Schiff base epoxy oligomer chain extender as well as preparation method and application thereof

Abstract

The invention discloses a Schiff base epoxy oligomer chain extender as well as a preparation method and application thereof. The preparation method of the Schiff base epoxy oligomer chain extender comprises the following steps: adding p-hydroxybenzaldehyde and p-hydroxyaniline into ethanol, reacting at 30-70 DEG C for 2-5 hours, and performing suction filtration to obtain a 4-((4-hydroxybenzylidene) amino) phenol intermediate; and mixing the 4-((4-hydroxybenzylidene) amino) phenol intermediate, epoxy chloropropane and a catalyst, reacting at 80-100 DEG C for 3-7 hours, cooling to 50-70 DEG C, dropwise adding a strong alkali solution, reacting for 4-8 hours after dropwise adding, washing with deionized water, retaining an organic layer, and carrying out rotary evaporation on the organic layer to obtain the product. The rigidity, toughness and heat resistance of the modified nylon 6 prepared by applying the Schiff base epoxy oligomer chain extender are improved, and the modified nylon 6 still has processability similar to that of nylon 6.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material modification additives, and in particular to a Schiff base epoxy oligomer, a preparation method thereof, and application of the oligomer as a chain extender in nylon 6. Background Art

[0002] Epoxy chain extenders are widely used due to their low cost, wide variety, safety, and high efficiency. Epoxy chain extenders contain highly active epoxy groups. During polymer melt processing, these groups can react with various groups at the ends of macromolecular chains, such as hydroxyl, amino, and carboxyl groups, to link the macromolecular chains and increase the molecular weight of the polymer. Furthermore, the epoxy groups react with secondary hydroxyl groups generated by the chain extension reaction to form branched structures, increasing molecular chain entanglement and intermolecular interactions, thereby improving the mechanical properties of the polymer. Therefore, epoxy chain extenders are often used as modifiers to increase the rigidity and toughness of polymers.

[0003] Nylon 6 (PA6), one of the most important thermoplastic polymers, accounts for a significant portion of industrial production and use. Its excellent mechanical properties, chemical resistance, and processing properties make it widely used in various fields. However, PA6 also has some shortcomings, such as its high sensitivity to notches, which results in low notched impact strength. The amide group is highly hydrophilic, making PA6 highly susceptible to moisture absorption. Residual moisture causes molecular chains to break during processing, reducing molecular weight and, consequently, mechanical properties. Because PA6 contains amino and carboxyl groups at its ends, the addition of epoxy chain extenders to PA6 can reconnect broken molecular chains, increasing its molecular weight and thus improving its mechanical properties.

[0004] Chinese invention patent CN201410108006.0 discloses a flame retardant extended chain nylon 6 composition and its preparation method, which uses diepoxy chain extender P-EP to extend and modify flame retardant PA6. When the amount of P-EP is 3.1wt%, the tensile strength, flexural strength and notched impact strength of the modified PA6 are increased by 34%, 24% and 19%, respectively. The existing technical literature (Effects of grafting and long-chain branching structures on rheological behavior, crystallization properties, foaming performance, and mechanical properties of polyamide 6.2022; 22(1): 249-263) uses triglycidyl isocyanurate (TGIC) containing three epoxy groups to extend and modify PA6. The tensile strength, notched impact strength and elongation at break of PA6 modified with 0.5wt% TGIC are increased by 14.2%, 265% and 26.3%, respectively, compared with unmodified PA6. Chinese invention patent CN202211067733.8 discloses modified polyamide 6, modified polyamide 6 products, their preparation methods, and applications. The modified PA6 is prepared using a tetraepoxy chain extender, N,N,N,N-tetraglycidyl 4,4'diaminodiphenylmethane. The tensile strength, modulus, and notched impact strength of the modified PA6 increased by 38%, 33%, and 97%, respectively. While the epoxy chain extenders used in the above prior art improve the mechanical properties of PA6, the chain extension modification increases the degree of entanglement of the PA6 molecular chains, restricting the movement of chain segments and increasing melt flow resistance, resulting in a decrease in the processing performance of PA6. Therefore, improving the processing performance of chain-extended modified PA6 and obtaining higher mechanical properties are of great significance to enhancing the application value of chain-extended modified PA6. Summary of the Invention

[0005] In response to the shortcomings of existing chain extension modification technologies that lead to a decrease in the processing performance of PA6, the present invention aims to provide an epoxy oligomer containing dynamically reversible covalent bonds, which, as a chain extender, has the effect of improving the molecular weight, rigidity, toughness and heat resistance of PA6, especially while maintaining the good processing performance of nylon 6. This Schiff base epoxy oligomer chain extender and its preparation method are also provided.

[0006] Another object of the present invention is to provide the use of the Schiff base epoxy oligomer chain extender in the preparation of chain-extended modified nylon 6.

[0007] The object of the present invention is achieved through the following technical solutions:

[0008] A Schiff base epoxy oligomer chain extender having the following structural formula:

[0009]

[0010] Among them, -CH=N- is a Schiff base structure, and n≥1.

[0011] The preparation method of the Schiff base epoxy oligomer chain extender comprises the following steps: adding p-hydroxybenzaldehyde and p-hydroxyaniline to ethanol, reacting at 40°C to 70°C for 2h to 5h, and filtering to obtain a 4-((4-hydroxybenzylidene)amino)phenol intermediate; mixing the 4-((4-hydroxybenzylidene)amino)phenol intermediate, epichlorohydrin, and an ammonium salt catalyst with a phase transfer effect, reacting at 80°C to 100°C for 3h to 7h, cooling to 50°C to 70°C, adding an aqueous solution of a strong base, reacting for 4h to 8h after the addition is complete, washing with deionized water, retaining the organic layer, and rotary evaporation to obtain the Schiff base epoxy oligomer chain extender.

[0012] To further achieve the purpose of the present invention, preferably, the molar ratio of p-hydroxybenzaldehyde to p-hydroxyaniline is 1:(1-1.3).

[0013] Preferably, the ammonium salt catalyst having a phase transfer effect is tetrabutylammonium bromide, tetrabutylammonium chloride or benzyltriethylammonium chloride.

[0014] Preferably, the molar ratio of the 4-((4-hydroxybenzylidene)amino)phenol intermediate to epichlorohydrin is 1:(5-12), and the amount of the catalyst used is 1% to 3% of the 4-((4-hydroxybenzylidene)amino)phenol intermediate in molar percentage; the molar ratio of the 4-((4-hydroxybenzylidene)amino)phenol intermediate to the strong base is 1:(2-4).

[0015] Preferably, the concentration of the alkaline solution is 10 wt% to 50 wt%, and the dropping time is 0.3 h to 1 h.

[0016] Preferably, the strong base is sodium hydroxide, potassium hydroxide or potassium carbonate.

[0017] Preferably, the volume of the deionized water is 1 to 3 times that of the liquid mixture formed by the 4-((4-hydroxybenzylidene)amino)phenol intermediate, epichlorohydrin and the catalyst.

[0018] Preferably, the rotary evaporation is carried out at 60°C to 90°C for 2 hours to 4 hours.

[0019] The application of the Schiff base epoxy oligomer chain extender in the preparation of chain-extended modified nylon 6 comprises the following components, calculated by mass: 97 to 99 parts of nylon 6 resin; and 1 to 3 parts of the Schiff base epoxy oligomer chain extender.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0021] 1) The compounds disclosed in the present invention can improve the molecular weight, rigidity, toughness and heat resistance of PA6 through chain extension reaction, and at the same time introduce a dynamically reversible covalent Schiff base into the molecular chain of PA6 through chain extension reaction. The Schiff base undergoes an exchange reaction at the processing temperature of PA6, and the melt equilibrium torque of the modified PA6 is almost unchanged compared with PA6, showing good processing properties. When the dosage is 3wt%, the intrinsic viscosity of PA6 increases from 1.32dl / g to 3.04dl / g; the melt equilibrium torque increases by only 0.6N·m compared with the unmodified state; the tensile strength increases from 65.6MPa to 75.8MPa; the tensile modulus increases from 2.4GPa to 2.7GPa; the notched impact strength increases from 3.2kJ / m 2 Increased to 7.9kJ / m 2 ; The heat deformation temperature increased from 74.9℃ to 76.1℃.

[0022] 2) The chain extender of the present invention can be used to modify PA6 through common processing equipment such as an extruder, with low cost and small addition amount. At the same time, the chain extender modifies PA6 through a chemical chain extension reaction, avoiding precipitation of the additive. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the FTIR spectrum of the Schiff base epoxy oligomer prepared in Example 1 of the present invention.

[0024] Figure 2 The Schiff base epoxy oligomer prepared in Example 1 of the present invention 1 H NMR spectrum. DETAILED DESCRIPTION

[0025] For a better understanding of the present invention, the present invention is further described below with reference to the accompanying drawings and specific embodiments, but the embodiments of the present invention are not limited thereto. The embodiments described are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0026] The present invention aims to address the shortcomings of existing chain extension and modification technologies that result in reduced processing properties of PA6. The present invention provides an epoxy oligomer containing dynamically reversible covalent bonds. This Schiff base-based epoxy oligomer chain extender, as well as a method for preparing the same, is effective as a chain extender for improving the molecular weight, rigidity, toughness, and heat resistance of PA6, while also maintaining the excellent processing properties of nylon 6. The key to improving the processing properties of chain-extended PA6 lies in promoting molecular chain disentanglement, thereby reducing the hindrance to macromolecular motion and improving melt fluidity. Introducing dynamically reversible covalent bonds into the chain extender is an effective approach. Schiff bases are one of the most common dynamically reversible covalent bonds. Therefore, the Schiff base-based epoxy oligomer chain extender of the present invention incorporates a Schiff base structure (-CH=N-). During the chain extension reaction, the Schiff base in the molecular chain undergoes an exchange reaction, promoting the disentanglement of entangled macromolecular chains, reducing the hindrance to macromolecular chain motion, and restoring melt fluidity, thereby improving the processing properties of the modified PA6. Furthermore, the rigid structure of the Schiff base imparts higher mechanical strength to PA6. At the same time, epoxy groups are retained at both ends of the chain extender of the present invention. However, it should be noted that the purpose of the present invention is not achieved by simultaneously containing Schiff bases and epoxy groups in the chain extender molecular chain. For example, Chinese invention patent application CN108484534A discloses an epoxy-type chain extender and its preparation method, in which the chain extender molecular chain contains a Schiff base structure and two or more epoxy groups at both ends of the molecule. However, the structure of this technology is quite different from that of the present invention, especially in that this technology connects the Schiff base structure to the polymer backbone, with the purpose of enabling the Schiff base structure to undergo a chemical self-crosslinking reaction at a higher temperature to form an expanded aromatized carbon layer structure, so as to increase the viscosity of the melt, play a role in flame retardancy and suppressing droplets. Therefore, the chain extender of this technology is to improve the flame retardant properties of PA6 and improve the deterioration of the mechanical properties of flame-retardant PA6 caused by the flame retardant, and is targeted at the flame retardant system, while the present invention is to improve the processing performance of chain-extended modified PA6 while improving the mechanical properties of chain-extended modified PA6. Therefore, this technology is very different from the present invention in terms of synthesis temperature and conditions. The chemical self-crosslinking reaction of this technology is a reaction of Schiff base in the polymer during combustion. The exchange reaction of Schiff base in the present invention is a reaction that occurs during the melt processing of the polymer. The melt processing temperature is 240 ° C, which is much lower than the temperature of Schiff base when the polymer involved in CN108484534A is burned. At the same time, this technology uses a solvent as a reaction medium, and the solvent is used for post-treatment steps. 1,4-dioxane and N,N-dimethylformamide are high-boiling point solvents, which are difficult to completely remove from the product. Removing high-boiling point solvents increases energy consumption in the preparation process. The solvents dichloromethane and chloroform are highly toxic and pose a hazard to human health. The preparation of the present invention does not require the addition of additional solvents. The intermediate and epichlorohydrin can be reacted by utilizing the liquid nature of epichlorohydrin itself, thereby reducing the use of solvents and reducing preparation costs.

[0027] The present invention provides a Schiff base epoxy oligomer chain extender having the following structural formula:

[0028]

[0029] Among them, -CH=N- is a Schiff base structure, and n≥1.

[0030] The preparation method of the Schiff base epoxy oligomer chain extender comprises the following steps: adding p-hydroxybenzaldehyde and p-hydroxyaniline to ethanol, reacting at 40-70°C for 2-5 hours, and filtering to obtain a 4-((4-hydroxybenzylidene)amino)phenol intermediate; mixing the 4-((4-hydroxybenzylidene)amino)phenol intermediate, epichlorohydrin and an ammonium salt catalyst with a phase transfer effect, reacting at 80-100°C for 3-7 hours, cooling to 50-70°C, dropping an aqueous solution of a strong base, reacting for 4-8 hours after the dropwise addition is complete, washing with deionized water, retaining an organic layer, and rotary evaporation to obtain the Schiff base epoxy oligomer chain extender.

[0031] In this preparation method, the amounts of the relevant raw materials can be determined based on the purpose of the invention and the reaction mechanism. For example, the molar ratio of p-hydroxybenzaldehyde to p-hydroxyaniline is preferably 1:(1-1.3); the molar ratio of the 4-((4-hydroxybenzylidene)amino)phenol intermediate to epichlorohydrin is preferably 1:(5-12). In terms of molar percentage, the amount of catalyst used is preferably 1% to 3% of the 4-((4-hydroxybenzylidene)amino)phenol intermediate; the molar ratio of the 4-((4-hydroxybenzylidene)amino)phenol intermediate to the strong base is preferably 1:(2-4); and the volume of deionized water used is preferably 1 to 3 times the volume of the liquid mixture formed by the 4-((4-hydroxybenzylidene)amino)phenol intermediate, epichlorohydrin, and catalyst.

[0032] The catalyst of the present invention is an ammonium salt catalyst with phase transfer effect. In view of the characteristics of phase transfer effect and ammonium salt, tetrabutylammonium bromide, tetrabutylammonium chloride or benzyltriethylammonium chloride can be preferably used.

[0033] As for the above preparation method, the concentration of the aqueous solution of the strong base is preferably 10wt% to 50wt%, and the strong base is selected from sodium hydroxide, potassium hydroxide or potassium carbonate, which is a conventional practice in the field; rotary evaporation is also a conventional practice in this field. Through experiments, it can be preferably performed at 60°C to 90°C for 2h to 4h.

[0034] In summary, the present invention provides a Schiff base epoxy oligomer, a preparation method thereof, and an application in chain extension modification of PA6. The Schiff base epoxy oligomer can not only improve the molecular weight, rigidity, toughness, and heat resistance of PA6, but also maintain the good processing performance of nylon 6.

[0035] Example 1

[0036] 0.2 mol of p-hydroxybenzaldehyde, 0.26 mol of p-hydroxyaniline, and 300 mL of ethanol were added to a three-necked flask equipped with a reflux condenser and stirred at 70°C for 2 hours. After the reaction, the resulting mixture was filtered, washed with ethanol, and dried to obtain a 4-((4-hydroxybenzylidene)amino)phenol intermediate.

[0037] 0.1 mol of 4-((4-hydroxybenzylidene)amino)phenol intermediate, 1.2 mol of epichlorohydrin and 0.003 mol of benzyltriethylammonium chloride were added to a three-necked flask and reacted under a nitrogen atmosphere with magnetic stirring at 100° C. for 3 h. The temperature of the reaction system was then lowered to 70° C., and a 50 wt % aqueous solution of potassium carbonate (0.4 mol of potassium carbonate) was added dropwise to the reaction system over 1 h. After the addition was complete, the reaction was continued under a nitrogen atmosphere at 70° C. with stirring for 4 h. The system was allowed to cool naturally to room temperature and then washed with 375 ml of deionized water. The mixture was allowed to stand for separation, and the organic phase was retained. The washing operation was repeated three times. The organic phase was dried over anhydrous sodium sulfate and finally rotary evaporated at 90° C. for 2 h to remove excess epichlorohydrin to obtain a Schiff base epoxy oligomer chain extender (DE-1). The epoxy value thereof was measured by a hydrochloric acid-acetone method and was 0.39 mol / 100 g.

[0038] Example 2

[0039] 0.2 mol of p-hydroxybenzaldehyde, 0.2 mol of p-hydroxyaniline, and 300 mL of ethanol were added to a three-necked flask equipped with a reflux condenser and stirred at 40°C for 5 hours. After the reaction, the resulting mixture was filtered, washed with ethanol, and dried to obtain a 4-((4-hydroxybenzylidene)amino)phenol intermediate.

[0040] 0.1 mol of 4-((4-hydroxybenzylidene)amino)phenol intermediate, 0.5 mol of epichlorohydrin and 0.001 mol of tetrabutylammonium chloride were added to a three-necked flask and reacted under a nitrogen atmosphere with magnetic stirring at 80° C. for 7 h. The temperature of the reaction system was then lowered to 50° C., and a 10 wt % aqueous potassium hydroxide solution (0.2 mol of potassium hydroxide) was added dropwise to the reaction system over 0.3 h. After the addition was complete, the reaction was continued under a nitrogen atmosphere at 50° C. with stirring for 8 h. The system was allowed to cool naturally to room temperature and then washed with 125 ml of deionized water. The mixture was allowed to stand for separation, and the organic phase was retained. The washing operation was repeated three times. The organic phase was dried over anhydrous sodium sulfate and finally rotary evaporated at 60° C. for 4 h to remove excess epichlorohydrin to obtain a Schiff base epoxy oligomer chain extender (DE-2). The epoxy value thereof was 0.42 mol / 100 g as measured by the hydrochloric acid-acetone method.

[0041] Example 3

[0042] 0.2 mol of p-hydroxybenzaldehyde, 0.21 mol of p-hydroxyaniline, and 300 mL of ethanol were added to a three-necked flask equipped with a reflux condenser and stirred at 60° C. for 4 h. After the reaction, the resulting mixture was filtered, washed with ethanol, and dried to obtain a 4-((4-hydroxybenzylidene)amino)phenol intermediate.

[0043] 0.1 mol of 4-((4-hydroxybenzylidene)amino)phenol intermediate, 1.0 mol of epichlorohydrin and 0.002 mol of tetrabutylammonium bromide were added to a three-necked flask and reacted under a nitrogen atmosphere with magnetic stirring at 90° C. for 5 h. The temperature of the reaction system was then lowered to 60° C., and a 40 wt % aqueous sodium hydroxide solution (0.3 mol of sodium hydroxide) was added dropwise to the reaction system over 0.5 h. After the addition was complete, the reaction was continued under a nitrogen atmosphere at 60° C. with stirring for 6 h. The system was allowed to cool naturally to room temperature and then washed with 250 ml of deionized water. The mixture was allowed to stand for separation, and the organic phase was retained. The washing operation was repeated three times. The organic phase was dried over anhydrous sodium sulfate and finally rotary evaporated at 80° C. for 3 h to remove excess epichlorohydrin to obtain a Schiff base epoxy oligomer chain extender (DE-3). The epoxy value thereof was 0.50 mol / 100 g as measured by the hydrochloric acid-acetone method.

[0044] Application Example 1

[0045] 99 g of PA6 resin (PA6) and 1 g of DE-1 prepared in Example 1 were weighed and added to a high-speed mixer and stirred at a speed of 300 r / min for 5 min. The mixture was then added to a twin-screw extruder, and the melt extrusion temperature was set at 230° C. to 250° C. and the screw speed was set at 60 r / min to 90 r / min. After extrusion, cooling, and pelletizing, modified PA6 was obtained.

[0046] Application Example 2

[0047] 98 g of PA6 resin (PA6) and 2 g of DE-2 prepared in Example 2 were weighed and added to a high-speed mixer and stirred at a speed of 300 r / min for 5 min; then added to a twin-screw extruder, the melt extrusion temperature was set at 230° C. to 250° C., and the screw speed was set at 60 r / min to 90 r / min; after extrusion, cooling, and pelletizing, modified PA6 was obtained.

[0048] Application Example 3

[0049] 97 g of PA6 resin (PA6) and 3 g of DE-3 prepared in Example 3 were weighed and added to a high-speed mixer and stirred at a speed of 300 r / min for 5 min. The mixture was then added to a twin-screw extruder, and the melt extrusion temperature was set at 230° C. to 250° C. and the screw speed was set at 60 r / min to 90 r / min. Modified PA6 was obtained after extrusion, cooling, and pelletizing.

[0050] Comparative Application Example 1

[0051] 100 g of PA6 resin was weighed and added to a twin-screw extruder. The melt extrusion temperature was set at 230° C. to 250° C. and the screw speed was set at 60 r / min to 90 r / min. Pure PA6 was obtained after extrusion, cooling, and pelletizing.

[0052] Application Comparative Example 2

[0053] 99 g of PA6 resin (PA6) and 1 g of epoxy resin E51 (excluding Schiff base) were weighed and stirred at a speed of 300 r / min for 5 min. The mixture was then added to a twin-screw extruder, and the melt extrusion temperature was set at 230° C. to 250° C. and the screw speed was set at 60 r / min to 90 r / min. Modified PA6 was obtained after extrusion, cooling, and pelletizing.

[0054] The test method is as follows:

[0055] Mechanical Properties: Tensile testing was performed on samples according to ASTM D-638 using a SHIMADZU AGX universal testing machine at a rate of 50 mm / min. Flexural testing was performed on samples according to ASTM D-790 using an AGS-10KNI universal testing machine at a rate of 20 mm / min. Notched impact testing was performed on samples according to ASTM D-256 using a Zwick 5113 digital pendulum impact tester at an impact load of 2.75 J.

[0056] Processing Performance Testing: The melt equilibrium torque of the sample was measured using a torque rheometer (RTOI-55 / 20, Guangzhou Putong). The mixing chamber temperature was set at 240°C and the rotational speed was set at 30 rpm. The test was conducted after the mixing chamber was kept at this temperature for 30 minutes. The sample was added to the torque rheometer and mixed until the melt torque remained constant. The melt equilibrium torque of the sample was then recorded.

[0057] Heat Deflection Temperature Test: Samples were tested using a Vicat softening point temperature tester (VTM1300-A1, Shenzhen Sansi Zongheng). The test temperature range was 30°C to 200°C, with a heating rate of 2°C / min. The sample deflection changed over time under a bending stress of 1.80 MPa, and the heat deformation temperature of the sample was recorded.

[0058] Molecular weight determination: Use an Ubbelohde viscometer to determine the intrinsic viscosity of the sample. Dissolve the sample completely in m-cresol using m-cresol as the solvent and record the time it takes for the solution to pass through the two scale lines on the Ubbelohde viscometer. Use the extrapolation method to determine the intrinsic viscosity of the sample.

[0059] Table 1 Performance of application examples and comparative examples

[0060]

[0061] As can be seen from Table 1, the modified PA6 obtained in the application examples of the present invention has higher molecular weight, rigidity, toughness and heat resistance, while maintaining good processing performance. When the addition amount of the Schiff base epoxy oligomer of the present invention to PA6 is 3wt% (Application Example 3), the intrinsic viscosity is 3.0dl / g, which is 1.7dl / g higher than that of the unmodified PA6 (Application Comparative Example 1), indicating that the Schiff base epoxy oligomer increases the molecular weight of PA6. The modified PA6 with 1wt% E51 added (Application Comparative Example 2) formed a local gel, and the sample could not be completely dissolved, so the intrinsic viscosity data could not be obtained. The tensile strength, tensile modulus, flexural strength, flexural modulus and notched impact strength of Application Example 3 were 75.8MPa, 2.7GPa, 99.4MPa, 2.8GPa and 7.9kJ / m respectively. 2 , respectively, increased by 15.5%, 12.5%, 3.1%, 3.7% and 147% compared with the unmodified PA6 (application comparison example 1). The melt equilibrium torque of application example 3 is 3.5N·m, while the melt equilibrium torque of application comparison example 2 (without Schiff base) is 13.8N·m. The melt equilibrium torque of application example 3 is significantly lower than that of application comparison example 2. Because the epoxy chain extender contains a Schiff base structure, while the chain extension reaction occurs, the Schiff base in the molecular chain undergoes an exchange reaction to promote the disentanglement of the entangled macromolecular chains, the movement resistance of the macromolecular chains is reduced, the melt fluidity is restored, the melt equilibrium torque decreases, and the processing performance is higher. At the same time, the melt equilibrium torque of application example 3 only increases by 0.6N·m compared with the unmodified PA6 (application comparison example 1). The heat deformation temperature of application example 3 is 76.1°C, which is 1.2°C higher than that of the unmodified PA6 (application comparison example 1).

[0062] The Schiff base epoxy oligomers of the present invention are highly effective in PA6 modification. At relatively low addition levels, they can improve the rigidity, toughness, and heat resistance of PA6 while maintaining good processing properties. Furthermore, the preparation method for the Schiff base chain extender of the present invention utilizes a wide range of raw materials, is inexpensive, has simple synthesis steps, and is highly efficient.

[0063] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A Schiff base epoxy oligomer chain extender, characterized in that It has the following structural formula: Among them, -CH=N- is a Schiff base structure, and n≥1.

2. The method for preparing the Schiff base epoxy oligomer chain extender according to claim 1, characterized in that: Add p-hydroxybenzaldehyde and p-hydroxyaniline to ethanol, react at 40°C to 70°C for 2h to 5h, and filter to obtain a 4-((4-hydroxybenzylidene)amino)phenol intermediate; mix the 4-((4-hydroxybenzylidene)amino)phenol intermediate, epichlorohydrin and an ammonium salt catalyst with a phase transfer effect, react at 80°C to 100°C for 3h to 7h, cool to 50°C to 70°C, add an aqueous solution of a strong base, react for 4h to 8h after the addition is complete, wash with deionized water, retain the organic layer, and rotary evaporate to obtain a Schiff base epoxy oligomer chain extender.

3. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The molar ratio of p-hydroxybenzaldehyde to p-hydroxyaniline is 1:(1-1.3).

4. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The ammonium salt catalyst with phase transfer effect is tetrabutylammonium bromide, tetrabutylammonium chloride or benzyltriethylammonium chloride.

5. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The molar ratio of the 4-((4-hydroxybenzylidene)amino)phenol intermediate to epichlorohydrin is 1:(5-12), and the amount of the catalyst used is 1% to 3% of the 4-((4-hydroxybenzylidene)amino)phenol intermediate in terms of molar percentage; the molar ratio of the 4-((4-hydroxybenzylidene)amino)phenol intermediate to the strong base is 1:(2-4).

6. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The concentration of the alkaline solution is 10-50 wt %, and the dropping time is 0.3-1 h.

7. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The strong base is sodium hydroxide, potassium hydroxide or potassium carbonate.

8. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The volume of the deionized water is 1 to 3 times that of the liquid mixture formed by the 4-((4-hydroxybenzylidene)amino)phenol intermediate, epichlorohydrin and the catalyst.

9. The method for preparing a Schiff base epoxy oligomer chain extender according to claim 2, wherein: The rotary evaporation is carried out at 60° C. to 90° C. for 2 h to 4 h.

10. Use of the Schiff base epoxy oligomer chain extender according to claim 1 in the preparation of chain-extended modified nylon 6, characterized in that: The composition comprises the following components in parts by mass: 97 to 99 parts of nylon 6 resin; 1 to 3 parts of Schiff base epoxy oligomer chain extender.

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