A low free formaldehyde phenol-formaldehyde resin and a method for synthesizing the same

Abstract

The application discloses a low-free-aldehyde phenol-formaldehyde resin and a synthesis method thereof. Amino sulfonic acid is used as a catalyst, and phenol and formaldehyde are reacted at 90-110 DEG C. The resin is prepared through normal-pressure dehydration, vacuum phenol removal, free-aldehyde capture treatment under acidic conditions, and secondary vacuum distillation, and has low free-formaldehyde content. The free-formaldehyde content of the obtained resin is 8-18 mg / kg, the softening point is 118-125 DEG C, the viscosity is moderate, and the resin has excellent environmental protection performance and processing performance.

Description

Technical Field

[0001] This invention relates to the technical field of phenolic resins, and in particular to a low-free-aldehyde phenolic resin and its synthesis method. Background Technology

[0002] Phenolic resins, as typical thermosetting polymers, are widely used in electronic packaging, automotive manufacturing, green building materials, and interior decoration due to their excellent heat resistance, mechanical strength, and electrical insulation properties. With increasingly stringent environmental regulations limiting volatile organic compound (VOC) emissions, reducing the free formaldehyde content in phenolic resins has become a crucial development direction in this field. As a key environmentally friendly material, the preparation technology of low-free-formaldehyde phenolic resins has gradually evolved from early alkaline-catalyzed hydroxymethylation reaction systems to directional condensation systems characterized by Lewis acid catalysis, achieving certain progress in improving formaldehyde conversion rates and reducing free formaldehyde release.

[0003] In existing technologies, excess phenol is typically reacted with formaldehyde at a suitable temperature to promote the full consumption of formaldehyde, thereby reducing the free formaldehyde content. For example, Chinese patent (CN111909332A) discloses a method for preparing phenolic resin using divalent metal salts (such as zinc acetate, calcium chloride, barium acetate, etc.) as catalysts. Through high ortho-position condensation reaction combined with the feeding of composite formaldehyde source and subsequent dehydration and dephenolization purification processes, a reduction in free formaldehyde content is achieved to a certain extent. However, the aforementioned existing technologies still have the following shortcomings in practical applications: First, formaldehyde has high reactivity and exhibits reversible reaction characteristics in the polycondensation system. Even with excess phenol and catalysts, it is still difficult to achieve complete conversion and stable fixation of formaldehyde, resulting in unstable control of the free formaldehyde content in the product and a risk of re-release during storage or use, thus affecting the environmental performance and application safety of the material. Second, although the divalent metal salt catalytic system can improve the efficiency of the condensation reaction, this type of Lewis acid catalyst has a strong interaction with the resin system, making it difficult to completely remove through conventional processes, and easily leaving trace amounts of metal ions in the product. These residues may cause problems such as metal migration, decreased electrical insulation performance, and reduced material reliability in high-end applications, while also failing to meet increasingly stringent environmental regulations. Summary of the Invention

[0004] This application provides a method for synthesizing low-free-aldehyde phenolic resin, comprising the following steps: using aminosulfonic acid as a catalyst, excess phenol and formaldehyde are reacted at 90℃~110℃; after the reaction, the resin is successively dehydrated at atmospheric pressure to 115~125℃ and dephenolized under vacuum to 130~140℃; then, the free aldehyde is bound by the amino groups of aminosulfonic acid, followed by secondary vacuum distillation or filtration, and finally cooled and cured to obtain the low-free-aldehyde phenolic resin.

[0005] It should be noted that, under conditions of 90–110 °C, the sulfonic acid groups in aminosulfonic acid provide a restricted protic acid environment in the phenol-dominated organic phase and the stepwise dehydration environment, preferentially promoting the hydroxymethylation reaction of phenol and formaldehyde to generate hydroxymethylphenol. Under kinetic control, the hydroxymethyl groups undergo dehydration and etherification, forming a prepolymer structure with dibenzyl ether bonds (-CH2-O-CH2-) as important connecting units. Subsequently, water and excess phenol are removed by atmospheric pressure dehydration and vacuum dephenolization, driving the polycondensation reaction towards the formation of... The reaction proceeds in the direction of molecular weight increase; under subsequent acidic conditions, although the amino group in aminosulfonic acid is mainly in the protonated form, a dynamic deprotonation equilibrium still exists in the system, and formaldehyde is activated into a highly electrophilic species in the acidic environment, which can rapidly undergo irreversible nucleophilic addition reaction with a small amount of free amino group to generate a stable N-hydroxymethyl structure, thereby continuously consuming free formaldehyde and breaking the equilibrium; finally, low molecular weight volatiles are removed by double vacuum distillation and the reaction is terminated to obtain a phenolic resin with a stable structure and significantly reduced free formaldehyde content.

[0006] In a preferred embodiment of a method for synthesizing a low-free-aldehyde phenolic resin, the molar ratio of phenol to formaldehyde is 1:(0.7-0.9).

[0007] It should be noted that by using an appropriate excess of phenol in the reaction, the conversion rate of formaldehyde can be effectively improved and the residue of free formaldehyde can be inhibited. At the same time, the increase in free phenol content caused by excessive phenol can be avoided, thus reducing free formaldehyde while taking into account both the environmental friendliness of the product and the economic efficiency of the process.

[0008] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the amount of aminosulfonic acid added is 0.5wt% to 3wt% of the mass of phenol.

[0009] It should be noted that the amount of aminosulfonic acid added is controlled to be 0.5wt% to 3wt% of the mass of phenol. This provides sufficient acidic catalytic sites to promote the condensation reaction, ensures the effective capture of free aldehydes by its amino groups, and avoids excessive acidification of the system or an increase in side reactions due to excessive dosage, thereby achieving a balance between catalytic efficiency and product performance.

[0010] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the stirring reaction time in the reaction step is 1 to 3 hours.

[0011] It should be noted that the stirring reaction time in the above reaction steps is controlled at 1 to 3 hours, which helps to ensure that phenol and formaldehyde are in full contact and complete the hydroxymethylation and preliminary condensation reaction, thereby improving the formaldehyde conversion rate. At the same time, it avoids excessive condensation of the system, widening of molecular weight distribution and decline in processing performance due to excessive reaction time.

[0012] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the system temperature is controlled at 115–125°C and maintained for 20–60 minutes during the atmospheric pressure dehydration step.

[0013] It should be noted that in the atmospheric pressure dehydration step, the system temperature is controlled at 115-125℃ and maintained for 20-60 minutes. This is beneficial for efficiently removing the water generated in the reaction and promoting the polycondensation reaction towards the product. At the same time, it avoids excessive polycondensation of the resin caused by excessively high temperature or time, which would affect the subsequent processing performance.

[0014] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the vacuum degree of the vacuum phenol removal step is -0.06 MPa to -0.095 MPa.

[0015] It should be noted that the vacuum degree of the vacuum phenol removal step is -0.06MPa to -0.095MPa, which means that within this moderate vacuum range, the system can stably and efficiently distill off excess phenol at 130 to 140°C. At the same time, it avoids excessive vacuum from causing resin foaming, partial volatilization of aminosulfonic acid, or molecular chain breakage. This provides a dry and stable reaction environment for the subsequent precise capture of residual free aldehydes using the amino groups of aminosulfonic acid, thereby ensuring that the final product has extremely low free aldehyde content and no heavy metal residue with low energy consumption.

[0016] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the system temperature is controlled at 130–140°C and the removal time is 30–120 min during the vacuum phenol removal step.

[0017] It should be noted that the system temperature is controlled at 130-140℃ and the removal time is 30-120min in the vacuum phenol removal step. This means that the system is heated to this range under vacuum and maintained for the corresponding duration to efficiently and stably evaporate excess phenol and residual moisture, avoid excessive resin condensation or premature decomposition of aminosulfonic acid, and create a dry and undisturbed environment for the subsequent precise capture of free aldehydes by amino groups. This achieves the goal of extremely low free aldehyde content, excellent flowability and no heavy metal residue in the final product with low energy consumption.

[0018] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the free aldehyde binding treatment temperature is 90–120°C, the treatment time is 30–90 min, and the pH of the reaction system is 1.5–3.0.

[0019] It should be noted that under conditions of 90–120℃ and pH 1.5–3.0, the system is in a strongly acidic and low-water-content organic reaction environment, where formaldehyde is preferentially protonated to form highly electrophilic activated species (such as CH2OH). +The reactivity of the amino group is significantly improved. At the same time, although the amino group in the amino sulfonic acid molecule is mainly in the protonated form, there is always a dynamic equilibrium of instantaneous deprotonation. A small amount of free amino group (–NH2) can rapidly undergo irreversible nucleophilic addition reaction with activated formaldehyde to generate stable N-hydroxymethylsulfonic acid derivatives, thereby continuously consuming free formaldehyde and driving the equilibrium to the bound state. Suitable temperature and time further improve the effective collision frequency and reaction rate between molecules, so that the free aldehyde is efficiently fixed into a non-volatile structure, while inhibiting its reverse reaction or re-release, ultimately achieving a significant reduction in the content of free formaldehyde in the system and an improvement in structural stability.

[0020] In a preferred embodiment of a method for synthesizing low-free-aldehyde phenolic resin, the vacuum degree of the secondary vacuum distillation step is -0.07 MPa to -0.098 MPa.

[0021] It should be noted that this method is beneficial for efficiently removing trace amounts of residual free aldehydes and low-molecular-weight volatiles from the system at lower temperatures, while avoiding thermal degradation or further condensation reactions of the resin, thereby ensuring product structural stability and low free aldehyde content.

[0022] This invention offers the following significant advantages: It utilizes aminosulfonic acid as a bifunctional catalyst to achieve directional ortho-condensation of phenol and formaldehyde at 90℃–110℃. Simultaneously, its amino groups are used to efficiently fix residual free formaldehyde through nucleophilic addition in a subsequent strongly acidic environment (pH 1.5–3.0). Combined with atmospheric pressure dehydration, vacuum phenol removal, and secondary vacuum distillation, the free formaldehyde content of the final product is stably controlled at 8–18 mg / kg, completely eliminating the heavy metal residue risk associated with traditional divalent metal salt catalytic systems. The product's softening point is stable at 118–125℃, and its melt viscosity at 120℃ is 2.5–3.5 Pa·s, exhibiting excellent processing fluidity and storage stability. This method is simple, energy-efficient, and requires no additional formaldehyde-reducing agents, significantly improving the environmental performance and material reliability of phenolic resins. It fully meets the stringent regulatory requirements for low VOC emissions in fields such as electronic packaging, green building materials, and interior decoration, providing a green and high-performance new solution for the thermosetting resin industry. Attached Figure Description

[0023] Figure 1 The infrared spectrum of the phenolic resin prepolymer prepared in Example 1; Figure 2 The infrared spectrum of the N-hydroxymethylsulfonic acid derivative prepared in Example 1. Detailed Implementation

[0024] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0025] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0026] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single embodiment or an embodiment selectively excluded from other embodiments.

[0027] Example 1 This embodiment provides a method for synthesizing low-free-aldehyde phenolic resin, comprising the following steps: Step 1: Condensation Reaction: 1000g of phenol was added to a reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. Then, 37wt% formaldehyde aqueous solution was added to maintain a phenol to formaldehyde molar ratio of 1:0.7. Subsequently, aminosulfonic acid catalyst was added at a rate of 0.5wt% of the phenol mass. Stirring was started and the temperature was raised to 90℃. The system was maintained at this temperature with continuous stirring for 1 h. Under the acid catalysis of the aminosulfonic acid sulfonic acid groups, phenol underwent ortho-hydroxymethylation with formaldehyde, followed by further condensation to form a phenolic resin prepolymer dominated by dibenzyl ether bonds.

[0028] Step 2: Dehydration under normal pressure: After the reaction is completed, the temperature is slowly raised to 115°C under continuous stirring and maintained at normal pressure for 20 minutes. The water and a small amount of low-boiling-point volatiles generated during the reaction are removed by evaporation, so that the system gradually changes from a low-viscosity reaction liquid to a prepolymer resin system with a certain viscosity.

[0029] Step 3: Vacuum phenol removal: The vacuum system is then turned on, and the vacuum level of the system is controlled at −0.06 MPa. The temperature is then increased to 130℃ and removed under these conditions for 30 min. This allows excess phenol in the system to evaporate under reduced pressure, thereby reducing the free phenol content in the system and further increasing the molecular weight of the resin.

[0030] Step 4: Free Aldehyde Binding Treatment: After phenol removal, the system temperature was adjusted to 90℃, and the pH was controlled at 1.5 by adding a small amount of dilute sulfuric acid solution. The reaction was then maintained for 30 min. Under these strongly acidic conditions, the amino groups in the residual aminosulfonic acid molecules in the system underwent a nucleophilic addition reaction with the residual free formaldehyde to generate a stable N-hydroxymethylsulfonic acid derivative, thereby fixing the free aldehyde into a non-volatile structure.

[0031] Step 5: Secondary vacuum distillation and curing: Vacuum distillation is then performed again, with the system vacuum controlled at −0.07 MPa. The vacuum distillation is maintained for 30 min to remove residual low-molecular-weight volatiles and a small amount of free phenol from the system. Then, heating is stopped and the system is cooled to room temperature to obtain a solid low-free-aldehyde phenolic resin product.

[0032] Example 2 This embodiment provides a method for synthesizing low-free-aldehyde phenolic resin, comprising the following steps: Step 1: Condensation reaction: Add 1000 g of phenol and 37 wt% formaldehyde aqueous solution to the reactor to make the molar ratio of phenol to formaldehyde 1:0.8. Then add aminosulfonic acid catalyst, which is 1.5 wt% of the mass of phenol. Under mechanical stirring, the temperature is raised to 100℃ and the reaction is maintained for 2 h to allow phenol and formaldehyde to undergo hydroxymethylation and condensation reaction under acid catalysis to generate phenolic resin prepolymer.

[0033] Step 2: Dehydration under normal pressure: After the reaction is complete, the system is heated to 120°C and maintained under normal pressure for 40 minutes to continuously evaporate the reaction water and some low-boiling substances in the system, thereby reducing the water content of the system and allowing the resin to gradually enter the pre-condensation stage.

[0034] Step 3: Vacuum phenol removal: Then turn on the vacuum system, control the vacuum degree at −0.08 MPa, and raise the temperature to 135℃. Maintain the phenol removal for 60 min. Gradually remove excess phenol from the system by vacuum distillation, so that the resin system can further condense and increase the solid content.

[0035] Step 4: Free aldehyde binding treatment: After the phenol removal is completed, the system temperature is adjusted to 105℃, and the pH of the system is adjusted to 2.2 by adding an appropriate amount of acid regulator. Under these conditions, the reaction is maintained for 60 min, so that the amino group in aminosulfonic acid undergoes a nucleophilic addition reaction with the residual free formaldehyde, thereby converting the free aldehyde in the system into a stable bound structure.

[0036] Step 5: Secondary vacuum distillation and curing: A secondary vacuum distillation process is then performed, with the system vacuum level controlled at −0.085 MPa and distillation continued for 40 min to further remove trace amounts of volatile impurities. After that, heating is stopped and the system is cooled and cured to obtain low free aldehyde phenolic resin.

[0037] Example 3 This embodiment provides a method for synthesizing low-free-aldehyde phenolic resin, comprising the following steps: Step 1: Condensation reaction: Add 1000 g of phenol to the reaction vessel, and add formaldehyde solution to make the molar ratio of phenol to formaldehyde 1:0.9. Then add aminosulfonic acid catalyst, the amount of which is 3 wt% of the mass of phenol. Under mechanical stirring, heat to 110℃ and maintain the reaction for 3 h to allow phenol to fully undergo hydroxymethylation and further condensation to form the phenolic resin backbone structure.

[0038] Step 2: Dehydration under normal pressure: After the reaction is completed, continue to heat to 125℃ under normal pressure and maintain for 60 min to fully evaporate the reaction water and low-boiling substances in the system, so that the viscosity of the system gradually increases and a more stable prepolymer resin system is formed.

[0039] Step 3: Vacuum removal of phenol: Then turn on the vacuum system, control the vacuum degree of the system at −0.095 MPa, and control the system temperature at 140℃, maintain the removal of phenol for 120 min, so that the excess phenol is fully evaporated under reduced pressure.

[0040] Step 4: Free aldehyde binding treatment: After the phenol removal is completed, the system temperature is adjusted to 120℃, and the pH of the system is adjusted to 3.0 by adding an acid regulator. This condition is maintained for 90 min, so that the amino group in the residual aminosulfonic acid in the system can undergo an addition reaction with the free formaldehyde, thereby achieving efficient capture of free aldehyde.

[0041] Step 5: Secondary vacuum distillation and curing: A secondary vacuum distillation process is then performed, with the system vacuum controlled at −0.098 MPa and distillation continued for 60 min to further remove trace amounts of volatiles. Heating is then stopped and the mixture is allowed to cool naturally to room temperature to obtain the low free aldehyde phenolic resin product.

[0042] Example 4 This embodiment provides a method for synthesizing low-free-aldehyde phenolic resin, comprising the following steps: Step 1: Condensation reaction: Add 1000 g of phenol and formaldehyde aqueous solution to the reaction vessel to make the molar ratio of phenol to formaldehyde 1:0.85. Then add aminosulfonic acid catalyst, the amount of which is 2 wt% of the mass of phenol. Under stirring, heat to 105℃ and maintain the reaction for 2.5 h to allow phenol and formaldehyde to fully undergo hydroxymethylation and condensation reaction.

[0043] Step 2: Dehydration under normal pressure: After the reaction is complete, the temperature is raised to 122℃ and maintained under normal pressure for 50 minutes to remove the reaction water and low-boiling substances from the system, so that the system gradually forms a stable prepolymer resin.

[0044] Step 3: Vacuum removal of phenol: The vacuum system is then turned on, the vacuum level of the system is controlled at −0.085 MPa, and the temperature is maintained at 138℃. Phenol is removed under these conditions for 90 min to further reduce the free phenol content in the system.

[0045] Step 4: Free aldehyde binding treatment: After the phenol removal is completed, the system temperature is adjusted to 110℃ and the system pH is adjusted to 2.5. Under these conditions, the reaction is continued for 70 min to allow the amino groups in aminosulfonic acid to react with the residual free formaldehyde to form a stable binding structure.

[0046] Step 5: Secondary vacuum distillation and curing: Finally, a secondary vacuum distillation process is performed to control the vacuum degree of the system at −0.095 MPa and maintain it for 50 min to remove residual volatiles in the system. Then, heating is stopped and the system is cooled and cured to obtain a low free aldehyde phenolic resin product.

[0047] Example 5 This embodiment provides a method for synthesizing low-free-aldehyde phenolic resin, comprising the following steps: Step 1: Condensation Reaction: 1000g of phenol was added to a reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. Then, 37wt% formaldehyde aqueous solution was added to maintain a phenol to formaldehyde molar ratio of 1:0.5. Subsequently, aminosulfonic acid catalyst was added at an amount equal to 2.0wt% of the phenol mass. Stirring was started and the temperature was raised to 90℃. The system was maintained at this temperature with continuous stirring for 1 h. Under the acid catalysis of the aminosulfonic acid sulfonic acid groups, phenol and formaldehyde underwent ortho-hydroxymethylation and further condensation to form a phenolic resin prepolymer dominated by dibenzyl ether bonds.

[0048] Step 2: Dehydration under normal pressure: After the reaction is completed, the temperature is slowly raised to 125°C under continuous stirring and maintained at normal pressure for 20 minutes. The water and a small amount of low-boiling-point volatiles generated during the reaction are removed by evaporation, so that the system gradually changes from a low-viscosity reaction liquid to a prepolymer resin system with a certain viscosity.

[0049] Step 3: Vacuum phenol removal: The vacuum system is then turned on, and the vacuum level of the system is controlled at −0.06 MPa. The temperature is then increased to 130℃ and removed under these conditions for 30 min. This allows excess phenol in the system to evaporate under reduced pressure, thereby reducing the free phenol content in the system and further increasing the molecular weight of the resin.

[0050] Step 4: Free Aldehyde Binding Treatment: After phenol removal, the system temperature was adjusted to 90℃, and the pH was controlled at 2.0 by adding a small amount of dilute sulfuric acid solution. The reaction was then maintained for 30 min. Under these strongly acidic conditions, the amino groups in the residual aminosulfonic acid molecules in the system underwent a nucleophilic addition reaction with the residual free formaldehyde to generate a stable N-hydroxymethylsulfonic acid derivative, thereby fixing the free aldehyde into a non-volatile structure.

[0051] Step 5: Secondary vacuum distillation and curing: Vacuum distillation is then performed again, with the system vacuum controlled at −0.085 MPa. The vacuum distillation is maintained for 30 min to remove residual low-molecular-weight volatiles and a small amount of free phenol from the system. Then, heating is stopped and the system is cooled to room temperature to obtain a solid low-free-aldehyde phenolic resin product.

[0052] Examples 6 to 11 The difference between Examples 6 to 11 and Example 5 is that the molar ratios of phenol and formaldehyde are shown in Table 1 below.

[0053] Table 1 Compare with Example 1 This comparative example is basically the same as Example 1, except that the treatment step of capturing free aldehydes with aminosulfonic acid amino groups is not performed.

[0054] Compare with Example 2 This comparative example is basically the same as Example 1, except that acid adjustment was not performed during the free aldehyde binding treatment.

[0055] Performance testing methods 1. Free formaldehyde content A certain amount of resin sample was weighed, and after distillation or water extraction, it was reacted with acetylacetone reagent to generate a yellow compound. The absorbance was measured at a wavelength of 412 nm using a UV-Vis spectrophotometer, and the free formaldehyde content was calculated using a standard curve. The results were expressed in mg / kg.

[0056] 2. Softening point The resin sample is placed in a standard copper ring and heated to a specified heating rate (usually 5°C / min). The temperature at which the steel ball falls to a specified distance is recorded as the resin softening point. 3. Viscosity Test the viscosity of the sample at a specified temperature (e.g., 120°C or a specified solvent system) and record the stable reading in Pa·s.

[0057] Table 2 In conjunction with Example 1 and Figure 1It can be seen that this phenolic resin prepolymer exhibits obvious absorption peaks at multiple characteristic wavenumbers. Among them, the peak at approximately 3400 cm⁻¹ is particularly prominent. -1 A broad and strong absorption peak appears at 2920 cm⁻¹, which is attributed to the stretching vibrations of phenolic hydroxyl and primary alcohol hydroxyl groups, indicating the presence of phenolic hydroxyl and hydroxymethyl structures in the system; -1 and 2850 cm -1 The absorption peak appearing nearby corresponds to the stretching vibration of CH in the methylene group, indicating that the phenolic rings have undergone condensation polymerization through the methylene structure; 1705 cm⁻¹ -1 The absorption peak at 1600 cm⁻¹ is attributed to the C=O stretching vibration of the aldehyde group, indicating that a small amount of unreacted aldehyde structures may still exist in the system; -1 and 1510 cm -1 The peak at 1260 cm⁻¹ is a characteristic peak of the C=C vibration of the aromatic ring skeleton, indicating that the resin matrix is ​​aromatic; -1 1210cm -1 and 1150~1100 cm -1 The distinct absorption peaks within the range correspond to the stretching vibrations of hydroxymethyl CO, aryl ether bonds, and methylene ether bridge structures, respectively, indicating the formation of numerous ether bond linkages in the system, especially the dibenzyl ether bond (-CH2-O-CH2-) formed by two benzyl structures linked by oxygen atoms, gradually forming a main chain structure with the dibenzyl ether bond as an important linking unit; furthermore, at 830 cm⁻¹... -1 and 750 cm -1 The absorption peaks appearing nearby correspond to the out-of-plane bending vibrations of CH in the ortho- and para-substituted benzene rings, respectively, further indicating that the benzene rings mainly undergo ortho- and para-substituted reactions. In summary, the infrared spectroscopy results show that the system simultaneously contains structural units such as phenolic hydroxyl groups, hydroxymethyl groups, methylene bridges, and dibenzyl ether bonds, indicating that the phenolic condensation reaction has occurred, forming a phenolic resin prepolymer molecular skeleton with dibenzyl ether bonds as an important connecting structure.

[0058] In conjunction with Example 1 and Figure 2 It can be seen that, in conjunction with Example 1 and Figure 2 It can be seen that this N-hydroxymethylsulfonic acid derivative exhibits distinct absorption peaks at multiple characteristic wavenumbers. Among them, the peak at approximately 3400 cm⁻¹ is particularly prominent. -1 A broad and strong absorption peak appears at 3300 cm⁻¹, which is attributed to the OH stretching vibration of the alcohol hydroxyl group (-CH₂OH), indicating the presence of a stable hydroxymethyl structure in the system; -1 The presence of an NH stretching vibration peak at 2920 cm⁻¹ indicates that the secondary amino group (-NH⁻) still retains some activity; -1 and 2850 cm -1 The absorption peak appearing nearby corresponds to the CH stretching vibration of the methylene group (-CH2-); 1550 cm⁻¹-1 The peak of the NH bending vibration is located at 1460 cm⁻¹. -1 The peak at 1375 cm⁻¹ represents the CH₂ bending vibration. -1 and 1180 cm -1 The positions correspond to the S=O asymmetric and symmetric stretching vibrations of the sulfonic acid group (-SO3H), respectively; 1100 cm -1 The peak value for CO / CN stretching vibration is located at 1040 cm⁻¹. -1 The peak at point 1 indicates the SO stretching vibration, confirming the complete presence of the three major functional groups: hydroxyl, secondary amino, and sulfonic acid groups. Infrared spectroscopy results show that under the strongly acidic conditions (pH 1.5, 90℃, 30 min) in step 4 of Example 1, the amino group of the residual aminosulfonic acid successfully underwent a nucleophilic addition reaction with the residual free formaldehyde, generating a stable N-hydroxymethylsulfonic acid derivative (HOCH2-NH-SO3H). This chemically fixes the free aldehyde into a non-volatile, non-free structure, fully verifying the feasibility of the low-free aldehyde technology of this invention.

[0059] As can be seen from Examples 1 to 4 and Table 2, the free formaldehyde content of the low-free aldehyde phenolic resin product prepared by the present invention is stably controlled at 8–18 mg / kg, the softening point range is 118–125℃, and the melt viscosity at 120℃ is 2.5–3.5 Pa·s. Overall, it exhibits characteristics of extremely low free aldehyde content, moderate softening point, and good fluidity, which fully verifies the stability and excellent performance of the aminosulfonic acid bifunctional catalysis and secondary vacuum distillation process under different molar ratios (1:0.7–0.9) and catalyst dosages (0.5 wt%–3 wt%).

[0060] As can be seen from Examples 5 to 11 and Table 2, as the molar ratio of phenol to formaldehyde gradually increases from 1:0.5 to 1:1.1, the softening point of the resin gradually increases from 112℃ to 130℃, the melt viscosity at 120℃ increases from 2.2 Pa·s to 4.2 Pa·s, and the free formaldehyde content increases significantly from 6 mg / kg to 60 mg / kg. Among these, within the range of 1:0.7 to 0.9, the free formaldehyde content remains at 8 to 18 mg / kg, while the softening point and viscosity are at a moderate level, exhibiting the best overall performance. The reason is that as the amount of formaldehyde increases, the degree of hydroxymethylation and condensation reaction in the system increases, and the number of methylene bridges and ether bonds increases, which in turn increases the molecular weight and cross-linking degree of the resin, resulting in an increase in softening point and viscosity. However, when formaldehyde is in excess, it exceeds the range that phenol can react and consume, and some formaldehyde fails to participate in effective condensation or exists only in an unstable form, resulting in an increase in free formaldehyde residue. At a lower formaldehyde ratio, although the free formaldehyde content is lower, the molecular weight is lower due to insufficient condensation reaction, which in turn leads to a decrease in softening point and viscosity.

[0061] As can be seen from Example 1, Comparative Example 1, and Table 2, Comparative Example 1, due to the absence of the amino group capture step of aminosulfonic acid, resulted in a free formaldehyde content as high as 450 mg / kg (Example 1 only 8 mg / kg), a slightly lower softening point of 115℃, and a viscosity of 2.0 Pa·s. In contrast, Example 1 converted the residual formaldehyde into a stable N-hydroxymethylsulfonic acid derivative through the nucleophilic addition reaction in step 4, thereby achieving the chemical fixation of free aldehydes, significantly reducing the risk of volatilization, while maintaining the integrity of the main chain structure and avoiding the problem of free aldehyde re-release in traditional processes.

[0062] As can be seen from Example 1, Comparative Example 2 and Table 2, although Comparative Example 2 underwent free aldehyde binding treatment, the pH of the system was not adjusted to 1.5-3.0, resulting in a free formaldehyde content of 180 mg / kg (compared to only 8 mg / kg in Example 1), a softening point of 117℃, and a viscosity of 2.3 Pa·s. In contrast, Example 1 significantly enhanced the nucleophilic addition ability of the amino groups under a strongly acidic environment, enabling the free aldehyde to be efficiently converted into a non-volatile structure. Therefore, the free aldehyde content was greatly reduced, while ensuring the stability of the resin molecular weight distribution and avoiding the decrease in amino activity or the increase in side reactions caused by excessively high pH, ​​ultimately obtaining a product with excellent storage performance.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for synthesizing a low-free-aldehyde phenolic resin, characterized in that, The process includes the following steps: using aminosulfonic acid as a catalyst, excess phenol and formaldehyde are reacted at 90℃~110℃; after the reaction, the mixture is successively dehydrated at atmospheric pressure to 115~125℃ and dephenolized under vacuum to 130~140℃; then, the amino groups of aminosulfonic acid are used to bind free aldehydes, followed by a second vacuum distillation or filtration, and finally cooled and cured to obtain the low free aldehyde phenolic resin.

2. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, The molar ratio of phenol to formaldehyde is 1:(0.7-0.9).

3. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, The amount of aminosulfonic acid added is 0.5wt% to 3wt% of the mass of phenol.

4. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, The stirring reaction time in the above reaction step is 1 to 3 hours.

5. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, In the atmospheric pressure dehydration step, the system temperature is controlled at 115–125°C and maintained for 20–60 minutes.

6. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, The vacuum degree of the vacuum phenol removal step is -0.06 MPa to -0.095 MPa.

7. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, In the vacuum phenol removal step, the system temperature is controlled at 130–140°C, and the removal time is 30–120 min.

8. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, The free aldehyde binding treatment temperature is 90–120°C, the treatment time is 30–90 min, and the pH of the reaction system is 1.5–3.

0.

9. The method for synthesizing low-free-aldehyde phenolic resin according to claim 1, characterized in that, The vacuum level of the secondary vacuum distillation step is -0.07 MPa to -0.098 MPa.

10. A low-free-aldehyde phenolic resin prepared by the synthesis method according to claim 1.

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