylene to BPM is 100:66.7. Let it stand for 30 minutes. After the xylene and resin in the reactor separate, aspirate the upper layer of xylene.

[0040] After repeating the above steps five times, the xylene was removed by distillation under reduced pressure at 140°C. The mixture was discharged at 140°C under normal pressure to obtain a thermoplastic phenolic resin having a number average molecular weight of 1437 g / mol, a weight average molecular weight of 2363 g / mol, and a PDI of 1.64.

[0041] Example 2

[0042] Step 1: Add BPM, a 38.0% formaldehyde aqueous solution, ethanol, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:1; the mass ratio of BPM to ethanol is 100:50; and the mass ratio of BPM to oxalic acid is 100:4;

[0043] Step 2: After purging with nitrogen, close the reactor, heat it to 90°C, control the pressure in the reactor to 0.108 MPa, and keep the reaction at this temperature for 3 hours; then heat it to 110°C, keep the reaction at this temperature for 3 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0044] Step 3: distill under normal pressure, pass nitrogen protection, raise the temperature to 130°C to remove most of the water and organic solvent; then distill under reduced pressure at -0.09 MPa, while heating to raise the temperature in the kettle to 162°C, and maintain it for 0.8h; finally, return to normal pressure and cool the resin to 135°C under nitrogen protection;

[0045] Step 4: extracting the product of step 3 with xylene. The specific operation is as follows:

[0046] Add xylene to the reactor, purge with nitrogen, stir at 60 rpm, heat to reflux (about 135°C), maintain for 20 minutes, and then stop stirring. The mass ratio of xylene to BPM is 100:200. Let it stand for 30 minutes. After the xylene and resin in the reactor are separated, aspirate the upper layer of xylene.

[0047] After repeating the above steps twice, the xylene was removed by distillation under reduced pressure at 140°C. The mixture was discharged at 140°C under normal pressure to obtain a thermoplastic phenolic resin with a number average molecular weight of 1174 g / mol, a weight average molecular weight of 2239 g / mol, and a PDI of 1.91.

[0048] After the resin is cured with hexamethylenetetramine, the carbonization rate can reach up to 60.0%, the interlaminar shear strength of the carbon fiber reinforced composite material reaches 50MPa, and the linear ablation rate is 0.022mm / s.

[0049] Example 3

[0050] Step 1: Add BPM, a 35.0% formaldehyde aqueous solution, ethanol, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:1; the mass ratio of BPM to ethanol is 100:50; and the mass ratio of BPM to oxalic acid is 100:4;

[0051] Step 2: After purging with nitrogen, close the reactor, heat it to 85°C, control the pressure in the reactor to 0.110 MPa, and keep the reaction at this temperature for 2 hours; then heat it to 100°C, keep the reaction at this temperature for 2 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0052] Step 3: distill under normal pressure, pass nitrogen protection, raise the temperature to 130°C to remove most of the water and organic solvent; then distill under reduced pressure at -0.08MPa, while heating to raise the temperature in the kettle to 165°C, and maintain for 0.5h; finally, return to normal pressure and cool the resin to 140°C under nitrogen protection;

[0053] Step 4: extracting the product of step 3 with xylene. The specific operation is as follows:

[0054] Add xylene to the reactor, purge with nitrogen, stir at 100 rpm, heat to reflux (about 135°C), maintain for 30 minutes, and then stop stirring; the mass ratio of xylene to BPM is 100:100; let it stand for 40 minutes, wait for the xylene and resin in the reactor to separate, and then aspirate the upper layer of xylene;

[0055] After repeating the above steps four times, the xylene was removed by distillation under reduced pressure at 140°C. The mixture was discharged at 145°C under normal pressure to obtain a thermoplastic phenolic resin with a number average molecular weight of 1333 g / mol, a weight average molecular weight of 2430 g / mol, and a PDI of 1.82.

[0056] After the resin is cured with hexamethylenetetramine, the carbonization rate can reach up to 63.0%, the interlaminar shear strength of the carbon fiber reinforced composite material reaches 57MPa, and the linear ablation rate is 0.021mm / s.

[0057] Example 4

[0058] Step 1: Add BPM, a 35.0% formaldehyde aqueous solution, tetrahydrofuran, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:1; the mass ratio of BPM to tetrahydrofuran is 100:50; and the mass ratio of BPM to oxalic acid is 100:4;

[0059] Step 2: After purging with nitrogen, close the reactor, heat it to 85°C, control the pressure in the reactor to 0.110 MPa, and keep the reaction at this temperature for 2 hours; then heat it to 100°C, keep the reaction at this temperature for 2 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0060] Step 3: distill under normal pressure, pass nitrogen protection, raise the temperature to 130°C to remove most of the water and organic solvent; then distill under reduced pressure at -0.08MPa, while heating to raise the temperature in the kettle to 165°C, and maintain for 0.5h; finally, return to normal pressure and cool the resin to 140°C under nitrogen protection;

[0061] Step 4: extracting the product of step 3 with xylene. The specific operation is as follows:

[0062] Add xylene to the reactor, purge with nitrogen, stir at 50 rpm, heat to reflux (about 135°C), maintain for 20 minutes, and then stop stirring; the mass ratio of xylene to BPM is 100:100; let it stand for 30 minutes, wait for the xylene and resin in the reactor to separate, and then aspirate the upper layer of xylene;

[0063] After repeating the above steps five times, the xylene was removed by distillation under reduced pressure at 140°C. The mixture was discharged at 145°C under normal pressure to obtain a thermoplastic phenolic resin with a number average molecular weight of 1498 g / mol, a weight average molecular weight of 2680 g / mol, and a PDI of 1.79.

[0064] Example 5

[0065] Step 1: Add BPM, a 35.0% formaldehyde aqueous solution, acetone, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:0.8; the mass ratio of BPM to acetone is 100:80; and the mass ratio of BPM to oxalic acid is 100:5;

[0066] Step 2: After purging with nitrogen, close the reactor, heat it to 85°C, control the pressure in the reactor to 0.110 MPa, and keep the reaction at this temperature for 2 hours; then heat it to 100°C, keep the reaction at this temperature for 2 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0067] Step 3: distill under normal pressure, pass nitrogen protection, raise the temperature to 130°C to remove most of the water and organic solvent; then distill under reduced pressure at -0.08MPa, while heating to raise the temperature in the kettle to 165°C, and maintain for 0.5h; finally, return to normal pressure and cool the resin to 140°C under nitrogen protection;

[0068] Step 4: extracting the product of step 3 with xylene. The specific operation is as follows:

[0069] Add xylene to the reactor, purge with nitrogen, stir at 50 rpm, heat to reflux (about 135°C), maintain for 30 minutes, and then stop stirring; the mass ratio of xylene to BPM is 100:50; let it stand for 20 minutes, wait for the xylene and resin in the reactor to separate, and then aspirate the upper layer of xylene;

[0070] After repeating the above steps 5 times, distill under reduced pressure at 140°C to remove xylene; and discharge the material at 145°C under normal pressure to obtain thermoplastic phenolic resin.

[0071] Example 6

[0072] Step 1: Add BPM, a 35.0% formaldehyde aqueous solution, methanol, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:1.2; the mass ratio of BPM to methanol is 100:100; and the mass ratio of BPM to oxalic acid is 100:0.5;

[0073] Step 2: After purging with nitrogen, close the reactor, heat it to 85°C, control the pressure in the reactor to 0.110 MPa, and keep the reaction at this temperature for 2 hours; then heat it to 100°C, keep the reaction at this temperature for 2 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0074] Step 3: distill under normal pressure, pass nitrogen protection, raise the temperature to 130°C to remove most of the water and organic solvent; then distill under reduced pressure at -0.08MPa, while heating to raise the temperature in the kettle to 165°C, and maintain for 0.5h; finally, return to normal pressure and cool the resin to 140°C under nitrogen protection;

[0075] Step 4: extracting the product of step 3 with xylene. The specific operation is as follows:

[0076] Add xylene to the reactor, purge with nitrogen, stir at 20 rpm, heat to reflux (about 135°C), maintain for 10 minutes, and then stop stirring. The mass ratio of xylene to BPM is 100:150. Let it stand for 30 minutes. After the xylene and resin in the reactor are separated, aspirate the upper layer of xylene.

[0077] After repeating the above steps 6 times, distill under reduced pressure at 140°C to remove xylene; and discharge the material at 135°C under normal pressure to obtain thermoplastic phenolic resin.

[0078] Comparative Example 1

[0079] Step 1: Add BPM, a 37.5.0% formaldehyde aqueous solution, ethanol, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:0.95; the mass ratio of BPM to ethanol is 100:50; and the mass ratio of BPM to oxalic acid is 100:2;

[0080] Step 2: After purging with nitrogen, close the reactor, heat it to 100°C, control the pressure in the reactor to 0.110 MPa, and keep the reaction at this temperature for 4 hours; then heat it to 120°C, keep the reaction at this temperature for 4 hours, and control the pressure in the reactor to be no more than 0.2 MPa;

[0081] Step 3: distill at normal pressure, pass nitrogen protection, raise the temperature to 140°C to remove most of the water and organic solvent; then distill under reduced pressure at 0.08 MPa, and heat to raise the temperature in the kettle to 165°C, and maintain it for 1.0 h; finally, restore normal pressure, and cool the resin to 140°C under nitrogen protection to obtain a resin with a number average molecular weight of 909 g / mol, a weight average molecular weight of 1716 g / mol, and a PDI of 1.89. The lower PDI indicates a narrower molecular weight distribution.

[0082] Comparative Example 2

[0083] Step 1: Add BPM, a 36.0% formaldehyde aqueous solution, ethanol, and oxalic acid into a reactor; wherein the molar ratio of BPM to formaldehyde is 1:1; the mass ratio of BPM to ethanol is 100:40; and the mass ratio of BPM to oxalic acid is 100:4;

[0084] Step 2: After purging with nitrogen, close the reactor, heat it to 80°C, control the pressure in the reactor to 0.105 MPa, and keep the reaction at this temperature for 1 hour; then heat it to 100°C, keep the reaction at this temperature for 1 hour, and control the pressure in the reactor to be no more than 0.2 MPa;

[0085] Step 3: Distill under atmospheric pressure with nitrogen protection, raising the temperature to 120°C to remove most of the water and organic solvent; then distill under reduced pressure at -0.09 MPa while heating the kettle to 160°C and maintain it for 0.2 h; finally, return the pressure to atmospheric pressure and cool the resin to 130°C under nitrogen protection. The resulting resin has a number average molecular weight of 880 g / mol, a weight average molecular weight of 1869 g / mol, and a PDI of 2.12.

[0086] Figure 1The GPC curves of the thermoplastic novolac resins obtained in Example 1 and Comparative Example 1 are shown. The peak on the right side of the GPC curves is formed by the BPM in the resin. In Comparative Example 1, despite containing 16.10% BPM, the resin still exhibits a relatively low PDI of 1.89, demonstrating the effectiveness of the synthesis process of the present invention for synthesizing low-PDI novolac resins. In Example 1, the extraction process, which follows the synthesis process, reduces the low-molecular-weight fraction in the resin, particularly reducing the BPM to 2.19%, resulting in a narrower molecular weight distribution. Consequently, while the average molecular weight of the resin increases, its PDI decreases to 1.64, demonstrating the effectiveness of the extraction process in reducing the PDI of the thermoplastic novolac resin.

[0087] Figure 2 The GPC curves of the thermoplastic novolac resins obtained in Comparative Example 2, Example 2, Example 3, and Example 4 are shown. The peak on the right side of the GPC curve is formed by BPM in the resin. As can be seen from the corresponding peak, Comparative Example 2 has the highest BPM content, while Examples 2, 3, and 4 show reduced BPM content. Specific numerical analysis shows that in Comparative Example 2, the resin has a PDI of 2.12 and a BPM content of 19.13%. In Example 2, due to the subsequent extraction process after the synthesis, the low-molecular-weight fraction in the resin is reduced, particularly BPM, to 8.07%, resulting in a narrower molecular weight distribution. Consequently, its PDI is reduced to 1.91, demonstrating the effectiveness of the extraction process in reducing the PDI of the thermoplastic novolac resin. In Example 3, the extraction process is further strengthened, reducing the BPM content in the resin to 4.47%, and its PDI to 1.82. In Example 4, the extraction process was further enhanced, reducing the BPM content in the resin to 2.67%, the resin's molecular weight reaching 2680 g / mol, and its PDI to 1.79. This demonstrates the effectiveness of combining the synthesis process of the present invention with the extraction process for synthesizing low-PDI thermoplastic novolac resins, particularly the xylene extraction process, which increases the average molecular weight while decreasing the PDI.

Claims

1. A method for preparing a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution, characterized in that: The following steps are involved: Step 1, adding dihydroxydiphenylmethane, formaldehyde aqueous solution, organic solvent and oxalic acid into a reaction kettle; wherein the molar ratio of dihydroxydiphenylmethane to formaldehyde is 1:(0.8-1.2); the mass ratio of dihydroxydiphenylmethane to organic solvent is 100:(40-100); the mass ratio of dihydroxydiphenylmethane to oxalic acid is 100:(0.5-5); the organic solvent includes an organic compound that is miscible with water and has good solubility in dihydroxydiphenylmethane; Step 2: After purging with nitrogen, close the reactor, heat it to 80-100°C, control the pressure in the reactor to 0.105-0.110 MPa, and keep the temperature for 1-4 hours; heat it to 100-120°C, keep the temperature for 1-4 hours, and control the pressure in the reactor to no more than 0.2 MPa; Step 3: Pass nitrogen protection, raise the temperature to 120-140°C, and perform atmospheric distillation to remove most of the water and organic solvent; then perform reduced pressure distillation at -0.09-0.08 MPa, while heating to raise the temperature in the kettle to 160-165°C, and maintain it for 0.2-1.0h; finally, return to normal pressure and cool down to 130-140°C under nitrogen protection; Step 4: extracting the product of step 3 with xylene to obtain a high molecular weight thermoplastic phenolic resin with a narrow molecular weight distribution.

2. The method for preparing a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution as claimed in claim 1, wherein The mass percentage concentration of the formaldehyde aqueous solution described in step 1 is 35.0-38.0%.

3. The method for preparing a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution as claimed in claim 1, wherein: The organic solvent includes any one of ethanol, methanol, acetone and tetrahydrofuran.

4. The method for preparing a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution as claimed in claim 1, wherein: The xylene described in step 4 includes any one of o-xylene, p-xylene or m-xylene, or a mixture of any of the above in any proportions.

5. The method for preparing a high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution as claimed in claim 1, wherein: The method of extracting the product of step 3 with xylene described in step 4 includes: S1: Add xylene to the reactor, pass nitrogen, stir at a speed of 20-100 rpm, heat to reflux for 10-30 minutes, stop stirring, let it stand for 20-40 minutes, wait for the xylene and resin in the reactor to separate, and then suck out the upper layer of xylene; wherein the mass ratio of xylene to dihydroxydiphenylmethane is 100:(50-200); S2 repeats the operation of S1 2 to 6 times, and at the same time determines the molecular weight, distribution and composition of the resin by gel permeation chromatography. When the molecular weight of the resin reaches the requirement, the extraction process is completed; S3 is distilled under reduced pressure at 140°C to remove xylene, and discharged at 135-145°C under normal pressure to obtain a high molecular weight thermoplastic phenolic resin with a narrow molecular weight distribution.

6. A high molecular weight thermoplastic novolac resin with a narrow molecular weight distribution prepared by the method according to any one of claims 1 to 5.

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