A method for preparing intrinsically flame-retardant nylon 6

Abstract

This invention discloses a method for preparing intrinsically flame-retardant nylon 6, belonging to the field of polymer material synthesis technology. The method includes the following steps: 1) reacting caprolactam, an initiator, and a heat stabilizer under nitrogen protection at 160-180℃ and 0.1-0.3 MPa for 2-3 hours to prepare a nylon prepolymer (oligopolymer) with a number-average molecular weight of 500-2000; 2) adding terminal amino-terminated organosilicon to the nylon prepolymer, and copolymerizing at 200-230℃ and 0.3-0.6 MPa for 3-5 hours, followed by atmospheric pressure reaction, vacuum devolatilization, cooling, and granulation to obtain intrinsically flame-retardant nylon 6 particles; 3) optionally, subjecting the particles to boiling and drying post-treatment to obtain the final product. This invention employs a two-step process of first preparing the prepolymer and then copolymerizing and modifying it, enabling the terminal amino-terminated organosilicon to be covalently embedded in the nylon 6 molecular chain, achieving intrinsic flame retardancy and solving problems such as easy migration of flame retardants, short-lasting flame retardant performance, and decreased mechanical properties in existing technologies. The resulting intrinsically flame-retardant nylon 6 has high flame retardant efficiency, excellent mechanical properties, and simple and controllable process, making it suitable for large-scale industrial production and widely used in electronics, automobiles, textiles and other fields.

Description

Technical Field

[0001] This invention belongs to the field of polymer material synthesis technology, specifically relating to a method for preparing intrinsically flame-retardant nylon 6, and more particularly relating to a method for preparing intrinsically flame-retardant nylon 6 with permanent flame-retardant properties by using nylon prepolymer (oligopolymer) generated by caprolactam polymerization as raw material and copolymerizing it with terminal amino organosilicon. Background Technology

[0002] Nylon 6 (polycaprolactam) is a core variety of nylon material, accounting for over 80% of consumption. It possesses excellent mechanical properties, thermal stability, oil resistance, abrasion resistance, and processability, and is widely used in civilian, industrial, and electronic applications. However, the linear aliphatic molecular structure of nylon 6 makes it flammable, with a limiting oxygen index of only 24%. Its combustion is characterized by intense exothermic reactions and severe dripping, making it highly susceptible to secondary fires. This severely limits its application in fire protection, electronics, aerospace, and other scenarios with high flame-retardant requirements. Therefore, flame-retardant modification of nylon 6 has urgent industrial value and practical significance.

[0003] Currently, flame retardant modification of nylon 6 is mainly divided into two technical routes: physical blending and chemical copolymerization. Both have significant drawbacks: (1) Physical blending modification: Flame retardant is melt-blended with nylon 6. The process is simple and the cost is low, but the amount of flame retardant added is large, the compatibility with the substrate is poor, and it is easy to migrate and precipitate. This not only greatly degrades the mechanical properties, but also causes the flame retardant performance to decay with the use time, and it is impossible to achieve permanent flame retardancy. When used in fibers, the large size and high amount of flame retardant will destroy the spinnability and make it difficult to industrialize. (2) Chemical copolymerization modification: Flame retardant groups are covalently attached to the molecular chain to achieve intrinsic flame retardancy. The flame retardant efficiency is high, the performance is long-lasting, and the compatibility is good. It is the mainstream development direction, but the existing technology has not yet solved the core problem. Organosilicon-modified nylon 6 is mostly produced by direct copolymerization of caprolactam monomer and terminal amino organosilicon. Terminal amino organosilicon is easily decomposed in high-temperature ring-opening polymerization above 250℃, resulting in poor modification effect and difficulty in stable control of molecular weight and flame retardant properties. The overall process is complex and has poor controllability, making it difficult to achieve both high flame retardant efficiency and good mechanical properties, and thus failing to meet the needs of high-end applications. Summary of the Invention

[0004] To address the aforementioned deficiencies in existing flame-retardant modification techniques for nylon 6, this invention provides a method for preparing intrinsically flame-retardant nylon 6. Using caprolactam as a raw material, and water and 6-aminohexanoic acid as initiators, the method involves ring-opening polymerization to generate a nylon prepolymer (oligopolymer). Simultaneously, a heat-stabilizing agent is added to inhibit degradation and stabilize the molecular weight. The nylon prepolymer is then copolymerized with terminal amino organosilicon. This method solves the problems of easy migration of flame retardants, short-lasting flame retardant performance, decreased mechanical properties, and poor controllability of the polymerization process in existing technologies. It achieves intrinsic flame-retardant modification of nylon 6 while simultaneously improving its flame-retardant, mechanical, and heat-resistant properties.

[0005] To achieve the above technical objectives, the present invention adopts the following technical solution: This invention provides a method for preparing intrinsically flame-retardant nylon 6, comprising the following steps: Preparation of nylon prepolymer (oligomer): Caprolactam, initiator, and heat stabilizer are added to a polymerization reactor. Nitrogen gas is introduced to replace the air in the reactor 3-5 times, maintaining the pressure for 3-5 minutes each time before releasing the pressure to atmospheric pressure to eliminate interference from air on the polymerization reaction. Then, under nitrogen protection, the reaction temperature is raised to 160-180℃, and the reaction pressure is adjusted to 0.1-0.3 MPa. The reaction is carried out for 2-3 hours to allow caprolactam to fully undergo ring-opening polymerization, forming a nylon prepolymer (oligomer) with a number average molecular weight of 500-2000. Copolymerization reaction of intrinsic flame-retardant nylon 6 modified with terminal amino organosilicon: Terminal amino organosilicon was added to the prepared nylon prepolymer (oligopolymer), and stirred evenly at a rate of 40-60 r / min. The reaction temperature was controlled to rise to 200-230℃, and the reaction pressure was maintained at 0.3-0.6 MPa for 3-5 hours. Then the reaction pressure was gradually reduced to atmospheric pressure and the reaction was continued for 1-2 hours. Then the vacuum was drawn to a vacuum degree of -0.08~-0.1 MPa and the vacuum state was maintained for 1-2 hours to remove small molecule by-products and trace water molecules generated by the copolymerization and polycondensation reaction. After the reaction was completed, nitrogen was purged to restore atmospheric pressure and establish a slight positive pressure. The melt was extruded, cooled, and granulated by a water-cooled granulator to obtain intrinsic flame-retardant nylon 6 particles.

[0006] Post-processing: The intrinsic flame-retardant nylon 6 particles obtained in the above steps are added to deionized water at a water-to-material ratio of 8:1. The mixture is continuously stirred during the boiling process, and the water is changed 1-2 times. The boiling temperature is 80-100℃ and the boiling time is 4-8 hours to remove residual small molecule impurities and unreacted monomers on the particle surface. Then, the boiled particles are placed in a vacuum oven and dried at a vacuum degree of -0.09~-0.1MPa and 80-120℃ for 4-6 hours until the particle moisture content is ≤0.05%, resulting in a more stable intrinsic flame-retardant nylon 6 product.

[0007] In the above steps, the initiator is water and 6-aminohexanoic acid, wherein the mass ratio of water to 6-aminohexanoic acid is 9:1. Water is the main catalyst for the ring-opening polymerization of caprolactam, and 6-aminohexanoic acid is a co-initiator and can also be used as a capping agent. The heat stabilizing agent is selected from one or more of calcium phosphite, potassium phosphite, sodium phosphite, boric acid, and phosphoric acid, preferably calcium phosphite. The heat stabilizing agent mainly plays the role of inhibiting thermo-oxidative degradation and stabilizing the molecular chain structure. It has no catalytic activity for the ring-opening of caprolactam and will not interfere with the molecular weight regulation of the prepolymer. The mass ratio of caprolactam, initiator, and heat stabilizing agent is 100:(0.5-2.0):(0.1-0.5).

[0008] The terminal amino organosilicon is a terminal amino polydimethylsiloxane, selected from aminopropyl-terminated polydimethylsiloxane (such as the DMS-A series), with a number average molecular weight of 850-5000 and an amino content of 0.05-3.8 wt%, preferably with a number average molecular weight of 850-5000 and an amino content of 0.6-3.8 wt%; the amount of terminal amino organosilicon added is 5%-20% of the mass of the nylon prepolymer (oligomer).

[0009] The limiting oxygen index was determined according to GB / T 2406.2-2009; the vertical flammability rating was determined according to GB / T2408-2021; the tensile strength was tested according to GB / T 1040.2-2022; the impact strength was tested according to GB / T1043.1-2008; and the flexural strength was tested according to GB / T 9341-2008.

[0010] Beneficial effects This invention employs a two-step process of first preparing a prepolymer and then copolymerizing it, enabling terminal amino-terminated organosilicon to be covalently embedded in the nylon 6 molecular chain, achieving intrinsic flame retardancy. This solves the problems of easy migration of flame retardants, short-lasting flame retardant performance, and decreased mechanical properties in existing technologies. The resulting intrinsically flame-retardant nylon 6 exhibits high flame retardant efficiency, excellent mechanical properties, and a simple and controllable process, making it suitable for large-scale industrial production and widely applicable in the electronics, automotive, and textile industries. Attached Figure Description

[0011] Figure 1 The intrinsic flame-retardant nylon 6 particles prepared in Example 1 of this invention; Figure 2 This is a flame retardant test diagram of Example 1. Detailed Implementation

[0012] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0013] Example 1 A method for preparing intrinsically flame-retardant nylon 6 includes the following steps: 100 kg of caprolactam, 0.45 kg of water, 0.05 kg of 6-aminohexanoic acid, and 0.1 kg of calcium phosphite were added to a polymerization reactor. Nitrogen gas was introduced to replace the air in the reactor three times. Under nitrogen protection, the reaction temperature was controlled at 160 °C and the reaction pressure at 0.1 MPa for 3 hours to remove most of the water molecules generated in the reaction, resulting in a nylon prepolymer (oligomer) with a number average molecular weight of 800.

[0014] Add 6 kg of terminal amino organosilicon (DMS-A21, number average molecular weight 5000, amino content 0.6-0.7 wt%) to the nylon prepolymer (oligopolymer) prepared in step 1, and stir evenly at a stirring rate of 40 r / min; control the reaction temperature to rise to 200℃, maintain the reaction pressure at 0.3 MPa, and carry out the copolymerization reaction for 5 hours; then gradually reduce the reaction pressure to atmospheric pressure and continue the reaction for 2 hours; then evacuate to a vacuum degree of -0.09 MPa, maintain the vacuum state for 2 hours, and remove small molecule by-products in the system; after the reaction is completed, purge with nitrogen to restore atmospheric pressure and establish a slight positive pressure, and extrude, cool, and granulate the melt through a water-cooled granulator to obtain intrinsic flame-retardant nylon 6 particles.

[0015] The intrinsic flame-retardant nylon 6 particles obtained in step 2 were added to deionized water at a water-to-material ratio of 8:1. The boiling process was carried out with continuous stirring at 80°C for 8 hours. Then, the boiled particles were placed in a vacuum oven and vacuum dried at 100°C for 6 hours until the particle moisture content was ≤0.05%, thus obtaining the intrinsic flame-retardant nylon 6 product.

[0016] The intrinsically flame-retardant nylon 6 product obtained in this embodiment was subjected to performance tests, and the test results are as follows: Limiting Oxygen Index (LOI) of 30%, Vertical Burning Rating of UL94V-0, Tensile Strength of 68 MPa, and Impact Strength of 5.5 kJ / m². 2 It has a flexural strength of 92 MPa and a number-average molecular weight of 26,000, exhibiting excellent and stable performance.

[0017] Example 2 100 kg of caprolactam, 0.9 kg of water, 0.1 kg of 6-aminohexanoic acid, and 0.2 kg of calcium phosphite were added to a polymerization reactor. Nitrogen gas was introduced to replace the air in the reactor three times. Under nitrogen protection, the reaction temperature was controlled at 170°C, the reaction pressure at 0.2 MPa, and the stirring rate was maintained for 2.5 hours to remove most of the water molecules generated in the reaction, resulting in a nylon prepolymer (oligomer) with a number average molecular weight of 1000.

[0018] Add 10 kg of terminal amino organosilicon (DMS-A21, number average molecular weight 5000, amino content 0.6-0.7 wt%) to the nylon prepolymer (oligopolymer) prepared in step 1, and stir evenly at a stirring rate of 50 r / min; control the reaction temperature to rise to 210℃, maintain the reaction pressure at 0.4 MPa, and carry out the copolymerization reaction for 4 hours; then reduce the reaction pressure to atmospheric pressure and continue the reaction for 1.5 hours; then evacuate to a vacuum degree of -0.09 MPa, maintain the vacuum state for 2 hours, and remove small molecule by-products in the system; after the reaction is completed, purge with nitrogen to restore atmospheric pressure and establish a slight positive pressure, and extrude, cool, and granulate the melt through a water-cooled granulator to obtain intrinsic flame-retardant nylon 6 particles.

[0019] The intrinsic flame-retardant nylon 6 particles obtained in step 2 were added to deionized water at a water-to-material ratio of 8:1. The boiling process was carried out with continuous stirring at 80°C for 8 hours. Then, the boiled particles were placed in a vacuum oven and vacuum dried at 100°C for 6 hours until the particle moisture content was ≤0.05%, thus obtaining the intrinsic flame-retardant nylon 6 product.

[0020] The intrinsically flame-retardant nylon 6 product obtained in this embodiment was subjected to performance testing. The test results are as follows: Limiting Oxygen Index (LOI) is 32%, Vertical Burning Rating is UL94V-0, Tensile Strength is 65 MPa, and Impact Strength is 5.8 kJ / m². 2 With a flexural strength of 89MPa and a number-average molecular weight of 25,000, it exhibits stable performance and meets the practical application requirements of conventional fire-resistant scenarios such as electronic and electrical appliance housings and general plastic parts.

[0021] Example 3 A method for preparing intrinsically flame-retardant nylon 6 includes the following steps: 100 kg of caprolactam, 1.8 kg of water, 0.2 kg of 6-aminohexanoic acid, and 0.5 kg of calcium phosphite were added to a polymerization reactor. Nitrogen gas was introduced to replace the air in the reactor five times. Under nitrogen protection, the reaction temperature was controlled at 180°C and the reaction pressure at 0.3 MPa. The reaction was carried out for 2 hours to remove most of the water molecules generated in the reaction, resulting in a nylon prepolymer (oligomer) with a number average molecular weight of 1500.

[0022] Add 15 kg of terminal amino organosilicon (DMS-A12, number average molecular weight 900-1000, amino content 3.0-3.2 wt%) to the nylon prepolymer (oligopolymer) prepared in step 1, and stir evenly at a stirring rate of 60 r / min; control the reaction temperature to rise to 230℃, maintain the reaction pressure at 0.6 MPa, and carry out the copolymerization reaction for 3 hours; then gradually reduce the reaction pressure to atmospheric pressure and continue the reaction for 1 hour; then evacuate to a vacuum degree of -0.09 MPa, maintain the vacuum state for 1 hour, and remove small molecule by-products in the system; after the reaction is completed, purge with nitrogen to restore atmospheric pressure and establish a slight positive pressure, and extrude, cool, and granulate the melt through a water-cooled granulator to obtain intrinsic flame-retardant nylon 6 particles.

[0023] The intrinsic flame-retardant nylon 6 particles obtained in step 2 were added to deionized water at a water-to-material ratio of 8:1. The boiling process was carried out with continuous stirring at 80°C for 8 hours. Then, the boiled particles were placed in a vacuum oven and vacuum dried at 100°C for 6 hours until the particle moisture content was ≤0.05%, thus obtaining the intrinsic flame-retardant nylon 6 product.

[0024] The intrinsically flame-retardant nylon 6 product obtained in this embodiment was subjected to performance tests, and the test results are as follows: Limiting Oxygen Index (LOI) is 35%, Vertical Burning Rating is UL94V-0, Tensile Strength is 61 MPa, and Impact Strength is 6.2 kJ / m². 2 It has a flexural strength of 86 MPa, a number-average molecular weight of 28,000, and excellent flame retardant properties.

[0025] Comparative Example 1 100 kg caprolactam, 0.45 kg water, 0.05 kg 6-aminohexanoic acid, 0.1 kg calcium phosphite, and 10 kg terminal amino organosilicon (DMS-A12) were added to the polymerization reactor, and nitrogen was introduced to replace the air four times. The reaction temperature was controlled to rise to 200°C, the reaction pressure was 0.4 MPa, and the reaction was carried out for 6.5 hours. The subsequent vacuum treatment and post-treatment steps were the same as in Example 1.

[0026] The product obtained from the comparative example underwent performance testing, and the results are as follows: Limiting Oxygen Index (LOI) of 27%, Vertical Flammability Rating of UL94V-1, Tensile Strength of 55 MPa, and Impact Strength of 4.3 kJ / m². 2 The product has a flexural strength of 78 MPa, a number-average molecular weight of 18,000, and contains residual decomposition of terminal amino organosilicon, resulting in poor performance stability.

[0027] Comparing Example 1 with Comparative Example 1, it can be seen that the present invention adopts a two-step process of first preparing a prepolymer and then copolymerizing and modifying it, which effectively avoids the decomposition of terminal amino organosilicon during high temperature and high humidity polymerization, and significantly improves the flame retardant properties, mechanical properties and molecular weight uniformity of the product, with obvious advantages.

[0028] Comparative Example 2 100 kg of pure nylon 6 granules and 10 kg of amino-terminated organosilicon (DMS-A12) were added to a twin-screw extruder and melt-blended at 230 °C. The mixture was then extruded and granulated to obtain flame-retardant nylon 6 granules. The post-processing steps were the same as in Example 1.

[0029] The product obtained from the comparative example underwent performance testing, and the results are as follows: Limiting Oxygen Index (LOI) of 25%, Vertical Flammability Rating of UL94V-2, Tensile Strength of 52 MPa, and Impact Strength of 4.0 kJ / m². 2 The bending strength is 75 MPa. After immersing the product in water at 80°C for 24 hours, the limiting oxygen index drops to 24%, and the flame retardant performance is significantly reduced, indicating that the flame retardant is prone to migration and cannot achieve permanent flame retardancy.

Claims

1. A method for preparing intrinsically flame-retardant nylon 6, characterized in that, Includes the following steps: (1) Caprolactam, initiator and heat stabilizer are reacted under nitrogen protection at 160-180℃ and 0.1-0.3MPa for 2-3 hours to prepare nylon prepolymer with a number average molecular weight of 500-2000; (2) Add terminal amino organosilicon to the nylon prepolymer, and copolymerize at 200-230℃ and 0.3-0.6MPa for 3-5 hours. React under normal pressure, vacuum devolatilization, and cooling granulation to obtain intrinsic flame-retardant nylon 6 particles.

2. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, In step (1), the initiator is water and 6-aminohexanoic acid, wherein the mass ratio of water to 6-aminohexanoic acid is 9:

1.

3. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, In step (1), the heat stabilizing agent is selected from one or more of calcium phosphite, potassium phosphite, sodium phosphite, boric acid, and phosphoric acid.

4. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, In step (1), the mass ratio of caprolactam, initiator, and heat stabilizer is 100:0.5-2.0:0.1-0.

5.

5. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, In step (2), the terminal amino organosilicon is an amino-terminated polydimethylsiloxane, selected from aminopropyl-terminated polydimethylsiloxane, with a number average molecular weight of 850-5000 and an amino content of 0.05-3.8wt%.

6. The method for preparing intrinsically flame-retardant nylon 6 according to claim 5, characterized in that, In step (2), the amino content of aminopropyl-terminated polydimethylsiloxane is 0.6-3.8 wt%.

7. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, The amount of terminal amino organosilicon added is 5%-20% of the mass of nylon prepolymer.

8. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, Step (1) The specific steps are as follows: Add caprolactam, initiator and heat stabilizer to the polymerization reactor, purge the air in the reactor with nitrogen 3-5 times, and release the pressure to atmospheric pressure after each pressurization for 3-5 minutes; then, under nitrogen protection, raise the reaction temperature of the system to 160-180℃, adjust the reaction pressure of the system to 0.1-0.3MPa, and react for 2-3 hours to form a nylon prepolymer with a number average molecular weight of 500-2000.

9. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, The specific steps of step (2) are as follows: add terminal amino organosilicon to the prepared nylon prepolymer, stir evenly at a rate of 40-60 r / min, control the reaction temperature to rise to 200-230℃, maintain the reaction pressure at 0.3-0.6 MPa, and carry out copolymerization reaction for 3-5 hours; then gradually reduce the reaction pressure to normal pressure, continue the reaction for 1-2 hours, then evacuate to a vacuum degree of -0.08~-0.1 MPa, maintain the vacuum state for 1-2 hours; after the reaction is completed, purge with nitrogen to restore normal pressure and establish a micro positive pressure, and extrude, cool and granulate the melt through a water-cooled granulator to obtain intrinsic flame-retardant nylon 6 particles.

10. The method for preparing intrinsically flame-retardant nylon 6 according to claim 1, characterized in that, The present invention also includes post-processing: the obtained intrinsic flame-retardant nylon 6 particles are added to deionized water at a water-to-material ratio of 8:1, and the boiling process is continuously stirred. The water is changed 1-2 times during the entire boiling process. The boiling temperature is 80-100℃ and the boiling time is 4-8 hours. Then, the boiled particles are placed in a vacuum oven and dried at a vacuum degree of -0.09~-0.1MPa and 80-120℃ for 4-6 hours until the particle moisture content is ≤0.05%, thus obtaining an intrinsic flame-retardant nylon 6 product with more stable performance.

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